Best Practice
SABP-A-005
28 December 2005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Document Responsibility: Energy Systems Unit/ESD/CSD
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Previous Issue: None
Next Planned Update: 1 January 2009
Primary Contact: nourelmm@aramco.com, phone +966 (3) 873-6045
Copyright©Saudi Aramco 2009. All rights reserved.
Page 1 of 54
Document Responsibility: Energy Systems Unit/ESD/CSD
Issue date: 28 December 2005
Next Update: 1 January 2009
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Table of Contents
Page
1.0
2.0
3.0
Introduction
3
1.1
Definition
3
1.2
Purpose and Scope
3
1.3
Intended Users
4
1.4
Initial Assessment Objectives
4
Quick Energy Assessment Overview
4
2.1
Energy Efficiency Optimization Task Description
4
2.2
Solution Method Using Decomposition Approach
5
Methodology
3
3.1
Energy Flow Diagrams
3.2
Steam and Power Diagram
11
3.3
Fuel Diagram
12
3.4
Energy Saving Housekeeping List
13
3.5
Energy Saving Generic Checklist
14
4A.0 Appendices for Short-cut Calculations and HGP Energy Study
7
16
4A.1
Basic Steam Mass Balance
16
4A.2
Basic Energy Utility Targeting Using Pinch Method
18
4A.3
Basic Formulas for Some Quick Savings Estimation
52
4A.4
HGP Detailed Energy Assessment Study
54
Page 2 of 54
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Document Responsibility: Energy Systems Unit/ESD/CSD
Issue date: 28 December 2005
Next Update: 1 January 2009
1.0
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Introduction
Energy conservation in Saudi Aramco became everyone’s business. It is mandatory for
each process facility to find cost effective solutions to save energy and achieve more
with less in their facilities. Saudi Aramco has constituted a committee called EMSC
Energy Management Steering Committee to direct and manage a sustainable process
for energy conservation.
A vital contribution towards the success of the company wide energy conservation
policy comes through documenting the company best practices in methodology, tools
and applications in the field of energy conservation and distributing such knowledge
among our facilities. Hence, a consistent effort has been exerted in Saudi Aramco to
produce Best Practices to help Saudi Aramco plants achieve their energy conservation
targets and disseminate energy conservation knowledge. This particular Best Practice
document for initial energy assessment is a contribution towards this goal. It is
expected to draw the line in conducting energy assessments through a user-friendly
methodology.
The theme of this quick energy assessment methodology for energy efficiency
optimization is, “Big Picture First, Details Later”.
Energy assessments, also called energy audits, may be primitive or comprehensive
and detailed. A variety of approaches, methods and tools are available to conduct such
“energy assessments” to improve the energy efficiency of industrial processes.
1.1
Definition
The term “Energy Assessment” refers to the methodology of collecting and analyzing
available energy utilities related data in order to establish the “big picture” of the
breakdown of energy consumption for a particular facility and identify componentbased-energy saving opportunities within the facility.
1.2
Purpose and Scope
The purpose of this best practice document is to describe a methodology and introduce
short cut tools by which quick assessment for energy efficiency improvement can be
conducted faster, cheaper and better. Its scope include quick energy assessment
methodology in a step-by-step manner, simple models for data representation,
checklists for identifying areas for energy savings and short cut tools for creating and
evaluating process initiatives for energy saving for common use in Saudi Aramco
plants.
Page 3 of 54
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Document Responsibility: Energy Systems Unit/ESD/CSD
Issue date: 28 December 2005
Next Update: 1 January 2009
1.3
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Intended Users
This Best Practice manual is intended for use by the energy engineers working in
Saudi Aramco plants, who are responsible for efficient operation of their facility. This
particular document will enable them to conduct quick energy assessments
systematically.
1.4
2.0
Initial Assessment Objectives
•
Preliminary review of plant process, system drawings and Data gathering
•
Understand the “Big Picture” of the plant-wide operations
•
Understand process energy and utility systems
•
From the available data, establish your current reality
•
Establish your desired result “Targeting”
•
Identify All Opportunities for energy savings
•
Propose Quick-hit savings
•
Propose scope for definitive assessment with some economic analysis
•
Propose plant-wide energy-utility strategy or “Total energy Management”
Quick Energy Assessment Overview
The first priority in any plant initial energy assessment effort is to define the “quick hits”
for energy cost savings that can be achieved with little or no investment.
The best practice document does not only help plants energy groups conduct a quick
energy assessment for both short and long term energy savings opportunities but also
establish the procedures and checklists that can enable process engineers and
operators make intelligent choices on hour-to-hour basis.
2.1
Energy Efficiency Optimization Task Description
Energy Efficiency Optimization Task aims to specify the near-optimal operation
targets/modes that minimize the plant’s energy consumption at minimum deficiency in
energy supply of the utility systems to the plant’s process. Following that, the task will
be to list all possible operational and design modifications necessary to achieve the
specified/desired target(s). This includes identification of all related engineering
activities in a minimum possible time using uncertain plant data and without any
interruption to the plant’s normal production. This task shall achieve the following two
objectives:
Page 4 of 54
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Document Responsibility: Energy Systems Unit/ESD/CSD
Issue date: 28 December 2005
Next Update: 1 January 2009
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
1. Minimum disruption in the energy utility supply
2. Minimum consumption of energy utility.
“Given an industrial facility that consists of several processes and utility plants, define;
at minimum deficiency in energy utility systems supply to the process; near-optimal
targets for energy consumption minimization, find a list of possible operational and
design modifications to achieve desired target(s) and conduct the engineered
solutions”
2.2
Solution Method Using Decomposition Approach
For simplicity and timely results, Decomposition Techniques will be used in lieu of the
time-consuming Mathematical Programming/Optimization Techniques.
The plant’s energy utility needs will be defined with reasonable level of flexibility and
the energy utility system; electricity, fuel, steam and other energy-related utilities will be
scrutinized one by one to find the near- optimal consumption of such utilities that
guarantee minimum deficiency in the utility supply to plant processes.
On the macro level the energy system components are generation, distribution and
utilization. The objective will be to minimize waste in energy fresh resources and
capital (de-bottlenecking) in these three components. This can be done via the
continuous upgrade of the efficiency of energy system components in generation,
distribution and utilization. However, the utilization component has a unique feature,
where its boundaries are not completely dictated by the process. Therefore the room of
improvement in this component is much wider than the others.
3.0
Methodology
There are four essential tasks that can be conducted by a small energy focus group of
three engineers:
1.
Data, Models and Targets
2.
Insights, Opportunities and Estimated savings potential
3.
Screen and Formulate Strategy
4.
Document, Report and Present
These tasks are exhibited in the 10-Steps procedures below.
1.
Site survey through templates, checklists and interviewing of process
owners/proponents to gather the right amount of data that enable the energy
team build the plant’s “big picture” and understand the goals and the constraints
of the facility( What to look for and what to ask)
Page 5 of 54
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SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
2.
Define the criteria for focusing on potential areas of interest (when to be rigorous
and get to the second level of details)
3.
Develop site energy/utility nominal design/normal operation models with the
appropriate level of details in a high level generic “path” diagrams for, power, fuel,
H2, steam, water, nitrogen and air. Preliminary purpose of these models will be to
understand what is going on in the energy utility system, locate the “energy
consumption elephants” (ECEs) in both process and utility plants and generate
insights for energy saving opportunities
4.
Add more depth in the level of details of the energy utility model for each ECE
and/or other criterion of focus
5.
Define the effect of disturbances and uncertainty on the energy utility system
models
6.
a.
Sources of disturbances
b.
Site energy utility balance under disturbances
c.
Nominal and dynamic targeting of energy utility systems
d.
(Check that the “big picture” depicted for the process and the utility plants is
correct with enough degree of confidence before you proceed)
Target (order of magnitude targeting)
a.
_ Identify main processing issues that affect utility utilization
b.
_ Link utility-utility interactions
c.
_ Integrate and qualitatively optimize site utilities (If we do not coordinate
between steam and fuel systems we may stop flaring steam but flaring
fuel) for energy utility saving
7.
Integrate core processes among themselves and with utilities
8.
Develop a comprehensive initiatives list via identifying and estimating energy
utility savings opportunities
a.
Housekeeping list
b.
Checklists (generic and process specific)
c.
Waste energy recovery (pinch method and others)
d.
In-process Modification (pinch method and others)
9.
Champion a cross-fertilizing discussion among plant disciplines to prioritize,
screen the initiatives and writing a project sheet, for each initiative, including a
description of the opportunity and energy utility savings estimate
10.
Develop “word” strategies for realizing savings from facility goals, analysis of the
results and the mapping of the opportunities onto the facility strategy (remember,
50% of something is better than 100% of nothing). Then report and present
results.
Page 6 of 54
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Issue date: 28 December 2005
Next Update: 1 January 2009
3.1
Energy Flow Diagrams
3.1.1
Overview
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Fuel
Energy Sources
Energy
Generation
Boilers & Cogeneration
Steam
Electricity
Buildings & Others
Process
Energy
Distribution
Energy
Utilization
Energy
Byproducts
The task now is to look for the “Elephants” in the energy-process system.
The graphs below will enable the Energy Engineer to find the “Elephants” to focus
his/her efforts and decide where to start and insist on accurate data collection to further
proceed with the analysis.
Money-based graphs will help quickly to pinpoint the areas of focus. Furthermore, the
BTU-based graphs will double check the pre-defined priority list, direct the study
towards the process reasons behind certain energy “elephant” and find more efficient
ways to satisfy the process target(s).
Page 7 of 54
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Document Responsibility: Energy Systems Unit/ESD/CSD
Issue date: 28 December 2005
Next Update: 1 January 2009
3.1.2
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Overall Energy Flow Diagram ($/yr)
Typical Oil Refinery
$ 0.5 MM
$ 0.9 MM
Other users
Process
Purchase
$ 24.4 MM
$ 0.0 MM
Steam
$ 52.4 MM
$ 0.8 MM
$ 45.9 MM
$ 41.5 MM
$ 21.5 MM
Machine
Drive
Re-boilers
heaters
$ 0.4 MM
Flare
$ 20.70 MM
$ 51.2 MM
Fuel
$ 73.1 MM
$ 0.0 MM
$ 0.3 MM
$ 2.2 MM
$ 16.9 MM
furnaces
refrigeration
$ 18.4 MM
power
Motors
$ 19.1 MM
$ 0.40 MM Other users
Total Purchased
$ 37.6 MM
$ 0.0 MM
Export
Page 8 of 54
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Document Responsibility: Energy Systems Unit/ESD/CSD
Issue date: 28 December 2005
Next Update: 1 January 2009
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Typical Gas Plant
Purchase
$ 20.018 MM/yr
“ 2.54 MM Ib/hr”
$ 0.0 MM/yr
$ 0.0 MM/yr
Export
SA Fuel
Gas
$19.618 MM/yr
Steam
Flare Area
$0.11MM/yr
$0.27 MM/yr
$ 19.618 MM/yr
$ 0.0 MM/yr
$ 20.018MM/yr
Fuel
$ 0.0 MM/yr
$ 0.0 MM/yr
$ 56.1 MM/yr
power
$ 56.1 MM/yr
Inlet Area
$1.72MM/yr
$2.8 MM/yr Utility Area
$0.244MM/yr
$ 0.4 MM/yr Sulfur Recovery
Area
$0.221MM/yr
$16.6MM/yr Gas Treat
$3.25 MM/yr Area
$0.335MM/yr
Gas Comp.
$18.08 MM/yrArea
Total Purchased
$ 56.1 MM/yr
“200 MW”
$0.609MM/yrLiquid Recovery
$29.04 MM/yr Area
Note: These Data are Shedgum’s
Gas Plant
Page 9 of 54
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Document Responsibility: Energy Systems Unit/ESD/CSD
Issue date: 28 December 2005
Next Update: 1 January 2009
3.1.3
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Overall Energy Flow Diagram (Trillion-Btu/yr)
Total Input= 10.56 TBtu
2 TBtu
Building
1.8 TBtu
Process
-0.19 TBtu
Boilers
steam
cogen
-0.19 TBtu
Boiler
Fuel
Area #1
Unit #1
0.5 TBtu
Building
0.0 TBtu
steam
5.46 TBtu
0.12 TBtu - 5.4 TBtu
2.11 TBtu 5.77 TBtu
Boilers
Boilers
Process
Process
Building
steam
steam
cogen
cogen
5.59 TBtu
Conventional
Electricity
#N
0.18 TBtu
Boiler
Fuel
Conventional
Electricity
#N
Area #1
Area #1
-5.4 TBtu
Boiler
Fuel
0.0 TBtu
Conventional
Electricity
#N
#N
Unit #1
Heating
7.56 TBtu
0.18 TBtu
Fuel
Electricity
0.39 TBtu
8.38 TBtu
Cooling/Ref
0.55 TBtu
Machine Drive
1.24 TBtu
#N
#N
Unit #1
Electro-Chem
0.0 TBtu
Other use
0.02 TBtu
Page 10 of 54
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Document Responsibility: Energy Systems Unit/ESD/CSD
Issue date: 28 December 2005
Next Update: 1 January 2009
3.2
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Steam and Power Diagram
Steam “big picture” shall exhibit at least steam sources, pressure levels, mass balance,
let down valves and turbines, main users and losses such as vents, let downs and
condensate. These information can be used as depicted in the way below to generate
ideas and opportunities for saving under the main theme of “Big picture first, and
details later.”
Steam Targets
103 t/h
Enhance the recovery of
Steam from condensate
Consider replacing turbines with motors
Enhance condensate recovery
By 10 %
Heat integration of process#1 and # 4
Estimated saving of 10%
98 t/h
HP Boiler
HP
Shut down MP boiler
21 t/h 8 t/h
68 t/h
0.0 t/h
0.0 t/h
Proc. #1
Reduce letdown via turbine
flow increase or new turbine
6.28 MW
Reduce it via
operating at
lower pressure
Proc. #2
4 t/h
27 t/h
Eliminate vent
Vent
2 t/h
18 t/h
40 t/h
0.0 t/h
Proc. #4
Proc. #1
Effluent
5 t/h
2 t/h
MP Process
Condensate
Vent
0.0 t/h
30 t/h
MP
13 t/h
30 t/h
Deaerator
BFW
49 t/h
Raw water
Make-up Treatment Plant
HP Process
Condensate
68 t/h
MP Boiler
chemicals
1 t/h
0.0 t/h
1 t/h
0.0 t/h
LP
7 t/h
Proc. #1
Process Condensate
Reduce and make use of boiler blow-down
Est. 50 % Returned
4 t/h
Proc. #3
LP Process
Condensate
Page 11 of 54
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Issue date: 28 December 2005
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3.3
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Fuel Diagram
Fuel “big picture” shall exhibit at least fuel sources, pressure levels, mass balance, let
down valves and compressors and main users. Hydrogen composition should also be
considered to define the opportunities for recycles.
Hydrogen
Off-Gas
Fuel Gas
400 psig
380 psig
PSA
350 psig
180 psig
NG
150 psig
150 psig
52 psig
50 psig
boilers
furnaces
15 psig
5 psig
Page 12 of 54
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Next Update: 1 January 2009
3.4
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Energy Saving “Housekeeping” list
1.
Establish steam trap auditing program to determine the number of traps (working,
leaking, blocking…etc.) and periodically check it and repair or replace the
defected ones
2.
Condensate recovery (maintenance and operation)
3.
Piping insulation (maintenance)
4.
Shutting off equipment when not required (load management)
5.
Minimize slow rolling of steam turbines
6.
Boilers and furnaces tuning through better combustion control
7.
Preventive maintenance of energy system components such as heat exchangers,
pumps, fans, compressors, turbines, furnaces, boilers (e.g., clean the fin fan tubes
of gas coolers)
8.
Improved cooling water system (maintenance and operation)
9.
Optimize air compressors operation
10. Optimize BFW pumps load allocation
11. Energy monitoring and indices/ energy management system
12. Shut down compressors at low feed rate (load management)
13. Steam trap management (consider self regulating electrical tracing)
14. Booster, shipper and condensate pumps load management
15. Compressors load management
16. Consider the use of Economizers and Pre-heater in the boilers
17. Compressed air leaks (maintenance)
18. Minimize boilers and water coolers blow downs
19. Utilize boiler blow-down
20. Turbines load management
21. Upgrade sluggish response control valves since the delay might result in extra
flaring
22. Enhance the efficiency of boilers and furnaces through tight control
23. Enhance boilers and furnaces insulation
24. Optimize the CHP system
Page 13 of 54
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SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
25. Maintain plant steam reserve
26. Minimize/Eliminate the use of steam reducing stations and vents
27. Consider the reuse of turbo-expander to generate power in the HP/LP pressure
control valves
28. Enhance the combustion process in boilers and furnaces via using full additives
as a catalyst
3.5
Energy Saving “Generic” Checklist
1.
Replace gas turbines with more efficient steam turbines
2.
Increase boiler steam pressure and temperature to the extent that matches
process needs unless electricity generation is the controlling factor
3.
Use auxiliary turbines to minimize steam let downs
4.
Use steam in the process instead of venting it
5.
Preheat combustion air
6.
Use ASD on BFW pumps
7.
Reduce process variability using stable ops program
8.
Trim/Optimize the fired heaters excess oxygen
9.
Optimize the fired heaters stack temperature
10. Re-use the flue gases in process heating
11. Optimize your waste heat boilers
12. Recover valuable gases from your fuel gases
13. Reduce the H2 wheel in your plant
14. Cool down the inlet temperature to compressors
15. Reduce cooling medium return temperature in refrigeration cycles
16. Upgrade, regenerate and replace your catalyst
17. Optimize let down stations and steam turbine operation
18. Use highest efficiency turbines
19. Maintain your steam turbines to reduce steam consumption
20. Give frequent attention to steam traps and leaks
21. Replace turbine drives with electric motors if more economical since they are
more efficient
Page 14 of 54
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Next Update: 1 January 2009
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
22. Boilers and furnaces efficiency enhancement through advanced process control,
on-line performance monitoring and optimization
23. Recover condensate
24. Thermal Heat and power integration (CHP)
25. Better control for dispersion steam to flare stacks
26. Optimize steam use in strippers
27. Minimize live steam utilization
28. Mechanical energy integration
29. Reduce natural gas consumption by understanding fuel gas sinks and constraints
30. Reduce fuel gas use with energy integration
31. Keep H2 separate from fuel gas system
32. Measure the composition of off-gas streams and recover C2 and C3+
33. Avoid unnecessary processing of off-gas
34. Avoid unnecessary processing of wastes and inert
35. Minimize the unnecessary production of off-gas
36. Use on-line monitoring and APC for furnaces and other control sensitive fuel users
37. Avoid unnecessary recycles
38. Avoid leaks in the pressure relieve valves to the fuel system
39. Adjust operating pressures and optimize process interaction
40. Clean and maintain pipelines and valves to minimize pressure drops
41. Clean and maintain Boiler tubes from deposits & scale for better operations
42. Treat and recycle blow down to force lower cycles of concentration in cooling
towers and boilers
43. Use lowest quality water
44. Maximize use of stripped sour water
45. Minimize generation of wastewater
46. Seek out and repair all hydrocarbon leaks
47. Eliminate direct water injection for cooling purposes
48. Eliminate live steam used for re-boiling and stripping where it is only used for BTU
value
49. Minimize or eliminate live steam consumption in sour water strippers by replacing
it with re-boilers
Page 15 of 54
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SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
50. Boiler blow-down could be considered for cooling tower make-up
51. Extract the low pressure steam from the boiler blowdown
52. Consider re-using the boiler blowdown for reserving the boiler
53. Use process water effluent as a source on the next lower water quality level
54. Eliminate live steam usage since it becomes water and follows an energy path
through the plant consuming more energy to process it
55. Should live steam becomes necessary optimize the amount used through
pressure manipulation
56. Use lowest quality water possible for desalter operation
57. Minimize water used in desalter
58. Automate desalter operation, avoid water slipping through with crude during
desalting/maximize the separation of free water upstream of the crude desalting
(each Ib of water will require roughly Ib steam for processing)
59. Minimize the water-wheel in the plant
60. Maximize utilization of treated oily-water from the waste-water treatment plant
61. Install low NOX burners
62. Consider the use of Cogeneration
63. Adjustable speed motors/devices for pumps, compressors, etc.
64. Increase waste heat steam generation
65. Insulate condensate return lines, valves, flanges, etc.
66. Cooling- tower blow-down should not be treated but segregated to sewer
67. Boiler blow-down should not be sent to wastewater treatment but segregate to
sewer.
28 December 2005
Revision Summary
New Saudi Aramco Best Practice.
Page 16 of 54
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Issue date: 28 December 2005
Next Update: 1 January 2009
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
4A.0 Appendices for Short-Cut Calculations and HGP Energy Study
4A.1 Basic Steam Mass Balance
The basic Steam Mass Balance does not require high accuracy as long as the
developed model still makes sound engineering sense. (i.e., output is much higher than
input).
Common engineering sense shall be used to estimate what the unknowns. For
example condensate return, blow-down and flares can be defined after getting good
idea about main consumers.
98+5 t/h
98 t/h
HP Boiler
HP
21 t/h 8 t/h
68 t/h
0.0 t/h
0.0 t/h
Proc. #1
1 t/h
6.28 MW
HP Process
Condensate
Proc. #2
0.0 t/h
0.0 t/h
68 t/h
MP Boiler
chemicals
27 t/h
Vent
Proc. #4
Proc. #1
18 t/h
Deaerator
30 t/h
BFW
(42+5) t/h
Raw water
Make-up Treatment Plant
Effluent
5 t/h
2 t/h
MP Process
Condensate
Vent
0.0 t/h
38 t/h
MP
9 t/h
30 t/h
1 t/h
0.0 t/h
0.0 t/h
LP
7 t/h
Proc. #1
Process Condensate
Est. 50 % Returned
4 t/h
Proc. #3
LP Process
Condensate
Page 17 of 54
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Next Update: 1 January 2009
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
4A.2 Basic Energy Utility Targeting Using Pinch Method
The purpose of this section is not to conduct a pinch study but to get some energy
targets regarding the utilities consumption for a desired plant area. This can be done
essentially via three methods, graphical, algebraic and using mathematical
programming/optimization. In this document the first two methods are going to be
explained.
Graphical Method:
Background:
Any heat exchanger can be represented as a hot stream that is cooled down by
another cold stream and/or cold utility and a cold stream that is heated up by a hot
stream and/or hot utility with a specified minimum temperature approach between the
hot and the cold called ∆Tmin.
The process exhibited below in the graph shows the situation when the two streams do
not have a chance of overlap that produce heat integration between the hot and the
cold.
Page 18 of 54
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Next Update: 1 January 2009
Feed
120
H
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
C
PROCESS
Product
HOT UTILITY
T
100
80
60
40
20
0
0
10
COLD UTILITY
20
30
40
50
60
H
Page 19 of 54
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SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Moving the cold stream to the left on the enthalpy axis without changing its supply and
target temperatures till we have small vertical distance between the hot stream and the
cold stream we obtain some overlap between the two streams that result in heat
integration between the hot and the cold and less hot and cold utilities. As been
depicted in the graph below with shrinkage in the hot and cold lines span.
Feed
H
Product
HOT UTILITY
T
120
C
PROCESS
100
HEAT
RECOVERY
80
Pinch
60
(MAT)
40
20
0
COLD UTILITY
0
10
20
30
40
50
60
H
For demonstration, all hot streams will be represented in the process by one long hot
stream to be called “the hot composite curve”. Same thing be done for all cold streams
in the process.
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The next step will be drawing the two composite curves/lines on the same page in
Temperature (T)-Enthalpy diagram with two conditions:
1- The cold composite curve should be completely below the hot composite curve,
and
2- The vertical distance between the two lines/curves in terms of temperature should
be greater than or equal to a selected minimum approach temperature called global
∆Tmin
The resulting graph is depicted below and known as thermal pinch diagram:
Net Heat Sink
Above the Pinch
Opportunity for
heat recovery
Net Heat Source
Below the Pinch
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1. Constructing the composite curves (step-by-step)
The above mentioned process can proceed as follows:
Stream Type
Supply Temperature (ºC)
Target Temperature (ºC)
FCp (kW/ ºC)
1-Hot
170
70
10
2-Hot
120
30
20
3-Cold
50
90
40
4-Cold
20
110
18
1- Draw the hot composite curve and the cold composite curve via developing the
following tables.
Note: The tables list all the hot and cold streams temperatures in an ascending order
with the cumulative enthalpy corresponding to the lowest hot temperature and
lowest cold temperature respectively equal to zero.
2- In every temperature interval the cumulative hot load is calculated using the
following formula:
H= FCp * (Tsupply – Ttarget)
3- In every temperature interval the cumulative cold load is calculated using the
following formula:
H= FCp * (Ttarget – Tsupply)
Hot streams temperature list
Cumulative Enthalpy (H)
T0=30
H0=0.0
T1=70
H1=800
T2=120
H2=2300
T3=170
H3=2800
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Cold streams temperature list
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Cumulative Enthalpy (H)
T0=20
H0=0.0
T1=50
H1=540
T2=90
H2=2860
T3=110
H3=3220
Temperature (T)- Enthalpy (H) Diagram
T
Hot composite curve
Cold composite curve
30
20
Cold composite curve is not completely below the hot composite curve
H
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As we mentioned before the cold composite curve shall lie completely below the hot
composite curve and this can be done via dragging the cold composite curve to the
right on the enthalpy axis (H). This process shall stop at a vertical distance between
the cold and the hot composite curve for a temperature equal to the minimum
temperature approach selected earlier.
Temperature (T)- Enthalpy (H) Diagram
Minimum Heating Utility
T
Qh=480 kW
Hot composite curve
Cold composite curve
Minimum Temperature Approach
30
20
Qc=60 kW
Minimum Cooling Utility
H
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Algebraic Method:
Information needed
Explained via another example
Given a unit with a list of hot streams to be cooled and cold streams to be heated
Stream ID
Type
1
2
3
4
hot
hot
cold
cold
Flowrate*Average Specific heat (FCp)
10
5
19
2
Supply temperature
Target Temperature
520
380
300
320
330
300
550
380
1. Constructing temperature interval diagram
1.1_ Draw two temperature scales one for the hot streams and another for the cold
streams
1.2_ Select reasonable minimum temperature approach between the hot streams and
the cold stream (for instance, 10ºC)
1.3_ Draw all the hot streams (in the table hot section) to be cooled according to the
hot steam scale as arrows that start at the supply temperatures and end at the
target temperatures
1.4_ Repeat step 1.3 for all cold streams in the cold section of the table
1.5_ Start at the highest temperature of any hot stream in the hot section and draw a
horizontal line that span along the two sections of the table, the hot and the cold.
1.6_ Draw horizontal lines again at the start and the end of any arrow representing the
hot streams in the hot section of the table
1.7_ Repeat step 1.6 for any arrow representing cold stream in the cold section (at the
start and the end of any arrow)
1.8_ Count the number of segments generated and number them starting at the
highest temperature (they are called temperature intervals)
1.9_ Make sure that each temperature interval has now temperature value on both the
hot temperature scale and cold temperature scale. The difference is the desired
minimum temperature approach (for instance the 10ºC used in this example)
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These procedures are depicted in the figure below:
Note: This structure means that within any temperature interval it is thermodynamically
feasible to transfer heat from the hot streams to cold streams. It is also feasible
to transfer heat from a hot stream in an interval “x” to any cold stream which lies
in an interval below.
The temperature Interval Diagram
∆ T minimum = 10 K
T*
555
515
Interval
1
Hot Streams
560
H1
T
t
Cold Streams
550
520
510
390
380
2
385
3
H2
380
370
4
330
320
305
5
310
300
295
6
300
290
375
310
Hot Streams:H1; F1Cp1= 10 kW/K
H2; F2Cp2= 5 kW/K
C2
C1
Cold Streams:C1; F1Cp1= 10 kW/K
C2; F2Cp2= 5 kW/K
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Note: The temperature symbol T* is interval inlet temperature used later on selecting
the suitable energy utility after calculating the targets using what is known as
grand composite curve.
To calculate T* we take the average interval inlet temperature of the hot and cold
temperature scale.
2. Constructing tables of exchangeable heat loads and cooling capacities
2.1 Determining individual heating loads and cooling capacities of all process streams
for all temperature intervals using this formula:
Qnm = F1Cp1* (Ts-Te) in energy units (kW)
Ts is the interval start temperature and
Te is the interval end temperature
“n” is stream number and “m” is the interval number
Example 1:
Interval # 1 in the hot section:
The interval start temperature is 560 K
The interval end temperature is 520 K
Q11(Q for stream #1 in interval #1) = F1Cp1*(560-520)
Since there is no H1 stream in this interval, hence, F1Cp1=0.0
Q stream # 1(exchangeable load) in this interval = 0.0*(560-520) = zero
Example 2:
Interval # 2 in the hot section:
The interval start temperature is 520 K
The interval end temperature is 390 K
The flow specific heat F1Cp1= 10 kW/K
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Then,
Q stream #1(exchangeable load) in interval #1 = 10*(520-390) = 1300 kW
Example 3:
Interval # 1 in the cold section:
The interval start temperature is 550 K
The interval end temperature is 520 K
The flow specific heat of this cold stream is F1Cp1 = 119 kW/K
Then,
Q stream #1(cooling capacity) in interval #1= 19*(560-520) = 760 kW
Upon the completion of this step
2.2 Obtain the collective loads (capacities) of the hot (cold) process streams.
These collective loads (capacities) are calculated by summing up the individual loads
of the hot process streams that pass through that interval and the collective cooling
capacity of the cold streams within the same interval.
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These calculations for the above problem are shown in the following tables:
Table of Exchangeable Loads for Process Hot Streams Intervals
Interval
1
Load of H1, kW
Load of H2, kW
0.0*(560-520)= 0.0
0.0*(560-520)= 0.0
Total Load, kW
0.0+0.0= 0.0
1300+0.0= 1300
2
10*(520-390)= 1300 0.0*(520-390)= 0.0
3
10*(390-380)= 100
0.0*(390-380)= 0.0
100+0.0= 100
4
10*(380-350)= 500
5*(380-330)= 250
500+250= 750
5
0.0*(330-310)= 0.0
5*(330-310)= 100
0.0+ 100= 100
6
0.0*(310-300)= 0.0
5*(310-300)= 50
0.0+50= 50
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Table of Cooling Capacities for Process Cold Streams Intervals
Interval
1
Capacity of C1, kW Capacity of C2, kW
19*(550-510)= 760
0.0*(550-510)= 0.0
Total Load, kW
760+0.0= 760
2
19*(510-380)= 2470 0.0*(510-380)= 0.0
3
19*(380-370)= 190
4
19*(370-320)=950
2*(370-320)= 100
950+100= 1050
5
19*(320-300)= 380
0.0*(320-300)= 0.0
380+ 0.0= 380
6
0.0*(300-290)= 0.0
2*(380-370)= 20
0.0*(300-290)= 0.0
2470+0.0= 2470
190+20= 210
0.0+0.0= 0.0
3. Constructing thermal cascade diagrams
This diagram is constructed using the total hot loads and cooling capacities obtained in
the previous step for each temperature intervals.
The temperature intervals are drawn as “rectangular” with two inlets and two outlets.
The inlet from the left is the total hot load available in this interval (for instance,
1300 kW in case of interval # 2).
The inlet from above is the utility input load, in case of the first interval, or the input
from interval above in case of second, third,……,N intervals.
The output from the right is the total cooling capacity of this interval (for instance,
2470 kW in case of interval #2).
The output from the bottom is the difference between the total inputs and the cooling
capacity of the interval.
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The heat balance around each interval will be conducted as follows:
First Interval Heat Balance
Hot Load From Utility Source
“Top Input”
Hot Load From Process Source
“Left Input”
Cooling Capacity From Process Source
1
“ Right Output”
Residual Hot to Subsequent Interval
“Bottom Output” from first interval
Heat Balance
Top Input+ Left Input- Right Output = Bottom Output
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Numerical Example of First Interval Heat Balance
Hot Load From Utility Source
“Top Input”= 0.0 kW
Hot Load From Process Source
“Left Input”= 0.0 kW
Cooling Capacity From Process Source
1
“ Right Output”=760 kW
Residual Hot to Subsequent Interval
“Bottom Output” from first interval
= - 760 kW
Heat Balance
Top Input+ Left Input- Right Output = Bottom Output
0.0
+ 0.0
- 760
= - 760 kW
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Subsequent Intervals Heat Balance
Hot Load From Above Interval
“Top Input”
Hot Load From Process Source
“Left Input”
Cooling Capacity From Process Source
N
“ Right Output”
Residual Hot to Subsequent Interval
“Bottom Output”
Heat Balance
Top Input+ Left Input- Right Output = Bottom Output
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Numerical Example for Subsequent Intervals Heat Balance
For instance; Interval # 2
Hot Load From Above Interval
“Top Input” = -760
Hot Load From Process Source
“Left Input”= 1300
Cooling Capacity From Process Source
2
“ Right Output”= 2470
Residual Hot to Subsequent Interval
“Bottom Output” = - 1930
Heat Balance
Top Input+ Left Input- Right Output = Bottom Output
- 760
+ 1300
-2470
= -1930
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Upon the completion of heat balance around each interval the following diagram will be
produced:
Thermal Cascade Diagram (Un-Balanced)
Note: During this step the input from Hot Utility to the first interval is equal to zero
0.0
0.0
1300
100
750
760
1
- 760
2
- 1930
3
2470
210
- 2040
1050
4
-2340
100
50
380
5
- 2620
0.0
6
- 2570
The maximum difference between the available hot loads and cooling capacities from
the heat balances of these intervals is – 2620 kW.
This deficiency in heat will be supplied via outside hot utility.
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This value will be the input (from the top of the first interval) and the same heat balance
calculation conducted above will be repeated to produce the balanced thermal
cascade diagram below.
Thermal Cascade Diagram (Balanced)
Note: During this step the input from Hot Utility to the first interval is equal to zero
Minimum Q-heating = 2620 kW
0.0
1300
100
750
760
1
1860
2
690
3
2470
210
580
1050
4
280
Thermal Pinch
100
380
5
0.0
50
6
0.0
Minimum Q-cooling = 50 kW
With the completion of this step, now the minimum heating utility and minimum cooling
utility required are 2620 kW and 50 kW respectively.
These targets can give some idea about the potential of utility saving in the facility.
To get better idea in terms of utility types needed, diagram known as grand composite
curve shall be drawn to be used in defining kind of utilities needed and compare it with
the current facility needs from such utility. This step will help in capturing some
potential savings upon the heat integration of certain process area.
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4. Constructing the grand composite curve (G.C.C)
This curve will be drawn between T* calculated before and the corresponding top heat
inputs to each interval.
These data are depicted below:
Data Required To Construct The G.C.C
T* (K)
Enthalpy ( kW)
2620 kW
555
T* (K)
1
515
1860 kW
2
690 kW
385
3
580 kW
375
4
280 kW
310
5
305
0.0 kW
Thermal Pinch
6
295
50 kW
Enthalpy ( kW)
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Drawing these data as T* versus Enthalpy results in the following diagram that can be
used to define different levels of utilities that can be used to satisfy the process heating
utility requirement as shown below.
Grand Composite Curve (G.C.C)
Should Be Drawn To Scale
Total hot utility required is equal to 2620 kW
T* (K)
600
Hu3
Hu2
500
Hu1
400
300
200
Enthalpy ( kW)
700
1400
2100
2800
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Multiple utility targeting/selection using Grand Composite Curve (GCC)
Upon maximizing heat recovery in the heat exchanger network, those heating duties
and cooling duties not serviced by heat recovery must be provided by external utilities.
