Refrigeration & Air-Conditioning

Module 6. Vapour absorption refrigeration system

Lesson 23
PRACTICAL VAPOUR ABSORPTION SYSTEM

23.1 Introduction

To improve the above simple system, especially to prevent water going along with ammonia vapours into condensor, to get more economy of heat, certain changes are made.

Analyser and Rectifier are added to reduce the passage of water into condenser.

23.2 Analyzer

Analyser is usually an arrangement of trays and may be an integral part of the generator itself. Here, both the strong solutions from absorber and aqua from rectifier are dropped and allowed to flow on the trays and then get exposed and cooled. This partial cooling condenses water vapour and only ammonia vapour leaves at the top

23.3 Rectifier

Rectifier is a double pipe, shell and coil or shell and tube type of vapour cooler. It is usually water cooled. Cooling is just sufficient to remove all the water and leave only ammonia vapours in the condenser. Temp is almost 100 – 120^ºC.

23.4 Heat Exchanger

This is located between absorber and generator, and serves to cool the weak aqua by heating the strong aqua. Double pipe system is used in small system, while shell and tube is used in large system. This will reduce the amount of heat added in the generator and decrease the amount of heat rejected in the absorber. The sizes of the generator and absorber will also be reduced.

23.5 Desirable Characteristics of Refrigerant-Absorbent Pair

1) Low viscosity to minimize pump work

(2) Low freezing point

(3) Good chemical and thermal stability. Also, the two main thermodynamic requirements.

23.5.1 Solubility requirement

The refrigerant should have more than Raoult’s law solubility in the absorbent so that a strong solution, highly rich in the refrigerant, is formed in the absorber by the absorption of the refrigerent vapour.

23.5.2 B.P. requirement

There should be a large difference in the normal binding points of the two substances; at least 200ºC, so that the absorbent exerts negligible vapour pressure at the generator temperature. Thus, almost absorbent free refrigerent is expelled off from the generator and the absorbent alone return to the absorber.

Ammonia forms a highly non ideal solution in water. So it satisfies solubility requirement. But B.P temperature. Difference is only 138°C. So vapour leaving generator contains some amount of water, which creates problem.

In Lithium Bromide – water system, water is the refrigerant. Hence, it is only used in air conditioning application. Since Lithium Bromide is of selt, it exerts no pressure. But it is corrosive and plant works in high degree of vacuum.

23.6 Maximum COP of Vapour Absorption System

VA system is heat operated refrigerating machine. It may be considered as a combination of a heat engine and refrigerating machine.

The energy supplied to the system is in the form of heat Q_g at T_g. The Thermodynamic cycle is considered to comprise heat engine cycle, operating between the heat source temperature Tg and the temperature of heat rejection Tc, and a refrigeration cycle operating between the refrigeration temperature Tr and temperature of heat rejection Tc. The work done in the heat engine part of the cycle is equal to the work requirement of the refrigeration part of the cycle. Thus one may write for the COP of the cycle.

COP =

The COP of a heat operated refrigerating machine should, therefore, be minimum when each of the two terms has a maximum value which unloads so when both are equal to their respective Carnot values.

=

In order that this should be high,

1) Temperature T_g of the heat source should be as high as possible.

2) Temperature T_c of the heat sink is as low as possible.

3) Temperature T_e of the refrigeration is as high as possible.

It may be noted that in case the condenser and absorber temperature are not the same and are equal to T_c and T_A respectively, then the minimum possible COP is given by

However, because of the presence of the H, and the partial pressure it exerts on the low pressure side of the system (Absorber and evaporator) the partial pressure exerted by the NH₃ vapour in these parts will be lower than that exerted by the ammonia vapour in the generator and condenser, when hydrogen is not present.

23.7 Absorbent System

23.7.1 Desirable characteristics

1. Solubility requirement

The refrigerant should have more than Raoult’s law solubility in the absorbent so that a strong solution, highly rich in the refrigerant, is formed in the absorber by the absorption of the refrigerant vapour.

2. Boiling points requirement

There should be a large difference in the normal boiling points of the two substances, at least 200^ºC, so that the absorbent exerts negligible vapour pressure at the generator temperature. Thus almost absorbent – free refrigerant is boiled off from the generator and the absorbent alone returns to the absorber.

In addition to these, some of the other less important ones are

1. Low viscosity – to minimize pump work
2. Low freezing point
3. Good chemical & thermal stability – decomposition, polymerization, corrosion etc.

In the ammonia – water systems, ammonia is the refrigerant and water is the absorbent. Ammonia forms a highly non – ideal solution in water. Hence it is satisfactory from the point of view of the solubility requirement. But the difference in their Boiling point is only 133^ºC. Hence the vapour leaving the generator contains some amount of water which results in many problems. Thus the ammonia – water system is not suitable from the point of view of the boiling point requirement.

In the lithium bromide – water system, water is the refrigerent and lithium boroxide the absorbent. Hence the mixture is used only in air – unlimited applications. The mixture is again non – ideal and is satisfactory from the point of view of the solubility requirement. Since lithium bromide is a salt, it exerts no vapour pressure. So the vapour leaving the generator is a pure refrigerant. The mixture satisfies the boiling point requirements also. However, it is corrosive and the plant work under high vacuum.

23.8 Using Analyzer-the Exhausting, Column, and Dephlegmotor-the Rectifying Column

In a system like that of ammonia – water, the vapours distilled from the generator contain a considerable amount of absorbent vapour which subsequently reaches the evaporator after condensation. As a result, evaporation would not be isothermal and the required low temperature would not be reached.

Not only is the evaporator pressure reduced due to concentration being less, it can be seen that the rich solution concentration at the given absorber temperature is also reduced, because of lower absorber pressure. Thus will ultimately result in a smaller difference in the rich and poor solution concentrations and hence a lower COP.

Hence, to return the absorbent to the generator and to allow, as for as possible only the refrigerant vapour to enter the condenser, two elements are added,

(1) Analyzer or the exhausting column.

(2) Dephlegmator and rectifier on the rectifying column.

The analyzer on the exhausting column is installed on top of the generator. The vapour learning the generator with certain ref. concentration in equilibrium with the boiling poor solution having certain concentration, enter this analyzer. As it travels upwards, counter flow to the entering rich solution, the vapours encounters heat and mass exchange with then falling rich solution ultimately learning their analyzer enriched in the refrigerant.

This method has the additional advantage of returning some heat from the vapour to the generator in the form of preheating of the rich solution with simultaneous cooling of the vapour.

The enriched vapour now enters the rectifying column where in heat is removed from the vapour by the circulation of the cooling medium. The leaving state of the vapour is determined by the temperature of the cooling medium. A part of the vapour is condensed and is returned as drip to the analyzer.

The latter method of increasing the refrigerant concentration of the vapour has a drawback in that it involves a loss of useful heat added in the generator which is rejected to the cooling medium in the rectifier. The drip returns to the generator and has to be evaporated again. The condition under which the use of rectifier would improve the COP would depend on the working pair being used and the operating conditions.

It may be noted that the use of an analyzer and rectifier is not necessary in the case of system such as lithium bromide – water in which case the absorbent or adsorbent doesn’t it exist any significant vapour pressure, if at all.