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re: frugal ac tips
5 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.
>> the output air temp changes from 60 to 40 when the airflow is reduced,
>> but i'm assuming the coil is just above freezing, ie not any colder.
>> goswami et al found an eer increase from 9 to 11 with evaporative
>> condenser cooling along with a decrease in compressor power from 3 kw
>> to 2.4, all other things being equal...
>
>non-sequitor. goswami was talking about evaporating water on the condenser
>coil.
goswami was talking about surrounding the coil with a swamp cooler box
to cool it with air at something close to the wet bulb temp, which seems
different (and less efficient) to me than cooling the coil with wet-bulb
water. for one thing, the water-coil thermal conductance is much higher
than the air-coil conductance. for another, the water can dissipate heat
100% of the time, even with the compressor running part time, like the
"heat accumulators" on sunfrost and 1930-era refrigerators.
>i was talking about the discussion of just changing the evaporator
>to run at 40 vs 60 and thus running a lower humidity in the house.
was it neon john who pointed out that typical dehumidifiers run with
a large "bypass" airflow, so a small fraction of the air is dehumidified
as it passes by the cold coil, but most of the air isn't cooled much?
he pointed out certain advantages to this arrangement, in the typical
series dehumidifier configuration, but acs are different. for one thing,
they dump the latent heat outdoors. for another thing, the evaporator
can have less airflow than the condenser.
>your calcs earlier show a 40 f evaporator and lower humidity would
>require less ac cooling load than 60 f and higher humidity (by 6%
>i believe it was).
yes...
>so, keeping all other things equal, do you believe the eer for a 40 f
>evaporator is higher than for a 60 f evaporator??
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, but
it seems to me that we can change the amount of airflow and the output
air temp without changing the coil temp.
>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.
>i agree that spraying/watering the condenser coil would improve system
>performance, but i was interested in which evaporator design would be
>better, a 40 f evaporator setting, or 60?? i'm thinking 60 is better
>because although it would require 6% higher cooling load (for 'harry' to
>feel comfy), the cop would be better by more than 6%.
agreed, if we are talking coil temp vs output air temp.
nick
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