Abby Normal wrote:

You were using the term 'perfect' earlier and in a previous thread
using it to describe air that was saturated before being blown through
hollow blocks under a floor.

A "perfect swamp cooler," as I used the phrase, would have RH and temp
controls, which has nothing to do with how close it can cool air to
the wet bulb temp. I was thinking a swamp cooler like that could achieve
the same performance as any indoor scheme...

But thinking further, that isn't true, for swamp coolers that don't
recirculate indoor air. Swamp coolers with RH and temp controls may
still be less efficient than indoor schemes for houses with natural
air leakage, ie all houses :-)

For instance, in this case, the indoor scheme required
1360 cfm of exhaust air and 44 pounds per hour of water:

Sensible heat gain of 10,000 Btu/hr -excludes make up air/infiltration

105F db 65F wb ambient

.... 100(Pw-Pa)/(105-65) = -1 (Bowen, 1926) makes the water vapor pressures
Pw-Pa = 0.4 "Hg, and Pw = e^(17.863-9621/(460+65)) = 0.6296, approximately,
using a Clausius-Clapeyron approximation, so Pa = 0.2296, and the absolute
outdoor humidity ratio wo = 0.62198/(29.921/Pa-1) = 0.00481 pounds of water
per pound of dry air.

maintain it at 80F inside.

.... (105-80)C+10K = 1000P makes P = 0.025C + 10 pounds per hour of water,
since C cfm of airflow moves about C Btu/h-F and evaporating each pound
of water takes about 1000 Btu. With wi = 0.0120 (an efficient corner of
the ASHRAE 55-2004 comfort zone), P = 0.075x60C(wi-wo) = 0.03236C, since
each cubic foot of air weighs about 0.075 pounds and there are 60 minutes
each an hour (want to argue about that? :-) so C = 1360 cfm and P = 44,
ie we evaporate 5.28 gallons per hour of water.

Now suppose the house leaks 200 cfm of air (about average in the US.)
In the indoor scheme, the fan would only move 1160 cfm, and the cooler
might reduce its airflow to 1160 cfm (real vs inflated cooler cfm :-),
so the cooler and the indoor scheme would have equivalent performance.

But what can the cooler do if the house leaks more air or we need less
cooling? Suppose we only need 200 cfm of outdoor air? It can't reduce
the airflow to zero and still evaporate water, so it will have to move
excess outdoor air through the house and use excess water, ie the indoor
scheme will use less air and water in this case.

For equivalent performance, it seems we also have to add a motorized
bypass damper to the swamp cooler to allow indoor air recirculation.

Nick