Most heat exchanger studies begin with a given flow
rate and temperature change. A certain amount of heat must be taken
out of a flow of material or put into it per unit of time. This is the
load.
Using common terms, we measure heat in BTU or
British Thermal Units, and we express heat transfer rate on an hourly
basis. Then, load - termed q in the following equations - is expressed
in BTU/hr.
In heat exchanger design two flows of materials are involved - one
in the tubes and one on the shell side surrounding the tubes - we
assume that all the heat given up by one material goes into the other
material. It does not matter which flow of material we use to measure
load if that assumption is made.
If either fluid vaporizes or is condensed from a vapor
in the heat exchanging operation, then temperature differences alone
do not account for the load. A large amount of heat - usually the
greatest part of it - will be latent heat used or given up during this
change of state.
