9: Why is counterflow more efficient?
In a shell and tube heat exchanger, coolant usually flows through the central ‘tube core’ to cool hot oil, water or air, which passes over and around the tubes. The direction in which the two fluids travel through the heat exchanger can be either ‘parallel flow’ or ‘counterflow’.
Parallel flow is where the fluid to be cooled, flows through the heat exchanger in the same direction as the cooling medium. Whilst this arrangement will provide cooling, it has limitations and can also create thermal stress within the heat exchanger, as one half of the unit will be appreciably warmer than the other.
In counterflow cooling, the incoming cooling medium absorbs more heat as the ‘hot’ fluid travels in the opposite direction. The cooling medium heats up as it travels through the heat exchanger, but as colder water enters the heat exchanger it absorbs more heat, reducing the temperature much lower than could be achieved with parallel flow.
The mean temperature difference between the cooling medium and the fluid being cooled is also more uniform along the length of the heat exchanger, reducing thermal stress.
Depending on flow rate and temperature, the heat transfer performance could be up to 15% more efficient with counterflow, possibly enabling a smaller heat exchanger to be used, saving space and money!