Removal of Moisture From Systems by Vacuum Pump

Standard Specification No: 107

L:\Standard Specifications\SS107.C. Removal of Moisture from Systems by Vacuum Pump.doc of 4

Removal of Moisture from Systems by Vacuum Pump 1.0 Introduction The purpose of this specification is to improve the effectiveness of our procedures for removing moisture from refrigerating systems at the commissioning stage on sites. It should be noted that this is not a factory vessel dehydration standard where different and rigorous procedures are required and where we apply heat to pressure vessels to remove water following hydrostatic pressure tests. 1.1 Notes 1.1.1 Evacuating a system before charging serves two purposes. The obvious one is that of

removing air and other non condensible gases from the system. The other equally important purpose is the removal of moisture.

This document is mainly aimed at all systems. HFC halocarbon refrigerants and R744 (CO2) are much less tolerant of moisture than

ammonia. 1.1.2 In the case of ammonia systems it is important to remove non-condensible gases before

charging but high quality vacuum pumps are not required. 1.1.3 Vacuum dehydration will not be effective at temperatures below 10ºC. If it is suspected that

a halocarbon or R744 system contains moisture and the temperature is below 10 ºC then it may have to be heated – consult the Technical Department.

1.1.4 Former practices such as “triple evacuation” using ordinary compressors and expendable

refrigerant are no longer allowable because they result in escape of refrigerants to the atmosphere.

1.1.5 The use of a vacuum pump alone to remove moisture will not work if there is a lot of water

left in the system either from the hydro test or following water ingress during transport or storage. In these instances all steps must be taken to remove water by draining the water from the low points of vessels and heat exchangers before connecting the vacuum pump. In exceptional circumstances heat may also have to be applied but always seek agreement from the Technical department before applying heat to a refrigeration system or component.

1.1.6 Special care is required where installing air coolers into a chill or cold store where the room

is already down in temperature. In this case it is very important that we never leave the cooler connections open in the roof void where water vapour can enter and condense inside the coolers. With the room down to temperature vacuum dehydration will be totally ineffective.



Also, where chillers are being installed or maintained it is important that chilled water is not

allowed to continue to circulate when the refrigerant circuit is open to atmosphere or again water can be condensed inside the system.

Degrees of vacuum are often described in units of mmHg or torr where: 1 atmosphere = 14.7 psia = 760mm Hg = 1.013 Bar (A) 1mm Hg = 1 torr = 1,000 microns = 1.33 mbar = 133 Pa 2.0 Equipment and Procedures 2.1 For the evacuation of an industrial refrigerating system, a vacuum pump of reasonable size

will be required. This should have a swept volume in the range of at least 280 to 425 litres/minute (10 to 15 cfm). The vacuum pump should be associated with a vacuum gauge capable of reading down to at least 1 torr or 1,000 microns. It is desirable to have a vacuum gauge on the vacuum pump side of the isolating valve to check pump performance and there must be another vacuum gauge on the plant side of the isolating valve to check behavior of the vacuum when the pump has been valved off from the system. It is very important to use good quality vacuum gauges and that these gauges are carefully stored and looked after when not in use. Gauges that have been subject to rough handling or that are in any way in doubt should be replaced or recalibrated.

2.2 The vacuum pump must be in good condition and capable of pulling a vacuum of 1000

microns against a shut valve. 2.3 The correct grade of vacuum pump oil to suit the pump must be used and must be changed

at least after every 10 hours operation or when it becomes “milky” or emulsified due to moisture. Always put new oil in the pump before starting to evacuate a system. Wherever possible we should avoid pumping refrigerant through the vacuum pump because the refrigerant will contaminate the oil and rot the pump valves.

2.4 The vacuum pump must be connected to both high and low sides of the system being

evacuated. 2.5 Solenoid valves in the system should be jacked open, have a magnetic coil valve lifter fitted

or their seats and armatures should be removed. Where check valves or thermostatic expansion valves are likely to prevent or restrict a flow path to the pump, system connections from either side of the valve will be required.

