Effect of fouling on pressure drop

The ratios given in Table 12.7, which are adapted from Devore s figures, can be used to make a rough estimate of the effect of fouling on pressure drop. 

Fouling coefficient Shell diameter/baffle spacing 

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In addition to the use of antifouling chemical agents to mitigate the effect of fouling on the tube-side, twisted tubes can be used rather than plain tubes. These have surface irregularities. Plain tubes can also be fitted on the inside with tube inserts. Twisted tubes and tube inserts promote additional turbulence and pressure drop and reduce the surface temperature of the tube to mitigate fouling. Tube inserts will be dealt with in more detail later. 

Effects of Velocity on Heat Transfer, Pressure Drop, and Fouling... 

The values of h, and AP, calculated by this procedure are for clean exchangers and are intended to be as accurate as possible, not conservative. A fouled exchanger will generally give lower heat-transfer rates, as reflected by the dirt resistances incorporated into Eq. (11-2), and higher pressure drops. Some estimate of fouling effects on pres-... 

The effect of the fouling on the shell-side flow is to increase the cross-flow and increase the overall heat transfer coefficient for a fixed pressure drop (assuming the same fouling coefficients in both cases). 

Performance of an RO system, specifically the permeate flow rate, salt rejection and pressure drop, is a function of membrane fouling, scaling, and degradation, as previously discussed (See Chapter 11.3). This chapter covers the detailed effects of membrane fouling, scaling, and degradation have on normalized product flow, normalized salt rejection, and pressure drop. 

Knitted wire mesh serves as an effective entrainment separator when it cannot easily be fouled by sohds in the hquor. The mesh is available in woven met wire of most alloys and is installed as a blanket across the top of the evaporator (Fig. ll-122d) or in a monitor of reduced diameter atop the vapor head. These separators have low-pressure drops, usually on the order of 13 mm V2 in) of water, and collection efficiency is above 99.8 percent in the range of vapor velocities from 2.5 to 6 m/s (8 to 20 ft/s) [Carpenter and Othmer, Am. Inst. Chem. 

An alternative insert for tubes is the spiral insert produced by Cal Gavin. The device consists of a matrix of wire loops on a twisted wire core. The diameter of the coils is such that it is a push fit. Laboratory studies using these wire matrix inserts with an Arabian crude oil demonstrate that the steady asymptotic value of the fouling resistance is reached in only 10 hr from start up and only of the order of 2-7% of the recommended TEMA value. The presence of the wires, particularly those in contact with the heat-transfer surface, creates turbulence that is largely responsible for the beneficial effects on the fouling. The benefits are not without the potential penalty of increased pressure drop and hence increased pumping costs. By suitable design for the same duty, this penalty can be substantially reduced. 

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