Trane TRG-TRC016-EN User Manual
Air Conditioning
Clinic
Chilled-Water Systems
One of the Systems Series
TRG-TRC016-EN
NO POSTAGE
NECESSARY
IF MAILED
IN THE
UNITED STATES
BUSINESS REPLY MAIL
FIRST-CLASS MAIL
PERMIT NO. 11
LA CROSSE, WI
POSTAGE WILL BE PAID BY ADDRESSEE
THE TRANE COMPANY
Attn: Applications Engineering
3600 Pammel Creek Road
La Crosse WI 54601-9985
NO POSTAGE
NECESSARY
IF MAILED
IN THE
UNITED STATES
BUSINESS REPLY MAIL
FIRST-CLASS MAIL
PERMIT NO. 11
LA CROSSE, WI
POSTAGE WILL BE PAID BY ADDRESSEE
THE TRANE COMPANY
Attn: Applications Engineering
3600 Pammel Creek Road
La Crosse WI 54601-9985
Comment Card
We want to ensure that our educational materials meet your ever-changing resource development needs.
Please take a moment to comment on the effectiveness of this Air Conditioning Clinic.
Chilled-Water Systems
Level of detail (circle one) Too basic Just right Too difficult
One of the Systems Series
Rate this clinic from 1–Needs Improvement to 10–Excellent
…
TRG-TRC016-EN Content 12345678910
Booklet usefulness 12345678910
Slides/illustrations 12345678910
Presenter’s ability 12345678910
Training environment 12345678910
Other comments? _________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
About me … Type of business _________________________________________________________
Job function _________________________________________________________
Optional: name _________________________________________________________
phone _________________________________________________________
address _________________________________________________________
Give the completed card to the
presenter or drop it in the mail.
Thank you!
The Trane Company • Worldwide Applied Systems Group
3600 Pammel Creek Road • La Crosse, WI 54601-7599
www.trane.com
An American-Standard Company
Response Card
We offer a variety of HVAC-related educational materials and technical references, as well as software tools
that simplify system design/analysis and equipment selection. To receive information about any of these
items, just complete this postage-paid card and drop it in the mail.
Education materials
❏
Air Conditioning Clinic series About me…
❏
Engineered Systems Clinic series Name ___________________________________________
❏
Trane Air Conditioning Manual Title ___________________________________________
❏
Trane Systems Manual Business type ___________________________________________
Software tools
❏
Equipment Selection Phone/fax _____________________ ____________________
❏
System design & analysis E-mail address ___________________________________________
Periodicals
❏
Engineers Newsletter Company ___________________________________________
Other?
❏
_____________________________ Address ___________________________________________
___________________________________________
___________________________________________
Thank you for your interest!
The Trane Company • Worldwide Applied Systems Group
3600 Pammel Creek Road • La Crosse, WI 54601-7599
www.trane.com
An American-Standard Company
Chilled-Water Systems
Chilled-Water Systems
One of the Systems Series
A publication of
The Trane Company
Preface
Chilled-Water Systems
A Trane Air Conditioning Clinic
Figure 1
The Trane Company believes that it is incumbent on manufacturers to serve the
industry by regularly disseminating information gathered through laboratory
research, testing programs, and field experience.
The Trane Air Conditioning Clinic series is one means of knowledge sharing.
It is intended to acquaint a nontechnical audience with various fundamental
aspects of heating, ventilating, and air conditioning. We have taken special
care to make the clinic as uncommercial and straightforward as possible.
Illustrations of Trane products only appear in cases where they help convey
the message contained in the accompanying text.
This particular clinic introduces the reader to chilled-water systems.
© 2001 American Standard Inc. All rights reserved
i
TRG-TRC016-EN
Contents
period one Types of Water Chillers ...................................... 1
Vapor-Compression Water Chillers .......................... 3
Air-Cooled or Water-Cooled Condensing .................. 6
Packaged or Split Components .............................. 11
Absorption Water Chillers ...................................... 15
Equipment Rating Standards ................................. 18
period two Chilled-Water System Design ........................ 26
Load-Terminal Control ........................................... 28
Parallel Configuration ............................................. 35
Series Configuration .............................................. 38
Primary-Secondary System Operation .................... 53
period three System Variations ............................................. 59
Alternate Fuel Choice ............................................ 59
Low-Flow Systems ............................................... 61
Variable-Primary-Flow Systems .............................. 64
Preferential Loading .............................................. 66
Heat Recovery ...................................................... 68
Asymmetric Design ............................................... 72
“Free” Cooling ...................................................... 75
Application Outside the Operating Range
of the Chiller
......................................................... 78
period four Chiller-Plant Control .......................................... 79
Chiller Sequencing ................................................ 82
Failure Recovery and Contingency Planning ........... 90
System Tuning ...................................................... 92
System Optimization ............................................. 96
Operator Interface ............................................... 100
period five Review ................................................................. 103
Quiz ....................................................................... 108
Answers .............................................................. 110
Glossary .............................................................. 112
TRG-TRC016-EN ii
iii TRG-TRC016-EN
notes
period one
Types of Water Chillers
Chilled-Water Systems
period one
Types of Water Chillers
Figure 2
Water chillers are used in a variety of air conditioning and process cooling
applications. They cool water that is subsequently transported by pumps
and pipes. The water passes through the tubes of coils to cool air in an air
conditioning system, or it can provide cooling for a manufacturing or industrial
process. Systems that employ water chillers are commonly called chilled-
water systems.
