Introduction
Exchange systems are widely used both in living organisms and industrial applications. An exchange process, whether it involves heat, gases, solutes, or water, is an important feature in many different physiological processes. Knowledge of these exchange principles is important for the understanding of many physiological principles like respiration, thermoregulation, osmoregulation, water balance,etc. One of the most effective exchange principles is the counter-current principle.
We can use the exchanger for transferring heat, gases, or water between two media separated by a thin membrane that has good thermal conductance or a high conductance for different gases, solutes or water. Depending on the flow arrangements, we can classify an exchanger as counter-exchanger, con-exchanger or cross-counter exchanger. A fourth type is the mixing-heat-exchangers, where we mix the two in the exchanger, such as the nasal passages in some animals and some human applications. The efficiency of the exchanger is dependent on a number of factors, such as the flow direction of the media, the flow rate of the media and diffusion distance, or conductance of the exchanger material.
Figure 1: Con and counter current flow in exchange systems
In this experiment a model heat exchanger is constructed and the water temperature is recorded using a series of thermocouples together with a TC-08 data logger. By altering the direction of water flow the exchange principles of both con-current and counter-current systems can be evaluated.
Experiment setup
The experiment is set up as shown in the Figure 2 below.
Figure 2: Experiment setup
Two thermostatted baths (1) are used — one is set to 5 °C and the other to 37 °C. Two peristaltic pumps (2) are then used to pump water from the two baths through the heat exchanger model (3). The computer (6) runs a custom LabVIEW program and is used to control the two pumps through a control box (5). The computer can also control an external alarm box (7) that can be used to notify the responsible teacher every time a person logs on if the experiment is performed over the internet.
The exchanger model (see Figure 3 below) consists of two 1 meter long copper tubes that are soldered together. In each tube 4 thermocouples are inserted to monitor the temperature along each tube using the TC-08 data logger. The output from the TC-08 is recorded by the computer.
Figure 3: Exchanger model
Carrying out the experiment
- Start one pump (the other pump should be switched off), set the speed of the pump to low and let the system equilibrate. Record the water temperature. Repeat with the pump speed set to medium and then with the pump speed to high.
- Repeat with the other pump.
- Start both pumps and select con-current flow with the pump speed set to low. Allow the system to equilibrate and record the temperature. Repeat with pump speeds set to medium and high.
- Start both pumps and select counter-current flow with the pump speed set to low. Allow the system to equilibrate and record the temperature. Repeat with pump speeds set to medium and high.
Further study
Try to answer the following questions:
Why is a reindeer that stands outside in the middle of the winter not losing heat via the rather thin and uninsulated legs?
How is this heat conserving strategy in the reindeer affected during heavy exercise?
Why does the dairy industry use counter-current exchangers instead of just boiling the milk to pasteurise it?
Credits, comments and further info
This experiment was written by Michael Axelson of the University of Göteborg in Sweden. It is possible to perform this experiment over the internet and so this experiment is suitable for distance education courses. For further information please contact Michael Axelson.
Please contact us if you have any comments about this experiment or suggestions for improvements.
