A shell and tube heat exchanger is to be designed to preheat the feed to a reactor with  = 800 W/K from 20°C to 90°C using waste-saturated steam at 1 atm absolute pressure. The overall heat transfer coefficient is = 300 W/m² K.

(a) Determine the heat transfer area required.

(b) The heat exchanger is built according to the specifications in part (a). However, when it is put in operation, it is found that it is gradually fouled as a function time on stream with a fouling coefficient given by:

where = 0.0004 m² K/W h and t is the time on stream in hours. Cleaning the heat exchanger restores the original conditions but takes 4 h. It is proposed to operate the existing heat exchanger on a cycle during which the heat exchanger spends a number of hours on stream followed by a 4-h cleaning period. Develop an expression to calculate the cumulative amount of heat exchanged during the time on stream and use this expression to find the cycle time that maximizes the amount of heat exchanged during 1 week (168 h). Note that the number of cycles per week will be .

Transcribed Image Text:

mcC P.C