Water Treatment Strategies for Evaporative Cooling Systems

Residential air conditioning is a challenging and costly peak load for utilities to meet. The load factor, defined as the average annual demand divided by peak demand, is only 7%1 . In California (CA), only 8% of air conditioning units surveyed in single family homes were evaporative coolers2 , even though evaporative systems have a large potential to reduce both the peak electricity demand and the energy use associated with cooling. Water management of evaporative cooling units is essential and particular care must be taken to reduce the effects of hard water on the system. Evaporative processes lead to the concentration of minerals in the cooling water reservoir, which eventually precipitates out of solution, scaling the condensing coil, pumps, piping, and other surfaces. The primary goal of this investigation is to provide long-term laboratory test data to assess the longevity of an evaporatively-cooled condensing unit (ECCU) and to provide recommendations for operation and maintenance of the system to maintain performance and energy efficiency. A secondary objective is to evaluate water management strategies to minimize the use of bleed water and reduce water consumption of the system. The first step toward completing the objectives was to run a full scale ECCU to “failure”, defined for the purposes of this project as a loss of efficiency of 25% or more. This accelerated test was accomplished by operating the 3-ton ECCU for 2,000 hours with no water treatment or bleed, which is against the manufacturer’s recommendation. Based on the water supply in Davis, CA the manufacturer recommends that the bleed be set such that 40% more water (over and above the amount of water evaporated and/or lost through drift) be supplied to the ECCU. This test identified modes of failure for the ECCU and the rate of performance and efficiency degradation.