Vapor Compression Heat Pumps with Refrigerant Mixtures

Reinhard Radermacher, Yunho Hwang

• Publisher: CRC Press
• Year: 2005
• Print ISBN: 9780367454111
• eBook ISBN: 9781000611274
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Construction & Architectural Engineering

Amidst tightening requirements for eliminating CFC's, HCFC's, halons, and HFC's from use in air conditioning and heat pumps, the search began for replacements that are environmentally benign, non-flammable, and similar to the banned refrigerants in system-level behavior. Although refrigerant mixtures have long been used in commercial products to improve environmental impact, there are few resources available that address the use of fluid mixtures in vapor compression systems. Vapor Compression Heat Pumps with Refrigerant Mixtures provides a comprehensive background and thorough discussion of the thermodynamics of working fluid mixtures and their applications. It covers the fundamentals of various refrigeration cycles as well as a basic background in the thermodynamics related to these mixtures. It also provides important data on heat transfer and pressure drop correlations as well as critical operational issues related to refrigerant mixtures.

1 Introduction

Due to growing environmental awareness and resulting concerns, refrigerants — the working fluids for refrigeration systems, heat pumps and air conditioners — have gained considerable attention. Well-known, proven fluids were banned and replaced with new ones. In the process, refrigerant mixtures were introduced to achieve acceptable properties reasonably well-matched to those of the fluids to be replaced. The thermodynamics of mixtures is considerably more complex than that of pure fluids. This text is dedicated to introducing the thermodynamic theory and background of working fluid mixtures and their practical implementations. It discusses the opportunities and challenges and aims at providing the practicing engineer, as well as the aspiring engineering student, with all the necessary information to successfully design systems that take the best possible advantage of fluid mixtures. Emphasis is placed on practical experience from laboratory investigations.

1.1 HEAT PUMPING

On many occasions, a fluid needs to be cooled below the temperature of the surroundings. Examples are the production of chilled water or brine for air-conditioning and refrigeration purposes or the cooling of air to condition a space. In this context, the term “cooling” usually implies a certain quantity of energy is extracted from a fluid while reducing its temperature. This energy is rejected to a second fluid at a higher temperature than that of the first fluid. For example, in a domestic refrigerator, energy is extracted from the air inside the cabinet, reducing its temperature and cooling the cabinet. In turn, energy is rejected from the refrigeration unit to the surrounding air, heating the exterior. When energy transfer is based on a temperature gradient, one speaks of heat transfer. In this strict sense, “heat” exists only as long as a temperature gradient enables this transfer of energy. However, in the field of refrigeration and air-conditioning, the term “heat” is used more loosely and is understood as energy in more general terms. This text adheres to this latter convention.

In general, the machines that accomplish the lift of a certain quantity of energy from a lower temperature level to a higher one are termed heat pumps. The thermodynamic processes involved in heat pumping are governed by the First and Second Laws of Thermodynamics. The First Law is the conservation of energy and reads

$\sum {\text{Q}}_i+W=0(1.1)$

where Q stands for the amounts of heat exchanged at various temperature levels T_i and W for the amount of work that is required in the process.

The Second Law states that heat cannot flow from a lower to a higher temperature without the expenditure of energy. One way of expressing this statement is shown in the following equation.

$\sum {\displaystyle \frac{{\text{Q}}_i}{T_i}}=0\text{for}\text{reversible}\text{processes}(1.2)$

Equation 1.1 is based on the assumption that the energy supplied to a system is counted as positive.

Figure 1.1.1 illustrates the situation. The vertical axis is the temperature axis with increasing temperature from the bottom to top, i.e., T₁ < T₂. A certain amount of heat is removed by the device M at temperature T₁ and, by using work that is supplied to M, heat is rejected at T₂. The amount of heat rejected is the sum of the amount of work W required by M and the amount of heat removed at T₁. This is a consequence of the First Law,

$Q_1+W=Q_2(1.3)$

However, does this relationship of Figure 1.1.1 fulfill the Second Law, Equation 1.2? Applying Equation 1.2 to the situation of Figure 1.1.1 yields

${\displaystyle \frac{Q_1}{T_1}}?{\displaystyle \frac{Q_2}{T_2}}=0(1.4)$

and using Equation 1.3,

${\displaystyle \frac{Q_1}{T_1}}?{\displaystyle \frac{Q_1+W}{T_2}}=0(1.5)$

FIGURE 1.1.1 Heat Pumping — The process of lifting energy Q₁ from a temperature level T₁ to T₂, releasing Q₂. The input of energy, in this case work W, is required.

After rearranging the terms, we find

${\displaystyle \frac{Q_1}{T_1}}?{\displaystyle \frac{Q_1}{T_2}}?{\displaystyle \frac{W}{T_2}}=0(1.6)$

Since T₂ > T₁, the difference of the first two terms is still larger than zero. Equation 1.6 requires that the Second Law is fulfilled only if the work input is sufficiently large. Thus, there is a minimum work requirement. This minimum amount of work is required for the system to operate reversibly. The Second Law does not say anything about an upper limit for the work, so the system may be very inefficient and operate irreversibly if W becomes much larger than necessary. There is no upper limit on the degree of irreversibility for the heat pumping operation.

