Heat transfer coefficient Totally Explained

Everything about The Heat Transfer Coefficient totally explained

The heat transfer coefficient, in thermodynamics and in mechanical and chemical engineering, is used in calculating the heat transfer, typically by convection or phase change between a fluid and a solid: ť

Delta Q=h cdot A cdot Delta T cdot Delta t

where ť ΔQ = heat input or heat lost, J

h = overall heat transfer coefficient, W/(m²K) ť A = heat transfer surface area, m²

Delta T

= difference in temperature between the solid surface and surrounding fluid area, K ť

Delta t

= time period, s

From the above equation, the heat transfer coefficient is the proportionality coefficient between the heat flux, Q/(AΔt), and the thermodynamic driving force for the flow of heat (for example, the temperature difference, ΔT).
The heat transfer coefficient has SI units in watts per meter squared-kelvin (W/(m²K)). Heat transfer coefficient can be thought of as an inverse of thermal resistance.
There are numerous correlations for calculation of heat transfer coefficient in different heat transfer modes, different fluids, flow regimes, and under different thermohydraulic conditions. Often it can be estimated by dividing the thermal conductivity of the convection fluid by a length scale. The heat transfer coefficient is often calculated from the Nusselt number (a dimensionless number).

Dittus–Boelter correlation (forced convection)

A common and particularly simple correlation useful for many applications is the Dittus–Boelter heat transfer correlation for fluids in turbulent flow. This correlation is applicable when forced convection is the only mode of heat transfer; for example, there's no boiling, condensation, significant radiation, etc. The accuracy of this correlation is anticipated to be +/-15%.
For a liquid flowing in a straight circular pipe with a Reynolds number between 10 000 and 120 000 (in the turbulent pipe flow range), when the liquid's Prandtl number is between 0.7 and 120, for a location far from the pipe entrance (more than 10 pipe diameters; more than 50 diameters according to many authors) or other flow disturbances, and when the pipe surface is hydraulically smooth, the heat transfer coefficient can be expressed as:
ť

h= + Sigma R

where ť R = Resistance(s) to heat flow in pipe wall (K/W)

   Other parameters are as above.
   The heat transfer coefficient is the heat transferred per unit area per Kelvin. Thus Area is included in the equation as it represents the area over which the transfer of heat takes place. The areas for each flow will be different as they represent the contact area for each fluid side.
   The thermal resistance due to the pipe wall is calculated by the following relationship:
R=x/k.A
   where ť x = The wall thickness (m)

   k = the thermal conductivity of the material (W/mk) ť A = The total area of the heat exchanger (m²

This represents the heat transfer by conduction in the pipe.
   The thermal conductivity is a characteristic of the particular material.
   Some typical themal conductivity values include:
ť - Polypropylene - k = 0.12 W/mk

   - Stainless steel - k = 21 W/mk
   As mentioned earlier in the article the convection heat transfer coefficient for each stream depends on the type of fluid, flow properties and temperature properties.
   Some typical heat transfer coefficients include:
ť - Air - h = 10 to 100 W/m²K

   - Water - h = 500 to 10 000 W/m²K

Resistance due to Fouling

Surface coatings known as foul can build up in heat exchangers, which add extra thermal resistance to the wall thus decreasing the overall heat transfer coefficent. Fouling also increases pumping costs. The resistance due to fouling is found by comparing calculations of the overall heat transfer coefficient from laboratory readings with calculations based on predicted theoretical correlations. The following relationship is used:
1/Uexp = 1/Upre+Rf
where, ť Uexp = Overall Heat transfer coefficient based on experimental data (W/m²K)

Upre = Overall Heat transfer coefficient based on predicted data (W/m²K) ť Rf = The resistance due to fouling (m²W.K)

Further Information

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