No Vacuum in a Multiple Effect Evaporator

[ReddeerC]

Dear Colleagues,

I am now working on a 3-effect evaporator in my plant. The evaporator is mainly composed of 3 flash drums, 3 heat exchangers, and 1 final condenser.

Steam and Vapor flow direction: steam from other production process goes to the 1st effect heat exchanger; the vapor from 1st flash drum goes to the 2nd effect heat exchanger; the vapor from 2nd flash drum goes to the 3rd effect heat exchanger; and the vapor from the 3rd flash drum goes to the final condenser.

Feed direction: the feed goes to the 3rd effect heat exchanger; then 2nd; and at last to the 1st effect heat exchanger. From the 1st effect heat exchanger, the feed is pumped into final product bin.

The 3-effect evaporator is using a 2-stage ejector to vent remaining vapor/steam to the ejector itself condenser through vent lines connecting the 3 heat exchangers and the ejector, and then discharge them to the atmosphere.

The problem I am facing is that there is no vacuum in all the 3 flash drums (According to the design, the drums are supposed to have vacuums in them). I am wondering: what should result in vacuums in the 3 flash drums? The vacuums in the flash drums are generated by the 2-stage ejector? Or, the vapor condensation process in the 2nd and 3rd heat exchangers and the final condenser causes the vacuums in the drums?

Any one can recommend me some books about multiple effect evaporators?

Thanks for your time and help in advance.

Sincerely.

 

[ReddeerC]

It seems this post is not in the right place!

 

[rmw]

Vacuum is not created by the 2 stage ejector. The ejector merely removes non condensables from the final condenser. The vacuum is created by the collapse of the vapor in the condenser (and in the various stages depending on their temperatures.

The ejector(s) do pull the condenser down to an operating vacuum point prior to the admission of feed, but once it is there, it is the condensation of the final vapor that is sustaining the vacuum.

You need to look at your design temperatures of your feeds and your cooling water (medium) in the condenser as well as the performance of the 2 stage jets.

Ejectors can have nozzle erosion or internal bypassing that don't allow them to remove the N/C's which will not permit the condenser to operate properly. Check the motive steam pressure and temperature. Operating far from the design points can cause the jets to perform poorly. Check the jets inter-condenser and after-condensers to make sure they have the proper spray patterns and to make sure that there is no problem with the tail legs. (Same with the main condenser.)

If the last stage jet exhausts into the sump, make sure its discharge area is baffle separated from the other tail leg(s). Air and steam from that jet can be pulled into the legs of other condensers if this isn't done.

Check all piping and condenser vessels for leaks.

I typically recommend

http://www.graham-mfg.com

as a good resource because you can find some good technical papers on their site (or could last time I visited some time back.) There are other good vacuum sites, some often recommended in these fora. Do a search.

Post back if you check all this and don't get results.

rmw