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[nastaran]

Dear all,

I'm supposed to design a water bath heater. I know how to calculate the coil length and number of passes as well as doing the fire tube thermal calculations too. The thing is, I can't size the shell, because I know how water volume or water + glyclol volume calculates. The water volume or(water + glycol) doesn’t appear in the heat transfer equations. I think that I should consider a minimum volume to avoid high temperature in bath and bath vaporizing am I right?

Should the heater shell be big enough to place coils and fire tubes or should the volume of water be cosidered for sizing of shell too?

Thanks in advance,

Nastaran

[Art Montemayor]

Nastaran:

You are correct; the amount of water volume contained in a water bath heater does not enter into the heat transfer calculations for the equipment. The equations are based on the RATE of heat transfer, not the quantity.

If your heating medium (water + glycol) is receiving sufficient heat by your burner and fire tube combination at a rate that keeps the mass of liquid at the design temperature so that it can, in turn serve as a heat source for the fluid you are heating in the coil, everything should go as expected and designed.

What the designer and/or fabricator of the water bath heater normally does is that he designs the unit such that it has sufficient space (of volume) to mechanically allow for ease of operation and removal of the fire tube and coil(s) in order to carry out inspections and maintenance in the future. He also takes care to ensure that the creation of natural and efficient convection currents are created in the heating medium such that a favorable film heat transfer coefficient is obtained on the heating medium side – especially if the fluid in the coil(s) is a gas. The gas side coefficient is usually the “controlling” coefficient (the weakest or most inefficient) while the liquid heating medium coefficient can be increased with favorable convection.

By taking the above into consideration, enough heating medium inventory is usually assured within the water bath heater to successfully carry out the designed operation. This type of specialized equipment always carries with its design a lot of empirical knowledge and “know-how” – engineering that is not found in academic text books or classrooms. The most successful and well-known designer/fabricators have spent many years accumulating the required knowledge and experience and are not willing to part with it or give it away. As a beginner, you should be prepared to learn and improve with experience.

Good Luck.