Piping and accessories
From a thermodynamic point of view, the only essential substance in the system is the refrigerant, anything else is undesirable. Of the other substances which may find their way into a system, only a lubricant is necessary, and this is only because the compressor requires it for proper operation. Thus air, water and dirt are not wanted, and every possible step must be taken to ensure that they are absent. Their total elimination may be impossible but care in the proper choice of materials, and in assembly and charging, can minimise their presence.
The piping used must be of a material which will not be attacked by the refrigerant (for example, copper is acceptable with the fluorinated hydrocarbons but not with ammonia) and, apart from having the necessary structural properties, it must be clean and dehydrated, prior to use. The same holds true for all fittings, valves, and other components used.
Piping must be large enough to permit the flow of the refrigerant without undue pressure drop and, most important, it must permit the lubricant in the system to be carried to the place where it is wanted, that is, to the crankcase. Inevitably, some is discharged with the high-pressure refrigerant from the compressor, into the condenser. When the refrigerant is in the liquid state, it mixes well with mineral oil, if it is a fluorinated hydrocarbon, and both are easily carried along. When the refrigerant is a gas, however, mixing does not occur. If mineral oil is used it turns into a mist and the pipes must be sized to give a refrigerant gas velocity high enough to convey the lubricant. It is clear from this that a problem may arise under conditions of partial load, when the system is not pumping the design quantities of gas. Mineral oil usually collects on the walls of the piping and drains down into the low points of the system. For this reason, proper oil return to the crankcase can be assisted by pitching the pipes. An oil separator between the compressor and the condenser is also useful, particularly in air-conditioning applications where large variations of load occur. Lubricants that are not mineral oils will have their own characteristics.
An oil pressure failure switch is essential. This is interlocked with the compressor motor starter so that if the pressure falls below a minimum value, the compressor is stopped and damage from lack of lubrication is prevented.
Water in a system can cause trouble in a number of ways and to minimise this a dryer is inserted in the piping. This should not be regarded as a valid substitute for proper dehydration in the assembly and setting to work, but should be considered in addition to it.
Strainers should be fitted upstream of the expansion valve and any solenoid valve which may accompany it.
Sight glasses are essential. They should be placed in the liquid line between the condenser or liquid receiver and the expansion valve. Their purpose is to verify the presence of an adequate charge in the system and to monitor the state of the liquid entering the expansion valve.
Gauges are also essential although they are not always fitted on some of the modem packaged plant. They permit a proper check to be kept on the running of the plant and they are of assistance in commissioning the plant and in locating faults.
Gauge glasses are necessary, although they may not always be present. Their purpose is to establish the levels of the liquid in the condensers and receivers, this being of particular use in charging the system.
If the condensing pressure is less than the ambient air pressure, air and possibly other non-condensible gases will leak into the condenser and an automatic purging system becomes necessary. (Note that some small leakage of refrigerant into the atmosphere will always occur when the automatic purge operates.) Non-condensible gases collect in the condenser and their presence raises the total pressure (in accordance with Dalton’s law), reducing the refrigeration capacity and increasing the power needed for compression. Oxygen in the gases may cause mineral oil to oxidise in the presence of a high temperature and the non-condensible gases can form a film over the heat transfer surfaces within the condenser. Webb et al. (1980) showed how small percentages of non-condensible gases cause a large decrease in the film heat transfer coefficient with shell and tube condensers.
Posted in Engineering Fifth Edition