May 2008
Disclaimer
This publication was prepared for the Canadian Association of Petroleum Producers, the Gas
Processing Association Canada, the Alberta Department of Energy, the Alberta Energy
Resources and Conservation Board, Small Explorers and Producers Association of Canada and
Natural Resources Canada by CETAC-West. While it is believed that the information contained
herein is reliable under the conditions and subject to the limitations set out, CETAC-West and the
funding organizations do not guarantee its accuracy. The use of this report or any information
contained will be at the user’s sole risk, regardless of any fault or negligence of CETAC-West or
the sponsors.
Acknowledgements
This Fuel Gas Efficiency Best Management Practice Series was developed by CETAC WEST
with contributions from:
•
Accurata Inc.
•
Clearstone Engineering Ltd.
•
RCL Environmental
•
REM Technology Inc.
•
Sensor Environmental Services Ltd.
•
Sirius Products Inc.
•
Sulphur Experts Inc.
•
Amine Experts Inc.
•
Tartan Engineering
CETAC-WEST is a private sector, not-for-profit corporation with a mandate to encourage
advancements in environmental and economic performance in Western Canada. The corporation
has formed linkages between technology producers, industry experts, and industry associates to
facilitate this process. Since 2000, CETAC-WEST has sponsored a highly successful ecoefficiency program aimed at reducing energy consumption in the Upstream Oil and Gas Industry.
Head Office
# 420, 715 - 5th Ave SW
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Canada T2P2X6
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Table of Contents
1. Applicability and Objectives .......................................... 1
2. Fundamentals of Refrigeration ...................................... 2
2.1 Refrigeration Circuit
2.2 Types of Refrigerant
2.3 Energy Consumption Targets
3. Inspection, Monitoring and Record Keeping................ 4
4. Efficiency Assessment and Adjustments..................... 7
4.1 Compression
4.2 Refrigerant Condenser
4.3 Economizer
4.4 Sub-Cooler
4.5 Chiller and Low Temperature Separator
4.6 Refrigerant Circuit Piping
5. Facility Engineering Input .............................................17
5.1 Water for Propane Condensing
5.2 Refrigeration – Lean Oil Absorption
5.3 Cascaded Refrigeration
6. Appendices.....................................................................20
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
Appendix F
Appendix G
Propane P-H Diagram
Taking Advantage of Climate
Chiller–LTS Coordination
Refrigeration and Other Processes
Variable Flow in Compressor
Condenser Performance
References
Figures
Figure 2.1 – Simple Refrigeration Circuit
Figure 2.2 – Complex Refrigeration Circuit
Figure 3.1 – Flow Plan for Reducing Energy in a Refrigeration Unit
Figure A.1 – Simple Refrigeration Circuit
Figure A.2 – Pressure-Enthalpy (P-H) Diagram for Propane
Figure A.3 – Propane P-H Diagram – Simple Refrigeration Circuit
Figure A.4 – Complex Refrigeration Circuit
Figure A.5 – Propane P-H Diagram – Economizer
Figure A.6 – Propane P-H Diagram – Subcooler
Figure B.1 – Compressor bhp as Function of Discharge Pressure
Figure C.1 – Refrigeration Unit Feed – Phase Map
Figure C.2 – NGL Recovery by LTS Choke Valve
Figure E.1 – Performance Curves – Centrifugal Compressor
Figure E.2 – Performance Curves – Centrifugal Compressor
Figure F.1 – Propane Temperature-Pressure Equilibrium
Tables
Table 2.1 – Efficiency of Generating Refrigeration
Table 3.1 – Check Sheet for Compressor Operation
Table 3.2 – Check Sheet for Condenser Operation
Table 3.3 – Check Sheet for Economizer Operation
Table 3.4 – Check Sheet for Subcooler Operation
Table 3.5 – Check Sheet for Chiller/LTS Operation
Table 3.6 – Check Sheet for Refrigeration Circuit Piping
Table E.1 – Compressor Comparison Chart
Background
The issue of fuel gas consumption is increasingly important to the oil and gas
industry. The development of this Best Management Practice (BMP) Module is
sponsored by the Canadian Association of Petroleum Producers (CAPP), the
Gas Processing Association Canada (GPAC), the Alberta Department of Energy,
Small Explorers and Producers Association of Canada (SEPAC) Natural
Resources Canada (NRC) and the Energy Resources and Conservation Board
(ERCB) to promote the efficient use of fuel gas in refrigeration units used in the
upstream oil and gas sector. It is part of a series of 17 modules addressing fuel
gas efficiency in a range of devices.
This BMP Module:
•
identifies the typical impediments to achieving high levels of operating
efficiency with respect to fuel gas consumption;
•
presents strategies for achieving cost effective improvements through
inspection, maintenance, operating practices and the replacement of
underperforming components; and
•
identifies technical considerations and limitations.
The aim is to provide practical guidance to operators for achieving fuel gas
efficient operation while recognizing the specific requirements of individual
refrigeration units and their service requirements.
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5.
EFFICIENT USE OF FUEL GAS
IN THE UPSTREAM OIL AND GAS INDUSTRY
MODULE 13 of 17: Refrigeration
1.
Applicability and Objectives
This module focuses on refrigeration units from the perspective of both the
process and the refrigerant sides. They are intimately linked and need to be
considered in tandem in order to achieve the best results.
While the overwhelming majority of refrigeration units operated in the upstream
oil and gas industry (UOG) use propane as the refrigerant, the concepts
discussed within this document are applicable to all refrigerants to varying
degrees. Nevertheless, the examples, unless specifically stated, refer to
propane refrigeration.
In this module the terms fuel use and energy use are used interchangeably.
Refrigeration units employ engines, turbines and/or electric motors to drive the
major equipment. For those plants where compression is motor-driven the
impact upon unit fuel consumption is obviously much reduced, but the savings
still exist, in the form of reduced electrical power consumption. The energy
management concepts that are outlined in this document are applicable all
cases.
The objectives of this module are to:
•
Describe the fundamental operation of a refrigeration unit in terms of the
thermodynamics involved. This description is for the overall unit and for
each piece of component equipment.
•
The target audience are the operators of the facilities, although input from
corporate and technical functions elsewhere within the company will be
necessary and of vital importance.
•
Investigate energy management opportunities for the overall process and
for each major piece of equipment both on the refrigerant side and on the
process gas side.
•
Extend the study to other processes that are affected by the refrigeration
unit and vice versa.
Efficient Use of Fuel Gas in Refrigeration Units
Module 13 of 17
Rev Date 27/05/2008
Page 1 of 52
2.
Fundamentals of Refrigeration
Refrigeration is a complex process, involving a long list of operations typically
found in upstream oil and gas facilities – compression, aerial cooling, heat
exchange, instrumentation and controls. Overlaying those operations are the
impacts of thermodynamic cycles and vapour–liquid equilibria.
2.1
Refrigerant Circuit
The most basic refrigeration circuit consists of a compressor, condenser,
expansion valve and a chiller/evaporator. Typically, in sour gas plants, ethylene
glycol (EG) will be injected into the chiller and upstream process gas exchangers,
such as the gas-gas exchanger, in order to prevent water freezing. The EG
regeneration equipment is typically located in the refrigeration unit. Energy
management of the EG loop is discussed in another module. Figure 2.1 is a
schematic of a simple (single-stage) refrigeration circuit.
Simple Refrigeration Circuit
Condenser
Compressor
Process Gas
Chiller
Receiver
Expansion Valve
Figure 2.1
Simple Refrigeration Circuit
More complex units will have an economizer or a propane sub-cooler, or both.
The increase in equipment complexity results in a much more energy-efficient
process. Figure 2.2 is a schematic of a complex (two-stage) refrigeration circuit.
Efficient Use of Fuel Gas in Refrigeration Units
Module 13 of 17
Rev Date 27/05/2008
Page 2 of 52
Analysis of the thermodynamics of the refrigeration process – using propane and how to determine equipment and process efficiency are discussed in
Appendix A.
Complex Refrigeration Circuit
Condenser
Compressor
Process Gas
Chiller
Receiver
Economizer
Subcooler
Expansion Valve
Figure 2.2
Complex Refrigeration Circuit
The design of refrigeration units is dictated by the need to compress and
subsequently condense propane during the hottest days of summer. However,
the thermodynamics are much more efficient in cold weather. The climate in
Western Canada is therefore very conducive to the realization of energy savings
through the effective operation of the refrigeration unit. Appendix B explores the
climatic effect on compression power requirements.
The refrigeration unit is a utility. However, it should not be considered
independent of the main process. Significant benefits can be achieved by
combining the thermodynamic properties of the process gas and the capabilities
of the refrigeration unit chiller. These benefits can be in the form of either
reduced energy consumption for a constant liquid yield, or increased liquid yield.
Appendix C discusses this concept.
Refrigeration units are sometimes operated in conjunction with lean oil absorbers
for NGL recovery. Plants that have deep-cut fractionation of the sales gas, which
require extremely cold condensing temperatures, combine two refrigeration
systems: the first uses ethane or ethylene for the main process refrigeration and
the second refrigeration loop uses propane to condense the first refrigerant. See
Appendix D for more information.
Efficient Use of Fuel Gas in Refrigeration Units
Module 13 of 17
Rev Date 27/05/2008
Page 3 of 52
2.2
Types of Refrigerant
The most common refrigerant used in the upstream oil and gas industry is
propane. Other refrigerants are used occasionally, such as ethane, ethylene,
propylene, ammonia and various derivatives of Freon.
The ultimate choice of refrigerant is based, to a certain extent, upon availability,
price, and compatibility with the rest of the process. However, the major factor
when choosing a refrigerant is its thermodynamic properties – what chilling
temperature can be achieved and what compression power is required.
