Trane SYS-APM001-EN User Manual
Applications
Engineering Manual
Chiller System Design and Control
May 2009
SYS-APM001-EN
Chiller System Design and Control
Susanna Hanson, applications engineer
Mick Schwedler, applications manager
Beth Bakkum, information designer
Trane, in proposing these system design and application concepts, assumes no
responsibility for the performance or desirability of any resulting system design. Design of
the HVAC system is the prerogative and responsibility of the engineering professional.
“Trane” and the Trane logo are registered trademarks, and TRACE, System Analyzer and
TAP are trademarks of Trane, a business of Ingersoll-Rand.
Preface
This manual examines chilled-water-system components, configurations,
options, and control strategies. The goal is to provide system designers with
options they can use to satisfy the building owners’ desires, but this manual
is not intended to be a complete chiller-system design manual.
System designers may get the most use from this manual by familiarizing
themselves with chilled-water-system basics and understanding the benefits
of various options. Thereafter, when a specific job will benefit from these
advantages, consult appropriate sections of the manual in detail.
The Engineers Newsletters that are referenced in this manual are available at:
www.trane.com/commercial/library/newsletters.asp
© 2009 Trane All rights reserved Chiller System Design and Control SYS-APM001-EN
Contents
Preface .................................................................................................. i
Primary System Components ................................................... 1
Chiller ............................................................................................... 1
Loads ................................................................................................ 7
Chilled-Water Distribution System .................................................. 10
Condenser-Water System ................................................................ 13
Unit-Level Controls .......................................................................... 15
Application Considerations ...................................................... 18
Small Chilled-Water Systems (1-2 chillers) ...................................... 18
Mid-Sized Chilled-Water Systems (3-5 Chillers) ............................. 21
Large Chilled-Water Systems (6+ Chillers, District Cooling) .......... 22
Chiller Plant System Performance ................................................. 24
System Design Options ............................................................ 27
Condenser-Water Temperatures ..................................................... 29
Chilled- and Condenser-Water Flow Rates ..................................... 29
Cost Implications ............................................................................ 38
Misconceptions about Low-Flow Rates ......................................... 39
System Configurations .............................................................. 42
Parallel Chillers ............................................................................... 42
Series Chillers ................................................................................ 44
Primary–Secondary (Decoupled) Systems ..................................... 45
Variable-Primary-Flow Systems ...................................................... 55
Chilled-Water System Variations ........................................... 70
Heat Recovery ................................................................................ 70
Condenser “Free Cooling” or Water Economizer .......................... 70
Preferential Loading ........................................................................ 73
Series–Counterflow Application ..................................................... 77
Unequal Chiller Sizing ..................................................................... 78
System Issues and Challenges ............................................... 79
Low T Syndrome .......................................................................... 79
Amount of Fluid in the Loop ........................................................... 79
Contingency ................................................................................... 81
Alternative Energy Sources ............................................................ 82
Plant Expansion .............................................................................. 83
Retrofit Opportunities ..................................................................... 84
Applications Outside the Chiller’s Range ....................................... 84
SYS-APM001-EN Chiller System Design and Control iii
System Controls ........................................................................... 87
Chilled-Water System Control ........................................................ 87
Condenser-Water System Control .................................................. 89
Failure Recovery ............................................................................. 95
Conclusion ...................................................................................... 96
Glossary ........................................................................................... 97
References ..................................................................................... 100
Index ................................................................................................ 103
iv Chiller System Design and Control SYS-APM001-EN
Primary System Components
For more details on the basic operation
and components of a chilled-water
system, consult another Trane
publication, Chilled-Water Systems, part
of the Air Conditioning Clinic Systems
Series (TRG-TRC016-EN).
Specific application considerations for
absorption chillers are addressed in
another Trane publication, Absorption
Chiller System Design (SYS-AM-13).
Chilled-water systems consist of these functional parts:
• Chillers that cool the water or fluid
• Loads, often satisfied by coils, that transfer heat from air to water
• Chilled-water distribution pumps and pipes that send chilled water to the
loads
• Condenser-water pumps, pipes, and cooling towers or condenser fans that
reject heat from the chiller to ambient air
• Controls that coordinate the operation of the mechanical components
together as a system
In most cases, the chiller’s purpose is to make water colder. Some chillers cool a
mixture of water and other chemicals, most commonly added to prevent
freezing in low-temperature applications. Other additives may be used to
modify the properties of the fluid, thereby making it more suitable for its
intended application. For the purposes of this manual, the term water can be
understood to be any such acceptable fluid, with recognition of the diverse
applications in which chillers are used.
The chiller rejects the heat extracted from the chilled water, plus the heat of
compression (in the vapor-compression cycle), or the heat of absorption (in the
case of an absorption chiller) to either the ambient air (air-cooled) or to another
circuit of water (water-cooled). If the compressor-motor is refrigerant cooled,
the chiller also rejects heat generated by motor inefficiency. Air-cooled
condensers use fans to facilitate cooling by the ambient air. Water-cooled
condensers typically use an evaporative cooling tower.
After the water has been chilled, it is distributed via pumps, pipes, and valves
(the distribution system) to the loads, where a heat exchanger—for example, a
cooling coil in an air-handler—transfers heat from the air to the chilled water,
which is returned to the chiller.
Each component of the chilled-water system is explained in more detail in the
following sections.
Chiller
There are a variety of water chiller types. Most commonly, they are absorption,
centrifugal, helical rotary, and scroll. Some reciprocating chillers are also
available. Chillers can be either air- or water-cooled. Major vapor-compression
chiller components include an evaporator, compressor(s), condenser, and
expansion device(s) (Figure 1). This manual discusses the chiller’s evaporator
and condenser and their relationship to the chilled-water system.
