Condenser optimization

[twr]

We have a shell and tube condenser off a flash chamber - product temperature into the flash chamber is 300ºF, we want to drop the temperature to 185ºF but no lower. If we go lower our solids increase and puts the product out of spec.
The condenser has been fouled. We ran an acid wash through the system and it is running better.
As part of this project I contacted the OEM. They provided engineering data that indicates a vapor temperature drop from 300ºF in to 100ºF out.

My first question is - if we run cooling water through the condenser to drop the vapor temperature to 100º F, will that also drop the product temperature out? Right now the vacuum on the condeser varies between the saturation pressure of the product out the flash chamber and the saturation pressure of the vapor into the condenser.

My second question - the engineered LMTD is calculated out to be 65. Our condenser had been running at LMTD of 125. After the cleaning, with a limited increase in shell side flow, the LMTD came down to 117. Should I try to get the LMTD back down to 65?

This site has helped me immensely get as far as I have. Now I'm looking to change the control around a bit further, possible optimization on the condenser cooling water loop to limit the flow of raw water in.

 

[DickRussell]

Tell us more about the exchanger configuration. Presumably it is horizontal, with CWS on the tube side, condensing on the shell. For the most part, if you get vapor out at 100 F, then the liquid will approach that too, except that condensate may run along the bottom without as much subcooling, depending on configuration. You don't want to throttle the CWS for control, assuming it is in a loop from a cooling tower, since lower velocity and higher outlet temperature leads to more fouling.

How about a tempered water system, with a closed loop of very clean water involving a tank and pump, with a slipstream going through another exchanger cooled by fixed flow of normal cooling water, so as to maintain control of the tempered water at some elevated level to cut down the MTD in your condenser as required to control liquid outlet temperature.

 

[twr]

This is a vertical tube and shell, single pass, the shell side has notoriously dirty tower water for cooling - the cheapest available. The tubes carry the vapor over from the flash chamber.
We have been running a restricted flow (ball valve control) on the cooling side - I am trying to get the cooling side ball valve out of there entirely - we have a back pressure valve downstream of the control valve.

 

[UmeshMathur]

Using ANY flow restriction on the coolant side is a great way to encourage fouling. You must design a better scheme for controlling the cooling. Dick Russell's tempered water system would be a great start to eliminate fouling on the coolant side in the main exchanger.

The secondary exchanger (to cool the tempered water) would use the water from the cooling tower and should be made redundant. Thus, you would have one exchanger in service and the other on standby when the unit in service goes out for cleaning.

 

[twr]

Thanks for the response - the control system is a nightmare with multiple loops on the cooling side. I plan on starting this effort by getting rid of the 2" bypass ball valve that has been the temperature control out of the condenser. The ball valve worked around a 4" butterfly that was taken out of the loop, that I've been manually opening.

No matter what I do with those valves, downstream there's a 2" globe valve for backpressure control. Cv of 54, max flow ~295 gpm (with a 30 psid). Normal pressure drop is 10 psi, so the flow is barely 200 gpm.

I like your ideas, this line has been in production for a few years and it runs just fine - no-one is entirely convinced there is a problem, so I need to generate the justification.