Overall heat transfer coefficient
An important parameter in the design and monitoring of heat exchangers is the overall heat transfer coefficient, U, between the two fluids. A value for U can be easily obtained by knowing the followings:
- Mass flow of the fluid,
- Specific heat of the fluid,
- Difference in temperature of the fluid across the heat exchanger,
- Inlet and outlet temperature of both the fluids involved in heat exchanger and
- Area of the heat transfer surface.
From the equation Q = m.cp. Δt
Calculating Q in watts:
Where m = mass flow in kg/hr of kg/sec
Cp= specific heat or heat content in KJ/Kg K
Δt= Temperature difference across the heat exchanger.
Calculating LMTD from the following equation with the inlet and outlet temperatures of the hot and cold fluid across the heat exchanger and with the knowledge of type of heat exchanger using:
(For multipass heat exchangers)
ΔT m = (T2-t1) – (T1 – t2)
ln [(T2-t1)/(T1-t2)]
Where ΔT m = log mean temperature difference,
T1 = inlet shell side fluid temperature,
T2 = outlet shell side fluid temperature,
t1 = inlet tube side temperature,
t2 = outlet tube-side temperature,
OR
(for single pass heat exchanger)
Δtm = (T1-t2) +(T2-t1)
2
We can calculate U from the equation Q = UA Δtm
Where Q = heat transferred per unit time (W)
U = the overall heat transfer coefficient (W/m2 oC)
A –heat-transfer area, m2,
Δtm = the mean temperature difference, the temperature driving force, oC.
U = . Q . (eq. 1.14)
A Δtm