Thermal
Design
The
following write-up provides general design information for those engineers not
familiar with the design of shell & tube heat exchangers.
Heat
Load Balance
The
first step in ensuring that a heat exchanger is properly designed is to calculate
the thermal duty. The thermal duty is the amount of heat transfered per unit
time (Btu/hr).
For fluids not undergoing a phase change (sensible heating/cooling), the
thermal duty is equal to :
Q
= Cp (T1 - T2) m
Q = Thermal Duty (Btu/hr)
Cp = Heat Capacity (Btu/lb F)
T1 = Inlet Temperature (F)
T2 = Outlet Temperature (F)
m = Mass Flow Rate (lb/hr)
For Fluids Undergoing a phase change, the thermal duty is equal to:
Q = m Lamda
Q = Thermal Duty (Btu/hr)
m = Mass Flow Rate (lb/hr)
Lamda = Latent Heat (Btu/lb)
In cases where the fluid undergoes both sensible heating and a phase change,
simply add the sensible duty (for both the liquid phase and in the vapor phase
as required) to the phase change duty.
The thermal duty on the hot side must equal the thermal duty on the cold side
therefore,
after calculating the duty for the process side, use this duty to calculate
the flow rate or outlet temperature for the heating or cooling medium.
Sizing
Formulae
After the thermal balance is confirmed, the next step in the process is to properly
size the heat exchanger. The governing equation for determining the size of
the exchanger is as follows:
Q = U A MTD
Q = Thermal Duty (Btu /hr)
U = Overall Heat Transfer Coefficient (Btu/hr ft2 F)
A = Area (ft2)
MTD = Mean Temperature Difference (F)
The thermal duty calculated from this formulae must equal (or exceed) the thermal
duty required from the heat balance.
Overall Heat Transfer Coefficient -("U" value) - The U value
is a function of the physical properties of both the hot fluid and cold fluid,
the fouling factors applied, and the fluid flow dynamics through the heat exchanger.
When designing a heat exchanger, the engineer tries to maximize the U value
in order to minimize the required surface area.
U Value versus Pressure Drop - When specifying the allowable pressure
drop through the heat exchanger, customers should give the heat exchanger manufacturer
as much allowable pressure drop as possible. Higher allowable pressure drops
allow higher velocities through the heat exchanger, increasing the heat transfer
coefficient.
Area - The area available is equal to the outside surface area of the
tubing. When finned tubes are used, this heat transfer area is substantially
increased however, often the U value will drop as the finned surface area is
not as effective is providing heat transfer as the surface of a bare tube.
MTD - The MTD is a measure of the temperature difference between the
hot fluid and the cold fluid. The greater the difference in temperature between
the hot fluid and the cold fluid, the lower the required surface area.