In the late spring of 2009 Shasta Eye Medical Group facility manager Don McComas had a problem. The air conditioning system on one of his facilities could not keep up with the cooling demand as outdoor temperatures rose steadily with the approach of summer. This situation was not entirely surprising given the requirements of the surgery center- 100% fresh outdoor air.
All compressor-based, direct expansion (DX) systems are designed for and function best in a re- circulatory mode. DX systems re-circulate indoor air and cool it a few degrees with each pass through the evaporator coil. A small percentage of hot outdoor “make-up air” is introduced as required, but the vast majority of the indoor air has already been cooled by many passes through the system. However, many medical facilities are required to use 100% fresh outdoor air to reduce the risk of infection and transmission of airborne pathogens. As the ambient temperature rose above 85°F the DX system at Shasta Eye could no longer cool the outdoor air from the ambient temperature to the required indoor temperature with a single pass through the system. And as the outdoor temperature rose the DX system actually became less efficient as the differential or delta T between the ambient temperature and the condenser coil narrowed.
Facility manager Don McComas sought help. A local engineer thought there was only one solution- to put a second DX unit in front of the first DX unit. The second unit would cool the air from the ambient temperature to 85°F and then the original system would be able to sufficiently cool the air to meet the indoor set point requirement. There was just one problem with this recommendation- not only would the second DX system be expensive to operate, it would require a large capital investment that Shasta Eye was reluctant to make.
The requirements were challenging:
- The existing DX system required 100% fresh outdoor air delivered at a temperature not greater than 85°F no matter what the ambient outdoor conditions.
- The second system could not add any humidity to the supply air since this would make the existing DX system less effective in addition to increasing the risk of airborne contaminates.
- The proposed system had to be inexpensive to install and run.
- The proposed system had to be designed, fabricated and installed within six weeks due to the approach of summer high temperatures.
The AMAX Solution
The AMAX Cooling Solutions engineers immediately understood that only evaporative cooling could cost effectively lower the ambient outdoor air temperature to the DX system requirement. Unlike DX systems, evaporative cooling systems do not become less efficient in higher temperatures. An evaporative system’s cooling effectiveness depends on ambient humidity rather than just temperature differential between condenser coil and air. Like much of the western United States Northern California has a dry climate that is ideal to take advantage of evaporative cooling.
The Shasta Eye Medical Group building located in Redding is in California Climate Zone 11. This area has an ASHRAE 1% design condition of 105°F with a mean coincident wet bulb of 68°F.
Most evaporative cooling systems use direct evaporative cooling- in which hot dry air is passed over a wetted media. The supply air becomes cooler as the heat in the air is absorbed by the evaporative process and becomes latent heat in the cooler more humid air. This basic system is simple and cost effective but it would not meet the requirements of the Shasta Eye Center. The first problem with this solution was that the DX would have to remove the latent heat present in the humid air before it could continue the cooling process. The second problem was that moisture could not added to the air due to concerns about mold, Legionella and other contaminants associated with humidity. The AMAX solution was to use the direct evaporative process to chill water rather than air. Using advanced ridged media and efficient blower fans the AMAX Indirect Evaporative Cooler (IEC) chills water to the wet bulb temperature. This chilled water is then pumped through the AMAX heat exchanger which is placed in the supply air stream of the DX system. The heat from the outdoor supply air is thus transferred to the AMAX exhaust air by way of a continuous water loop. This process results in an adiabatic effectiveness of 75% of the difference between the ambient dry bulb and wet bulb conditions (greater cooling effectiveness can be achieved, however this adds to unit cost, which was another important constraint). In the 1% ASHRAE Design conditions for Climate Zone 11 of 105°F dry bulb and 68°F wet bulb the AMAX IEC delivered supply air at a temperature of 77°F, well below the 85°F required by the existing DX system.
For more information, please download the Case Study