AR No. ____ - Install Evaporative Cooling
Estimated Electric Energy Savings = _____ kWh/yr; _____MMBtu/yr
Estimated Electric Energy Cost Savings = $_____/yr
Estimated Implementation Cost = $_____
Simple Payback = _____ months
Recommended Action
Evaporative cooling units could be installed in the ______________ area to offset mechanical
cooling energy usage. The evaporative units could be used to provide cooling whenever outdoor
conditions are favorable, particularly when the outdoor relative humidity is low. During periods
when outdoor temperature and/or relative humidity are too high to allow useful operation of the
evaporative units, existing mechanical cooling can be used to provide additional cooling.
Background
Evaporative cooling units, sometimes referred to as "swamp coolers", can significantly reduce space
cooling energy usage (as compared with mechanical cooling) in two ways. Mechanical (vapor
compression) equipment requires more energy input to provide a unit of heat removal than
evaporative cooling. A compressor system, condenser fans and main system fans are required in the
mechanical system, while a small sprayer pump and fan provides the cooling effect in the
evaporative system. In addition, duct (static pressure) losses are significantly reduced with
evaporative cooling. Evaporative cooling systems (some with mechanical cooling backup) have
been successfully installed in restaurants, office buildings, schools and production areas.
In this region of the country, free cooling may be used for a significant number of hours due to the
relatively cool morning hours and low humidity levels. Typically, evaporative cooling is not used
in areas where the design wet bulb temperature is greater than about 65F. For the Denver area, the
design wet bulb temperature is only about 59F.
The cooling effect produced by evaporative coolers comes at the expense of increased humidity, but
this humidity can be controlled to maintain an optimal comfort level. Also, the large air flows
associated with evaporative cooling give improved comfort at higher humidity levels, due to the
increased air movement.
The units considered are available from a number of HVAC manufacturers, and are constructed
from galvanized or stainless steel. A mesh of plastic tubes, wood or plastic fibers (depending on the
manufacturer), across which the supply air flows, is wetted with a small pump sprayer to produce
the evaporative cooling effect. Some units incorporate use of a fine water spray, across which air
flows directly. They can be installed and controlled to operate in tandem with most types of
mechanical cooling systems, and it appears that the system in the ______________________ area
can be retrofitted with little difficulty.
Anticipated Savings
The evaporative units are assumed to provide cooling as long as a temperature of __ F (the current
setpoint in the area) can be maintained. Once the temperature rises above __ F (with a small
deadband), mechanical cooling will be considered activated. Savings are calculated assuming that
the supply air can be delivered for cooling at a dry bulb temperature much less than that of the
outdoor air, calculated as follows:
T s  T db  ( T db  T wb )  EFF ev
where
Ts
Tbd
Twb
EFFev
=
=
=
=
supply air temperature available from swamp cooler, F
outdoor dry bulb temperature1, F
outdoor wet bulb temperature2, F
evaporator efficiency, no units (75% from manufacturers' data)
Using the above expression and weather data for Denver, Colorado, the available supply air
temperatures (on average) were determined. Building load and energy savings calculations were
performed using an hourly approach. Standard ASHRAE algorithms3, were used to perform the
calculations. The energy savings, ES, and the cost savings, ECS, are estimated as follows:
ES  CCU  PCU
ECS  ES  avoided cost of electricity
where
CCU =
PCU =
current annual energy usage for cooling in building area being considered
(determined from simulation), kWh/yr
proposed annual energy usage for cooling in building area being considered
(determined from simulation), kWh/yr
The current energy usage for space cooling by each building area is calculated as follows:
CHU 
where
QC
QI
QV
QG
QS
COP
C1
=
=
=
=
=
=
=
QC  QI  QV  QG  QS
 C1
COP
load due to conduction through envelope of building, Btu/yr
load due to infiltration and natural ventilation for building, Btu/yr
load due to mechanical ventilation for building, Btu/yr
gain from internal effects (people, equipment, etc.), Btu/yr
gain from solar effects, Btu/yr
seasonal coefficient of performance of existing system, no units
conversion factor, 0.0002928 kWh/Btu
1
National Weather Service Data, Denver, CO.
2
Ibid.
3
Procedure for Determining Heating and Cooling Loads For Computerizing Energy Calculations, American
Society of Heating, Refrigeration and Air Conditioning Engineers, 1975.
Values calculated for each of these effects include both occupied and unoccupied periods. The
projected energy usage for space cooling in the ____ area is determined in a similar fashion, but the
implementation of evaporative coolers to replace the vapor compression systems during appropriate
periods (i.e., when they can maintain the area at __ F and ____ maximum relative humidity) is
considered in these calculations.
Currently, the ____________ area is operated from ___ a.m. to ___ p.m., Monday through Friday
and ____________________. The cooling setpoint is about __ F, and cooling is currently
provided by the __________________________ units. Using the hourly method described with
construction, occupancy, lighting and equipment data for the ___________ area, the current and
proposed cooling energy usage and potential savings were determined. These values include both
mechanical cooling savings and fan energy savings. The table below summarizes these results.
Summary of Building Cooling Energy Usage and Potential Savings
Area
Considered
Current
Cooling
Usage
kWh/yr
Projected
Cooling
Usage
kWh/yr
Cooling
Energy
Savings
kWh/yr
Cooling
Cost
Savings
per year
Marine Products Building
87.0
36.0
51.0
$750
Terrestrial Products Building
87.0
36.0
51.0
$750
TOTALS
87.0
36.0
51.0
$750
If the existing mechanical equipment is to provide all cooling when the temperature cannot be
maintained at __ F by the evaporative system, then in most cases, the mechanical units will operate
at least once during each billing period. To be conservative, this analysis assumes that this
eliminates any demand-related savings. Thus, cost savings are calculated using only the effective
cost of electricity $___/kWh.
As shown in the table, the total annual energy savings to be realized by implementing the
recommended thermostat setback procedure are ______ kWh/yr, and the total annual cost savings
$____/yr.
Implementation Cost
Implementation requires purchase and installation of ________ evaporative cooler units. Roof
penetrations for mounting the new units must also be made, and barometric relief must be provided.
Considering the cooling load provided, data supplied by evaporative cooler equipment
manufacturers suggest that units with a total air flow of approximately ______ cfm and a fan size of
__ hp will be required. The estimated cost for _______ evaporative cooler units which could
provide this flow is $_____, including installation and contractor overhead and profit. Costs for
roof penetrations are estimated as $___________ (mostly labor). (*** CHECK IF THE EXISTING
DAMPER SYSTEM COULD PROVIDE ADEQUATE BAROMETRIC PRESSURE RELIEF
***) Thus, the estimated annual cost savings of $_____/yr would pay for the estimated
implementation cost of $_____ in about ___ months (_____ years).
*** NOTES ON USING THIS AR ***
1.
This typically only pays off when high humidity levels are not a serious concern, as in
production areas, warehouses, or other areas where people are active.
2.
It typically requires that existing equipment be near the end of its useful life, or that cooling
load is increasing, as in the case of an expansion.
3.
In areas where humidity is needed, as in the paper/printing industry, use of these units can
improve humidity levels (winter mostly, some fall and spring) and can reduce mechanical
cooling load.
4.
Consider either add-on packs or stand-alone units.
Contact Information
Vendor contact name:
Vendor company name:
Address:
City:
State:
Phone:
Fax:
E-mail:
Internet:
Make and Model Number:
Other Info: