Trane TRG-TRC010-EN User Manual
Air Conditioning
Clinic
Centrifugal
Water Chillers
One of the Equipment Series
TRG-TRC010-EN
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FIRST-CLASS MAIL LA CROSSE, WIPERMIT NO. 11
POSTAGE WILL BE PAID BY ADDRESSEE
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Attn: Applications Engineering
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La Crosse WI 54601-9985
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Centrifugal
Water Chillers
One of the Equipment Series
A publication of
The Trane Company—
Worldwide Applied Systems Group
Preface
Centrifugal Water Chillers
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’s intended to acquaint a nontechnical audience with various fundamental
aspects of heating, ventilating, and air conditioning.
We’ve 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 concept of centrifugal water chillers.
© 1999 American Standard Inc. All rights reserved
ii
TRG-TRC010-EN
Contents
Introduction ........................................................... 1
period one Components .......................................................... 3
Compressor ............................................................ 4
Condenser .............................................................. 7
Expansion Device .................................................... 8
Economizer ............................................................. 9
Evaporator ............................................................ 11
Motor ................................................................... 12
Controls and Starter .............................................. 13
period two Refrigeration Cycle ............................................ 15
Refrigerants .......................................................... 19
Purge System ....................................................... 20
period three Compressor Capacity Control ........................ 22
period four Maintenance Considerations .......................... 30
period five Application Considerations ............................. 36
Condensing Temperature Control .......................... 37
Constant or Variable Evaporator Water Flow .......... 39
Short Evaporator-Water Loops ............................... 40
Heat Recovery ...................................................... 42
Free Cooling .......................................................... 44
Equipment Certification Standards ......................... 46
period six Review ................................................................... 48
Quiz ......................................................................... 52
Answers ................................................................ 54
Glossary ................................................................ 55
TRG-TRC010-EN iii
iv TRG-TRC010-EN
notes
Introduction
Chilled
Water
System
Figure 2
Water chillers are used in a variety of air conditioning and process cooling
applications. They are used to make cold water that can be transported
throughout a facility using pumps and pipes. This cold water can be passed
through the tubes of coils to cool the air in an air conditioning application, or it
can provide cooling for a manufacturing or industrial process.
Systems that employ water chillers are commonly called chilled water
systems.
absorption
helical-rotary
There are several types of water chillers. They differ from each other based on
the refrigeration cycle or the type of compressor they use.
Absorption water chillers make use of the absorption refrigeration cycle and do
not have a mechanical compressor involved in the refrigeration cycle.
centrifugal
Figure 3
TRG-TRC010-EN 1
Introduction
notes
Water chillers using the vapor-compression refrigeration cycle vary by the type
of compressor used. Reciprocating and scroll compressors are typically used in
small chillers. Helical-rotary (or screw) compressors are typically used in
medium-sized chillers. Centrifugal compressors are typically used in large
chillers.
As mentioned earlier, this particular clinic discusses centrifugal water
chillers.
Centrifugal Water Chillers
Figure 4
Centrifugal water chillers can also be divided into two types based on the
method used to reject heat to the atmosphere: water-cooled or air-cooled. Since
most centrifugal chillers are water-cooled, they are the primary focus of this
clinic. Water-cooled centrifugal chillers are generally available from 100 to 3,000
tons [350 to 10,500 kW] as prefabricated machines, and up to 8,500 tons
[30,000 kW] as built-up machines.
2 TRG-TRC010-EN
notes
period one
Components
Centrifugal Water Chillers
period one
Figure 5
Many of the components of the centrifugal water chiller are similar to those of
other chiller types.
components of a
Centrifugal Water Chiller
compressor
compressor
condenser
condenser
control
control
panel
panel
evaporator
evaporator
This particular centrifugal water chiller makes use of a shell-and-tube
evaporator where refrigerant absorbs heat from the water flowing through the
tubes. The compressor is made up of 1 or more centrifugal impellers. A second
shell-and-tube heat exchanger serves as the water-cooled condenser, where
refrigerant is condensed inside the shell and water flows inside tubes.
