How to find surface area of heat exchanger without the value of overall heat transfer coefficient?
Answers
Heat transfer through a surface like a wall can be calculated as
q = U A dT (1)
where
q = heat transfer (W (J/s), Btu/h)
U = overall heat transfer coefficient (W/(m2K), Btu/(ft2 h oF))
A = wall area (m2, ft2)
dT = (t1 - t2)
= temperature difference over wall (oC, oF)
The overall heat transfer coefficient for a multi-layered wall, pipe or heat exchanger - with fluid flow on each side of the wall - can be calculated as
1 / U A = 1 / hci Ai + Σ (sn / kn An) + 1 / hco Ao (2)
where
U = the overall heat transfer coefficient (W/(m2 K), Btu/(ft2 h oF))
kn = thermal conductivity of material in layer n (W/(m K), Btu/(hr ft °F))
hc i,o = inside or outside wall individual fluid convection heat transfer coefficient (W/(m2 K), Btu/(ft2 h oF))
sn = thickness of layer n (m, ft)
A plane wall with equal area in all layers - can be simplified to
1 / U = 1 / hci + Σ (sn / kn) + 1 / hco (3)
Thermal conductivity - k - for some typical materials (not that conductivity is a property that may vary with temperature)
Polypropylene PP : 0.1 - 0.22 W/(m K)
Stainless steel : 16 - 24 W/(m K)
Aluminum : 205 - 250 W/(m K)
Convert between Metric and Imperial Units
1 W/(m K) = 0.5779 Btu/(ft h oF)
1 W/(m2 K) = 0.85984 kcal/(h m2 oC) = 0.1761 Btu/(ft2 h oF)
Conductive heat transfer
Thermal conductivity of commonly used materials
The convection heat transfer coefficient - h - depends on
type of fluid - if its gas or liquid
flow properties like velocity
other flow and temperature dependent properties
Convective heat transfer coefficient for some common fluids:
Air - 10 to 100 W/m2K
Water - 500 to 10 000 W/m2K
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The heat transfer coefficient or film coefficient, or film effectiveness, in thermodynamics and in mechanics is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat.
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