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Name: Jane
Status: Other
Grade: Other
Location: N/A
Country: United States
Date: June 2006


Question:
In central air conditioning systems why is the low pressure tube (the larger one coming back from the evaporator to the compressor) insulated and the high pressure tube (the small one from the condenser to the evaporator) is not? Would it not be better to wrap the two together to promote heat exchange between them, thereby increasing the efficiency of the system? They do this on refrigerators. Any ideas how an ordinary person could measure efficiency of a home central air system?



Replies:
No, it would not be better. The idea is to move heat out to the condenser and leave it there. If the tubes were thermally connected, some of the heat moving out to the condenser would cross over to the incoming tube and get back into the house

Tim Mooney
Advanced Photon Source, Argonne National Lab.


Jane -

ideas:

- the refrigerator is trying to push air to a lower temperature (~-10C) than the air conditioner (~10C). Pre-cooling the fluid before it expands would help the chosen refrigerant fluid to reach its lowest temperature when expanded. Do they both use the same fluid these days? I do not know yet, but I would not be surprised if they do.

- maybe it represents a job-size trade-off:

The refrigerator is a small job (~700W, with 100% on-time), and public attention is focused on efficiency. The air-conditioning is a big job, and horsepower for a given plant-size and cost matters about as much as efficiency. Without a heat-exchanger to transfer left-over coolness from the return fluid to the compressed fluid, the compressed fluid is warmer and perhaps readier to expand and evaporate faster. Perhaps not having heat-exchange gives more cooling power (in BTU's/hr)

for a given minimal-sized expander. (probably not too important) And the insulated return tube keeps the return fluid cold, maximizing the flow of fluid that can be re-compressed by a given-sized compressor-motor to send back out to the expander.

- somewhere there must be a heat-dump from the compressor output, even if the tubes are heat-exchanged. The heat-dump could be some un-insulated length of tube between the compressor and the heat-exchange, preferably with radiator-fins and a fan blowing across it and the location being "outside" the container being cooled. Central air-conditioning's uninsulated length of high-side tubing running through the building may provide a little final cooling of the fluid.

- manufacturability:
Condensation on cold things is always a durability and maintenance problem. Much easier and cleaner when pushing tubes through a building to insulate one neat round bendable tube and have the other be warm and non-condensing, than to provide perfect waterproof insulation around some oval pair. A coupled-pair's extra stiffness would make routing much more difficult too. A refrigerator, on the other hand is fully designed and made in a factory, repetitively.

A refrigerator's exchanger needs to be accommodated only "once" per design, and it always has the protection of the refrigerator's shell. Air conditioning's routing through the building is always a new design, and the building is not designed primarily for the purpose of cooling. The refrigerator's exchanger can be picked up in one hand, but the air-conditioning tubes are tens of feet long...

So close and yet so many practical differences....

Jim Swenson



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