Air Conditioning
Clinic
Refrigeration
Compressors
One of the Fundamental Series
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La Crosse WI 54601-9985
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The Trane Company • Worldwide
Applied Systems Group
3600 Pammel Creek Road • La Crosse, WI 54601-7599
An American-Standard Company
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The Trane Company • Worldwide Applied Systems Group
3600 Pammel Creek Road • La Crosse, WI 54601-7599
An American-Standard Company
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Preface
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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 concept of refrigeration compressors.
© 2000 American Standard Inc. All rights reserved
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Contents
Introduction ........................................................... 1
period one
period two
Compressor Types ............................................... 3
Reciprocating Compressor ...................................... 4
Scroll Compressor ................................................... 7
Helical-Rotary (Screw) Compressor ........................ 10
Centrifugal Compressor ......................................... 13
Compressor Capacity Control ........................ 18
Cylinder Unloaders ................................................ 19
Cycling On and Off ................................................ 24
Slide Valve ............................................................ 26
Inlet Vanes ............................................................ 27
Variable Speed ...................................................... 29
period three The Compressor in a System ......................... 30
System-Level Control ............................................ 30
Preventing Evaporator Freeze-Up ........................... 33
period four Review ................................................................... 38
Quiz ......................................................................... 43
Answers ................................................................ 44
Glossary ................................................................ 45
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Introduction
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Figure 2
The purpose of the compressor in a refrigeration system is to raise the pressure
of the refrigerant vapor from evaporator pressure to condensing pressure. It
delivers the refrigerant vapor to the condenser at a pressure and temperature at
which the condensing process can be readily accomplished, at the temperature
of the air or other fluid used for condensing.
A review of the refrigeration cycle, using the pressure–enthalpy chart, will help
to illustrate this point.
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Introduction
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The pressure–enthalpy (P–h) chart plots the properties of a refrigerant:
refrigerant pressure (vertical axis) versus enthalpy, or heat content (horizontal
axis). A diagram of the basic vapor-compression refrigeration cycle can be
superimposed on a pressure–enthalpy chart to demonstrate the function of
each component in the system.
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
be cooled to the refrigerant, causing the liquid refrigerant to boil and in some
cases superheat (B). The resulting vapor (B) is then pumped from the
evaporator by the compressor, which increases the pressure and temperature
of the refrigerant vapor. Notice that during the compression process (B to C),
the heat content (enthalpy) of the vapor is increased. The mechanical energy
used by the compressor to increase the pressure of the refrigerant vapor is
converted to heat energy, called the heat of compression. This causes the
temperature of the refrigerant to also rise as the pressure is increased.
The resulting hot, high-pressure refrigerant vapor (C) enters the condenser
where heat is transferred to ambient air or water at a lower temperature. Inside
the condenser, the refrigerant desuperheats (C to D), condenses into a liquid (D
to E), and, in some cases, subcools (E to F). The refrigerant pressure inside the
condenser is determined by the temperature of the air or water that is available
as the condensing media.
This liquid refrigerant (F) then flows from the condenser to the expansion
device. The expansion device 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 (A). The cool mixture of liquid and vapor
refrigerant travels to the evaporator to repeat the cycle.
2
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period one
Compressor Types
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This period is devoted to the discussion of the different types of compressors.
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There are primarily four types of compressors used in the air-conditioning
industry: reciprocating, scroll, helical-rotary (or screw), and centrifugal.
The traditional reciprocating compressor has been used in the industry for
decades. It contains cylinders, pistons, rods, a crankshaft, and valves, similar to
an automobile engine. Refrigerant is drawn into the cylinders on the
downstroke of the piston and compressed on the upstroke.
Scroll and helical-rotary (or screw) compressors have become more
common, replacing the reciprocating compressor in most applications due to
their improved reliability and efficiency.
These three types of compressors (reciprocating, scroll, and helical-rotary) all
work on the principle of trapping the refrigerant vapor and compressing it by
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period one
Compressor Types
gradually shrinking the volume of the refrigerant. Thus, they are called
positive-displacement compressors.
notes
In contrast, centrifugal compressors use the principle of dynamic
compression, which involves converting energy from one form to another in
order to increase the pressure and temperature of the refrigerant. The
centrifugal compressor uses centrifugal force, generated by a rotating
impeller, to compress the refrigerant vapor.
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Figure 6
Reciprocating Compressor
The first type of compressor to be discussed is the reciprocating
compressor. The principles of operation for all reciprocating compressors are
fundamentally the same. The refrigerant vapor is compressed by a piston that
is located inside a cylinder, similar to the engine in an automobile. A fine layer
of oil prevents the refrigerant vapor from escaping through the mating
surfaces. The piston is connected to the crankshaft by a rod. As the crankshaft
rotates, it causes the piston to travel back and forth inside the cylinder. This
motion is used to draw refrigerant vapor into the cylinder, compress it, and
discharge it from the cylinder. A pair of valves, the suction valve and the
discharge valve, are used to trap the refrigerant vapor within the cylinder
during this process. In the example reciprocating compressor shown, the
spring-actuated valves are O-shaped, allowing them to cover the valve
openings around the outside of the cylinder while the piston travels through the
middle.
During the intake stroke of the compressor, the piston travels away from the
discharge valve and creates a vacuum effect, reducing the pressure within the
cylinder to below suction pressure. Since the pressure within the cylinder is
less than the pressure of the refrigerant at the suction side of the compressor,
the suction valve is forced open and the refrigerant vapor is drawn into the
cylinder.
4
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period one
Compressor Types
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Figure 7
During the compression stroke, the piston reverses its direction and travels
toward the discharge valve, compressing the refrigerant vapor and increasing
the pressure within the cylinder. When the pressure inside the cylinder exceeds
the suction pressure, the suction valve is forced closed, trapping the refrigerant
vapor inside the cylinder.
