The overall heat-transfer coefficient is value that identifies the ease of heat transfer between objects. The higher the value, the more heat transfer is taking place. (In a heat exchanger this value should be high)
Typical values1
|
Heat Exchanger Duty |
U (W/m2·K) |
|
Water-to-Water |
800-2000 |
|
Water-to-Oil |
100-350 |
|
Water-to-Fuel |
200-1000 |
|
Water-to-heat-transfer
liquids |
650-1500 |
|
Steam
condenser |
1000-6000 |
|
Refrigerant
condenser |
300-1000 |
|
Water-to-gas |
40-75 |
|
Steam-to-gas |
20-300 |
There are several different methods by which the overall heat transfer coefficient (U) can be found.
Experimental
= heat transfer rate (W)
= mass flow rate of
the fluid (kg/s)
Cp = heat capacity of the fluid (J/kg·K)
DT = temperature difference between the inlet and outlet of the fluid (K)
A = surface area of heat transfer (m2)
DTlm = log mean temperature difference (K)
for counter-current flow, DT1 = Thotin - Tcoldout and DT2 = Thoutout - Tcoldin
for co-current flow, DT1 = Thotin - Tcoldin and DT2 = Thoutout - Tcoldout
Empirical
Uo = overall heat-transfer coefficient based on the outside surface area
hi = local inside heat-transfer coefficient
ho = local outside heat-transfer coefficient
Ao= area
Literature
Hagen, Kirk D. Heat Transfer with Applications. Upper Saddle River, NJ: Prentice
Hall, 1999.