How you are heating the steam? Heating through pipes or a separate heater? what is the heater capacity?

First u need to know heat capacity of stream which can be obtain by standard data tables.now find the heat transfer took place by finding the enthalpy difference in stream before heating and after heating.now calculate velocity , reynold no. ,prandalt no. Of stream flowing.then use sidder tate equation to find heat transfer equation from nusset no. And then find area of heat transfer.equate the area with area of cylinderical pipe.now u can find length.

Heating though pipes. Heater Capacity of cast iron is 55 W/(m·K)

Obviously this length is quite long. This is a hypothetical.

You should know the mass flow rate of steam, then By using energy equation (first law of thermodynamica) you can calculate the rate of heat transfer required to heat the steam. The reqiured length of the pipe depends on the heat source also, like for instance the source temperature, the mode of heat transfer....... etc. So please tell us how do you want to heat the steam,

I think the steam would be heated by thermal radiation of the iron pipe in which the steam is in (moving as this question evolves). Lets say the temperature of the iron pipe is 625 degrees Kelvin.