Vacuum Rise Test Misconceptions

[DTLarca]

I think I have now been able to upload the images...

Pre-Commissioning Tasks

Prior to commissioning a refrigeration or air conditioning system we carry out three key tests on the pipework. A strength pressure test, a tightness pressure test and a vacuum rise test. The tightness test is to prove the system does not have leaks and the vacuum rise test is to prove the system is moisture free.

The tradition is to carry out the vacuum rise test only at the end of the vacuuming period. Commonly 24hrs vacuuming is applied to smaller systems. The first 4 hours to reach the target Torr level and the other 20 to remove the small quantities of pipe and component impregnated moisture.

A recent disappointing experience has motivated me to ask a few questions. The answers so far have made me think the traditional sequence of method followed by myself, and those I have spoken to, should probably change.


My Question

If we vacuumed a newly installed system to 0.5 Torr, over a period of 24 hours, is it possible to then see no rise in the vacuum within 30 minutes, after the vacuum pump has been isolated, even if there in fact is still free water in the pipework?


Time and Heat

At 0°C and 1 atmosphere liquid water has an arbitrary heat content, enthalpy, of 0kJ/kg and then at 0.5 Torr and -27°C on the sublimation line, where vapour can deposit as ice crystals, the enthalpy is -388kJ/kg. At 0°C and 0.5 Torr water can only exist stably as a vapour shown as point A in figure 2. Vapour at 0.5 Torr has an enthalpy of 2451kJ/kg. As such, on starting a vacuum pump, the 0°C water mass only has enough heat to vapourise 16% (388/2451x100) of itself taking it to point B. The other 84% of the heat required has to be conducted and radiated from the pipework. A major time constraint is the saturated vapour’s specific volume down at 0.5 Torr where it would take a 5CFM vacuum pump nearly 11 days to remove the volume of vapour produced by each kg of ice. The idea is to instead keep up the heat supply which keeps up the vapour pressure and therefore vapour density where, with proficiency, can result in a full kg of water being removed in a single working day.


Vacuum Ballast Valve

Ice originally in contact with the pipe walls would sublimate leaving a large remaining portion reliant purely on radiant energy for its heat source. Ice subliming under vacuum of course by definition does not pass through a liquid phase on taking heat so there would be no liquid pools ever bridging the thermal gap between suspended ice and the lower pipe walls. Ideally the vacuum pump gas ballast valve would not allow pressures to fall below 4.6 Torr in order that freezing is avoided in addition to reducing moisture condensation in the pump oil. Unfortunately, with most modern vacuum pumps, the difference in achievable Torr with and without the gas ballast valve open is a mere 0.2Torr, not sufficient to prevent the water freezing. Instead a difference of about 4Torr is required.


Vacuum Rise Test Illusion

Upon closing the service gauges for a vacuum rise test if, during that motion, the vacuum happens to rise just 0.3 Torr from point B to point C from 0.5 to 0.8 Torr, as is commonly seen when free water exists but is as commonly ignored, then the sublimation line moves 5K higher above the ice temperature to point D instantly subcooling the remaining ice. This means it is quite possible that any quantity of ice still in the piping could avoid detection by a mere 30 minute vacuum rise test and even for as long as 120 minutes under winter conditions.


Proposed Method Modifications

To ensure ice quantities formed do not also hide below a transitional sublimation temperature it seems that a 30 minute vacuum rise test should instead be carried out earlier after the first say 4hrs of vacuuming immediately after the first instant say 2 Torr is reached. Even with free water in the system it can take 20 minutes for the vacuum rise test to reveal its presence despite a prior vacuum period of merely 2 to 4 hours. If the initial 30 minute rise test detects moisture then another rise test done patiently over perhaps 24hrs should be carried out after each subsequent vacuuming session. Until the system passes any follow up vacuum rise test vacuuming should be done with the gas ballast valve open and the oil changed frequently. Each subsequent vacuum rise test need only be done over 24 hours if the pressure attained during each vacuuming session was below the triple point of water, approximately 4.6 Torr, under which all remaining water is likely to have frozen and so escape the detection by the traditional test method.