# Evaporative cooler question

I also suspect that swamp cooler cfms are inflated, as fans used to be. Air King's 9166 whole house fan is listed as 8908 cfm in Grainger's 394 catalog, but Air King says it only delivers 3560 under the current test method, which measures the actual cfm flowing through the fan, without adding the room air entrained by that airstream after it leaves the fan.
Most of the swamp coolers in Grainger's catalog are now listed with an "industry standard rating" ("a numerical rating for comparing similar models and sizing purposes") instead of "CFM." For instance, their chart says a 5NV70 WisperCool model RW5000 cooler with an "industry standard rating" of 5000 can cool a 1200 ft^2 house in Las Vegas.

Moving lots of air wastes water and electricity. A Las Vegas homeowner might do better with Sam's portable 797895 Arctic Breeze cooler mounted inside a house near an open low window and an exhaust fan in a higher window with a one-way plastic film damper. Turn on the cooler when the house temp reaches 80 F and turn on the exhaust fan when the RH reaches 60% to keep the house air at the upper right corner (80 F and w = 0.012) of the ASHRAE 55-2004 comfort zone.
A 1200 ft^2 house with R30 walls and R40 ceiling and 96 ft^2 of R3 windows and 0.2 ACH (I suppose nobody builds houses like that in Las Vegas) would have a thermal conductance of 1200ft^2/R30 for the ceiling + 96/2 for the windows + 1024/30 for the walls + 0.2x1200x8/60 for air leaks, totaling 128 Btu/h-F. July looks like the worst-case month for cooling, when it's 91.1 F over an average day, with an average low and high of 76.2 and 105.9 and humidity ratio w = 0.0066 pounds of water per pound of dry air.
Keeping that house 80 F while evaporating P lb/h of water into C cfm of outdoor air means 1000P = (91.1-80)(128+C). P = 60C(0.075)(0.012-0.0066) = 0.0243C makes C = 108 cfm and P = 2.62 lb/h, ie 7.6 gallons per day. If the house has significant thermal mass (eg a floorslab), we can save more water and energy by only running the cooler at night.
Why do we need 5000 cfm???
Nick
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Like an air conditioner you want to have one rated above what you need. I run mine at the lowest speed most of the time. Only if the humidity rises do I find it necessary to use high speeds.

Why do you say that? They use relatively little water - only that which is evaporated and a bit more if some is bled off.

Breeze
You can't recycle the air in the house. You have to get rid of the moist air as it will not cool.

Your house would be like a steam bath. The maximum cooling is when the humidity is very low. Once it gets to 30% cooling becomes marginal.

I think you need to live in the South West deserts for a summer and try cooling with a swamp cooler. You soon find out why most houses have refrigeration if they can afford it.
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From:
http://www.consumerenergycenter.org/homeandwork/homes/inside/heatandcool/evaporative_coolers.html
"With evaporative cooling, a complete air change in a home occurs every one-to-three minutes. This flow of fresh air means that evaporative coolers can be operated without using the water pump to replace warm stale air with cooler nighttime air, much like a whole house fan does. That's an added benefit."
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"Evaporative cooler water use" by Martin Karpiscak and Mary H. Marion at http://ag.arizona.eduj/pubs/consumer/az9145.pdf cites a survey showing an average water usage of 7.6 gph, which seems like a lot compared to the 7.6 gpd below. And these things have 1/2 to 1 HP motors.

That's what the exhaust fan does, controlling the indoor RH precisely with a humidistat, vs a swamp cooler without a humidistat.

The 80 F comes from a room temp thermostat.

I disagree, as do 21,000 ASHRAE comfort survey participants.

You are thinking about the air outside vs inside the house.

Air-leaky houses with no insulation, poor controls, ignorant owners and HVAC criminals? :-)
Nick
It's a snap to save energy in this country. As soon as more people become involved in the basic math of heat transfer and get a gut-level, as well as intellectual, grasp on how a house works, solution after solution will appear. Tom Smith, 1980
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Or a soaker hose on a slab and a solenoid valve from an old washing machine and a ceiling fan with a room temp thermostat and an occupancy sensor? :-)

Herbach and Rademan (800) 848-8001 http://www.herbach.com sell a nice brass \$4.95 Navy surplus humidistat, item number TM89HVC5203, with a 20-80% range, a 3-6% differential, and a 7.5A 125V switch that can be wired to open or close on humidity rise.

