Overall Heat Transfer Coefficient

The overall heat transfer coefficient (U) is a measure of the ability of a material to conduct heat. It is calculated as the reciprocal of the sum of the individual thermal resistances of the layers of the material through which heat must pass.

Factors affecting overall heat transfer coefficient:
- Type of fluid
- Temperature difference between fluids
- Velocity of fluids
- Surface area and geometry of heat transfer surface
- Material of heat transfer surface

Lowest value of U

The lowest value of U is found in the case of air to low viscosity liquids.

Explanation

When air is used as one of the fluids in a heat exchanger, the overall heat transfer coefficient is relatively low. This is due to the low thermal conductivity of air, which means that heat is transferred more slowly through it than through other materials.

Low viscosity liquids, on the other hand, have a lower thermal conductivity than more viscous liquids. This means that they are less able to conduct heat, leading to a lower value of U.

When air is used to cool low viscosity liquids, therefore, the overall heat transfer coefficient is particularly low. This is because both fluids have low thermal conductivities, resulting in a slow rate of heat transfer between them.

In comparison, steam condensers and feed water heaters typically have higher values of U, as they involve more conductive materials and higher temperature differences between fluids. Air condensers may also have higher U values, depending on the specific materials and conditions involved.