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Technical Reference eBook
DISTRICT
COOLING
DC
DISTRICT COOLING REFERENCE EBOOK
01 INTRODUCTION TO DISTRICT COOLING
01 INTRODUCTION TO DCP
DISTRICT COOLING
District Cooling refers to the centralized production and distribution of cooling energy.
The cooling energy is produced in a central cooling plant as chilled water and is
distributed to consumers in a closed piping circuit, also referred to as a reticulation
system. The concept of District Cooling is being implemented more often worldwide
by many different organizations, both private and public, due to the multiple benefits
that brings to a high density demand area. More and more often it is incorporated
just as another facility for a building, like electricity, fresh water or natural gas may be.
Traditionally, DC Plants have been designed following residential standards and
practices. However, the most recent Plants with a capacity of several tens of thousands
of Tons of Refrigeration (TR) cannot be designed following this philosophy. The idea
behind central cooling is applying economies of scale to the residential refrigeration.
Traditionally, the building refrigeration systems have been designed following
residential standards and practices. Summing up the refrigeration demand from
several consumers enables a larger production of refrigeration energy. That implies
the use of industrial-grade equipment and industrial practices in order to ensure the
reliability and safety of the District Cooling Plant.
Efficiency is a crucial parameter for these kinds of plants as the amounts of energy
consumed throughout the year is very high. Any improvement in efficiency results in
huge savings in any developer’s running costs. Reliability is also a must as so many
residences, hotels, people and businesses depend on it.
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DISTRICT COOLING REFERENCE EBOOK
01 INTRODUCTION TO DISTRICT COOLING
01 INTRODUCTION TO DCP
BUILDING A DISTRICT COOLING
Building a District Cooling is not always feasible. The main factor for the DC
implementation is the consumer density. Having a high demand in a small area makes
it a better solution compared with the traditional ones. If the consumers are too spread
apart, the DC may not be possible due to high energy loses in the cooling energy
transport.
Just as any other utility, the construction of the District Cooling is more difficult to
achieve in an already existing development. This is the reason why the developments
of new construction are already implementing this technology, providing the necessary
space for its elements.
The concentration of heterogeneous consumers with different land uses will make
necessary the study of the simultaneity of the cooling load profiles. Not all consumers
have their peak cooling load at the same time; for example offices will have their
maximum consumption during the working hours while the maximum consumption
for the hotels will be in the afternoon. Therefore the cooling plant shall be designed
according to this peak of the connected load.
Regarding the rest of the factors surrounding the potential District Cooling, both the
other utilities and the local legislation (such as water regulations, electricity regulations,
etc.) should be compatible with the system implementation. ARANER studies the
particular contour conditions in order to provide a tailor-made solution.
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DISTRICT COOLING REFERENCE EBOOK
02 BENEFITS OF DISTRICT COOLING
02 BENEFITS OF DISTRICT COOLING
STAKEHOLDERS
PRODUCERS
DC is a safe business with a quite stable demand and
relatively low payback periods. As production takes place
in an industrial environment, it is always subjected to
optimization and improvement of efficiencies and installed
cooling capacity.
Reduction in
Optimization
peak Electricity
of the installed
Demand
cooling capacity
Possibility of
Energy Storage
CONSUMERS
The capital cost and operation cost are reduced. In addition,
the cooling equipment which can be noisy is usually
placed outside the building, sometimes remotely, and the
maintenance operations are often outsourced.
SOCIETY
The implantation of DC System for sustainable cities, since
it is a more efficient solution than a stand-alone chiller. This
means a reduction of the energy consumption and of the
CO2 footprint. Also the city skyline benefits by eliminating the
installation of individual cooling equipment systems.
4
Recognized Green
Technology with
carbon credit
Operation and
Maintenance
Reduction in
Services Provided
CO2 Emissions
by experts
Higher
Higher
Reliability
Efficiency
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DISTRICT COOLING REFERENCE EBOOK
02 BENEFITS OF DISTRICT COOLING
02 BENEFITS OF DISTRICT COOLING
TYPICAL POWER DEMAND
TYPICAL PEAK POWER DEMAND FOR DIFFERENT COOLING SOLUTIONS
When a DC is designed for several building with different usage, the demand can
greatly differ among the different heating consumers. Building energy efficiency
2
legislation has traditionally focused on space cooling energy consumption. However,
in the future when more renewable energy is used both on site and in energy systems,
and energy security.
If the network is well balanced, the production plant size can be optimized adding
some thermal storage technology.
This reduction in the plant production power will not only reduce the total amount of
energy consumed, but also the primary energy needed to feed the District Cooling.
P E AK P OWE R DE M AND ( KW / T R )
the peak energy demand becomes more important with respect to CO2 emissions
1.75
1.5
1.25
1
0.75
0.5
0.25
0
5
Air Cooled Building
Systems
District Cooling
(Electric)
District Cooling (Electric
with TES)
District Cooling (100%
Gas-fired)
DISTRICT COOLING REFERENCE EBOOK
03 EFFICIENT DISTRICT COOLING SYSTEMS
03 EFFICIENT DC SYSTEMS
GREEN BUILDING
Green building (also known as green construction or sustainable building) refers to
a structure that is designed, constructed and operated using processes that are
environmentally responsible and resource-efficient. Efficient District Cooling Systems
contribute to the Green building as the total electrical consumption of the building is
reduced.
LEED Certification (Leadership in Energy & Efficient Design), which is recognized across
the world as the premier standard for Green buildings, recognizes the use of DC systems
as a significant contributor towards achieving a highly efficient building. This is a point
based system that categorizes Green buildings into four different levels – Certified,
Silver, Gold and Platinum.
Selecting an environmentally friendly refrigerant is also important in order to receive
a higher quantity of LEED points to achieve a higher certification level. The refrigerant
used must have a very low Global Warming Potential and Ozone Depletion Potential.
CFC and HCFC refrigerants have been already phased out in most of the countries
around the world and R-134 will be phased out in the very near future (phase out
is already planned in USA and Europe); given its favorable properties R-717 remains
a preferred refrigerant for efficiency and environmental reasons and is expected to
remain so indefinitely.
6
LEED CERTIFICATIONS
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USGBC
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DISTRICT COOLING REFERENCE EBOOK
04 DESIGN CRITERIA & SIMULATIONS
04 DESIGN CRITERIA
DC SIMULATION
The first stage is to design the DC Plant as efficient as possible, in this regards studying the simultaneity of the cooling demand of
the different buildings/consumers can result in a great optimization of the system. Not all consumers have their peak cooling load
at the same time. As an example, offices will have their maximum consumption during the working hours while the maximum
consumption for the hotels will be in the afternoon.
Studying the simultaneity of the different buildings/consumers will determine the peak of the connected cooling load of the entire
DC system. The cooling plant in order to be efficient shall be designed according to this peak of the connected cooling load
and maximize energy production.
A simulation of the District Cooling for a complete year will be very useful in order
100%
to evaluate the different technologies and select the optimum solution for every
90%
80%
factors:
•
•
•
The ambient temperature
The building occupancy depending on the hour and the building type
(office, residential, hotel, retail…)
The seasonal occupancy (in case of resorts, universities or any other special
cases which have very low occupancy during certain months of the year)
Loads % of Peak
case. The simulation will estimate the Hourly Cooling Demand based on the following
70%
60%
50%
40%
30%
Residential
20%
Offices
Hotel
Retail
10%
0%
0
2
4
6
8
10
12
Hours of the Day
7
14
16
18
20
22
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DISTRICT COOLING REFERENCE EBOOK
04 DESIGN CRITERIA & SIMULATIONS
04 DESIGN CRITERIA
For this third stage, it is recommended to involve a DC Designer with
parameters in order to achieve an optimum design. The natural following
important point is to wonder whether it is advisable to install a Thermal
Energy Storage (TES) system in our plant.
Then, how can we reject the heat from our refrigeration process?