When designing a chilled-water system, one of the first issues that must be
addressed is to determine which type of water chiller to use. This period
discusses the primary differences in chiller types.
absorption
water chiller
centrifugal
water chiller
The refrigeration cycle is a key differentiating characteristic between chiller
types. The vapor-compression and absorption refrigeration cycles are the two
most common cycles used in commercial air conditioning.
Figure 3
TRG-TRC016-EN 1
period one
Types of Water Chillers
notes
Water chillers using the vapor-compression refrigeration cycle vary by the type
of compressor used. Reciprocating, scroll, helical-rotary, and centrifugal
compressors are common types of compressors used in vapor-compression
water chillers.
Absorption water chillers make use of the absorption refrigeration cycle.
Driving Sources
compressor-driven
Vapor-compression water chillers use a compressor to move refrigerant around
the system. The most common energy source to drive the compressor is an
electric motor.
heat-driven
Figure 4
Absorption water chillers use heat to drive the refrigeration cycle. They do not
have a mechanical compressor involved in the refrigeration cycle. Steam, hot
water, or the burning of oil or natural gas are the most common energy sources
for these types of chillers.
2 TRG-TRC016-EN
notes
period one
Types of Water Chillers
Vapor-Compression Cycle
reject heat
reject heat
expansion
expansion
device
devi ce
D
condenser
condenser
evaporator
A
evaporator
absorb heat
absorb heat
C
compressor
compressor
B
energy in
energy in
Figure 5
Vapor-Compression Water Chillers
In the vapor-compression refrigeration cycle, refrigerant enters the evaporator
in the form of a cool, low-pressure mixture of liquid and vapor (A). Heat is
transferred from the relatively-warm air or water to the refrigerant, causing
the liquid refrigerant to boil. The resulting vapor (B) is then drawn from the
evaporator by the compressor, which increases the pressure and temperature
of the refrigerant vapor.
The hot, high-pressure refrigerant vapor (C) leaving the compressor enters
the condenser, where heat is transferred to ambient air or water at a lower
temperature. Inside the condenser, the refrigerant vapor condenses into a
liquid. This liquid refrigerant (D) then flows to the expansion device, which
creates a pressure drop that reduces the pressure of the refrigerant to that of
the evaporator. At this low pressure, a small portion of the refrigerant boils
(or flashes), cooling the remaining liquid refrigerant to the desired evaporator
temperature. The cool mixture of liquid and vapor refrigerant (A) travels to the
evaporator to repeat the cycle.
The vapor-compression refrigeration cycle is reviewed in detail in the
Refrigeration Cycle Air Conditioning Clinic.
TRG-TRC016-EN 3
notes
period one
Types of Water Chillers
Compressor Types
scroll
reciprocating
helical-rotary
The type of compressor used generally has the greatest impact on the efficiency
and reliability of a vapor-compression water chiller. The improvement of
compressor designs and the development of new compressor technologies
have led to more-efficient and -reliable water chillers.
The reciprocating compressor was the workhorse of the small chiller market
for many years. It was typically available in capacities up to 100 tons [350 kW].
Multiple compressors were often installed in a single chiller to provide chiller
capacities of up to 200 tons [700 kW].
Scroll compressors have emerged as a popular alternative to reciprocating
compressors, and are generally available in hermetic configurations in
capacities up to 15 tons [53 kW] for use in water chillers. As with reciprocating
compressors, multiple scroll compressors are often used in a single chiller to
meet larger capacities. In general, scroll compressors are 10 to 15 percent more
efficient than reciprocating compressors and have proven to be very reliable,
primarily because they have approximately 60 percent fewer moving parts than
reciprocating compressors. Reciprocating and scroll compressors are typically
used in smaller water chillers, those less than 200 tons [700 kW].
Helical-rotary (or screw) compressors have been used for many years in
air compression and low-temperature-refrigeration applications. They are now
widely used in medium-sized water chillers, 50 to 500 tons [175 to 1,750 kW].
