What is the difference between a boiler and a thermic fluid heater?

                       The primary difference is the medium being heated.  In a regular boiler water is heated and steam is generated directly from the water being heated.  In a thermic fluid heater a fluid, such as Terminol, Servotherm is heated.  The fluid runs through tubes which are submerged in water.  The heated water generates steam.  Both a boiler and a thermic fluid heater are usually considered, but not always, ASME Section I pressure vessels.

            

                                                                Boiler

Boilers are used to heat water & produce steam. This is commonly done by 2 ways:
1. Fire-tube: Hot flue gases (combustion gases) pass through tubes submerged in water. Water gets heated due to conduction through the fire-tubes.
2. Water-tube: Here,water flows through the tubes, which are heated by - (i) Radiation from hot flames, & (ii) Convection from hot flue gases.

                                  Thermic Fluid Heater

Thermic fluid heaters are used just to heat the water, not necessarily producing steam. Water is heated by passing hot thermic fluid in tubes submerged in water. This arrangement is similar to the fire-tube boiler described above,except that thermic fluid is passed through the tubes instead of flue gases.