For the same rate of heat transfer, the surface area of heat exchanger required with thermic fluid greater that what is required with steam. With larger heating surfaces the radiation losses also proportionally increase.
Steam provides excellent heat transfer; the condensation process effectively transfers the heat to the product being heated. Steam will supply heat by condensing at a constant temperature.
The overall heat transfer co-efficient during the heat transfer when steam is used a heating medium is much greater as compared to thermic fluid. This is because steam undergoes a phase change while it gives the heat to the process.
Thus to transfer a given quantity of heat energy, the steam based heat exchangers will be smaller as compared to thermic fluid based heat exchanger. The smaller the heat exchanger, the lesser will be the radiation loss from the surface. Also, the cost of the heat ex and the size also reduces.
For the same heat transfer, thermic fluid flow is 20 times that of steam flow!