It should be pointed out that each refrigerant has its own thermodynamic
properties. While they are essentially identical in form, the shape/slope of each
curve is somewhat different so that the impact of equipment, such as an
economizer and a sub-cooler, is different.
2.3
Energy Consumption Targets
Power consumption in a refrigeration unit will be a function of the type of
compressor, the operating mode (dew point control, NGL recovery) and the
ambient conditions.
The following Table 2.1 lists the brake power input to the compressor per ton of
refrigeration in the chiller. 1 (A ton of refrigeration – TR - is an old unit of
measurement, based upon the amount of heat required to melt one ton of ice in a
day. It is equivalent to 12,000 BTU/hr.) The values in the table are those for
relatively small Mycom compressors – as such, they are representative of many
of the refrigeration compressors in Western Canada.
The left column assumes a condensing temperature of 48.9°C, which is
equivalent to an ambient temperature of about 32-35°C. The right column
assumes a condensing temperature of 20°C, which corresponds to an ambient
temperature of 4-6°C. This would be fairly representative of spring and autumn
conditions and is typical of wintertime operation where pressure control is used to
maintain a minimum operating pressure.
The chiller has been set at -10°C, for dew point control, and at -22.7°C, for
increased NGL recovery.
The term “TR at Full Load” indicates the amount of refrigeration that can be
produced when the compressor is at full load. That number multiplied by the
number “brake kW/TR” will allow the calculation of the compressor power
requirement. Power requirements are for brake power output (in kW). For the
amount of input energy required, as well as means to reduce that energy
consumption, are discussed in other module chapters.
Efficient Use of Fuel Gas in Refrigeration Units
Module 13 of 17
Rev Date 27/05/2008
Page 4 of 52
Because of the large number of factors that dictate the amount of refrigeration
that is required and the ability to generate that amount, a precise quantification of
an energy benchmark is impossible in a simple table. Nevertheless, the table
does give an indication of the approximate energy intensity that should be set as
an initial target.
Table 2.1
Efficiency of Generating Refrigeration
Condensing Temperature, °C
48.9
Reciprocating Compressor
Single Stage
20.0
Mycom F12WB
Chiller Temperature, °C
-10.0
-10.0
TR at Full Load
121.7
186.6
Brake kW/TR
1.58
0.75
Chiller Temperature, °C
-22.7
-22.7
TR at Full Load
63.2
108.4
Brake kW/TR
2.22
1.13
Screw Compressor
No Economizer
Mycom P200 VS-M
Chiller Temperature, °C
-10.0
-10.0
TR at Full Load
117.6
163.6
Brake kW/TR
1.72
0.84
Chiller Temperature, °C
-22.7
-22.7
TR at Full Load
69.9
101.5
Brake kW/TR
2.63
1.17
Screw Compressor
With Economizer
Mycom P200 VS-M
Chiller Temperature, °C
-10.0
-10.0
TR at Full Load
146.0
177.2
Brake kW/TR
1.62
0.82
Chiller Temperature, °C
-22.7
-22.7
TR at Full Load
95.1
116.8
Brake kW/TR
2.27
1.12
Efficient Use of Fuel Gas in Refrigeration Units
Module 13 of 17
Rev Date 27/05/2008
Page 5 of 52
Note from Table 2.1 how the ability to generate refrigeration, both in terms of
quantity and efficiency of generation, decreases as the condensing temperature
rises and/or the chiller temperature is reduced.
Note also how the use of an economizer will improve the quantity of refrigeration
and generally increase the efficiency of that generation. The same screw
compressor model was used in both the no economizer/economizer cases.
Efficient Use of Fuel Gas in Refrigeration Units
Module 13 of 17
Rev Date 27/05/2008
Page 6 of 52
3.
Inspection, Monitoring and Record Keeping
Operators should have a record keeping program to support the company’s
refrigeration testing and improvement program. Proper record keeping should
assist in ensuring that operations are maintained at peak efficiency more
discussion on efficient operations can be found in section 4. Record and retain
the following efficiency related information:
Table 3.1
Check Sheet for Compressor Operation
Frequency
Pressure and Temperature Suction (Readings)
Daily
Pressure and Temperature Discharge (Readings)
Daily
Brake Power (Reading or Calculation)
Daily
Unit Performance (HC Dew Point and/or NGL Recovery)
Daily
Chiller Pressure (Reading) and delta P to Compressor
Daily
Receiver Pressure (Reading) and delta P from Compressor
Daily
Is there Pressure Control Activated on Receiver (Y / N)
Daily
Is there Heating of the Refrigerant in the Suction Scrubber (Y/N)
Weekly
Is this Heating Required (Y / N)
Weekly
Efficient Use of Fuel Gas in Refrigeration Units
Module 13 of 17
Rev Date 27/05/2008
Page 7 of 52
Table 3.2
Check Sheet for Condenser Operation
Frequency
Temperature Approach (Value)
Monthly
Propane Purity (% C2, %C4s)
Seasonally
Pressure Control Setting on Receiver (Reading)
Fan Pitch (Setting)
Monthly
Seasonally
Condenser Bundle Condition
Extent of Fouling (Qualitative Assessment)
Extend of Fin Damage (Qualitative Assessment)
Monthly
Seasonal
Top Louvre Position (% Open)
Weekly
Side Louvre Positions (Open / Shut)
Weekly
Recirculation Air (Y / N)
Weekly
Augmented Cooling (Y / N)
Weekly
Fan Blade Damage (Y / N)
Monthly
Fan Belts (Loose? / Damaged?)
Weekly
Hot Air Draw-in From Other Plant Equipment (Y / N)
Table 3.3
Check Sheet for Economizer Operation
Frequency
Economizer Pressure Setting (Reading)
Monthly
Pressure Drop between Economizer and Compressor (Value)
Monthly
Table 3.4
Check Sheet for Sub-Cooler Operation
Frequency
Accuracy of Temperature Readings
Monthly
Temperature Approach (Value)
Monthly
Efficient Use of Fuel Gas in Refrigeration Units
Module 13 of 17
Rev Date 27/05/2008
Page 8 of 52
Table 3.5
Check Sheet for Chiller/LTS
Frequency
Temperature and Pressure in Chiller
Daily
Opening in LCV into Chiller
Daily
Is Chiller LCV Modulating (Y / N)
Daily
Level in Chiller
Daily
Level in Compressor Suction Scrubber
Daily
Temperature and Pressure in LTS
Daily
Hydrocarbon Dew Point of Sales Gas
Daily
NGL Production
Daily
Table 3.6
Check Sheet for Refrigeration Circuit Piping
Frequency
Control Valve Openings
Daily
Eliminate Unnecessary Pressure Restrictions
Weekly
Check Compressor Oil Addition Rates
Weekly
Evaluate Condenser Performance
Monthly
The information collected in record keeping will assist in establishing the required
checking frequency and demonstrate improvements in fuel gas efficiency.
Efficient Use of Fuel Gas in Refrigeration Units
Module 13 of 17
Rev Date 27/05/2008
Page 9 of 52
4.
Efficiency Assessment and Adjustments
Whether the plant has a simple or complex configuration, energy use
optimization is performed in a similar manner. Figure 3.1 is a flow plan for
evaluating a refrigeration unit.
Confirm Measurement
Accuracy
Repair, Replace,
Install
Set Study
Goals
Investigate
Opportunities
Data Collection
Compressor
(Section 3.1)
Condenser
(Section 3.2)
Evaluate
Opportunities
Y
Poor/Missing
Data?
Economizer
(Section 3.3)
Subcooler
(Section 3.4)
Opportunities
Feasible?
N
N
Discard
Data Analysis/
Reconciliation
Y
Identify Opportunities/
Calculate Benchmark
Implement
Opportunities
Form Refrig'n Unit
Energy Mgmt Team
Determine/Confirm
Operating Targets
Chiller/LTS
(Section 3.5)
Piping
(Section 3.6)
Figure 4.1
Flow Plan for Reducing Energy in a Refrigeration Unit
When conducting an energy management study of the refrigeration unit the main
parameters to track are the absolute amount of energy consumed. Absolute
amount of energy consumed should be represented in terms of brake power on
the compressor and refrigeration obtained, for example, tons of refrigeration per
brake compressor power. In thermodynamics, this is often referred to as the
coefficient of performance.
The energy consumed can also be expressed and tracked as a function of the
plant processing conditions – the amount of power compared with the product
quality. For a unit that is designed for dew point control, the function would be
power versus sales gas dew point. For a plant that wants NGL recovery, it would
be power versus the NGL production.
Ambient conditions need to be accounted for, in view of the tremendous impact
that condensing temperature has upon the entire refrigeration process.
Efficient Use of Fuel Gas in Refrigeration Units
Module 13 of 17
Rev Date 27/05/2008
Page 10 of 52
4.1
Compressors
Compression is the largest energy input into a refrigeration unit. As such, it is
imperative that attention be directed at reducing energy consumption on the
compressor(s). Much of the reduction will be the direct result of actions taken
elsewhere in the unit and even in other operating units. The reader is referred to
those parts of this module for guidance. This sub-section looks at the
compressor(s) specifically.
The three types of compressors found in refrigeration units are screw,
reciprocating and centrifugal. They operate on different principles and these
dictate, to varying extents, the type and amount of fuel savings that can be
achieved. The comparison between the different compressors is shown in Table
E.1 Appendix E.
The fundamental problem with refrigeration compressors is that they are all
designed to work best at one point. The unit design point is the maximum
operating temperature that will be experienced at the plant. The climate in
Western Canada dictates that the compressor(s) could be working at a point
considerably away from that design, with the consequent loss of energy
efficiency. Every effort should be made to take advantage of the optimum
operating point which is attained by minimizing the refrigeration flow and
maximizing the suction pressure.
