Tank Heater Rules of Thumb 1

[dogbertcountry2]

I was wondering if anyone had some general rules of thumb for design of insertable tank heaters especially in regards to quantifying/identifying required heat loads. I have looked at examples that Tranter offers and got a few ideas...

I am faced with replacing an existing heater in an existing tank with no past engineering calcs to justify the size. It has been suggested that we decrease the length of the tubes by 10% due to tube availability (bayonnet U-bend type heater). I am just trying to put together some technical backing to whatever we decide to do.

 

[TD2K]

I'm not sure about rules of thumb.

Sizing a tank heater will depend on the heat losses from the tank, eg. temperature maintenance, as well as any heat loads associated with bringing fresh fluid entering the tank from an initial temperature to the final temperature.

You really just need to work through the numbers. In addition to Tranter, Brown Fintube is another company you may want to talk to.

 

[Montemayor]

dogbertcountry2:

You say you're looking for rules of thumb for tank heaters. If so, I wouldn't worry about a 10% difference in heat transfer area making a difference in the results. Such a "rough" device would normally have 25 -50% contingency due to a lot of unknowns - as TD2K implies. A lot of the heat transfer efficiency and rate will depend on the convection currents in the tank: with an agitator these will increase dramatically; with heavy, viscous and static liquids these will be poor.

I would use the same heater area as the existing - if this has proven to be historically sufficient for what you need. If need be, I could increase the efficiency by hooking up a recirculation pump with an eductor to increase convection in the tank and achieve mixing. I could also employ a suction heater: a bayonet-type device which is connected to the suction of your tank's pump-out/pump-around pump. These devices are much more efficient than the conventional immersion types because they use the convection current created by the pump's suction.

The rules of thumb always involve:
1. Keep the heating elements off the bottom and out of any precipitated sludge or solids in the tank; this reduces corrosion and heat transfer fouling;
2. Keep the tank well mixed; an agitator is great - but expensive and maybe prohibitive; a pump-around may be possible;
3. Use heavy gauge tubes/pipe for the element or coil in order to reduce possible leaks into the tank, causing contamination.
4. Slope your tubes/coils well enough to ensure ready and easy condensate drainage; this avoids condensate hammering inside the element.

I hope these help.

Art Montemayor
Spring, TX

 

[FredGarvin]

This may help a bit. It's worth a read if you have nowhere to start from:

http://www.chromalox.com/literature/tank.pdf