Overall Heat Transfer Coefficient

Calculate overall heat transfer coefficient - walls or heat exchangers

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

1 / U A = 1 / h1 A1 + dxw / 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)

dxw = 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

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 hair = 50 W/m2K.

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

U = 1 / (1 / hA + dxw / k + 1 / hB)         (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 : U = 10 - 50 W/m2K (gas boiler)
  • Liquid Free Convection - Free Convection Liquid : U = 25 - 500 W/m2K (oil bath for heating)
  • Liquid Free Convection - Forced Liquid flowing (Water) : U = 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 : U = 300 - 1000 W/m2K (steam jackets around vessels with stirrers, water), 150 - 500 W/m2K (other liquids)
  • Forced liquid (flowing) water - Free Convection Gas : U = 10 - 40 W/m2K (combustion chamber + radiation)
  • Forced liquid (flowing) water - Free Convection Liquid : U = 500 - 1500 W/m2K (cooling coil - stirred)
  • Forced liquid (flowing) water - Forced liquid (flowing) water : U = 900 - 2500 W/m2K (heat exchanger water/water)
  • Forced liquid (flowing) water - Condensing vapor water : U = 1000 - 4000 W/m2K (condensers steam water)
  • Boiling liquid water - Free Convection Gas : U = 10 - 40 W/m2K (steam boiler + radiation)
  • Boiling liquid water - Forced Liquid flowing (Water) : U = 300 - 1000 W/m2K (evaporation of refrigerators or brine coolers)
  • Boiling liquid water - Condensing vapor water : U = 1500 - 6000 W/m2K (evaporators steam/water)

Related Topics

  • Heat Loss and Insulation - Heat loss from pipes, tubes and tanks - with and without insulation - foam, fiberglass, rockwool and more
  • Insulation - Heat transfer and heat loss from buildings and technical applications - insulation methods and coefficients to reduce energy consumption
  • Heat Loss and Insulation - Steam and condensate pipes - heat loss uninsulated and insulated pipes, insulation thickness and more
  • Thermodynamics - The effects of work, heat and energy on a system

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