Here we have three main options:
•
•
•
Air cooled
Water cooled
Chiller Consumption (kW·h)
extensive experience and the capability to evaluate the relevant
16,000,000
16,000,000
14,000,000
14,000,000
12,000,000
12,000,000
10,000,000
10,000,000
8,000,000
8,000,000
6,000,000
6,000,000
4,000,000
4,000,000
2,000,000
2,000,000
Sea water / river water cooled
Water Cooled Chillers Consumption (kW·h)
After the preliminary configuration of the plant is selected, it is advisable to run a simulation
for a complete year in order to check the overall benefit of the system. Once the cooling
demand is simulated, the cooling plant consumption will be also evaluated using local
costs for energy and water. The optimum solution will depend on the Owner’s criteria:
•
•
•
8
0
0
Reduce the yearly operation cost
Reduce the yearly electrical consumption
Reduce the yearly water consumption
Air Cooled Chillers Consumption (kW·h)
Cooling Demand (TR·h)
Cooling Demand
DC SIMULATION
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DISTRICT COOLING REFERENCE EBOOK
05 DISTRICT COOLING PLANT COMPONENTS
05 DISTRICT COOLING PLANTS
COMPONENTS
>
HEAT REJECTION
TECHNOLOGIES
THERMAL ENERGY
STORAGE TANK
<
CONSUMPTION
PUMPS
HIGH - EFFICIENT
INDUSTRIAL CHILLERS
9
>
<
PRODUCTION
PUMPS
>
DISTRICT COOLING REFERENCE EBOOK
05 DISTRICT COOLING PLANT COMPONENTS
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The chillers are one of the most important elements of the plant, since
they have the mission of producing the cooling energy, performing the
05 DCP COMPONENTS
MECHANICAL ROOM
> HIGH- INDUSTRIAL EFFICIENCY CHILLERS
When designing the cooling plant the chillers selection play an important role for
lowering the energy consumption. These are one of the most important elements of
the plant, since they have the mission of producing the cooling energy, performing
the refrigeration cycle. The common practice is to use vapor compression chillers, but
other technologies can be applied depending on the project conditions and cooling
requirements.
ARANER´s chillers are robust industrial grade machines designed and manufactured
to improve the District Cooling plant efficiency, reducing the total water and
electrical consumption in the plant. Energy consumption is one of the major concerns
worldwide in these days. Lowering energy consumption not only results in big savings in
operation cost but also contributes to the environment by reducing the CO2 footprint.
Furthermore, the tailor-made chillers developed by ARANER are very flexible and will
be suitable despite the project specifications and site conditions:
•
•
•
•
•
•
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Different refrigerant types (R717, R134a, R504a)
Electrical, diesel or gas driven motors
Dry cooled or water cooled solutions and sea water cooled
Absorption chillers
Integration with Thermal Energy Storage Technologies
Centrifugal or Screw type
refrigeration cycle
DISTRICT COOLING REFERENCE EBOOK
05 DISTRICT COOLING PLANT COMPONENTS
05 DCP COMPONENTS
MECHANICAL ROOM
CENTRIFUGAL VS. SCREW
These are one of the most important elements of the plant, since they have the
mission of producing the cooling energy, performing the refrigeration cycle. The
common practice is to use vapor compression chillers, but other technologies can be
applied depending on the project conditions and cooling requirements. These vapor
compressors could be:
•
CENTRIFUGAL: They are a non-positive displacement type and therefore
more sensible to a pressure differential lift between evaporation pressure and
condensing pressure. The main advantage of centrifugal compressors is their
high flow rates capability and good efficiency characteristics
•
SCREW: They are positive displacement compressors encase a quantity of
refrigerant in a decreasing volume during the compression process. They
provide excellent lift characteristics. ARANER screw machines have an extra
improved efficiency for the oil injection system.
A key factor when designing a District Cooling is the proper chiller selection. The
selection shall come together with the District Cooling plant design, in order to ensure
the smooth integration of the system.
11
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DISTRICT COOLING REFERENCE EBOOK
05 DISTRICT COOLING PLANT COMPONENTS
05 DCP COMPONENTS
MECHANICAL ROOM
CENTRIFUGAL CHILLER
SCREW CHILLER
Centrifugal compressors are a non-positive displacement type and therefore more
Screw compressor or positive displacement compressors encase a quantity of
sensible to a pressure differential lift between evaporation pressure and condensing
refrigerant in a decreasing volume during the compression process. They provide
pressure.
excellent lift characteristics
•
Centrifugal chillers are designed with fewer moving parts and straightforward
with efficient engineering, and have proven durability records in hospitals,
district cooling systems, and in other applications where minimal downtime is
a crucial concern
•
High strength aluminum-alloy compressor impellers feature backward curved
vanes for high efficiency. Airfoil shaped pre-rotation vanes minimize flow
disruption for the most efficient partial load performance
•
The advantage of centrifugal compressors is their high flow rates capability
and good efficiency characteristics.
12
•
They are low speed machines with less wear and so, long trouble free life with
less maintenance.
•
Screw compressors are able to accommodates the pressure lift increasing
performance for different conditions (day/night, Summer/winter)
•
ARANER screw machines have an extra improved efficiency for the oil injection
system
DISTRICT COOLING REFERENCE EBOOK
05 DISTRICT COOLING PLANT COMPONENTS
05 DCP COMPONENTS
MECHANICAL ROOM
ABSORPTION CHILLER
Absorption chillers, instead of using electricity, use ‘heat’ as the energy source, a low
grade energy. The energy source may be steam or hot water, or it may even be waste
heat like in exhaust gases from an engine (gas or oil based). ARANER offers a wide
range of solutions for each of these energy sources, all of which represent a major
advance in Absorption Refrigeration Technology.
ARANER absorption heat pump uses tested components and procedures for industrial
refrigeration applications. Their main characteristics are:
•
•
•
•
•
•
•
Minimum maintenance due to few moving parts
Ready for dry condensation with ZERO water consumption
Modular solution, factory assembled
LiBr-water technology as the most proven for heating
Lower electrical energy consumption compared with compression heat pumps
Higher CAPEX than compression heat pumps
Extra investment is recovered on mid/long-term basis
Our absorption heat pumps can be installed in a co-generation plant as a heat
recovery system, in order to reduce steam consumption. It can be also used on the
flue gas condensation for improving the system efficiency.
13
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DISTRICT COOLING REFERENCE EBOOK
05 DISTRICT COOLING PLANT COMPONENTS
05 DCP COMPONENTS
MECHANICAL ROOM
> HEAT REJECTION
Due to the nature of the cooling process, the heat extracted from one system should
be transferred to another one, thus rejected from the District Cooling plant. Learn the
main advantages and disadvantages in our White-paper and discover the technology
that best suits your project. In the following chapters you can find out how to improve
the District Cooling plant performance, taking into account the most important factors.
Other than that, heat rejection is closely bounded to the chiller design and chiller
selection.
> PUMPING SYSTEMS
Usually there are two pumping groups for the chilled water and one for the condenser
water (if they are required). Regarding the chilled water pumps, they are usually
divided into production pumps, pumping into the chillers the necessary return of chilled
water, and consumption pumps, responsible of moving the water along the Distribution
Piping Network (DPN) and the Energy Transfer Stations (ETS). Nevertheless, the pumping
strategy may vary depending on the DPN & ETS of each particular project.
14
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DISTRICT COOLING REFERENCE EBOOK
05 DISTRICT COOLING PLANT COMPONENTS
05 DCP COMPONENTS
ELECTRICAL & CONTROL ROOM
> ELECTRICAL ROOM
> CONTROL ROOM
Electrical rooms are very important for projects successfully operation, providing a hub
The diverse components of each project need to operate and perform all together as
to supply electrical power for equipment and other components of the DCP. The main
a single and fully integrated system in order to achieve the optimum performance of
advantage of a well-designed electrical room is providing a central location where
the plant, up to client satisfaction.
technical staff members can manage and service building power systems and ensure
an optimized operating work space day after day, even in blackouts or emergency
situations.
ARANER designs, implements and develops a Safety and Redundant Systems with the
highest specifications from the industry according to the project requirement, including
since the most basic devices up to the highest performance equipment. Our control
Our Electrical Department is formed by highly skilled engineers with relevant experience
systems are based on latest PAC (Programmable Automation Controller) technology
in most electrical engineering fields, such as; high and low voltage substations,
for highest performance and durability and are equipped with a suite of tools that
Overhead lines, Variable Frequency Drives and Soft Starters...Our engineers design the
allow the creation of full customizable features, such as HMI alarms, historical and real-
electrical facilities as per the most recent industry standards from IEC, IEEE, NEC, BS,
time trending, built in reporting. With a very intuitive and easy-to-use Human Machine
and other Electrical Engineering institutions.
Interfaces (HMI) and SCADA’s, full control and plant integration are achieved
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DISTRICT COOLING REFERENCE EBOOK
05 DISTRICT COOLING PLANT COMPONENTS
05 DC COMPONENTS
DISTRIBUTION PIPING NETWORK (DPN)
As the principle behind District Cooling lies in the production of cooling energy in a
Depending on the particular conditions of the area where the District Cooling will be
centralized location, this cooling energy should be delivered to the costumers. Chilled
deployed, the distribution network can be above ground or underground. Regarding
water is conducted then to the consumption points by means of a Distribution Piping
the underground piping, it could be directly buried, laying on a gallery or trench, or
Network (DPN). The routing of the distribution pipe implies two different conductions:
traveling along a basement. Any of those arrangements should provide an access
one for chilled water supply and another one for chilled water return. The temperatures
to the piping every few meters, in case maintenance is required. In order to do that,
of the supply and return water will vary depending on the cooling energy deliver:
isolating cutting points shall also be foreseen.
