Charging the system

Before any charge of refrigerant is put into a system, it must be thoroughly pressure-tested for leaks on both the low and high side of the compressor. Anhydrous carbon dioxide should be used for this and all joints and connections carefully inspected, soapy water being used for a bubble test. A watch on the pressure gauges will indicate if a serious leak is present. Following this, a small amount of refrigerant should be added to the system, and all joints, pipework and connections gone over with a hallide torch. (This is a trace technique for detecting leaks when fluorinated hydrocarbons are used. Any escaping refrigerant reacts with the flame of the torch to produce a green colour.)

The system is regarded as free from leaks if, after having been left under pressure for 24 hours, no variation is observed in the gauge readings, due attention being paid to variations in the ambient temperature, which will alter the gauge readings.

When the system is free from leaks, it must be dehydrated. This is accomplished by using an auxiliary vacuum pump (never the refrigeration compressor itself) to pull a hard vacuum on the entire system. Dehydration can be accomplished in this way only if liquid water in the system is made to boil, the vapour then being drawn off. At 20°C, the saturation vapour pressure is 2.337 kPa absolute. That is, a vacuum of about 99 kPa. The application of external heat to the system may be of help in attaining the required amount of dehydration.

Following the above testing and dehydration procedure, the ancillary systems should be set to work. By this is meant the associated air handling and water handling plant, together with their automatic controls. It is not possible to charge and set to work a refrigeration system unless this is done. For example, unless the cooling-water pump is circulating an adequate amount of water through the condenser, the refrigerant plant will fail continually on its high-pressure cut-out.

The system should be charged from a cylinder or drum of refrigerant through a dehydrator. For any given system, if the design duty is to be obtained, only one weight of refrigerant is the correct amount for it to operate between the design condensing and evaporating pressures. For packaged plants of well-known size and performance, a measured weight of refrigerant should give the desired operation. However, for plants of a more bespoke character, sight glasses and gauge glasses may be used to advantage in assessing the necessary amount of the charge.

The location of a charging point varies according to the size and type of plant. Very small reciprocating machines may have no special point for charging; this then has to be done through the gauge connection at the suction valve. In this case it is important that only vapour (i. e. no liquid) refrigerant is charged. For larger installations the charging point is either between the expansion valve and the evaporator or in the liquid line or at the liquid receiver. In these cases liquid can be fed in.

Notation

Posted in Air Conditioning Engineering