## Flare radiant heat transfer on atmospheric tank - heat transfer calc

## Flare radiant heat transfer on atmospheric tank - heat transfer calc

(OP)

I am installing a flare fairly close to an atmospheric tank, and am trying to determine the minimum height of the flare. I have generated isopleths for the governing cases, which give thermal radiation flux values (btu/hr sq ft). I wish to estimate the amount of vaporization of liquid in the tank. It would seem that using the flux rate solely might be too conservative, as this does not take into account the resistance to heat transfer through the insulation, tank wall, and inside coefficient.

Anyone have an opinion on this? If you agree, then do you have a suggestion for how to convert the flux value into a resistance term?

Anyone have an opinion on this? If you agree, then do you have a suggestion for how to convert the flux value into a resistance term?

## RE: Flare radiant heat transfer on atmospheric tank - heat transfer calc

If you need more rigorous results, some advanced modeling tools will be required.

Dejan IVANOVIC

Process Engineer, MSChE

## RE: Flare radiant heat transfer on atmospheric tank - heat transfer calc

## RE: Flare radiant heat transfer on atmospheric tank - heat transfer calc

For example, I know that a certain point on the tank that the flux from the flame center is 200 btu/hr sq ft. I believe I can come up with an absorptivity coefficient (probably .25), but I can't think of how to integrate this with the remaining resistantcies to heat transfer. I suppose if I could estimate the temperature of the flame that might help.

Thanks again for the help.

## RE: Flare radiant heat transfer on atmospheric tank - heat transfer calc

temperatureof that metal cover. As radiation pores heat into the cover, its temp will rise until it either melts (if it's aluminum) or until it reaches an equilibrium temperature, at which point the heat radiated into the cover equals the heat lost from the cover (through radiation and conduction) to the atm.## RE: Flare radiant heat transfer on atmospheric tank - heat transfer calc

The 200btu/hr/ft2 arriving at the tank insulation comprises of the following subsequent modes of heat transfer, which depends strongly on the surface temp of aluminum jacket

a) Heat absorbed which depends on the absorption coeff of the surface

b) Heat reradiated which depends on the emissivity of the surface

c) Heat dissipated by natural convection along the outside vertical surface of the aluminum jacket, and we could assume no wind cross currents

Each of these terms can be computed if we know what Ts is.

Further, component (a) must also travel across the insulation layer, the tank wall (assume zero resistance) and the not insignificant internal natural convection htc/ resistance, assuming there are are no forced internal convection currents.

Start with some trials on Ts, compute (a), then compute (b) and (c) and see if the total sums up to 200btu/hr/ft2 of external surface. For initial trials, we could assume the internal nat convection htc inside the tank to be negligible.

## RE: Flare radiant heat transfer on atmospheric tank - heat transfer calc

Normal convention would be to disregard any heat transferred to the dry walls of the tank (roof and top of side walls), but since the btu/hour in the roof is 10-15 higher than on the wetted walls, I thought I should consider the affect of the roof flux.

Should I consider a htc on from the vapor to liquid? If so, any recommendations on a method to calculate the htc?

Bill

## RE: Flare radiant heat transfer on atmospheric tank - heat transfer calc

Radiation component may be significant, since the liquid ammonia would be at -35degC or so , and will depend on inside roof wall temp. Radiation view factors (F) for this mode are in Perry Chem Engg Handbook in the chapter on Radiation Heat Transfer - I used the graph for view factors between parallel planes / discs on a similar calc some years ago. Also include emissivity values for the 2 surfaces as required.

Startup tank cooldown vaporisation rates would usually be the controlling case for boil off gas compressor duty estimates, but agree in this case, these external heat leaks into the tank may be large also. Also account for heat leak from the soil into tank bottom, and any other tank surfaces that may be deliberateley left uninsulated.

## RE: Flare radiant heat transfer on atmospheric tank - heat transfer calc

## RE: Flare radiant heat transfer on atmospheric tank - heat transfer calc

## RE: Flare radiant heat transfer on atmospheric tank - heat transfer calc

Thanks for the input. I had not thought of the radiation inside the tank (from tank walls to liquid surface). I have several old copies of Perry's, plus the 8th edition. Trying to come up with 6th edition to be sure I am looking at the equations/figure you referenced. Thanks,

Bill

## RE: Flare radiant heat transfer on atmospheric tank - heat transfer calc