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

• More about Convective Heat Transfer

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

• Overall Heat Transfer Coefficients for combinations of Heat Exchangers

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