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re: frugal ac tips
7 aug 2002
daestrom wrote:
>> >> >...what does running the ac with 60 degree rise from evaporator
>> >> >to condenser, versus 40 degrees, do to the cop?? i would think
>> >> >the 150% increase in delta t would hurt cop by more than the 6% gain.
>>
>> the ac might consider this a 100(460+60)/(460+40)-1 = 4% increase.
>
>nope. the 40 and 60 are differential temperatures. to convert them to
>absolute is not appropriate here.
it is in the thermodynamical college of knowledge.
>in the 60 f evaporator, the cycle is moving heat across a 40 degree
>differential. in the 40 f evaporator, the cycle is moving heat
>across a 60 degree differential.
i'd estimate that the evaporator coil is about the same temp in each case.
perhaps it's warmer in the 40 f case, with less heatflow and less airflow
and thus more thermal resistance from coil to room air.
>> no. my understanding is that the theoretical (carnot) cop of a heat pump
>> is thot/(thot-tcold) = (460+100)/(100-60) = 14 if it's 100 f outdoors and
>> we make 60 f air, and (460+100)/(100-40) = 9.3 if we make 40 f air,
the real-world cops are closer to 3.
>> it seems to me that we can change the amount of airflow and the output
>> air temp without changing the coil temp.
>
>so that means a 9.3/14 * 100 = 66% lower cop
i'd say it's about the same, because
>> it seems to me that we can change the amount of airflow and the output
>> air temp without changing the coil temp.
let me repeat that.
>> it seems to me that we can change the amount of airflow and the output
>> air temp without changing the coil temp.
otoh, the condenser runs cooler with water or wet-bulb air cooling.
>> >i was thinking that since the compressor would have to work harder
>> >(larger dp to outside condenser pressure), the cop would go down
>> >with the 40 f design, wiping out the 6% savings.
>>
>> it seems to me that would be correct if the coil temp changed,
>> but it's contrary to goswami's real-world measurements.
>again, you're confusing goswami's experiments with changing the condenser
>temperature (by using water or wet-bulb air)
i'm confused? :-) he only tried air.
>with the discussion about changing the evaporator temperature (which
>you started in your earlier post). if you leave the outside condenser
>alone, the question is whether to run a 40 f evaporator or a 60 f evaporator.
doing both seems like a good idea. i worry about the corrosion issue,
but if david's ac coil degraded over 3 years of trickling (corrosive,
pure, unrecycled?) di water in tucson, mine may last a lot longer in
phila, if it's only used for 2 or 3 weeks per year.
>...a 40 f evaporator means a poorer cop so it takes more energy to move
>each btu of cooling. i believe the poorer cop will actually result in a 40
>f evaporator design requiring more power to maintain the home 'comfy' even
>though fewer btu/hr cooling requirement.
may i repeat this again?
>> it seems to me that we can change the amount of airflow and the output
>> air temp without changing the coil temp.
nick
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