Refrigerant is metered through the system using an expansion device such as a
fixed orifice plate. An economizer can be used to enhance the efficiency of a
chiller with multiple compressor impellers. A control panel is also provided on
the chiller and a starter is either mounted on the chiller or located remotely.
motor
motor
starter
starter
Figure 6
TRG-TRC010-EN 3
notes
period one
Components
Compressor
Figure 7
Compressor
The centrifugal compressor uses the principle of dynamic compression,
which involves converting energy from one form to another, to increase the
pressure and temperature of the refrigerant. It converts kinetic energy to static
energy.
Impeller
blades
blades
Figure 8
The core component of a centrifugal compressor is the rotating impeller. The
center, or eye, of the impeller is fitted with blades that draw refrigerant vapor
into radial passages that are internal to the impeller body.
4 TRG-TRC010-EN
notes
period one
Components
Centrifugal Compressor
volute
volute
diffuser
diffuser
passage
passage
impeller
impeller
passages
passages
Figure 9
The rotation of the impeller causes the refrigerant vapor to accelerate within the
impeller passages, increasing its velocity and kinetic energy.
The accelerated refrigerant vapor leaves the impeller and enters the diffuser
passages. These passages start out small and become larger as the refrigerant
travels through them. As the size of the diffuser passages increases, the
velocity, and therefore the kinetic energy, of the refrigerant decreases.
The first law of thermodynamics states that energy is not destroyed—only
converted from one form to another. Thus, the refrigerant’s kinetic energy is
converted to static energy or static pressure.
Refrigerant, now at a higher pressure, collects in a larger space around the
perimeter of the compressor called the volute. The volute also becomes larger
as the refrigerant travels through it. Again, as the size of the volute increases,
the kinetic energy is converted to static pressure.
Due to its pressure and temperature, the refrigerant leaving the compressor is
in a condition that allows its heat to be rejected from the chiller.
TRG-TRC010-EN 5
notes
period one
Components
centrifugal compressor
Energy Conversion
refrigerant
refrigerant
enters
enters
diffuser
diffuser
velocity
velocity
refrigerant
refrigerant
refrigerant
enters impeller
enters impeller
static pressure
static pressure
path through compressor
path through compressor
Again, in the passages of the rotating impeller, the refrigerant vapor
accelerates, increasing its velocity and kinetic energy. As the area increases in
the diffuser passages, the velocity, and therefore the kinetic energy, of the
refrigerant decreases. This reduction in kinetic energy is offset by an increase in
the refrigerant’s static energy or static pressure. Finally, the high pressure
refrigerant collects in the volute around the perimeter of the compressor, where
further energy conversion takes place.
refrigerant
enters volute
enters volute
Figure 10
The resulting pressure and temperature of the refrigerant is now high enough
that its heat can be rejected from the chiller.
Multistage Compressor
Figure 11
Centrifugal compressors use 1 or more impellers to compress the refrigerant. A
multistage compressor uses 2 or 3 impellers to increase the pressure of the
refrigerant in steps instead of performing the task within a single impeller.
Compressed refrigerant vapor travels from the outlet of the first-stage
compressor impeller to the inlet of the second-stage compressor impeller. After
6 TRG-TRC010-EN
period one
Components
notes
the accelerated refrigerant vapor leaves the last impeller, it collects in the
compressor volute and travels on to the condenser.
Condenser
refrigerant vapor
refrigerant vapor
baffle
baffle
cooling
cooling
tower
tower
water
water
liquid
tube
tube
bundle
bundle
Condenser
The high-pressure refrigerant vapor is discharged from the compressor into a
heat exchanger that acts as a condenser.