The First and Second Laws state for lifting heat from one temperature level to a higher one, the expenditure of energy is required and that the amount of energy rejected at the higher temperature level is the sum of the heat removed from the lower temperature level plus the energy added to accomplish the lift. The energy required to drive the system can be supplied in the form of work, as assumed here, or it can be supplied in the form of heat. Examples are absorption heat pumps, desiccant systems and engine-driven heat pumps.

One of the most common implementations of a heat pump is shown in Figure 1.1.2. Liquid refrigerant evaporates in the evaporator at a given pressure level while it absorbs heat. In other terms, this is the cooling effect. The compressor compresses the vapor that exits from the evaporator and delivers the compressed, high-pressure vapor to the condenser where the vapor is cooled and condensed. The liquefied vapor leaves the condenser and enters the expansion valve. This valve reduces the pressure level and meters the fluid so that as much refrigerant enters the evaporator as the compressor removes.

The concept shown in Figure 1.1.2 represents a vapor compression heat pump. It is the underlying concept of the type of heat pumps discussed in this text. All of these systems have to fulfill the First and Second Law in the form of the Equations 1.1 and 1.2.

It should be noted that although both the First and Second Law must be observed in heat pump design, the reader will notice that in subsequent chapters energy and mass balances are discussed but rarely entropy balances. This implies that the Second Law could be violated in idealized situations. However, in all examples we are using measured data of actual working fluids that do exist. These data inherently ensure that the Second Law is not violated.

FIGURE 1.1.2 Schematic of a vapor compression heat pump (or refrigeration system).

The following sections provide an overview of various heat pump applications. Note that whenever energy is lifted from a lower temperature to a higher temperature, we speak in general of a heat pumping process. However, for many applications, the process is not as important as its result. In all refrigeration and air-conditioning applications, energy is removed from a lower temperature level and rejected at a higher temperature level. The main effect that interests the user is the refrigeration or cooling effect, i.e., heat removal. On the other hand, in what is customarily called heat pumping, the user is interested in the heat delivered at the high temperature level. In both cases, the underlying thermodynamic process is the same; only the application or the desired benefit are different. In this text, the term heat pumping is used as the general description for the thermodynamic process independent of the application. The terms refrigeration and air-conditioning refer specifically to the application.

1.2 OVERVIEW OF CURRENT PRODUCTS

Vapor compression heat pump systems have found many applications spanning a wide range of capacities. This chapter will give an overview of applications, system configurations and the economic importance of vapor compression systems.

The global market for air-conditioning and refrigeration equipment is estimated to be $42 to 45 billion, with the United States and Japan each covering one third of the worldwide production followed by Europe and China. In terms of applications, the market is split into thirds as well. One third covers residential air-conditioning, one-third refrigeration (mostly food preservation), and one-third commercial air-conditioning. In the United States, stationary refrigeration and air-conditioning systems are used for 45 million homes and commercial buildings as well as 100 million refrigerators and 30 million freezers. Mobile refrigeration and air-conditioning systems are used for 90 million air-conditioned cars and trucks and 200,000 refrigerated trucks and rail cars. In 1985, approximately 420 million pounds of refrigerants were used for these applications.

1.2.1 Residential Air Conditioners And Heat Pumps

In the United States, residential air-conditioning has become very common and is even used in the colder regions of the north. The units are usually “split” systems, referring to the fact that the air conditioner is split into two units. The evaporator together with the expansion device is located inside the building and the condensing unit, consisting of compressor and condenser, is located outside. The piping connection is installed in the field, which is also where the system is charged with refrigerant. The cooling capacity ranges from 3 to 18 kW (1 to 5 refrigeration tons). Around 18% of these units are designed as so-called heat pumps (ARI, 2000). They have a four-way reversing valve that allows the same unit to serve as an air conditioner in the summer months and as a heat pump in the winter. Since traditionally the system is designed and sized for the air-conditioning function, the heat pumps turn out to have insufficient heating capacity in most climates. Supplemental heat in the form of electric or gas heat is usually required.

Another form of residential air-conditioning is the window air conditioner. It is designed to cool just one room and the largest capacity usually does not exceed 7 kW (2 refrigeration tons). All the components are integrated within one chassis tha...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Preface
• Acknowledgment
• About the Authors
• Table of Contents
• Chapter 1 Introduction
• Chapter 2 Properties of Working Fluids
• Chapter 3 Vapor Compression Cycle Fundamentals
• Chapter 4 Methods for Improving the Cycle Efficiency
• Chapter 5 Experimental Performance Measurements
• Chapter 6 Refrigerant Mixtures in Refrigeration Applications
• Chapter 7 Refrigerant Mixtures in Heat Pump Applications
• Chapter 8 Heat Transfer of Refrigerant Mixtures
• Chapter 9 Operational Issues
• Index