2 replies to this topic

[adolfoOR]

Hi,
This is my question:

In a heat exchanger:
Why it is controlled the flow of steam at the inlet (not in the condensate line) and on the contrary when a product is cooled in a heat exchanger the control of cooled water is at the exit of the heat exchanger?

Thanks

[Art Montemayor]

Adolfo:

I like this type of practical question. It is an important topic that is often overlooked by a lot of professional engineers.

Actually, some steam exchangers are installed with the condensate being controlled – at least in liquid level. I remember one of our members, Katmar (Harvey Wilson) wrote me some years ago regarding an old application he had come across in one of his projects that presumably carried my name on it. It turned out to be a Furfural Stripper reboiler that I had designed and installed in the original Furfural plant ever built in Cedar Rapids, IA when I worked for Quaker Oats Chemicals in 1969. I was, at that time, assigned to replace direct low pressure steam (approximately 35 psig) sparging into a Furfural stripper column with a new thermosyphon reboiler. We wanted to eliminate dilution of the bottoms product and conserve the steam condensate. Instead of throttling the low pressure steam into the reboiler chest and suffer a low thermal driving force, I decided to apply full steam pressure to the reboiler shell and control the condensate level in the vertical shell. I installed a condensate pot beside the reboiler and used a Fisher-Governor Level-trol to control the condensate liquid level. The steam condensate was recovered and sent back to the powerhouse. The system worked flawlessly from the very outset and everyone was pleased. Since that time, I have used the same technique on other applications.

Controlling the flow rate of cooling water on the outlet of a process cooler is one of custom, tradition, and also personal preference. I have never found anything basically wrong with controlling the water inlet rather than the outlet, but my preference has always been to control the outlet. I hope that some of our heat exchange experts on the Forum – such as SRFish (Dave Gulley) and Chris Haslego – have an opportunity and contribute their valued comments to this thread on these subjects. They may have more and better explanations than I have offered.

[Chris Haslego]

I'm a big fan of condensate level control for steam heaters, especially if the heaters are not too large (ie. a large condensate volume). This control scheme really cuts down on temperature / pressure variations that can cause mechanical problems with equipment. It's very good at avoiding over-heating the product as well. Just make sure that the condensate trap is upstream of the condensate control valve so that the control valve only encounters liquid. Also, make sure that you install a vacuum breaker on the steam side as well to ensure that the condensate can always drain. If your steam heater is large (I don't have a good way to define exactly what "large" is in this case), and you need a fast response from the heater, then liquid level control may not be the very best option. In those cases, I may prefer liquid side bypass or steam side control or a combination of these.

As far as placing cooling water control valves downstream of the heat exchangers....I always preferred that so that in case the water gets very cold (in the winter) or a significant reduction in duty is experienced, and the valve throttles down under 50%, I didn't have to worry about distribution of the water inside the heat exchanger after the throttling. If the valve were located upstream of the heat exchanger at a distance of say 15 feet or more, then distribution would likely not be an issue. However, often times, these valves are placed very close the heat exchangers, thus the preference for them being downstream.