•
•
DIRECT CHILLED WATER UTILIZATION: the water from the DPN is pumped directly
The deployment area, together with the chilled water network analysis, conditions the
through the end user heat exchangers. It is used for small installations where
piping material selection. Suitable materials could be for example PCV or carbon steel.
any failure on the end user side can be easily solved (usually they have the
In any case, the pipe should be thermally isolated, so that energy losses are minimized
same owner). The typical temperatures for this method are 7 ºC for the supply
It is usual that there are several District Cooling plants supplying energy to the network
and 12 ºC for the return.
from different locations. Therefore, the selection of an appropriate topology is a key
ENERGY TRANSFER STATION (ETS): the energy is transferred in a heat
exchanger to another water circuit. It is extensively used, as it allows a
normal performance of the network in case of a failure on the end user
side. The typical temperatures for this method are 4.4 ºC for the supply and
13.3 ºC for the return.
16
aspect for the supply of critical loads.
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DISTRICT COOLING REFERENCE EBOOK
05 DISTRICT COOLING PLANT COMPONENTS
05 DCP COMPONENTS
ENERGY TRANSFER STATION (ETS)
An Energy Transfer Station (ETS) takes cold energy from the network by means of a
This valve ensures a determined flow rate, which will vary according to the ETS
heat exchanger for delivering it to the end user. On one side of the heat exchanger
demand. The demand flow rate is usually calculated measuring the chilled water
consequently the chilled water supply will be heated up to the return temperature.
return temperature from the ETS: if the demand rises, the return temperature will be
On the other side, an independent water circuit is cooled – this circuit is called tertiary
lower and it will require a higher flow rate in order to maintain a constant temperature.
system.
The ETS, composed by the heat exchanger and the pumping group, has usually a
The tertiary system delivers the cooling energy direct to the end user heat exchangers,
dedicated space inside the building, most often in the basement. It distributes the cold
as it takes it from the network. It has its own pumping group, since it is not hydraulically
water to the different end users just as another building utility. In that sense, each ETS
connected to the network. This means they can be individually designed according to
has their own metering equipment and the building owner can decide if having also
the building characteristics, with no influence on the rest of the District Cooling design.
partial metering devices in his tertiary system for the different end users. A common
practice is the installation of partial redundancy in the ETS heat exchanger. That means
As the supply and return temperature in the DPN are constant, the energy transferred
installing two heat exchanger with 75% each of the total capacity required. The two
to the ETS depends on the flow rate that passes through it. The flow rate adapts to the
heat exchangers should work together for achieving the peak capacity, but one of
demand by partially opening or closing a Pressure Independent Control Valve (PICV).
them can be stopped for maintenance while the other is running during low demand
periods, avoiding service interruption.
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DISTRICT COOLING REFERENCE EBOOK
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06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
COOLING TOWERS
The District Cooling plant may reject the heat derived from the cooling process with
The water quality is an important factor in the cooling tower design. Depending on this
a cooling tower. Cooling towers are available in different types and sizes depending
parameter, the cooling water will have a different number of cycles of concentration
on the load configuration: an important reason to outline the options available. Note
(CoC). A good water quality would imply a high CoC, typically around 10 or 11, so less
that despite the different designs, the basic function remains the same as that of
blow-down water would be required and consequently less make-up consumption.
dissipating heat from the process to the air through evaporation. Defining the design
On the other hand, with a bad water quality the CoC would be low, around 3-5. The
wet bulb temperature is a key aspect for a proper cooling water dimensioning. The
latter is usual when direct TSE water is used as make-up water.
wet bulb temperature is the minimum theoretical temperature that can be achieved
at the cooling tower outlet, as heat is rejected by evaporation. The fact is that the
outlet temperature is higher: the difference between this two temperatures is called
approach. Selecting a smaller approach will result in a bigger cooling tower, which
may be not feasible in economical or technical terms. The approach selected is
usually higher than 3 ºC.
The placement of the cooling towers is also very important, as the air should be allowed
to flow inside and outside the tower, with less obstructions as possible. If several cooling
towers are arranged in the immediate vicinity, the outlet wet air of one cooling tower
could enter in another cooling tower (which will reduce drastically its performance),
if they are not carefully located depending on the direction of the prevailing wind.
The same phenomenon could occur with one cooling tower itself. This recirculation
depends on the relation between the plume discharge velocity and the ambient
wind velocity. In order to ensure a suitable cooling tower performance, ARANER can
develop a CFD model simulating the interaction between the wind and the cooling
tower air current.
18
The air current that flows through the cooling tower can drag away water droplets
due to its speed. For avoid water losses, a drift eliminator is installed. This also helps to
prevent the formation of a plume, as the moist outlet air meets the ambient.
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DISTRICT COOLING REFERENCE EBOOK
06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
AIR CONDENSATION
With this technology, heat is rejected to the ambient air by means of a fin and tubes
temperature approach between the refrigerant and the cooling water and
heat exchanger. A current of ambient air is forced through the heat exchanger by
another one between the cooling water and the dry bulb temperature.
a fan installed on top of it. This air current flows between the heat exchanger tubes,
arranged in coils. The tubes have fins attached to them, so that the surface of thermal
exchange is enhanced. Therefore, heat is transferred from the fluid inside the tubes to
the air current. The tubes are normally made of copper and the fins material is usually
aluminum, nevertheless different treatments such as protective coating and different
alloys with good thermal transmission properties can be selected.
The minimum theoretical temperature that can be achieved with this technology will be
the ambient dry bulb temperature. This is not feasible as the size of the heat exchanger
would be infinite, therefore the fluid outlet temperature will be higher than the dry bulb
temperature. The difference between the minimum theoretical temperature (in this
case the dry bulb) and the fluid outlet temperature is called approach. The dry bulb
temperature is always higher than the wet bulb temperature, and this difference is
even higher in dry climates. This temperature conditions the condensation temperature
of the refrigeration machine (the condensation temperature must be higher than the
dry bulb temperature, so that heat exchange takes place), and consequently the
plant efficiency: the lower the condensation temperature is, the better efficiency can
be accomplished. Depending on the type of air condensation used, the efficiency
can vary:
•
INDIRECT CONDENSATION. The fluid sent to the fin and tubes heat exchanger
is water. This water is circulated in a closed circuit, rejecting the heat from
the condenser of the refrigeration machine. There would exist hence a
19
•
DIRECT CONDENSATION. The refrigerant is sent directly to the fin and tubes
heat exchanger. Compared with the indirect condensation, one temperature
approach will be avoided, so that the condensation temperature will be
able to be lower, leading to a higher plant efficiency. The problem with
direct condensation is the fact that the fin and tubes heat exchanger should
be integrated into the refrigeration machine, as the exchanger is now the
condenser, substituting the usual shell and tubes heat exchanger. This is not
a solution proposed by centrifugal chiller manufacturers because it requires
a complex calculation of the refrigeration cycle for each particular scenario
instead of a packaged solution.
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DISTRICT COOLING REFERENCE EBOOK
06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
SEA WATER CONDENSATION
A good alternative for condensation technologies is rejecting the heat to the sea.
Around 70 % of the Earth’s surface is covered by water, making it an unlimited,
constant and low-cost resource. Moreover, the water mass has a lower temperature
than the ambient temperature and it also suffers less temperature variations. There is
no contact between sea water and other fluids (such as refrigerant, oils, etc.) During
the process, so water can be released back into the sea with no changes but a slight
increase of temperature.
However, this resource is not always available depending on the project location
(obviously it is not available everywhere) and its particular constrains. The local
regulations protect as well the environment from the uncontrolled used, such as
limiting the temperature rise and the sea water interfaces (intake and discharge),
preserving the zones with high submarine value.
Water is taken from the sea with a sufficient height from the sea bed in order to avoid
Even if most solid particles entrance is avoided with a good sea water intake design, a
sand to come inside the system, and deep enough to prevent the entrance of lower
filtration system is necessary for protecting the pumping system and the heat exchanger.
density debris. Furthermore, the intake water speed should be low enough for fish to
The filtration system selected should adequate to the sea water characteristics and to
be able to swim away when they get the feeling that they are dragged by the current.
the project specific conditions. The sea water quality usually has more influence in
The stability of the submerged system should be also carefully studied, as it is subject
the number of filtration stages and the free span of the mesh, whether the contour
to mechanical stresses caused by the tides and the geotechnical conditions. Without
conditions usually influence more in the technology selected (screen coarse, rotating
a proper structural dimensioning the system components could flip, turn, fall or even
filters, disk filters, etc.). In order to avoid the growth of marine life inside the system,
break, causing important malfunctions downstream.
which could deposit and result of great efficiency losses, antibiofouling measures must
be installed.