A 30'x40' 4" slab has at least 4/12x30x40x25 = 10K Btu/F of capacitance, making RC = C/G = 10K/128 = 78 hours. Outdoor air is about 84 F at night and 99 F for 12 hours per day in Las Vegas in July. If the slab cools to to Tmin by dawn and warms to 80 by dusk, 80 = 99+(Tmin-99)e^-12/78, so Tmin = 77. If 12hx1000P = 12h(84-78.5)(128+C)+(80-77)10K and P = 0.0243C, C = 170 cfm and P = 4.14 lb/h, ie 6 gpd.
Nick
...a research study initiated by the Office of Arid Lands Studies at the University of Arizona and the Water Services Department at the City of Phoenix with funding from the Arizona Department of Water Resources monitored evaporative coolers at 46 homes in Phoenix. Preliminary data from this study indicates that water usage... was about 7.6 gallons for each hour that the cooler was operated (4.4 gallons per hour for systems without bleed-off and 10.4 gallons per hour for systems with bleedoff.)
from "Evaporative cooler water use" by Martin Karpiscak and Mary H. Marion at http://ag.arizona.eduj/pubs/consumer/az9145.pdf
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Most home coolers have 1/2 HP or less.

That unit may be able to cool a 9 x 9 room. Not much more than that.

Have you lived in Las Vegas or anyplace in the Southwest?

But the air is already quite humid when it enters the house.

For an average good cooler pulling in 110 degree air at 10% humidity will lower the temperature to 80 degrees. The exhaust fan is working all the time if that dinky portable cooler can handle the load. Anyway a normal cooler which costs about the same will do the job.

No. At 110 degrees and 30% humidity the air coming out of the cooler is at 90 degrees. You can calculate it's relative humidity at that time.

Have you ever lived in the Southwest where the overnight temperature does not get below 90? Most all houses in that area already have slab floors. The solution would be to move underground for the summer. And, I will agree that during the part of the season where the temperature does not get below 90 you may indeed us 7 gallons an hour to cool a 2000 square foot house. In that climate we usually set the thermostat at 80 or above.
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You have my numbers. Would you have any evidence for your article of faith?

Think "portable swamp cooler, indoors."

Think "portable swamp cooler, indoors."

You have my numbers. Would you have any evidence for your article of faith?

Nick
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wrote:

Arctic
faith?
I have no faith in your numbers because I think you have drastically oversimplified the problem. What is the efficiency of your heat exchanger. How much area is required to evaporate that much water? How much air movement is required? I have been indirectly associated with cooling problems for aircraft and space vehicles for 40 years. I know how complex such a system can become.
If you really believe you can do it you are wasting your time trying to convice us. You should be in business building those units.
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You have none whatsoever! :-)
Nick
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wrote:

faith?
I have no faith in someone who can not answer questions. "Numbers" are meaningless unless you can explain how you derived them. Your arithmetic appears to be OK but you did not even explain how you derived the formula. You would have received a maximum of an incomplete in my class. I am trying to see if you can think.
You are somehow going to magically evaporate 2.62 lb/h of water. As far as I know that can only be achieved with the largest of coolers using extremely hot and dry air. I want to know what apparatus will do such a thing in an environment such as you describe. The portable unit you picked can't evaporate more than about two gallons a day using 10%, 100 degree air. How are you going to make it evaporate 7 gallons using air that already has a higher water content and temperature approaching 80 degrees.
It is rather simply to state that evaporating a gallon of water will remove 8100 BTUs of energy from the air. The mechanism is something else.
A well designed pad for a whole house cooler has 3000 to 4500 square foot of surface area for evaporation.
That portable one probably has about 500 square foot.
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Amusing. Try some premises, Rich. Do we agree that:
1. Most people find 80 F at 60% RH (w=0.012) comfortable. 2. A C cfm airstream has an effective thermal conductance of about C Btu/h-F. 3. It takes about 1000 Btu to evaporate a pound of water.
Cooling more outdoor air than we need to wastes water and energy. What's the minimum outdoor airflow C in cfm and water evaporation rate P lb/h needed to cool a house with a 128 Btu/h-F conductance to 80 F at w=0.012 if the outdoor air is 91.1 F with w=0.0066? Air weighs about 0.075 lb/ft^3.

But your "5000 cfm" cooler might evaporate 5 gallons per hour.

