Two stage indirect/direct
evaporative coolers are a relatively
new addition to the family of
alternative cooling appliances.
A conventional swamp cooler, by its
nature, adds moisture to the air it
cools. To offset this, a pre-cooling, or
indirect stage, was added.
The advantage two stage coolers
have over conventional, single stage
models is the production of cool air
with 10%-30% less humidity
(depending on your climate).
The stages of a two stage cooler are described as "indirect" for the first stage, and
"direct" for the second stage.
In the indirect stage, warm, dry air is pre-cooled by passing through a heat exchanger
cooled by evaporation on the outside. Because the air supply to the second stage
evaporator is pre-cooled, the end result is cool air with 50%-70% relative humidity
compared to 80% humidity from a conventional single stage cooler.
An additional benefit of first stage pre-cooling and reduced humidity is extending the
product's usefulness into hotter and more humid conditions.
Two stage evaporative coolers are most often installed
directly into an outside wall or at ground level supported by
a small concrete pad.
The units require a 115 VAC electrical connection, a supply
water connection and, if they have a water bleed-off
mechanism to reduce mineral deposits, a method for water
The largest units have a cooling capacity of 3 tons, a supply
air flow of 1,400+ cubic feet per minute and consume around
3 gallons of water per hour. This is sufficient cooling capacity
for up to 1,700 square feet of living space.
Two Stage Indirect/Direct Evaporative Coolers
Since a two stage evaporative cooler brings outdoor air into the home at a relatively
high rate and doesn't recycle indoor air, positive air pressure builds up inside the
home. In order for the system to function effectively, the pressure must be relieved by
opening a few windows a couple of inches, or installing an exhaust duct or pressure
operated damper in the attic or an outside wall.
According to the American Society of Heating and Engineers (ASHRAE), two-stage
evaporative coolers can reduce energy consumption by 60 to 75 percent over
conventional air conditioning systems.
Coincidently, I found a study by the Department of Energy simulating the annual
power consumption of different cooling systems in a prototype house in Borrego
The two systems I chose for comparison from the study are a 13 SEER, 3 ton, DX
(Direct Expansion) air conditioner and a variable speed, 3 ton Oasys brand two stage
According to the study, the air conditioner consumed 3248 kWh (kilowatt hours) of
electricity during the cooling season. Using the average California retail electric rate
(2007) of $0.1435 per kWh, the resulting cost is $466.
By comparison, the two stage evaporative cooler consumed 667 kWh of electricity for
a seasonal cooling bill of $96, or 79% less than the air conditioner.
The new two stage evaporative coolers have successfully taken the "swamp" out of
swamp cooler, but may face some resistance to the $3,000 price tag until the next
wave of electric rate increases hits home.