Adjusting which valve?

[DieselJunky]

Hello,

I'd like to know what is the best to do when you want the highest gain for heat tranfer when heating a tank with thermal oil or low pressure steam. It concerns the valves before and after the tank. Which valve is best used to control the tank temperature and have the best heat transfer?
For steam i think it's the inlet valve to make the best use of the expansion heat, but as thermal oil is a liquid and doesn't expand very much, does it matter which valve you restrict?

Bart

 

[Montemayor]

DieselJunky:

If you are trying to control the amount of heat input into a tank coil (or a heat exchanger) you don't necessarily have to throttle the inlet (or outlet) of the heating fluid.

For example, if I were using a heating oil (i.e., Dowtherm) I normally would use a 3-way by-pass valve around the tank or exchanger. If you're using a heat fluid other than steam, I would expect that you would normally be circulating it and not subjecting it to a throttle. But, if you want (or need) to throttle, I would throttle at the inlet section. I'm presuming the coil inventory is rather small and doesn't make for a large band in the residual heating when the stream is throttled. This is the usual case. If you're dealing with a liquid heating fluid, I see no measurable, practical difference whether you throttle on either side of the exchanger - except that throttling the outlet keeps the heating surface totally pressurized 100 % of the time - if that makes any difference in mechanical stress on the unit.

But, I prefer to by-pass instead of throttling heating fluid. This keeps my circulating pump always pumping at the same flowrate (more or less) and keeps it from undergoing dead-head pressure stress and installing minimum flow instrumentation.

Art Montemayor
Spring, TX

 

[DieselJunky]

Well, as i'm a maritime engineer on merchant ships, i don't have too much choice. The heating-medium is going to sevarel tanks and systems. When one tank or system is adjusted, this will evidently influence the rest, but normally these fluctuations are hardly noticable. Furthermore there are low-flow alarms and trips in the system, just as an overpressure valve between suction and pressure of the pump. Most thermal oil regulating valves are either normal or flow-control valves, which should be mounted on the right place. Here comes the probles, because most of the time i've sailed Chinese built ships (not my first choice, but i've got to deal with it as it is) and Chinese people just dont care what they place where, so its hard to find out things like that in situa sometimes.

But thanks for your quick response, in fact it is as i thought is would be, i just didnt have the right logic behind it.

Bart, NL