By Marc Sandofsky
On a hot morning in July, three men climbed onto the roof of a Stater Bros. Market in San Jacinto, CA, to check up on a recent installation. One was a Stater Bros. refrigeration technician. The others were from MicroCool, Palm Springs, CA.
MicroCool had recently added a technology to the store’s air-cooled condensers to improve efficiency under upper ambient conditions, which was exactly what they were experiencing on that blistering July morning. Although it was barely 11 a.m., the ambient temperature was 97F. Later that afternoon, it would exceed 105F.
The technology that MicroCool added was a C3 Evaporative Fogging system. In the past, MicroCool would engineer each application and then ship the appropriate components to the job site to be assembled in the field. To simplify the installation process, MicroCool developed the C3 system, a pre-assembled single skid module that combines a high-pressure pump, an RO unit, and all the necessary controls.
The C3 system had been placed in a mechanical room on a mezzanine just below the roof. High-pressure stainless steel piping was run from the C3 up to the condensers. When the C3 was energized, fogging nozzles, placed at roof level around the perimeter of the condensers, injected a water mist into the air entering the condensers, thereby reducing the temperature of the air. It was expected that this would lower peak demand and energy usage while increasing cooling capacity.
The Stater Bros. technician knew that under these high ambient conditions, the head pressure would normally be around 240 lbs. Now, however, with the fogging system turned on, the head pressure on one system was down to 142 lbs., while a second system was down to 168 lbs.
“These systems are operating like it’s 60F outside,” the technician said. That was exactly what they had been hoping to see, since the reduction in head pressure would likely mean a drop in demand (kW) and energy usage (kWh), while the cooler liquid would mean an increase in cooling capacity.
Ambient Temperatures, System Efficiencies
The Stater Bros. San Jacinto store lies in an arid area between Los Angeles and Palm Springs. During much of the summer, the ambient temperature is above 100F, which places a strain on the refrigeration and air conditioning systems. Both are air-cooled, for, as ambient temperatures rise, the cooling loads on refrigeration and air conditioning systems increase, while cooling capacity falls and energy usage increases. This is demonstrated in Table 1, which is based on manufacturer’s data for a 100-ton chiller.
As shown in Table 1, the same system that produces 104.6 tons of cooling when the ambient temperature is 85F produces just 83.6 tons of cooling when the temperature is 115F. That represents a 20% reduction in cooling capacity. Over that same temperature range, the system’s power draw rises from 118.3 kW to 156.2 kW, a 32% increase. Overall, the kW per ton of cooling increases by 65%, which represents a sizeable falloff in performance.
To understand the cause of this degradation in performance, remember that on hot summer days, the condenser is not capable of rejecting as much heat. Cooling capacity is reduced, and the system must run longer to satisfy the load. This increase in run time leads to increased energy usage.
Furthermore, since the system must operate under higher head pressures, it draws more power when it runs. That also increases energy usage as well as demand. In addition, the higher head pressures can increase maintenance costs by significantly shortening the life of the compressors.
Understanding Evaporative Fogging
Evaporation is the process through which water changes from a liquid to a vapor. For this to occur, the bond that holds water molecules together must be broken. Ordinarily, this is done through the absorption of heat. Therefore, when fog is injected into the air entering an air-cooled condenser and the moisture is absorbed into the air, heat is also absorbed and the temperature of the condenser air falls.
To gain a better understanding of this process, one need only reflect on the aftereffect of a rain shower on a hot summer day: the ambient air temperature is reduced rapidly and dramatically. Replicating this process can have the same effect on condenser air.
Evaporative fogging should not be confused with the all too common practice of sprinkling water on undersized condenser coils on hot summer days to increase cooling capacity. While at first glance the benefits might seem similar, an uncontrolled water flow spray will cause water and sewage costs to rise dramatically, In addition, unfiltered water can leave deposits on the condenser coil that can further inhibit proper operation. A properly designed evaporative fogging system prevents these undesirable consequences.
The evaporative fogging process begins by passing water through a filtration (reverse osmosis) system. The filtered, PH-neutral water then flows through a high-pressure water pump where its pressure is elevated to 1,000-1,200 psi. Finally, the clean, pressurized water travels through highpressure tubing to fogging nozzles strategically placed around the system to be fogged. The mixture of water and air reaching the condenser coil will be at a considerably lower temperature. This will cause cooling capacities to rise and energy usage to fall.
Results and Conclusions
Southern California Edison contracted with Alternative Energy Systems Consulting (AESC), Carlsbad, CA, to monitor energy usage before and after the installation of the MicroCool C3 system. The measured energy (kWh) savings on the refrigeration racks over the course of the test was 17.6%. In addition, the measured energy savings on the condenser fans was 13.9%.
The condenser fan savings was achieved when the refrigeration system head pressures reached the targeted goal of 140 pounds and the VFDs ramped down the speed of the fans to maintain that pressure.
As previously noted, the actual savings from the application of a C3 system is dependent on ambient conditions. In general, as the weather cools and relative humidity increases, the percentage of savings will fall, although the absolute savings may increase due to longer run hours.
It’s also important to reiterate that there are additional benefits to be provided by the C3 system. Elevated head pressures are one of the leading causes of premature compressor failure. The C3 system can greatly reduce head pressures during upper ambient conditions, thereby extending compressor life and lowering maintenance costs. Furthermore, when applied to systems that are lacking in capacity under peak conditions, the C3 can often add sufficient capacity to eliminate the need to add supplemental cooling.