Direct Fired Heaters are used for specific applications. We could consider Direct Fired Heaters basically as custom-designed equipment, given that the heat is directly transmitted to the product of the process, and any change in this characteristic could render the equipment ineffective for the new conditions of the process or even destroy the product or equipment due to unsuitability.
The determination of the heat transfer rates to or from a system and, therefore, the heating or cooling times and temperature variation is the subject of the science of heat transfer.
The term convection comes from the Latin convectare, which means to take to a place. In heat transfer, the concept of convection refers to the process of moving thermal energy from or to a solid by means of an adjacent fluid in motion, in the presence of a temperature gradient
P&ID is defined as a diagram of piping and instrumentation and it shows the process flow in pipes, as well as, the equipment installed and instrumentation.
In the analysis of heat transfer processes, some bodies can be considered to behave as a unit, with an internal temperature essentially uniform (although not constant) at all times during the heat transfer process.