The following pros and cons of the main compressor types assume that the
machines are operating at design efficiency. Opportunities for fuel savings
through maintenance repair are discussed in other sections.
Refrigerant Flow Control
The actions taken to make the refrigeration unit more efficient generally result in
a reduction in the circulation of refrigerant. Operators should therefore direct
attention towards greater refrigerant flow control.
Where there is automatic flow control, such as through engine speed
modulations, there is no real activity required by operators except to ensure that
the desired control is achieved. Where flow control is manually adjusted, this will
generally be done using variable pockets and/or unloaders. Since there will be
fluctuations in the plant process and the weather, there will be a constantly
changing process demand. To attempt to attain very tight control would require
an inordinate amount of operation attention. It is preferable to adjust the pockets
(and unloaders) at a setting that will ensure all fluctuations can be handled but
still result in reduced energy consumption. While the savings are less, so is the
required attention.
Efficient Use of Fuel Gas in Refrigeration Units
Module 13 of 17
Rev Date 27/05/2008
Page 11 of 52
If adopting this strategy, the situation should be reviewed periodically (for
example, seasonally) and further adjustments made. The important point is to
make as frequent adjustments as can be conveniently handled by the staff and
equipment on site.
This action ideally requires that the operators are able to determine the flow of
propane through the compressor and the amount flashing across the LCV into
the chiller. The best way would be to have a meter on the propane, combined
with a chart to show the percentage that flashes.
Metering is often not included in the design of the unit. Typically, though, the
alternative is a qualitative determination where the operators monitor the
compressor performance relative to the results achieved – the hydrocarbon dew
point or the NGL production.
Appendix E contains examples of the effects of controlling variable flow in the
three types of compressors.
Compressor Suction Pressure
It is desirable to keep the compressor suction pressure as high as possible while
still attaining process requirements. Generally speaking, the suction pressure
will be roughly 7-14 kPa (1-2 psi) below the chiller pressure. The impact of chiller
operation upon refrigeration is discussed in Section 4.5 of this module.
Operators should ensure that there are no pressure restrictions – leading to
untoward pressure drops – in the piping between the chiller and the compressor
suction scrubber. The same check should be done to ensure the piping from the
scrubber to the compressor is similarly free of restrictions, such as plugged
witch’s hats.
Any avoidable restrictions should be eliminated since even small pressure drops
will result in disproportional increases in the compression ratio, and thus the
power required.
Reduction of suction pressure is sometimes used as a method of flow control.
Compressors handle actual volumes of gas and by reducing the pressure the
density of the incoming gas, and hence the mass of gas, is reduced. However,
the compression ratio is increased and this counterbalances the reduction in
power requirements.
Depending upon the particular operating conditions and the compressor design,
a reduction in suction pressure could lead to rod load and/or rod reversal
problems.
Efficient Use of Fuel Gas in Refrigeration Units
Module 13 of 17
Rev Date 27/05/2008
Page 12 of 52
Some compressors heat their refrigerant in the suction scrubber in order to
prevent liquids build-up in the scrubber and possible carryover into the
compressor. While this prevents damage to the compressor, it does result in
increased fuel use since it superheats the compressor feed and adversely
impacts the isentropic compression of the refrigerant.
It is recommended that the use of heating coils in the compressor suction
scrubber be minimized wherever possible, consistent with safe operation of the
compressor.
4.2
Refrigerant Condenser
The propane condenser determines the discharge pressure on the compressor
and thus the power requirements. Nearly all propane condensers in Western
Canada use aerial cooling. A few have water cooling and there can be benefits
to using a combination of both air and water as cooling media (See Appendix D).
It is critical that the condenser be operated as efficiently as possible. Fouling of
the condenser or impurities in the circulating refrigerant are two areas where
condenser performance can be seriously impacted. They are discussed in more
detail in Appendix F.
The condenser directly influences the discharge pressure of the compressor. As
such, the condenser should be monitored constantly. The following energy
management action items are recommended.
•
Monitor the temperature approach. Compare the outlet temperature
versus the ambient air temperature when the condenser has just been
cleaned. Using this as the basis of comparison, measure the temperature
approach regularly. A widening temperature gap is an indication that the
cooling efficiency is declining. Note that precipitation will improve the
temperature approach.
•
Ensure that the fan pitch is appropriate. If the fan motor is controlled by a
VFD, set the fan blades at the optimum pitch and allow the VFD to adjust
the speed and hence the air flow. Normally, it is desirable to maximize the
air flow until the minimum system operating pressure is reached.
•
Ensure adequate air flow. Check the gap between the fan blade tip and
the fan housing – if more than about one inch, backflow of air will occur,
which reduces cooling. Check the fan blades for damage. Ensure that
the fan belts are not worn or damaged.
•
Clean the condenser bundle. It is recommended that aerial coolers be
cleaned after significant fouling events (such as poplar fluff) but prior to
hot weather so that there is maximum cooling capability over the summer.
Efficient Use of Fuel Gas in Refrigeration Units
Module 13 of 17
Rev Date 27/05/2008
Page 13 of 52
•
Avoid practices that lead to fin damage. Use of high pressure water spray
can lead to fin damage, which reduces the effective area of the
condenser.
•
Avoid the use of water spray to augment cooling. If absolutely necessary,
consider spraying underneath the bundle and have the water drawn up
through the bundle. Consult the cooler manufacturer prior to using water
spray.
•
Check to make sure that hot air from another part of the plant process is
not being drawn into the condenser fan. This phenomenon can be
demonstrated using smoke bombs or even sensed by positioning oneself
around the perimeter of the cooler underneath the bundle. If there is air
being drawn in, determine the source of the hot air and investigate ways to
resolve the problem. In some plants, the outlet plenum of the offending
cooler is extended approximately 6-8 feet.
•
Check the refrigerant purity on a regular basis and when there is any
suspicion that purity has changed. Indications of a change can be quickly
checked by plotting the temperature and pressure for the condenser,
economizer and chiller versus the expected curve (for pure propane that
would be Figure F.1 or the curve for the actual and for industrial-grade
refrigeration). Preferably, analyse the refrigerant. If possible, ask the
supplier for the composition.
•
Check the pressure control setting on the receiver (also called the
accumulator). In order to keep sufficient pressure in the refrigerant circuit
for maintaining circulation, a small stream of hot refrigerant vapour is
bypassed around the condenser to the receiver and flow-controlled by the
pressure in the receiver. Ensure that the pressure setting is not higher
than needed. (This presupposes that the minimum discharge pressure
required/allowed by the circuit is known.)
4.3
Economizer
An economizer can significantly reduce energy needs in a refrigeration unit. It
removes vapour from the refrigeration circuit at an intermediate pressure, thereby
reducing the amount that would have to be compressed up from the suction
scrubber pressure.
Not all units will have an economizer. As discussed in Section 4.1, some
compressors are not configured to allow the installation of an economizer.
Others have not been so equipped due to capital expenditure consideration
during the original plant construction.
The following energy management opportunities are suggested concerning the
operation of the economizer.
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•
Operate the economizer at the optimum pressure. Typically, the optimum
pressure is the geometric average of the compressor discharge and the
compressor suction pressures. That can be determined by multiplying the
discharge and suction pressures – expressed in absolute terms (kPaa or
psia) – and then taking the square root of that number. For example, if the
suction pressure is 70 kPag and the discharge is 1380 kPag, the
approximate optimum pressure would be
SQ RT [(70 + 90) x (1380 + 90)] = 485 kPa abs = 395 kPag
The optimum pressure could then be more precisely found by making
small iterative adjustments until the minimum compression power is
achieved, while maintaining the process requirements.
•
Check the pressure drop between the economizer and the compressor.
Unnecessary pressure restrictions should be eliminated in order to reduce
compressor horsepower.
From the method of obtaining the optimum economizer pressure two
fundamental conditions are obvious.
•
The plant must have a method of precisely measuring compression
power. Otherwise, the approximate optimum must be used as a target.
•
The optimum economizer pressure will vary, if the compressor discharge
pressure varies with ambient temperature.
4.4
Sub-Cooler
The purpose of the sub-cooler is to cool the liquid refrigerant stream leaving the
receiver or the economizer. This increases the percentage of liquid that is
entering the chiller. By getting more refrigeration for the same refrigerant flow,
the efficiency of the circuit is improved and energy savings can be realized.
Not all refrigeration units are equipped with sub-coolers. There has to be a cool
stream available. The exchanger is typically installed downstream of the
economizer (if the unit has one). Otherwise, it is installed downstream of the
receiver.
Being a heat exchanger, there are limited actions that operators can/need to do
in order to ensure energy savings. Primarily, they need to ensure that the
exchanger does not become fouled. Fouling reduces the heat transfer and
increases pressure drop through the exchanger. If that occurs on the refrigerant
side, it could reduce the fraction of liquid entering the chiller.
Checking for deterioration in performance is best done by observing the
temperature approach (the outlet of the sub cooler on the refrigerant side versus
the inlet side on the cooling medium). This assumes counter-current cooling,
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which is the configuration in the overwhelming number of cases. A widening
difference is a sign of fouling.
It is highly recommended that the plant check the accuracy of the temperature
dials on the sub cooler (and throughout the unit in general). Thermometers often
tend to be quite inaccurate, making analysis of the performance very difficult.
4.5
Chiller and Low Temperature Separator
The chiller is core of the refrigeration process. The condenser determines the
observable power requirements within the unit, and the economizer/sub cooler
arrangement improves the overall efficiency of the circuit. But, poor chiller
operation will seriously alter the overall economics and thus the energy
consumption. Unfortunately, many of the adverse effects are not obvious and
therefore may be inadvertently overlooked.
The LCV on the chiller drops the pressure to achieve the desired chiller
temperature. Some plants set the valve opening at a fixed level (typically 50%),
instead of letting it modulate (typically between 30-70%).