As the piston continues to travel toward the discharge valve, the refrigerant
vapor is compressed, increasing the pressure inside the cylinder.
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Figure 8
When the pressure within the cylinder exceeds the discharge (or head)
pressure, the discharge valve is forced open, allowing the compressed
refrigerant vapor to leave the cylinder. The compressed refrigerant travels
through the headspace and leaves the compressor through the discharge
opening.
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period one
Compressor Types
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Figure 9
In the reciprocating compressor shown, the refrigerant vapor from the suction
line enters the compressor through the suction opening. It then passes around
and through the motor, cooling the motor, before it enters the cylinder to be
compressed. The compressed refrigerant leaves the cylinder, travels through
the headspace, and leaves the compressor through the discharge opening.
Most reciprocating compressors have multiple piston–cylinder pairs attached to
a single crankshaft.
In the air-conditioning industry, reciprocating compressors were widely used in
all types of refrigeration equipment. As mentioned earlier, however, scroll and
helical-rotary compressors have become more common, replacing the
reciprocating compressor in most of these applications because of their
improved reliability and efficiency.
6
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period one
Compressor Types
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Figure 10
Scroll Compressor
Similar to the reciprocating compressor, the scroll compressor works on the
principle of trapping the refrigerant vapor and compressing it by gradually
shrinking the volume of the refrigerant. The scroll compressor uses two scroll
configurations, mated face-to-face, to perform this compression process. The
tips of the scrolls are fitted with seals that, along with a fine layer of oil, prevent
the compressed refrigerant vapor from escaping through the mating surfaces.
The upper scroll, called the stationary scroll, contains a discharge port. The
lower scroll, called the driven scroll, is connected to a motor by a shaft and
bearing assembly. The refrigerant vapor enters through the outer edge of the
scroll assembly and discharges through the port at the center of the stationary
scroll.
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period one
Compressor Types
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Figure 11
The center of the scroll journal bearing and the center of the motor shaft are
offset. This offset imparts an orbiting motion to the driven scroll. Rotation of the
motor shaft causes the scroll to orbit—not rotate—about the shaft center.
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Figure 12
This orbiting motion causes the mated scrolls to form pockets of refrigerant
vapor. As the orbiting motion continues, the relative movement between the
orbiting scroll and the stationary scroll causes the pockets to move toward the
discharge port at the center of the assembly, gradually decreasing the
refrigerant volume and increasing the pressure.
Three revolutions of the motor shaft are required to complete the compression
process.
8
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period one
Compressor Types
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Figure 13
During the first full revolution of the shaft, or the intake phase, the edges of
the scrolls separate, allowing the refrigerant vapor to enter the space between
the two scrolls. By the completion of first revolution, the edges of the scrolls
meet again, forming two closed pockets of refrigerant.
During the second full revolution, or the compression phase, the volume of
each pocket is progressively reduced, increasing the pressure of the trapped
refrigerant vapor. Completion of the second revolution produces near-
maximum compression.
During the third full revolution, or the discharge phase, the interior edges of
the scrolls separate, releasing the compressed refrigerant through the
discharge port. At the completion of the revolution, the volume of each pocket
is reduced to zero, forcing the remaining refrigerant vapor out of the scrolls.
Looking at the complete cycle, notice that these three phases—intake,
compression, and discharge—occur simultaneously in an ongoing sequence.
While one pair of these pockets is being formed, another pair is being
compressed and a third pair is being discharged.
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period one
Compressor Types
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Figure 14
In this example scroll compressor, refrigerant vapor enters through the suction
opening. The refrigerant then passes through a gap in the motor, cooling the
motor, before entering the compressor housing. The refrigerant vapor is drawn
into the scroll assembly where it is compressed, discharged into the dome, and
finally discharged out of the compressor through the discharge opening.
In the air-conditioning industry, scroll compressors are widely used in heat
pumps, rooftop units, split systems, self-contained units, and even small water
chillers.
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Figure 15
Helical-Rotary (Screw) Compressor
Similar to the scroll compressor, the helical-rotary compressor traps the
refrigerant vapor and compresses it by gradually shrinking the volume of the
refrigerant. This particular helical-rotary compressor design uses two mating
screw-like rotors to perform the compression process.
10
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period one
Compressor Types
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Figure 16
The rotors are meshed and fit, with very close tolerances, within the
compressor housing. The gap between the two rotors is sealed with oil,
preventing the compressed refrigerant vapor from escaping through the mating
surfaces.
Only the male rotor is driven by the compressor motor. The lobes of the male
rotor engage and drive the female rotor, causing the two parts to counter-
rotate.
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Figure 17
Refrigerant vapor enters the compressor housing through the intake port and
fills the pockets formed by the lobes of the rotors. As the rotors turn, they push
these pockets of refrigerant toward the discharge end of the compressor.
After the pockets of refrigerant travel past the intake port area, the vapor, still at
suction pressure, is confined within the pockets by the compressor housing.
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period one
Compressor Types
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Figure 18
Viewing the compressor from the opposite side shows that continued rotation
of the meshed rotor lobes drives the trapped refrigerant vapor (to the right),
toward the discharge end of the compressor, ahead of the meshing point. This
action progressively reduces the volume of the pockets, compressing the
refrigerant.
Finally, when the pockets of refrigerant reach the discharge port, the
compressed vapor is released and the rotors force the remaining refrigerant
from the pockets.
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Figure 19
In this example helical-rotary compressor, refrigerant vapor is drawn into the
compressor through the suction opening and passes through the motor,
cooling it. The refrigerant vapor is drawn into the compressor rotors where it is
compressed and discharged out of the compressor.
12
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period one
Compressor Types
In the air-conditioning industry, helical-rotary compressors are most commonly
used in water chillers ranging from 70 to 450 tons [200 to 1,500 kW].
notes
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Figure 20
Centrifugal 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 (velocity)
to static energy (pressure).