20
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DISTRICT COOLING REFERENCE EBOOK
06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
SEA WATER CONDENSATION
The materials of the sea water system should be selected according to the water
•
SUBMERGED DISCHARGE: similar considerations as for the sea water intake must
characteristics. As a clear example, the salinity of the Dead Sea, which is around 337
be taken into account. Additionally, some nozzles at the end of the discharge
g/kg, has nothing to do with the World Ocean salinity, which has a mean of 34.7 g/
pipe shall be placed. The nozzles will just allow the flow of water to the sea, not
kg. The presence of sodium chloride makes the water corrosive against some usual
letting marine life to enter the pipe when the system is not working
construction materials like SEA copper. Organic materials are normally inert to this
water, just like PE or PVC, making them often suitable for water conduction. In addition
to water corrosion resistance, the material selected for each component should
meet their own requirements. Pumps material should guarantee their mechanical
performance, and the heat exchangers material should have a good heat transmission
coefficient.
Regarding the sea water discharge, the determination of the water outlet point
is essential. Recirculation must be avoided so that the system can work properly,
therefore a careful study of the currents and the project particularities must be held.
The discharge point shall comply with the local regulation and with this regard it could
be differently performed:
21
•
ABOVE WATER DISCHARGE: it causes more visual impact as the previous one.
Nevertheless, the discharge could be used with aesthetically purposes, and
the complex developer may take advantage of the DC plant discharge for
applying it to a decorative space.
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06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
RIVER WATER CONDENSATION
River water cooling is a system very similar to the sea water condensation, also with no
water consumption and low visual impact. In addition to the advantages of the sea
water systems, river water condensation implies the following:
•
Less corrosive water, which benefits the system components
•
Constant direction of the river flow prevents the recirculation of water, ensuring
a good performance of the plant
•
Alternative to sea water condensation in locations further away from the sea
side
As always when designing a cooling solution that uses water from the environment,
special attention should be paid to the particularities of the project. A river water
current study, an environmental impact study, a water quality study, etc. should be
elaborated, which are required in order to comply with the environmental legislation.
Not all the rivers are suitable to be used in this application.
Water is taken from the river with a sufficient height from the river bed in order to avoid
sand to coming inside the system, and deep enough to prevent the entrance of lower
density debris. Moreover, the intake water speed should be low enough for fish to be
able to swim away when they get the feeling that they are dragged by the current.
The water is also filtrated, so that suspension solids are removed from the cooling water.
22
After that, water should be treated to avoid the growth of water life inside the system,
which could deposit and result of great efficiency losses (great pressure losses, lower
heat transmission, etc.). Therefore an antibiofouling system may be used. Water is then
pumped into the chiller condenser by means of a pumping group. Both pumps and
condenser materials in contact with water should be adequate in order to not present
corrosion.
Finally, water is returned to the river with a slight increase of temperature, as per
environmental legislation. As stated before, the discharge point should be carefully
placed with the support of studies. It is also important to diffuse the discharge along
the river and not in a single point, since depending on the particular conditions it could
have an impact over the river life.
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DISTRICT COOLING REFERENCE EBOOK
06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
REFRIGERANT SELECTION
No refrigerant fits the requirements 100 % for all cooling plants. An ideal refrigerant
would need to have it all in terms of material compatibility, chemical stability,
performance, non-toxicity, non-flammability, boiling point among other criterion
industrial refrigeration engineers, our designs, manufactures and installations are
suitable for the most popular refrigerants.
> HFC
HFC is the third generation of fluorinated refrigerants. These are manmade organic
> AMMONIA R717
Ammonia, a member of the so-called halogen-free chemicals, it’s perhaps the
most common refrigerant in industrial cooling plants and among the oldest ones. Its
heat absorption per volume is unrivaled. This alone allows its application in smaller
components- no need for huge cooling plants. Other attractive features include
high critical point, high coefficient of performance and low molecular weight. Highly
energy-efficient refrigerant with minimal environmental problems, for that, ammonia
should be always selected, when possible, for industrial refrigeration installations.
compounds having fluorine and hydrogen atoms. The main difference in comparison
with HCFC refrigerants is that HFC do not contain chlorine. Chlorine can cause harm
to the ozone layer, whereas HFC is free of chlorine so they do not harm the ozone
layer. Therefore HFC are considered more environmentally friendly than HCFC. The
most common HFC refrigerant is R134a. R407A, R410a, R507A, etc. are also part of the
HFC group..
> HFOS (HYDROFLUORO-OLEFINS)
HFOs are the fourth generation of fluorinated refrigerants. HFOs are composed of the
same atoms like HFCs: carbon (C), hydrogen (H) and fluorine (F), but are unsaturated
organic compounds – hence the suffix “olefin”. They are based on alkenes like
propene and butane. Their Global Warming Potential (GWP) is very low, much lower
than HFC’s GWP; however when HFOs decompose in the atmosphere, trifluoroacetic
acid (TFA(A)) is formed which produce trifluoroacetate (TFA), a salt very difficult to
remove from drinking water. The most utilized HFOs refrigerants are HFO-1234yf, HFO1234ze, HFO-1233zd.
23
> CO2 R744
CO2 scores highly because of its minimal environmental impact. The refrigerant is
also non-flammable and non-toxic. However, despite these pleasant attributes, the
refrigerant needs careful handling. First, the chemical is heavier meaning that in case
of leakage, it would displace oxygen from the room. Combining that with the fact that
it is odorless forms a very dangerous scenario. It mainly revolves around efficiency, size
and cost of system. A pressure of about 4,000 psi presents a huge cost and technical
challenge for heat exchangers and compressors.
DISTRICT COOLING REFERENCE EBOOK
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06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
REFRIGERANT SELECTION
> WATER
Water has been used as a refrigerant for many decades and it never ceases to
impress. Apart from being readily available, this substance has impeccable chemical
and thermodynamic properties. It cannot be considered as a refrigerant itself, but
The debate over what is “the best” commercial refrigerant has been around and is
unlikely to end anytime soon. Intensity on this matter is even increasing as leakage
studies reveal the adverse effects of HFC emissions. The only way out for centralized
systems is emissions reduction, which while possible, is costly.
it is chilled in cooling plants and introduced into the circuit to lower temperatures.
It presents several technical challenges though. These include high-pressure ratios and
The industry is committed to reducing environmental impact even as it promotes safety
outlet temperatures at the compressor.
and efficiency. Governments are continually requiring equipment manufacturers
to shift to chemicals that are safer to the environment. In the cooling/ refrigeration
> HCFC-HYDROCHLOROFLUOROCARBONS
These compounds are slowly being phased out because of their high GWP. Some
places have already banned the use of this refrigerant in new equipment. If you are
looking to recover, maintain or replace your HCFC refrigeration equipment, ARANER
qualified personnel can help. Specifically, they will help you handle the dangerous
refrigerant with expert guidance and suggest better options for you. Some examples
of specific refrigerants under this group are R2, R22, R123 and R124. Others include R133
and R151.
> HYDROCARBONS (HCS)
HCs are usually available as either R600a (isobutene) or R290 (propane). You will find
these chemicals in domestic refrigeration systems, commercial refrigeration systems
and air conditioning systems. The flammability of these substances requires special
safety installations, but some plants are willing to take the extra precaution. Propane
has zero ODP making it suitable for industrial cooling. As a natural refrigerant, the
chemical does not make any effect on global warming either.
24
market, the result is that dangerous chemical refrigerants such as chlorofluorocarbons
(CFCs), hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) are being
replaced with environmental friendly ones. The industry is committed to reducing
environmental impact even as it promotes safety and efficiency
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DISTRICT COOLING REFERENCE EBOOK
06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
THERMAL ENERGY STORAGE IMPLEMENTATION
DISTRICT COOLING LOAD PROFILE AND TES TANK OPERATION
TES Tank is a thermal accumulator that allows the storage of chilled water or ice
60,000
200,000
Cooling Demand, TR
produced during off-peak time. This energy is later used during on-peak time. A TES
Chiller Output, TR
•
•
•
•
•
Maximum efficiency in simultaneous chilled water production and consumption,
usage of what you need, storage of excess
average loads and not for peaks
Energy production and storage during electricity low-cost periods.
Ice storage option available where site space constraints would not allow for
120,000
30,000
100,000
80,000
20,000
60,000
40,000
10,000
20,000
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
0
Hour of the Day
a chilled water TES tank
lower due to the cooling energy already accumulated in the tank during the previous
25
140,000
40,000
Smaller refrigeration equipment sizes and costs as they are designed for
In District cooling plants with TES Tank, the refrigeration capacity of the chillers will be
hours.