SYS-APM001-EN Chiller System Design and Control
1
Primary System Components
Compressor
Condenser
Evaporator
Tube Bundle
Liquid Level
Sensor
Chilled
Water
Return
Liquid
Refrigerant
Refrigerant
Vapor
Chilled
Water
Supply
Figure 1. Typical vapor-compression chiller
Water-cooled chillers are typically installed indoors; air-cooled chillers are
typically installed outdoors—either on the roof or next to the building. In cold
climates, air-cooled chillers may have a remote evaporator inside the building
for freeze protection.
Chiller evaporator
The evaporator section of a water chiller is a shell-and-tube, refrigerant-towater heat exchanger. Depending on the chiller’s design, either the
refrigerant or the water is contained within the tubes.
• In a flooded shell-and-tube evaporator (Figure 2), cool, liquid refrigerant
at low pressure enters the distribution system inside the shell and moves
uniformly over the tubes, absorbing heat from warmer water that flows
through the tubes.
Figure 2. Flooded evaporator cut-away
2 Chiller System Design and Control SYS-APM001-EN
Primary System Components
Chilled Water
Supply
Baffles
Tube Bundle
Chilled
Water
Return
Refrigerant
Vapor
Liquid
Refrigerant
• In a direct-expansion (DX) shell-and-tube evaporator (Figure 3), warmer
water fills the shell while the cool, lower-pressure liquid refrigerant flows
through the tubes.
Figure 3. Direct-expansion evaporator cut-away
In either design, there is an approach temperature, which is the temperature
difference between the refrigerant and exit water stream temperatures. The
approach temperature is a measure of the heat transfer efficiency of the
evaporator.
Effect of chilled-water temperature
For a given chiller, as the leaving chilled-water temperature drops, the
refrigerant temperature and pressure must also drop. Conversely, as the
leaving chilled-water temperature rises, so do the refrigerant temperature
and pressure. When the leaving chilled-water temperature changes, the work
a compressor must do also changes. The effect of leaving chilled-water
temperature change on power consumption can be 1.0 to 2.2 percent per
degree Fahrenheit [1.8 to 4.0 percent per degree Celsius]. Always consider
the energy consumption of the entire system—not only the chiller. It is
important to remember that although reducing leaving chilled-water
temperature penalizes the chiller, it may reduce the overall system energy
because less water is pumped through the system. System interactions are
covered in more detail in “System Design Options” on page 27.
Effect of chilled-water flow rate and variation
The evaporator is sensitive to the water flow rate. Excessive flow may result
in high water velocity, erosion, vibration, or noise. Insufficient flow reduces
heat-transfer efficiency and causes poor chiller performance, which might
cause the chiller controls to invoke safeties. Some designers have concerns
over low flow rates causing fouling. Generally, as Webb and Li
concerns are unwarranted since the chilled-water loop is a closed system,
1
noted, these
thus reducing the chances of materials entering the system and causing
fouling. Chilled-water flow through the evaporator must be kept within
specific minimum and maximum limits. Contact the manufacturer for these
limits.
SYS-APM001-EN Chiller System Design and Control 3
Primary System Components
Some chiller controls can accommodate very little flow variation during
machine operation.
flow variation. Some chillers can tolerate flow-rate variations—as much as 50
percent per minute or greater—while others can only tolerate up to 2 percent
per minute. It is important that chiller capabilities are matched to system
requirements. Contact the chiller manufacturer to determine the allowable
rate of flow variation before varying the flow through the evaporator in a
chiller. Flow variation is discussed in detail in the section “Variable-PrimaryFlow Systems” on page 55.
2
Other, more sophisticated, chiller controls allow some
Water-cooled condenser
To cool a building or process, the transferred heat must ultimately be rejected
outdoors or to another system (heat recovery). The total amount of heat
rejected includes the sum of the evaporator load, the compressor work, and
the motor inefficiency. In a hermetic chiller, where the motor and compressor
are in the same housing, these loads are all rejected through the condenser.
In an open chiller, where the motor is separate from the compressor and
connected by a shaft, the motor heat is rejected directly to the surrounding
air. The evaporator load and the compressor work are rejected through the
condenser, and the motor heat must be taken care of by the equipment
room’s air-conditioning system.
Effect of condenser-water temperature
For a given chiller, as the leaving condenser-water temperature rises,
refrigerant temperature and pressure also rise. Conversely, as the leaving
condenser-water temperature drops, so do refrigerant temperature and
pressure. As the refrigerant pressure and temperature changes, the work a
compressor must do also changes. The effect of leaving-condenser-water
temperature change on power consumption can be 1.0 to 2.2 percent per
degree Fahrenheit [1.8 to 4.0 percent per degree Celsius]. Always consider
the energy consumption of the entire system—not just the chiller. It is
important to remember that although raising the leaving condenser-water
temperature penalizes the chiller energy, it may reduce the energy used by
the condenser pumps and cooling tower through the use of reduced flow
rates and higher thermal driving-forces on the tower. System interactions are
covered in more detail in “System Design Options” beginning on page 27.
Effect of condenser-water flow rate
The condenser is sensitive to the water flow rate. Excessive flow may result
in high water velocity, erosion, vibration, or noise, while insufficient flow
reduces heat transfer efficiency and causes poor chiller performance.
Therefore, condenser-water flow through the chiller should be kept within a
specific range of limits, except during transient startup conditions. Contact
the manufacturer for these limits. Some chillers may allow extended
operation below the selected flow rates.