When the LCV modulates, the chiller pressure tends to stay relatively constant,
regardless of the compressor discharge pressure. However, when the LCV is set
at a fixed value, the chiller pressure drops when the compressor discharge
pressure drops, although obviously not to the same degree. Nevertheless, the
chiller is already at a very low pressure so any drop in pressure causes a
significant rise in the compression ratio, which dictates the compression power
required.
It is a rule of thumb that the chiller should be operated at the highest temperature
(and consequently) pressure that is consistent with the plant’s production goals.
In other words, do not over-chill the process gas. The chiller temperature will be
dictated by whether the goal is to meet sales gas hydrocarbon dew point or to
maximize NGL yield.
Assuming that the chiller temperature that meets the plant’s production goal has
been identified:
•
Operate the chiller level control valve in order to achieve the desired target
conditions in the chiller.
•
Confirm that there is no refrigerant carryover into the compressor suction
scrubber. Excessive carryover usually results in lost refrigerant since it is
often flared. Carryover is an indication that the refrigerant circulation is
too high relative to the process demand or that the LCV is letting too much
liquid too fast into the chiller.
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Depending upon the thermodynamic properties of the process gas and the sales
gas compression, significant energy savings could be realized if the chiller were
considered in conjunction with the low temperature separator (LTS). The
optimum energy use would be achieved when those two vessels were operated
as one, by taking advantage of the Joule-Thompson effect and the process gas
was chilled at stream pressure and then flashed down to the optimum pressure in
the LTS. The reader is referred to Appendix C for a discussion of this
opportunity.
Before the optimum chiller/LTS conditions could be implemented, the following
energy management studies would have to be done. These action items are
generally outside the scope of operators. They usually involve computer
simulations, or similar complex calculations. Obviously, if product values change
significantly, it would be necessary to repeat the above steps using the new data.
•
Determine the optimum process temperature,
•
Determine the optimum LTS pressure, and
•
Determine the chiller temperature that gives the optimum temperature in
the LTS at the optimum LTS pressure.
The reader is referred to Section 4 for more detail regarding these studies.
Assuming that the operators are provided with the target temperature and
pressure:
•
4.6
Operate the chiller level control valve in order to achieve the desired target
conditions in the LTS.
Refrigerant Circuit Piping
The piping in the refrigeration circuit is sized to handle the original design flow
rates. Subsequent to plant start-up, conditions could have changed considerably
and the existing piping may be causing a loss of energy efficiency.
Restricted piping causes unwanted pressure drops. These, in turn, cause
generation of vapour according to the refrigerant vapour-liquid equilibrium curve.
Vapour reduces the amount of liquid refrigerant to the chiller, which reduces the
amount of possible chilling. Ultimately, it could limit the circulation of refrigerant
at low circuit pressure by causing the chiller LCV to go wide open, thereby losing
control capability.
It is necessary to determine the minimum compressor discharge pressure that
the refrigeration unit can handle, since the unit works more efficiently at lower
discharge pressures (for the same suction pressure). There are a number of
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limiting conditions, the first-encountered of which dictates the minimum operating
pressure.
•
The chiller control valve goes wide open, thereby losing the ability to
control.
•
The pressure drop through the piping (and control valve) circuit is greater
than the desired discharge pressure. The only way to balance the
delivered pressure and required pressure drop would be to reduce the
flow rate of refrigerant.
•
The reduced pressure causes refrigerant vapour breakout upstream of the
chiller thereby reducing the amount of refrigeration capable in the chiller.
•
The lower pressure causes excessive gas velocity through the oilrefrigerant separator on the compressor discharge such that the oil is not
entirely separated.
The first three limits are readily observable in normal operation. The last,
however, shows no immediate signs of a problem. However, there will be a loss
of oil inventory which is the first indication of trouble. In the long-term, the lost oil
will coat the internal side of the tubes of the condenser and potentially even the
external side of the tubes of the chiller. The oil coating will reduce heat transfer
and seriously impact performance – compressor discharge pressures will rise
and the refrigeration effect will drop.
The following action is recommended in order to determine the operating
boundary:
•
Determine the minimum discharge pressure on the refrigerant
compressor. This is best done by consulting with the designer of the unit
(to first determine conditions which absolutely must be avoided) and
experimenting to see how far the pressure can go without adversely
affecting performance. While Facility Engineering should take the lead
role in this exercise, operator input is extremely important and useful.
Once the minimum operating pressure has been determined, the following
actions, by the operating, are recommended:
•
Ensure there are no unwanted pressure restrictions in the refrigerant
circuit. Some restrictions such as level control valve pressure drops into
the economizer and the chiller are desired. On the other hand, control
valves should not be wide open. Inadvertent pressure drops caused by
partially-closed valves or material in the piping should be eliminated
wherever possible.
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•
Frequently note the inventory of oil in the compressor. Be aware of
increased need for additional oil. The loss of oil is an indication of
carryover to the condenser, which leads to the following action.
•
Ensure that the condenser tube internals are clean. This is necessary if
there are indications of loss of condenser performance that cannot be
explained by conditions on the condenser external surface (fouling, fin
damage, etc.) or fan performance (See Section 4.2). It may be necessary
to extend the cleaning to downstream exchangers such as the sub cooler
and the chiller. Operator involvement in this step is vital in order to ensure
that the deteriorating condition is identified as quickly as possible.
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5.
Facility Engineering Input
The foregoing review of energy management in refrigeration units has illustrated
that there is a wide range of opportunities for savings and/or improved product
value. The ability for a plant to take advantage of them falls into three
categories:
•
immediate, low/no capital investment,
•
minor capital investment,
•
major capital investment.
Experience has shown that roughly half of the savings identified in energy audits
are in the first category. A well-run unit will accrue significant savings. This
requires mostly operator attention and this module has been prepared with that
goal in mind.
On the other hand, capital investment – coupled with good operating practices –
will greatly improve the energy efficiency of the unit. While facility engineering
should take the lead role, operators can provide valuable input in view of their
experiences and observations from running the specific, or a similar, unit.
This section discusses the role that facility engineering can play in ensuring that
the refrigeration unit is operated as efficiently as possible.
5.1
Strategic Goals and Operating Parameters
The conventional refrigeration unit consists of a simple circuit – no economizer
and no sub-cooler. While that configuration involves the least capital investment,
it forgoes substantial savings in operating costs and/or liquid recovery. Retrofits
will bring about those savings, but at a much higher installation cost than if the
same configuration had been incorporated into the original design.
When considering a new refrigeration unit, or modification of an existing unit, it is
important to establish the strategic goals and the consequent operating
parameters required to meet those goals.
•
Clearly establish the goal(s) of the unit. Is the unit to be operated for
hydrocarbon dew point control or liquid recovery?
•
Clearly determine the operating parameters. Dependent upon the unit
goals, what range of operating conditions are desired? Generally, with a
wide the range of conditions, there will be deterioration of efficiency at
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some point within that range. It is also important to know how those
conditions will change over time.
•
Design with maximum unit complexity consistent with unit goals. If
increases in complexity – such as installation of an economizer – are
forecasted, try to avoid costly retrofits by increased capital investment at
the time of original design.
•
Ensure that staff are well trained in energy management concepts with
respect to the process and to the equipment.
5.2
Retrofits and New Units
The following actions should be considered, primarily by facility engineers, when
designing a new refrigeration unit or modifying an existing one. They have been
listed by equipment. Where appropriate, facility engineering should seek
operator input into the specific design.
•
Compressor
Determine the minimum discharge pressure on the refrigerant
compressor. This is best done by consulting with the designer of the unit
(to first determine conditions which absolutely must be avoided) and
experimenting to see how far the pressure can go without adversely
affecting performance.
•
Economizer
Consider the feasibility of installing one, especially if the compressor(s)
can accept an intermediate feed. Space, especially in small units, is also
an important factor, due to the normally-congested nature of the
refrigeration skid.
•
Sub-Cooler
If the plant is considering the installation of a sub-cooler, and there are
multiple choices for the cooling medium, consider the options from the
point of view both the refrigerant circuit and the rest of the plant. In other
words, where is the best place within the entire facility for taking the
cooling stream(s)?
•
Chiller/LTS
Determine the optimum process temperature. Usually, that is dictated by
whether the plant is cooling the process gas in order to achieve
hydrocarbon dew point control or whether the goal is NGL recovery. In
the first case, the temperature is generally around -17°C to -10°C (0°F14°F). In the second case, the will often be run at -30°C or colder.
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Determine the optimum LTS pressure. That is typically done using a
phase envelope and an overall holistic study wherein compression
efficiencies (on the refrigeration unit and on the process side), relative
product values for sales gas and NGL/NGL individual components are
considered. If there is an incentive to choke the process gas into the LTS,
it will be necessary to determine the chiller temperature that gives the
optimum temperature in the LTS at the optimum LTS pressure.
•
Piping
Ensure there are no unwanted pressure restrictions in the refrigerant
circuit. Some restrictions such as level control valve pressure drops into
the economizer and the chiller are desired. Inadvertent pressure drops
caused by partially-closed valves or material in the piping should be
eliminated wherever possible.
When modifying the unit, ensure adequately-sized piping and the piping
runs. Obviously, there will be pressure drops in any piping as long as
there is a flow. But where there is some flexibility in the design and
project economics, consider larger-diameter piping.
The piping
configuration – such as rises in elevation - is important also. Once vapour
has broken out of the mix, such as due to a rise in piping elevation, it
does not turn back into liquid on a subsequent drop in elevation unless
there is a change in the energy of the fluid (i.e., from cooling or
compression).
