Refrigeration & Air-Conditioning

Module 6. Vapour absorption refrigeration system

Lesson 22
SIMPLE ABSORPTION REFRIGERATION SYSTEM

22.1 Introduction

The Refrigeration by mechanical vapour compression system is an efficient method. But, the energy input is the shaft work, which is a high – grade energy (One that can be easily converted to other forms )and therefore very expensive. And the work required is relatively large because of compression of the vapours which undergo large changes in specific volumes. If same gas is available in liquid form, to pump that to higher pressure, the energy required is less. Hence in order to achieve this, the system designed is called Absorption Refrigeration system, in which the refrigerant vapour is dissolved in an inert liquid at the same pressure as the evaporator and the solution so formed is pumped to a container at condenser pressure. This liquid which is practically incompressible and undergoes very little change, in specific volume, requires very little work in raising its pressure. After raising the pressure, the refrigerant is separated from the solution by heating, and increasing the temperature.

The shaft work involved in both the cases, if you campare for compressing and just pumping for a particular case was 1.225% to that of V.C system.

Combination

Absorbent Refrigerant

Water Ammonia

Lithium bromide Water

22.2 Simple Absorption Refrigeration System

The essential components are: an Evaporator, an absorber, a generator, a condenser, an expansion value, a pump and a reducing value.

Fig. 22.1 Vapour compression and vapour absorption system

Table 22.1 Comparison of vapour absorption and compression

Thus it is seen that the compressor in the VC system is replaced by an absorber, generator, a reducing value and a pump.

The choice of the absorbent depends on its -vc deviation from, Raoult’s law (At a given temperature the ratio of the partial pressure of a volatile component in a solution to the vapour pressure of the pure component at the same temp is equal to its mole fraction in the solution. The mole fraction is equal to the number of moles of the component divided by the total no. of moles present. Mol = wt. of substance/ mol.wt . A negative deviation is said to be there when observed vapour pressure is less than the calculated pressure.)

In case of Ammonia and water, the total volume is approximately equal to the volume of water above plus 85% of volume normally occupied by the Ammonia. The combination of water and ammonia is called Aqua Ammonia. Is Absorption of ammonia to water is a chemical reaction?

22.2.1 Absorber

It may be in a shell, in which weak solution containing considerably less ammonia is sprayed or imposed and which absorbs ammonia from evaporator. Absorption of ammonia lowers the pressure in the absorber and as a result more ammonia vapour in drawn from the evaporator, creating lower pressure in evaporations.

There is some heat evolved due to vapours going into liquid and the heat due to condensation. To take this heat away, there will be a cooling water coil inside. The stronger solution is pumped by a pump into the generator, which is at a higher pressure than absorber.

22.2.2 Generator

In generator heat is added to the system by some heat source, may be a gas burner, steam or electric heating. Connections are provided for weak solution from generator and return to absorber through a reducing value.