The core component of a centrifugal compressor is the rotating impeller.
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Compressor Types
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Figure 21
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. The rotation
of the impeller causes the refrigerant vapor to accelerate within these 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 passage 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 (velocity) 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.
14
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Compressor Types
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Figure 22
This chart plots the conversion of energy that takes place as the refrigerant
passes through the centrifugal compressor. In the radial 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 (velocity) 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.
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Figure 23
In this example centrifugal compressor, refrigerant vapor is drawn into the
compressor and enters the center of impeller. This particular centrifugal
compressor uses multiple impellers to perform the compression process in
stages. The impellers rotate on a common shaft that is connected to the motor.
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period one
Compressor Types
In the air-conditioning industry, centrifugal compressors are most commonly
used in prefabricated water chillers ranging from 100 to 3,000 tons [350 to
10,500 kW]. They are also used in field-assembled water chillers up to
8,500 tons [30,000 kW].
notes
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Figure 24
In addition to the different methods of compression, compressors can be
classified as open, hermetic, and semihermetic. A reciprocating compressor
will be used to explain these terms.
An open compressor is driven by an external power source, such as an
electric motor, an engine, or a turbine. The motor is coupled to the compressor
crankshaft by a flexible coupling. Since the shaft protrudes through the
compressor housing, a seal is used to prevent refrigerant from leaking out of
the compressor housing.
This motor is cooled by air that is drawn in from the surrounding space. The
heat removed from the motor must still be rejected from the space, either by
mechanical ventilation or, if the space is conditioned, by the building’s cooling
system.
16
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period one
Compressor Types
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Figure 25
A hermetic compressor, on the other hand, seals the motor within the
compressor housing. This motor is cooled by the refrigerant, either by
refrigerant vapor that is being drawn into the compressor from the suction line
or by liquid refrigerant that is being drawn from the liquid line. The heat from
the motor is then rejected by the condenser.
Hermetic compressors eliminate the need for the shaft couplings and external
shaft seals that are associated with open motors. The coupling needs precise
alignment, and these seals are a prime source of oil and refrigerant leaks. On
the other hand, if a motor burns out, a system with a hermetic compressor will
require thorough cleaning, while a system with an open compressor will not.
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Figure 26
Similarly, the motor for a semihermetic compressor is also contained within
the compressor housing and is cooled by the refrigerant. The term
“semihermetic” means that the sealed housing is designed to be opened to
repair or overhaul the compressor or motor.
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period two
Compressor Capacity Control
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Figure 27
The capacity of a compressor is defined by the volume of evaporated
refrigerant that can be compressed within a given time period. The compressor
needs a method of capacity control in order to match the ever-changing load on
the system.
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Figure 28
Capacity control is commonly accomplished by unloading the compressor. The
method used for unloading generally depends on the type of compressor.
Many reciprocating compressors use cylinder unloaders. Scroll compressors
generally cycle on and off. Helical-rotary compressors use a slide valve or a
similar unloading device. Centrifugal compressors typically use inlet vanes or a
variable-speed drive in combination with inlet vanes. In addition, all four types
of compressors could use variable speed to control their capacity.
18
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period two
Compressor Capacity Control
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Figure 29
Cylinder Unloaders
Most large reciprocating compressors (above 10 tons [35 kW]) are fitted with
cylinder unloaders that are used to match the compressor’s refrigerant-
pumping capacity with the falling evaporator load, by progressively
deactivating piston–cylinder pairs.
The cylinder unloader shown in this example reciprocating compressor uses an
electrically-actuated unloader valve to close the suction passage to the cylinder
that is being unloaded.
In response to a decreasing load, an electronic controller sends a signal to open
a solenoid valve. This solenoid valve diverts pressurized refrigerant vapor from
the compressor discharge to the top of the unloader valve, causing the
unloader valve to close and shut off the flow of refrigerant vapor into the
cylinder. Even though the piston continues to travel back and forth inside this
cylinder, it is no longer performing compression since it cannot take in any
refrigerant vapor.
TRG-TRC004-EN
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period two
Compressor Capacity Control
notes
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Figure 30
In response to an increasing load, the controller sends a signal to close the
solenoid valve. This closes the port that allows the pressurized refrigerant
vapor to travel to the top of the unloader valve. A controlled leakage rate
around the unloader valve relieves the pressure, allowing the valve to open and
refrigerant vapor to once again flow to the cylinder to be compressed.
Another type of cylinder unloader uses either pressure or electrically-actuated
valving mechanisms to hold open the suction valve of the piston–cylinder pair.
Since the suction valve is prevented from closing, no compression occurs in
that cylinder and the discharge valve does not open. Still other types of cylinder
unloaders divert the compressed refrigerant vapor back to the suction side of
the compressor. In contrast to the cylinder unloaders shown, these other
methods expend energy in moving refrigerant vapor during both the upward
and downward piston strokes within the unloaded cylinders.
20
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period two
Compressor Capacity Control
notes
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A plot of compressor capacity versus suction temperature (assuming a constant
condensing temperature) reveals that the capacity of the compressor increases
as the suction temperature increases. As the suction temperature, and,
therefore, the suction pressure, increases, the refrigerant vapor becomes
denser. A greater quantity of refrigerant can be compressed in a given
compression cycle and the capacity of the compressor is higher.
For an example nominal-30-ton [105 kW] reciprocating compressor that has six
cylinders, Figure 31 shows the capacity produced by the various stages of
unloading. Four of the six cylinders are equipped with unloaders, and two
cylinders are unloaded as a pair. The compressor, therefore, can operate with
all six cylinders loaded, with four cylinders loaded, with only two cylinders
loaded, or it can shut off. Again, these capacity curves assume the compressor
is operating at a constant condensing (discharge) pressure.