In a water-cooled condenser, water is pumped through the tubes of the shell-
and-tube heat exchanger while refrigerant vapor fills the shell space
surrounding the tube bundle. A baffle inside the condenser helps distribute the
refrigerant evenly. As heat transfers from the hot, high-pressure refrigerant
vapor to the water, refrigerant condenses on the tube surfaces.
liquid
refrigerant
refrigerant
Figure 12
Cooling water flows first through the lower tubes and then through the upper
tubes. This produces a nearly constant temperature difference between the
downward-moving refrigerant and the tube surfaces, resulting in a uniform
heat transfer rate within the tube bundle.
Condensed liquid refrigerant collects in the bottom of the shell and flows
through the liquid line to the expansion devices and economizer.
TRG-TRC010-EN 7
notes
period one
Components
expansion device
Orifice Plates
orifice plates
orifice plates
H
H
1
1
Expansion Device
An expansion device is used to maintain the pressure difference between the
high-pressure (condenser) and low-pressure (evaporator) sides of the
refrigeration system, as established by the compressor. This pressure
difference allows the evaporator temperature to be low enough for the
refrigerant to absorb heat from the water being cooled, and the condenser
temperature to be high enough for the refrigerant to reject heat to water at
normally available temperatures. High-pressure liquid refrigerant flows through
the expansion device, causing a pressure drop that reduces the refrigerant
pressure to that of the evaporator. This pressure reduction causes a small
portion of the liquid to boil off, or “flash,” cooling the remaining refrigerant to
the evaporator temperature.
to
to
evaporator
evaporator
Figure 13
The expansion device is also used as a liquid refrigerant metering system,
balancing the refrigerant flow rate with the evaporator load condition. In our
example centrifugal chiller, the expansion device used is a set of 2 orifice
plates. At full load, a large amount of refrigerant is moving through the chiller.
The column of liquid refrigerant in the liquid line pressurizes the liquid at its
base. During passage through the orifice plates, the liquid refrigerant
undergoes a pressure drop equal to the head (H
vapor.
8 TRG-TRC010-EN
) before some of it flashes to
1
notes
period one
Components
expansion device
Orifice Plates
orifice plates
orifice plates
H
H
2
2
As the load decreases, less refrigerant moves through the chiller and the level
of the liquid column drops. Now, as the liquid refrigerant passes through the
orifice plates, it only undergoes a pressure drop equal to the lower head (H
before some of it flashes to vapor. This causes additional flashing at the orifice
plate which, in turn, feeds less liquid to the evaporator.
to
to
evaporator
evaporator
Figure 14
)
2
Other types of expansion devices found in centrifugal chillers include: float
valves, expansion valves (thermostatic or electronic), and variable orifices.
2-stage chiller
Economizer
refrigerant vapor
refrigerant vapor
to second stage
to second stage
of compression
of compression
liquid refrigerant
orifice
orifice
liquid refrigerant
liquid refrigerant
from condenser
from condenser
Economizer
An economizer can be used in conjunction with multiple expansion devices to
improve the efficiency of a multistage chiller. In a chiller with a 2-stage
compressor, the expansion process can be separated into 2 steps with an
economizer chamber between.
liquid refrigerant
to evaporator
to evaporator
Figure 15
TRG-TRC010-EN 9
period one
Components
notes
Liquid refrigerant from the condenser enters the first expansion device, which
reduces the pressure of the refrigerant to that of the second-stage impeller
inlet. This pressure drop causes a portion of the liquid refrigerant to evaporate,
or flash, and the resulting mixture of liquid and vapor enters the economizer
chamber. Here, the vapor is separated from the mixture and is routed directly to
the inlet of the second stage impeller. The remaining liquid travels on to the
second expansion device and evaporator.
Just before entering the evaporator, the liquid refrigerant flows through a
second expansion device that reduces its pressure and temperature to
evaporator conditions.
Flashing a portion of the refrigerant prior to the economizer reduces the
amount of compressor power required, since the refrigerant vapor generated in
the economizer only needs to be compressed by the second-stage impeller.
The benefit of the economizer will be discussed in greater detail in Period 2.
3-stage chiller
Economizer
refrigerant vapor
refrigerant vapor
to third stage
to third stage
of compression
of compression
refrigerant vapor
refrigerant vapor
to second stage
to second stage
of compression
of compression
liquid refrigerant
orifice
orifice
liquid refrigerant
liquid refrigerant
from condenser
from condenser
In a chiller with a 3-stage compressor, the expansion process can be separated
into 3 steps with separate economizer chambers between the steps.
Liquid refrigerant from the condenser enters the first orifice (expansion device),
which reduces the pressure of the refrigerant to that of the third-stage impeller
inlet. This pressure drop causes a portion of the liquid refrigerant to flash, and
the resulting mixture of liquid and vapor enters the high-pressure chamber of
the economizer. Here, the vapor is separated from the mixture and is then
routed directly to the inlet of the third-stage impeller. The remaining liquid
travels on to the second expansion device.
The second expansion device further reduces the pressure of the refrigerant to
that of the second-stage impeller inlet. This pressure drop causes a portion of
the liquid refrigerant to flash, and the resulting mixture of liquid and vapor
enters the low-pressure chamber of the economizer. Here, the vapor is
separated from the mixture and routed directly to the inlet of the second-stage
impeller. The remaining liquid travels on to the third expansion device and
evaporator.
10 TRG-TRC010-EN
orifice
orifice
liquid refrigerant
to evaporator
to evaporator
Figure 16
period one
Components
notes
Again, the final expansion device reduces the pressure and temperature of the
refrigerant to evaporator conditions.
Evaporator
tube bundle
tube bundle
liquid
liquid
refrigerant
refrigerant
chilled
chilled
water
water
return
return
liquid
liquid
distributor
distributor
eliminator
eliminator
Evaporator
In the flooded shell-and-tube evaporator shown, the low-pressure mixture
of liquid refrigerant and refrigerant vapor enters the distribution system that
runs the entire length of the shell. Small openings and baffles in the passage of
the liquid distributor provide an even spray of refrigerant over the surfaces of
the tubes inside the evaporator shell, where the refrigerant absorbs heat from
relatively warm water flowing through the tube bundle. This transfer of heat
boils the liquid refrigerant on the tube surfaces. The resulting vapor passes
through an eliminator that prevents liquid from being drawn upward. The vapor
collects in a large chamber at the top of the shell and is drawn back to the
compressor. The now-cool water can be used in a variety of comfort or process
applications.
orifice system
orifice system
Figure 17
Some chiller designs may make use of a direct expansion (DX) shell-andtube evaporator. In this type of evaporator, liquid refrigerant flows through
the tubes and water fills the surrounding shell. As heat is transferred from the
water to the refrigerant, the refrigerant boils inside the tubes and the resulting
vapor is drawn to the compressor.
TRG-TRC010-EN 11
notes
period one
Components
Motor
motor
motor
gear-drive
impeller
impeller
motor
motor
impellers
impellers
direct-drive
gears
gears
Figure 18
Motor
A motor is used to rotate the impeller(s). A direct-drive motor is connected
directly to the impeller shaft and the impeller rotates at the same speed as the
motor. A gear-drive motor transfers its energy to the impeller shaft using a set
of gears. This allows the impeller to rotate at a higher speed than the motor.
The direct-drive motor requires fewer bearings and does not incur gear losses.
Additionally, since the compressor rotates at a lower speed, it can be much
quieter.
Direct-drive compressors are, however, only practical in centrifugal chillers that
use low-pressure refrigerants.
Hermetic Motor Cooling
stator
stator
rotor
rotor
liquid
liquid
refrigerant
refrigerant
Another important difference in compressor motors is the issue of hermetic
versus open. A hermetic motor is totally enclosed within the chiller’s
refrigeration system. An open motor is mounted externally—outside of the
drain
drain
Figure 19
12 TRG-TRC010-EN
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