Question

A chemical process stream enters a shell-and-tube exchanger at a temperature of 200 °F and does two passes on the shell side, exiting the exchanger at 170 °F. For this particulate scenario, the heat exchanger has 200 stainless steel tubes that are 2-in. OD and 10 ft long. Indicate whether the temperature of the process stream will increase, decrease, or remain the same under the following scenarios. Justify your answer. a) The flow rate of the cooling fluid is increased. b) There are 100 tubes that are 1.5-in. OD and 26.6 ft long. c) The number of shell passes is doubled. d) The tube material is changed to copper.

Answer 1

Heat transferred , Q= UALMTD

U= overall heat transfer coefficient = 1/hi +1/ho

where hi and ho are shell side and tube side heat transfer coefficients.

a) A= heat transfer area = n*PI*d*L, n= number of tubes, d= diameter of tube and L= length of tubes

A= 200*3.142*2/12ft*10   = 1047.33

a) steam is expected to have high heat transfer coefficient (3000 btu/hr.ft2.F) than water (1000 btu/hr.ft2.F) and hence controlling resisntace is on the water side. when water flow rate is doubled, the heat transfer coefficient of water increases and hence overall heat transfer coefficient increases and this also decreases the process stream temperature.

b) when there are 100 tubes of 1.5 in dia and 26.6 ft long. heat transfer area= 100*3.142*1.5/12* 26.6=1044.72 ft2

heat transfer area has decreased and hence temperature of exiting stream decreases.

c) when number of passes on the shell side are doubled, already controlling resistance is on water side and hence two passes increases the heat transfer coefficient of shell side steam resulting in less overall heat transfer coefficient and hence a decrease in outlet steam temperature.

d) Copper is having high thermal conductivity and this leads to better heatt transfer on the surface between steam and water and this leads to outlet temperature of steam being lower than 170 deg.F