Heating Vessels

L. Koss

I am designing a closed-loop hot water system to heat several agitated, jacketed vessels. All heat transfer calculations were made based on a 311 F supply temperature. At this temperature, the saturation pressure is 65 psig. In order to avoid flashing into steam, system pressure must be above 65 psig (maybe less, because return temperature is actually lower). I have calculated the total pressure loss (mainly friction) through the loop to be about 22 psi. Flow rate at the pump is about 15 GPM. My questions are:
Q1. Do I need to specify a pump that delivers 15 GPM @ 22+65=87 psi in order to overcome losses and have min 65 psi in the system? If so, how do I obtain that pressure at the pump discharge? With a control valve at the pump discharge or inlet? Other device?
Q2. I have seen several loop configurations for hot water: a) air separator>pump>boiler>load; or b) boiler>air separator>pump>load. The advantage of a) (pump before boiler) is that the pump works with colder water. Is there an advantage in using b) besides the fact that the separator will be running hotter and separation could improve (lower air solubility)?

Thanks for the help.

Luis

Brad White

Differential versus static pressure

If you are installing the circulator (circulator not pump by definition) entirely within the pressurized system, all it does is impose a differential pressure in the circuit to induce flow. It does not add pressure to the system per se, just a differential across it. The means to pressurize the system would be external, a true pump, expansion vessel, or system fill setup.

Your expansion tank should be on the suction side of the circulator and that is where your minimum static pressure is defined. The discharge of the circulator will increase this but only (if calculated correctly) to the friction needs of the circuit.

I suspect you will need a special circulator, certainly not a wet rotor type with those temperatures. Most commercial circulators have a rating of 125 or 175 PSIG. I would go at least 250 at those temperatures.

A mechanical engineer can guide you vis a vis the ASME code. Title VIII covers unfired pressure vessels by the way. The circulator, valves, glands, seals, expansion tanks, all components in fact have to be duty rated for "pressure at temperature".

cattledog

why pressurized water?

Luis,

Why are you designing this application around pressurized hot water and not a thermal transfer fluid like Dowtherm which would run at low pressure? The system design may be simplified and the safety improved with a different approach.

Alternatively, have you considered using steam for your application.

My experience with industrial process heating would lead me to consider one of these approaches rather than pressurized hot water.

L. Koss

The product temperature inside the vessels is relatively low (70 C) so I thought that hot water could do the job. I did consider a thermal fluid but water was more familiar.

Agree that a thermal fluid would work at lower pressures and would make system components simpler. I will go back and study the possibility of using it.

Thanks to all for the comments. I will get back to you if any daubts arise.