Air Compressor - Supe'ing Up Question

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It may be how the particular compressor is working. You are correct that it takes a certain amount of energy to compress the air. The compressor, however, may react differently by drawing less amps, but for a longer time. It may be less efficient due to blow by of the piston too.
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On Sun, 6 Dec 2009 12:21:43 -0800, "Roger Shoaf"

I have done this already, when I got the compressor. I know that is how it works. I will get some pictures tomorrow.
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snipped-for-privacy@aol.com wrote:

It is counter-intuitive. Real interesting.
Power is basically pressure times flow rate. As Clare wrote, if flow drops faster than pressure rises the power goes down. With a piston pump the flow rate would seem to be the same. But the piston does not contact the head. At high pressure there is more air left in the cylinder between the piston and the head, so the flow rate is lower. Wouldn't be surprised if this is an intentional design with more space than needed.
----------------------- A separate issue - with a higher start pressure the compressor starts against a higher head pressure which makes it harder to start the motor which produces more motor heating when starting. Or are these things unloaded to start?
As others have said, the motor also operates on a shorter cycle with more starts which can be a major problem. I would guess motor heating is the limiting factor.
--
bud--

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air compressors are unloaded at the end of a run cycle. That's what that short pssst you get at the end is, unloading the line form the compressor to the tank.
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wrote:

They are DEFINITELY unloaded to start. No way a standard compressor duty motor could start a normal compressor head against even FIFTY pounds of pressure.

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I think you are a bit confused. As the piston goes up, the pressure increases until the pressure in the cylinder is greater than the pressure on the other side of the discharge valve. High pressure air now flows until the pressure is equal and the discharge line closes. (This occurs when the piston is at top dead center)
At top dead center in the space between the valve and the piston, there is a volume of air. As the piston goes down, that volume of air decompresses, and the intake valve will open when the pressure inside the cylinder is less than the pressure. when the piston is at the bottom of the stroke, the valve shuts and the pressure inside the cylinder is equal to the air pressure outside the compressor.
Now the work begins. There are two extreme conditions one where the tank is empty, or equal to atmospheric pressure, and the other extreme is where the tank is "full". "Full" is used here to describe a condition where the pressure inside the cylinder when the piston is at top dead center is equal to the tank pressure. The valve can not open and the pressure will not rise as no air is being added.
Consider the full condition situation. Energy is required to compress all of the air in the cylinder even though none of the air that is compressed moves into the tank. After the piston begins it's descent, the air pressure helps push down the piston requiring less energy to be applied by the motor.
Now consider the empty condition. The air pressure at the bottom of the stroke equals the atmospheric pressure. The piston goes up, and so does the pressure inside the cylinder so the discharge valve opens and the air flows to the tank.
If we were to have a hole in the tank that would allow all of the air pumped into the tank to escape while the piston was going down, each cycle of the compressor would consume the same amount of power.
The same thing is true when the "full" condition exists, each cycle of the compressor would consume the same amount of power.
The power consumption at "full" is more than the power consumption at empty.
If you were to add a third condition of "half full", the piston starts at the same condition, but as it goes up the pressure rises inside the cylinder until the cylinder pressure exceeds the pressure in the tank and the valve opens to the tank. If a hole smaller than the one in the empty condition, but just large enough to bleed off all the increase in pressure that you gained on the previous cycle you would consume more energy than at the empty condition than you would at the "full" condition.
At all three conditions described no energy was being expended at the end pf the compressor hose, yet power was being used to one degree or another and the power was used to create heat. (Waste)
The temperature of the compressed air increases with the pressure. and that heat is transferred to the walls of the compressor and dissipated outside of the compressor. Since the air inside the compressor will be at the highest pressure at the "full" condition it will be hotter than it would be at "half full" or empty conditions. Since the rate of heat transferred increases with the difference in temperature more energy has to be consumed at the "full" condition than anything less than "full".
So what would explain this:
"As for your motor, upping the pressure won't hurt it a bit. The load on the motor is actually less at 135 PSI than it is at 100. This is because the compressor is not moving as much air. That sounds counter intuitive but if you put an amp meter on it you will see."
observation? I think what explains this is that his amp meter has some lag.
The measurement was taken from an inductive load which is quite dynamic. Magnetic fields are being created and collapsing as the motor turns and as the incoming power alternated from positive to negative and back to positive.
To measure the load it would be better to use a Watt meter.
Consider the full condition once more. The air is compressed and the piston starts to go down, The energy in the air is being expended as heat as long as it is hotter than the compressor and it is expanding pushing the piston down. Since some energy is expended as heat and some is gained as heat (you will have a net loss) some energy is being sent back to the motor as during the down stroke the piston is actually pushing the motor a bit, making the motor an alternator, and sending current backwards. So it depends where exactly in the compressor cycle you capture the measurement of the electrons flowing.
Upon the completion of the compressor cycle you will have a net flow of Watts from the power grid to the motor even though you may have some points in time during the cycle when the flow is actually reversed.
Hope this helps.
--

Roger Shoaf

About the time I had mastered getting the toothpaste back in the tube, then
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On Tue, 8 Dec 2009 12:30:25 -0800, "Roger Shoaf"

If the air is compressed and does NOT flow out the "exhaust" valve the pressure pushes the piston back down. On a single cyl or parallel cyl compressor this does not do anything, but on an opposed or V or alternate twin, this reduces the amount of power required to turn the compressor.

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snipped-for-privacy@snyder.on.ca wrote:

On a single cylinder compressor it does do something. It pushes the piston down, speeding up the motor, reducing the current drawn.
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Actually they are all very similar. An HVAC compressor just compresses the vapor that is presented at the inlet valve, if you were to open the inlet valve to the atmosphere it would compress the air just like a bicycle pump or an air compressor. The differences come in to the design of how they are powered and how they are cooled, but they all do the same thing.
--

Roger Shoaf

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On Sun, 6 Dec 2009 12:31:01 -0800, "Roger Shoaf"

Here you go
http://gfretwell.com/electrical/compressor%20amps.jpg
short answer 14.3a @ 80 psi 16.1a @ 110 psi 13.2 a @ 150psi
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Mine just went up, watts went up with pressure.
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Christopher A. Young
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On Thu, 10 Dec 2009 17:56:46 -0500, "Stormin Mormon"

Not ALL compressors exhibit this counterintuitive behaviour, but a surprising percentage most likely will.
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On Dec 10, 7:44pm, snipped-for-privacy@snyder.on.ca wrote:

I noticed this a while back I think it is because the cheaper compressor have crappy motors. I tore one apart that burned up and it tlooked like it had half the wire it should in it.
Jimmie
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The machine shut off at 100, so I didn't have much choice.
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On Thu, 10 Dec 2009 17:56:46 -0500, "Stormin Mormon"

110 PSI seems to be the break point.
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Mine shuts off at 100PSI. So, I'm unable to test that.
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I wonder if there is a way to run the compressor continuous. And then let the relief valve do its thing when the pressure is too high. I would also think of adding a cooling fan.
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