There are three ways that heat may be transferred between substances at different temperatures viz. - conduction, convection, and radiation.
The flow of heat by conduction occurs via collisions between atoms and molecules in the substance and the subsequent transfer of kinetic energy
convection is the flow of heat through a bulk, macroscopic movement of matter from a hot region to a cool region, as opposed to the microscopic transfer of heat between atoms involved with conduction. Heating a pot of water on a stove is a good example of the transfer of heat by convection. When the stove is first turned on heat is transferred first by conduction between the elements through the bottom of the pot to the water. However, eventually the water starts bubbling - these bubbles are actually local regions of hot water rising to the surface, thereby transferring heat from the hot water at the bottom to the cooler water at the top by convection. At the same time, the cooler, more dense water at the top will sink to the bottom, where it is subsequently heated.
The third and last form of heat transfer we shall consider is that of radiation, which in this context means light (visible or not). This is the means by which heat is transferred, for example, from the sun to the earth through mostly empty space - such a transfer cannot occur via convection nor conduction, which require the movement of material from one place to another or the collisions of molecules within the material.
convection form of heat transfer is the basis of Fluid Heat Transfer System. Fluid Heat Transfer Systems provide an indirect heating source that circulates the fluid through the process equipment, to maintain an even and consistent temperature profile. They have several advantages over steam boilers viz.-water treatment facilities not required, no problem of scale deposition on heat transfer tubes; due to low operating pressures it is safe from explosion hazards etc.
Heat Transfer fluids are mineral oil based as the temperature of oil remains fairly constant thereby higher efficiency of heat utilization can be achieved.
Selection of Heat Transfer Fluids
To ensure reliable performance of heat transfer system proper selection of thermic fluid is very important. The basic criteria for selection of thermic fluids are as under:
a. It should have good oxidation and thermal stability.
b. It must have low volatility characteristics.
c. Prevent formation of sludge, rust and corrosion.
d. It should have sufficiently high Viscosity Index, preventing too much viscosity drop during its hot run.
e. The specific heat should not exceed 0.70 Kcal/Kg/°C at high temperature, as otherwise the thermic fluid may loose its heat rapidly.
f. It should have low pour point so that the system does not get wax clogged at low ambient temperatures.