tdp_06-796-084_preview

(1)

REFRIGERATION

CYCLE

Principles of

Mechanical

Refrigeration

Level 2:

Cycle Analysis

Technical Development Program

(2)

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 topics. 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.

This is the second module devoted to the mechanical refrigeration cycle. In the first module the basic cycle is explained in terms of the four basic principles and the four basic components that are

required for the refrigeration cycle. The first module also introduced the terminology used to de-scribe this process. Module two assumes this information is understood. In this module the pressure-enthalpy diagram is developed and it is used to explain and analyze the mechanical

refrig-eration process. The pressure-enthalpy diagram is the tool used by designers of HVAC equipment to determine the size and performance of each system component. Most HVAC system designers do not select each of these components; particularly since packaged type equipment has become the norm. However, the pressure-enthalpy diagram gives the system designer a very useful tool for understanding refrigeration system modification or options and their impact on cycle performance. The diagram is also a valuable tool for troubleshooting the refrigeration cycle. This TDP module will develop the required understanding of the pressure-enthalpy diagram. This TDP has five major sections describing the pressure-enthalpy diagram and demonstrating how it is used for refrigeration cycle analysis. First the pressure-enthalpy diagram is developed. The second section explains the use of refrigerant data tables. The third section explains the mechanical refrigeration cycle by plotting it on a pressure-enthalpy diagram. The final two sections use the pressure-enthalpy diagram

to analyze several common cycles and demonstrate its use with examples.

The information in this m anual 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 r esulting system design.

The information in this publication is subject to change without notice. No part of this publication may be reproduced or transmit-ted 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

(3)

Introduction

This is the second TDP module concerning the study of the mechanical refrigeration cycle. The first module introduced you to the mechanical refrigeration process and explained the key refrigeration terms. This module continues the study of the mechanical refrigeration process and develops the use of the pressure-enthalpy diagram to explain mechanical refrigeration. The pressure-enthalpy diagram (p-h) is a very useful tool for describing, analyzing, and

troubleshoot-ing mechanical refrigeration systems.

The first section of this TDP reviews two key concepts from the first module - the principles and basic components of refrigeration systems. These concepts will be built upon in this book. If the concepts from the first module (TDP-400, Principles of Mechanical Refrigeration, Level 1: Introduction) are not clear, it is recommended that you review the first module before beginning this module. This TDP starts by developing the p-h diagram and then explains the basic cycle using the p-h diagram. The last two sections use the p-h diagram to explain common operating conditions and options encountered in the refrigeration cycle. The p-h diagram is used to explain conditions such as part load, head pressure control , and the use of system options to improve performance and control. The last section analyzes four different system types and describes how

each of the system components is selected.

It is important for a designer to understand the operation of the refrigeration system. As the use of packaged units has become typical, designers may not need to select each individual component of the refrigeration system. However, proper selection and application of all equip-ment depends on a working knowledge of the mechanical refrigeration process. This module will help the reader develop an understanding of refrigeration systems and will demonstrate how to analyze a refrigeration system’s performance. When a designer understands the p-h diagram, it can be used to evaluate the performance benefits of different refrigeration system options, such as adding a desuperheater . The p-h diagram can also be used to check a system’s performance and determine if components of the system are working properly.

Four Principles of Refrigeration

As you may recall from module one (TDP-400, Principles of Mechanical Refrigeration, Level 1: Introduction), four basic principles can be used to explain the mechanical refrigeration process. See Figure 1. Each component of the system uses one or more of these principles in its operation.

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

Figure 1

Four Principles of System Operation

The four basic principles can be summarized as:

Heat energy will only flow from a warmertemperatureto a cooler temperature.

Refrigeration Cycle

(6)

Mechanical Refrigeration

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.

Two types of heat energy exist in a substance, sensible and latent. Thelatent heat , which is the

changes of state, is the larger amount.

The temperature at which a change of state occurs is dependent on the pressure.

Flow of aliquid or vapor will occur only if a pres-sure difference exists, unless mechanical work is added.

Four Components of Mechanical Refrigeration System

As shown in Figure 2, the mech-anical refrigeration cycle requires four components for continuous operation. The first component, a section where the refrigerant absorbs the heat from the fluid being cooled, is called the evaporator. The second component, a device that raises the vapors pressure to a temperature where it can be condensed, is called the compressor . The third component, a section where the absorbed heat is rejected from the refrigerant, is called the condenser . The fourth component, a device that controls the flow and drops the refrig-erant pressure back to the temperature required in the evaporator, is called the metering device.

Figure 2

Four Components of a Mechanical Refrigeration System

Temperature and Enthalpy

We will use water as our refriger-ant to explain the concept of the p-h diagram. The relationship between temperature and enthalpy (total heat ) was introduced in the Level 1 module also by using water. Let’s use the chart introduced in the Level 1 mod-ule. This chart is a convenient way to visualize the total heat content in a substance as it is heated and cooled. See Figure 3.