If water velocity through the condenser tubes is too low for significant
periods of time and the water is extremely hard, long-term fouling of the
tubes may also occur. Webb and Li
condenser tubes at low velocity (3.51 ft/s [1.07 m/s]) and high water hardness.
1
tested a number of internally-enhanced
4 Chiller System Design and Control SYS-APM001-EN
Primary System Components
Packaged or Split System?
A number of different options are
available for packaging and splitting the
components of an air-cooled chiller . There
is an excellent discussion in Chilled-Water
Systems, part of the Air Conditioning
Clinic Systems Series (TRG-TRC016-EN).
While they found that some of the internally-enhanced tubes fouled in the
long term, they concluded:
Because of the high hardness and low water velocity used in these
tests, we do not believe that the fouling experienced is typical of that
expected in commercial installations. With use of good maintenance
practices and water quality control, all of the tubes tested are
probably suitable for long-term-fouling applications.
It is important to remember that a chiller selected for low flow does not
necessarily have low velocity through its tubes, as discussed in the chapter
“System Design Options” on page 27. If tube fouling is a major concern,
consider the use of smooth, rather than internally-enhanced, tubes in the
condenser for ease of cleaning.
Air-cooled condenser
Air-cooled chillers do not use condenser-water, since they reject their heat by
passing ambient air across refrigerant-to-air heat exchangers. In packaged
air-cooled chillers, the manufacturers improve performance by staging fans
in response to chiller load and ambient, dry-bulb temperature. Air-cooled
chillers can also be split apart. One technique is to use an indoor remote
evaporator with a packaged air-cooled condensing unit outdoors. Another
technique is to locate the compressor(s) and the evaporator indoors (also
known as a condenserless chiller) with an air-cooled condenser outdoors. It is
also possible to have an indoor air-cooled condenser.
SYS-APM001-EN Chiller System Design and Control 5
Air-cooled versus water-cooled condensers
One of the most distinctive differences in chiller heat exchangers continues to
be the type of condenser selected—air-cooled versus water-cooled. When
comparing air-cooled and water-cooled chillers, available capacity is the first
distinguishing characteristic. Air-cooled condensers 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 nearly 4,000 tons [35 to
14,000 kW].
Maintenance
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. However, see “Energy efficiency” below.
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.
Primary System Components
Low-ambient operation
Air-cooled chillers are often selected for use in systems with year-round
cooling requirements that cannot be met with an airside economizer. Aircooled 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 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.
Energy efficiency
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 design 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 drybulb temperature tends to drop faster than the wet-bulb temperature (see
Figure 4). 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 aircooled and water-cooled systems.
6 Chiller System Design and Control SYS-APM001-EN
Primary System Components
12
midnight
12 12
midnight
dry bulb
wet bulb
Dry Bulb
Wet Bul b
Outdoor Temperature
12 12 12
Midnight MidnightNoon
Figure 4. Air-cooled or water-cooled efficiency
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, aircooled chiller advantages include lower maintenance costs, a pre-packaged
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.
Loads
In comfort-cooling applications, cooling loads are often satisfied by air
handlers equipped with coils to transfer heat from the conditioned space air
to circulating chilled-water. Air is cooled and dehumidified as it passes across
the finned surface of the cooling coils. Since the psychrometric process of
conditioning air takes place at the coils, selection of the optimum coil size
and type from the wide variety available is important for proper system
performance.
Some specialized process loads do not involve cooling air. Instead, they may
involve heat transfer directly within a piece of process equipment, such as the
cooling jacket of an injection-molding machine.
SYS-APM001-EN Chiller System Design and Control 7
Primary System Components
Airflow
Bypass
Pipe
Three-Way
Modulating
Valve
Heat transferred from the loads can be controlled in a number of ways:
• Three-way valve
•Two-way valve
• Variable-speed pump
• Face-and-bypass dampers
Three-way valve load control
A three-way control valve (Figure 5) regulates the amount of water passing
through a coil in response to loads. The valve bypasses unused water around
the coil and requires a constant flow of water in the system, regardless of
load. A drawback of this bypass is that the temperature of the water leaving
the three-way valve is reduced at part-load conditions. This can be a major
contributor to so-called “low T syndrome” discussed on page 79. Three-way
valves are used in many existing systems, especially in those with constantvolume pumping.
Figure 5. Three-way valve
Two-way valve load control
A two-way, water modulating valve (Figure 6) at the coil performs the same
water throttling function as the three-way valve. The coil sees no difference
between these two methods. The chilled-water system, however, sees a great
difference. In the case of the two-way valve, all flow in the coil circuit is
throttled. No water is bypassed. Consequently, a system using two-way
valves is a variable-flow chilled-water system. The temperature of the water
leaving the coil is not diluted by bypass water so at part-load conditions, the
system return-water temperature is higher than with three-way valve control.
8 Chiller System Design and Control SYS-APM001-EN
Primary System Components
Two-Way
Modulating
Valve
Airflow
Airflow
Variable
Speed Pump
Figure 6. Two-way valve
Variable-speed pump load control
By using a pump for each coil (Figure 7), the flow may be controlled by
varying the pump speed. In such systems, there may be no control valves at
the coil. This can reduce both the valve and the valve installation costs, but
increases coil pump and maintenance costs.
Figure 7. Variable-speed pump load control
Face-and-bypass dampers
Figure 8 shows a control variation using an uncontrolled or “wild” coil. In
this system, control of the conditioned air supply is executed by face-andbypass dampers that permit a portion of the air to bypass the coil surface.
