[Tfug] *Way* OT: Swamp coolers, ACbrrs, etc.

[Bexley Hall]

--- Adrian <choprboy at dakotacom.net> wrote:

> Well now.... I would go out on a limb and say that,
> until the humidity exceeds 
> ~75% (WAG), a swamp cooler will ALWAYS be more
> efficient than an AC unit. Now 
> notice... I said more efficient, not more
> comfortable...

*assuming* the cooler can actually *get* to the
desired "indoor temperature"...
 
> The power used by the circulation fan in both cases
> is approximately the same. 

In our case, the cooler fan draws considerably
more power than the ACbrrr... it moves two or three
times the volume of air that the furnace does
(air conditioning is designed for low velocity
delivery of air).  E.g., cooler motor is 1HP.
I suspect the furnace is 1/3 or smaller (I will
check tonight -- I took some power readings earlier
and will run all the math later)

> The power used by the water pump is negligible,
> while the AC compressor power 
> usage is very significant. Typical power usage is
> going to very depending on 
> AC SEER rating, size, ducting, etc. as well as the
> required duty cycle to 
> maintain temperature. The more humid it is, the

Of course!

> better the AC unit works 
> (more heat capacity per volume of air over the
> condensor coils), but the 
> worse the swamp cooler is (reduced evaporation
> rate). A guess for high temp, 
> low humidity summer is something like:
> 
> Swamp -
>   Fan 120V*2.5A = 300W
>   Pump 120V*0.25A = 30W
>   40% duty cycle == 3.2kWh/day

I'd disagree with the duty cycle assessment -- esp.
at this time of year.  Ours runs full out to maintain
an indoor temperature of ~78-80.  Note that swamp
cooler can't "coast" as long as the ACbrrr -- because
the windows are open (especially on windy days where
the hot wind blows *in* through all those windows
as soon as the cooler shuts off!)

[did I mention this is a complicated algorithm?? :> ] 

> AC -
>   Fan 120V*2.5A = 300W
>   Compressor 240*18A = 4300W
>   40% duty cycle == 44kWh/day

Our AC exhibits a much *lower* duty cycle than that.
Even on the days we've seen lately.  I think the
real numbers (in terms of electricity usage) are
a lot closer than you (i.e. *I*) would guess.  :<
 
> So... basically the only case a swamp cooler is
> inefficient is when humidity 
> rises such that it is run continuously to make a
> small temperature 
> difference. I think you really need to plot 3 axises
> to find the convergence 
> points of swamp/AC efficiency: temperature delta,
> humidity, duty cycle.

But, again, that only describes an instantaneous
assessment of which is better.  It ignores any
predictive/historical measures you may have
(i.e. if you have a device controlling this
thing, it sure can *learn* what conditions are
like instead of following a simple set of rules)
 
> Now... comfort wise, that is left up to the user.
> And I would also guess users 
> very greatly. But again, a WAG, I would propose that
> a rule of thumb would 
> be:
> 
> if (make_me_cold ){
>   delta_T = max(0,min((outdoor_T - indoor_T),30));
> 
>   if ( dew_point < (indoor_T - delta_T) ){
>     use_swamp_cooler;
>   }else{
>     use_ac;
>   }
> }



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