When servicing or commissioning any equipment it is necessary to record the system pressures by fitting pressure gauges. To make this possible, all commercial systems generally include at least three service valves: suction, discharge and liquid shut-off.
Suction and discharge service valves may be located on the compressor body of both reciprocating open-type compressors and semi-hermetic motor compressors, but on some compressor designs the service valves may be an integral part of the compressor head assembly. Hermetic motor compressors and some semi-hermetic models do not feature a discharge service valve, but the high side pressure may be obtained from the service valve on the liquid receiver or via a Schraeder-type valve fitted into the discharge line or on the receiver itself.
Service valves can be set to three different positions (Figure 3):
Front seated position The valve stem is turned fully clockwise to effectively stop the flow of refrigerant vapour from the suction line union on the low pressure side of the compressor and to the discharge line union on the high pressure side of the compressor.
Back seated position The valve stem is turned fully counter-clockwise to stop the flow of refrigerant vapour to the gauge port of the service valve.
Midway position The valve stem is turned either clockwise or counter- clockwise to leave the valve unseated. Thus refrigerant vapour can flow from the suction line and also to the discharge line and at the same time pass through the gauge port, to the gauge hose and to the relevant pressure gauge.
The liquid shut-off valve may be located at the outlet of the receiver. It is a single seating valve, i.e. it is either open or closed (Figure 4). On systems which do not have a receiver the valve design will be similar to that of a suction or discharge service valve and will have a gauge connection. When closed or fully front seated, this valve will stop the flow of liquid refrigerant from the condenser or receiver to the expansion device.
Performing service operations, carrying out repairs, commissioning a system and diagnosing faults involve the use of these valves in addition to test equipment, which will be dealt with separately.
This is also called a system analyser. It consists of a manifold on the top of which are mounted two pressure gauges (Figure 5). Underneath the manifold
A colour code has been introduced for the pressure gauges, hoses and shut- off valves.
The left hand pressure gauge is known as a compound gauge because it will record both positive and negative pressures, since the gauge is cali- brated to read zero at atmospheric pressure. The gauge pressure range is from 30 in Hg to 0 psi (0.9 to 0 bar) for pressures below atmospheric pressure, and from 0 to 250 psi (0 to 10.7 bar) or more for those above. This gauge is coded blue.
The right hand gauge is called a pressure gauge. It only records pressures above atmospheric, from 0 to 500 psi (0 to 35 bar). This gauge is coded red.
The hoses are also colour coded to correspond to the gauges and shut-off valves. The blue hose should be connected to the compound gauge, the yellow to the centre connection and the red to the pressure gauge.
When the manifold is assembled it is not necessary to open the valves. Pressure will be recorded as soon as the system pressure is passed to the hose after setting the compressor service valve. When either valve is opened, and
assuming pressure is available from the system, it will pass to the centre hose. It is advisable to keep the centre hose plugged at all times, or the centre connection capped when the hose is removed.
With the centre connection capped and both valves open, it will be seen that the pressures will equalize on both gauges. When both valves are closed and the centre connection is capped, it will be seen that both negative and positive pressures can be recorded when the compressor service valves are set
to operating positions, assuming that the system is designed to operate in such a manner.
The function of the valves on the manifold is shown in Figure 5.
The service gauges are a vital part of a service engineer’s equipment. They are invaluable for performing service operations or diagnosing faults, which will be dealt with later.
Although the task of fitting gauges is a simple one, it must be realized that any refrigerant has a pressure/temperature relationship: the higher the temperature of the refrigerant, the greater the pressure. It will be seen from the pressure/temperature chart at the end of this manual that any refrigerant, if subjected to even a normal ambient temperature, will generate sufficient pressure to present a danger. Thus the engineer should be conversant with the safety procedures for handling refrigerants, and should observe them.