Advantages of this strategy are the elimination of control valves and
improved part-load dehumidification. A disadvantage is that all the water is
SYS-APM001-EN Chiller System Design and Control 9
Primary System Components
Bypass
Damper
Face
Damper
Airflow
Additional reference information on the
components of a chilled-water
distribution system is available in the
2008 ASHRAE HVAC Systems and
Equipment Handbook, chapter 12,
“Hydronic Heating and Cooling System
Design.”
3
pumped all the time; however, in systems with very small water pressure
drops, this system arrangement may work economically.
Figure 8. Uncontrolled water flow with bypass damper
10 Chiller System Design and Control SYS-APM001-EN
Chilled-Water Distribution System
Chilled water is circulated through fixed piping—most commonly steel,
copper, or plastic—that connects the chiller with various load terminals.
Piping is sized to meet pressure loss, water velocity, and construction cost
parameters.
Chilled-water pump
The chilled-water pump creates pressure to circulate chilled water within the
loop. Generally, the pump must overcome the frictional pressure losses
caused by the piping, coils, and chiller and the pressure differential across
open control valves in the system. The pump, while working at the system
static pressure, does not need to overcome this static pressure. For example,
in a forty-story building, the pump need not overcome the static pressure due
to those forty stories.
The chilled-water pump is typically located upstream of the chiller; however,
it may be anywhere in the system, provided that the pump:
• meets the minimum pump net positive suction-head requirements. That
is, the system pressure at the pump inlet must be both positive and high
enough to allow the pump to operate properly;
• maintains the minimum dynamic pressure head at critical system
components (usually the chiller). If the dynamic pressure head is not high
enough at these components, proper flow will not be established through
them;
Primary System Components
Figure 9. Pump per chiller
Load
Pump
Pump
Figure 10. Manifolded pumps
Manifolded
Pumps
Load
• accommodates the total pressure (static head plus dynamic head) on
system components such as the chiller’s evaporator, valves, etc.
Note that the pump heat is added to the water and must be absorbed by the
chiller. Generally, this represents a very small temperature increase.
Multiple pumps are often used for redundancy. Depending on the terminal
control devices and system configurations, the chilled-water pumps may be
either constant- or variable-flow.
As previously stated, pumps may be either on the inlet or the outlet of the
chiller, as long as the inlet of the pump experiences an adequate, positive
suction pressure. In applications where there is a significant liquid column
head (for example, a high-rise building), the pump is often located at the
chiller’s outlet so that the evaporator bundle is subject only to the static head
(rather than the static head plus the dynamic head added by the pump). The
need for high-pressure water boxes on the chiller can be eliminated.
Conversely, an advantage of locating the pump at the chiller’s inlet is that if
the pump motor rejects its heat to the water, the heat can be removed directly
by the chiller. The chiller does not need to compensate for the pump heat by
making colder water.
Pump per chiller
In either a primary–secondary or variable-primary-flow system, using one
pump per chiller simplifies system hydraulics (Figure 9). The pump can be
selected to produce the flow and pressure drop necessary for the specific
chiller. Bringing on additional pumps changes system hydraulics, but only
minimally. One drawback of such a system is a lack of redundancy, since the
pump and chiller are dedicated to one another. This may be overcome by
using a spare pump, pipes, and valves so that the spare pump could work
with any chiller during emergency conditions.
Manifolded pumps
In an effort to resolve the redundancy consideration, some designers prefer
to manifold pumps and provide n+1 pumps, where n is the number of chillers
(Figure 10). Such an arrangement allows any pump to be used with any
chiller. However, system hydraulics become more complicated. Unless all
piping runs and evaporator pressure drops are equal, the amount of water
flowing to each chiller will differ. As discussed in “Moderate ’low T
syndrome’" on page 68, manifolded pumps present a control opportunity
when low T is experienced.
Either pump configuration can be successful; one pump per chiller simplifies
the hydraulics, while manifolded pumps allow redundancy.
Distribution piping
By itself, the distribution system is easy to understand. Figure 11 shows a
simplified distribution system consisting of multiple cooling coils, each
controlled by a thermostat that regulates the flow in its respective coil. The
SYS-APM001-EN Chiller System Design and Control 11
Primary System Components
Expansion
Tank
Pump
Chiller
Distribution
Piping
Loads
Figure 12. Constant flow system
Load
Chillers
CV
Pump
Three-Way
Control
Valve
valves may be either three-way or two-way. As previously discussed, threeway valves require constant water flow, while two-way valves allow the water
flow in the system to vary. As flow varies, the pump may simply ride its curve
or use a method of flow control such as a variable-speed drive. Refer to the
chapter “System Configurations” on page 42 for a detailed discussion of
distribution-system options.
Figure 11. Simplified distribution system
12 Chiller System Design and Control SYS-APM001-EN
The distribution system may contain other components, such as an
expansion tank, control valves, balancing valves, check valves, and an air
separator, to name a few. The density, and therefore the volume, of the water
in a “closed” chilled-water distribution system varies as it undergoes
changes in temperature. The expansion tank allows for this expansion and
contraction of water volume.
Pumping arrangements
Variations on three basic pumping arrangements are common. They are
referred to as constant flow, primary-secondary (decoupled) flow, and
variable-primary flow (VPF). The implications and nuances of each of these is
discussed in greater detail in “System Configurations” on page 42.