Like the scroll compressor, helical-rotary compressors have a reliability
advantage due to fewer moving parts, as well as better efficiency than
reciprocating compressors.
centrifugal
Figure 6
Centrifugal compressors have long been used in larger water chillers.
High efficiency, superior reliability, reduced sound levels, and relatively low
cost have contributed to the popularity of the centrifugal chiller. Centrifugal
compressors are generally available in prefabricated chillers from 100 to
3,000 tons [350 to 10,500 kW], and up to 8,500 tons [30,000 kW] as built-up
machines.
4 TRG-TRC016-EN
period one
Types of Water Chillers
notes
These various types of compressors are discussed in detail in the Refrigeration
Compressors Air Conditioning Clinic.
Variable-Speed Drives
variable--
variable
speed
speed
drive
drive
Figure 7
The capacity of a centrifugal chiller can be modulated using inlet guide vanes
(IGV) or a combination of IGV and a variable-speed drive (adjustable-frequency
drive, AFD). Variable-speed drives are widely used with fans and pumps, and as
a result of the advancement of microprocessor-based controls for chillers, they
are now being applied to centrifugal water chillers.
Using an AFD with a centrifugal chiller will degrade the chiller’s full-load
efficiency. This can cause an increase in electricity demand or real-time pricing
charges. At the time of peak cooling, such charges can be ten (or more) times
the non-peak charges. Alternatively, an AFD can offer energy savings by
reducing motor speed at low-load conditions, when cooler condenser water
is available. An AFD also controls the inrush current at start-up.
Certain system characteristics favor the application of an adjustable-frequency
drive, including:
n A substantial number of part-load operating hours
n The availability of cooler condenser water
n Chilled-water reset control
Chiller savings using condenser- and chilled-water-temperature reset, however,
should be balanced against the increase in pumping and cooling-tower energy.
This is discussed in Period Four. Performing a comprehensive energy analysis
is the best method of determining whether an adjustable-frequency drive is
desirable. It is important to use actual utility costs, not a “combined” cost, for
demand and consumption charges.
Depending on the application, it may make sense to use the additional money
that would be needed to purchase an AFD to purchase a more efficient chiller
instead. This is especially true if demand charges are significant.
TRG-TRC016-EN 5
notes
period one
Types of Water Chillers
Condenser Types
air-cooled
water-cooled
Figure 8
Air-Cooled or Water-Cooled Condensing
The heat exchangers in the water chiller (the condenser and evaporator) have
the second greatest impact on chiller efficiency and cost. One of the most
distinctive differences in chiller heat exchangers continues to be the type of
condenser selected—air-cooled versus water-cooled.
Air-Cooled or Water-Cooled
wat er--
cooled
wat er
cooled
air--
cooled
air
cooled
0 tons
0 tons
[0 kW]
[0 kW]
500 tons
500 tons
[1,759 k W]
[1,759 kW]
1,000 tons
1,000 tons
[3,517 kW]
[3,517 kW]
1,500 tons
1,500 t ons
[5,276 k W]
[5,276 k W]
chiller capacity
chiller capacity
2,000 t ons
2,000 t ons
[7,034 k W]
[7,034 k W]
When comparing air-cooled and water-cooled chillers, available capacity is the
first distinguishing characteristic. Air-cooled chillers are typically available in
packaged chillers ranging from 7.5 to 500 tons [25 to 1,580 kW]. Packaged
water-cooled chillers are typically available from 10 to 3,000 tons [35 to
10,500 kW].
2,500 t ons
2,500 t ons
[8,793 k W]
[8,793 k W]
3,000 tons
3,000 t ons
[10,551 kW]
[10,551 kW]
Figure 9
6 TRG-TRC016-EN
notes
period one
Types of Water Chillers
air-cooled or water-cooled
Maintenance
▲ Water treatment
▲ Condenser tube brushing
▲ Tower maintenance
▲ Freeze protection
▲ Makeup water
cooling tower
cooling tower
A major advantage of using an air-cooled chiller is the elimination of the cooling
tower. This eliminates the concerns and maintenance requirements associated
with water treatment, chiller condenser-tube cleaning, tower mechanical
maintenance, freeze protection, and the availability and quality of makeup
water. This reduced maintenance requirement is particularly attractive to
building owners because it can substantially reduce operating costs.
Figure 10
Systems that use an open cooling tower must have a water treatment program.
Lack of tower-water treatment results in contaminants such as bacteria and
algae. Fouled or corroded tubes can reduce chiller efficiency and lead to
premature equipment failure.
TRG-TRC016-EN 7
notes
period one
Types of Water Chillers
air-cooled or water-cooled
Low Ambient Operation
air--
cooled
air
cool ed
chiller
chiller
Figure 11
Air-cooled chillers are often selected for use in systems that require year-round
cooling requirements that cannot be met with an airside economizer. Air-cooled
condensers have the ability to operate in below-freezing weather, and can do so
without the problems associated with operating the cooling tower in these
conditions. Cooling towers may require special control sequences, basin
heaters, or even an indoor sump for safe operation in freezing weather.
