Vacuum Systems

Mr. Lieberman – Which should be hotter – the vapor or liquid outlet of my vacuum tower pre-condenser? Currently the liquid outlet is approximately 20°C and the vapor outlet is 40°C. That difference is increasing and our tower top vacuum is becoming progressively worse.

--L.T. Lee

            To Mr. L.T. Lee – By design, the vapor outlet of a normal surface condenser (where the vapor outlet is located on the side of the condenser shell) is cooler by about 20°F-30°F. This is accomplished by an internal baffle located inside the tube bundle. It’s called the air or vapor baffle. It forces the vapor to flow down and then up across the tubes.

            There are two possible reasons for your problem. First this air baffle may be leaking. This is always caused by defective air baffle seal strips (please see my book, Process Equipment Malfunctions, McGraw Hill, 2011.

            Secondly, there may be condensate back-up due to inadequate drainage from the seal leg. Your seal leg may be plugging or your seal drum may be filling with corrosion products or sludge. Blowing out the seal legs with steam will help in this case. Perhaps the seal leg is sucking in air, due to a seal leg leak. Such a leak will prevent proper drainage through the seal leg.

            Regardless of the cause, an increase of the vapor outlet temperature is certain to cause a loss of vacuum and could possibly result in the jets making a surging (i.e., erratic) sound, which is a certain indication of a loss in the sonic boost of the jets.

--Norm