Constant flow system
When a chiller is on, a constant speed pump dedicated to it is on, and there
need not be any other pumps operated in the system (Figure 12). This is a
simple system and makes the most sense when there will only be one chiller
operated at a time in the system. Challenges with this system arise at part
load when chillers are in the parallel arrangement (refer to “Parallel Chillers”
on page 42). To solve some of these problems, the chillers can be placed in
Primary System Components
~
Figure 13. Primary-secondary system
CV
Pump
CV
Pump
Bypass (Decoupler)
VV
Pump
Load
Chillers
Two-Way
Control
Valve
~
~
Figure 14. Variable-primary system
VV
Pump
VV
Pump
Chillers
Minimum Flow Bypass Valve
Load
Two-Way
Control
Valve
the series, or another pumping arrangement can be considered. Reducing the
flow rate affects this system type’s energy use all the time, so careful
attention to flow rates and temperature is critical (refer to “System Design
Options” on page 27).
Primary-secondary system
In this configuration (Figure 13), the distribution piping is decoupled from the
chiller piping and is known as the primary-secondary or decoupled system.
There is constant primary flow through the operating chiller(s) and variable
secondary flow through the loads. A bypass pipe between the two balances
the primary flow with the secondary flow. Because there are more pumps
and a bypass, this system costs more than a constant flow system to install.
Details on this system type are in “Primary–Secondary (Decoupled) Systems”
on page 45.
Variable-primary system
This pumping arrangement (Figure 14) was made possible in recent years by
advanced chiller controls that permit varying the flow through the chillers.
Like a constant flow system, the distribution piping is directly connected to
the chiller piping. Flow is varied through at least most of the loads and the
chillers. A smaller bypass (compared to the primary-secondary system)
ensures chiller minimum flow rates are avoided. Fewer pumps and smaller
bypass lead to lower first costs compared to the primary-secondary system.
Operation costs can also be lower, but the plant is controlled differently than
in other pumping arrangements and operator training is essential. This
system type is covered in detail in “Variable-Primary-Flow Systems” on
page 55.
SYS-APM001-EN Chiller System Design and Control 13
Condenser-Water System
As in chilled-water distribution systems, condenser-water system piping—
most commonly steel, copper, or plastic—is sized to meet a project’s
operating pressure, pressure loss, water velocity, and construction cost
parameters. Pressure drop through piping and the chiller’s condenser, plus
the cooling tower static lift, is overcome by use of a condenser-water pump.
To ensure optimum heat transfer performance, the condenser-heat transfer
surfaces must be kept free of scale and sludge. Even a thin deposit of scale
can substantially reduce heat transfer capacity and chiller efficiency. Specifics
of cooling-tower-water treatment are not discussed in this manual. Engage
the services of a qualified water treatment specialist to determine the level of
water treatment required to remove contaminants from the cooling tower
water.
Cooling tower
To reject heat, water is passed through a cooling tower where a portion of it
evaporates, thus cooling the remaining water. A particular cooling tower’s
effectiveness at transferring heat depends on water flow rate, water
temperature, and ambient wet bulb. The temperature difference between the
Primary System Components
Cooling
Towers
Manifolded
Pumps
Chillers
Figure 15. Manifolded condenserwater pumps
water entering and leaving the cooling tower is the range. The temperature
difference between the leaving water temperature and the entering wet-bulb
temperature is the approach.
Effect of load on cooling tower performance
As the building load—or heat rejection—decreases, range and approach also
decrease. This means that when the building is at part load, the cooling tower
can provide colder water at the same ambient wet-bulb temperature.
Effect of ambient conditions on cooling tower performance
As ambient wet-bulb temperature drops, the approach—at a constant load—
increases. This is counter-intuitive to many, and it must be considered when
cooling-tower-control strategies are developed. Detailed descriptions of these
conditions appear in “Chiller–tower energy balance” on page 91. For
additional information, refer to 2008 ASHRAE HVAC Systems and Equipment
Handbook, chapter 39, “Cooling Towers.”
3
Condenser-water pumping arrangements
Water-cooled chillers require condenser-water-system variations to be
considered. For a discussion of condenser-water temperatures and flow
rates, refer to “System Design Options” on page 27. Since air-cooled-chiller
condenser controls are part of the chiller design, they are not discussed in
this manual.
Most important, the inlet to the pump must have sufficient net positive head.
This often means locating the pump below the cooling-tower sump.
Single tower per chiller
In some applications each chiller has a dedicated cooling tower. This is most
likely to occur when chillers, and their accompanying towers, are purchased
at different times during the facility’s life—such as when additions are made.
Manifolded pumps
A much-used pumping arrangement has a single cooling-tower sump with
manifolded pumps, one condenser water line, and separate, smaller, pipes
for each chiller as shown in Figure 15. This provides a number of advantages:
• Pumping redundancy
• If cooling towers cells can be isolated, any cooling-tower cell can run with
any chiller.
• Hydraulics are generally less problematic than on the chilled-water side.
• Cooling towers can be located remotely from chillers, with only a single
supply and return pipe to connect them.
14 Chiller System Design and Control SYS-APM001-EN
Primary System Components
Unit-Level Controls
The chilled-water supply temperature is usually controlled by the chiller.
Most commonly, supply water temperature is used as the sensed variable to
permit control of chiller capacity to meet system load demand. Supplytemperature control strategies may be used on either constant- or variableflow systems. As previously discussed, flow control is executed at the load
terminals using three-way or two-way valves, or separate pumps for each
coil. Control capabilities run the gamut from slow-acting pneumatic controls,
to electromechanical controls, to sophisticated digital controls that use "feedforward" algorithms tuned to give superior performance.
Chiller control
Today’s chiller controls are capable of doing more than simply turning the
chiller on and off. At a minimum, these controls should monitor:
• Safety points, such as bearing temperatures and electrical points, that
may cause motor failure when out of range.
• Data points that may cause operational problems if corrective action is
not taken. An example is low chilled-water or refrigerant temperature,
which may result in freezing in or around the evaporator tubes.
