The equation for convection can be expressed as:
q = hc A dT (1)
q = heat transferred per unit time (W, Btu/hr)
A = heat transfer area of the surface (m2, ft2)
hc = convective heat transfer coefficient of the process (W/(m2oC, Btu/(ft2 h oF))
dT = temperature difference between the surface and the bulk fluid (oC, F)
Convective heat transfer coefficients - hc - depends on type of media, if its gas or liquid, and flow properties such as velocity, viscosity and other flow and temperature dependent properties.
Typical convective heat transfer coefficients for some common fluid flow applications:
- Free Convection - air, gases and dry vapors : 0.5 - 1000 (W/(m2K))
- Free Convection - water and liquids: 50 - 3000 (W/(m2K))
- Forced Convection - air, gases and dry vapors: 10 - 1000 (W/(m2K))
- Forced Convection - water and liquids: 50 - 10000 (W/(m2K))
- Forced Convection - liquid metals: 5000 - 40000 (W/(m2K))
- Boiling Water : 3.000 - 100.000 (W/(m2K))
- Condensing Water Vapor: 5.000 - 100.000 (W/(m2K))
The convective heat transfer coefficient for air flow can be approximated to
hc = 10.45 - v + 10 v1/2 (2)
hc = heat transfer coefficient (kCal/m2h°C)
v = relative speed between object surface and air (m/s)
1 kcal/m2h°C = 1.16 W/m2°C
- (2) can be modified to
hcW = 12.12 - 1.16 v + 11.6 v1/2 (2b)
hcW = heat transfer coefficient (W/m2°C)
Note! - this is an empirical equation and can be used for velocities 2 to 20 m/s.
A fluid flows over a plane surface 1 m by 1 m. The surface temperature is 50oC, the fluid temperature is 20oC and the convective heat transfer coefficient is 2000 W/m2oC. The convective heat transfer between the hotter surface and the colder air can be calculated as
q = (2000 W/(m2oC)) ((1 m) (1 m)) ((50 oC) - (20 oC))
= 60000 (W)
= 60 (kW)
Heat Transfer = (W)
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