TRG-TRC004-EN
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period two
Compressor Capacity Control
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Figure 32
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At design conditions, the capacities of the evaporator coil and compressor
balance (A) at a suction temperature of 45°F [7.2°C] and a capacity of 31 tons
[109 kW]. As the cooling load decreases below this balance point, assuming a
constant condensing pressure, the compressor pumping capacity decreases
with the falling suction temperature along the six-cylinder curve until it reaches
B. Here, the compressor unloads the first set of two cylinders.
When the first set of two cylinders is unloaded, the compressor operates with
only four active cylinders and the compressor capacity falls immediately to
19 tons [66.8 kW] along the four-cylinder curve (C). As the load continues to
decrease, the capacity and suction temperature follow the four-cylinder curve
until it reaches D. Here, the second set of two cylinders is unloaded, decreasing
the compressor capacity to 9.5 tons [33.4 kW] along the two-cylinder curve (E).
As the load continues to decrease, the suction temperature reaches the
minimum set point, 28°F [-2.2°C] in this example (F), and the two remaining
cylinders are deactivated by shutting off the compressor. The minimum
capacity of the compressor in this example is 7 tons [24.6 kW].
This illustrates how cylinder unloading extends the stable part-load range of a
reciprocating compressor. The example compressor is able to perform over
77% of its capacity range (31 tons to 7 tons [109 kW to 24.6 kW]). An increasing
load reverses the sequence.
22
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period two
Compressor Capacity Control
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Figure 33
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In the case of comfort-cooling applications, however, the load generally
changes slowly in small intervals. For example, assume that the load decreases
from 28 tons [98.5 kW] (B) to 25 tons [88 kW]. In response to the decreasing
load, the compressor unloads to C on the four-cylinder capacity curve where it
has a pumping capacity equivalent to 19 tons [66.8 kW]. The 25-ton [88-kW]
evaporator load causes the suction temperature to rise and the capacity of the
compressor increases toward D. When the load reaches D the compressor
reloads the first set of two cylinders and the compressor capacity jumps to
31 tons [109 kW]. Because, at this point, the available compressor capacity
exceeds the evaporator load, the suction temperature decreases toward B
where the compressor is again unloaded to C.
From this example, it becomes obvious that the compressor and evaporator
cannot reach a balance point while the evaporator load remains between these
stages of compressor loading. This example compressor can produce a
pumping capacity of 28 tons [98.5 kW] (B) with six cylinders loaded or 22 tons
[77.4 kW] (D) with four cylinders loaded. It cannot exactly match the 25-ton
[88-kW] evaporator load. As long as the evaporator load remains between the
capacities produced by four and six cylinders, the compressor will alternate
between the two stages of loading in an effort to produce an “average”
capacity of 25 tons [88 kW].
Alternating between these stages of loading does not harm the reciprocating
compressor. The only time it should be avoided is when the compressor must
cycle between off and on to balance a load that is less than the minimum stage
of compressor loading. Excessive starting and stopping of large reciprocating
compressor motors is generally discouraged due to the mechanical wear on a
motor of that size.
TRG-TRC004-EN
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period two
Compressor Capacity Control
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Figure 34
Cycling On and Off
Scroll compressors do not have valves or unloaders. A piece of equipment that
uses scroll compressors generally unloads by using multiple compressors and
turning them on and off, as needed, to satisfy the evaporator load.
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Figure 35
Cycling multiple scroll compressors is very similar to the use of cylinder
unloaders on a single reciprocating compressor. As an example, a large 40-ton
[140.6-kW] reciprocating compressor may have eight cylinders with unloaders
on six of them, allowing it to unload in equal steps of 10 tons [35.2 kW] each,
with a minimum nominal capacity of 10 tons [35.2 kW].
A similar 40-ton [140.6-kW] unit using scroll compressors would include four
separate 10-ton [35.2-kW] scroll compressors. Just as the reciprocating
compressor unloads in equal intervals by unloading a pair of cylinders, the
scroll compressor unit unloads in the same 10-ton [35.2-kW] intervals by
shutting off individual compressors.
24
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period two
Compressor Capacity Control
notes
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Figure 36
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At design conditions, the capacities of the evaporator and this four-compressor
unit balance at a suction temperature of 43°F [6.1°C] and a capacity of 44 tons
[154.7 kW] (A). As the cooling load decreases below this balance point,
assuming a constant condensing pressure, the capacity of the unit decreases
with the falling suction temperature along the four-compressor curve until it
reaches B. Here, the first scroll compressor is shut off and the capacity of the
unit decreases immediately to 30 tons [105.5 kW] (C) along the three-
compressor curve.
As the load continues to decrease, the individual compressors shut off in a
similar manner until the suction temperature reaches a minimum set point and
the final compressor is shut off. The minimum capacity of the four-compressor
unit in this example is 8 tons [28.1 kW].
Excessive starting and stopping of scroll compressors is not a concern. The
reciprocating compressor system on Figure 35 includes a single large
compressor with a single large motor. In contrast, the scroll compressor system
has four small compressors, each with its own small motor. These small motors
are designed to cycle, just like those used with small reciprocating
compressors.
TRG-TRC004-EN
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period two
Compressor Capacity Control
notes
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Figure 37
Slide Valve
The helical-rotary compressor used as the example in this clinic is unloaded
using a slide valve that is an integral part of the compressor housing. Other
helical-rotary compressor designs may use a variety of methods to vary
capacity. Some of these methods are similar in function to the slide valve
presented in this clinic. One major determining factor is whether the
compressor is designed to unload in steps, like a reciprocating compressor, or if
it has variable unloading.
The position of the slide valve along the rotors controls the volume of
refrigerant vapor delivered by the compressor, by varying the amount of rotor
length actually used for compression. By changing the position of the slide
valve, the compressor is able to unload to exactly match the evaporator load,
instead of unloading in steps like the reciprocating compressor discussed
earlier.
26
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period two
Compressor Capacity Control
notes
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Figure 38
At full load, the slide valve is closed. The compressor pumps its maximum
volume of refrigerant, discharging it through the discharge port.
