Re: On board air

[jlandry@domain.elided (John A. Landry)]

On Tue, 23 Dec 1997 11:51:36 -0500, tsm1@domain.elided (Tom Mandera) wrote:

>BUT.. my 196 is *idling*, and now the compressor has the clutch engaged
>(need to up that PSI), but the compressor is NOT putting out 150psi.. it's
>pushing 50psi... or trying to.

What are you talking about Tom???  You're thinking too much again.  That
compressor is capable of (guessing) 250+ psi no matter what speed it's
turning.  The difference in compressor speed dictates how is how long it
takes to reach that pressure.  You could plug the output port and attach a
pressure gauge and then turn it by hand, and assuming you were strong
enough (and disregarding any piston or valve leakage) you could reach 250
psi... it might take awhile, but you could do it.

So the engine speed (and hence compressor speed) dictates how *fast* the
air gets moved and therefore has a direct result on your cfm rating.  Your
cfm rating of the compressor will be much higher when the engine is turning
at highway speeds verses at idle.  Certain factors do place a limit on how
high of a cfm you can reach... such as the design of the pump and
restrictions in the fittings, filters, and hoses.

Think of air like electricity... cfm is like amperage, it's the flow.  The
air pressure is like voltage, it's the potential.  The check valve used in
the system is like a diode.  Pressure and cfm are tied to each other but
are separate things.  So ignoring the compressive nature of air, you could
*almost* say that the York compressor has the same "voltage" capability no
matter what speed it's turning (within reason), but the "amperage" is
directly related to it's speed.

>If I let the truck idle for a long time (maybe I'm warming it up on a cold
>morning after using the air tools last night), the compressor will never get
>above 50psi.. it'll just sit there and FIGHT with the tank until I jump in,
>punch the throttle and tear out of the driveway to work. :-)
>
>Or, maybe I'm sitting on a trail, idling up, or just waiting in "traffic" at
>the foot of an obstacle, and this situation occurs.
>
>This looks like bad news for the compressor, possible premature clutch
>failure, maybe some seals.. what do you think?
>
>This might be another good argument for moderation.. and a 90/125psi switch. :-)

As I described above... this simply doesn't happen the way you envision.
The air system has to have a check valve after the pump, but before the
tank and pressure switch.  That way your pump pressurizes the tank and when
it stops, the air doesn't reverse course and leak back out through the
pump.  It's a very smart idea to use a combination (or separate) check
valve and unloader valve after the pump.  If the pump valves aren't "leaky"
and the pump head remains pressurized, then without the unloader valve the
pump will have to start against the residual line pressure.  I'm sure the
big gasoline engine of our rigs could easily overcome this head pressure,
but there is a lot of momentary strain on both the pump and v-belt as the
compressor clutch engages.  If you didn't even have a check valve holding
the pressure in the tank, then the pump would have to start against the
full force of the line pressure (in your case you were talking about 250
psi... not good for the pump!).

But to talk a little more about what actually happens... Let's say that the
pump head pressure is zero and you have 2 feet of air line after the pump
until the check / unloader valve (which beyond that let's say the line
pressure is 100 psi).  The way this works is that when the clutch engages,
the pump brings the pressure in that 2 feet of hose up from zero to 100 psi
(in seconds) and *then* when the pressure increases over 100 psi, the check
valve opens and the air flows into the lines beyond.

If you didn't have a check valve, then the pump would always be at the same
pressure as the entire system (and you'd probably have air leaking back
through the pump causing the system to cycle even when you aren't using the
air).  If your system pressure was at 50 psi, then the when the pump was
started, it would take the pressure from there.  If the system pressure
were at 200 psi, then when started the pump would go from there... get the
idea?  Without a check valve the pump can't be 50 psi and the system at 200
psi!

The basic compressor system is...

Intake -> pump -> check/unloader valve -> filter (optional) -> pressure
switch -> tank (optional) -> safety valve -> filter (optional) -> regulator
(optional) -> air line.

The safety valve can go anywhere on the pressurized part of the system.
Most people mount it on the tank (if they have one).

Happy holidays,

John

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