For process applications, such as computer centers that require cooling yearround, this ability alone often dictates the use of air-cooled chillers.
8 TRG-TRC016-EN
notes
period one
Types of Water Chillers
air-cooled or water-cooled
Efficiency
dry bulb
dry bulb
wet bul b
wet bul b
outdoor temperature
outdoor temperature
12
12
midnight
midnight
Water-cooled chillers are typically more energy efficient than air-cooled chillers.
The refrigerant condensing temperature in an air-cooled chiller is dependent
on the ambient dry-bulb temperature. The condensing temperature in a
water-cooled chiller is dependent on the condenser-water temperature, which
is dependent on the ambient wet-bulb temperature. Since the wet-bulb
temperature is often significantly lower than the dry-bulb temperature, the
refrigerant condensing temperature (and pressure) in a water-cooled chiller
can be lower than in an air-cooled chiller. For example, at an outdoor design
condition of 95°F [35°C] dry-bulb temperature, 78°F [25.6°C] wet-bulb
temperature, a cooling tower delivers 85°F [29.4°C] water to the water-cooled
condenser. This results in a refrigerant condensing temperature of
approximately 100°F [37.8°C]. At these same outdoor conditions, the refrigerant
condensing temperature in an air-cooled condenser is approximately 125°F
[51.7°C]. A lower condensing temperature, and therefore a lower condensing
pressure, means that the compressor needs to do less work and consumes
less energy.
This efficiency advantage may lessen at part-load conditions because the
dry-bulb temperature tends to drop faster than the wet-bulb temperature
(see Figure 12). As a result, the air-cooled chiller may benefit from greater
condenser relief.
Additionally, the efficiency advantage of a water-cooled chiller is much less
when the additional cooling tower and condenser pump energy costs are
considered. Performing a comprehensive energy analysis is the best method
of estimating the operating-cost difference between air-cooled and water-cooled
systems.
12
12
noon
noon
12
12
midnight
midnight
Figure 12
TRG-TRC016-EN 9
notes
period one
Types of Water Chillers
air-cooled or water-cooled
Comparison
air-cooled
▲ Lower maintenance
▲ Packaged system
▲ Better low-ambient
operation
Another advantage of an air-cooled chiller is its delivery as a “packaged
system.” Reduced design time, simplified installation, higher reliability, and
single-source responsibility are all factors that make the factory packaging of
the condenser, compressor, and evaporator a major benefit. A water-cooled
chiller has the additional requirements of condenser-water piping, pump,
cooling tower, and associated controls.
Water-cooled chillers typically last longer than air-cooled chillers. This
difference is due to the fact that the air-cooled chiller is installed outdoors,
whereas the water-cooled chiller is installed indoors. Also, using water as the
condensing fluid allows the water-cooled chiller to operate at lower pressures
than the air-cooled chiller. In general, air-cooled chillers last 15 to 20 years, while
water-cooled chillers last 20 to 30 years.
To summarize the comparison of air-cooled and water-cooled chillers, air-cooled
chiller advantages include lower maintenance costs, a prepackaged system for
easier design and installation, and better low-ambient operation. Water-cooled
chiller advantages include greater energy efficiency (at least at design
conditions) and longer equipment life.
water-cooled
▲ Greater energy efficiency
▲ Longer equipment life
Figure 13
10 TRG-TRC016-EN
notes
period one
Types of Water Chillers
Packaged Air-Cooled Chiller
air-cooled chill er
Figure 14
Packaged or Split Components
Water-cooled chillers are rarely installed with separable components. Air-cooled
chillers, however, offer the flexibility of separating the components in different
physical locations. This flexibility allows the system design engineer to place
the components where they best serve the available space, acoustic, and
maintenance requirements of the customer.
A packaged air-cooled chiller has all of the refrigeration components
(compressor, condenser, expansion device, and evaporator) located outdoors.
A major advantage of this configuration is factory assembly and testing of all
chiller components, including the wiring, refrigerant piping, and controls.
This eliminates field labor and often results in faster installation and improved
system reliability. Additionally, all noise-generating components (compressors
and condenser fans) are located outdoors, easing indoor noise concerns.
Finally, indoor equipment-room space requirements are minimized.
TRG-TRC016-EN 11
notes
period one
Types of Water Chillers
Remote Evaporator Barrel
condensing unit
condensing unit
remote
evaporator
refrigerant
piping
Figure 15
An alternative to the packaged air-cooled chiller is to use a packaged
condensing unit (condenser and compressor) located outdoors, with a remote
evaporator barrel located in the indoor equipment room. The two components
are connected with field-installed refrigerant piping. This configuration locates
the part of the system that is susceptible to freezing (evaporator) indoors and
the noise-generating components (compressors and condenser fans) outdoors.