As the load on the compressor decreases, the slide valve modulates toward the
open position. The opening created by the valve movement allows refrigerant
vapor to bypass from the rotor pockets back to the suction side of the
compressor. This reduces the volume of vapor available for the compression
process. It also reduces the amount of rotor length available for compression.
In this manner, the volume of refrigerant that is pumped by the compressor is
varied, unloading it to balance the existing load.
,QOHWꢀ9DQHV
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Figure 39
Inlet Vanes
A common method of modulating the capacity of a centrifugal compressor is to
use a set of vanes installed at the inlet of the compressor impeller. While a
survey of other centrifugal compressor designs shows that there are other
TRG-TRC004-EN
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period two
Compressor Capacity Control
methods of capacity control, many of them function in a manner similar to the
inlet vanes presented in this section of the clinic.
notes
Inlet vanes “preswirl” the refrigerant before it enters the impeller. By
changing the refrigerant’s angle of entry, these vanes lessen the ability of the
impeller to take in the refrigerant. As a result, the compressor’s refrigerant-
pumping capacity decreases to balance with the evaporator load.
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Figure 40
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These curves represent the performance of a typical centrifugal compressor
over a range of inlet vane positions. The pressure difference between the
compressor inlet (evaporator) and outlet (condenser) is on the vertical axis and
compressor capacity is on the horizontal axis. The surge region represents the
conditions that cause unstable compressor operation.
As the load on the compressor decreases from the full-load operating point (A),
the inlet vanes partially close, reducing the flow rate of refrigerant vapor and
balancing the compressor capacity with the new load (B).
Less refrigerant, and therefore less heat, are transferred to the condenser. Since
the available heat rejection capacity of the condenser is now greater than
required, the refrigerant condenses at a lower temperature and pressure. This
reduces the pressure difference between the evaporator and the condenser.
Continuing along the unloading line, the compressor remains within its stable
operating range until it reaches C.
Inlet vanes on a centrifugal compressor allow it to unload over a broad capacity
range while preventing the compressor from operating in the surge region.
28
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period two
Compressor Capacity Control
notes
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Figure 41
Variable Speed
Alternatively, the capacity of a compressor can be controlled by varying the
rotational speed of the compressor motor. This is accomplished using a device
called an adjustable-frequency drive (AFD) or variable-speed drive.
On a reciprocating compressor, this would vary the speed at which the
crankshaft rotates, thus controlling the rate at which the piston travels back and
forth inside the cylinder. On a scroll compressor, this would vary the speed at
which the driven scroll rotates. If applied to a helical-rotary compressor, this
would vary the speed at which the rotors rotate. Applied to a centrifugal
compressor, this would vary the speed at which the impeller rotates.
Although variable-speed capacity control could be applied to all four types of
compressors discussed in this clinic, it is most often applied to centrifugal
compressors. Because speed variation reduces both the flow rate of refrigerant
through the compressor and the pressure differential created by the
compressor, it is used in conjunction with inlet vanes. This requires fairly
complex control strategies to balance refrigerant flow rate, pressure
differential, and load.
TRG-TRC004-EN
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period three
The Compressor in a System
notes
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Figure 42
Period Two presented several methods used to control the capacity of a
compressor. This next section considers the entire system in order to determine
how the capacity of the compressor is controlled to maintain desired space
conditions.
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Figure 43
System-Level Control
The method of controlling compressor capacity to maintain desired space
conditions depends on 1) whether the system is a chilled-water or a direct-
expansion system, and 2) how the airside system responds to changes in space
loads.
Generally, in air-conditioning applications, compressors will be applied in either
a chilled-water or a direct-expansion (DX) system. A chilled-water system
uses water as the cooling media. The refrigerant inside the evaporator absorbs
heat from the water, and this water is pumped to coils in order to absorb heat
30
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period three
The Compressor in a System
from the air used for space conditioning. In contrast, the refrigerant inside the
evaporator of a direct-expansion (DX) system absorbs heat directly from the
air used for space conditioning.
notes
The airside system responds to changing space loads by varying either the
temperature or the quantity of air delivered to the conditioned space. A
constant-volume system provides a constant quantity of variable-
temperature air to maintain the desired conditions in a space. A variable-air-
volume (VAV) system, however, maintains the desired space conditions by
varying the quantity of constant-temperature air.
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Figure 44
Again, a constant-volume system supplies the same quantity of air to the space
and varies the temperature of this air to respond to changing loads.
In this example single-zone, constant-volume DX system, in order to respond to
changing space loads, the capacity of the compressor is controlled by directly
sensing space temperature. The compressor is loaded or unloaded based on
how close the actual space temperature is to the set point temperature.
Loading and unloading the compressor results in a temperature change of the
air leaving the evaporator coil.
TRG-TRC004-EN
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period three
The Compressor in a System
notes
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Figure 45
As mentioned previously, a VAV system varies the quantity of air supplied to the
space in order to satisfy the load. The supply temperature is held constant in
this system.
In a VAV DX system, the capacity of the compressor is controlled by sensing the
temperature of the air being supplied to the system. The compressor is loaded
or unloaded based on how close the actual supply air temperature is to the set
point.
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Figure 46
In contrast to the DX system examples shown previously, a chilled-water
system responds to changing space loads by controlling the capacity of the
chilled-water cooling coil. Although there are various methods of controlling
the capacity of this coil, this discussion will assume the use of a modulating
water valve.
32
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period three
The Compressor in a System
In a VAV chilled water system (shown in Figure 46), the capacity of the chilled-
water coil is controlled to maintain the desired supply air temperature. By
sensing the supply air temperature, a controller varies the flow of water
through the coil by modulating the valve. Varying the water flow maintains the
temperature of the air as the flow rate of the air changes to match the space
load.
notes
In a constant-volume chilled-water system, the capacity of the chilled-water coil
is controlled by directly sensing space temperature and varying the flow of
water through the coil by modulating the valve. Varying the water flow changes
the temperature of the air leaving the coil to match the space load.
