Monday, July 1, 2013
Improving indoor evaporative cooling
ASHRAE says 80 F air with a 0.0120 max humidity ratio (pounds of water per pound
of dry air) is comfortable. Vapor pressure Pi = 29.921/(0.62198/0.0120+1) =
0.5663 "Hg, and RH = 0.5663/e^(17.863-9621/(460+80)) = 0.54, ie 54% max.
In a dry climate, a portable indoor evaporative cooler (eg
can be efficient if a line-voltage thermostat (
QAAEAE ) runs the cooler when the room air temp rises to 80 F and a humidistat (
http://www.zorotools.com/g/Humidity%20Controls/00053793/ ) runs an exhaust fan (
ef=sr_1_1?ie=UTF8&qid=1372590555&sr=8-1&keywords=lasko+2155a ) when or if the
room RH rises to 54%.
If the outdoor air temp is Ta = 86.7 F with a wa = 0.008 humidity ratio (an
average July afternoon in Billings MT) and the cooler evaporates 10 pounds of
water per hour and the exhaust fan moves C cfm of 0.075lb/ft^3 air and 10 =
60C0.075(wi-wa), C = 555 cfm, and the net cooling is 10x1000 - (86.7-80)555 =
6278 Btu/h, like a typical window AC with about 10X less electrical power.
Cool exhaust air could desirably fill a house attic ("up ducts") with a one-way
plastic film damper to prevent warm outdoor air from flowing down through the
house when the fan isn't running and allow cooler outdoor air to flow up through
the house when it is available. If the house has enough natural air leakage, the
fan may never run.
A cooler near an outdoor air inlet can evaporate more water than one handling
cooler and moister house air. A supply plenum could enclose the exhaust fan and
the cooler inlet, with a plastic film damper that allows house air to enter the
plenum when the exhaust fan is not running. The exhaust fan could become a
2-watt motorized damper, raising the COP.
A solenoid valve scrounged from an old washing machine and some misters that
spray water on a rock pile in a tub could replace the indoor evaporative cooler
and lower the cost and raise the SEER to about 6278Btu/h/(90Wx555/2470) = 310 vs
10 for a window AC, using the 90 watt 2470 cfm Lasko fan...
Fancier controls could turn off evaporation and run conventional AC when the
weather is unsuitable.