Refrigeration Load, TR
temperature is low and chillers have better performance.
160,000
Stored Energy, TR·h
tank reduces refrigerant plant capacity and operational cost, producing chilled
water when demand is low, which usually coincides with the night, when ambient
180,000
TES Charge, TR·h
50,000
In other words, a smaller refrigeration system can be used to satisfy an specified peak
demand. Therefore, ARANER engineers put a great effort to design the most efficient
Thermal Energy Storage Tanks for District Cooling Plants.
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DISTRICT COOLING REFERENCE EBOOK
06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
THERMAL ENERGY STORAGE IMPLEMENTATION
> OPERATING PHASES
Thermal Energy Storage (TES) has two operational phases: charging and discharging.
During the discharge phase, the warmer, less dense returning water floats on top of the
stored chilled water. The water from storage is supplied and withdrawn in low velocity,
in essentially horizontal flow, so that buoyancy forces dominate inertial effects.
When the stratified storage tank is charging, chilled supply water, enters through the
diffuser at the bottom of the tank, and return water exits to the chiller unit through the
diffuser at the top of the tank.
CFD (Computational Fluid Dynamics) uses numerical algorithms and computational
software to simulate how a fluid flows within a certain boundary domain. The
numerical model must be accurate and truthful to ensure the proper performance of
the prototype. The CFD techniques offer the capacity of studying a fluid system under
conditions over its limits and understanding the features of the result.
Through the use of CFD analysis ARANER provides a real conditions-tested design that
fulfills specifications and assures thermal stratification of water inside the TES Tank.
The use of CFD software ensures authenticity and quick turnaround time, reducing cost
and adding significant value to the final plant and thus, to the customers.
> DESIGN CRITERIA - CFD
In District cooling plants with TES Tank, the refrigeration capacity of the chillers will be
lower due to the cooling energy already accumulated in the tank during the previous
hours. In other words, a smaller refrigeration system can be used to satisfy an specified
peak demand. In Thermal Energy Storage Tanks, a proper diffuser geometry helps the
water volumes to stratify uniformly inside the tank, via water velocities and densities,
and thus provide the District Cooling System with the required cooling capacity.
ARANER sets a great effort to design the most efficient Thermal Energy Storage Tanks
for its cooling plants. To ensure that the cooling needs of the plants are absolutely
fulfilled, it is extremely important to test the performance of the designed TES in real
conditions before erection.
26
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06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
THERMAL ENERGY STORAGE IMPLEMENTATION
> BENEFITS AND ADVANTAGES
1
2
The overall cost of the
installation is lower due to the
reduction of the refrigeration plant
installed capacity. The refrigeration
equipment size is smaller because it
is designed for average loads and
not for peaks
The operational cost is reduced
in comparison with an online
cooling system because peak
consumption can be avoided
during high electrical tariff periods
TYPICAL CONFIGURATION FOR COOLING PLANT WITH TES
•
•
•
3
Stores the thermal energy and transforms it later on electrical energy
Maximum net electricity production during the most profitable moments
The plant dimension of the
refrigeration system could be
reduced. This is given by the fact
that the refrigeration capacity of
energy to be produced by the plant
will be lower due to the energy
already accumulated in the tank.
TYPICAL CONFIGURATION FOR ONLINE COOLING
•
•
•
Deferred cooling production
4
The
environmental
impact
is reduced because of the
reduction of CO2 storing the energy
and improving operating efficiency
with TES tank helps to reduce even
more the CO2 emissions of a district
cooling system
Coinciding cooling production/use
Heating cost varies with energy cost
Net electricity production reduction during the most profitable
PRODUCTION
PUMPS
LOAD
T
T
27
CHILLER 2
CHILLER 1
CHILLER 1
CONSUMPTION
PUMPS
LOAD
LOAD
T
T
T
P
T
CHILLER 2
T
P
PUMPING
GROUP
LOAD
T
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DISTRICT COOLING REFERENCE EBOOK
06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
ENERGY TRANSFER STATION · ETS
Normally, every District Cooling System is supplying chilled water to its consumers
CHILLED WATER DISTRICT SIDE (NETWORK)
through a sub-station, which we will call Energy Transfer Station (ETS). The Energy
Transfer Station can be located in some machinery room inside the building to be
serviced. Every building residential towers, hotels, hospital, shopping boulevard, etc.)
which is serviced by the District Cooling will require an Energy Transfer Station.
This ETS will consist of pumps (for controlling the building side), heat exchangers (the
heart of the ETS; they transfer the cooling energy from the DC Network), control valves
(PICV’s), strainers, instrumentation and PLC. In order to identify the parts of the ETS, we
can identify two side:
28
The chilled water district side shall tie directly into the district’s main chilled water
distribution header. The standard chilled water district side shall be comprised of the
following equipment that directly ties into the automation system:
•
•
•
•
•
•
Energy quantifiers: Energy meter, Flow meter, Supply temperature & Return
temperature
Chilled Water supply strainer differential pressure transmitter
Chilled Water supply/return differential pressure transmitter
Modulating temperature control valve
Pressure Independent Control Valve
Network to Building System Plate Heat Exchangers
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06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
ENERGY TRANSFER STATION · ETS
CHILLED WATER CUSTOMER SIDE (BUILDING)
The chilled water customer side shall tie directly into the customer’s equipment.
Cooling energy shall be transferred from the CHW district loop to the CHW customer
side through the use of the CHW to CHW heat exchangers listed above. The CHW
As a possibility for good design and improvement in the capabilities of control for
efficiency enhancement, the PLC’s central processing unit (CPU) can communicate
with the local operator interface terminal via a local ETS Ethernet network and shall
communicate with the SCADA system via Fiber.
customer side shall be comprised of the following equipment that directly ties into the
Going through the operation philosophy of the ETS, the control can be done in several
automation system:
ways and even it is possible to have configured different modes to act depending on
•
•
•
•
•
•
•
different conditions (weather, season, demand, etc.). The purpose of the control in the
Supply temperature transmitter in Building side
Return temperature transmitter in Building side
In case there is more than one PHE, supply and return temperature off each
CHW heat exchanger
Return strainer differential pressure transmitter
Differential pressure transmitter
Circulation pumps to supply water to the building
Network to Building System Plate Heat Exchangers
ETS pay attention to 2 factors mainly:
•
•
The supply water temperature in the Building (consumer side)
The delta T in the Network (district side)
In order to have a proper performance in the District Cooling System, it is necessary
that the consumers allows the ETS to have the chilled water return temperature as per
design. In addition, the supply water temperature in the Building is usually guaranteed
by the DC Services Provider to the consumers by contract.
29
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06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
SPECIAL & TAILORED SOLUTIONS
> ENGINE DRIVEN CHILLERS
The refrigeration machines require a work input in order to have the capability of
performing the refrigeration cycle. In compression chillers, mechanical energy is
transmitted to the chiller compressor by an engine. Generally the use of an electrical
motor as the compressor mover is the most extended solution, due to its versatility and
cost. However, in some cases the access to electricity is limited and there is no power
available for the chiller. In such occasions the motor may use some of the following
technologies: Diesel piston engine, Natural gas piston engine & Natural gas turbine
engine
The heat rejection in these cases would be greater than in the case of an electrical
chiller. Also different requirements and standards should be met, often more restrictive
in comparison with electrical chillers. The use of this technologies would be bound to
the particular constrain conditions of each particular project.
> ABSORPTION CHILLERS
The working principle of the absorption chillers is similar to the compression chillers,
as they both perform the refrigeration cycle. Nevertheless, the compression chillers
perform the refrigerant pressurization over a gas, while the absorption chillers do it over
a liquid by means of a pump. As the refrigerant exits the evaporator in its gas phase,
it is absorbed by a liquid absorbent. After the pressurization, the absorption process is
reversed and solely the refrigerant enters in the condenser in the form of gas.
In order to reverse the absorption of the refrigerant, the absorbent solution shall be
30
heated, therefore the machine requires a heat input. These machines demand more
energy that the compression chillers. However, pressurizing a liquid instead of a gas
requires less energy and produces less noise. That makes the absorption chiller to have
a very low electrical consumption. The electrical consumption of an absorption chiller
is often despicable when compared with the heat input that requires; consequently it
is often referred to the heat consumption of the absorption chiller as the only energy
consumption that it has.
The absorption technology is mainly used when a suitable heat source is available.
When this heat source is a waste product of another industrial process, the system is
highly energy-efficient, and the operation costs can be greatly reduced. If there is no
heat available, the use of absorption technology is not preferred over compression
due to its higher cost and higher energy consumption.
