Overall Heat Transfer Coefficient

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The overall heat transfer coefficient for a wall or heat exchanger can be calculated as:

1 / U A = 1 / h₁ A₁ + dx_w / k A + 1 / h₂ A₂         (1)

where

U = the overall heat transfer coefficient (W/m²K)

A = the contact area for each fluid side (m²)

k = the thermal conductivity of the material (W/mK)

h = the individual convection heat transfer coefficient for each fluid (W/m²K)

dx_w = the wall thickness (m)

The thermal conductivity - k - for some typical materials (varies with temperature)

• Polypropylene PP : 0.1 - 0.22 W/mK
• Stainless steel : 16 - 24 W/mK
• Aluminum : 205 - 250 W/mK

• More about conductive Heat Transfer
• Thermal Conductivity for Several Materials

The convection heat transfer coefficient - h - depends on

• the type of fluid - gas or liquid
• the flow properties such as velocity
• other flow and temperature dependent properties

Heat transfer coefficient for some common fluids:

• Air - 10 to 100 W/m²K
• Water - 500 to 10 000 W/m²K

Thermal resistance

The overall heat transfer coefficient can also be calculated by the view of thermal resistance - R. The wall is split in areas of thermal resistance where

• the heat transfer between the fluid and the wall is one resistance
• the wall it self is one resistance
• the transfer between the wall and the second fluid is a thermal resistance

Surface coatings or layers of "burned" product adds extra thermal resistance to the wall decreasing the overall heat transfer coefficient.

Some typical heat transfer resistances

• static layer of air, 40 mm (1.57 in)  - R = 0.18 m²K/W
• inside heat transfer resistance, horizontal current - R = 0.13 m²K/W
• outside heat transfere resistance, horizontal current - R = 0.04 m²K/W
• inside heat transfer resistance, heat current from down upwards - R = 0.10 m²K/W
• outside heat transfere resistance, heat current from above downwards - R = 0.17 m²K/W

Example - Heat Transfer in a Heat Exchanger

A single plate exchanger with media A transfers heat to media B. The wall thickness is 0.1 mm and the material is polypropylene PP, aluminum or stainless steel.

Media A and B are air with a convection heat transfer coefficient of h_air = 50 W/m²K.

The overall heat transfer coefficient U per unit area can be expressed as:

U = 1 / (1 / h_A + dx_w / k + 1 / h_B)         (1b)

Using the values from above the overall heat transfer coefficient can be calculated to:

• Polypropylene PP : U = 24.5 W/m²K
• Steel : U = 25.0 W/m²K
• Aluminum : U = 25.0 W/m²K

Typical Overall Heat-Transfer Coefficients 

• Free Convection Gas - Free Convection Gas : U = 1 - 2 W/m²K  (typical window, room to outside air through glass)
• Free Convection Gas - Forced liquid (flowing) water : U = 5 - 15 W/m²K  (typical radiator central heating)
• Free Convection Gas - Condensing Vapor Water : U = 5 - 20 W/m²K  (typical steam radiators)
• Forced Convection (flowing) Gas - Free Convection Gas : U = 3 - 10 W/m²K (superheaters)
• Forced Convection (flowing) Gas - Forced Convection Gas : U = 10 - 30 W/m²K (heat exchangers gases)
• Forced Convection (flowing) Gas - Forced liquid (flowing) water  : U = 10 - 50 W/m²K (gas coolers)
• Forced Convection (flowing) Gas - Condensing Vapor Water  : U = 10 - 50 W/m²K (air heaters)
• Liquid Free Convection - Forced Convection Gas : 10 - 50 W/m²K (gas boiler)
• Liquid Free Convection - Free Convection Liquid : 25 - 500 W/m²K (oil bath for heating)
• Liquid Free Convection - Forced Liquid flowing (Water) : 50 - 100 W/m²K (heating coil in vessel water, water without steering), 500 - 2000 W/m²K (heating coil in vessel water, water with steering)
• Liquid Free Convection - Condensing vapor water : 300 - 1000 W/m²K (steam jackets around vessels with stirrers, water), 150 - 500 W/m²K (other liquids)
• Forced liquid (flowing) water - Free Convection Gas : 10 - 40 W/m²K (combustion chamber + radiation)
• Forced liquid (flowing) water - Free Convection Liquid : 500 - 1500 W/m²K (cooling coil - stirred)
• Forced liquid (flowing) water - Forced liquid (flowing) water : 900 - 2500 W/m²K (heat exchanger water/water)
• Forced liquid (flowing) water - Condensing vapor water : 1000 - 4000 W/m²K (condensers steam water)
• Boiling liquid water - Free Convection Gas : 10 - 40 W/m²K (steam boiler + radiation)
• Boiling liquid water - Forced Liquid flowing (Water) : 300 - 1000 W/m²K (evaporation of refrigerators or brine coolers)
• Boiling liquid water - Condensing vapor water : 1500 - 6000 W/m²K (evaporators steam/water)

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Related Topics

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• Heat Loss and Insulation - Steam and condensate pipes - heat loss uninsulated and insulated pipes, insulation thickness and more
• Insulation - Heat transfer and heat loss from buildings and technical applications - insulation methods and coefficients to reduce energy consumption
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