REFRIGERATIONCYCLE

Compressor

Types

Technical Development Program

Technical Development Programs (TDP) are modules of technical training on HVAC theory, system

design, equipment selection and application topics. They are targeted at engineers and designers who wish to develop their knowledge in this field to effectively design, specify, sell or apply HVAC equipment in commercial applications.

Technical Development Programs (TDP) are modules of technical training on HVAC theory, system design, equipment selection and application topics. They are targeted at engineers and designers who wish to develop their knowledge in this field to effectively design, specify, sell or apply HVAC equipment in commercial applications.

Although TDP topics have been developed as stand-alone modules, there are logical groupings of top-ics. The modules within each group begin at an introductory level and progress to advanced levels. The breadth of this offering allows for customization into a complete HVAC curriculum – from a complete HVAC design course at an introductory-level or to an advanced-level design course. Advanced-level modules assume prerequisite knowledge and do not review basic concepts.

Although TDP topics have been developed as stand-alone modules, there are logical groupings of top-ics. The modules within each group begin at an introductory level and progress to advanced levels. The breadth of this offering allows for customization into a complete HVAC curriculum – from a complete HVAC design course at an introductory-level or to an advanced-level design course. Advanced-level modules assume prerequisite knowledge and do not review basic concepts.

The compressor is one of the four basic components required for the mechanical refrigeration process.

While most system designers will not normally select a compressor by itself, the compressor is one of the major components of the air-conditioning systems that they design. An understanding of compressors and how they operate will help the designer better select and apply air-conditioning units. This TDP has four major sections describing compressors and their operation and application. First, the different refrigeration cycles are discussed. These include the positive displacement cycle, the dynamic cycle (centrifugal cycle), and the absorption cycle. The second section explains terminology associated with compressors and de-scribes design and operational issues that relate to several of the types of compressor designs. The third section explains each of the five major compressor types used in comfort cooling applications in terms of the principle of operation, construction, and performance. The final section addresses system impacts on compressors and how compressors are selected to match the design requirements. A knowledge of the ba-sics of the mechanical refrigeration cycle is necessary to understand this TDP.

© 2006 Carrier Corporation. All rights reserved. The information in this manual is offered as a general guide for the use of industry and consulting engineers in designing systems. Judgment is required for application of this information to specific installations and design applications. Carrier is not responsible for any uses made of this information and assumes no responsibility for the performance or desirability of any resulting system design.

The information in this publication is subject to change without notice. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, for any purpose, without the express written permission of Carrier Corporation. Printed in Syracuse, NY

CARRIER CORPORATION Carrier Parkway Syracuse, NY 13221, U.S.A.

Table of Contents Introduction.....................................................................................................................................................1

Air Conditioning and Refrigeration............................................................................................................1 The Refrigeration Cycle .............................................................................................................................2

Four Basic Principles of the Mechanical Refrigeration Cycle ...............................................................3 Refrigeration Cycles .......................................................................................................................................5

Positive Displacement Vapor Compression Cycle .................................................................................6 Positive Displacement p-h Cycle Diagram...........................................................................................10

Reverse Cycle Positive Displacement ......................................................................................................11 Heat Pump p-h Cycle Diagram ............................................................................................................13

Dynamic (Centrifugal) Refrigeration Cycle .............................................................................................13 Dynamic Cycle p-h Cycle Diagram .....................................................................................................16

Absorption Cycle......................................................................................................................................17 Single-Effect Absorption Cycle ...........................................................................................................19 Double-Effect Absorption Cycle..........................................................................................................21 Equilibrium Diagram............................................................................................................................23 Cycle Diagram......................................................................................................................................24 Capacity Control ..................................................................................................................................26 Purge Cycle ..........................................................................................................................................27 Dilution Cycle ......................................................................................................................................27 Application...........................................................................................................................................28

Compressor Terms and Ratings ....................................................................................................................29 Compressor Designation...........................................................................................................................29 Compressor Terms....................................................................................................................................29

Compressor Efficiency Terms..............................................................................................................32 Measures of Performance.....................................................................................................................33 Displacement........................................................................................................................................33 Mass Flow ............................................................................................................................................34 Compression Ratio ...............................................................................................................................34 Compressor Capacity ...........................................................................................................................35 Riding with Load..................................................................................................................................36

