Sight glasses can be used to indicate whether gas is present in a pipe which should be carrying only liquid. The main application in refrigeration is in the liquid line from the receiver to the expansion valve. Close to the expansion valve is best because observation here can determine liquid presence at the valve, and being downstream of the filter drier, it can be used to indicate the need for filter change. If the equipment is running correctly, only liquid will be present and any gas bubbles seen will indicate a refrigerant shortage (see also Chapters 11 and 29).
Sight glasses for the halocarbons are commonly made of brass, and should have solder connections, Figure 9.11. For ammonia, they are made of steel or cast iron.
Figure 9.11 Sight glass/moisture indicator (Henry)
Since the interior of the system can be seen at this point, advantage is taken in most types to insert a moisture-sensitive chemical which will indicate an excess of water by a change of colour. When such an indication is seen, the drier needs changing or recharging, and the colour should then revert to the ‘dry’ shade.
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