I have a 3-stage steam jet air ejector system that produces vacuum for a process. The two inter-condensers are direct contact, not surface.

The condensers currently operate on 75-80 F closed loop water. A big problem in the hot summer months is that our cooling tower cannot make water cold enough for the condensers. When the water temperature climbs to 90-95 F, we have to slow down the process significantly, or turn it off.

In theory, increasing the water flowrate through the condenser should allow us to operate at a higher water temperature (i.e. 90-95 F), since this is just a heat transfer problem. Does this hold true in practice, or are there other factors that need consideration?

I can also tell you, that the water inlet valves to the condensers are only 30% open right now. This is because the drain pipe was never sized correctly and water starts to back up into the condenser if the valve(balance vavle) is opened too much. The maintenance guys found that 30% open gives the "best performance" with the current setup.

Also, the small amount of documentation I have on these VERY old condensers is: Maximum condensing water temperature 95 F.

I have ruled out purchasing a chiller to bring the water temperature down because of excessive cost! Our flow rates are in the order of 500 gpm.

What type of internals do the intercondensers have?  There are a variety of types and styles some better in my opinion than others.  Whatever type you have, you need to maximize your heat transfer within the device that you have.  That may be changing the sprays, curtains, trays, or whatever is in there.  If you have any room for change there you may get more heat transfered to the condensers that you have.

Now, how about the jets?  Are the steam pressures per the nozzle design?  The steam temperature?  A jet sized for a given pressure saturated steam trying to operate on superheated steam can do different things than it is designed for.

What about wet steam?  Wet steam can cut things to ribbons in jets causing them to lose performance.

What about steam leakage around your nozzles?  Leakage around the nozzle ends up as load for the condensers without contributing to jet performance.

What about strainers in the steam line in front of the jets?  They can rob the jets of pressure needed for proper performance.

Some fine tuning may bring the jets back to design and solve your problem.

I have encountered few pristine jet systems that were operating at the design conditions on the nameplates.

There have been some good comments regarding the steam and the jet nozzles, but if the problem manifests only in summer the chances are that your first bottleneck is in the condensers as you originally suspected. Fix that first and then fine tune the jets.

The water flow will have an influence on the condenser capacity. The outlet temperature is what you need to work with. A well designed condenser will achieve an approach of 5 or 6 degrees F - i.e. the difference between the outlet temperature and the saturation temperature of the steam.  Increasing the water flow keeps the outlet temperature lower and helps the approach.

If there are no horizontal sections in your drain pipe (i.e. all are 45 degrees or steeper) you should be able to get away with it.  I have done that successfully.  But it makes the requirement for self venting sizing more critical.

If your 500 gpm flow is to each condenser then you need a 12" drain.  If it is split between the condensers then each drain can be 10".  I agree with CJKruger that self venting design is important, but I believe he is being unnecessarily conservative with his recommended superficial velocity.  In a 12" pipe you can go up to 1.7 ft/s and still be self venting.  And to 1.5 ft/s in a 10" pipe.

Some condenser designers do deliberately design the drains small to get siphon flow rather than self venting flow.  The theory is that siphon flow will suck some bubbles out with the draining water and increase the condenser capacity. I don't do this for 2 reasons.  Firstly the entrained bubbles make the fluid in the drain pipe less dense and it therefore has to be longer than if filled with liquid water.  Secondly, the siphon regime is fairly narrow and if you don't have just the right water flow you can have all sorts of problems.  Its a bit more expensive to install self venting drain lines, but more reliable IMO.

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