Single stage vs. 2-stage air compressor - advantages at ...

Clive603 said:

Just recalled where the manual for my retired Atlas KE Vee twin compressor was hiding. These units came in 3 sizes and 15 versions. A quick look shows the single stage KE series are rated at 100 psi and the otherwise identical two stage KT versions are rated at 200 psi. Interestingly the output is given as piston displacement at specified run speed not directly as air delivery. I guess this avoids many potential specification mis-read problems as its then down to the user to figure out exactly how much air he (or she) gets at what temperature. My 3 hp KE2 was rated at 16.2 cfm piston displacement running at 1000 rpm. The lower pressure KE23 version running at 1200 rpm delivers 45 psi from 19.5 cfm piston displacement.

The KT2 version has two piston displacement ratings 7.32 and 9.75 cfm covering 2 HP 900 rpm and 3 hp 1200 rpm drive. The manual also lists a higher pressure 18 suffix version rated at 255 psi and 8.13 cfm piston displacement running at 1000 rpm.

Its interesting to see that specified running speed varies from 720 to 1200 rpm for various versions despite similar design. My mate Andy also got a KE2 from the same scrappy and ran his at 130 psi but the last 10 psi were very slow. I haver seen single stage units installed to deliver 150 psi. Hot air!

Clive

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Heating the air in compression can actually help. If you start with room temperature air and compress it, the heating makes it expand so it has a larger volume and lowered viscosity, making it push out the exhaust valve faster. If the tank is large enough that the air in it is reasonably cool, that air temperature defines the back pressure, not the air going through the exhaust port. If the heat conduction between the intake and exhaust ports is high enough to significantly heat the incoming air, then the compression stroke starts with a smaller charge and resultant loss in output. Cool in, hot out improves the throughput.

The manufacturer only listing displacement (swept volume) is just dodging the issue. A good spec sheet gives the actual output at various pressures. These compressors never deliver the full swept volume. Running at slow speeds and against low back pressure, they may come close, but all drop off when they have to work hard. Configured as a two stage where the low pressure cylinder was only working against about 15 PSI in the manifold between the cylinders, at 62 CFM swept volume and 100 PSI it actually delivered 56 CFM. That is about the best you could expect. I sized the orifice in the bead gun to match and sent the air through the cooler and to it, only feeding the 200 gal. tank when not blasting. With the large storage, other people in the shop never noticed.

Another factor usually neglected is the force needed to open an atmospheric intake valve. Running my Gardner Denver ACR as a vacuum pump, it would rapidly go up to 25 in hg gauge and stop. The approximately 4-5 in difference between that and atmospheric pressure was the differential needed to actuate the valve. That difference is also subtracted from the fill pressure when it is used as a compressor, although that is mitigated a bit because once the valve snaps open, it will tend to stay.

The viscosity of the air can have a surprising effect. I once repaired some huge heaters for an aluminum annealing oven big enough to handle truck loads. The operator commented that even though he had the blower motors rewound for the maximum performance possible, he had to start with the blowers off and heat the air until it got thin enough to avoid overloading them.

All these factors get in the equation, which isn't simple.

Bill
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Heating the air in compression can actually help. If you start with room temperature air and compress it, the heating makes it expand so it has a larger volume and lowered viscosity, making it push out the exhaust valve faster. If the tank is large enough that the air in it is reasonably cool, that air temperature defines the back pressure, not the air going through the exhaust port. If the heat conduction between the intake and exhaust ports is high enough to significantly heat the incoming air, then the compression stroke starts with a smaller charge and resultant loss in output. Cool in, hot out improves the throughput.The manufacturer only listing displacement (swept volume) is just dodging the issue. A good spec sheet gives the actual output at various pressures. These compressors never deliver the full swept volume. Running at slow speeds and against low back pressure, they may come close, but all drop off when they have to work hard. Configured as a two stage where the low pressure cylinder was only working against about 15 PSI in the manifold between the cylinders, at 62 CFM swept volume and 100 PSI it actually delivered 56 CFM. That is about the best you could expect. I sized the orifice in the bead gun to match and sent the air through the cooler and to it, only feeding the 200 gal. tank when not blasting. With the large storage, other people in the shop never noticed.Another factor usually neglected is the force needed to open an atmospheric intake valve. Running my Gardner Denver ACR as a vacuum pump, it would rapidly go up to 25 in hg gauge and stop. The approximately 4-5 in difference between that and atmospheric pressure was the differential needed to actuate the valve. That difference is also subtracted from the fill pressure when it is used as a compressor, although that is mitigated a bit because once the valve snaps open, it will tend to stay.The viscosity of the air can have a surprising effect. I once repaired some huge heaters for an aluminum annealing oven big enough to handle truck loads. The operator commented that even though he had the blower motors rewound for the maximum performance possible, he had to start with the blowers off and heat the air until it got thin enough to avoid overloading them.All these factors get in the equation, which isn't simple.Bill

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