Practical considerations

Figure 14.5 is a more practical flow diagram for a single effect lithium bromide-water absorption refrigeration system. It represents a water chiller producing water at 6.7°C. The evaporator works at 10.13 kPa, or 4.4°C, and is equipped with a recirculating pump which sprays the liquid refrigerant (water) over the bundle of tubes carrying the water to be chilled. The absorber operates at 40.6°C and is also provided with a recirculating pump which sprays concentrated solution over the bundle of tubes carrying water from the cooling tower.

Weak solution from the absorber is pumped to the generator via a heat exchanger, where its temperature is raised from 40.6°C to 71.1 °C. The generator is supplied with steam at 1.841 bar and in it the solution temperature is 104.4°C and the pressure 10.13 kPa. The heat exchanger effects an economy in operation since the cold solution is warmed from

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Practical considerations

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Fig. 14.5 Practical flow diagram for a single effect lithium bromide-water system.

подпись: fig. 14.5 practical flow diagram for a single effect lithium bromide-water system. Practical considerationsCO

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40.6°C to 71.1 °C before it enters the generator for further heating to 104.4°C, and warm solution is cooled from 104.4° to 73.9°C before it enters the absorber for further cooling to 40.6°C.

In the example illustrated, cooling water from the tower or spray pond at 30°C first passes through the absorber and then the condenser, which it leaves at 39.4°C. The machine shown in the diagram consumes, at full load, from 0.70 to 0.72 g s-1of steam per kW of refrigeration and the cooling water requirement is about 0.07 litre s-1 kW“1. If control is by varying the evaporation rate, steam consumption increases from about 0.72 to about 1.08 g s“1 kW-1, as the load falls from 50 to 10 per cent of full capacity. With solution control, on the other hand, there is a reduction in steam consumption from 0.72 to about

0. 61 g s“1 kW-1 as the load falls from 100 to 30 per cent and a small subsequent rise of

0. 02 g s"1 kW“1 to 0.63 g s-1 kW-1, when the load falls further through the range 30 to 10 per cent.

The output of the machine is controlled in two ways: (i) by varying the rate at which the refrigerant boils off in the generator, or (ii) by allowing some of the solution leaving the heat exchanger to by-pass the generator. The first method involves either modulating the heat supply to the generator or varying the flow of cooling water through the condenser.

Practical considerations

Fig. 14.6 Flow diagram for an aqua-ammonia system.

Electronic controls are used with modem machines and give better results than pneumatic or electric controls. Chilled water flow temperatures from 4°C to 15°C are possible.

High temperature hot water is best not used as a heat source in the generator as an alternative to steam. This is because the temperature drop that accompanies the water flow as it gives up heat introduces thermal expansions and stresses for which the heat exchanger was probably not designed. With steam this does not occur because it condenses at a constant temperature as it gives up its heat.

Lithium bromide machines are used with steam at pressures from 60 kPa to 80 kPa for single effect operation or 550 kPa to 990 kPa for double effect, according to ASHRAE (1998) and are available with cooling capacities from about 350 kW to about 6000 kW of refrigeration. Water-cooled systems are favoured, one reason being that there is less risk of crystallisation than with air-cooled machines. Crystallisation occurs with lithium bromide machines when the concentration of the absorbent in the solution becomes too high and the solution solidifies or turns into a slush, reducing the flow rate and causing the refrigeration process to fail. It can occur when the cooling water flow temperature falls rapidly during

Operation and causes liquid to be carried over from the generator to the condenser. Most machines automatically limit the heat input at the generator, according to changes in the cooling water temperature. The ability to use lower cooling water temperatures, without crystallisation occurring, is desirable, because it improves operational efficiency.

Air can leak into the system and hydrogen form therein, as a result of corrosion. The presence of these condensible gases reduces refrigeration capacity and increases the risk of crystallisation. A purge system is therefore needed. Corrosion inhibitors of various sorts are also used as are performance enhancers; these are necessary because of the low values of the heat and mass transfer coefficients of lithium bromide machines.

Posted in Air Conditioning Engineering


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