Capacity Control Factors ..........................................................................................................................38 Compressor Displacement....................................................................................................................38 Refrigerant............................................................................................................................................39 Pressure Levels.....................................................................................................................................39

Compressor Applications..............................................................................................................................40 Compressor Motors ..................................................................................................................................40 Compressor Lubrication ...........................................................................................................................42 Compressor Motor Cooling ......................................................................................................................43 Compressor Protection..............................................................................................................................43

High-Pressure Protection......................................................................................................................43 Low-Pressure Protection ......................................................................................................................44 Overcurrent Protection .........................................................................................................................44 Pressure Relief .....................................................................................................................................45 High Discharge Gas Temperature ........................................................................................................45 Short Cycle Protection .........................................................................................................................46 Low Voltage Protection........................................................................................................................46 Phase Reversal Protection ....................................................................................................................46 Crankcase Heater..................................................................................................................................46 Oil Protection .......................................................................................................................................47

Liquid Refrigerant Issues..........................................................................................................................47 Slugging ...............................................................................................................................................47 Floodback.............................................................................................................................................48 Flooded Starts.......................................................................................................................................48

Sound and Vibration.................................................................................................................................49 Compressor Testing and Certification ......................................................................................................49

Compressor Types ........................................................................................................................................ 50Rotary Vane and Rolling Piston Compressors ......................................................................................... 50

Principle of Operation .......................................................................................................................... 51 Construction ......................................................................................................................................... 53 Performance ......................................................................................................................................... 53 Applications ......................................................................................................................................... 55 Capacity Control .................................................................................................................................. 55

Reciprocating Compressors...................................................................................................................... 55 Principle of Operation .......................................................................................................................... 55 Types.................................................................................................................................................... 60 Construction ......................................................................................................................................... 60 Performance ......................................................................................................................................... 61 Application........................................................................................................................................... 62 Capacity Control .................................................................................................................................. 62 Accessories........................................................................................................................................... 64

Scroll Compressors................................................................................................................................... 65 Principle of Operation .......................................................................................................................... 65 Construction ......................................................................................................................................... 66 Performance ......................................................................................................................................... 67 Application........................................................................................................................................... 67 Capacity Control .................................................................................................................................. 68 Accessories........................................................................................................................................... 68

Screw Compressors .................................................................................................................................. 68 Operation.............................................................................................................................................. 69 Construction ......................................................................................................................................... 70 Performance ......................................................................................................................................... 70 Application........................................................................................................................................... 71 Capacity Control .................................................................................................................................. 71 Accessories........................................................................................................................................... 72

Centrifugal Compressors .......................................................................................................................... 73 Operation.............................................................................................................................................. 73 Construction ......................................................................................................................................... 74 Performance ......................................................................................................................................... 74 Application........................................................................................................................................... 75 Capacity Control .................................................................................................................................. 75

Compressor Operation In Systems................................................................................................................ 76 Refrigerants .............................................................................................................................................. 76 Type of Air-Conditioning System and Compressor Impact ..................................................................... 76

Constant Volume (CV) Systems .......................................................................................................... 77 Variable Air Volume (VAV) Systems ................................................................................................. 77 Chilled Water Systems......................................................................................................................... 78 Heat Pumps .......................................................................................................................................... 78 Low Temperature Refrigeration........................................................................................................... 79

Impacts of Part Load Control ................................................................................................................... 80 Evaporator Coil Frost Control.............................................................................................................. 80 Hot Gas Bypass .................................................................................................................................... 81 Head Pressure Control.......................................................................................................................... 83

Selecting Compressors.................................................................................................................................. 84 Selection Input Data ................................................................................................................................. 84 Example Selection .................................................................................................................................... 85

Summary....................................................................................................................................................... 87 Work Session 1 – Cycles .............................................................................................................................. 89 Work Session 2 ............................................................................................................................................. 91 Work Session 1 – Answers ........................................................................................................................... 94 Work Session 2 – Answers ........................................................................................................................... 95 Glossary ........................................................................................................................................................ 96

COMPRESSOR TYPES

Introduction Compressors are one of the four basic system components required in the mechanical refrig-

eration process. The other three components are the evaporator, condenser, and metering device. The compressor’s function is to create the pressure differential and resulting saturated tempera-ture (pressure) difference between the evaporator and condenser so that heat transfer can take place. This module will discuss the types of compressors, their construction, application, part load control and performance in a system.

