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
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.
Purging hoses and manifold
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.
Once the gauges have been fitted, proceed as follows:
1. Crack off the discharge service valve (turn the valve stem one turn clockwise).
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.
Further points on fitting
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.
System without a discharge service valve
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.