By Chad Edmondson
Cooling towers for HVAC systems come in either crossflow or counterflow configurations. This is defined by the way the water meets the heat transfer surface, more commonly known as the “fill” surface. Crossflow cooling towers distribute the hot water perpendicularly to the air flow. Water flows from the top of the cooling tower through the hot water gravity distribution basin and into the fill while the cooling tower fan draws air horizontally across the fill. Counterflow cooling towers also distribute water from the top of the tower thru pressurized spray nozzles, however the air flow is parallel (but in opposing direction) to the flow of water. Air is drawn from the bottom of the cooling tower, passing over the fill surface and exiting out the top.
The obvious question is: Why would one specify one type of cooling tower configuration over the other?
Very simply, it typically comes down to footprint. In most cases, a crossflow tower is preferred for several reasons that include, most importantly, overall efficiency and serviceability. However, crossflow towers have a larger footprint so if you are limited in real estate, a counterflow tower, which is taller but has a smaller footprint, may be your only option. A counterflow tower could also be a better option because of its air intake configurations if you are installing the tower next to one or more walls.
Now let’s explore the design characteristics of each type of tower.
Serviceability is a big bonus when it comes to crossflow towers. Crossflow towers have an internal access plenum that can be fitted with an internal platform from which the drive system can be serviced, and the entire fill assembly inspected. These towers can also be fitted with an external service platform or a ladder with handrails that provides safe maintenance access to the hot water basin.
Water flow from the top of a crossflow tower is by gravity only. The spray nozzles do not require any additional pressurization, which saves pump energy. At reduced water flow rates, weir dams help to fully distribute the water across the fill surface. Counterflow towers, however, require pressurized spray nozzles to ensure even distribution of water at part load.
This is an important distinction to remember since ASHRAE 90.1-2010 requires that chiller systems be configured either with multiple or variable speed condenser water pumps so that the cooling tower can be operated:
(1) With flow that is produced by the smallest pump at its minimum expected flow rate, or
(2) At 50% of design flow per cell.
The intent of this requirement is to improve energy efficiency by maximizing the number of towers operating for a given flow rate while also maintaining the minimum required flow rate through the tower. This is because it is more efficient to run a single 500 ton chiller across two 500 ton towers. More tower tonnage means more heat transfer surface, which increases the amount of evaporative cooling.
The challenge is reducing flow without risking insufficient water distribution across the fill. Dry areas across the heat transfer surface are bad for two reasons:
(1) Dry areas create less resistance to air flow, so more air will flow over dry areas than wet areas, which limits the potential for evaporative cooling
(2) Dry areas tend to increase the amount of mineral deposits that get left behind from evaporated water, increasing the potential for dry air disease. [Link to Cooling Tower and Condenser Water Design Part 11: Avoiding Common Pitfalls.]
In addition to being taller and more compact than crossflow towers, counterflow towers have pressurized hot water nozzles which increases the pump head requirement and total system operating costs. Counterflow towers also have limited internal accessibility for service and inspection. An external service platform and ladder is usually required for access to the hot water spray distribution and drive system.
To operate a counterflow cooling tower at half flow you will probably need to modify the spray nozzle pattern to ensure a fully wetted fill. Of course, before lowering the flow through any cooling tower, you should always check with the manufacturer to determine what the minimum required flow is. Otherwise, you run the risk of underflowing the tower and eventually building up scale on heat transfer surfaces.
(Note: All the above features and benefits primarily apply to factory assembled cooling towers, not field erected cooling towers.)
Fan Options for Cross and Counterflow Towers
Both cross and counterflow towers can be built with either axial or centrifugal fans. For indoor applications and/or those with extremely low height requirements, centrifugal fans can be used on counterflow towers to keep the vertical height of the overall unit to a minimum. This option, however, is typically limited to smaller applications that are 272 tons or less and under 1400 USGPM.