When is a BTU not a BTU?

I'm thinking that it's not the BTUs, but the difference in how each unit is designed to work.

http://www.ehow.com/info_8129123_difference-system-air-conditioning-system.html

A BTU is a BTU. It is the amount of energy required to heat one pound of water 1 °F. The correct term for refrigeration would be BTU/hour. Its origin is British.

I expect the difference may be the size of the heat exchangers used in the two units.

There are some refrigeration experts around here who can provide much more insight.

Lyn said, "I expect the difference may be the size of the heat exchangers used in the two units."

That was my first thought. But the British Thermal Unit/hour rating should be -- I would think -- a rating based on the thermal transfer ability of a particular component.

My Air conditioner is rated at 10K BTU/hr, as a *component* consisting of compressor, heat transfer unit, fan and evaporator.

The Commercial cooling system has a compressor rated at 7.5K BTU/hr. I have no idea what the evaporator or heat excahnger are rated at. But I think that is irrelevant. Even with 100% efficient heat transfer in the evaporator and the heat excahanger, the MAXIMUM BTU/hr of the complete system could never be greater than the 7.5K of the compressor.

Yes? No?

But empirically the Commercial unit out-performs the Air Conditioner.

BTW the CoolBot seems to be working perfectly at "fooling" the air conditioner. It does this by heating (slightly) the temperature sensor so the unit "thinks" the air is warmer than it really is. There is also a "frost" sensor to override the cooling if the cooling coil hits, say, 33 degrees.

Commercial refrigeration units are rated based on the evaporator which is a heat exchanger in BTU. Another heat exchanger condenser, capacity is always higher than evaporator capacity as per the HRR( Heat rejection ratio). For air cooled units it is 25 to 30 % higher .

I think the www.storeitcold.com site itself provides some good explanation--read some of the pages like "How it Works" and "Who It Won't Work For". Here's a quote from one of those pages:

"It will work better, but still not very well, because while BTU's are BTU's (it's a strict measure of cooling/heating capacity), your ability to actually ACCESS that cooling power drops drastically as you approach only 60?? F. This is because you don't have the fans and extra surface area built into normal walk-in cooler compressor/condensor/evaporator units which dissipate the cold without freezing up."

BTW, that is what lyn said. ;-)

Tell us KT4YE,

Which one consumed more electricity to cool? Can you give us a kWh cost per BTU?

As the evaporating temperature decreases, the capacity of the compressor also decreases (because it is moving a less dense fluid). The AC might be rated 10,000 Bth/h at a 35°F evaporating temperature suitable for AC, but if operating at 25°F evaporating temperature, its capacity might be only 6000 Btu/h. These numbers are illustrative only; you would have to get the performance curves of each compressor to really compare.

The issue relates to the end use of the two devices. Both are heat pumps, meant to remove heat from a (relatively) cooler region and deliver it to the ambient. However, the design and the rated capacity depends on the the difference in temperatures in the two cases.

Lyn asked the crucial question about consumed wattage.

Just as a water pump working against a larger level difference consumes more power for the same volume of water pumped than a low pressure pump, the walk in cooler designed to work at a larger temperature difference will draw more power than an AC designed to cool a room for human occupancy comfort.

It is not just BTU but BTU multiplied by the difference in temperature that must be considered.

GA. Good explanation!

Is it just compressor capacity, or do the size of the evaporator and condenser coils come into play?

It must be a combination of factors. And any (or all) of them can be a limiting factor. Compressor capacity, size of evaporator and condenser coils, AIR FLOW (fan size/shape and fan motor capacity), and delta T.

If this is so, then, for a given compressor size, improvement might be possible by increasing the size of the coils or by increasing air flow.

In a room air conditioner, compressor size, condenser size and evaporator size are fixed. Air flow might be varied, however by the use of an external fan to either blow more air over the evaporator coils OR an external fan to "suck" more air out through the condenser coils.

Any ideas as to which might be the most effective. Intuitively, I'm guessing that more air over the evaporator might be an exercise in "mouse milking." That leaves the condenser coils.

Any ideas?

Thanks! Bill

Our discussion of hopping up a window unit reminds me of my childhood days and hopping
up a 53 Chevy
There is always another weak point in the line next Hop up the engine and clutch goes out increase the clutch size and tranny lays down, beef up the tranny and the driveshaft falls down in the street, correct that and the axles break

I guess your bankroll is the only limit as to what you can do