This usually eliminates any requirement to protect the chilled-water loop from
freezing during cold weather.
This configuration is particularly popular in schools and other institutional
applications, primarily due to reduced seasonal maintenance for freeze
protection. A drawback of splitting the components is the requirement for
field-installed refrigerant piping. The possibility of system contamination
and leaks increases when field-installed piping and brazing are required.
Additionally, longer design time is generally required for the proper selection,
sizing, and installation of this split system.
12 TRG-TRC016-EN
notes
period one
Types of Water Chillers
Remote Air-Cooled Condenser
air--
cool ed
air
cooled
condenser
condenser
refrigerant
ref riger ant
piping
piping
condenserless
condenserless
chiller
chiller
Figure 16
Another popular configuration is to use an outdoor air-cooled condenser
connected to a packaged compressor and evaporator unit (also called a
condenserless chiller) that is located in the indoor equipment room. Again,
the components are connected with field-installed refrigerant piping.
The primary advantage of this configuration is that the compressors are located
indoors, which makes maintenance easier during inclement weather and
virtually eliminates the concern of refrigerant migrating to the compressors
during cold weather.
TRG-TRC016-EN 13
notes
period one
Types of Water Chillers
Indoor Air-Cooled Condenser
indoor
indoor
air--
cooled
air
cooled
condenser
condenser
refrigerant
refr igerant
pipi ng
piping
condenserless
condenserless
chiller
chiller
The final configuration includes a packaged compressor-and-evaporator unit
that is located in an indoor equipment room and connected to an indoor,
air-cooled condenser. The air used for condensing is ducted from outdoors,
through the condenser coil, and rejected either outdoors or inside the building
as a means for heat recovery. Indoor condensers typically use a centrifugal fan
to overcome the duct static-pressure losses, rather than the propeller fans used
in conventional outdoor air-cooled condensers. Again, the components are
connected with field-installed refrigerant piping.
Figure 17
This configuration is typically used where an outdoor condenser is
architecturally undesirable, where the system is located on a middle floor of a
multistory building, or where vandalism to exterior equipment is a problem.
A disadvantage of this configuration is that it typically increases condenser fan
energy within compared to a conventional outdoor air-cooled condenser.
Similarly, a packaged cooling tower in a water-cooled system can also be
located indoors. This configuration also requires outdoor air to be ducted to
and from the cooling tower, and again, typically requires the use of a centrifugal
fan. Centrifugal fans use about twice as much energy as a propeller fan, but can
overcome the static-pressure losses due to the ductwork. Alternatively, the
tower sump can be located indoors, making freeze protection easier.
14 TRG-TRC016-EN
notes
period one
Types of Water Chillers
Absorption Refrigeration Cycle
reject heat
reject heat
D
expansion
expansion
device
device
A
condenser
condenser
evaporator
evaporator
absorb heat
absorb heat
C
B
Absorption Water Chillers
So far, we have discussed water chillers that use the vapor-compression
refrigeration cycle. Absorption water chillers are a proven alternative to vaporcompression chillers. The absorption refrigeration cycle uses heat energy as the
primary driving force. The heat may be supplied either in the form of steam or
hot water (indirect-fired), or by burning oil or natural gas (direct-fired).
There are two fundamental differences between the absorption refrigeration
cycle and the vapor-compression refrigeration cycle. The first is that the
compressor is replaced by an absorber, pump, and generator. The second is
that, in addition to the refrigerant, the absorption refrigeration cycle uses a
secondary fluid called the absorbent. The condenser, expansion device, and
evaporator sections, however, are similar.
Warm, high-pressure liquid refrigerant (D) passes through the expansion
device and enters the evaporator in the form of a cool, low-pressure mixture of
liquid and vapor (A). Heat is transferred from the relatively-warm system water
to the refrigerant, causing the liquid refrigerant to boil. Using an analogy of the
vapor-compression cycle, the absorber acts like the suction side of the
compressor—it draws in the refrigerant vapor (B) to mix with the absorbent.
The pump acts like the compression process itself—it pushes the mixture of
refrigerant and absorbent up to the high-pressure side of the system. The
generator acts like the discharge of the compressor—it delivers the refrigerant
vapor (C) to the rest of the system.
generator
generator
absorber
absorber
heat energy i n
heat energy in
pump
pump
reject heat
reject heat
Figure 18
The refrigerant vapor (C) leaving the generator enters the condenser, where
heat is transferred to cooling-tower water at a lower temperature, causing the
refrigerant vapor to condense into a liquid. This high-pressure liquid refrigerant
(D) then flows to the expansion device, which creates a pressure drop that
reduces the pressure of the refrigerant to that of the evaporator, repeating
the cycle.