This module begins with a description of the two basic vapor compression refrigeration cy-cles and how the compressor fits into the cycle. The absorption cycle is also covered in this section. Even though the absorption cycle does not use a compressor, the cycle still operates on the vapor compression principle. This TDP will not cover additional construction or features of absorption units. Subsequent sections of this module address the vocabulary associated with compressors, a description of the construction, operation and control of the five basic compressor types, and criteria used in compressor selection. An understanding of the basic refrigeration cycle is assumed. To review these principles see TDP-400 and TDP-401, Principles of Mechanical Re-frigeration, Level 1: Introduction and Level 2: Analysis.

Compressors are frequently packaged with other refrigerant system components and systems. Designers are rarely required to select only the compressor. However, the compressor is an inte-gral part of comfort system performance and design. A thorough knowledge of compressors will allow the designer to better apply the packaged units containing compressors, such as rooftop units, chillers, and water source heat pumps.

Air Conditioning and Refrigeration

A good place to start learning about com-pressor types is by defining two important terms, air conditioning and refrigeration. Air conditioning is often considered to be cooling the indoor air for the comfort of people; how-ever, it is much more. Air conditioning is the process of treating air to control simultaneously its temperature, humidity, cleanliness, and dis-tribution to meet the requirements of the conditioned space. See Figure 1. This process can be applied to controlling the air for the pur-pose of providing comfort or to provide conditioning for process applications like food storage or manufacturing.

.

Figure 1 Requirements of an Air-Conditioning System

Refrigeration Cycle

1

COMPRESSOR TYPES

The term refrigeration is often considered to be controlling temperature for food preservation or industrial processes. Refrigeration is the process of using a volatile fluid to absorb heat from a

lower temperature place, raising the fluid’s pressure and temperature, so that then the heat can be rejected to a higher temperature place. The mechanical refrigeration process is the way cooling is provided by the air-conditioning system. The process of transferring heat is the mechanical refrigeration process.

In this TDP module we will learn how compressors are used in the process of transferring heat. This heat transfer may be for comfort cooling refrigeration or for process cooling, at the associated temperature levels re-quired for these applications. The impact on the compressor choice for each of these applications is dif-ferent and will influence the compressor design and options. This module is concerned primarily with com-

fort cooling applications at temperature and pressure levels associated with comfort cooling duty.

Definition

Air conditioning is controlling simultaneously the temperature, humidity, cleanliness, and distribution of air. Mechanical refrigeration is the process of using a volatile fluid to absorb heat from a lower temperature place, raising the fluid’s pressure and temperature so it can be rejected to a higher temperature place.

In this TDP, when the text refers to “air conditioning,” it implies comfort-cooling duty. When “refrigeration” is used, it means the mechanical refrigeration process. The text will specifically state when the discussion applies to low temperature or process applications.

The Refrigeration Cycle

The compressor is just one component in a refrigeration system, so it is important to under-stand the refrigeration cycle before beginning an in depth discussion of the compressor. In order to fully understand compressor opera-tion, you must understand the basic refrigeration cycles. The refrigeration cycle that is used for almost all com-fort cooling applications is the vapor compressor cycle.

Figure 2 Comfort Cooling Using a Direct Expansion (DX) System

There are three basic types of va-por compression refrigeration cycles: positive displacement, dynamic (cen-trifugal), and absorption.

The vapor compression refrigera-tion cycle transfers heat from the space or process load in one of two ways: direct expansion systems or in-direct systems.

Positive displacement cycles are used for both direct expansion systems and indirect expan-sion systems. Dynamic and absorption cycles are used only with indirect systems.

In a direct expansion (DX) system, the air or water from the space or load passes over a series of tubes. Inside the tubes, liquid refrigerant evaporates (boils), absorbing the heat. The compres-sor draws the resulting vapor out of the evaporator, and then raises its pressure and temperature above the condensing pressure and temperature by compressing it. The vapor is discharged into

Refrigeration Cycle

2

COMPRESSOR TYPES

the condenser where the hot vapor refrigerant cools and condenses into a warm liquid as its heat is released to the air or water passing through the condenser. A metering device provides the pressure drop from saturated condensing pressure to saturated evaporating pressure and regulates the flow of refrigerant to the evaporator. The cycle is then repeated. See Figure 2.