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Appendix A
Propane P-H Diagram
This appendix illustrates how the thermodynamics of a refrigeration circuit, in this
case propane, can be used to analyze the operation of the propane side of the
unit. In doing so, the concepts discussed in the text of the module are illustrated
graphically. For more information regarding refrigeration units, the reader is
referred to the GPSA Engineering Data Book, 11th Edition, Chapter 14.
While this investigation can be done using a simulation package and will produce
more precise results, the use of the P-H diagram is favoured in this case for two
reasons:
•
•
A.1
The graphical method illustrates the concepts more clearly,
The method has much more availability for operating staff.
Simple Refrigeration Cycle
The simplest refrigeration cycle consists of a compressor, condenser, expansion
valve (commonly called a JT valve) and the chiller. Figure A.1 outlines this cycle.
Simple Refrigeration Circuit
Condenser
Compressor
Process Gas
Chiller
Receiver
Expansion Valve
Figure A.1
Simple Refrigeration Circuit
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The calculation basis of the refrigeration cycle is the P-H diagram which outlines
the thermodynamic properties of the refrigerant. (The x-axis is the enthalpy - the
symbol for which is “H” and the y-axis is pressure.) Figure A.2 shows this
information for propane2.
Figure A.2
Pressure-Enthalpy (P-H) Diagram for Propane
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In Figure A.3 a simple, or single-stage, propane refrigeration loop has been
overlaid onto the P-H diagram.
Figure A.3
Propane P-H Diagram – Simple Refrigeration Circuit
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For this loop, the following conditions apply:
•
the ambient temperature is 80°F,
•
the circulating rate of propane is 40,000 lbs/hour, and
•
the chiller operates at 20 psia (-32°F).
The (reciprocating) compressor discharge conditions are 200 psia and 163°F.
The compressor has two stages. Assume that the starting point is the chiller,
which is operating at 20 psi absolute, (-32°F). The outlet of the chiller is 100%
vapour. The enthalpy of the propane is 413 BTU/lb.
There is a 1.5 psi drop between the chiller and the compressor suction. Since no
work is done, the total enthalpy of the propane remains at 413 BTU/lb but there is
a drop in temperature of roughly 3 degrees Fahrenheit, i.e., to -35°F.
Normally, compression in screw and reciprocating machines is isentropic.
Therefore, the theoretical discharge temperature would be 125°F. However,
since the compressor does not operate at constant entropy (the sloping lines
labelled “s” show lines of constant entropy), the discharge temperature is higher.
In this case, the discharge temperature is 163°F.
This information can be used to determine the compressor horsepower and the
overall efficiency of the compressor. The enthalpy of the propane into the
compressor is 413 BTU/lb. The theoretical enthalpy of the discharge is 462
BTU/lb (200 psia and 125°F) and the actual enthalpy is 480 BTU/lb (200 psia and
163°F).
The compression ratio of the compressor is 200 / 20 = 10. This means that the
compression ratio for each of the two stages is 3.16. This is a relatively high
value and potential for rod loading/rod reversal issues should be investigated.
The brake power to the compressor is 480-413 = 67 BTU/lb. For the circulating
load, the total power is 40,000*67 = 2,680,000 BTU/hour, which is equivalent to
1,053 horsepower. The theoretical power input is 462-413 = 49 BTU/hour. The
overall efficiency of compression is therefore 49 / 67 = 73.1%.
Note, strictly speaking, the enthalpy of the circulating oil (if applicable) must be
taken into consideration. The net result of the circulating oil is to reduce the
discharge temperature. This is especially the case with screw compressors
where the amount of heat absorbed by the oil can be very significant.
Typically, there is roughly a 10 psi pressure drop between the compressor
discharge and the condenser, which is therefore operating at 190 psia. The duty
of the condenser is based on the assumption that the outlet of the condenser is
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100% liquid. Therefore, the outlet has an enthalpy of 312 BTU/lb, for an overall
duty of 480 – 312 = 168 BTU/lb, or 6,720,000 BTU/hour.
The temperature approach of the condenser is the difference between the
condenser outlet temperature and the ambient temperature. Since the outlet
pressure is 190 psia, this is equivalent to an equilibrium temperature of 99°F.
The temperature approach is therefore 99 – 80 = 19 degrees Fahrenheit, which
is typical of design values, although in many units much better performance is
achieved.
The chiller conditions are achieved by dropping the pressure through the
expansion valve, which is the level control valve on the chiller. Again, because
there is no work done, there is no loss of enthalpy in the propane as it goes
through the LCV. Graphically, this is shown by the vertical line dropping from the
condenser outlet (P=190 psia, H=312 BTU/lb) to the chiller inlet (P=20 psia,
H=312 BTU/lb.
At the chiller conditions, the enthalpy of the propane when liquid is 234 BTU/lb
and the enthalpy of propane vapour is 413 BTU/lb. The difference – 179 BTU/lb
– is the latent heat of vaporization of propane at 20 psia. The weighted enthalpy
of the vapour-liquid mixture entering the chiller is 312 BTU/lb. The proportion
that is liquid can be calculated by the following formula
(Hvapour – Hmixture) / (Hvapour – Hliquid)
or
(413 – 312) / (413 – 234) = 56.42%
Nearly half of the propane entering the chiller does so in the form of vapour.
While the low temperature of the vapour will contribute to the chilling of the
process gas, the amount is negligible compared to the chilling achieved by
vaporizing the liquid propane entering the chiller. In other words, the amount of
chilling, or refrigeration effect, is related to the amount of liquid propane entering
the chiller. The refrigeration effect is therefore
40,000 lb/hr * 0.5642 * (413 – 234) = 4,040,000 BTU/hour
or
40,000 * (413 – 312) = 4,040,000 BTU/hour
Two terms are frequently used with reference to refrigeration units:
•
Tons of Refrigeration. This is defined as the amount of heat required to
melt 1 ton of ice in 24 hours. It is, by definition, equal to 12,000 BTU/hour.
In our example, the tons of refrigeration total 4,040,000 / 12,000 = 336. 7
TR.
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•
Coefficient of Performance. The coefficient of performance (COP)
equals the amount of chilling divided by the amount of work added in the
compressor. In our example, the COP is 4,040,000 / 2,680,000 = 1.507.
It can be seen from Figure A.3 that any steps that reduce the compression ratio
will increase the proportion of liquid entering the chiller. Fuel requirements will
decrease due to the lower compression ratio and the need to circulate less
propane in order to achieve the same amount of chilling.
A.2
Economizer and Sub-Cooler
The compression power requirements can be reduced by adding either an
economizer, or a sub-cooler, or both. Figure A.4 shows the revised configuration
– in this case, two-stage refrigeration - and Figure A.5 illustrates the impact of the
new economizer on the P-H diagram.
Complex Refrigeration Circuit
Condenser
Compressor
Process Gas
Chiller
Receiver
Economizer
Subcooler
Expansion Valve
Figure A.4
Complex Refrigeration Circuit
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Economizer.
In this example, the compressor consists of two stages and can thus handle an
economizer. The economizer pressure has been set at 70 psia. The enthalpy of
the saturated vapour and the saturated liquid are 432 BTU/lb and 270 BTU/lb,
respectively. The enthalpy of the liquid out of the condenser/receiver is 312
BTU/lb (see previous sub-section). Therefore, the fraction of liquid in the
propane entering the economizer is (432-312) / (432-270) = 74.07%.
The vapour removed in the economizer is returned to stage 2 of the compressor.
Power/fuel savings are achieved because the economizer vapour does not have
to be compressed from chiller pressure. From the viewpoint of the compressor,
the optimum discharge pressure on stage 1 would be roughly 63.3 psia. Note
that the economizer pressure is 6.7 psi higher, that difference being taken up by
the back pressure control valve on the gas off the economizer. This observation
will be discussed later.
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Figure A.5
Propane P-H Diagram - Economizer
The theoretical discharge temperature, assuming constant entropy would be
104°F and the theoretical change in enthalpy would be 450 BTU/lb (at 200 psia,
104°F) minus 432 BTU/lb (at 63.3 psia and vapour into stage 2, which equals 18
BTU/lb. The efficiency of the compressor, from before, was 73.1%, meaning that
the actual input was 18 / 0.731 = 24.6 BTU/lb of propane going to stage 2.
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Since the chiller and compressor suction conditions are identical to the simple
refrigeration cycle scenario discussed in the previous subsection, the change in
enthalpy of the propane vapour into stage 1 is 67 BTU/lb. However, the fraction
of propane entering the chiller has changed dramatically. The vapour that would
have been generated from dropping the pressure from 190 psia (in the receiver)
to 70 psia (in the economizer) has been removed.
The enthalpy of the propane liquid entering the expansion valve is the same as
that of the liquid in the economizer, 270 BTU/lb. Therefore, the fraction of liquid
entering the chiller is (413-270) / (413-234) = 79.89%.
Prior to the use of the economizer, the refrigeration effect was estimated to be
4,040,000 BTU/hour. Since the latent heat of evaporation in the chiller is 413 –
234 = 179 BTU/lb, the required amount of liquid propane in the chiller is 22,570
lb/hour. In view of the fraction of liquid entering the chiller, the required amount
of propane circulation is:
22,570 / 0.7989 = 28,251 lb/hour via the chiller
plus
28,251 / 0.7407 – 28,251 = 9,890 lb/hour removed in the economizer.
The total propane flow is 38,141 lb/hour, whereas the original propane circulation
rate was 40,000 lb/hour. The total power required in the compressor is
28,251 lb/h*67 BTU/lb+9,890 lb/h*24.6 BTU/lb = 2,136,111 BTU/hr = 840 bhp.
The installation of the economizer reduced the overall propane flow by 4.4%, but
reduced the power required by 20.3% (from 1,053 bhp to 840 bhp).