In either case, the capacity of the compressor is generally controlled by sensing
the temperature of the water leaving the evaporator and comparing it to the set
point.
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Preventing Evaporator Freeze-Up
In addition to unloading the compressor in order to match the ever-changing
system load, a second system-related concern involves maintaining the suction
temperature above the conditions where evaporator freeze-up may occur. This
can be illustrated by returning to an earlier example. Assume that, in response
to a decreasing load, the capacity of the 40-ton [105.5-kW] scroll-compressor
unit is progressively reduced to a minimum of 8 tons [28.1 kW], corresponding
to a suction temperature of 28°F [-2.2°C] (H). If the load on the evaporator
decreases no further, the suction temperature is maintained within safe
operating limits. However, if the system must be operated at loads below this
minimum stage of unloading, the suction temperature may fall to the point (I)
where evaporator freeze-up can occur.
In a direct-expansion (DX) application, where the refrigerant in the evaporator is
cooling air, a suction temperature of approximately 28°F [-2.2°C] can cause the
moisture that condenses out of the air to form frost on the surface of the
evaporator coil. In a chilled-water application, where the refrigerant in the
evaporator is cooling water, a suction temperature of approximately 30°F
[-1.1°C] can cause the water to freeze inside the evaporator. (This minimum
TRG-TRC004-EN
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period three
The Compressor in a System
suction temperature for a specific application depends on the system operating
conditions and the evaporator design.)
notes
Evaporator freeze protection in a chilled-water application is accomplished by
sensing the temperature of the water in the evaporator. If the water approaches
32°F [0°C], the compressor is shut off to protect the evaporator from freezing.
Most chilled water-equipment includes this protection as part of the controls for
the equipment.
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Figure 48
In a direct-expansion (DX) application, where the refrigerant in the evaporator is
cooling air, frost protection can be accomplished in a number of ways. As
mentioned, if the surface temperature of the coil gets too cold, the moisture
that condenses out of the air can form frost on the surface of the coil. This “coil
frosting” is detrimental to system performance and compressor reliability.
Historically, in DX air-conditioning applications, hot gas bypass, coil pressure
regulators, and defrost cycles initiated by a timer, pressure sensor, or
temperature sensor are a few of the methods that have been used to prevent
evaporator frosting. This clinic will focus on two of these—a defrost cycle
initiated by a temperature sensor and hot gas bypass.
A temperature sensor on the suction line leaving the evaporator is used to
determine if the coil reaches a frosting condition. Compressors are turned off
and the supply fan continues to run to de-ice the coil. Timers prevent the
compressors from rapid cycling.
This control scheme (referred to by Trane as FROSTAT™) is especially well
suited for equipment using scroll compressors, which are designed to start and
stop much more often than large reciprocating compressors.
34
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period three
The Compressor in a System
notes
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Figure 49
Hot gas bypass may be another solution for preventing evaporator frosting in
DX applications. Hot gas bypass diverts hot, high-pressure refrigerant vapor
from the discharge line to the low-pressure side of the refrigeration system.
This added “false load” helps to maintain an acceptable suction pressure and
temperature. Hot gas bypass, however, fails to reduce energy consumption
because it does not allow the compressor to shut off at these low load
conditions.
In a DX application, there are two bypass methods used. The first method
bypasses refrigerant vapor from the compressor discharge line to the inlet of
the evaporator coil. Sensing a decrease in suction pressure, a pressure-
actuated valve opens to bypass hot refrigerant vapor from the compressor
discharge line to the inlet of the evaporator coil, between the expansion valve
and the liquid distributor. This increases the rate at which liquid refrigerant is
boiled off within the evaporator coil and causes the temperature of the
refrigerant leaving the coil to rise. Sensing this increased temperature, the
expansion valve feeds additional refrigerant to the coil, increasing the suction
pressure and temperature.
The principal advantage of hot gas bypass to the evaporator inlet is that the
refrigerant velocity in the evaporator and suction line is higher at low loads.
This promotes a uniform movement of oil through the evaporator coil and
suction piping. When the evaporator is located above the compressor, as
shown, the holdup of oil within the vertical hot-gas-bypass riser must be
considered. Since the flow rate within the hot-gas-bypass line modulates over a
wide range, no size of pipe can ensure adequate velocity to carry oil up the
riser. Oil will collect at the base of the vertical riser when the bypass valve
throttles to lower flow rates. This problem is commonly addressed by adding a
small oil return line between the base of the riser and the suction line.
TRG-TRC004-EN
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period three
The Compressor in a System
notes
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Figure 50
The second method bypasses refrigerant vapor from the compressor discharge
line to the suction line. This method requires the service of an additional
expansion valve, called a liquid injection valve. The remote bulb of this valve is
attached to the suction line near the compressor. When reduced suction
pressure causes the bypass valve to open, the expansion valve senses the
resulting rise in suction temperature (superheat) at its remote bulb. A rising
suction temperature causes this expansion valve to open, mixing liquid
refrigerant with the hot, bypassed refrigerant vapor. The heat content of this
refrigerant vapor causes the liquid refrigerant to evaporate, thus cooling the
mixture. This increase in the refrigerant flow rate stabilizes the compressor
suction pressure (temperature).
The principal advantage of hot gas bypass to the suction line is that the amount
of refrigerant piping is generally less than the other method. A key
disadvantage is that the refrigerant velocity in the evaporator and suction line
drops very low when the bypass valve is open. This creates a problem of oil
hanging up in the evaporator coil and suction piping. For this reason, this
method is not acceptable in applications where the evaporator is located below
the compressor.
