Assessing the Performance of Cooling Towers and Their Effect on Chiller Efficiency

Traditionally, energy recommendations from energy audits have been concerned with the energy use of cooling towers for point of use energy, such as variable speed fans, and pumping savings. The actual energy use of a cooling tower is small, and is only a portion of the energy involved in the entire cooling system. The majority of the energy used is to perform the work necessary to compress the refrigerant at the chiller, and is most commonly defined by the term coefficient of performance. The efficiency of a tower is based on the original design conditions as well as the type of tower (evaporative, draft driven). Inefficiencies can occur due to under or over sizing, controls that are not properly functioning, flawed strategies for operation, and basic lack of maintenance that can create fouling of the exchangers. This paper explores the efficiency relationship between the tower and the chiller, but more specifically develops a methodology to measure and estimate this efficiency on a short one or two day assessment. Modern inexpensive devices allow for measurement and monitoring of air temperature, humidity, process cooling temperatures and sump temperature to be measured on a short term basis. More importantly, these devices can be installed quickly by plant personnel and can be left in place for a week or more and possibly even mailed back to the auditor with the collected data. This paper includes information about how to determine the appropriate design conditions for the tower, its temporal performance, as well as tips from major tower manufactures on design, performance, and maintenance.