VAPOUR ABSORPTION REFRIGERATION AND AIR REFREGIRATION

VAPOUR ABSORPTION SYSTEM

The vapour absorption refrigeration is heat operated system. It is quite similar to the vapour compression system.

VAPOUR COMPRESSION SYSTEMS

The challenge in refrigeration and air conditioning is to remove heat from a low temperature source and dump it at a higher temperature sink. Compression refrigeration cycles in general take advantage of the idea that highly compressed fluids at one temperature will tend to get colder when they are allowed to expand. If the pressure change is high enough, then the compressed gas will be hotter than our source of cooling (outside air, for instance) and the expanded gas will be cooler than our desired cold temperature. In this case, we can use it to cool at a low temperature and reject the heat to a high temperature.

Vapour-compression refrigeration cycles specifically have two additional advantages. First, they exploit the large thermal energy required to change a liquid to a vapour so we can remove lots of heat out of our air-conditioned space. Second, the isothermal nature of the vaporization allows extraction of heat without raising the temperature of the working fluid to the temperature of whatever is being cooled. This is a benefit because the closer the working fluid temperature approaches that of the surroundings, the lower the rate of heat transfer. The isothermal process allows the fastest rate of heat transfer

Vapour compression refrigeration is the primary method to provide mechanical cooling. All vapor compression systems consist of the following four basic components along with the interconnecting piping. These are the evaporator, condenser, compressor and the expansion valve.

The evaporator and the condenser are heat exchangers that evaporate and condense the refrigerant while absorbing and rejecting the heat. The compressor takes the refrigerant from the evaporator and raises the pressure sufficiently for the vapor to condense in the condenser. The expansion device controls the flow of condensed refrigerant at this higher pressure back into the evaporator. Some typical expansion devices are throttle valves, capillary tubes and thermostatic expansion valves in case of large refrigeration systems.

Figure shows the T–S plot of the working of such a system. Here, the dry saturated working medium at state 1 is compressed isentropically to state 2.

Constant pressure heat transfer occurs from state 2 until the compressed vapor becomes saturated liquid or condensate at state 4. The compressed vapor is next throttled from the high pressure region in the condenser (state 4) to the low pressure region in the evaporator (state 5). Since throttling is an irreversible process, it is represented by a broken line. After throttling to evaporator pressure, the heat transfer in the evaporator causes vaporization of the working medium until state 1 is reached, thus completing the cycle. The process 4-5 is assumed to be adiabatic during throttling, an isenthalpic process.

CARNOT VAPOR COMPRESSION SYSTEMS

Here, the compression is imagined to take place in two stages: isentropic compression up to state 2 and isothermal compression from state 2 to 3. 

The working medium is condensed in a heat exchanger giving saturated liquid at state 4. The isentropic expansion from state 4 to state 5 gives the refrigeration effect, the area under line5- 1.

Comparing figs, we can see that the Carnot vapor compression cycle gives a greater refrigeration effect than the vapor compression cycle. It can be seen that the refrigeration system working on the Carnot vapor compression cycle has the highest COP.

LIMITATIONS OF CARNOT VAPOR COMPRESSION SYSTEMS WITH VAPOR AS REFRIGERANT

Although in theory, the Carnot vapor compression cycle has the highest COP; it is not suited for use in practical refrigeration systems. This is because it is virtually impossible to compress the refrigerant isothermally from state 2 to state 3 in a finite time interval. To offset this difficulty, we can follow the alternate path 1′-3- 4-5. However, this results in other difficulties which are mentioned in detail below:

Dry vs. Wet Compression

If the Carnot vapour cycle follows the path 1-2-3-4, then there is dry compression of the refrigeration vapor since the refrigerant is dry saturated at state 1. This type

of compression is desirable in the compressor. But, in this case we see that the refrigerant now has to be compressed isothermally from state 2 to state 3, which is impossible to achieve in practice. The alternate path 1′-3-4-5 involves a wet compression of the vapor from state 1′ to state 3. Wet compression is highly undesirable as the compressor now has to deal with two different fluid phases. Besides, the liquid droplets present in the vapor would now react with the lubricant in the compressor which is highly undesirable. Thus, we see that both the paths of the Carnot vapor cycle are not suitable for use in practical refrigeration systems.

Throttling vs. Isentropic Compression

In the Carnot vapour compression cycle, there is isentropic expansion from state 4 to state 5. This is achieved by the use of a turbine. However, in actual cycles, the expansion from saturated liquid at state 4 to liquid-vapor mixture at state 5 produces very little work. A turbine working under such conditions would have very low efficiency which would not justify the cost involved in using a turbine. Also, the refrigeration system would become very bulky and not suitable for domestic use.

In actual practice, an expansion valve is used to achieve the desired expansion from state 4 to state 5. The refrigerant gets throttled in the expansion valve from saturated liquid to liquid- vapor mixture. The expansion no longer remains isentropic. The expansion now becomes an isenthalpic process.