• General points to ensure proper chiller performance and refrigerant
containment.
Table 1. Recommended chiller-monitoring points per ASHRAE Standard 147
Flow
Inlet Pressure Inlet Pressure
Chilled Water (or other
secondary coolant)
Evaporator
Oil
Vibration Levels PPM Refrigerant Monitor Level
Purge
Ambient Temperatures
Inlet Temperature Inlet Temperature
Outlet Pressure Outlet Pressure
Outlet Temperature Outlet Temperature
Refrigerant Pressure
Refrigerant Temp. Refrigerant Temp.
Level
Pressure Compressor Discharge Temp.
Temperature Compressor Suction Temp.
Addition of Addition of (in Refrigerant Log)
Exhaust Time
Discharge Count Signature of Reviewer
Dry Bulb
Wet Bulb Volts Per Phase
Condenser
Water
Condenser
Refrigerant
Logs
Motor
Flow
Refrigerant Pressure
Level
Date and Time Data
Amperes Per Phase
4
SYS-APM001-EN Chiller System Design and Control 15
Primary System Components
For more information about chillers,
Trane Air Conditioning Clinics are
available for centrifugal (TRG-TRC010EN), absorption (TRG-TRC011-EN) and
helical-rotary (TRG-TRC012-EN) chiller
types.
Adjustable
Frequency
Drive
Inlet Guide Vanes
Compressor
Evaporator
Condenser
In addition to monitoring data, it is vital that the chiller controls alert
operators to possible problems. Diagnostic messages are necessary for the
operator to respond to safety issues and data points that are outside normal
operating ranges. While communicating these diagnostic messages is a
requirement, some chiller controls include factory-installed programming
that responds to the issue causing the diagnostic messages. For example,
when the chilled-water temperature nears freezing, the chiller sends a
diagnostic message and adapts its operation by reducing the compressor
capacity, raising the chilled-water temperature to a safer condition.
Finally, the chiller controls should communicate with a system-level
controller. There are many system aspects that are outside the chiller’s direct
control, such as condenser-water temperature and the amount of fluid
flowing through the evaporator and condenser. To minimize the system
energy costs, the system controls must coordinate chiller, pump, coolingtower, and terminal-unit controls. This can only be done if adequate
information is communicated from each system component to the systemlevel controls. System-level control is discussed in detail in “System
Controls” beginning on page 87.
16 Chiller System Design and Control SYS-APM001-EN
Centrifugal chiller capacity control
The capacity of a centrifugal chiller can be modulated using inlet guide vanes
(IGV) or a combination of IGV and a variable-speed drive (adjustablefrequency drive, AFD)(Figure 16). 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 being applied to centrifugal water chillers.
Figure 16. Centrifugal chiller with AFD
ASHRAE 90.1 requires a chiller to meet both full and part-load efficiency
requirements. Using an AFD with a centrifugal chiller degrades the chiller’s
full-load efficiency. This causes 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. In return, an AFD can offer energy savings
Primary System Components
by reducing motor speed at “low-lift” conditions, when cooler condenser
water is available.
Certain system characteristics favor the application of an AFD, including:
• A substantial number of part-load operating hours (for example, when an
air- or water-economizer is not installed in the system)
• The availability of cooler condenser water (condenser-water reset)
• Chilled-water reset control
Chiller savings using condenser- and chilled-water-temperature reset,
however, should be balanced against the increase in pumping and coolingtower energy. Performing a comprehensive energy analysis is the best
method of determining whether an AFD is desirable. It is important to use
actual utility costs, not a “combined” cost, for demand and consumption
charges. It is also important to include drive maintenance and replacement
costs, since the drive life is shorter than the chiller life. See “Energy and
economic analysis of alternatives” on page 26.
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, or if the condenser water is close to its design temperature most
of the time (e.g., in a hot and humid climate such as Miami).
Consider the following analysis of an 800-ton office building with two
chillers. The analysis compares equally priced high efficiency or AFDequipped chillers, as one or both of the chillers. Utility costs for the combined
or “blended” rate are $0.10 per kWh and for the actual rate are $12 per kW
and $0.06 per kWh.
Simple paybacks using the combined rate analysis show almost no difference
between the two options (Table 2). However, when utility costs with an actual
consumption and demand component are used, the difference between the
alternatives is much more pronounced. The conclusion is that using actual
energy rates matters a great deal.
Table 2. Analysis of high-efficiency chiller options with combined vs actual rates
Simple paybacks, humid climate Combined rate Actual rate
AFD on both chillers 7.2 12.7
High efficiency on both chillers 7.1 8.3
AFD on one chiller 6.1 10.8
High efficiency on one chiller 6.3 7.7
SYS-APM001-EN Chiller System Design and Control 17
Application Considerations
Air-Cooled Chiller
Pump
Control
Valve
Load
Chiller system size affects design and control considerations. Each size comes
with its own set of advantages and challenges.
Small Chilled-Water Systems (1-2 chillers)
Figure 17. Small chilled-water system schematic
A common design goal for the small chilled-water system with one or two
chillers (Figure 17) is to minimize complexity while balancing energy
consumption goals. Smaller chilled-water systems may have smaller budgets
allotted for operation and maintenance and may run unattended more often
than larger systems. Keeping it simple, while capitalizing on chilled water
advantages, is the hallmark of a successful project.
18 Chiller System Design and Control SYS-APM001-EN
The first cost of a small system is a common hurdle faced by a building owner.