When hot gas bypass is applied to a water chiller containing a direct-expansion
evaporator, hot gas bypass to the evaporator inlet is always used. In a direct-
expansion evaporator, liquid refriegerant flows through the tubes and water
fills the surrounding shell. Oil holdup within the tubes can be a problem at part
load when refrigerant velocity is reduced. The increased velocity brought about
by bypassing to the evaporator inlet solves this problem for water chillers.
Finally, when hot gas bypass is applied to a system, the need for condensing
pressure control must be considered. Sufficient condensing pressure must be
available to ensure adequate refrigerant flow to produce a bypass load when
the hot gas bypass valve is to be opened. If a decreasing load is accompanied
by a corresponding reduction in condensing pressure, the hot-gas-bypass valve
may not be capable of bypassing refrigerant vapor at the rate required to
stabilize the suction temperature within reasonable limits. The result is that the
36
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period three
The Compressor in a System
suction temperature falls, and coil frosting or chiller freezing may occur. Since
the hot-gas-bypass valve is sized to pass a given quantity of refrigerant vapor at
a particular condensing–suction pressure difference, some means of
maintaining the condensing pressure within limits must be provided. Various
methods of controlling condensing pressure are discussed in the Refrigeration
System Components clinic.
notes
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period four
Review
notes
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Figure 51
We will now review the main concepts that were covered in this clinic on
refrigeration compressors.
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Figure 52
Period One introduced the four types of compressors commonly used in air-
conditioning applications: reciprocating, scroll, helical-rotary (or screw), and
centrifugal.
The first three types are called positive-displacement compressors. They work
on the principle of trapping the refrigerant vapor and compressing it by
gradually shrinking the volume of the refrigerant. Centrifugal compressors use
the principle of dynamic compression, which involves converting energy from
one form to another, to increase the pressure and temperature of the
refrigerant.
38
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period four
Review
notes
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Figure 53
Period Two reviewed various methods of varying compressor capacity.
Reciprocating compressors typically use cylinder unloaders that match the
compressor capacity to the evaporator load by deactivating piston–cylinder
pairs.
Refrigeration systems using scroll compressors generally unload by using
multiple compressors, cycling them on and off as needed to satisfy the
evaporator load.
A common method of unloading a helical-rotary compressor is to use a slide
valve that is an integral part of the compressor housing. By changing the
position of the slide valve along the compressor rotors, the volume of
refrigerant vapor being delivered by the compressor can be controlled to match
the evaporator load.
Finally, centrifugal compressors generally use inlet vanes to “preswirl” the
refrigerant before it enters the impeller, lessening the ability of the impeller to
take in the refrigerant. As a result, the compressor’s refrigerant pumping
capacity decreases to balance with the evaporator load.
Alternatively, the capacity of any of these types of compressors can be
controlled by varying the rotational speed of the compressor motor. It is most
often applied, however, to centrifugal compressors.
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period four
Review
notes
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Figure 54
Period Three considered the entire system and discussed how the capacity of
the compressor is controlled to maintain desired space conditions.
In a constant-volume DX system, in order to respond to changing loads, the
capacity of the compressor is controlled by directly sensing space temperature.
In a VAV DX system, the capacity of the compressor is controlled by sensing the
supply air temperature. In a chilled-water system, the capacity of the
compressor is typically controlled by sensing the temperature of the water
leaving the evaporator.
Period Three also discussed sensing suction temperature and hot gas bypass as
methods for preventing evaporator freeze-up.
40
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period four
Review
notes
Figure 55
For more information, refer to the following references:
▲ Trane Air Conditioning Manual
▲ Trane Reciprocating Refrigeration Manual
▲ Helical-Rotary Water Chillers Air Conditioning Clinic (Trane literature order
number TRG-TRC012-EN)
▲ Centrifugal Water Chillers Air Conditioning Clinic (Trane literature order
number TRG-TRC010-EN)
▲ Hot Gas Bypass Control Applications Engineering Manual (Trane literature
order number AM-CON10)
▲ ASHRAE Handbook – Refrigeration
▲ ASHRAE Handbook – Systems and Equipment
For more information on additional educational materials available from Trane,
contact your local Trane office (request a copy of the Educational Materials
catalog – Trane order number EM-ADV1) or visit our online bookstore at
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Quiz
Questions for Period 1
1
2
What is the purpose of the compressor in a refrigeration system?
List the four primary types of compressors used in air-conditioning
applications.
3
4
What causes the suction valve to open on a reciprocating compressor?
True or False: The intake of refrigerant vapor in a scroll compressor occurs
at the outer edge of the scroll assembly and discharge occurs through the
port at the center of the scroll.
5
What is the term for the type of compressor that has the motor sealed
within the compressor housing?
Questions for Period 2
6
7
8
Assuming a constant condensing temperature, does the capacity of a
compressor increase or decrease as the suction temperature decreases?
What method of capacity control is commonly applied to scroll
compressors?
What method of capacity control is commonly applied to centrifugal
compressors?
Questions for Period 3
9
In a VAV DX system, the capacity of the compressor is typically controlled
by sensing _____. (space temperature, supply air temperature, chilled-water
supply temperature)
10 In a constant-volume chilled-water system, the capacity of the compressor
is typically controlled by sensing _____. (space temperature, supply air
temperature, chilled-water supply temperature)
11 In a constant-volume DX system, the capacity of the compressor is typically
controlled by sensing _____. (space temperature, supply air temperature,
chilled-water supply temperature)
12 What are the two common methods of preventing evaporator frosting in a
direct-expansion (DX) system?
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Answers
1
To elevate the pressure, and, therefore, the temperature, of the refrigerant
vapor high enough that it can reject heat to air, or some other fluid, at
normally available temperatures.
2
3
Reciprocating, scroll, helical-rotary (or screw), and centrifugal
During the intake stroke, the piston travels away from the discharge valve
and creates a vacuum effect, reducing the pressure within the cylinder to
below suction pressure. Since the pressure within the cylinder is less than
the pressure of the refrigerant at the suction side of the compressor, the
suction valve is forced open and the refrigerant vapor is drawn into the
cylinder.
