[TriLUG] OT: thermodynamics of A/C question

[Joseph Mack NA3T]

On Sat, 23 Jun 2012, Matt Flyer wrote:

> On 06/23/2012 10:01 AM, Joseph Mack NA3T wrote:
>> this is for evaporative water cooling, or regular freon 
>> type A/C? (I'm sure you must have said this, but I've 
>> forgotten sorry.)

> This is for regular type A/C.  We use a combination of 
> machines that are mostly R134a, but have a few left over 
> R22, and even an R11 machine or two that are tools of last 
> resort.  The condenser side does make use of evaporative 
> cooling via cooling towers for efficiency.

still trying to get a handle on the connection between wet 
bulb temp and power required for cooling. Is it the 
evaporative cooling that does it?

When I was a kid, I tried evaporative cooling on a towel, 
but the dissolved minerals crusted up the towel eventually. 
Is this a problem with your setup?

> The big electricity driver is being how low you can get 
> the refrigerant temperature on the hot side as this causes 
> the pressure to go up which is what the compressor needs 
> to pump against, which takes horsepower.

This would also point to doing your cooling at night then?

> From this 
> perspective, if you can place the condenser in the shade 
> you will save money.

My house was built with the heatpack sitting in the sun :-( 
They could have moved it around the corner about 10' away, 
with only minor rerouting of the air handling tubes in the 
crawl space, and the heatpack would have been in the shade.

> Just be careful to not restrict the air flow.

yup.

>> I'd forgotten about this. I remember back in the 70's 
>> seeing solar/energy_efficient houses with large (3" ?) 
>> diameter columns in the house, full of water, to store 
>> heat/cold (depending on the season).

> That is a rather neat idea.  I've seen discussions on how 
> some places are actually making ice at night,

so do all your cooling at night eh?

> storing it, and then blowing air over it during the day. 
> I've also seen that in places like Brazil, they design the 
> buildings so that cold air flows over the building steel 
> at night, cooling it down, and then chilling the warmer 
> air the flows over it during the day.
>
> Another tactic, though I am not sure how well it would 
> work in a residential environment, at least without a 
> capital expenditure, would be to use variable speeds on 
> the machines.  As power is a cubic function of the speed, 
> it is often times better to run two machines, slower than 
> one machine.

When you vary the speed, what are you varying? The speed of 
the compressor? If so, then you're dropping the pressure and 
hence the temperature in the condenser. If you drop the 
temperature in the condenser, then you aren't going to be 
able to dump as much heat either.

I understand that A/C works most efficiently when it runs 
continuously, that it takes about 10 mins for everything to 
come to thermal equilibrium. On the ramp up (for some reason 
I don't understand), efficiency is low. In that case you are 
supposed to get an A/C, that on a typical hot day, has just 
enough capacity to handle the heat, when running 
continuously. ie don't get a bigger unit than you need.

I imagine then if you had two smaller units, that for cooler 
days you would have one of the units run continuously. When 
it got hot, a 2nd small unit would kick in. I wonder how 
economies of scale work here. I can imagine that a 2x sized 
unit doesn't cost 2x, but maybe 1.5x. In that case you'd 
have to make up the difference in running costs. If a unit 
costs 5k$ and your annual bill is 1k$, then your savings 
might be 200$ for having two units. This savings would take 
a while to pay back.

Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
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