MODULE III Air-conditioning and Cooling technology Page 73

Absorption cooling

The solution circulation comprised of Absorber A, Solution Pump SP, the Solution Heat Exchanger SHE, the Generator G and the Solution Expansion Valve SEV replaces the compressor. A solution of refrigerant and a suitable solvent runs in this circulation pattern (see Fig. 59).

The fluid refrigerant in the Evaporator E is evaporated at temperature T0 when it absorbs heat Q0 from the heat source. This heat is again the available heat. The refrigerant steam is now absorbed into the absorber from a solution which is poor in refrigerant, thus to a certain extent sucked in by the absorber. The absorption heat Q1' is passed on to the heat sink at temperature T1. The solution, which has now been enriched with refrigerant through the absorption process, is pumped by the solution pump SP into the generator, which is found on the condenser pressure.

The mechanical work involving the solution pump is relatively minor. Depending upon the material system, this amounts to between a few thousandths and a few hundredths of the generator heat. The pump work is therefore not taken into account here. It can, however, be very easily integrated into the calculation.

After the solution pump, the rich solution flows into the solution heat exchanger SHE (before it reaches the generator G), where it is heated up by the hotter solution flowing back from the generator into the absorber. In generator G the rich solution is brought to the boiling point by the actuation heat Q2 at the highest possible temperature T2, whereby the refrigerant, which has been absorbed in absorber A, is again driven out of the solution. The refrigerant condenses in the condenser C. The condensation heat Q1" is passed on to the heat sink at temperature T1. The surface tension of the condensate is then reduced in the expansion valve EV to the evaporator pressure. The work material circulation is now complete.

In generator G, due to the supply of actuation heat Q2, a solution rich in refrigerant becomes a solution poor in refrigerant. This poor solution flows into the solution heat exchanger SHE, in which it passes on heat to the rich solution flowing in the opposite direction, and cools off as a result. Following this, the surface tension of the poor solution is reduced in the solution expansion valve SEV to the evaporator pressure, after which it then flows again into absorber A, where it can once again absorb refrigerant. The solution circulation is thus now closed.


Energetic parameter

As a parameter, we will define the heat relationship as the relationship between available heat and the actuation heat Q2 to be expended. With the actual heat pump, the available heat is the passed on heat Q1, which is comprised of the absorber heat Q1' and the condenser heat Q1", thus

1 = Q1/Q2 = (Q1'+ Q1")/Q2.

With operation as a refrigerator, the use of the heat Q0 supplied to the evaporator is thus 0 = Q0/Q2.