By Chad Edmondson
“Would my project be better served by an open loop cooling tower or a closed loop cooling tower?”
It’s a question that anyone in the early stages of designing a hydronic system requiring heat rejection is certain to ask. The answer is partly intuitive but also based on application.
If you have a hydronic system that requires protection of heat transfer surfaces of building cooling equipment such as a water source heat pumps, you probably want a closed loop cooling tower. Also, if an open tower would need to be located in an area where it will be exposed to a lot of environmental debris (leaves, dirt, the occasional bird, etc.) or debris from an adjacent factory, a closed loop tower is probably the better choice.
Open Cooling Towers
If the tower is serving a chiller and debris is not a significant concern, open cooling towers generally offer owners a lower first cost. They also typically provide colder water. This is because so much of the cooling effect from an open cooling tower is the result of evaporative cooling. (Remember, the evaporation of a single pound of water releases 1,000 BTUs into the atmosphere.) Depending on the design condition for the geographical area, open cooling towers offer a “closer approach,” which is a good thing because it means the tower is able to cool the leaving water closer to the wet bulb temperature. (For more information on this read Cooling Tower and Condenser Water Design Part 3: Understanding Tonnage, Range, and Approach.)
While open cooling towers may have a lower first cost than closed cooling towers, engineers must remember to factor in any filtration and other equipment that may be required. This is of particular importance if you are looking at a water source heat pumps system or other application requiring it to be a closed loop system.
At least initially, operating costs of open cooling towers tend to be lower. However, these can increase rather dramatically if heat exchangers become fouled. To maintain optimum heat transfer, it is essential to maintain a rigorous cleaning schedule. Open cooling towers also require additional water treatment, which increases the operating costs.
Closed Cooling Towers
Closed circuit cooling towers operate similarly to open towers except there is a heat exchanger coil within the tower that separates the cooling tower water from the building cooling water. During operation cooling tower water is pumped from the basin up to a spray header where it is then sprayed onto a coil containing the building cooling water, thereby cooling it down. Since building cooling water is contained in the coil, it is not exposed to the atmosphere and not subject to fouling caused by oxygen and dirt. Another advantage is that by creating a closed building loop, we minimize the NPSH requirements of the building pumps.
When comparing a closed cooling tower’s cost to that of an open cooling tower with a plate and frame heat exchanger, the closed tower tends to have a higher first cost unless you look at the complete cooling tower system with adjacent components. This is because one must remember that open cooling towers often require filtration equipment and plate and frame heat exchangers are highly susceptible to fouling due to the small pathways within the unit. An open tower with a plate and frame heat exchanger will also require an extra set of pumps. When you add in the plate and frame heat exchanger, filtration and additional sets of pumps the open tower system can actually be a higher first cost than that of a closed loop. Operationally, closed cooling towers maintain their efficiency better than open towers with plate and frame heat exchangers simply because their heat transfer capability is far less impaired by surface fouling.
That’s a brief overview of the differences between open and closed cooling towers that an engineer should consider in the earliest design stages.