We haven't talked about that. More premises:
4a. An A ft^2 pool of water loses about 100A(Pw-Pa) Btu/h by evaporation, where Pw is the vapor pressure ("Hg) at the water temp and Pa is the vapor pressure of the air around the pool (ASHRAE), or if you prefer,
4b. The ratio of heat loss by evaporation to heat gain by convection R 100(Pw-Pa)/(Tw-Ta), in T (F) degrees, regardless of windspeed (Bowen, 1926). (R = -1 defines the wet bulb temp.)
5. A ft^2 in a V mph wind has about A(2+V/2) Btu/h of airfilm conductance.

Try 4a.

It helps if the cooler is near a window with incoming air. More premises:
6. Pa = 29.921/(1+0.62198/w) "Hg and
7. Pw = e^(17.863-9621/(Tw+460)) "Hg (a Clausius-Clapeyron approximation.)

But that's way too much air and water, with poor controls.
Nick
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A portable swamp cooler near an open window comes to mind, or some Humidifalls (tm) with the heaters turned off, or a rock cairn with a \$10 10 W fountain pump in a corner or a few toilet tank fountains or an indoor greywater wetland or (my favorite), a concrete slab.
My PA neighbor has a 20-year old basement slab that was probably placed with no vapor barrier underneath, unlike present practice. The water table is high (my old hand-dug well has water 9' below ground) and water seems to be moving up through the slab and evaporating into basement air. We never see puddles or damp spots on the slab. The gutters and downspouts are in good shape, but after 3 blower-door tests and air sealing, the house air is never less than 60% RH in wintertime, even with a bathroom exhaust fan in series with a 60% humidistat. It's probably time to paint the slab.

Pw = e^(17.863-9621/(80+460)) = 1.047 "Hg and Pa = 29.921/(1+0.62198/0.012) = 0.566 "Hg, so each square foot of 80 F wet surface will evaporate 48 Btu/h (about 0.048 lb/h), if it's only exposed to house air. In that case, we need about 2.62/0.048 = 54 ft^2.
I can imagine a 50' soaker hose in series with a solenoid valve making a 1'x50' perimeter strip of slab dampish. A new house might have plastic film on the caliche, under stone, under concrete, with a hole in the slab for water to enter and flow under the slab.
Cooling thermal mass directly is more efficient than cooling air that cools mass, and a cool slab with a ceiling fan and a room temp thermostat and an occupancy sensor might allow good room air temp control with a cooler slab with a larger temp swing that stores more coolth and allows effective "AC setbacks" when a room is unoccupied.
Nick
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On 15 Jan 2005 23:39:50 -0500, snipped-for-privacy@ece.villanova.edu wrote:

What you can't seem to understand is that your formulaes mean nothing if they don't result in the same outcome as occurs in the real world. People who use evap coolers know how much CFM is needed to be comfortable on a 110 degree day with humidity of 20%. You're like the theorist who can "prove" that bees can't fly. That guys theory was inadequate, just as yours seems to be. -- Elbridge Gerry, of Massachusetts:
"What, sir, is the use of militia? It is to prevent the establishment of a standing army, the bane of liberty. . . Whenever Government means to invade the rights and liberties of the people, they always attempt to destroy the militia, in order to raise a standing army upon its ruins." -- Debate, U.S. House of Representatives, August 17, 1789
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I'm not aware that anyone's tried this yet. Seems easy enough. Why don't you?

How much, for a house with a 128 Btu/h-F conductance?

Shades of Harry Thomason. See "Solar Heat in Snow Country" at http://www.ece.villanova.edu/~nick
Nick
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Pa = 0.2e^(17.863-9621/(110+460)) = 0.535 "Hg, so w = 0.62198/(29.921/Pa-1) = 0.0113. At 80 F and w=0.012, 1000P=(110-80)(128+C) and 60C(0.075)(0.012-w) = P make C = -142 cfm, so it can't be done. Do people who use evap coolers know that? :-)
It's easy to see why it can't be done. Air has a specific heat of 0.24 Btu/lb-F. Each pound of air needs sensible cooling from 110 to 80 F, which takes 0.24(110-80) = 7.2 Btu, but we can only add 0.012-0.0113 = 0.0007 pounds of water per pound of air to raise its humidity ratio from 0.0113 to 0.012, which only provides 1000x0.0007 = 0.7 Btu of latent cooling.
But there's still hope for evap cooling with cooler outdoor air at night or outdoor air that's passed through earthtubes, as well as indirect schemes.
Nick
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wrote:

you?
This is done to some extent all over the southwest. They sell those portable coolers everywhere as they are often used to cool one room. A family member had one for a while. It was worthless when sitting in the middle of the room. They would exhaust air with a fan but the cooling was minimal. Finally they devised a way to mount it by a window so they could use only dry outside air. Even then it was good for only a few feet near the cooler. Another problem is the intensity of the sun. Just a few years ago I was in Maryland during the hottest part of the day. I noted that my car air conditioner worked better there than it did in the Southwest. The difference being that the suns rays were so much more intense in the SW. I was doing something in the sun so I moved into the shade. It did not make any difference. The surface and interior of the car stayed much cooler there than in the SW. I have had tires rot from the heat before they wore out.
In the SW the sun is so intense you can burn in a few minutes (Thus Cowboys with their wide brimmed hats and long sleeves). In the SW it can be like an inferno in the sun but quite comfortable in the shade.
This also reminds me of a story about a friend from Europe who was on his way to Los Angles. He was traveling by train and got off at Albuquerque. he said he went around asking if it got that hot in California. He had never experience heat that intense.

You are also using a rather small house for an example. Maybe that is the problem as most people in the Southwest live in houses twice the size you are using for an example. Most have slab floors.

Nothing unusual there. Perhaps only that he is using trickling water instead of a network of pipes, but he still has 700 sq. feet of collector. They can be about 80% efficient. During the energy shortage of the 70s solar heat was very popular in Colorado where the sun is quite a bit more intense and more available (very few cloudy days) than in the northern part of the country. Now most of those units have been abandoned. They are effective when replacing electric heat. At present prices it is still cheaper to heat with natural gas.
A plastic cover on a swimming pool will very rapidly result in a pool that is too hot. Some heat their pools by running their filter water through some black irrigation pipe on a patio roof.
But those have nothing to do with cooling by evaporating water.
One place where I worked the temperature often got to as high as 120 degrees. When those temperatures occurred the humidity was often about 2%. At 90 degrees the humidity would normally be about 20%. They had three large ponds between the buildings with multiple heads spraying water into the air. Surrounded by grass it was quite pretty. The ponds were actually cooling ponds for the buildings refrigeration units. Initially the heads were about a foot above the pond surface. The freon began to get too warm so they raised the heads about six feet to get more evaporation. Later they added another ponds.
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There are many ways to do something badly :-) I suspect a lot more insulation and airtightness and more precise controls would help a lot.

We seem to have shifted from engineering to poetry.
Nick
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On 16 Jan 2005 13:55:30 -0500, snipped-for-privacy@ece.villanova.edu wrote:

Really?? You don't think anyone who lives in the SW has tried using evap to cool their homes???

For a house of about 1200 SF you need at least 2500. For a large home you'd need to go to the 4000 or more. And the 128 Btu/h-F conductance doesn't mean squat. Once you start using an evap cooler you generally run them pedal to the metal during the day once temps are over about 93.487 degrees F. If things cool down at night you might run them on low speed then and at the beginning and end of the summer when it's not real hot. But as soon as you start pumping all that humidity into the air from the EVAP you need to keep a lot of air moving or it becomes stifling regardless of what the thermometer says.

-- Elbridge Gerry, of Massachusetts:
"What, sir, is the use of militia? It is to prevent the establishment of a standing army, the bane of liberty. . . Whenever Government means to invade the rights and liberties of the people, they always attempt to destroy the militia, in order to raise a standing army upon its ruins." -- Debate, U.S. House of Representatives, August 17, 1789
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I doubt anyone's tried controlling an indoor moisture source with an 80 F thermostat and an exhaust fan with a 60% humidistat in an airtight house.

The numbers in my previous posting indicate that's utterly useless at 110 F and 20%. Where are your numbers?

We seem to have different approaches to numbers :-)
Nick
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On 17 Jan 2005 06:02:58 -0500, snipped-for-privacy@ece.villanova.edu wrote:

I can hardly wait to be in an airtight house with 80 degree temps and 60% humidity.... I think they have a spot for you over at Abu Grabi.

Don't need numbers. It will be less then comfy but bearable. I've lived it, you obviously have not.

That's not the difference. I actually know how these things work in the real world. You still have your nose stuck in a book.

-- Elbridge Gerry, of Massachusetts:
"What, sir, is the use of militia? It is to prevent the establishment of a standing army, the bane of liberty. . . Whenever Government means to invade the rights and liberties of the people, they always attempt to destroy the militia, in order to raise a standing army upon its ruins." -- Debate, U.S. House of Representatives, August 17, 1789
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