Service gauges may be fitted when the plant is at rest or when it is operating. In the latter case the high side system pressure will be much greater, and it is recommended that the following procedure be adopted for either condition (Figure 6):
1 Remove the valve caps from the suction and discharge service valves on the compressor.
2 Set both valves to the back seat position.
3 Remove the seal caps from the gauge connections of both service valves (normally a flare nut and bonnet).
4 Fit the blue hose from the manifold to the suction service valve connection.
5 Fit the red hose from the manifold to the discharge service valve connection.
6 Ensure that the centre hose connection on the manifold is capped or that the yellow hose is plugged.
Having fitted the gauges to the service valves, the next step is to remove any air from the hoses and manifold. Air contains moisture and is a contaminant, and its entry to the system must be prevented.
1 Crack off the discharge service valve (turn the valve stem one turn clock- wise).
2 Open both shut-off valves on the manifold.
3 Loosen the hose connection (blue) on the suction service valve and allow the pressure to leak out slowly for one or two seconds.
4 Tighten the hose connection.
5 Close both shut-off valves on the manifold.
6 Crack off the suction service valve.
When fitting gauges to service valve gauge connections, always use two spanners of the correct size to avoid breaking the vapour seal of the gauge unions.
The plant, if operating with a low pressure control, may have the control tubing or capillary connected with a tee union to which the gauge hose is also fitted. Thus when the service valve is back seated, the pressure will be trapped in the control line. When pressure is released during the purging operation, the plant will stop; it will restart when the suction service valve is cracked off to pressurize the control.
When gauges are fitted to a plant which has been closed down, the liquid shut-off valve must be opened so that refrigerant will circulate around the system.
The procedure for fitting gauges is the same, except that the red hose will be connected to the gauge connection of the service valve on the receiver (Figure 6). When this valve is back seated, the refrigerant will circulate.
After fitting gauges allow a short time to elapse before recording the pressures, i.e. when the pressures stabilize on the gauges.
Once again, bearing in mind that high pressures are generated whilst a plant is operating, care must be taken not to pollute the atmosphere when gauges
are removed. The following procedure will minimize the loss of refrigerant and discharge to atmosphere:
1 Back seat the discharge service valve.
2 Open both shut-off valves on the manifold and allow the pressures to equalize in the gauges.
3 Close high side valve on the manifold and operate the compressor until a zero pressure is indicated on the compound gauge. Close the low side valve on the manifold.
4 Remove the hose from the discharge service valve union, replace the seal cap and tighten.
5 Back seat the suction service valve.
6 Remove the hose from the suction service valve union, replace the seal cap and tighten.
7 Set both service valves to the operating positions (cracked off the back seat).
8 Replace the service valve caps and tighten.
9 Test for leaks at the valve caps and gauge unions (see Chapter 3).
The low side pressure in systems charged with refrigerants R22 and R502 can be much higher than that in systems charged with refrigerant R12. Thus when removing the hose from the low side, loosen the connection and release the pressure slowly. Although the pressure may be high, the volume of vapour in the hose will be small.
It is common practice for systems to be installed with pressure safety controls. When gauges are removed, the service valves must be left in the operating positions cracked off the back seat to allow the controls to be pressurized (see Chapter 6).
Modern service manifolds incorporate additional hand valves, connections for a vacuum pump, and a liquid-indicating sight glass. Some feature Schraeder
The Schraeder valve is similar to that used in cycle or car tyres (Figure 7). The fitting into a system can vary: it may be brazed in, screwed into a component or supplied as a flare connector.
When connection is made to this type of valve it is necessary for the service hose to include a quick release adaptor. This depresses the valve stem and allows passage of refrigerant into the hose.
These are sometimes referred to as piercing valves. They consist of a small clamp which is in two sections fitted to the pipework.
Totally sealed systems will involve the use of this type of valve in order to gain access to the system. A special charging/gauge adaptor is screwed on to the piercing valve assembly (see Figure 8).
The valve assemblies are available for a wide range of pipe sizes and the adaptor is a standard fitting to the assemblies.