Our chart will explain several important refrigeration terms. As ice melts, the latent heat involved is called the heat of fusion. The latent

Figure 3

Temperature-Enthalpy Chart

Refrigeration Cycle

(7)

heat involved in changing a liquid to a vapor is called the latent heat of vaporization. This is sometimes also referred to as the latent heat of condensation when the process condenses the vapor to a liquid. Enthalpy is a measure of the total heat content. This includes both the latent and sensible heat energy.

When the vapor is heated above the boiling point, it is referred to as superheated. When the liquid is at a temperature below the boiling point, it is referred to as subcooled. Superheat is the number of degrees above the boiling point. Subcooling is the number of degrees below the boiling point. For example, water at 180º F could be said to have 32 degrees of subcooling. (212º F –180º F = 32º F)

Two other important points are on our chart in Figure 3. The point at which the liquid just be-gins to turn to a vapor is called saturated liquid . When heat is added to a fluid so it has entirely evaporated, the fluid reaches a point identified as saturated vapor . The fluid is saturated with all the heat it can contain at that pressure and still remain at the saturation temperature.

After the change of state has been completed, any additional heat results in a rise in tempera-ture of the vapor. This additional heat is called superheat. Between saturated liquid and saturated vapor exists a mixture of liquid and vapor.

Notice the heat energy involved in each step of the process, 970 Btu are required to change liquid to a vapor, 144 Btu are required to change solid ice to liquid, 1 Btu is required to change the liquid temperature 1 degree and 0.45 Btu are required to change the vapor temperature 1 degree. The greatest heat energy is in changing from a liquid to a vapor or vapor to a liquid.

Refrigerants

We used 32º F as our datum point where the enthalpy is zero. In fact, all matter contains heat or energy down to a temperature of absolute zero (

−

460º F), but, for convenience purposes, other

datum lines are selected arbitrarily.

Because water freezes at 32º F, the datum line for water could be taken at 32º F. Any values of heat content below 32º F are thus negative. With many common refrigerants the datum line would be much lower.

To avoid the use of heat contents with negative values down to lower temperatures, the heat content of most refrigerants is taken at a datum line of

−

40º F.

Total energy in a fluid involves not only the internal energy represented by heat, but also the external energy or work required to reach that state. The sum of these energies is called enthalpy. Since it is the more accurate term, enthalpy is used in refrigerant tables. Enthalpy is used in this text to indicate total heat content.

We should recall that the function of the refrigerant is to carry heat. So the most important property a refrigerant can have is to absorb lots of heat as it changes from a liquid to a vapor.

Water is very good for this change of state. But water has other properties that make it less suit-able as a refrigerant. One of the four principles is that boiling points change with pressure. To get water to boil at temperatures low enough to absorb heat for comfort cooling requires pressures well below atmospheric pressure. These low pressures result in some other design and operational problems.

Refrigeration Cycle

(8)

1. Non-toxic and non-flammable 2. Reasonable operating pressures 3. Leakage resistance

4. Large heat of vaporization 5. Relatively low specific volume

6. Low liquid specific heat (reduced flash gas) 7. Easy to detect leaks

8. Compatible with oils (vapor side) 9. High coefficient of heat transfer 10. Easy to handle and cost effective 11. Non-corrosive and chemically stable 12. Environmentally sound

Safe • Efficient • Stable • Cost Effective • Compatible

Figure 4

Properties of a Good Refrigerant

The characteristics of refrigerants are shown in Figure 4. There is no one perfect refrigerant that has all the properties listed in Figure 4. As a result, the refrigerants used are a compromise between different re-quired properties. The best refrigerant to use is dependent on the application, the cycle, the refrigerant type, and the influence that the refrigerant has on the design and performance of the system components.

In module one we looked at the choices available for refrigerants. These choices are summa-rized in Figure 5. The refrigerants that contain chlorine are being phased out because of their adverse effect on the atmospheric

ozone layer. This has lead to even greater reliance on refrigerants that are blends. By blending different refrigerants, the characteristics of the mixed refrigerant can be changed to have characteristics that better meet the needs of a given application.

Figure 5

Refrigerant Choices Based on Environmental Impact

In this TDP we will use HCFC R-22 for many of our descriptions and examples. That is because it is still in wide use. However, in many cases, other refrigerants, primarily HFC R-410A, are also shown to illustrate the same principle with a non-ozone depleting refrigerant. Blend refriger-ants have some characteristics (in terms of saturation temperatures and pressures) that influence some of the calculations in this TDP. When the use of a blended refrigerant will change the calculation procedure, we will illustrate its use.

Refrigeration Cycle

tdp_06-796-084_preview

REFRIGERATION

REFRIGERATION

CYCLE

CYCLE

Principles of

Principles of

Mechanical

Mechanical

Refrigeration

Refrigeration

Level 2:

Level 2:

Cycle Analysis

Cycle Analysis

Technical Development Program

Table of Contents

Introduction

Four Principles of Refrigeration

Four Components of Mechanical Refrigeration System

Temperature and Enthalpy

Refrigerants

−

−