Surface condensers shell-side fouling

Whereas shell-side fouling in a surface condenser is uncommon, loss of heat transfer coefficient due to air binding is as common as tube-side fouhng due to poor-quality cooling water. 

The cooled surface may be of any orientation although vertical and horizontal arrangements are most common. Condensation operations often are carried out inside shell and tube heat exchangers and the condensing vapors may be fed either to the shell side or to the tube side depending on the nature of the fluids involved, their pressure, and their corrosion and fouling characteristics (Webb and McNaught, 1980). 

Air leaking into the surface condenser results in vapor binding. This causes an increase in the shell-side heat transfer thermal resistance. In effect, the steam does not make good contact with the exterior of the surface condenser tubes. This vapor-binding effect often appears to the operators as if the tubes are fouled with cooling-water deposits. 

For moderate-temperature and low-pressure condensers, such as vacuum surface condensers (see Chap. 25), a single pass on the tube side is not uncommon. These exchangers are typically fixed-tubesheet designs. Such exchangers are fine from a purely process point of view. However, there is no practical way to disassemble the exchanger to clean the outside (i.e., the shell side) of the tubes. This inability to clean the shell side frequently leads to tremendous loss of efficiency after the condensers foul. 

Estimate the overall heat transfer coefficient U, based on the inside tube surface area, of a shell-and-tube-type vapor condenser, in which cooling water at 25 °C flows through stainless steel tubes, 25 mm i.d. and 30 mm o.d., at a velocity of 1.2 m s 1. It can be assumed that the film coefficient of condensing vapor at the outside tube surface is 2000kcalh m 2 C, and that the fouling factor of the water side is 5000 kcal h-1m-2oC 1. 

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