BASICS OF MECHANICAL ENGINEERING (ME-101 F)Lectures on

REFRIGERATION AND AIR-CONDITIONING

ByPriyanka Singh

M.Tech (Applied Mechanics)Motilal Nehru National Institute of Technology, Allahabad

•Introduction to RAC•Rating of refrigeration machines•Coefficient of performance•Simple refrigeration cycle•Vapour compression cycle•Psychrometric charts and its use.•Human comforts

Process of producing and maintaining temperature of a region below the surrounding temperature by transfer of heat from lower temperature regions to higher temperature ones is called refrigeration. Devices that produce refrigeration are called refrigerators, and the cycles on which they operate are called refrigeration cycles. The working fluids used in refrigerators are called refrigerants. Refrigerators used for the purpose of heating a space by transferring heat from a cooler medium are called heat pumps.

Introduction

The performance of refrigerators and heat pumps is expressed in terms of coefficient of performance (COP), defined as

The standard of comparison for refrigeration cycles is the reversed Carnot cycle. A refrigerator or heat pump that operates on the reversed Carnot cycle is called a Carnot refrigerator or a Carnot heat pump, and their COPs are

Refrigerator and Heat Pump Objectives

(fig. 10-1)

The objective of a refrigerator is to remove heat (QL) from the cold medium; the objective of a heat pump is to supply heat (QH) to a warm medium

The power cycles can be used as refrigeration cycles by simply reversing them. Of these, the reversed Brayton cycle, which is also known as the gas refrigeration cycle, is used to cool aircraft and to obtain very low (cryogenic) temperatures after it is modified with regeneration. The work output of the expanding device can be used to reduce the work input requirements to the compressor. Thus the COP of a gas refrigeration cycle is

Simple Gas (Air) Refrigeration Cycle

Simple Gas (Air) Refrigeration Cycle

Schematic and T-s Diagram

Many of the impracticalities associated with the reversed Carnot cycle can be eliminated by vaporizing the refrigerant completely before it is compressed and by replacing the turbine with a throttling device, such as an expansion valve or capillary tube. The cycle that results is called the ideal vapor-compression refrigeration cycle, and it is shown schematically and on a T-s diagram in Figure. The vapor-compression refrigeration cycle is the most widely used cycle for refrigerators, air-conditioning systems, and heat pumps. It consists of four processes:1-2 : Isentropic compression in a compressor2-3 : Constant-pressure heat rejection in a condenser3-4 : Throttling in an expansion device4-1 : Constant-pressure heat absorption in an evaporator

Vapor-Compression Refrigeration Cycle

Vapor-Compression Refrigeration Cycle

(Fig. 10-3)

Schematic and T-s Diagram

In an ideal vapor-compression refrigeration cycle, the refrigerant enters the compressor as a saturated vapor and is cooled to the saturated liquid state in the condenser. It is then throttled in Expansion valve to the evaporator pressure and vaporizes as it absorbs heat from the refrigerated space.Very low temperatures can be achieved by operating two or more vapor-compression Systems in series, called cascading. The COP of a refrigeration system also increases as a result of cascading.

Vapor-Compression Refrigeration Cycle

P-h Diagram of an Ideal Vapor-Compression Refrigeration Cycle

Schematic and T-s Diagram for Actual Vapor-Compression Refrigeration Cycle

(Fig. 10-7)

Another form of refrigeration that becomes economically attractive when there is a source of inexpensive heat energy at a temperature of 100 to 2000ºC is absorption refrigeration, where the refrigerant is absorbed by a transport medium and compressed in liquid form. The most widely used absorption refrigeration system is the ammonia-water system, where ammonia serves as the refrigerant and water as the transport medium. The work input to the pump is usually very small, and the COP of absorption refrigeration systems is defined as

Vapour Absorption Refrigeration Cycle

Vapor Absorption Refrigeration Cycle

(Fig. 10-21)

The maximum COP an absorption refrigeration system can have is determined by assuming totally reversible conditions, which yields

where T0, TL, and Ts are the absolute temperatures of the environment, refrigerated space, and heat source, respectively.

Ordinary Household Refrigerator

Heat Pump Heats a House in Winter and Cools it in Summer

(Fig. 10-9)

Air conditioning is the process of maintain the quality of air. Quality of air includes temperature, velocity, humidity and purity (oxygen content) of air maintained at the perfect level. Properties of airAir is the sole working substance of an air conditioning rig. It is actually the mixture of gases in which water vapour exists in superheated or saturated state, hence air is considered as binary mixture and hence requires three independent variables to fix its state. These properties of moist air is known as psychrometric properties. These are as follows1.Specific humidity or humidity ratioSpecific or absolute humidity or humidity ratio or moisture content(ω) is defined as the ratio of the mass of water vapour present to the mass of dry air in a given volume of mixture.

Introduction to air-conditioning

Air-conditioning System

All data essential for the complete thermodynamic and psychrometric analysis of air-conditioning process when summarised in a graph format is known as psychrometric chart. Plotting of psychrometric chart is done as dry bulb temperature that is normal temperature to specific humidity and water vapour pressure.

Psychrometric Chart

Human comfort is the condition of supplied air which in turns proved suitable for people experiencing it, that is to maintain the quality and velocity of supply so that person should feel comfortable within the atmosphere created.Governing parameters: These are air temperature, humidity and air-velocity along with air purity. It is too hard to define or fix human comfort with four different variables, hence, effective temperature is used as an index, that is temperature at 100% relative humidity in which subject will feel the same experience as in actual environment.Because human comfort varies person to person and by age group and gender, and many such factors affect it, vast study is needed. Such studies have shown correlation between comfort level, temperature, humidity, gender, length of exposure, age group and other causes. These are tabulated in the form of comfort charts.

Human Comfort

Human comfort usually depends upon certain causes as follows:

1.Metabolic rate: The rate at which body produces heat.

2.Mechanism of body heat loss.

3.Heat exchange between man and environment.

END