Thus, we see that the Carnot vapour refrigeration cycle is not suitable for use in refrigeration systems. A better ideal cycle is the vapor compression refrigeration cycle.

VAPOUR ABSORPTION SYSTEMS

In both the systems, there are evaporator and condenser. The process of evaporation and condensation of the refrigerant takes place at two different pressure levels to achieve refrigeration in both the cases. The method employed to create the two pressure levels in the system for evaporation and condensation of the refrigeration makes the two processes different. Circulation of refrigerant in both the cases is also different.

In the absorption system the compressor of the vapour compression system is replaced by the combination of „absorber‟ and „generator‟. A solution known as the absorbent, which has anaffinity for the refrigerant used, is circulated between the absorber and the generator by a pump (solution pump). The absorbent in the absorber draws (or sucks) the refrigerant vapour formed in the evaporator thus maintaining a low pressure in the evaporator to enable the refrigerant to evaporate at low temperature.

In the generator the absorbent is heated. There by releasing the refrigerant vapour (absorbed in the absorber) as high pressure vapour, to be condensed in the condenser. Thus the suction function is performed by absorbent in the absorber and the generator performs the function of the compression and discharge. The absorbent solution carries the refrigerant vapour from the low side (evaporator– absorber) to the high side (generator-condenser). The liquefied refrigerant flows from the condenser to the evaporator due to the pressure difference between the two vessels; thus establishing circulation of the refrigerant through the system.

The absorbent solution passing from the generator to the absorber is hot and ha to be cooled. On the other hand the absorbent solution sent to the generator is cooled and has to be heated in the generator for the regeneration of the refrigerant. A shell and tube heat exchanger is introduced between the generator and the absorber.

There is number of vapour absorption system depending on the absorbent e.g. ammonia absorbent system, lithium bromide absorption system etc. Ammonia absorbent systems were used in the early stages of refrigeration. This system uses ammonia as the refrigerant and water as absorbent. In lithium bromide absorption system lithium bromide salt solution absorbent and water as the refrigerant. A concentrated solution of lithium bromide has a great affinity for water. Since water is the refrigerant, the refrigerant operating temperature in the evaporator has to be above the freezing point of water (0oC) of water

AIR REFREGIRATION

INTRODUCTION

The art of air conditioning developed only gradually from the predecessor arts of cooling, cleaning, heating and ventilating. Towards the latter half of the 19th century, the developments in the art of humidifying air went along with the progress of textile industry in England. It is worth mentioning here the name of

Wolff who designed air-conditioning systems for as many as hundred buildings during his life- time. But it is W.H.Carrier (1876-1950) who is known as the „Father of Air Conditioning‟. He engineered and installed the first year-round air-conditioning system, providing for the four major functions of heating, cooling, humidifying and dehumidifying.

He made use of air washers for controlling the dew point of air by heating or chilling recirculated water. Carrier presented his remarkable paper „Rational Psychrometric Formulae‟ in an ASME meeting. Carrier also employed the centrifugal compressor for refrigeration in 1922. As far as air conditioning for comfort is concerned, it got off the ground in motion-picture theatres in 1920 in Chicago employing CO2 machines and in 1922 in Los Angeles employing NH3 compressors.

In the following chapters, attention will henceforth be focused on the art and science of air conditioning which is the greatest single application of refrigeration, in addition to that of heating and ventilation. For this purpose it is necessary to study the properties of the working substances in air conditioning, viz ., moist air.

WORKING SUBSTANCE IN AIR CONDITIONING

An important thing for the student of air conditioning is to appreciate that the working substance under study, viz., moist air, is a mixture of two gases. One of these is dry air which itself is a mixture of a number of gases and the other is water vapour which may exist in a saturated or superheated state.

One might ask whether moist air can be considered as a pure substance. But a pure substance is homogeneous and invariable in chemical composition. Thus, a homogeneous mixture of gases is a pure substance until its components do not change in phase. Dry air is a good example of such a kind of pure substance.

Water vapour is certainly a pure substance. But moist air is not a pure substance in any process in which condensation or evaporation of moisture occurs. In such a case, regular charts have to be developed to describe the thermodynamic properties of the mixture under different conditions and compositions.

It is, thus, seen that moist air consists of two parts: one, comprising dry air, considered as the fixed part, and the other, solely of water vapour, considered as the variable part.

The dry air part is a mixture of a number of permanent gases with approximate compositions as given in Table 6.1.

Both dry air and water vapour can be considered as perfect gases since both exist in the atmosphere at low pressures. Hence, perfect gas laws can be applied to them individually. In addition, Gibbs-Dalton laws for non-reactive mixtures of gases can be applied to the dry air part only to obtain its properties as a single pure substance, before establishing the properties of moist air.