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 ‘counter flow’. In this article we are looking at why counter flow is more efficient and why Bowman recommend this method when installing their heat exchangers.
Parallel or counter flow – what is the difference?
The clue is in the name; parallel flow is where the fluid that needs 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.
As an example, if the incoming hot fluid has a temperature of say 100 °C and the incoming cooling medium is 30 °C, the mean temperature difference between the two fluids decreases. This is because the incoming cooling medium, travelling in parallel with the hot fluid, is being gradually warmed along the length of the heat exchanger and with cold water only being introduced adjacent to the hottest area of the unit, the heat exchanger cannot cool to a lower temperature than the cooling medium it is itself, as shown in the illustration below.
This arrangement can also create thermal stress within the heat exchanger, as one half of the unit will be appreciably warmer than the other.
How is counter flow different?
In a counter flow cooling, the incoming cooling medium absorbs 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.
As the illustration below shows, the mean temperature difference between the cooling medium and the fluid being cooled is much more uniform along the length of the heat exchanger, significantly reducing thermal stress to the unit.
So, what does it mean?
Whilst a heat exchanger installed with parallel flow will reduce temperature, it is nowhere near as efficient as a counter flow arrangement and, to achieve the required outlet temperature, a larger heat exchanger may possibly be required.
By contrast, counter flow is significantly more efficient and, depending on the flow rate and temperature, the heat transfer performance could be up to 15% more efficient, possibly enabling a smaller heat exchanger to be used, saving space and money!
To ensure the correct product is always specified, Bowman offer computer aided product selection for all their heat exchangers. Call +44 (0)121 359 5401 or email [email protected] for more information.