4
5
6
7
8
9
True
Hermetic or semihermetic
Decreases
Cycling individual scroll compressors on and off
Inlet vanes or variable-speed drive with inlet vanes
Supply air temperature
10 Chilled-water supply temperature
11 Space temperature
12 Sensing the suction temperature and hot gas bypass
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Glossary
adjustable-frequency drive (AFD) A device used to vary the capacity of a
compressor by varying the speed of the compressor motor.
ASHRAE American Society of Heating, Refrigerating and Air-Conditioning
Engineers
centrifugal compressor A type of compressor that uses centrifugal force,
generated by a rotating impeller, to compress the refrigerant vapor.
chilled water system Uses water as the cooling media. The refrigerant inside
the evaporator absorbs heat from the water, and this water is pumped to coils
in order to absorb heat from the air used for space conditioning.
compressor A mechanical device in the refrigeration system used to increase
the pressure and temperature of the refrigerant vapor.
condenser A component of the refrigeration system where refrigerant vapor is
converted to liquid as it rejects heat to air, water, or some other fluid.
constant-volume system A type of air-conditioning system that varies the
temperature of a constant volume of air supplied to meet the changing load
conditions of the space.
cycling The practice of turning a compressor on and off to match the system
load.
cylinder unloader A device used to unload the capacity of a reciprocating
compressor by either closing the suction passage to the cylinder, holding open
the suction valve of a piston–cylinder pair, or diverting the compressed
refrigerant vapor back to the suction side of the compressor.
diffuser passages Passages inside the centrifugal compressor that 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. This kinetic energy is converted to static energy or
static pressure.
direct-expansion (DX) system Uses the refrigerant directly as the cooling
media. The refrigerant inside the evaporator absorbs heat directly from the air
used for space conditioning.
discharge line A pipe that transports refrigerant vapor from the compressor to
the condenser in a mechanical refrigeration system.
dynamic compression A method of compression that involves converting
energy from one form to another to increase the pressure and temperature of
the refrigerant vapor.
enthalpy A measure of heat quantity, both sensible and latent, per pound [kg]
of refrigerant.
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Glossary
evaporator A component of the refrigeration system where cool, liquid
refrigerant absorbs heat from air, water, or some other fluid, causing the
refrigerant to boil.
expansion device A component of the refrigeration system used to reduce the
pressure and temperature of the refrigerant to the evaporator conditions.
flash The process of liquid refrigerant being vaporized by a sudden reduction
of pressure.
heat of compression The amount of heat added to the refrigerant vapor by the
compressor during the process of raising the pressure of the refrigerant to
condenser conditions.
helical-rotary compressor A type of compressor that uses two mated rotors to
trap the refrigerant vapor and compress it by gradually shrinking the volume of
the refrigerant.
hermetic compressor A type of compressor that has the motor sealed within
the compressor housing. The motor is cooled by refrigerant.
hot gas bypass A method used to prevent evaporator freeze-up by diverting
hot, high-pressure refrigerant vapor from the discharge line to the low-pressure
side of the refrigeration system.
impeller The rotating component of a centrifugal compressor that draws
refrigerant vapor into its internal passages and accelerates the refrigerant as it
rotates, increasing its velocity and kinetic energy.
inlet vanes A device used to vary the capacity of a centrifugal compressor by
“preswirling” the refrigerant in the direction of rotation before it enters the
impeller, lessening its ability to take in the refrigerant vapor.
liquid line A pipe that transports refrigerant vapor from the condenser to the
evaporator in a mechanical refrigeration system.
open compressor A type of compressor that is driven by an external power
source, such as an electric motor or a turbine. The motor is coupled to the
compressor crankshaft by a flexible coupling, and a seal is used to prevent
refrigerant from leaking out of the compressor housing.
ported compressor A type of compressor where the refrigerant vapor enters
and exits through ports—no valves are used.
positive-displacement compressor A class of compressors that works on the
principle of trapping the refrigerant vapor and compressing it by gradually
shrinking the volume of the refrigerant.
pressure–enthalpy chart A graphical representation of the properties of a
refrigerant, plotting refrigerant pressure versus enthalpy.
46
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Glossary
reciprocating compressor A type of compressor that uses a piston that travels
up and down inside a cylinder to compress the refrigerant vapor.
refrigerant A substance used to absorb and transport heat for the purpose of
cooling.
rotor The part of the helical-rotary compressor used to trap and compress the
refrigerant vapor. The male and female rotors mesh together, forming pockets
of refrigerant to move through the compressor.
scroll compressor A type of compressor that uses two opposing scrolls to trap
the refrigerant vapor and compress it by gradually shrinking the volume of the
refrigerant.
semihermetic compressor A type of compressor that has the motor sealed
within the compressor housing. The sealed housing may be opened to repair or
overhaul the compressor or motor.
slide valve The part of the helical-rotary compressor used to vary the flow rate
of refrigerant vapor through it.
suction line A pipe that transports refrigerant vapor from the evaporator to
the compressor in a mechanical refrigeration system.
variable-air-volume (VAV) system A type of air-conditioning system that
varies the volume of constant temperature air supplied to meet the changing
load conditions of the space.
variable-speed drive See adjustable-frequency drive.
volute A large space around the perimeter of a centrifugal compressor that
collects refrigerant vapor after compression.
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Literature Order Number
File Number
TRG-TRC004-EN
E/AV-FND-TRG-TRC004-0200-EN
2803-2-385 and 2803-13-587
Inland-La Crosse
The Trane Company
Supersedes
Worldwide Applied Systems Group
3600 Pammel Creek Road
La Crosse, WI 54601-7599
Stocking Location
An American Standard Company
Since The Trane Company has a policy of continuous product improvement, it reserves the right to change
design and specifications without notice.
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