Question

Consider a counter-flow plate HX with channels of 12 in length, 6 in width, and 3/8 in thickness. The heat exchanger consists of three flow channels with air at 200oC entering the center channel, and air at 25oC entering the 2 outer channels, above and below the center channel. The center channel has a flow rate of 3 x 10-4 m3/s and the outer two channels have a flow rate of 1.67 x 10-4 m3/s in each channel. The plates used in the heat exchanger are commercial steel and have a thickness of 0.0625 in.

1.State/show all assumptions

Answer 1

Following assumption should be considered in above case:

1) Heat loss to surrounding via any of the heat transfer mode (conduction, convection and radiation) is negligible.

2) Kinetic and potential energy changes during the flow through the channels are negligible.

3) The materials used in heat exchanger have constant properties in all the directions at all the times.

4) Air has equivalent values of hot and cold specific heats.

5) Steady state operation to be considered.

6) No fouling occurs in the heat exchanger.

7) Air flow is fully developed.