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Old 06-15-13, 07:58 PM   #1
NiHaoMike
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Default Compressor difference between different efficiency window A/Cs?

When comparing a 9.7 EER window A/C to a 10.7 EER window A/C of the same age and BTU rating, how much difference (if any) is there between the compressors? Do older window A/Cs have much less efficient compressors?

I'm starting to pick out a window A/C or dehumidifier for my heat pump water heater project. Since I'm going to use ES22a (mostly R290), how will the efficiency change if I started with a R410a unit? (I know that the effective capacity would be reduced and the expansion valve would have to be replaced. Also, if I started with a R410a unit, I would reuse the R410a to make a freeze drying machine.)

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Old 06-16-13, 02:32 AM   #2
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The main difference in window AC units that use the same refrigerant but higher SEER/EER in old units is usually the condensor. Either it has more surface area or more layers of tubing through the same surface area. The compressors are mostly all constant-speed rotary and are highly alike. Watts in per watts of cooling is mainly determined by the effectiveness of the outdoor coil.

Newer units are mostly the same in the cheaper lines. As cost increases, so does the design. Mini-split units are the epitome of this trend. The expensive units have electronic expansion valves, inverter-driven scroll compressors, and sophisticated digital controls. With mid-line models, you never know what's inside until you do your homework.

If you are going to start with an R-410a unit and fill it with ES-22a, please take lots of before and after measurements and tell us how it does. Other members have done this with relative success, but the jury is still out as to what gas or blend of gases works best. But it has been proven to work with propane. Just remember that the oil in these units absorbs water.

As for me, I'll stick with R-22 based units for donor units. They are cheap and plentiful, and so is bbq gas refrigerant and mineral oil.

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Old 06-16-13, 08:03 AM   #3
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The window units I'm looking at definitely won't have inverters. As nice as it would be, it most likely would be running for hours at a time every day and not on at all othertimes. A Danfoss Little Kim would be awesome just from an efficiency standpoint, but just finding one is a challenge...

If the condenser is the main difference and the compressors are basically the same, then there is little value starting with a more efficient unit since I'm going to replace that with a water cooled condenser. The existing condenser would become a second evaporator (with an ejector cycle) so the evaporator would then be sized to the point of diminishing returns. The fans would be replaced with variable speed ECM to allow fine control over the evaporator temperature for better dehumidification, so the efficiency of the existing fan motor is irrelevant. I probably won't use Cindy Wu technology for the fans since that excels at high airflow (great for cooling computers and other electronics) and I probably won't need that for this application. (I can always upgrade later...)

The main advantage I see going with a R410a compressor is that it would be working under less stress so it will last longer and allow for higher hot water temperatures if the need arises. (I plan to set the outlet water temperature to 140F for washing dishes. The actual condensing temperature would probably be lower since the inlet water is going to be on the order of 80F or so worst case.)
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Old 06-16-13, 12:41 PM   #4
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Ok, so I've been studying these "mixed flow ejector" configured systems since you posted in the previous thread that you were considering building one from scratch. It seems the mfr's have two types of these systems in production: one for med/low temp refrigeration (reefer trucks) and one for A/C (passenger vehicles).

The first design looks eerily like a flooded evaporator system, with the ejector feeding the flash tank (sometimes through an evaporator, sometimes not) instead of a level control valve. The evaporator runs at near zero superheat, and discharges into the venturi side of the ejector. Major energy savings is said to be accomplished by supercooling the condenser. Sounds great for low condenser discharge temps (near ambient), but not for heating water.

The second design reminds me of an Acadia cold climate heat pump, only the ejector provides the pressure drop required instead of a second compressor. With the Acadia design, on cold heating days, a compressor of large capacity and designed for R- 134a systems was used to provide added pressure and mass flow to raise condensor output. A cap-tube fed intercooler ran from the discharge line to the intermediate line between the two compressors. The "prius design" has only one compressor, but the similar "intercooler" evaporator runs between the discharge line and the venturi side of the ejector. It serves the same purpose, only in reverse: the superheated gas from the "intercooler" evaporator would increase the effective area of the "main" evaporator.

This looks like it would have great potential for energy savings, especially in your application. At higher water temperatures, the "intercooler" evap would help diffuse the warm stream of high pressure liquid and atomize it immediately. With a near-constant air temp going through the evap assembly, it would not be difficult for an HVAC engineer to optimize the flows through each branch of the suction side. Mechanical metering would be best done with a "subcooling" expansion valve like some of our members are integrating into their present geothermal designs. However, with a digital control circuit, you could use an electronic expansion valve to meter your mass flows, both at the condensor discharge and at the intercooler input. This could result in highly improved COP and capacity if done right.

However, there is great potential in this system to push the main evaporator too hard and flood it. With the intercooler gurgling and/or burping a foggy or foamy mixture into the ejector, it could cavitate or stall if overfed. This effect would domino in the main evaporator, causing compressor slugging without ample accumulator volume. At less violent conditions, the refrigerant would rush through the first portion of the evaporator, ruining heat transfer.

It looks like you might have your homework cut out for you if you pursue this configuration.


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