There are ways to minimize first costs without sacrificing operating costs. For
example, a wider design T reduces flow rates, which in turn reduces pipe and
pump sizes. In addition to reducing pump and pricing costs, this may also allow
the designer to avoid installing a storage tank to meet the required chiller “loop
times.” (See “Amount of Fluid in the Loop” on page 79.) On a system with
multiple chillers, using a variable-primary-flow design (“Variable-Primary-Flow
Systems” on page 55) can reduce the number of pumps, starters, electrical
equipment, and space required.
Application Considerations
Constant flow
Constant flow is simple and often applied to small systems up to 200 tons—
as long as the system pressure drop is fairly low and a wider T is applied to
reduce the system flow rate. In constant flow systems, appropriate chilledwater reset reduces chiller energy. These two strategies for saving energy
(reducing flow rates and/or chilled-water temperature reset) can be used
successfully in the constant flow designs more common in small chilledwater systems. These two strategies are covered in “Selecting Chilled- and
Condenser-Water Temperatures and Flow Rates” on page 27 and “Chilled
water reset—raising and lowering” on page 87.
Constant flow systems use either a balancing or pressure-reducing valve or,
in a few cases, trim the pump impeller to set the system design flow.
Pressure-reducing valves waste pump energy. Another option designers use
to reduce pumping energy and increase system flexibility is to install a
variable frequency drive on the pump motor and set it at a constant speed
during system commissioning.
If, instead, system flow is balanced by trimming the pump impeller, flow
adjustment is much more difficult. Using a variable frequency drive at a set
speed allows the flow to be decreased or increased in the future if necessary.
This approach is more cost effective because the cost of variable frequency
drives has dropped. Any incremental cost will be offset by the elimination of
the balancing valves and pump starter.
Variable flow
Although a variable-primary-flow system may cost more than a constant flow
system, it is growing in popularity because it is less expensive than installing
a decoupled system. Another reason for its increased popularity is that pump
energy is reduced.
Some owners are concerned that the controls are more complex, but variable
flow systems can work very simply in the small chilled-water system when
there is only one chiller or when two chillers are piped in series. Key control
issues for variable flow systems are discussed in “Variable-Primary-Flow
Systems” on page 55, and variable flow with series chillers in “Series
Chillers” on page 44.
Condensing method
Many small chilled-water systems use air-cooled chillers because of the lower
maintenance requirements of the condensing circuit. Water-cooled systems
are generally more energy efficient and have more options for features such
as heat recovery, though some air-cooled chillers have partial heat recovery
options.
To help the owner decide on the system selection, a comprehensive energy
analysis is the best method of estimating the life cycle cost difference
between air-cooled and water-cooled systems. Energy analysis is likely
required for many facilities seeking LEED certification, so it may already be
SYS-APM001-EN Chiller System Design and Control 19
Application Considerations
part of those jobs. See “Energy and economic analysis of alternatives” on
page 26.
Number of chillers
The number of chillers to install is a function of redundancy requirements
and first cost. In general, the more chillers installed, the higher the initial cost.
Therefore, many small systems only use one chiller. Most chillers in the 20
through 200 ton range use multiple compressors with multiple refrigeration
circuits and provide a reasonable level of cooling redundancy. The only
system controls installed on a single chiller installation may be a clock and
ambient lockout switch to enable and disable the chilled-water system. If only
one chiller is used, a system that varies the flow rate through the chiller can
be quite simple to operate. Minimum and maximum flows and maximum
rate of change for the flow would still need to be addressed (see “VariablePrimary-Flow Systems” on page 55).
As systems get larger, the owner may require more redundancy, leading
them to install multiple chillers. Some designers use 200 tons as the
maximum job size for a single chiller.
When there is more than one chiller, there are many more system control
decisions to be made including:
• enabling the second chiller,
• turning the second chiller off, and
• failure recovery.
Two-chiller plants require higher system control intelligence than single
chiller plants. Sequencing logic, discussed in “System Configurations” on
page 42, varies based on system configuration, and failure recovery is
discussed on page 95.
Parallel or series
Parallel configurations are more common than series configurations. (See
“Parallel Chillers” on page 42.) In chiller systems with an even number of
chillers, there are advantages to putting them into a series configuration,
especially if low or variable water flow is desired. This offers the benefits of
better system efficiency and higher capacity because the upstream chiller
produces water at a warmer temperature. Series chillers should not be
applied with low system Ts, because the maximum flow through the chillers
may be reached. Efforts to eliminate the so-called “Low T syndrome” (page
79) must be addressed for both configurations. The energy and control
requirements of series chillers are covered in “Series Chillers” on page 44.
Part load system operation
For small chilled-water systems, especially those with only one chiller, part
load system energy use may be dominated by ancillary equipment,
especially in a constant flow system. At low loads, constant speed pumps
and tower fans constitute a much larger portion of the chiller plant energy
20 Chiller System Design and Control SYS-APM001-EN
Application Considerations
Loads
Distribution Pumps
than at full load. Variable frequency drives for unloading tower fans and
chilled-water pumps may provide benefits, depending on the costs, system
operating hours, system type, and outdoor air conditions. (See “System
Controls” on page 87.)
Mid-Sized Chilled-Water Systems (3-5 Chillers)
Figure 18. Mid-sized chilled-water system schematic
In addition to the design decisions faced by the small chilled-water system
designer, the following objectives may be encountered by the mid-sized
system designer.
Managing control complexity
As chilled-water systems get larger (Figure 18), control system design and
execution become more critical and more complex. There are simply more
combinations of equipment and operating scenarios. On the other hand,
systems this size generally have more highly-skilled operators who can
understand proper operation and maintenance. To help operators
understand expected system operation, chiller plant controls are usually
more customized and sophisticated.
