Overall Heat Transfer Coefficient

Calculating the overall heat transfer coefficient in walls or heat exchangers

The overall heat transfer coefficient for a wall 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: .

• Polypropylene PP - 0.12 W/mK
• Stainless steel - 21 W/mK
• Aluminum - 221 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. 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.

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

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