Heat Exchangers 2

Fouling is a result of Precipitation Corrosion Chemical reaction Growth of algae (Biological fouling)

Factors affecting fouling Operating temperature (higher temperature => higher fouling) Fluid velocity (lower velocity => higher fouling) Length of service (longer service => higher fouling)

Modified overall heat transfer coefficient The overall HTC must be modified to account for the effects of fouling on both the inner and the outer surfaces of the tube. For a shell-and-tube HEX, it can be expressed as:

Log Mean Temperature Difference (LMTD) is best suited for when the temperature difference is known. The Effectiveness-NTU is best used when the surface area is known.

Assumptions for HEX analysis Steady-state, steady flow No heat generation in the HEX Negligible ΔPE, ΔKE Adiabatic processes No phase change Constant specific heats and other physical properties.

Under these assumptions, the first law of thermodynamics requires that the rate of heat transfer from the hot fluid equals the rate of heat transfer to the cold fluid.

Notes

Example