Preferential vs. equalized loading and run-time
With more chillers, sequencing options might include preferentially loading
the most efficient chiller or equalizing the run time of chillers. The decision
hinges on how different these chillers are and the preferred maintenance
routine. For example, a chiller plant with one quite old—though still reliable—
chiller may periodically enable that chiller to ensure it continues to function,
SYS-APM001-EN Chiller System Design and Control 21
Application Considerations
~
~
~
~
~
but use it sparingly due to its lower efficiency. Or, a chiller may have a
different fuel source, used as a hedge against either high demand or high
energy consumption charges for other energy sources. (See “Alternative
Energy Sources” on page 82.) Some chilled-water systems have unequally
sized chillers, allowing fewer chillers to operate. (See “Unequal Chiller
Sizing” on page 78.)
Large Chilled-Water Systems (6+ Chillers, District Cooling)
Figure 19. Large chilled-water system schematic
Large chilled-water systems with six or more chillers (Figure 19) have
different challenges than smaller systems. Examples of these types of
systems are commonly found on campuses with multiple buildings,
downtown districts, and mixed-use residential and commercial
developments.
22 Chiller System Design and Control SYS-APM001-EN
Application Considerations
Creating one centralized chilled-water system takes significant foresight,
initial investment, and building development with a multi-year master plan. If
the initial plant is built to accommodate many future buildings or loads, the
early challenge is operating the system efficiently with much lower loads
than it will experience when the project is complete. The system may need to
blend parallel and series configurations (“Series–Counterflow Application”
on page 77) to accommodate the wide range of loads the plant experiences
during phased construction.
Another type of large chilled-water system could actually start out as more
than one chilled water-system. An existing set of buildings can be gradually
added to the central system, or two geographically distant chilled-water
systems can be connected. “Plant Expansion” on page 83 discusses the
unique control and hydraulic challenges of “double-ended” chilled-water
systems.
Operating large chilled-water systems can be different as well. As system
load drops, chillers are turned off. Individual chiller unloading characteristics
are not as important, because operating chillers are more heavily loaded.
Pipe size
Practical pipe size limitations start to affect the maximum size of a chilledwater system. As the systems get larger, it becomes more difficult to
accommodate the increasing pipe sizes, both in cost and in space. Large Ts
can help reduce flow and required pipe size. (See “Selecting Chilled- and
Condenser-Water Temperatures and Flow Rates” on page 27.) In general, the
larger the system, the higher the T should be.
Water
Large systems are almost always water-cooled. Both chilled water (a closed
loop) and condenser water (usually an open loop) pipes will have to be filled
with water. In some locations, it is difficult to find enough fresh water to fill a
very large system with water, especially if the chilled-water system is quite
distant from the loads. Cooling towers consume water, which can be
significant and difficult to obtain in some locations. The search for both
locally available make-up water and energy savings can lead to the
exploration of alternative condensing sources like lake, river, or well water.
(See “Well, river, or lake water” on page 72.) In rare instances, salt water or
brackish water can be applied if the system uses an intermediate heat
exchanger, or if the chiller is constructed with special tubes.
Power
Large chilled-water systems can be challenged by site power availability.
Transformer size may be dictated by local regulations. On-site power
generation may be part of the project, leading to using higher voltages inside
the chilled-water system to avoid transformer losses and costs. Alternative
fuels for some or all of the chillers may be attractive (“Alternative Energy
Sources” on page 82).
SYS-APM001-EN Chiller System Design and Control 23
Application Considerations
For more information about chiller plant
controls, consult the Trane applications
guide, Tracer Summit™ Chiller Plant
Control Program (BAS-APG004-EN).
To minimize power, large systems must be very efficient. The upside of a
large system is the amplification of energy savings. A relatively small
percentage of energy saved becomes more valuable. For this reason, the
highly efficient series-counterflow arrangement is popular for large systems.
(See “Series–Counterflow Application” on page 77.)
Controls
The designers of medium and large chilled-water systems are more likely to
consider the pros and cons of direct-digital controls (DDC) versus
programmable-logic controls (PLC). These platforms deliver similar results,
depending on proper design, programming, commissioning, and operation.
One way to think of PLC is “fast, centralized control with redundancy.” PLC
has a faster processing speed, with some hot-redundancy features—such as
an entirely redundant system processor that is ready to take over if the main
system processor fails.
Conversely, DDC can be considered “steady, distributed control with
reliability.” DDC controls feature easy programming and user-friendly
operation. In the DDC environment, a failure of the system processor results
in the lower-level processors defaulting to a pre-determined operating mode.
The speed of the PLC system can be one of its challenges. Controls that are
steady and do not overreact to minor changes work very well, even in large
chilled-water systems.
Chiller Plant System Performance
Chiller performance testing
All major chiller manufacturers have chiller performance test facilities in the
factory, in a laboratory, or both. A chiller performance test in accordance with
the test procedures in ARI Standard 550/590
under controlled conditions, with industrial grade instrumentation and
computerized data collection devices. This test ensures that the chiller meets
its promised performance criteria. If it does not, corrections are made before
it leaves the factory.
Limitations of field performance testing
After the chiller is installed at the job site, the system conditions will be less
controllable than in a test facility, and therefore unsuitable for chiller
acceptance testing. While measuring the performance of the entire chiller
plant is more difficult, it can help identify operating problems or evaluate the
effectiveness of system control methods and setpoints.
5
can be performed at the factory
24 Chiller System Design and Control SYS-APM001-EN
The goal is to operate as efficiently as possible and to sustain a high level of
individual equipment and coordinated operation. A proper energy
management system can help trend and diagnose problems or changes over
time.
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