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06 DISTRICT COOLING DESIGN OPTIMIZATION
06 DESIGN OPTIMIZATION
SPECIAL & TAILORED SOLUTIONS
> SOLAR COOLING
The sun is normally associated with heating applications but, what about cooling?
Solar energy is a renewable energy that is normally available during the peak cooling
demand hours, so it could be used to produce cold energy. Traditionally there are two
ways of exploiting solar energy.
•
•
PHOTOVOLTAIC SOLAR COOLING. It consists on obtaining electricity out of the
sun. This electricity would be used to feed the compression chiller.
THERMAL SOLAR COOLING. It consists on the use of the heating capacity of
the sun for feeding an absorption chiller. As seen before, the absorption chiller
needs a heat supply for the cold production.
> COMPLEMENTARY HEATING : HEAT PUMPS
Most often the consumers of the District Cooling network also have a heat demand
(e.g. heating, sanitary water). Depending on the nature of the project, this demand
could be satisfied locally or with centralized heat production. The latter would use its
own District Heating network, often next to the DC one. The combination of District
Cooling and Heating is regularly referred to as District Energy.
In order to provide central heating, a heat pump solution may be used. A heat pump
is a machine that performs the refrigeration cycle just as a chiller does, transferring
heat from a cold environment into a hot one, opposite to the natural heat flow. The
difference is that in this case the heat produced in the condenser is used for heating
purposes, while the cold energy produced in the evaporator is rejected.
31
Both heat pump and chiller may work together if both the heating and
cooling demand profile are suitable, so that one can use part of the energy
that the other rejects. Also depending on this load profile difference, the
use of a Thermal Energy Storage may enhance the efficiency of the system.
In case of marked seasonal environments, the chiller can be designed as a reversible
machine. That would mean that the machine will work as a chiller during the summer
season, delivering cold to the network, and as a heat pump during the winter season,
delivering heat to the network. This would imply a deep study of the project conditions
and technologies selected.
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DISTRICT COOLING REFERENCE EBOOK
07 DCP’S PERFORMANCE
07 DCP’S PERFORMANCE
COOLING LOADS CALCULATIONS
When sizing a District Cooling plant, cooling load calculations play a critical role. This
It is clear, therefore, that this profile simulation leans towards a clearer picture of a
is a complex process; calculate the cooling demand and the plant performance for
complete cooling plant performance. For the best results in District Cooling design and
thousands of different ambient conditions involves great efforts and resources. With
decision-making process, a detailed cooling load profile simulation is essential. Only
ARANER’s conscientious and experienced, clients can be sure of getting a District
Detailed Cooling Load Profile calculations can guarantee that the resulting District
Cooling system that will serve perfectly for many years. ARANER opts to use the more
Cooling system is optimally energy efficient, comfortable and satisfactory.
strenuous and reliable complex simulation method because its high accuracy helps
predict the energy performance of a District Cooling system with minimal errors.
60,000
We recognize that the cooling equipment may not operate at maximum capacity
180,000
means that the cooling demand and the cooling plant performance is calculated for
8,760 different points.
Our collection of data is straightforward, but thorough at the same time. The most
important thing about our detailed cooling load profile is that it describes the variation
of the load within time. Some issues may affect this profile and they include:
•
•
•
32
Refrigeration load (TR)
hour per day, we consider real weather data hour-by-hour for a complete year. This
50,000
Base loads- computer rooms, telecom closets, etc.
140,000
40,000
120,000
30,000
100,000
80,000
20,000
60,000
40,000
10,000
20,000
0
0:00
Hours of operation of a facility
Climate of plant’s location
160,000
Time
0:00
2:00
4:00
4:48
6:00
8:00
9:36
Demand: (TR)
10:00 12:00 14:00 16:00 18:00 20:00 22:00
14:24
Chiller: (TR)
19:12
0:00
TES Status: (TR·h)
0
4:48
Stored energy (TR·h)
throughout the day, leave alone the month or year. That is why we use powerful
computer software to determine the ideal cooling load profile. Instead of picking one
200,000
DISTRICT COOLING REFERENCE EBOOK
07 DCP’S PERFORMANCE
07 DCP’S PERFORMANCE
STUDY OF CHILLED WATER NETWORK
The recently constructed District Cooling plants, especially in Middle East, have a
total cooling capacity of several dozen thousands of Refrigeration Tons. These high
capacities result in large chilled water flows which require a thorough analysis of the
water pipe network:
•
Pressure Drop. This is the difference of pressure between two points of a fluid
in the piping network due to frictional forces. This calculation is necessary to
•
select the correct size of the pumps for the system
Pipe Stress Calculation. It is necessary to determine if the pipes accomplish
with the stress code, determine the forces that will act in the nozzles of the
equipment and in the pipe supports, etc. This analysis ensures the mechanical
•
life cycle of the pipe system
Water Analysis. The water is studied to prevent corrosion in pipes and equipment.
With this data we make a properly selection of equipment materials, required
•
chemical products and water treatments
Water Hammer Analysis. Water hammer is a pressure surge or wave caused
when a fluid in motion is forced to stop or change direction suddenly. This
pressure wave can cause major problems, from noise and vibration to pipe
collapse. We make simulations in the design phase to avoid these problems
•
during operation
Hydraulic Balance. We do studies of hydraulic balance to avoid shortage of
service in any point of the network and to have the equipment in its optimal
working point, which is translated in energy efficiency, saving costs and client
satisfaction
33
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DISTRICT COOLING REFERENCE EBOOK
08 POTENTIAL DC FAILURES & PROBLEMS
08 POTENTIAL DC FAILURES & PROBLEMS
LOW DELTA-T SYNDROME
Low Delta T Syndrome is a common problem in chilled water cooling plants in general,
Both reasons produce that the opening percentage of the PICV is higher than actually
which occurs more often than expected when the system is not properly designed
needed, increasing the water flow and reducing the Delta T. In order to fight against
and/or operated. For different reasons, the Delta T of the actual chilled water flow
this problem, the chilled water supply and return temperatures must be monitored for
(supply water temperature minus return water temperature) is lower than the Design
both the Cooling Plant side and Developers/Buildings side. If low Delta T is detected,
value. As the water flow needed is inversely proportional to the Delta T, low Delta T
and Alarm must inform the District Cooling Plant operator in order to proceed with the
results in higher required water flow and increase in the required pumping energy,
required counter-measures.
which adversely affects system efficiency. The Low Delta T phenomenon can be found
on both the Cooling Plant side and Developers/Buildings (consumers) side of the water
The consequence of the low Delta T is an increase in the electrical consumption of the
circuit. The most common causes of low Delta T are:
DC plant and reduction in the efficiency. In addition, more wear of the main equipment
•
•
•
occurs, due to longer operation running status of the water pumps and compressors.
Non-proper functioning of the control system of the Energy Transfer Station
Non-proper interface between the control system of the ETS and the control
system of the DC Plant. Normally, due to an improper or lack of communication
between them
Incorrect selection of the Pressure Independent Control valve (PICV)
ARANER DCPs are much less susceptible to suffer the Low Delta T Syndrome due to:
•
•
•
•
34
Real-time monitoring and control of the ETS from the DC Plant Control Room.
Different modes for ETS control depending on several factors like demand or
season conditions
Proper selection of PICV in the ETS’s and deep design of the equipment of the
ETS’s
Proper operation guidelines are provided and followed
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08 POTENTIAL DC FAILURES & PROBLEMS
08 POTENTIAL DC FAILURES & PROBLEMS
FOULING
Heat exchangers are totally necessary in the refrigeration cycle of a District Cooling
However, in this post we will focus on shell & tube and plate heat exchangers. These
Plant. As it has mentioned before, they are the interface equipment between
are two of the most used types of heat exchangers in District Cooling Plants.
refrigerant system and chilled water system, as well as between the refrigerant system
and ambient to dissipate (either by mean of dry condensation or via cooling towers or
In both of them, the efficiency is related directly to fouling. Fouling could be defined
any other fluid that can absorb the heat rejected by the refrigeration cycle. Two out
as the accumulation of deposits in the heat exchanger surfaces. There are many
of the four basic steps in the refrigerant cycle are heat exchangers. Their operation
causes of this: scaling, biological, suspended particles in the fluids… This accumulation
affects greatly to the overall plant efficiency, so their maintenance is very important.
produces a loss of heat transfer, reducing its efficiency and, in the case of a District
Cooling Plant, reducing the overall plant efficiency. Also fouling can increase the
Heat exchangers, like pressure vessels, are classified as static equipment as they don’t
head loss due to the reduction of the water section pass. Proper operation guidelines
have any moving parts. They don’t require too much maintenance, but it is important
are provided and followed
to make visual inspections to detect leaks or corrosion and control its efficiency as it
can decrease with time due to fouling.