As indicated earlier, stage 2 of the compressor would have an inlet pressure of
about 63.3 psia, but the economizer was being operated at 70 psia. Assume that
the economizer pressure could be reduced to 65 psia by opening the back
pressure control valve. Repeating the calculation, the new power required would
be 834 bhp – a reduction of 6 bhp, worth $3,120/year in fuel at $6.00/GJ.
Interestingly, there is a small increase in the total amount of propane being
circulated. However, the increase is in the economizer off gas and there is a
decrease in the chiller off gas. The savings in stage 1 power (due to reduced
flow) makes up for the small increase in stage 2 power (caused by higher
compression ratio).
The savings in a real situation would depend upon the economizer pressure level
relative to the compressor interstage pressure. For instance, an economizer
pressure set at design summertime levels would be at considerable variance
from the interstage pressure when running in the winter.
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As an example, a 2-stage compressor operating at a suction pressure of 20 psia
and a discharge of 250 psia would have an economizer pressure in the range of
70-71 psia. In the winter, when the discharge could be as low as 120 psia (this
value would depend upon a variety of factors but could be determined
experimentally by the plant), the economizer would be at roughly 50 psia.
Sub-Cooler. A sub-cooler increases the proportion of liquid entering the chiller
by shifting the equilibrium line into the sub-cooled propane region (to the left of
the saturated liquid line). In the experience of this document’s author, a typical
sub-cooler will deliver 10-20 degrees Fahrenheit cooling. For this exercise, 15
degrees is assumed. It is further assumed that the sub-cooler is installed in
conjunction with, and after, the economizer. Graphically, the sub-cooler is shown
in Figure A.6.
The economizer pressure is 65 psia. Therefore, the equilibrium temperature is
28°F. With the installation of a sub-cooler the propane going to the expansion
valve would be at 13°F. This means the enthalpy of the propane into the
expansion valve is 259 BTU/lb and the fraction of liquid entering the chiller is
86.03%.
As a result of installing a sub-cooler, the compressor power required is 786 bhp,
a reduction of 5.8% from the economizer case.
A.3
Economics
In summary, the installation of an economizer and sub-cooler has the following
results:
•
The compressor power requirements dropped 267 bhp, from 1,053 bhp to
786 bhp, or 25.4%.
•
The total propane flow dropped by 10% from 40,000 lbs/hr to 36,016
lbs/hr.
•
The propane condenser duty is correspondingly reduced by 10%. In
terms of power requirements, this represents a reduction in condenser fan
power of approximately 100 – 100 * (.9004 3) = 27.0%. Based on a typical
condenser design, the original installed condenser power requirement was
about 160 bhp. Power requirements on the condenser would be reduced
by 0.27*160=43 bhp (32 kW) on the fan motor output.
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Figure A.6
Propane P-H Diagram - Subcooler
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At an engine firing efficiency of 33.5% (i.e., 7,600 BTU fired/bhp) and a fuel value
of $6.00/GJ, the savings in engine firing would be:
267 bhp * 7,600 BTU LHV/bhp-hr * 8,760 hr/yr * 1.1 HHV/LHV
* 1,055 J/BTU / 1,000,000,000 J/GJ * 6 $/GJ HHV = $123,770/year
Other economic factors to consider are the following:
•
The fuel saved would be equivalent to approximately 1,082 tonnes of
CO2/year. At a value of $15.00/tonne, the value of the emissions is
$16,230.
•
The electrical savings on the condenser are 43 bhp, which is equivalent to
about 47 input hp. At a cost of $0.10/kWh, the cost of electric power
would be $30,700 annually.
•
A reduction of 267 bhp in engine use will result in a reduction in engine
maintenance.
At a rate of $60/year/bhp, the savings in
maintenance/spare parts would be $16,020 annually.
If considering a retrofit installation of an economizer/sub-cooler, the savings in
fuel, emissions, electrical costs and engine maintenance total $186,720.
If considering a new refrigeration unit, the above-mentioned costs would apply.
In addition, there would be a reduced cost of roughly $1,000/hp for the
equipment and $1,000/hp for the installation cost. The total savings in capital
expenditure are thus $534,000 plus the $186,720 in annual operating expenses.
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Appendix B
Taking Advantage of Climate
The propane P-H diagram can be used to estimate, with reasonable accuracy,
the impact of climate upon the refrigeration unit.
As the ambient temperature falls, the back pressure on the compressor will
decrease since the vapour pressure of the propane falls. However, many plants
operate at compressor discharge pressures that are consistent with summertime
conditions because of no or insufficient adjustment of the back pressure valve on
the condenser. Wherever possible, plants should determine the minimum
discharge pressure that the compressor can deliver and still result in good
operation of the refrigeration unit.
Assume that the simple refrigeration loop described in Appendix A is designed to
operate at 250 psia. This corresponds to a design condenser temperature of
120°F, which is typical for many plants. The chiller runs at 20 psia. The overall
efficiency of isentropic compression is 73.1%. The refrigeration effect is the
same – 4,040,000 BTU/hour.
The enthalpy of the liquid leaving the condenser is 327 BTU/lb. This means that
the proportion of the propane entering the chiller as liquid is
(413 – 327) / (413 – 234) = 48.04%.
The amount of propane that must be vaporized in the chiller is 4,040,000 / 179 =
22,570 lb/hour. Therefore, the amount of propane that must be circulated is
22,570 / 0.4804 = 46,981 lbs/hour. For an isentropic efficiency of 73.1% and a
discharge pressure of 250 psia, the change in enthalpy is 486 – 413 = 73 BTU/lb.
This equates to a power input of 73 * 46,981 / 2,544 = 1,348 bhp.
Repeating this calculation procedure for various discharge pressures yields the
following graph, Figure B.1. The large triangle corresponds to the design point
for the unit. It has been assumed that there is a 10 psi drop between the
compressor discharge and the condenser outlet.
Note that the range of pressures shown on the graph is in no way meant to imply
that a refrigeration unit can operate across that entire range. The maximum
operating pressure will be determined by two factors:
•
The point at which compressor power requirements will exceed the
capacity of the compressor/engine in terms of volume handling, power
output capability and/or mechanical stresses on the compressor or other
equipment.
•
The point at which the circulating volume of propane is so great that there
is a bottleneck elsewhere within the refrigeration loop. Two common
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pinch points would be the condenser capability and the opening on the
expansion valve into the chiller.
The rationale behind a minimum operating pressure has been discussed in
Section 10. Based on the author’s experience, the minimum operating pressure
is typically found by monitoring the unit operation while small downward
adjustments in discharge pressure are made.
Compressor bhp as Function of Discharge Pressure
3000
Compressor hp
2500
2000
1500
1000
500
0
0
50
100
150
200
250
300
350
400
450
Compressor Discharge, psia
Figure B.1
Compressor bhp as Function of Discharge Pressure
Regardless of the actual operating range for the plant’s refrigeration unit, it is
clear that significant savings in compression power – the largest energy
consumer within the refrigeration unit – can be achieved by taking advantage of
the cool weather throughout much of the year and by maintaining as low a
compressor discharge pressure as possible.
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Appendix C
Chiller–LTS Coordination
Considerable energy savings can be achieved by setting operating conditions in
the chiller and the low temperature separator (LTS) at the optimal values. This
appendix outlines a conceptual approach – experience has shown that each
plant and/or gas is unique and no definitive and all-inclusive quantitative
recommendations can be made.
The conventional approach is to cool the process gas in the chiller and flash it in
the LTS, which would be at a pressure not much different from the process side
of the chiller. The proposed approach is to partially chill the gas and then choke
it prior to flashing it in the LTS. In other words, there will be a significant
difference in pressure between the chiller and LTS.
Due to the complexity of vapour-liquid equilibrium calculations, this exercise is
best done by a computer simulation package.
The starting point for this analysis is the phase map of the process gas. Figure
C.1 shows the phase map for three hypothetical gas feeds, the major variation
being the amount of methane in the stream, the rest of the components being
adjusted accordingly.
Refrigeration Unit Feed - Phase Map
Dew Point
1800
80% C1
75% C1
1600
85% C1
1400
All Liquid
Pressure, psia
1200
Bubble Point
1000
800
600
400
200
All Vapour
0
-300
-250
-200
-150
-100
-50
0
50
100
150
Temperature, °F
Figure C.1
Refrigeration Unit Feed – Phase Map
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On the left hand side of the graph is the bubble point curve. At temperatures to
the left of the curve the fluid is all liquid. On the right of the graph is the dew
point curve, and at temperatures to the right of the curve, the fluid is all vapour.
Note that the bubble point curves (where the fluid starts to vaporize) are
essentially identical but the dew point curves (where condensing starts) are quite
distinct. As expected, the richer the gas the higher the temperature at which
liquid starts to condense.
At temperatures that lie between the bubble and dew point curves, there is a
mixture of vapour and liquid. As with the case of the P-H diagram for propane
(see Appendix A), when in a two-phase region, the closer the point is to the dew
point curve the smaller the fraction of liquid in the mix. It should be pointed out
that the fraction in this case is the molar fraction, whereas in the P-H diagram,
which is shown in Appendix A, the fraction can be either molar or mass (since it
is for pure propane).
Figure C.2 illustrates the effect of choking the process gas as it exits the chiller.
NGL Recovery by LTS Choke Valve
1200
20 mol% Liquid
30 mol% Liquid
40 mol% Liquid
1000
Bubble Point
Pressure, psia
800
600
Flashing Through Choke Valve
400
200
Dew Point
0
-150
-100
-50
0
50
100
Temperature, °F
Figure C.2
NGL Recovery by LTS Choke Valve
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Two sets of lines have been added to the phase envelope (which in this graph is
for the 85 mol % methane gas stream shown in Figure C.1).
•
The solid pink lines show lines of constant liquid content. For example, at
800 psia and 43.8°F, the liquid content is 20 mol %; at 200 psia and 6.4°F,
the liquid content is also 20 mol %.
