Wheeler on HVACR: The new dual-capacity condensing units
Over the past two years I've noted with considerable interest the race between compressor manufacturers to come up with new, inexpensive and reliable dual-capacity compressors. Oh, we've had many types of configurations over the years, such as the compressor with the two-speed motor and another with two compressors in tandem, but they were expensive and reliability was sometimes a problem. So, at last year's Air Conditioning, Heating and Refrigeration Exhibition, I was quite interested when I saw what two of these manufacturers were doing.
I'll have to admit that when I saw Copeland Corp.'s dual-capacity scroll, I thought I was looking at the true future of high-efficiency equipment in our industry. The company's idea of putting a valve in the middle of the scroll mechanism to allow the compressor to work at half capacity was simple and inexpensive. However, at this year's show in Atlanta it seemed the concept had been scrapped because the mechanism couldn't achieve a true 50% capacity reduction. But don't count Copeland's scrolls out yet; the company is still working on the concept.
The other idea came from Bristol Compressor Corp. The company had a unique design for a reciprocating compressor -- a standard two-piston system allowing both pistons to pump when running in one direction. However, the single-phase motor it's using allows the compressor to operate in both directions. And when the compressor rotation is reversed, only one piston operates while the other ratchets. Voila! A true 50% capacity reduction.
To tell you the truth, I didn't think Bristol's new compressor had much of a future a year ago. After all, this is outdated reciprocating technology. And besides, the circuitry to make such a compressor operate in two directions is more complicated. However, at this year's Industry Exhibition, I found Bristol's new compressor already in production, with one major customer, Carrier Corp., using it. Several more companies, including Bristol's parent company, York International, were waiting in the wings to buy the new product.
Why was the reciprocating design so readily accepted, despite my scholarly mental predictions to the contrary? Well, what Bristol didn't mention a year ago is that the new design was being co-developed by Carrier. And because of the investment it had, the success of this new compressor was virtually assured. Also, the availability of a working two-capacity compressor with a true 50% reduction was really tempting, since Carrier was looking for ways to continue to improve the efficiency of its chlorine-free refrigerant line, Puron. The new Bristol unit is made to handle the higher pressures of R-410a, so it filled the bill perfectly.
But why the step backward to reciprocating compressors? The folks at Carrier apparently don't think it's a step backward. Although the company truly pioneered the use of scroll compressors in North America (I saw scrolls coming down its assembly lines back in 1986), I was told that it will be using reciprocating compressors in place of scrolls in several new products. Part of this trend is due to continuing advances in reciprocating technology, and I'm sure that another part has to do with simple availability. After all, the big three small-tonnage compressor manufacturers -- Copeland, Bristol and Tecumseh -- all produce reciprocating compressors and, to date, Copeland is the only one that is offering a line of scrolls.
I hope I'm right this time, but I believe we'll see a lot more of two-capacity compressors in the future. Why? Because the U.S. Department of Energy is calling for a new minimum efficiency standard of somewhere in the 13-SEER range. While the industry is trying to hold the line at a minimum of 12-SEER, the fact is, residential cooling-equipment efficiencies will soon rise again. And because nobody is interested in larger condensing units, the most logical solution is to use a more efficient compressor. This is where dual-capacity compressors come in. And even if they can reach the new higher efficiencies by using standard compressors, the better models will have to go to staged operation to get much above the new minimum efficiency, no matter what it will be.
I'm sitting here as I write looking at a specification sheet for Carrier's Bryant line of Two-Speed Puron Plus air conditioners that use the new Bristol compressor. It appears that it's using a variable-speed indoor fan to achieve these results, but here are some of the advertised features:
- More than double the dehumidification of a standard unit (25 gallons per day, as opposed to 11);
- Smaller indoor-air temperature swings (2?F as opposed to 4?F);
- Much higher efficiencies (the unit will run at half capacity 75% of the time with efficiencies hitting 17-SEER); and
- Fewer on-off cycles (twice per hour on average as opposed to the current three times per hour).
Now I realize that I'm probably talking about one of your competitors, and I'm not trying to sell you on a new brand. I'm letting you know that something new will soon be available from Carrier, followed possibly by a similar offering from your manufacturer. And since more than one compressor manufacturer is working on the concept, I doubt you'll have to wait for several years. This is a good industry direction. The technology isn't mind-boggling, there's only one compressor to fail, and it does a better job of humidity control. So when your company finally sees such products, I would promote them heartily and without reservations.