Heat Exchanger

In this blog entry we will look into the thermodynamics of a heat exchanger. To learn more about types of heat exchangers and factors affecting performance of a heat exchanger, you can read a previous blog entry on Heat Exchangers here.

To recap, a heat exchanger is a device with two flowing streams that exchange heat without mixing. The most basic form of a heat exchanger is a double-pipe heat exchanger that consists of two concentric pipes with differing diameters. One of the fluids flows in the inner pipe while the other flows in the annular space between the two pipes. The two fluid should have a considerable temperature difference to facilitate heat transfer from hot to cold fluid through the pipe walls. 

The figure below illustrates the layout of co-current and countercurrent flow heat exchangers:

No work is produced in a heat exchanger and both kinetic and potential energy changes are negligible. The energy equation considering a heat exchanger as a system thus reduces to:

The outer shell of the heat exchanger is usually well insulated to prevent heat loss to the surroundings.

Note: For the following example, Appendices B.2 and B.4 for steam values that I have referred to in these questions was obtained from: 

M.D Koretsky, Engineering and Chemical Thermodynamics, Wiley, 2004.

Example

High pressure steam at 0.4 kg/s and 2 MPa and 450°C enters an adiabatic steady flow turbine. The work produced by the turbine is 400 hp. The exit stream from the turbine is at 40 kPa and it is fed to a heat exchanger where it is condensed at constant pressure to obtain saturated liquid. As cooling medium liquid water is used which enters the heat exchanger at 15°C and leaves at 55°C. Assume no heat losses from the heat exchanger to the surroundings.

a) What is the condition of the steam leaving the turbine?

b) Calculate the mass flow rate of the cooling water used

Solution 1

Solution 2

An alternate solution can be as follows:

Choosing the turbine and the heat exchanger as the system, the energy equation can be written and solved as follows: