What are the differences between cross-flow and counter-flow heat exchangers?
Cross-flow and counter-flow heat exchangers are two configurations used in heat transfer in HVAC applications. The main difference between the two lies in the direction of airflow within the heat exchanger and the overall size of the exchanger.
Let's compare.
Cross Flow Heat Exchanger
In a cross-flow heat exchanger, the hot and cold air flows are perpendicular (90o angle) to each other. The hot and cold air flows typically enter the heat exchanger from opposite ends and flow across each other. Heat transfer occurs through the separation walls between the hot and cold air streams.
The temperature difference between the air promotes heat transfer, but the heat transfer effectiveness may vary across the heat exchanger. The heat exchange is most effective in the half of the exchanger where the two air streams enter the exchanger. The opposite side has less opportunity to exchange the temperature.
Counter-Flow Heat Exchanger
In a counter-flow heat exchanger, the hot and cold air flows in opposite directions. Although it appears that the exchanger has a similar cross pattern, it does not. The counter flow exchanger is longer and provides the opportunity for the air to be turned at the entrance and exits to the exchanger. The airflow within the exchanger is along the same plane but in opposite directions.
As the air flows in opposite directions, there is a continuous temperature difference between them along the entire length of the heat exchanger. This method provides more surface area and time for temperature exchange.
Heat transfer occurs through the separation walls, and the temperature difference allows for efficient heat transfer from the hot air to the cold air. Counter-flow heat exchangers generally offer higher overall heat transfer efficiency compared to cross-flow heat exchangers.
Key differences and considerations between cross-flow and counter-flow heat exchangers
Heat Transfer Efficiency: Counter-flow heat exchangers generally offer better heat transfer efficiency because they maintain a larger temperature difference between the hot and cold air along the entire length of the exchanger.
Effectiveness: Counter-flow heat exchangers tend to have a higher heat transfer effectiveness, meaning they can transfer a greater amount of heat between the air streams.
Temperature Approaches: In counter-flow heat exchangers, the outlet temperatures of the hot and cold air streams are closer to each other compared to cross-flow heat exchangers
Pressure Drop: Cross-flow heat exchangers typically have lower pressure drops compared to counter-flow heat exchangers, which can be advantageous in some applications.
Size: Cross-flow heat exchangers are longer. They require some space to make the change in airflow direction at the entrance and exit.
Equipment Cost: Counter-flow heat exchangers are typically more costly than cross-flow heat exchangers.
The choice between cross-flow and counter-flow heat exchangers depends on factors such as application, fit and space within HVAC unit, desired heat transfer efficiency, pressure drop limitations, and installation cost. Engineering calculations and simulations can help determine the most suitable heat exchanger configuration for a given situation.
