HIGH-PRESSURE FLOAT VALVES
On a single-evaporator flooded system, a float valve can be fitted which will pass any drained liquid from the condenser direct to the evaporator. The action is the same as that of a steam trap. The float chamber is at condenser pressure and the control is termed a high-pressure float (Figure 8.9).
The high pressure float switch keeps the condenser drained without the need for a high pressure receiver. The level in the evaporator is fixed by the system charge. Low charge systems using shell and plate heat exchangers and spray chillers are possible with this method. The type of float valve in Figure 8.10 can work with ammonia or carbon dioxide refrigerants. Economizer circuits with the float switch expanding the liquid to an intermediate flash expansion vessel are used for low-temperature applications. This control cannot feed more than one evaporator, since it cannot detect the needs of either.
The difficulty of the critical charge can be overcome by allowing any surplus liquid refrigerant leaving the evaporator to spill over into a receiver or accumulator in the suction line, and boiling this off with the warm liquid leaving the condenser. In this system, the low-pressure receiver circuit, liquid is drained
Figure 8.9 High-pressure floatvalve circuit
Figure 8.10 Sectioned view of Witt high-pressure float valve (Titan)
From the condenser through the high-pressure float, but the final step of pressure drop takes place in a secondary expansion valve after the warm liquid has passed through coils within the receiver. In this way, heat is available to boil off surplus liquid leaving the evaporator (see Figure 8.11). Two heat exchangers carry the warm liquid from the condenser within this vessel. The first coil is in the upper part of the receiver, and provides enough superheat to ensure that gas enters the compressor in a dry condition. The lower coil boils off surplus liquid, leaving the evaporator itself. With this method of refrigerant feed, the evaporator has a better internal wetted surface, with an improvement in heat transfer.
The rate of refrigerant circulation depends only on the pumping rate of the compressor. The quantity evaporating will depend on the cooling load and
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Liquid valve + some flash gas
Figure 8.11 Low-pressure receiver circuit
Provided there is sufficient charge to flood the evaporator at all conditions, and there is sufficient heat transfer surface, the system is self-adjusting.
The low-pressure receiver system can be adapted to compound compression and can be fitted with hot gas defrost by reverse gas flow. In both circuits the low-pressure receiver provides the safety vessel to prevent liquid entering the compressor. Providing the high-pressure float is correctly sized, this system can operate at low condenser pressures, saving compressor energy in cool weather. Where the halocarbon refrigerants are used in this system, an oil distillation device is fitted.
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