- Water does not change state while it is being used as a heating medium. As it gives up heat energy to the secondary medium, its temperature drops. If one pound of water drops one degree Fahrenheit in temperature, it produces approximately 1 British thermal unit (BTU) of heat.
- Steam also does not change state while it is used as a heating medium, and it gives up heat energy to the secondary medium. During the process, its temperature drops, but the fluid condensate remains at the same temperature. One pound of steam at a pressure of 30 psi gives up approximately 929 BTUs of heat. Steam gives up more energy per unit mass than water.
Factors That Influence Heat Transfer Rates
- Surface area
- Flow characteristics
- Film coefficient of fluid
- Thermal conductivity of metal
How to Find Rate of Heat Transfer
For fluids that change state, the rate of heat transfer “Q” is given by:
If the rate of heat transfer is higher, then the heat exchanger’s efficiency is higher and vice versa.