An ideal refrigerant system consists on one compressor, one condenser, one expansion
valve and one evaporator. Condenser and evaporator are both of them heat
exchangers. In the evaporator the refrigerant absorbs the heat of the water so it is
chilled. In the condenser the refrigerant rejects that heat and the compressor heat to
other media, usually air or water. There are many types of heat exchangers.
35
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08 POTENTIAL DC FAILURES & PROBLEMS
08 POTENTIAL DC FAILURES & PROBLEMS
FOULING
This process is unavoidable. With time every heat transfer system will see a reduction
An overstated fouling factor increases the heat exchanger initial cost and substated
on his efficiency, but cleaning the surfaces will revert the heat exchanger to its original
fouling factor increases the maintenance and operation costs during its life, so it is
efficiency. The velocity of the fouling process depends mainly on the characteristics of
important to set the correct fouling factor during design.
the fluid used: fouling will not be the same using treated water than using sea-water:
the second one will carry more particles and biological contamination.
Depending on the type of fouling there are different methods to solve it. All of them
have the same purpose: to clean the surfaces of the heat exchangers. Heat exchanger
During design of the heat exchanger a fouling factor is provided to ensure that the
cleaning supposes a costly maintenance time and depending on the installations also
heat exchanger can work during certain time at design conditions, but after some
means the shutdown of the plant. So every investment in the reduction of the number
time fouling will increase and a cleaning operation will be required. The fouling factor
of these maintenance operations along the time will be profitable. There are many
used in design will increase the initial cost of the equipment as this fouling factor is
ways to reduce the fouling problem that can be divided in two types:
translated in an increase of the heat transfer area.
ONLINE METHODS
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OFFLINE METHODS
Consist on stopping the heat exchanger
Online methods don’t require the stop
service and dismount and clean the surfaces
of the plant. These methods are applied
directly. There are many methods used:
during operation and we can find upstream
manual cleaning, jet cleaning, blasting…
filtration, back flushing, flow excursion…
These methods are simple but require
These methods increase the initial cost of the
stopping the plant. They suppose a cost
plant but avoid plant shutdowns and reduce
during the operation of the plant.
the maintenance costs of the plant..
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DISTRICT COOLING REFERENCE EBOOK
08 POTENTIAL DC FAILURES & PROBLEMS
08 POTENTIAL DC FAILURES & PROBLEMS
CORROSION
There are many causes that can lead to corrosion in a water piping system like the
one existing in district cooling networks. Sometimes the corrosion is due to one or more
factors. The following are the main causes that may be related:
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•
Water temperature: The hotter the water, the more pronounced the corrosion.
Water’s chemical make-up: Minerals in water can either mitigate or increase
corrosive levels. For instance, if there are high levels of calcium in water, it may
•
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cause harm the corrosion levels.
Water velocity: Excessive velocity and/or sudden changes in direction (turns,
elbows) can lead to erosion corrosion because of water turbulence.
The pH of the water: In copper piping systems, if the pH is under a certain
level, it harms the protective barrier of the pipe and leads to corrosion. This
fact is especially important in case there is any copper coil or braze plate heat
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exchanger in the piping system.
Oxygen in the water: Oxygen degrades metals, gradually converting the metal
to rust. As this happens, impurities are deposited into water lines or collected
on the piping wall, creating restrictions and blockages.
These water issues are some of the things that can lead to corrosion in the piping
system and DC network. Careful attention needs to be paid to these facts, as the
effects of that corrosion may lead to several health problems as well as wasting the DC
operator’s money in many ways.
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THE EFFECTS OF CORROSION
It is essential to mention that corrosion effects can be very expensive to repair and it
can be a waste of money in many ways:
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•
•
•
Corroded water can hurt the efficiency of heat exchangers in cooling and
heating systems, and cause premature failure.
It can cause premature failure of plumbing systems and fixtures (facility of the
DC building)
It can result in stained fixtures and potential odors.
Proper operation guidelines are provided and followed
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08 POTENTIAL DC FAILURES & PROBLEMS
08 POTENTIAL DC FAILURES & PROBLEMS
CORROSION
PREVENTION METHODS
> PROTECTIVE COATINGS
> METAL TYPE
The application of a paint coating is a cost-effective way of preventing corrosion.
One simple way to prevent corrosion is to use a corrosion resistant metal such as
aluminum or stainless steel. Depending on the application, these metals can be used
to reduce the need for additional corrosion protection. These metals are usually used
in heat exchangers in order to improve the exchange efficiency. However, these
materials are very expensive in order to use it in large water systems like DC Plants
> ENVIRONMENTAL MEASURES
Corrosion is caused by a chemical reaction between the metal and gases in the
surrounding environment. By taking measures to control the environment, these
unwanted reactions can be minimized. This can be as simple as reducing exposure to
rain or seawater, or more complex measures, such as controlling the amounts of sulfur,
chlorine, or oxygen in the surrounding environment.
> CATHODIC PROTECTION
The most common example of cathodic protection is the coating of iron alloy steel
with zinc, a process known as galvanizing. Zinc is a more active metal than steel, and
when it starts to corrode it oxides which inhibits the corrosion of the steel. This method
is known as cathodic protection because it works by making the steel the cathode
of an electrochemical cell. Cathodic protection is used for steel pipelines carrying
water or fuel, water heater tanks, ship hulls, and offshore oil platforms. However it is not
commonly used in District Cooling water system network, due to its high cost.
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Paint coatings act as a barrier to prevent the transfer of electrochemical charge from
the corrosive solution to the metal underneath.
> ANODIC PROTECTION
Anodic protection involves coating the iron alloy steel with a less active metal, such
as tin. Tin will not corrode, so the steel will be protected as long as the tin coating is
in place. This method is known as anodic protection because it makes the steel the
anode of an electrochemical cell. Anodic protection is often applied to carbon steel
storage tanks used to store sulfuric acid and 50% caustic soda.
> CORROSION INHIBITORS
Corrosion inhibitors are chemicals that react with the surface of the metal or the
surrounding gases to suppress the electrochemical reactions leading to corrosion.
They work by being applied to the surface of a metal where they form a protective
film. Inhibitors can be applied as a solution or as a protective coating using dispersion
techniques. Corrosion inhibitors are commonly applied via a process known as passivation
In passivation, a light coat of a protective material, such as metal oxide, creates a
protective layer over the metal which acts as a barrier against corrosion.
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DISTRICT COOLING REFERENCE EBOOK
08 POTENTIAL DC FAILURES & PROBLEMS
08 POTENTIAL DC FAILURES & PROBLEMS
LEGIONELLA
Legionella bacteria can infect humans and cause legionellosis and Legionnaires’
For this reason, all cooling towers should be treated with a dual biocide program that
disease. The bacteria can grow on the wet surfaces of cooling towers, evaporative
uses both an oxidizing and non-oxidizing biocide whenever possible.
condensers (cooling plant) and scrubbers. Poorly positioned air intakes for air
conditioning units can also capture the bacterial plume and draw it into buildings. In
According to OSHA, cooling towers and evaporative condensers should be
District Cooling Plants, a disinfection system should always be installed in cooling water
inspected and thoroughly cleaned at least twice a year. Algae and accumulated
system as cooling towers are often subject to microbacterial growth.
scale should be removed. All metal surfaces should be treated with a biocide.
Corroded parts, such as drift eliminators, should be replaced.
Because of the
Cooling towers and evaporative condensers are used to dissipate unwanted heat to
potential dangers associated with them, all cooling tower cleanings should be
the atmosphere through water evaporation. Water is sprayed into the cooling tower
done under the supervision of a technician trained in legionella remediation.
through spray nozzles and tiny airborne droplets are formed. While falling through the
At Clarity Water Technologies, we take legionella prevention and legionella
tower, some of the water evaporates but some droplets, known as drift, are carried out
remediation very seriously.
of the tower by the air stream produced by the fans. The presence of drift has been
detected as far as 6 km away from the cooling tower.
Legionella bacteria grow often in the water and are easily dispersed together with
the drift. This water mist can be breathed into the respiratory system, causing risk of
Legionella disease and Pontiac fever. The likelihood of legionella infection can be
significantly reduced by good engineering and water treatment practices in the
installation, operation and maintenance of air and water handling systems.
39
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DISTRICT COOLING REFERENCE EBOOK
09 INTEGRATED CONTROL SYSTEMS
09 INTEGRATED CONTROL SYSTEMS
ARANER PERFORMANCE OPTIMIZATION
Different components of the District Cooling system need to operate and perform all
ARANER designs, implements and develops a Safety and Redundant Systems with the
together as a single and fully integrated system. The best practice is to integrate into
highest specifications from the Industry according to the project requirement, including
one single Control System all the information regarding the main equipment (chillers,
since the most basic devices up to the highest performance equipment. Our control
pumps, heat rejection devices) and the Energy Transfer Stations (ETSs). Even Preventive
systems are based on latest PAC (Programmable Automation Controller) technology
maintenance management system and ETSs Consumption billing system can be
for highest performance and durability. Our Control Systems are equipped with a suite
integrated in the same Control System
of tools that allow the creation of full customizable features, such as HMI alarms, color
schemes, historical and real-time trending, built in reporting.
