Following pressure testing and prior to evacuation, all air and moisture should be removed, as for as possible, by draining, purging with oxygen-free nitrogen and possibly using heated air. Significant quantities of water in a system cannot be removed in practice by a vacuum pump as this would take far too long. The principle of evacuation is to reduce the pressure of any moisture left in the piping to a saturation temperature well below the ambient temperature, shut off the vacuum pump and observe any pressure rise which indicates a leak or large quantity of moisture in the system which needs to be boiled off. Then the vapour pressure can be lowered as much as possible. The vapour pressure of water, expressed in a number of units, all as absolute pressures, is shown in Table 11.1 (see also units of measurement in Appendix).
The vacuum pump should be connected to draw from all parts of the circuit. This may require two connections, to bypass restrictions such as expansion valves, and all valves must be opened within the circuit, requiring electrical supplies to solenoid valves and the operation of jacking screws, where these are fitted.
The final vacuum pressure should be lower than the water vapour pressure at the lowest part of the system, and is therefore dependent on the system temperature at its coldest point. The process may be speeded by using a triple evacuation procedure that involves breaking the vacuum with oil-free nitrogen to help absorb the moisture and re-evacuating.
On small systems, such as factory packages, a typical final pressure of 50 |xm Hg (7 Pa) should be reached. Vacuum pumps of good quality which will tolerate refrigerant gas must be used. Evacuation of a large system may take a couple of days. During this time, checks should be made around the pipework for cold patches, indicating water boiling off within, and heat applied to get this away.
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