The absorption refrigeration cycle is discussed in more detail in the Absorption
Water Chillers Air Conditioning Clinic.
TRG-TRC016-EN 15
notes
period one
Types of Water Chillers
Absorption Chillers Offer Choice
▲ Avoid high electric
demand charges
▲ Minimal electricity
needed during
emergency situations
▲ Waste heat recovery
▲ Cogeneration
Figure 19
Absorption water chillers generally have a higher first cost than vaporcompression chillers. The cost difference is due to the additional heat-transfer
tubes required in the absorber and generator(s), the solution heat exchangers,
and the cost of the absorbent. This initial cost premium is often justified when
electric demand charges or real-time electricity prices are a significant portion
of the electric utility bill. Because electric demand charges are often highest at
the same time as peak cooling requirements, absorption chillers are often
selected as peaking or demand-limiting chillers.
Because the absorption chiller uses only a small amount of electricity, backupgenerator capacity requirements may be significantly lower with absorption
chillers than with electrically-driven chillers. This makes absorption chillers
attractive in applications requiring emergency cooling, assuming the alternate
energy source is available.
Some facilities, such as hospitals or factories, may have excess steam or hot
water as a result of normal operations. Other processes, such as a gas turbine,
generate waste steam or some other waste gas that can be burned. In such
applications, this otherwise wasted energy can be used to fuel an
absorption chiller.
Finally, cogeneration systems often use absorption chillers as a part of their
total energy approach to supplying electricity in addition to comfort cooling
and heating.
16 TRG-TRC016-EN
notes
period one
Types of Water Chillers
Absorption Chiller Types
single-effect
There are three basic types of absorption chillers. They are typically available
in capacities ranging from 100 to 1,600 tons [350 to 5,600 kW].
Indirect-fired, single-effect absorption chillers operate on low-pressure
steam (approximately 15 psig [205 kPa]) or medium-temperature liquids
(approximately 270°F [132°C]), and have a coefficient of performance (COP) of
0.6 to 0.8. In many applications, waste heat from process loads, cogeneration
plants, or excess boiler capacity provides the steam to drive a single-effect
chiller. In these applications, absorption chillers become conservation devices
and are typically base-loaded. This means that they run as the lead chiller to
make use of the “free” energy that might otherwise be wasted.
Indirect-fired, double-effect absorption chillers require medium-pressure
steam (approximately 115 psig [894 kPa]) or high-temperature liquids
(approximately 370°F [188°C]) to operate and, therefore, typically require
dedicated boilers. Typical COPs for these chillers are 0.9 to 1.2.
The direct-fired absorption chiller includes an integral burner, rather than
relying on an external heat source. Common fuels used to fire the burner are
natural gas, fuel oil, or liquid petroleum. Additionally, combination burners are
available that can switch from one fuel to another. Typical COPs for direct-fired,
double-effect chillers are 0.9 to 1.1 (based on the higher heating value of the
fuel). Higher energy efficiency and elimination of the boiler are largely
responsible for the increasing interest in direct-fired absorption chillers. These
types of absorption chillers have the added capability to produce hot water for
heating. Thus, these “chiller–heaters” can be configured to produce both chilled
water and hot water simultaneously. In certain applications this flexibility
eliminates, or significantly down-sizes, the boilers.
double-effect
direct-fired
Figure 20
TRG-TRC016-EN 17
notes
period one
Types of Water Chillers
Equipment Rating Standards
▲ Air-Conditioning &
Refrigeration Institute (ARI)
◆ Standard 550/590–1998:
centrifugal and helical-rotary
water chillers
◆ Standard 560–1992:
absorption water chillers
Figure 21
Equipment Rating Standards
The Air-Conditioning & Refrigeration Institute (ARI) establishes rating standards
for packaged HVAC equipment. ARI also certifies and labels equipment through
programs that involve random testing of a manufacturer’s equipment to verify
published performance. These equipment rating standards have been
developed to aid engineers in comparing similar equipment from different
manufacturers. Chiller full-load efficiency is described in terms of kW/ton and
coefficient of performance (COP). Additionally, two efficiency values developed
by ARI that are receiving increased attention are the Integrated Part-Load
Value (IPLV) and Non-Standard Part-Load Value (NPLV).
ARI’s part-load efficiency rating system establishes a single number to estimate
both the full- and part-load performance of a stand-alone chiller. As part of ARI
Standard 550/590–1998, Water-Chilling Packages Using the Vapor-Compression
Refrigeration Cycle, and ARI Standard 560–1992, Absorption Water Chilling-
Heating Packages, chiller manufacturers may now certify their chiller part-load
performance using the IPLV and NPLV methods. This gives the engineering
community an easy and certified method to evaluate individual chillers.
