Basically, they are pressure-operated valves.
You know how a Haskel works, right? Low pressure on a big piston pushes a small piston, which produces more pressure and less volume (the old gas-law game)
There are made valves used for switching air (and other fluids) under the same concept. They are available in normally "open" (pass when no control pressure) and normally "closed" (closed when no control pressure) versions. These devices are basic hydraulic components and available relatively cheaply, but air-rated ones for high pressure tend to be a bit tougher to find.
In hydraulic systems their basic use is to "lock" a cylinder when a machine is shut down (or it is desired to prevent a loaded arm or positioner from moving - say, while transporting something that has been lifted) to prevent the cylinder from moving. Basically, the idea is that when the control pressure is removed the valve is "locked" closed, and therefore, the cylinder cannot move. You want ones for high pressure air service as "bleed down" valves, not "lock" valves, since you want them OPEN when pressure is not applied to the control port.
The idea here is that you rig it like this:
1. Your lowest pressure drain has a solenoid valve. UPSTREAM from this (before the valve!) is a "T", which provides "drive air" to the pilot valve(s) for the subsequent stages. These are "normally open" valves - no control pressure, they are open. (Pneumatic systems folks think of these as "unloaders" or "bleed down" valves.)
2. The compressor starts. The first stage line pressure comes up, and as it does control pressure is applied to the pilot valves. The close, pressurizing the other stages.
3. OK, now its time to drain. The timer actuates the drain solenoid. This collapses the pressure in the control line at the same time it "blows down" the lowest-pressure drain. The result is that the second (and third, if you have one) drains open, as the control pressure has been relieved. Thus, all drains open at once and the pressure in those separators blows clear the accumulated condensate.
4. The first drain closes (timer again); the pressure builds back up in the control line, and the other stage(s) close.
Very simple. The first drain only has to hold back its pressure; solenoid valves that can handle "lower pressures" are available for $100 or so. You need bubble tight valves, obviously.
The timer can be built from a relatively simple electronic circuit (556 dual astable multivibrator) driving a power relay to trip the solenoid. You want a pretty short "on" time - a couple of seconds every 10 minutes or so - to actuate the drains. You DO need to select the "on" time so that the third stage feed pressure doesn't collapse entirely, because that will "unsync" the third stage piston on a floating-final-stage compressor and lead to knocking and excessive wear during bleed cycles - you only need to actuate the bleed long enough to blow clear any accumulated condensation. Some tuning of the timer will be required.
If you rig the first stage bleed "backwards" as well - so the solenoid valve is continuous duty and "energize to close", you also get an automatic unloader out of this design. When the compressor shuts down the solenoid is de-energized, which collapses the pilot valve control pressure and all the condensate drains open, dumping interstage pressure and unloading the compressor. (An "energize to open" unit will also eventually automatically unload, as eventually the first condensate canister will leak down, but that's a potentially dangerous design since it could happen minutes or even hours after the compressor is shut off without warning, and you do not want to start up into a pressurized accumulator - that's VERY hard on the motor and other parts of the compressor.)
Wired to a magnetic starter and final pressure switch you now have a fully automatic system that can be run unattended to fill a storage bank, and will turn off and unload when the final pressure is reached.
Here's one that LOOKS like it might work to 5,000 PSI (Note that I have NOT studied the specs on this unit carefully; this is just a quick search of some available sources for such things! High pressure gas can be dangerous; don't exceed ratings!)
http://www.doering.com/pdf/1200755.pdf
You know how a Haskel works, right? Low pressure on a big piston pushes a small piston, which produces more pressure and less volume (the old gas-law game)
There are made valves used for switching air (and other fluids) under the same concept. They are available in normally "open" (pass when no control pressure) and normally "closed" (closed when no control pressure) versions. These devices are basic hydraulic components and available relatively cheaply, but air-rated ones for high pressure tend to be a bit tougher to find.
In hydraulic systems their basic use is to "lock" a cylinder when a machine is shut down (or it is desired to prevent a loaded arm or positioner from moving - say, while transporting something that has been lifted) to prevent the cylinder from moving. Basically, the idea is that when the control pressure is removed the valve is "locked" closed, and therefore, the cylinder cannot move. You want ones for high pressure air service as "bleed down" valves, not "lock" valves, since you want them OPEN when pressure is not applied to the control port.
The idea here is that you rig it like this:
1. Your lowest pressure drain has a solenoid valve. UPSTREAM from this (before the valve!) is a "T", which provides "drive air" to the pilot valve(s) for the subsequent stages. These are "normally open" valves - no control pressure, they are open. (Pneumatic systems folks think of these as "unloaders" or "bleed down" valves.)
2. The compressor starts. The first stage line pressure comes up, and as it does control pressure is applied to the pilot valves. The close, pressurizing the other stages.
3. OK, now its time to drain. The timer actuates the drain solenoid. This collapses the pressure in the control line at the same time it "blows down" the lowest-pressure drain. The result is that the second (and third, if you have one) drains open, as the control pressure has been relieved. Thus, all drains open at once and the pressure in those separators blows clear the accumulated condensate.
4. The first drain closes (timer again); the pressure builds back up in the control line, and the other stage(s) close.
Very simple. The first drain only has to hold back its pressure; solenoid valves that can handle "lower pressures" are available for $100 or so. You need bubble tight valves, obviously.
The timer can be built from a relatively simple electronic circuit (556 dual astable multivibrator) driving a power relay to trip the solenoid. You want a pretty short "on" time - a couple of seconds every 10 minutes or so - to actuate the drains. You DO need to select the "on" time so that the third stage feed pressure doesn't collapse entirely, because that will "unsync" the third stage piston on a floating-final-stage compressor and lead to knocking and excessive wear during bleed cycles - you only need to actuate the bleed long enough to blow clear any accumulated condensation. Some tuning of the timer will be required.
If you rig the first stage bleed "backwards" as well - so the solenoid valve is continuous duty and "energize to close", you also get an automatic unloader out of this design. When the compressor shuts down the solenoid is de-energized, which collapses the pilot valve control pressure and all the condensate drains open, dumping interstage pressure and unloading the compressor. (An "energize to open" unit will also eventually automatically unload, as eventually the first condensate canister will leak down, but that's a potentially dangerous design since it could happen minutes or even hours after the compressor is shut off without warning, and you do not want to start up into a pressurized accumulator - that's VERY hard on the motor and other parts of the compressor.)
Wired to a magnetic starter and final pressure switch you now have a fully automatic system that can be run unattended to fill a storage bank, and will turn off and unload when the final pressure is reached.
Here's one that LOOKS like it might work to 5,000 PSI (Note that I have NOT studied the specs on this unit carefully; this is just a quick search of some available sources for such things! High pressure gas can be dangerous; don't exceed ratings!)
http://www.doering.com/pdf/1200755.pdf
