Many factors to consider when choosing a swamp cooler
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Question:
We are considering installing evaporative cooling systems for buildings on a church campus in California. Are "swamp coolers" a good strategy for economically cooling commercial buildings?
Answer:
The California Energy Commission adopted energy credits for evaporative air conditioning as part of its Title 24 code compliance program. Perhaps these credits will be helpful to your client. The publication Advanced Evaporative Cooling White Paper (458KB PDF) provides an overview of the technology, as well as eligibility and installation criteria for obtaining the credits.
Evaluating options
There are several evaporative cooling options that you might want to consider:
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Direct evaporative coolers are the least expensive. They directly evaporate water into the distribution air. They work best in consistently dry climates that do not get too severely hot. But, they can cause indoor air to become too warm and muggy when outdoor conditions are severe.
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Indirect evaporative coolers use evaporative cooling to cool air, but then run that air or the water cooled by evaporation through a heat exchanger to cool supply air without increasing its humidity. The supply air doesn't get quite as cool, but it may feel more comfortable because it is drier.
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The indirect/direct system combines the two systems by first using indirect evaporative cooling, then direct evaporative cooling on the air stream. This can produce supply air that is cooler than either direct or indirect alone at a relative humidity similar to that of direct.
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The hybrid system uses some combination of the above systems with a conventional vapor cycle cooling system to help during hotter or more humid ambient conditions when evaporative cooling cannot get supply air cool and dry enough.
Even within the above configurations, there are variations. In hot muggy environments, hybrid systems use the building exhaust air for evaporative cooling. Cooling occurs because exhaust air is both cooler and dryer than outdoor ambient air. Evaporation cools it even more before it runs through an air-to-air heat exchanger to accomplish what might be called "indirect evaporative pre-cooling for a vapor cycle system.
Climate matters
In your area—Santa Rosa, Calif.—July 31 was the hottest day of summer 2000. The dry bulb temperature was 100 degrees Fahrenheit with a dew point temperature of 55 degrees F. Theoretically, a direct evaporative cooler could give you a supply-air temperature of 75 degrees F at 80 percent relative humidity. With indirect, the supply temperature could get down to about 80 degrees F at 42 percent RH. With indirect/direct, you could get a supply-air temperature of about 68 degrees F at 80 percent RH.
You may find some of the following articles helpful in making your decision:
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"Energy Savings from Indirect Evaporative Pre-Cooling: Control Strategies and Commissioning;" American Council for an Energy-efficient Economy. Proceedings from ACEEE 1998 Summer Study on Energy Efficiency in Buildings. This article describes control of an indirect evaporative cooling system in the northern Sacramento Valley.
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"Field Measurement and Evaluation of Indirect-DX Air-Conditioning Systems;" ACEEE. Describes hybrid indirect/direct systems used in the hottest areas of south central California.
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"Out of Thin Air;" Engineered Systems, September 1998. Describes three stages of cooling in succession—indirect evaporative, vapor-cycle (conventional) and direct evaporative. This achieves very cool supply air that leads to low relative humidity in the conditioned space.
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"A Case for Evaporative Cooling;" Engineered Systems, November 1998. This describes a number of situations where various types of evaporative cooling are effective and appropriate.
Remember that space temperature is always warmer than supply air temperature on hot days. Exactly how much warmer depends upon air exchange rate, outside temperature, number of building occupants and magnitude of electric loads.
Articles and fact sheets:
Reports:
Software:
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Air Conditioning System Cost Calculator; Western Area Power Administration Energy Services
Compare the yearly operating costs of two different air conditioning systems, based on the efficiency of the unit, the distribution system and the cost of fuel in your area.