The indirect system transfers heat from the space or load to a secondary fluid, normally water. For this reason, indirect systems are often referred to as chilled water systems. The heat from the space load is transferred to the chilled water coils in an air han-dler or fan coil unit. The water from the coil is then pumped back to the chiller where the water is cooled using the same vapor compression process as in the direct expansion (DX) sys-tem. The heat rejection by the chiller may be to the outdoor air or by a water-cooled condenser normally connected to a cooling tower. See Figure 3.

Figure 3 Comfort Cooling Using and Indirect System

In the indirect system the air or process is cooled by water that in turn is cooled by the refrig-eration cycle. Both direct and indirect systems rely on a compressor to raise the pressure from the evaporator level to the condenser level.

Vapor compression cycles may be classified as mechanical or non-mechanical. See Figure 4. Mechanical cycles use a compressor. Positive dis-placement and dynamic cycles are mechanical vapor compression cycles. The absorption cycle is non-mechanical compression.

Figure 4 Vapor Compression Cycles

Four Basic Principles of the Mechanical Refrigeration Cycle

The mechanical refrigeration cycle is a process of transferring thermal energy from a low temperature source to a higher temperature sink. To understand this process, four principles can be used to explain how each component functions to transfer thermal energy.

The first principle is that heat transfer can only occur from places of higher temperature to places of lower temperature. For the refrigeration process, this means that to remove heat from the air circulating over the evaporator, the refrigerant temperature needs to be lower than the required discharge air temperature to control temperature and humidity in comfort cooling. The

Refrigeration Cycle

3

COMPRESSOR TYPES

discharge air temperature and the refrigerant temperatures must be below the required dew point temperature. Also, for the condenser to reject heat, the temperature of the refrigerant needs to be higher than the temperature of the air or water that the heat will be rejected to.

The second principle is that when refrigerant changes state (liquid to vapor or vapor to liquid) it absorbs or re-jects a large quantity of heat without a change in its temperature. In fact, the heat transfer that occurs as a re-sult of a change of state is much greater than the heat transfer that occurs by heating or cooling a liquid or va-por. Since this is the case, the ideal coil would have as much of its heat transfer surface dedicated to change of phase as possible. The change of phase allows the me-chanical refrigeration cycle to move large quantities of heat per pound of refrigerant.

Third, there is a direct relationship between pressure and the evaporating (boiling) temperature. If the pressure is controlled, then the boiling point can also be controlled. This principle is used in the in refrigeration process to cause a refrigerant to boil at a low temperature and pres-sure in the evaporator where it will absorb heat. The principle also applies to the condenser, where pressure is increased high enough so that the boiling temperature is above the condenser air temperature. When the refrigerant gives up its heat to the air, the refrigerant condenses back into a liquid.

The 4 principles from mechanical refrigeration:

1. Heat only moves from a higher temperature to a lower temperature

2. A large amount of energy is required to change the state of matter

3. The temperature and energy required to change state are a function of pressure

4. Fluid flow only occurs if a pressure difference exists

Fourth, for fluids to flow there must be a pressure difference. Therefore, a low pressure con-dition must be created which will cause the refrigerant to flow out of the evaporator. The compressor creates a suction to draw the refrigerant vapor out of the evaporator. The same is true in the condenser where a higher pressure is required at the compressor discharge to force the re-frigerant through the condenser.

With these four principles, the conditions which exist at any point in the mechanical refrig-eration cycle can be explained.

The pressure-enthalpy diagram is a useful tool to explain the refrigeration cycle. The per-formance of the components used in the mechanical refrigeration cycle are based on the total heat transfer per pound of refrigerant. The total heat content per pound is called enthalpy (h Btu/lb). A diagram that plots enthalpy against absolute pressure is called the pressure-enthalpy (p-h) dia-gram. Each refrigerant has a unique pressure-enthalpy diagram. This diagram plots the properties of refrigerants with pressure (vertical axis) against the enthalpy (horizontal axis). The chart can be used to determine enthalpy in all three operating zones: subcooled, mixed phase, and super-heated.

Refrigeration Cycle

4