ARANER designs and integrates our own control systems. With a very intuitive and
easy-to-use Human Machine Interfaces (HMI) and SCADA’s, full control and plant
Control solutions used by ARANER allows our customers to improve their process and
integration are achieved. ARANER built a manage control system architecture utilizing
become agiler by simplifying integration, streamlining commissioning and providing
redundant EtherNet/IP as their communication backbone, as well as other common
increased operational flexibility and performance.
industrial communication protocols such Modbus, Profinet or BacNet. As a result, they
can seamlessly integrate with other automation systems and software platforms.
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DISTRICT COOLING REFERENCE EBOOK
09 INTEGRATED CONTROL SYSTEMS
09 INTEGRATED CONTROL SYSTEMS
ARANER PERFORMANCE OPTIMIZATION
Nowadays, industrial plants are full of sensors providing massive data.
•
ARTIFICIAL INTELLIGENCE: According to performance optimization, predictive
Data without control is unavailing information.
behaviors are developed by ARANER AI. Learning capabilities is one the
Collect, save, sort and analyze data represents a complex process that has
challenges of the new era: knowing how the cooling load is going to be in the
to be developed fast and efficient in order to provide valuable information:
following hours or predict the electricity cost in the following days is a money
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saver. ARANER AI learns with trends and tendencies during District Cooling
BILLING SYSTEM: All the Energy Transfer Stations (ETS) of each client are under
ARANER control system, enabling data collection and analyzation. This feature
allows to detect any abnormal condition in a remote area in real time, at the
•
CLOUDING, IOT: Cybersecurity is one of the basics in ARANER Control Systems.
Cybersecurity is not at odds with accessibility. ARANER DCP provide the owner
ARANER Billing System provides the owner the capability to extend weekly
safety access to his plant from any place in the world if it is required. Supervise
or monthly invoices to each client with a fix cooling water price or variable
KPIs or check the plant performance during a meeting is now a real possibility.
one, depending on the electricity cost or the time scheduled consumption.
Receive emails in case of important alarms, or start a pump from a tablet in
front of it to monitor the pressures are others of the smart features.
REPORTING SERVICES: The DC plant maintenance team has a deep knowledge
and performing maintenance activities. ARANER Reporting Services allows the
maintenance team to receive a daily report of the maintenance activities to be
done as well as it advises if any equipment is having an unusual performance.
PERFORMANCE OPTIMIZATION: The target of a District Cooling plant is to reduce
the client’s cooling costs with a larger centralized plant that provides a better
performance than small individual ones. ARANER Performance Optimization
System checks several parameters such as the weather forecast, the weekday,
the electricity price, etc. and selects how many chillers and when have to be
running, in order to reduce energy costs.
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•
same time that the system is recording the client consumption in every instant.
of the complete plant and they are capable of managing technical mishaps
•
Plant life in order to predict future behaviors and optimize the performances.
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DISTRICT COOLING REFERENCE EBOOK
10 OPERATION AND MAINTENANCE
10 OPERATION AND MAINTENANCE
ACTIVITIES TO BE PERFORMED
Depending on the topology, technology of the plant, and the refrigerant used, the
The other important element in the chilled water system are the evaporators (these
operation and maintenance activities may vary in a slight way. Below, it is briefly
are part of the refrigerant system too) and heat exchangers in general. When the heat
explained the maintenance activities to be performed in the 3 main systems of a
exchangers are plates type, if gasketed, these would be opened after 4-5 years of
District Cooling Plant:
operation in order to clean the plates and re-gasket the plate heat exchanger. There
> CHILLED WATER SYSTEM
may be other plate heat exchangers without gaskets (brazed or welded). These will
The most important equipment in this system are the circulating pumps. In case
exchangers used in a District Cooling Plant are the shell and tube heat exchanger.
the District Cooling Plant has a TES Tank, there will be also production pumps. The
Some of them may be inspected due to an access point. Those which cannot, a CIP
water pumps used by Araner are always cutting-edge technology from renowned
shall be done after 4-5 years of operation.
manufacturers. Thus, the maintenance activities need to be programmed less often.
The main maintenance activities required for the water pumps are the following:
•
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•
•
•
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Inspection for leaks (monthly)
Monitor vibrations and temperature (monthly)
Re-greasing bearings (yearly)
Check alignment (yearly)
Replace gasket kit (every 2 years)
Replace mechanical seal (every 3 years or when damaged)
Impeller and shaft (only if it is required)
Another important element are the butterfly valves or ball valves. These valves need to
be serviced from time to time. Thus, gaskets, O-rings and seals shall be replaced after 3
years of operation. In addition, gear-boxes for these valves shall be greased annually.
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be cleaned by mean of a CIP (clean-in-place). Last but not least, other type of heat
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DISTRICT COOLING REFERENCE EBOOK
10 OPERATION AND MAINTENANCE
10 OPERATION AND MAINTENANCE
ACTIVITIES TO BE PERFORMED
> CONDENSATION SYSTEM
> REFRIGERANT SYSTEM
The condensation of the refrigerant can generally be done by means of air (dry
The heart of the refrigerant system in a District Cooling Plant are the compressors.
condensation) or water in a District Cooling Plant. If it is done by means of air, air-
Compressor usually used in ARANER District Cooling Plant are twin screw compressors
condensers have to be installed in our District Cooling Plant, and these are the main
or centrifugal compressors. For the compressors, an overhaul shall be done every 5
maintenance activities to be performed:
years. However, there is an annual service kit to be replaced every year in order to
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•
•
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Inspection for noises (daily)
Monitor vibrations and temperature (weekly)
Re-greasing motor bearings (yearly)
Re-greasing fan bearings (yearly)
Check alignment (monthly)
Check belt tension (monthly)
When the refrigerant is condensed by water, a heat exchanger is necessary. Thus, it
applies what has been explained before about the heat exchangers (plate type and
shell and tube type). Furthermore, additional information will be provided in another
section related to fouling in heat exchangers. When condensed by water, these water
may come from a cooling tower. There are some activities to be done in order to
prevent legionella and to have the system clean.
assure the proper performance of the seal, gaskets, O-rings and mechanical seals. The
main activities for the compressor and compressor unit/chiller are:
•
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•
•
•
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Inspection for leaks (daily)
Monitor vibrations and temperature (weekly)
Inspect lube oil strainers (monthly)
Replace oil super-filters (yearly)
Inspection of refrigerant strainers (yearly)
Check alignment of the compressor (yearly)
Replace gasket kit (every 1 years)
Replace mechanical seal (every 3 years or when damaged)
Impeller or screws and shaft (only if it is required)
Major overhaul (every 4 years)
Another important equipment of the refrigeration system are the refrigerant valves,
which shall be serviced after some years of operation. In addition, the heat exchangers
are part of the refrigerant cycle. 2 out of 4 steps implies the use of heat exchangers:
evaporation and condensation. For evaporation applies what has been explained in
the sub-epigraph of chilled water system.
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ABOUT
ARANER
ARANER Group of Companies constitutes an innovative and multi-disciplinary
ARANER’s main strength lies in its manpower, with the team committed to
EPC
engineering,
aim and accept challenges that bring value in any of the fields where we
procurement and construction activities. ARANER gets the know-how and
are present. ARANER faces each project with the utmost professionalism. This
experience of several pioneering and innovative companies from different
allows our clients to be confident with our engineering and to be satisfied with
areas of the world.
our solutions.
business
group,
involving
design,
manufacturing,
The company provides a wide range of products, solutions and professional
services to clients all across the world, where its leading-edge capabilities
cover several disciplines, mostly linked to Energy systems and Cooling and
Heating plants. Our goal is to create value for our stakeholders by meeting the
stated and implied needs of our customers, employees and the communities
where we exist and do business. We provide the best people working on the
most advanced engineering systems.
The company’s international structure, incorporated around ARANER Global
DMCC, as a Holding Company, and with subsidiaries in the most relevant
areas, allows the group to provide its solutions to all the markets where they
are required.
ARANER guarantees success in each project implementation. The company
develops pioneering and innovative solutions tailored to customer needs that
go beyond the usual requirements of the industry, focused on high efficiency
and reliability.
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Contact ARANER
>
DISTRICT COOLING REFERENCE EBOOK
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For continual development, ARANER
reserves the right to change
specifications or designs without notice.
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