2.6 A good place to connect the vacuum pump is in place of a relief valve on either the high or

low side of the system. For this purpose a ¾” male NPT to ¾” male flare fitting would be appropriate. The connections to the vacuum pump must be a generous size, minimum ½”. 6mm tube may be used to connect to the high and low sides of the system if it is a very small system (say internal volume less than about 200 litres).

2.7 Before starting to pull a vacuum, the complete system should be subjected to a strength

pressure test and a tightness test using oxygen-free nitrogen from a cylinder with a reducing valve. The pressure test should be at the pressure stated on the refrigerant circuit diagram (Diagram of Connections or P & ID). The pressure test has the additional benefit of removing scale or flux which might seal a leak under vacuum.



2.8 Blow off all the pressure. 2.9 Connect the vacuum pump and commence evacuation using the gas ballast if the pump has

this facility. 2.10 Evacuate until a steady condition of 8 torr (8,000 microns) or less is achieved. 2.11 Shut the vacuum pump suction valve and observe the vacuum on the system side, using a

good quality vacuum gauge. Wait for 30 minutes. If the pressure rises by more that 2 torr then this is not acceptable and it is likely that either there is moisture in the system or a leak or both. If the pressure rise is due to moisture it will be slower than if there is a leak and will tend to level off at about the values given in the table in 2.12 for a given temperature. If moisture is present stop the pump and remove the vacuum gauge. Use OFN to raise the pressure to atmospheric and leave the system to stand for at least 1 hour to allow the dry nitrogen to absorb moisture. Change the vacuum pump oil and run the pump again with vacuum gauge fitted. The vacuum pressure achieved should be lower than the first evacuation. This process will need to be repeated until the level of vacuum achieved and held after stopping the pump is less than the values in the table, 2.12 only then will all the moisture have been removed. The final maximum vacuum required and to be held can then be attained.

2.12 Once all possibility of leakage has been eliminated the pump should be operated without gas

ballast and run until a vacuum of better than 2 torr (2,000 microns) or 2.66 mbar has been achieved and maintained for several hours.

Note, virtually all of the air in the system can be removed in about 10 to 15 minutes.

However moisture takes much longer. It is very important that a sufficient length of time is allowed. The length of time required to remove moisture depends on the quality of the vacuum pump and the temperature of the system at the time of the evacuation. The table below is a good guide but only if you are using a high quality vacuum pump with good oil. The “temperature” represents the coldest part of the system.

System

Temperature ºC

Water Vapour Pressure

Torr Minimum Time

Hours

24 22.38 3 18 15.47 8 13 11.22 50 10 9.2 72

<10 Not applicable In case of ammonia (where we are mainly interested in getting rid of non condesible gases) the pump should be run until a vacuum of better than 10 torr has been maintained for a minimum time of 1 hour. 2.13 When it is clear that there is no moisture in the system, the vacuum pump should be isolated

and the vacuum should be broken with refrigerant vapour. Particular care is required when charging CO2. to prevent dry ice forming.



2.14 If the vacuum pump has a gas ballast facility it is good practice to run it for some time after

dehydration is complete, with its suction valve shut and the gas ballast fully open. This tends to remove contaminants from the oil. Vacuum pumps are only as good as the vacuum oil which they contain. When moisture has been removed by use of a vacuum pump, some of the moisture contaminates the oil and must be drained off or driven off to restore the effectiveness of the pump.

Remember, it takes time and heat to remove moisture by vacuum. The process cannot be hurried. It is much easier in summer than it is in winter. Starting to charge a refrigerating system which has moisture in it will waste a lot more time and money than waiting for the vacuum pump to do its job properly.

Date Issue No Originator Change Details Section

Sept. 1998 1 DJH March 1998 2 DJH May 2009 C DJH New requirements on starting with new

vacuum pump oil and on changing it added. Extensive rewrite and emphasis placed on use with halocarbon and carbon dioxide refrigerants. Extra guidance on vacuum pressures. We should aim to achieve and hold times added.

All

Removal of Moisture From Systems by Vacuum Pump

Documents

Transcript of Removal of Moisture From Systems by Vacuum Pump