The most common utility is steam. It is usually available at several levels. High
temperature heating duties require furnace flue gas or a hot oil circuit. Cold utilities
might be refrigeration, cooling water, air cooling, furnace air preheating, boiler feed
water preheating, or even steam generation at higher temperatures.
Although the composite curves can be used to set energy targets, they are not a
suitable tool for the selection of utilities. The grand composite curve drawn above is a
more appropriate tool for understanding the interface between the process and the
utility system. It is also as will be shown in later chapters a very useful tool in studying
of the interaction between heat-integrated reactors, separators and the rest of the
process.
The GCC is obtained via drawing the problem table cascade as we shown earlier.
The graph shown above is a typical GCC. It shows the heat flow through the process
against temperature. It should be noted that the temperature plotted here is the shifted
temperature T* and not the actual temperature. Hot streams are represented by
∆Tmin/2 colder and the cold streams ∆Tmin/2 hotter tan they are in the streams
problem definition. This method means that an allowance of ∆Tmin is already built into
the graph between the hot and the cold for both process and utility streams. The point
of “zero” heat flow in the GCC is the pinch point. The open “jaws” at the top and the
bottom represent QHmin and QCmin respectively.
The grand composite curve (GCC) provides convenient tool for setting the targets for
the multiple utility levels of heating utilities as illustrated above.
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The graphs below further illustrate such capability for both heating and cooling utilities.
The above figure (a) shows a situation where HP steam is used for heating and
refrigeration is used for cooling the process. In order to reduce utilities cost,
intermediate utilities MP steam and cooling water (CW) are introduced. The second
graph (b) shows the targets for all the utilities. The target for the MP steam is set via
simply drawing a horizontal line at the MP steam temperature level starting from the
vertical axis until it touches the GCC. The remaining heat duty required is then satisfied
by the HP steam. This maximizes the MP steam consumption prior to the remaining
heating duty be fulfilled by the HP steam and therefore, minimizes the total utilities
cost. Similar logic is followed below the pinch to maximize the use of the cooling water
prior the use of the refrigeration.
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The points where the MP steam and CW levels touch the GCC are called utility
pinches since these are caused by utility levels. The graph (C) below shows a different
possibility of utility levels where furnace heating is used instead of HP steam.
Considering that furnace heating is more expensive than MP steam, the use of the MP
steam is first maximized. In the temperature range above the MP steam level, the
heating duty has to be supplied by the furnace flue gas. The flue gas flowrate is set as
shown in graph via drawing a sloping line starting from the MP steam to theoretical
flame temperature Ttft.
If the process pinch temperature is above the flue gas corrosion temperature, the heat
available from the flue gas between the MP steam and pinch temperature can be used
for process heating. This will reduce the MP steam consumption.
In summary, the GCC is one of the basic tools used in pinch technology for the
selection of appropriate utility levels and for targeting for a given set of multiple utility
levels. The targeting involves setting appropriate loads for the various utility levels by
maximizing cheaper utility loads and minimizing the loads on expensive utilities.
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(C)
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T-tft
T*
MP
CW
Refrigeration
H
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Normally, Plant’s Operations have choices of many hot and cold utilities and the graph
below shows some of available options. Generally, it is recommended to use hot
utilities at the lowest possible temperature while generating it at the highest possible
temperature. And for the cold utilities it is recommended to use it at the highest
possible temperature and generate at the lowest possible temperature. These
recommendations are best addressed systematically using the grand composite curve.
Hot and cold utilities
Boiler House
And Power Plant
Fuel
Steam
Turbines
W
Gas
Turbines
Hot Oil
Circuit
W
BFW
preheat
Heat
Pump
Process
W
Furnace
W
Cooling
Towers
Air preheat
Refrigeration
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Understanding the Grand Composite Curve:
The graph below shows that utility pinches are formed according to the number of
utilities used. Each time a utility is used a “utility pinch” is created. It also shows that
the GCC right noses sometimes known as “pockets” are areas of heat
integration/energy recovery. In other words it does not need any external utilities.
These right noses/pockets are caused by;
-
Region of net heat availability above the pinch
-
Region of net heat requirement below the pinch
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Applying the Grand Composite Curve:
GCC curve can be used by engineers to select the best match between utility profile
and process needs profile. For instance, the steam system shown below needs to be
integrated with the process demands profile to minimize low pressure steam flaring and
high or medium pressures steam let downs. Besides it helps selecting steam header
pressure levels and loads.
HP Boiler
HP
Proc. #1
HP Process
Condensate
Proc. #2
chemicals
MP Boiler
MP
Vent
Proc. #4
MP Process
Condensate
Proc. #1
Vent
Deaerator
BFW
Raw water
Make-up Treatment Plant
LP
Effluent
Proc. #1
Process Condensate
Proc. #3
LP Process
Condensate
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Superimposed Utility Profile with Process Profile
Nominal Case Supply-Demand Matching Problem
HP
T
MP
Process GCC
LP
BFW
CW
H
The superimposed steam system on the process grand composite curve shows that
while process heating needs can be achieved electricity can also be generated to
satisfy process demands and/or export the surplus to the grid.
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The graph below shows how we can use the GCC not only to select utility type, load
but also to define the steam headers minimum pressure/temperature to minimize
driving force and save energy.
T
Qh
HP
MP
LP
BFW
CW
Qc
H
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Grand Composite Curve can also be utilized to select the load and return temperature
of hot oil circuits. The graph below shows that while in many cases the process pinch
can be our limiting point in defining the load (slop of the hot oil line) and the return
temperature of the heating oil. In some other cases the topology of the GCC is the
limiting point not the process pinch. This is also shown in the second graph below. This
practical guide to select the load and the target temperature of the hot oil circuits is
also applicable to furnaces as will be shown later in this chapter.
Process Pinch temperature is the Limiting temperature for the Hot oil return temperature
T*
T supply
Hot Oil
T return
Process
Pinch
CW
Refrigeration
H
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Process Pinch temperature is not the Limiting temperature for the Hot oil return temperature
But the topology of the GCC curve
T*
T supply
Hot Oil
CP-min
T return
Process
Pinch
CW
Refrigeration
H
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Grand composite curve (GCC) can also be used to select the process refrigeration
levels and the synthesis of the multiple-cycles refrigeration systems as we did in the
steam system. The schematic graph below shows a simplified refrigeration system.
