If you are being thorough, evaluate the heat loss through each surface - liquid surface, tank bottom, shell, fixed roof and floating roof if there is one. Loss through the shell can be further separated into loss above the liquid level and below the liquid level. Each loss quantity may have several mechanisms - conduction, convection and radiation. Even when done casually, there are many factors that come into play for each so you have to know the properties of the liquid as well as the local meteorological conditions (temperature, insolation, wind, etc) everything about your tank and of course the insulation. From my experience (on mostly large tanks), loss through a single nozzle is overwhelmed by the overall losses through the tank shell but they can be calculated in a similar fashion as the tank shell.

Designing heating coils is rather simple after you know the heat balance. You need to know the characteristics of the (probably fin-tube) heaters, your steam properties and the liquid properties. That mostly gets you the footage of coil needed. Design and locate the coil packs with slope for condensate return, to provide good coverage and to create convective swirls that don't interfere with each other. Coordinate with fill, suction and recirculation piping, air sparging and any blending equipment.

For a quick and most likely successful installation, engage your coil vendor to do the calculations for you. Ask for and follow their recommendations to avoid coking.

My opinion only - remember - you read it on the World's Wild Web !!!

@IFRs i was hoping you would answer!
Thank you that’s very helpful!
And also when you said “locate the coil packs with slope”, did you mean that the coil should be helical downward attached on the tank shell?
Can you recommend any book?

Detailing is a hobby,

Jay - I only meant that the steam inlet to the tank should be higher than the condensate outlet for each coil, to promote flow of both. I usually see fin-tube coils in packs that start high and end up lower. Then the condensate line runs at a slope to the tank shell. Also, make sure you have allowed for thermal expansion in the piping running to the coils - a 90 degree angle with some length on both legs really helps. I'm not aware of a book on the subject but I'm sure Google could find some. For vendors in the USA, try American Heating, Heatec, Ryan Process Equipment, Armstrong-Hunt, RW Holland - there are tons of them and they do this for a living.

i'm going to check them out!
Thank you again @IFRs

Detailing is a hobby,