Single stage vs. two stage compressor efficiency at 100 PSI

Circa 1988, I acquired a Gardner Denver tyoe ACR compressor for a small amount of money. The ACR has two cylinders with 62.5 cu feet per minute swept volume each connected in parallel to produce low pressures 25-35 PSI at near 125 CFM. I found that I could switch the ports to make the cylinders run in series. Of course, that would make a two stage compressor with both stages the same. All the compressor experts said that I could never get it balanced, whatever that means, and it would burn out the bearings, collapse the pistons and cause the total destruction of Western Civilization.

I looked up the loads on splash lubricated automotive bearings and found that even at the higher pressure, I would be well within the limits. I called the regional sales manager for Shell Oil, whom I happened to know because he was a fellow Indian owner. He coached me on the oil grade to use and I started experiments with the idea. First, I found that by 90-100 PSI the output of the cylinders in parallel had dropped to less than 60 CFM, half the swept volume. The reason is that there is necessarily some space between the top of the piston and head, along with space in the passages and valves. Some of the air being compressed fills those spaces instead of being exhausted and expands back in the cylinder. No new air will be pulled in until that air has expanded to about 4 in hg below the ambient pressure, the 4 in being the differential required to open the spring loaded intake valves. This residual air just is compressed and expanded without going anywhere, reducing the output. Somewhat surprisingly, the power required to run the compressor falls off because this air pushes on the piston as it expands, partially paying back the energy used to compress it.

Run as a two stage unit, the output at 100 PSI was about 56 CFM, about what you would expect from a 62.5 CFM swept volume. It continued at about that output up to 130 PSI, which was as high as I went, not wanting to abuse the bearings more than necessary.

Then I made a dryer and decided to feed the output directly into a glass bead gun, sizing the orifice to maintain the desired pressure and control it with a foot operated valve. This time the shrieks from the experts were comparable to Edward II's when he was executed by having a red hot poker stuck up his backside. It would build up terrible pressures, possible fatal and probably turning Webster Groves into a crater. Of course, we mounted a second valve on the pedal that would open when it was released and dumped the air.

The explanation for its operation is that when both cylinders are pumping against no back pressure, the air flows through at nearly the swept volume of a cylinder. When back pressure builds up on the outlet, the second stage's output drops and it does not exhaust all the air the first stage feeds it. Therefore, pressure rises in the manifold connecting them until the first stage is supercharging the second stage sufficiently to make it pass the full volume. "Balancing" is self adjusting. Most of the compression was in the second stage, which is a bit weird, but it worked well.

We used the setup for some 15 years until I came by a more conventional system. I gave the compressor to LST 325, but I don't think they ever got it running. No doubt they consulted an expert.

If you are going to stay at 100 PSI or less, probably the simplicity of a single stage is a better choice. My compressor was designed to pump a large volume at low pressures, so the passages were larger than they would be on one intended to go to 100 PSI, so the drop off would be less. Above that, you will get a lot more from two stages.

Bill

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How a Two-Stage Air Conditioner & Variable-Speed Air Conditioner Operate

Two-stage air conditioners and variable-speed air conditioners have a stark difference in performance capabilities. Variable-speed air conditioners are known to have as many as 700 different compressor settings. This is significant given that the number of settings directly relates to the various levels of air flow.

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Although two-stage air conditioners offer two separate compressor speeds, rather than the one speed offered by single-stage air conditioners, they still can’t compete with the features from a variable-speed compressor. The different settings allow you to run your air conditioner by using only as much power as you want or need.

Two-Stage Air Conditioning Systems: Benefits & Drawbacks

Since a two-stage air conditioner can run at different speeds, it’s more efficient than single-stage air conditioners that only run at full power. Two-stage systems are offered at great prices and offer value with mid-level productivity. However, a drawback of a two-stage air conditioner is that you don’t get as many customization options as far as the total amount of energy the system uses to cool down your home with the compressor.

Variable-Speed Air Conditioners: Benefits & Drawbacks

Variable-speed air conditioners are designed to bring you maximum energy efficiency and a quiet operation. This is especially true when they’re using only a small portion of the power capacity. You’ll save on energy costs and maintain a more consistent temperature around the house. However, the initial purchase of a variable-speed air conditioner is expensive.Not to mention the high price of its replacement parts during repairs and maintenance too.

Tips & Insights: How to Clean a Central Air Conditioner

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Differences Between Variable-Speed Compressors and Variable-Speed Blowers

Although they sound similar, variable-speed compressors and variable-speed blowers are not the same things. Read below for more details on their differences.

What Is a Variable-Speed Blower?

When it comes to your air conditioner, a “blower” is just the unit that’s found inside your house. More specifically, this indoor unit has a large fan that blows air to circulate through your ducts. Since it’s a variable-speed blower, the fan can run faster or slower to circulate the exact amount of cold air you need. This means that you can pair a variable-speed blower with a two-stage or variable-speed air conditioner. The blower will be limited to the full power capacity of a central air conditioning system.

What Is a Variable-Speed Compressor?

Variable-speed compressors are located in the outdoor air conditioner unit and enhance the performance of variable-speed blowers. So although a variable speed blower can operate with any other kind of air conditioner compressor, the blower and compressor are powerful when they are integrated together. They make it easy to gain excellent climate control, humidity control, operating efficiency, and a quiet operation.

Air Conditioning Repair Services

Nothing is worse than opening the windows in the summertime and the house getting hotter. Sometimes, you’ve got no choice but to run that air conditioner. But it doesn’t have to cost you a fortune. We offer reliable air conditioner repair and installation services for one-stage, two-stage, or variable-speed systems. In addition, our staff of certified technicians offers trustworthy air conditioning tune-up services to prepare you for the warm weather. Our contractors offer air conditioning repair services in Reston, VA and other areas such as Alexandria and Arlington. Our licensed, certified technical team can help you save while still getting the best cooling output possible. Book an appointment with our staff of technicians by phone at (703) 543-9649 or on our website.

Tips & Insights: What Size Air Conditioner Should I Buy For My Home?

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