Basic Refrigeration Cycles

Vapour Absorption Refrigeration Systems

John Leslie in 1810 kept H2SO4 and water in two separate jars connected together. H2SO4 has very high affinity for water. It absorbs water vapour and this becomes the principle of removing the evaporated water vapour requiring no compressor or pump. H2SO4 is an absorbent in this system that has to be recycled by heating to get rid of the absorbed water vapour, for continuous operation. Windhausen in 1878 used this principle for absorption refrigeration system, which worked on H2SO4. Ferdinand Carre invented aqua-ammonia absorption system in 1860. Water is a strong absorbent of NH3. If NH3 is kept in a vessel that is exposed to another vessel containing water, the strong absorption potential of water will cause evaporation of NH3 requiring no compressor to drive the vapours. A liquid pump is used to increase the pressure of strong solution. The strong solution is then heated in a generator and passed through a rectification column to separate the water from ammonia. The ammonia vapour is then condensed and recycled.

The pump power is negligible hence; the system runs virtually on low- grade energy used for heating the strong solution to separate the water from ammonia. These systems were initially run on steam. Later on oil and natural gas based systems were introduced. Figure 1.4 shows the essential components of a vapour absorption refrigeration system. In 1922, Balzar von Platen and Carl Munters, two students at Royal Institute of Technology, Stockholm invented a three fluid system that did not require a pump. A heating based bubble pump was used for circulation of strong and weak solutions and hydrogen was used as a non-condensable gas to reduce the partial pressure of NH3 in the evaporator. Geppert in 1899 gave this original idea but he was not successful since he was using air as non-condensable gas. The Platen-Munters refrigeration systems are still widely used in certain niche applications such as hotel rooms etc. Figure 1.5 shows the schematic of the triple fluid vapour absorption refrigeration system.

Vapour Compression Refrigeration System

As shown in the figure the basic system consists of an evaporator, compressor, condenser and an expansion valve. The refrigeration effect is obtained in the cold region as heat is extracted by the vaporization of refrigerant in the evaporator. The refrigerant vapour from the evaporator is compressed in the compressor to a high pressure at which its saturation temperature is greater than the ambient or any other heat sink. Hence when the high pressure, high temperature refrigerant flows through the condenser, condensation of the vapour into liquid takes place by heat rejection to the heat sink.

To complete the cycle, the high pressure liquid is made to flow through an expansion valve. In the expansion valve the pressure and temperature of the refrigerant decrease. This low pressure and low temperature refrigerant vapour evaporates in the evaporator taking heat from the cold region. It should be observed that the system operates on a closed cycle. The system requires input in the form of mechanical work. It extracts heat from a cold space and rejects heat to a high temperature heat sink.

QUIZZZZZ

Q. In the evaporator of a vapour compression refrigeration system:

a) A low temperature is maintained so that heat can flow from the external fluid

b) Refrigeration effect is produced as the refrigerant liquid vaporizes

c) A low pressure is maintained so that the compressor can run

d) All of the above

Q. The function of a compressor in a vapour compression refrigeration system is to:

a) To maintain the required low-side pressure in the evaporator

b) To maintain the required high-side pressure in the condenser

c) To circulate required amount of refrigerant through the system

d) To safeguard the refrigeration system

Q. In a vapour compression refrigeration system, a condenser is primarily required so that:

a) A high pressure can be maintained in the system

b) The refrigerant evaporated in the evaporator can be recycled

c) Performance of the system can be improved

d) Low temperatures can be produced

Q. The function of an expansion valve is to:

a) Reduce the refrigerant pressure

b) Maintain high and low side pressures

c) Protect evaporator

d) All of the above

Q. In a domestic icebox type refrigerator, the ice block is kept at the top because:

a) It is convenient to the user

b) Disposal of water is easier

c) Cold air can flow down due to buoyancy effect

d) None of the above