Vacuum Systems

Dear Mr. Norman – Our vacuum tower top pressure is 40-45 mm of mercury. We have two large vacuum jets on top of the tower. The front part of both jet bodies is around 80°F.  The steam supply is 150 psig and 360°F. The temperatures are obtained using my infrared gun. Is that 80°F normal, or good, or bad?

-- Henry R.

           

Henry – The 150 psig, 360°F motive steam may be saturated or it may be wet. When steam expands through the ejector nozzle, its temperature is converted to velocity. The more efficient the conversion, the higher the velocity of the steam. The greater the velocity of the steam, the more efficient the compression of the vacuum tower off-gas. Hence, in that sense the cooler the mixing chamber (i.e., the front end of the jet), the better.

            However, if the mixing chamber is cool, that could be due to moisture in the supply steam, flashing as it enters the mixing chamber. This conversion of latent heat to sensible heat just robs energy from the motive steam and thus slows down the motive steam as it enters the diffuser. And this is bad.

            If the part of the diffuser immediately downstream of the mixing chamber doesn’t get too hot to touch within one or two feet of the mixing chamber, that’s an indication of wet steam.

--Norm