Understanding the scope and application limits of IPLV and NPLV is, however,
crucial to their validity as system performance indicators.
18 TRG-TRC016-EN
notes
period one
Types of Water Chillers
Part-Load Efficiency Rating
▲ Integrated Part-Load Value (IPLV)
◆ Weighted-average load curves
◆ Based on an “average” single-chiller installation
◆ Standard operating conditions
▲ Non-Standard Part-Load Value (NPLV)
◆ Weighted-average load curves
◆ Based on an “average” single-chiller installation
◆ Non-standard operating conditions
Figure 22
The IPLV predicts chiller efficiency at the ARI standard rating conditions, using
weighted-average load curves that represent a broad range of geographic
locations, building types, and operating-hour scenarios, both with and without
an airside economizer. The NPLV uses the same methods to predict chiller
efficiency at non-standard rating conditions. Although these weighted-average
load curves place greater emphasis on the part-load operation of an average,
single-chiller installation, they will not—by definition—represent any
particular installation.
Additionally, ARI notes that more than 80 percent of all chillers are installed in
multiple-chiller systems. Chillers in these systems exhibit different unloading
characteristics than the IPLV weighted formula indicates. Appendix D of
Standard 550/590–1998 explains this further:
The IPLV equations and procedure are intended to provide a singlenumber, part-load performance number for water-chilling products.
The equation was derived to provide a representation of the average
part-load efficiency for a single chiller only. However, it is best to use a
comprehensive analysis that reflects the actual weather data, building
load characteristics, operational hours, economizer capabilities, and
energy drawn by auxiliaries, such as pumps and cooling towers, when
calculating the chiller and system efficiency.
Here is the important part:
This becomes increasingly important with multiple-chiller systems
because individual chillers operating within multiple-chiller systems
are more heavily loaded than single chillers within single-chiller
systems.
TRG-TRC016-EN 19
notes
period one
Types of Water Chillers
Standard Rating Conditions
chiller type
vapor-compression
• reciprocating
•scroll
• helical-rotary
• centrifugal
absorption
• single-effect
• double-effect,
indirect-fired
• double-effect,
direct-fired
water leaving evaporator = 44°F [6.7°C]
water entering condenser = 85°F [29.4°C]
evaporator
flow rate
2.4 gpm/ton
[0.043 L/s/kW]
2.4 gpm/ton
[0.043 L/s/kW]
2.4 gpm/ton
[0.043 L/s/kW]
condenser
flow rate
3.0 gpm/ton
[0.054 L/s/kW]
3.6 gpm/ton
[0.065 L/s/kW]
4.0 gpm/ton
[0.072 L/s/kW]
4.5 gpm/ton
[0.081 L/s/kW]
The standard rating conditions used for ARI certification represent a particular
set of design temperatures and flow rates for which water-cooled and air-cooled
systems may be designed. They are not suggestions for good design practice
for a given system—they simply define a common rating point to aid
comparisons.
rating
standard
ARI
550/590–1998
ARI
560–1992
Figure 23
In fact, concerns toward improved humidity control and energy efficiency have
changed some of the design trends for specific applications. More commonly,
chilled-water systems are being designed with lower chilled-water
temperatures and lower flow rates. The water flow rate required through the
system is decreased by allowing a larger temperature difference through the
chiller.
20 TRG-TRC016-EN
notes
period one
Types of Water Chillers
Flow Rates and Temperatures
Q
Btu/hr
[ Q
W
The temperature difference (∆T) through the chiller and the water flow rate are
related. For a given load, as the flow rate is reduced, the ∆T increases, and vice
versa.
Q 500 flow rate ∆T××=
Q 4,184 flow rate ∆T××=[]
where,
n Q = load, Btu/hr [W]
n flow rate = water flow rate through the chiller, gpm [L/s]
n ∆T = temperature difference (leaving minus entering) through the chiller,
ºF [°C]
Realize that 500 [4,184] is not a constant! It is the product of density, specific
heat, and a conversion factor for time. The properties of water at conditions
typically found in an HVAC system result in this value. Other fluids, such as
mixtures of water and antifreeze, will cause this factor to change.
