ChE 250

Spring 2000

Problem Set #11

Due Thursday, April 20

Read Felder & Rousseau, Chapter 9, at least through §9.5a.

1. Heat exchanger installation

Heat is to be transferred between two liquid streams using a spare heat exchanger. Stream A is flowing in at 20 kg/min and 130°C, while stream B enters the exchanger at 15 kg/min and 15°C. The heat capacities are c_pA = 2.4 kJ/kg°C and c_pB = 4.5 kJ/kg°C, respectively. The heat exchanger has an exchange area of 4.0 m², and a heat transfer coefficient of 400 kJ/m²hr°C.

a) Calculate the outlet temperatures and the overall heat transfer rate for this exchanger operated in cocurrent flow.

b) Calculate the outlet temperatures and the overall heat transfer rate for this exchanger operated in countercurrent flow.

c) Comment on the difference in behavior seen above.

d) Repeat these calculations for a heat exchanger with 10 times the surface area, and for an exchanger with 1/10 the original area. How does the difference caused by the flow pattern change for different heat exchanger sizes ? (Use a computer program such as Excel or Mathcad for the repetitive calculations.)

2. Cooling hot oil
A hot oil stream is to be cooled from 200 to 100°F using cooling water available at 60°F. The overall heat transfer coefficient for the heat exchanger is U=100 Btu/h/°F/ft².
Data on the streams are as follows:

Hot Oil Cooling Water

Flow rate: M (lb/hr) 10,000 6000

Heat capacity: c_p (Btu/lb/°F) 0.8 1.0

Input temperature: (°F) 200 60

Outlet temperature: (°F) 100 ??

a) How much heat must be exchanged, what is the outlet temperature of the cooling water, and how large a heat exchanger is required?

b) The water exit temperature is quite high, and you are to give consideration to the benefits of increasing the cooling water flow rate. Repeat your heat exchanger design calculations for a range of water flow rates between 6000 and 25000 lb/h. Present your results in tabular or graphical form, and comment.

3. Do F&R 9.7 - Heats of reaction

4. Do F&R 9.11 - Butane isomerization