•
The dashed lines show the decrease in temperature as the pressure in the
gas leaving the chiller is choked down to the LTS pressure. Two cases
have been shown. In the first the process gas in the chiller was at 40°F
and 1,000 psia line pressure prior to choking and, in the second case, at
40°F and 800 psia. Those conditions were chosen since the outlet
temperature of the gas-gas exchanger is generally around that value and
the line pressure is often in the 800-1,000 psia range.
As the process gas is choked, there is an increase in the liquid recovery. When
dropping from 1,000 psia, the maximum recovery is about 34% at 400-450 psia
in the LTS. When the initial pressure is 800 psia, the maximum is 28% at 350400 psia. (Experience has shown that the maximum liquid recovery is often in
the range of 350-450 psig.)
The greatest improvement in recovery is gained through the temperature drop
from the Joule-Thomson effect on the process gas, instead of through lower
temperatures/pressures on the propane side of the chiller. In other words, the
propane side of the chiller can be run at a higher pressure/temperature.
Choking the chiller process gas outlet pressure allows the plant either to achieve
a greater recovery for the same input to the propane compressor or to maintain
the same recovery for less compressor power consumption.
Working on the basis that the goal is to maintain the same liquid recovery, the
higher pressure in the propane side of the chiller allows the savings on the
propane circuit to accrue from three sources:
•
The actual volume of propane entering the compressor suction is less.
Therefore, the physical size of the machine can be less – a savings in
capital investment.
•
The compression ratio on the propane compressor is considerably less.
•
From the P-H diagram it can be seen that the proportion of liquid propane
entering the chiller is greater, meaning that the propane circulation rate
can be decreased.
Choking the pressure into the LTS will result in improved operation of the
refrigeration unit. Besides the reduced refrigerant compression, there will be a
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smaller condenser duty, as well as changes in the reboiler duty on the deethanizer and any subsequent fractionation towers.
On the other hand, it will be necessary to recompress the gas coming off the LTS
back up to the line pressure that is upstream of the LTS choke valve.
In determining the optimum LTS pressure, it is necessary to factor in the cost of
recompressing the sales gas back to the stream pressure, the savings in
refrigeration compression and the net change in value of sales gas versus NGL
(or propane, butane, etc.)
It is recommended that any study of choked LTS operation include an estimate of
the hydrocarbon dew point of the sales gas expected at the new LTS conditions,
in order to ensure that the sales gas specification is still met. If necessary,
repeat the calculations.
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Appendix D
Refrigeration and Other Processes
The refrigeration unit is a major component of a gas processing facility. Its
energy consumption is significant and the impact upon overall product value is
paramount as it is in the refrigeration unit where the essential split between sales
gas and liquid product(s) is achieved.
Considerable energy savings will be realized by optimizing the operation of the
refrigeration unit within the context of the overall facility. Energy management
studied have repeatedly pointed out the great benefits from integrating energy
flows between process units.
The most obvious opportunities for energy flow integration are to bring in hot
streams for use as reboiler heat in the de-ethanizer, cold streams for a
condensing medium in the de-ethanizer and a cool/cold stream for sub-cooling
the propane.
Where multiple choices exist for such duties, evaluate the candidate streams in
terms of the intensity of the heat/cold, quantity of the energy, availability and
whether those streams could be used more profitably elsewhere within the
facility.
In this section, three potential energy integration opportunities are briefly
discussed:
•
use of water for propane condensing,
•
use of refrigeration in conjunction with lean oil absorption,
•
use of cascaded refrigeration to achieve deep-cut separation in the sales
gas.
D.1
Water for Propane Condensing
The overwhelming majority of refrigeration units in Western Canada use aerial
cooling in order to condense the propane following compression. This places a
considerable load on the compressor in the summer when maximum ambient
temperatures are experienced.
Water to the facility in the summer will generally be much cooler than ambient air.
Thus the potential exists to condense the propane at a lower temperature (and
pressure), thereby reducing the compressor power. Also, as outlined in
Appendix A, the fraction of propane ultimately entering the chiller as liquid will
increase. This means that less propane has to be circulated, again reducing
power consumption. The third source of energy savings is due to the fact that a
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water condenser uses an exchanger whereas an aerial cooler requires an
energy-consuming fan (and associated maintenance costs).
It has been estimated that water cooling reduces compression requirements by
20% compared with aerial cooling, all other conditions being equal.3 There is
even the possibility of twinning water cooling (for summertime) with aerial cooling
(for wintertime).
The water could be a cooling water stream but that requires a cooling tower, i.e.,
energy input. As an alternative, water ultimately going to boiler feedwater makeup, could be used. The heat taken out in the condenser would now be retained
in the overall plant energy envelope rather than rejected into the atmosphere.
D.2. Refrigeration – Lean Oil Absorption
Lean oil absorption is sometimes used for recovery of NGL components. A
detailed description of the process is outside the scope of this present document.
Suffice it to say that the effectiveness of the lean oil absorber can be improved by
reducing the temperature of the absorber. Low temperatures reduce the
vapour/liquid equilibrium values, making the removal of heavier components from
the gas easier. This means that lower lean oil rates are needed.
When a refrigeration unit is operated in conjunction with a lean oil absorber, the
refrigeration unit chiller can be used to cool the lean oil and at the same time to
prepare the NGL for absorption by partially dropping out NGL – this also reduces
the amount of lean oil circulation. On the other hand, lower lean oil temperatures
mean that there has to be increased firing on the lean oil regeneration tower.
Optimization of an integrated refrigeration/lean oil absorption unit entails a tradeoff between increased power requirements for propane compression due to lower
chiller pressures and decreased fuel firing on the lean oil regenerator.
D.3 Cascaded Refrigeration
Deep-cut fractionation of the sales gas, in order to remove ethane from the gas,
is often done using a turbo-expander. An alternative is to use cascaded
refrigeration, which involves two refrigerants. The first, such as ethane, is used
to achieve the extremely low temperatures required for ethane recovery from the
gas. The second, such as propane, is required in order to achieve cooling
temperatures low enough to condense the first refrigerant. Both refrigeration
loops operate as conventional units, except that the ethane loop condenser is the
propane loop chiller. For more details on this process, the reader is referred to
the GPSA Engineering Data Book, Chapter 14.
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Appendix E
Variable Flow in Compressors
The following examples illustrate the effects of varying flow in refrigeration
compressors upon the potential for energy savings.
E.1
Screw Compressor
A Mycom P160 VS-M single-stage screw compressor circulates 1,596 kg/hr of
propane with a suction pressure of 174.5 kPaa and a discharge pressure of
1489.7 kPaa. Power requirements are 83.4 bkW.
Cutting back the circulation rate to 80% (1,277 kg/hr) by reducing the speed
reduces the power requirements to 73.8 bkW, or 88.6% of the base case.
Cutting back the circulation rate to 80% by reducing the suction pressure to
142.1 kPaa reduces the power requirements to 80.9 bkW, or 97.0% of the base
case. The small reduction in power is due to the increased heat to the circulating
oil.4
E.2
Reciprocating Compressor
The following example summarizes three methods of flow control on a propane
reciprocating compressor:
A small 2-stage 1800-rpm reciprocating compressor handles 14.007
e3m3/day of propane. Pressure into the first stage is 172.8 kPaa and
the discharge of the second stage is 1481.4 kPaa. Stage 2 is limiting
so its pocket is closed but that on Stage 1 is 29.7% open. At the
overall compression efficiency of 75%, total brake power is 54.7 bkW.
Refrigeration demand is reduced to 80%, or 11.205 e3m3/day. The
plant has three choices for reducing compression power:
•
Reduce the compressor speed to 1,440 rpm (0.80*1,800).
•
Open the pockets on both stages and leave the suction and discharge
pressures as is.
•
Leave the pockets at their current positions and let the suction
pressures drop until the compressor capacity is reached.
In the first choice, the power required is 43.6 bkW, or 80.0% of the original.
In the second the power is 44.4 bkW, or 81.4% of the original conditions. In
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the third choice, the stage 1 suction pressure drops to 148.8 kPaa. Power
requirements are 48.1 bkW, or 88.2% of the original.5
E.3 Centrifugal Compressor
Like virtually all centrifugal machines, as the flow increases the power
requirements also increase6. At the same time, the delivered head declines.
Therefore, in order to meet the extreme summertime demands, where maximum
head and maximum refrigerant circulation rate are needed, the machine is
consequently oversized for the rest of the year.
Figure E.1 shows the performance curves for the first stage of a centrifugal
compressor handling propane. The major points of flow control are illustrated in
the graph.
Flow control on centrifugal machines is best done by slowing the machine. In the
example below, the speeds are 1 and 2.
Assume that the flow rate of the machine is 2,400 ACFM. The compressor will
line out where the pressure developed by the machine equals the system curve,
which is the process pressure that the compressor must overcome. In this case,
the system curve is the vapour pressure of the propane at the condenser outlet
temperature. At 2,400 ACFM the head developed is roughly 18,420 feet and the
efficiency of compression is 77.4%. The required brake horsepower is 486.0 hp.
Reducing the speed (from Speed 1 to Speed 2) causes the compressor flow to
re-equilibrate at 2,070 ACFM and a head of 18,420 feet. The efficiency curve
shifts as a consequence of the speed change. There is a loss of efficiency but
less than the loss if the flow reduction had been achieved by choking in the
discharge valve at the original compressor speed. An efficiency of 76.3% is
assumed.
This results in a power requirement of 426.2 bhp, which is 87.7% of the base
case whereas the new propane circulation is 86.2% of the old. Note, when
making the speed reduction, be very careful of avoiding compressor surge.
Figure E.2 investigates the power savings if the ACFM flow is kept constant but
the suction pressure is reduced in order to reduce the mass of propane.
