Residence time for electric gas heater 2

[9492975209]

I am trying to size electric air heater. I am looking for formula or procedure. This is a horizontal heater with Watlow heaters. I need to estimate cross sectional area and heater density.

 

[Montemayor]

What you need to do is get the Watlow heater flux curves or data. Watlow publishes these in their catalogs or can tell you the correct flux to use in this application.

Chromalox electric heaters (Watlow's competitor) also furnishes the same information. Gases and vapors have very low thermal conductivities and film coefficients and, as a result, the heat transfer is relatively slow. If you don't design for the correct flux (heat density), you can easily burn out the electric elements. There is no residence time involved, just the correct flux. I don't know what you mean by cross-sectional area; all you'll wind up with is the exposed elements' surface heating area - which is what conducts and radiates the heat developed away from the element.

You should be able to pick out the correct size and model from a Watlow catalog.

 

[9492975209]

Thanks for your advice. I think residence time is still a factor to ensure that enough time is allowed for (hc + hr) in a confined space heater to reach to the desired temperature.

 

[Montemayor]

Fine. But I would stress - for anyone else reading this thread - that the Fourier equation (the basis for heat transfer equations) is based on a heat rate which takes in the element of time. There is no so-called
"residence time" involved in the derived and recognized:

Q = U A (t2-t1)

I've applied numerous electrical heating elements to gas streams using the basic heat transfer technology and - if anything else - the important principle has been to keep the "heat sink" flowing (the gas rate) or else the elements would overheat and short out. I know the Chromalox brochures point this fact out very clearly and I would assume the Watlow variation follows suit.

I would suggest you employ the Watlow customer technical service and confirm the design procedure and what parameters to look out for in order to protect your heater and utilize it in the most efficient and safest manner.

 

[9492975209]

Thanks again for quick and valuable advice.