Tank Heaters - W/m

[landrover77]

Gents

I have a tank of lubricating oil that i wish to pre heat before it can be used to lubricate a compressor.
i wish to pre-heat to 40degC from a min of 0degC, within a 12hr period. I have accounted for losses through the walls of the vessel and also had a rough estimate from the surface of the oil. The tank is not agitated.

I wish to check whether the heater size and length (which is limited) may result in local over heating of the oil.
This presumably is a function of the..
Surface temperature of the heating element. - which is related to the local convection currents.
Effectiveness of natural convection though the tank away from the heater.
Max oil temperature 120degC.

Which approach would you take to estimate the effective of natural convection though the tank?

Are there rules of thumb in terms of w/mm for heaters?

Thanks in advance

 

[Compositepro]

Look in the Watlow or Chromalox catalogs (on-line)for recommendations. They have two or three standard watt densities they offer for different applications. High viscosity and lower decomposition temperature requires lowering the watt density.

 

[landrover77]

thanks much appreciated

 

[TD2K]

You know the duty and the heater size so I'm assuming you have some idea of the area involved, that gives you a heat flux.

Calculate the natural convection coefficient for the oil, you can do this at different temperatures as the tank warms up and see what film temperature that back calculates for. If you exceed your 120C, you can try a staggered heating approach, less duty when the oil is cold and will have a lower heat transfer coefficient and up the duty later when the oil is warmer.

Any chance of using the lube oil pumps for circulating the oil through the heater, that's going to significantly increase the oil side heat transfer coefficient.