If the suction pressure is reduced, the required polytropic head will want to rise to
reach the discharge pressure. Being a centrifugal machine, the compressor will
cut back flow and could pass the surge line – a situation which must be avoided
at all costs.
In order to prevent surge, the operators should keep the flow 10-15% higher than
the surge point and drop the pressure in very small steps. In this case, a change
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in suction pressure of only 4.75 kPa (0.69 psi) brought about the desired
reduction in propane flow. The required power dropped to 440.1 bhp.
20000
78
19500
77
Polytropic Head, feet
19000
76
Surge
Line
18500
75
18000
74
17500
73
Efficiency
Head - Speed 1
17000
Polytropic Efficiency, %
Performance Curves - Centrifugal Compressor
72
16500
71
System Curve
Head - Speed 2
16000
70
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
Flow, ACFM x 100
Figure E.1
Performance Curves – Centrifugal Compressor
20000
78
19500
77
Polytropic Head, feet
19000
76
Surge
Line
18500
75
18000
74
17500
73
Efficiency
Head - Speed 1
17000
Polytropic Efficiency, %
Performance Curves - Centrifugal Compressor
72
16500
71
System Curve
16000
70
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
Flow, ACFM x 100
Figure E.2
Performance Curves – Centrifugal Compressor
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It should be noted that this compressor had a second section and any changes in
suction pressure would have to take into consideration the entire machine.
However, this example was only to illustrate the concept that would be supplied.
In summary, reduction in suction pressure is not as effective in saving energy as
reducing the compressor speed. Of course, if there is no ability to change the
speed, suction pressure reduction should be considered. In any event, operators
must be fully cognizant of compressor surge and take action in carefullymonitored steps.
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Table E.1
Compressor Comparison Chart
Compressor Type
Screw
Reciprocating
Centrifugal
Advantages
Disadvantages
- Most often used in small - Must circulate high volumes
refrigeration units
of oil which must be prevented
from entering the condenser
- Can run at very low suction - Designed for a specific
pressure and can deliver very compression ratio – there is a
high compression ratios
loss
of
efficiency
when
operated above or below
design
- Capable of having an
economizer installed
- Most often used in medium - Must have multiple stages
sized refrigeration units
(two or three) to handle
summer operation
- Versatile, commonly used - Not always capable of having
and well understood
an economizer installed
- Can be efficient over a wide - Tight flow control is not
range of operating conditions
always
possible
when
refrigerant demand is reduced
- Most often used in medium - Operating principle runs
and large sized refrigeration counter to a refrigeration
units
circuit. If refrigerant discharge
drops, due to lower condensing
temperature, the compressor
tries to supply more flow,
whereas less refrigerant is
required.
- Can circulate large volumes - Flow control can be achieved
of refrigerant
by reducing the speed of the
compressor but the driver will
lose efficiency. This is most
significant with turbines.
Can handle flows from Efficiency can decrease
economizers
substantially as the circulation
rate varies from design
- Critical to circulate at a rate
above the surge point and
below the maximum or choke
flow
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Appendix F
Condenser Performance
F.1
Fan and Bundle
The condenser is designed to condense propane during the hottest days of the
year. For much of Western Canada, the design air temperature is 32.2°C (90°F)
although some sites in the northern section use 26.7°C (80°F). When assessing
the performance of an aerial cooler the most important parameter is the
temperature approach – the difference between the condenser outlet
temperature and the ambient air temperature. Typically, the design temperature
approach is 10-15 degrees Celsius (18-27 degrees Fahrenheit). In other words,
if the ambient temperature is 32.2°C, the condenser outlet temperature will be
42.2-47.2°C.
For a propane condenser, those outlet temperatures would translate to pressures
of 208.3-233.3 psia. Even a 2 degree Celsius decline in approach temperature
(i.e., to 12-17 degrees Celsius) due to fouling, etc., would add 10-12 psia to the
discharge pressure.
Condensers can lose effectiveness for a wide variety of reasons:
•
Fouling. Poplar fluff, chaff, dust, animal matter can lead to considerable
fouling. Water on the tubes will also leave scale and cause corrosion
where there are different metals for the fins and tubes.
•
Damage. Care should be taken so that the fins on the tube bundles are
not damaged.
•
Fin Delamination. This can occur when cold water is sprayed onto a hot
bundle. The use of water spray is a common practice in an effort to
improve cooling. The result is short-term and can lead to later problems.
The water causes the fin to “pop” off the tube, leaving a small gap which
reduces heat transfer from the tube into the air stream via the fins.7
Unfortunately, once it occurs, delamination cannot be reversed.
The damage occurs when cold water is sprayed on the hot tubes. Some
plants attempt to get around this by spraying the water underneath the fan
and being drawn up through the bundle. In this case, the cold water
contacts colder tubes minimizing delamination. It is recommended that
the aerial cooler manufacturer be contacted when considering the use of
water spray.
•
Reduced Air Flow. This can occur if the fan pitch has not been properly
set. Very often, the pitch is adjusted before winter in order to minimize air
flow during very cold weather and readjusted for summertime air flow.
•
Inefficient Air Flow. This occurs when there is a large gap between the
fan blades and the fan housing. The result is an actual downflow of air
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through the fan.
reversal.
•
F.2
Damage to the fan blades can also cause this flow
Proximity to Other Coolers. Studies by cooler manufacturers and some
plants have shown that air exiting an adjacent cooler (or a heat source)
can be drawn into the inlet of an aerial cooler.
Refrigerant Purity
It is important to know the purity of the circulating refrigerant. The purity affects
the temperature-pressure relationship in the refrigerant receiver or accumulator;
in the economizer (if there is one); and in the chiller or evaporator. Figure F.1
illustrates this concept, using propane as the refrigerant.
The operation of the condenser is dictated by the bubble point curve. The
operation of the chiller is dictated by the dew point curve. For pure propane,
these two curves are identical.
In Figure F.1 the temperature-pressure curve for pure propane is shown in bold
font. The dashed line to the right of the pure propane line is the bubble point
curve for typical industrial-grade propane as would be used in a plant. (The dew
point curve lies, coincidently, virtually on top of the pure propane line.) The
dashed line to the left of the pure propane line is the dew point for propane
contaminated by butanes. (In this case, the bubble point curve is virtually
identical to the pure propane curve.)
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Propane Temperature-Pressure Equilibrium
120
Pure Propane
100
Bubble Point - Industrial Propane
Temperature, °F
80
60
Dew Point - Propane with C4's
40
20
0
0
20
40
60
80
100
120
140
160
180
200
220
240
260
-20
Assumes Patm = 13.5 psia
-40
Pressure, psig
Figure F.1
Propane Temperature-Pressure Equilibrium
In summary, if the contaminants are lighter than propane (methane and/or
ethane) the curve shifts to the right, meaning that for a given outlet temperature
from the condenser, the compressor discharge pressure will be higher. Some
reciprocating compressors cannot handle significant quantities of impurities
during warm weather because they are limited in the compression ratio that they
can deliver.8
On the other hand, if the contaminants are heavier (butanes and/or pentanes) the
pressure in the chiller will be much lower for the same chiller temperature. This
in turn, significantly affects the compression ratio and thus the power required.
It should be pointed out that, if the contaminants include both butane (and
heavier) and ethane (and lighter), the butanes will probably be drained from the
circuit, most likely from the suction scrubber. This could result in an increase in
ethane concentration and a rise in power requirements.
Contaminants will also affect the amount of refrigerant entering the chiller as
liquid, which, in turn, affects the amount of refrigeration that can be achieved.
However, this impact is usually very small and can be ignored in most day-to-day
operation unless the amount of contamination is very large or if the contaminant
is very light (such as methane).
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If contaminants are found, they can be removed by either:
•
venting the light ends from the receiver.
•
draining the heavy contaminants from a de-oiler (also known as an oil
reclaimer). This can consist of a boot in the chiller or as a small separator
upstream of the chiller9.
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Appendix G
References
The following documents were referenced while preparing this Best Management
Practice.
http://www.patchingassociates.com/lan/newsletter_10.htm, Patching Associates
Newsletter #10, March 1999
Improving Operational Efficiency [name withheld] Gas Plant Pilot Audit; prepared for
[name withheld]; prepared by CETAC-West; Calgary, AB; February 24, 2003
GPSA Engineering Data Book, 11th Edition, Chapters 14 and 24; Gas Processors
Suppliers Association, Tulsa, OK; 1998
Schaum’s Theory and Problems – Thermodynamics for Engineers; by M.C.
Potter and C.W. Somerton; McGraw-Hill Inc., 1995, page 71
Mycom 3.22ep; Screw Performance 3-22; Recip Performance 3.2; Mayekawa
Mfg. Co. Ltd; 1998-11-04.
Endnotes
1
Power consumption and refrigeration effect values found using the Mycom 3.22ep software.
P-H diagram from GPSA Engineering Data Book, 11th Edition, Figure 24-27, with refrigeration
loop overlaid on it in Figures A.3, A.5, A.6..
3
http://www.patchingassociates.com/lan/newsletters/newsletter_10.htm, March 1999
4
Mycom 3.22ep; Screw Performance 3-22; Recip Performance 3-2; Mayekawa Mfg. Co. Ltd.;
1998-11-04
5
Based upon GPSA calculation techniques outlined in the Engineering Data Book, 8th, 9th and
10th Editions.
6
The word “virtually” is used because very large centrifugal pumps sometimes have sections of
their pump curves, towards the end of their capacity, where the required power levels off and may
even decrease.
7
http://www.patchingassociates.com/lan/newsletters/newsletter_010.htm
8
Mycom 3.22 ep
9
GPSA Engineering Data Book, 11th Edition, Figure 14-26.
2
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