By Chad Edmondson
Most commercial HVAC professionals are probably already aware that there are two different types of cooling towers – crossflow and counterflow. While both operate on the principle of evaporative cooling by blowing air across water, the physical configuration of these two types of cooling towers differs quite a bit.
Counterflow cooling towers pull air up from the bottom of the cooling tower, through the fill media and out the top, while pressurized spray nozzles spray water downward in the opposite direction of the airflow. The fill media is located above the dry air inlet and beneath the spray nozzles and runs the entire width of the cooling tower. Air flow and water flow run in parallel with one another, but in opposite directions.
Crossflow cooling towers pull air in through louvers located all along the exterior sides of the tower. The fan draws the air inside the cooling tower horizontally across the fill media on either side and then up through an open space in the center of the tower. Hot condenser water enters the crossflow cooling tower from basins located at the top, and drains through small holes into the spaces containing the fill media. Crossflow cooling towers do not have any spray fixtures – the water drains vertically into the tower simply by gravity. Also, the airflow in a crossflow cooling tower runs perpendicular to the flow of the water, then makes a 90 degree turn to exit out the center of the unit.
Because of their more vertically oriented design, counterflow cooling towers typically have a smaller footprint than crossflow cooling towers. After all, their heat exchange/fill media area runs across the entire width of the cooling tower, while crossflow cooling towers have an open space in the center through which the air exits out. For a crossflow cooling tower this open space can lead to a more maintenance-friendly design. On the other hand, counterflow cooling towers can require a higher head pump due to the spray nozzles, which typically increase the head requirement on the condenser pump by 3 to 6 psig. Therefore it is critically important that engineers complete a detailed pump head loss calculation on all of the condenser water/cooling tower piping. This is required by ASHRAE regardless of the type of cooling tower you choose, but it can be an especially unfortunate omission if you have chosen a counterflow tower and do not take this into account. Depending on the total capacity of the system, spray nozzles can have a significant impact on your pump head requirement. It is also important to include this added pump head into life cycle cost of the condenser water system.
For more information, please view our video series on Cooling tower and Condenser Water Piping Design.