Heat exchangers fouling resistances

In MTG, the steady state simulation models were tuned to actual plant performance by adjusting compositions, flows and heat exchanger fouling resistances. 

Shell-and-Tube Heat Exchangers Heat-Transfer Fouling Resistance... 

Figure 10-40B. Fouling resistance for various conditions of surface fouling on heat exchanger surfaces. Thermal resistance of typical uniform deposits. Note that the abscissa reads for either the inside, r or outside, r , fouling resistance of the bulidup of the resistance layer or film on/in the tube surface. (Used by permission Standards of Tubular Exchanger Manufacturers Association, 6 Ed, p. 138, 1978. Tubular Exchanger Manufacturers Association, Inc. All rights reserved.)...

It generally is considered that resistance due to fouling is lower with plate heat exchangers than with tubular units. This is the result of four advantages of plate-type exchangers ... 

For the three heat exchangers from Exercise 1, make a first estimate of the order of magnitude of the overall heat coefficients from tabulated values of film transfer coefficients and fouling coefficients. Neglect the resistance from the tube walls. 

The practical heat-transfer coefficient is the sum of all the factors that contribute to reduce heat transfer, such as flow rate, cocurrent or countercurrent, type of metal, stagnant fluid film, and any fouling from scale, biofilm, or other deposits. The practical heat-transfer coefficient ((/practical) is, in reality, the thermal conductance of the heat exchanger. The higher the value, the more easily heat is transferred from the process fluid to the cooling water. Thermal conductance is the reciprocal of resistance (/ ), to heat flow ... 

The first term depends entirely on the physical properties of the reactor contents and degree of agitation. It represents resistance to heat transfer of the internal film and of eventual deposits at the wall, which may determine the overall heat transfer [3], Therefore, the reactor should be regularly cleaned with a high pressure cleaner. Both last terms depend on the reactor itself and on the heat exchange system, that is, reactor wall, fouling in the jacket, and external liquid film. They are often grouped under one term the equipment heat transfer coefficient (cp) [4, 5],... 

The performance of a heat exchanger depends upon the transfer surfaces being clean and uncorroded. The performance deteriorates with time due to accumulation of deposits on the heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer. In the design of heat exchangers, this added resistance is accounted for by a fouling factor or dirt factor, R. ... 

Oil is to be heated from 300 K to 344 K by passing it at 1 m/s through the pipes of a shell-and-tube heat exchanger. Steam at 377 K condenses on the outside of the pipes, which have outer and inner diameters of 48 and 41 mm respectively, though due to fouling, the inside diameter has been reduced to 38 mm, and the resistance to heat transfer of the pipe wall and dirt together, based on this diameter, is 0.0009 m2 K/W. 

After a period of operation the heat-transfer surfaces for a heat exchanger may become coated with various deposits present in the flow systems, or the surfaces may become corroded as a result of the interaction between the fluids and the material used for construction of the heat exchanger. In either event, this coating represents an additional resistance to the heat flow, and thus results in decreased performance. The overall effect is usually represented by a fouling... 

The performance of heat exchangers usually deteriorates with time as a result of accumulation of deposits on heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer and causes the rate of heat transfer in a heat exchanger to decrease. The net effect of these accumulations on heat transfer is represented by a fouling factor Rj, which i.s a measure of the thermal resistance introduced by fouling. 

A double pipe (shell-and-tube) heat exchanger is constructed of a stainless steel [k = 15.1 W/m O inner lube of inner diameter O/ = 1.5 cm and outer diameter 1.9 cm and an outer shell of inner diameter 3,2 cm. The convection heat transfer coefficient is given to be h,- = 800 W/m °C on the inner surface of the tube and h = 1200 W/m °C on the outer surface. For a fouling factor of f f, - 0.0004 m °C/W on the tube side and Ri =- 0.0001 m °C/W on the shell side, determine (a) the thermal resistance of the heat exchanger per unit iength,and (6) the overall heat transfer coefficients, Ujand U based on the inner and puter surface areas 0) the tube, respectively. 

SOLUTION The heat transfer coefficients and the fouling factors on the tube and shell sides of a heat exchanger are given. The thermal resistance and the overall heat transfer coefficients based on the inner and outer areas are to be determined. 

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