Refrigerant may be charged as a liquid through the connection shown in Figure

11. 3 . The cylinder with dip tube is connected as shown and the connecting pipe purged through with a little of the gas to expel air from it. It will be necessary to invert the cylinder if there is no dip tube. For small charges, the bottle may be supported on a weighing machine, or a calibrated charging cylinder may be used.

The compressor must not be started while the system is under vacuum, so refrigerant is admitted first up to cylinder pressure. At this point, the compressor


Figure 11.3 Charging connection

May be started, assuming that all auxiliary systems (condenser fan, pump, tower, cooler fan, etc.) are running. A liquid-line valve upstream of the charging con­nection is partially closed to reduce the line pressure at this point below that of the supply cylinder, and the refrigerant will continue to flow in. While the refrig­erant can be safely admitted in this way, the system is not running normally, since the throttle valve is reducing the pressure across the expansion valve. At intervals during charging, the cylinder valve must be closed and a throttle valve opened fully. Only under these conditions can correct running be observed. When fully charged, the sight glass will be clear.

As an alternative method, the cylinder pressure can be increased by gently heating it. Any heating of this sort should only be done using a bath of hot water at less than 40°C or a thermal jacket while keeping a careful watch on the cylin­der pressure. Raising the cylinder pressure in this way avoids the use of the throt­tle valve and the charging process is much quicker.

If a receiver is in circuit, this should be about one-sixth full under normal running conditions. Refrigerant may also be charged as gas into the suction line. This is usually done when the system is running and is being topped up. The liquid must be evaporated before it goes into the compressor. This can be done by charging through a manifold set as shown in Figure 11.3 and throttling its low side valve. With the valve just open the liquid is turned into gas before going into the system.

Records of refrigerant quantities added to systems should be kept, and the cylinder is placed on scales for this purpose.

Systems having high-pressure float expansion valves, and those without sight glasses, must be charged gradually, observing the frost line or using a contact thermometer to measure superheat.

Small packages will have the charge marked on the nameplate and must be carefully charged to this weight, which will be critical.

Systems may need to have further lubricating oil added, to make up for that which will be carried around with the refrigerant. In the absence of any firm guidance from the supplier, the crankcase must be topped up gradually during normal running, until it is level with the middle of the sight glass under operating conditions. This is not so with the small hermetic systems, where there is usually sufficient oil in the compressor to supply the needs of the circuit, and which, in many cases, have no oil level sight glass.

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