Density of water = 8.33 lb/gal [1.0 kg/L]
= 500 × flow rate ×∆∆∆∆T
= 4,184 × flow rate ×∆T ]
equation for water only
Figure 24
Specific heat of water = 1.0 Btu/lb°F [4,184 J/kg°K]
8.33 lb/gal 1.0 Btu/lb° F 60 min/hr 500=××
1.0 kg/L 4,184 J/kg° K4,184=×[]
TRG-TRC016-EN 21
notes
period one
Types of Water Chillers
Flow Rates and Temperatures
95°F
95°F
[35°C]
[35°C]
85°F
85°F
[29.4°C]
[29.4°C]
54°F
54°F
[12.2°C]
[12.2°C]
ARI conditions
evaporator
flow rate
condenser
flow rate
2.4 gpm/ton
[0.043 L/s/kW]
3.0 gpm/ton
[0.054 L/s/kW]
44°F
44°F
[6.7°C]
[6.7°C]
100°F
100°F
[37.8°C]
[37.8°C]
85°F
85°F
[29.4°C]
[29.4°C]
57°F
57°F
[13.9°C]
[13.9°C]
low-flow conditions
evaporator
flow rate
condenser
flow rate
1.5 gpm/ton
[0.027 L/s/kW]
2.0 gpm/ton
[0.036 L/s/kW]
In the example system shown in Figure 25, the chilled water is cooled from 57°F
[13.9°C] to 41°F [5°C] for a 16°F [8.9°C] ∆T. This reduces the water flow rate
required from 2.4 gpm/ton [0.043 L/s/kW] to 1.5 gpm/ton [0.027 L/s/kW].
41°F
41°F
[5°C]
[5°C]
Figure 25
Reducing water flow rates either: 1) lowers system installed costs by reducing
pipe, pump, valve, and cooling tower sizes, or 2) lowers system operating costs
by using smaller pumps and smaller cooling tower fans. In some cases, both
installed and operating costs can be saved. Low-flow systems will be discussed
in more detail in Period Three.
The two ARI rating standards mentioned previously, as well as ASHRAE/IESNA
Standard 90.1–1999 (the energy standard), allow reduced chilled-water
temperatures and flow rates. System design engineers should examine the use
of reduced flow rates to offer value to building owners.
22 TRG-TRC016-EN
notes
period one
Types of Water Chillers
ASHRAE/IESNA Standard 90.1–1999
▲ Energy Standard
◆ Building design and
materials
◆ Minimum equipment
efficiencies
◆ HVAC system design
Figure 26
ASHRAE/IESNA Standard 90.1–1999, Energy Standard for Buildings, Except
Low-Rise Residential Buildings, went into effect in October 1999. ASHRAE
is the American Society of Heating, Refrigerating and Air-Conditioning
Engineers and IESNA is the Illuminating Engineering Society of North America.
This standard addresses all aspects of buildings except low-rise residential
buildings. It contains specific requirements for both water chillers and
chilled-water systems.
standard 90.1-1999 efficiency requirements
Electric Vapor-Compression Chillers
chiller type
chiller type
air cool ed
air cool ed
water-cool ed
water-cool ed
recipr ocating
recipr ocating
heli cal-rotar y, scroll
heli cal-rotar y, scroll
centri fugal
centri fugal
* as of October 29, 200 1
* as of October 29, 200 1
Standard 90.1 contains minimum full- and part-load efficiency requirements
for packaged water chillers. The table in Figure 27 is an excerpt from Table
6.2.1C of Addendum J to the standard. It includes the minimum efficiency
requirements for electric vapor-compression chillers operating at standard ARI
conditions. The standard also contains tables of minimum efficiency
requirements for these chillers operating at nonstandard conditions. The test
procedure for these chillers is ARI Standard 550/590–1999. Notice that these
requirements go into effect on October 29, 2001.
TRG-TRC016-EN 23
capacity
capacity
all capacit ies
all capacit ies
all capacit ies
all capacit ies
< 150 tons [ 528 kW]
< 150 tons [ 528 kW]
150 to 300 tons [528 t o 1,056 kW]
150 to 300 tons [528 t o 1,056 kW]
> 300 tons [1,056 kW]
> 300 tons [1,056 kW]
< 150 tons [ 528 kW]
< 150 tons [ 528 kW]
150 to 300 tons [528 t o 1,056 kW]
150 to 300 tons [528 t o 1,056 kW]
> 300 tons [1,056 kW]
> 300 tons [1,056 kW]
minimum efficiency*
minimum efficiency*
2.8 COP 3.05 IPLV
2.8 COP 3.05 IPLV
4.2 COP 5.05 IPLV
4.2 COP 5.05 IPLV
4.45 COP 5.2 IPLV
4.45 COP 5.2 IPLV
4.9 COP 5.6 IPLV
4.9 COP 5.6 IPLV
5.5 COP 6.15 IPLV
5.5 COP 6.15 IPLV
5.0 COP 5.25 IPLV
5.0 COP 5.25 IPLV
5.55 COP 5.9 IPLV
5.55 COP 5.9 IPLV
6.1 COP 6.4 IPLV
6.1 COP 6.4 IPLV
Figure 27
Loading...