Air Conditioning
Clinic
Refrigeration
Compressors
One of the Fundamental Series
TRG-TRC004-EN
BUSINESS REPLY MAIL
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PERMIT NO. 11 |
LA CROSSE, WI |
POSTAGE WILL BE PAID BY
THE TRANE COMPANY
Attn: Applications Engineering
3600 Pammel Creek Road
La Crosse WI 54601-9985
BUSINESS REPLY MAIL
FIRST-CLASS MAIL |
PERMIT NO. 11 |
LA CROSSE, WI |
POSTAGE WILL BE PAID BY
THE TRANE COMPANY
Attn: Applications Engineering
3600 Pammel Creek Road
La Crosse WI 54601-9985
NO POSTAGE NECESSARY IF MAILED IN THE
UNITED STATES
NO POSTAGE NECESSARY IF MAILED IN THE
UNITED STATES
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Refrigeration Compressors
One of the Fundamental Series
TRG-TRC004-EN
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The Trane Company • Worldwide Applied Systems Group
3600 Pammel Creek Road • La Crosse, WI 54601-7599 www.trane.com
An American-Standard Company
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The Trane Company • Worldwide Applied Systems Group 3600 Pammel Creek Road • La Crosse, WI 54601-7599 www.trane.com
An American-Standard Company
Refrigeration
Compressors
One of the Fundamental Series
A publication of
The Trane Company—
Worldwide Applied Systems Group
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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TRG-TRC004-EN |
Contents
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Introduction ........................................................... |
1 |
period one |
Compressor Types ............................................... |
3 |
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Reciprocating Compressor ...................................... |
4 |
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Scroll Compressor ................................................... |
7 |
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Helical-Rotary (Screw) Compressor ........................ |
10 |
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Centrifugal Compressor ......................................... |
13 |
period two Compressor Capacity Control ........................ |
18 |
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Cylinder Unloaders ................................................ |
19 |
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Cycling On and Off ................................................ |
24 |
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Slide Valve ............................................................ |
26 |
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Inlet Vanes ............................................................ |
27 |
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Variable Speed ...................................................... |
29 |
period three The Compressor in a System ......................... |
30 |
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System-Level Control ............................................ |
30 |
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Preventing Evaporator Freeze-Up ........................... |
33 |
period four |
Review ................................................................... |
38 |
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Quiz ......................................................................... |
43 |
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Answers ................................................................ |
44 |
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Glossary ................................................................ |
45 |
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Introduction
notes
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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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1 |
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.
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period one
Compressor Types
notes
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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
TRG-TRC004-EN |
3 |
period one
Compressor Types
notes
gradually shrinking the volume of the refrigerant. Thus, they are called positive-displacement compressors.
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.
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period one
Compressor Types
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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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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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Compressor Types
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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.
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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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Compressor Types
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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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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.
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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 nearmaximum 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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Compressor Types
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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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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.
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