Schematic Diagram for multi-level Refrigeration System
Condenser
25ºC
CW
Process
0ºC
Process
-35ºC
Process
-65ºC
-5ºC
-40ºC
-70ºC
Work
Compressor
Page 50 of 54
Copyright©Saudi Aramco 2009. All rights reserved.
Document Responsibility: Energy Systems Unit/ESD/CSD
Issue date: 28 December 2005
Next Update: 1 January 2009
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
The GCC as we mentioned before can be used to place the refrigeration levels as we
did with steam levels. The graph below shows how we can do that.
We can place the refrigeration levels like steam levels.
Maximizing the highest temperature load to minimize the lower temperature loads
T
Tcw
- 5 ºC
- 40 ºC
- 70 ºC
H
Page 51 of 54
Copyright©Saudi Aramco 2009. All rights reserved.
Document Responsibility: Energy Systems Unit/ESD/CSD
Issue date: 28 December 2005
Next Update: 1 January 2009
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
When a hot utility needs to be at a high temperature and/or provide high heat fluxes,
radiant heat transfer is used from combustion of fuel in furnace. Furnace designs vary
according to the function of the furnace, heating duty and type of fuel, and method of
introducing combustion air.
4A.3 Basic Formulas for Some Quick Savings Estimation
_Compression Energy % Savings Due to Decrease in compressors Inlet temperature
% Energy saving in a compressor energy consumption = {1- (Tnew/Told)} * 100
Tnew is the new inlet temperature
Told is the old inlet temperature
_ Turbines gland steam leakage:
Steam leakage in kg/hr = Flow of steam measured- Flow of steam
utilized/required
Flow of steam required= work shaft required/ {Estimated Isentropic efficiency*(Inlet
enthalpy- outlet isentropic enthalpy)
Outlet isentropic enthalpy can be obtained from steam tables knowing outlet isentropic
entropy and outlet temperature or pressure
Outlet isentropic entropy = inlet entropy
Inlet entropy can be obtained from steam tables at inlet temperature and pressure
_ Back pressure turbines energy available for integration
Thermal energy available for Integration (Q) = Outlet steam flow*
(Vapor enthalpy- liquid enthalpy)
Outlet steam flow= Inlet steam flow (1- actual wetting factor)
Actual wetting factor can be assumed between (8 to 15) %
_ % Energy saving in heat pumps/refrigeration cycles due to decrease in reject
temperature
Page 52 of 54
Copyright©Saudi Aramco 2009. All rights reserved.
Document Responsibility: Energy Systems Unit/ESD/CSD
Issue date: 28 December 2005
Next Update: 1 January 2009
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
W2/W1 = (T reject 2 – Tc)/ (T reject 1 – Tc)
Treject is the temperature at which heat is rejected to the cooling medium (water)
Tc is the temperature at which heat is taken into the refrigeration) cycle
_ Material balance for the cooling tower
Assuming the system is at equilibrium
Make-up = Evaporation+ Blowdown+ Windage loss
Cooling towers cycles of concentration (C)
C= concentration of solids in the circulating water/ concentration of solids in
make-up water
_ Energy saving in adjusting of combustion in a natural gas fired boiler/furnace
Analysis on the exhaust from the boilers sometimes or most of the times in some
places reveals excess oxygen levels which result in unnecessary energy consumption.
This excess air defines a low efficiency on the combustion process. Portable or even
on-line flue gas analyzer can be used as a part of a rigorous boilers/furnaces
inspection program.
The optimum amount of O2 in the flue gas of a gas fired boiler is 2%, which
corresponds to 10% excess air. Controlling the combustion process could lead to at
least 2% fuel saving upon bringing the excess O2 from for instance 6% to 2%.
Cost saving $/year= Energy usage per year* % possible fuel savings* fuel cost
per unit of energy
Energy saving in adjusting of combustion in a an oil fired boiler/furnace.
Analysis on the exhaust from the boilers sometimes or most of the times in some
places reveals excess oxygen levels which result in unnecessary energy consumption.
This excess air defines a low efficiency on the combustion process. Portable or even
on-line flue gas analyzer can be used as a part of a rigorous boilers/furnaces
inspection program.
The optimum amount of O2 in the flue gas of an oil fired boiler is 4%. Controlling the
combustion process could lead to at least 1% fuel saving upon bringing the excess O2
from for instance 6% to 1%.
Page 53 of 54
Copyright©Saudi Aramco 2009. All rights reserved.
Document Responsibility: Energy Systems Unit/ESD/CSD
Issue date: 28 December 2005
Next Update: 1 January 2009
SABP-A-005
Quick Energy Assessment Methodology
for Energy Efficiency Optimization
Cost saving $/year= Energy usage per year* % possible fuel savings* fuel cost
per unit of energy
Energy saving in preheating boiler/furnace combustion air with stack’s waste heat:
If the intake air (air drawn from outside into the natural gas boiler) is at ambient outdoor
temperature unnecessary fuel will be consumed to heat up this combustion air. In order
to reduce fuel consumption, it is recommended to install recuperative air pre-heater on
the air intake of the boiler to preheat combustion air using heat which is exhausted
along with the combustion from the boiler.
Stack exhaust losses are part of all fuel-fired processes. They increase with the
exhaust temperature and the amount of excess air the exhaust contains. A high quality
air pre-heater could recover more than 40% of this waste heat. Therefore, the potential
savings from the installation of air pre-heater on the boiler is:
Cost saving = fuel cost ($)/unit of energy* Energy Consumed/year*(boiler
efficiency)*percent of energy recovered by air pre-heater
Note: The following attached pages are two curves and a table that can be used to
estimate the percentage of fuel savings.
4A.4 HGP Detailed Energy Assessment Study
Final Report, SAER # 5995,”Detailed Energy Assessment at Hawiyah Gas Plant”,
June 26, 2005
Page 54 of 54
Copyright©Saudi Aramco 2009. All rights reserved.