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Tech Tip - Losing Vacuum when Steam is Applied
Why do I lose vacuum when steam is applied?
Picture here is a single effect evaporator with a vertical tubular condenser showing vapor entering the condenser and condensing the vapor into a liquid which is removed by the condensers condensate pump. This is how it should work if all vapor is condensed. So if you can pull a good vacuum without stem applied but lose vacuum when you start steam here is why.
When steam is applied and heat energy is transferred to the condenser. The vapor must condense thereby reducing it volume or space that it occupies. Think of it this way one pound of steam at 135 F occupies 140 cubic feet. when it condenses it in to one pound of water it occupies only 0.016 cubic feet of space. This allows more steam to enter the condenser and condense. If the condenser cannot handle that much vapor the pressure begins to rise and so does the temperature because all the vapor is not being condensed. The pressure builds and vacuum is lost. Therefor, when no steam is applied and you attain a good deep vacuum and you lose vacuum when steam is applied, it's not a vacuum leak, it's your condenser, condensate pump, or cooling tower that isn't removing the heat energy as fast as it is coming in..
Why can’t the vacuum reach or hold set-point?
There are two main physical possibilities assuming the vacuum pump and controls are working correctly and the vacuum pump has adequate service water applied and it is below 65 F. The first main reason would be excessive vacuum leaks in which case the control valve would be completely closed not allowing any outside air in. In this case it would present itself if before steam is applied.