Need to size a water tank heating coil to prevent icing. I evaluated the heat load of the tank and get the heat transfer coefficient between pipe and water. One question is how to get the temperature difference: ambient is -40degF, water is to maintain 32-35degF and steam temperature after control valve is 400degF. Is it right just to assume steam temperature constant and use (400-35)degF as deltT ? The area calculate in this way is fairly small.

This is a two-step calculation, and it seems you are trying to do it in a single step.  The way to go about this is:

Step 1:  Determine the heat loss from the tank to the surroundings.  This will be made up of convection and radiation from the outer shell of the tank.  The delta-T for these calculations will be 35-(-40).  Don't forget that radiation calcs need to use absolute temperature.  This will be the amount of heat that you need to add to the tank to prevent freezing.

Step 2:  Calculate the steam flow rate needed supply this amount of heat.  You know the steam inlet conditions, so look up the enthalpy.  The steam outlet conditions are going to depend on your control devices and where the discharged steam is going.  For example, if you have a metering valve and are controlling the flow of live steam through your heating coil and into a condensate system then your outlet condition is going to be saturated steam at whatever the pressure of the condensate system is.  If you have a steam trap at the discharge then the outlet condition will be pretty close to saturated water at 210F.

Condensate piping pressure will have to be MUCH lower than the steam pressure. If they're the same pressure, the heating coil will simply water-log, since you won't have any pressure differential across the trap. Remember that steam traps are nothing more than automatic valves, that open when they sense air and/or water, and close when they sense steam.

If you install the heating coil near the bottom of the tank, the convection flow will likely provide enough circulation.

I once installed a steam heater as a retrofit in a fat tank. We bought an off-the-shelf shell & tube heat exchanger, removed the head and bundle. We cut the shell off a few inches past the flange. We cut a corresponding hole in the steel tank, and welded the flange section of the heat exchanger shell to it. We shoved the bundle through it, into the tank, and bolted the head back on. (We left enough of the shell on the flange to enable us to do-up the bolts to re-connect the head.) The tank was now, in effect, the shell of the heat exchanger. We ran the steam through the bundle, and it worked like a charm.

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