Slow Opening Valve

A total guess on my part, but can't you just set up your air dryer to start 10-15 minutes before the compressor kicks on, or just leave the air dryer on all of the time?

Kramarat's comment is quite fair considering (s)he is not aware of the total picture. But the air compressor is situated in a different plant room and it, together with its air dryer will normally be swtiched off. It will only be managed to run by the control system when the main energy efficient compressor fails, or is stopped for maintenance. This will happen so infrequently that it is not feasible to keep the dryer running "just in case". We want to install a failsafe system to maintain low dewpoint even when this 'emergency' compressor will start.

I am not familiar with desiccant type air dryers. do you mean that these dryers do not need some minutes after started, to commence drying?

Yes he does.

These types of dryers are basically plug and play, and yes it would start drying the air immediately, without drawing unnecessary power during down time.

An instrument control valve such as what these guys sell could be used and pilot controlled by a metering valve and a 4-20 or 0-10 signal to regulate your time.

I am one of the developers of the PMC electro pneumatic controller line. _{I had to leave the company to attend to family matters in 2000 and haven't had substantial contact with them since} This is a perfect application for a PMC controller. You can "dial" the speed of the system from instant full on down to full off with an amazing range of resolution. There are also optional pneumatic valve sets and configurations that can be used on very large bonnet/piston actuators or controlled remotely. Tuning of the system is a snap. For our advertising, I coined the phrase "If you can tune a radio, you can tune a PMC". Knowledge of PID is not necessary. The controls and their layout were designed to compare to a radios tuning (set point), volume (response speed) and tone (sensitivity) controls. In one of my pneumatic valve qualification tests, the eventual winner operated over 800 million cycles, at 10 cps, without leaking at a rate exceeding 1 bubble per minute at 100 psi. It was still operating satisfactorily at that point but upon inspection of its "innards", one of the three "spider leg" springs was cracked although the other two were working well enough to maintain sealing integrity. Check page 13 of their ad. Tell them BOB sent you.

Just curious, but it sounds like you have no air for 15 mins either way, so why not just plumb the emergency compressor into the main compressor output, so using it's dryer, that presumably is up and running?

(GA to "desiccant")

Our factory is so big the emergency compressor is located 250 m away from the running compressor (in a different self-contained compressor room with its own dryers, filters, vessel, etc). With the newly-purchased energy-efficient large compressor we no longer need to run this equipment unless for emergencies.

In the Navy we had some air compressors that needed to be free wheeling until they got to speed to reduce the motor loading. They used an unloading valve that was actuated by the system oil pressure and sized to close with normal oil pressure.

I haven't been involved with air compressors for years so I don't know if those valves are still around but it wouldn't take a lot to put a timer on this type of valve.

I think using a proportional valve motor normally used on HVAC systems may or can be adapted, retrofitted or modified to achieve the desired effect or results? This motor valves normally drives the air ducts / vents to close, open and anywhere in-between depending on thermostat settings. It can be engineered to directly linked on a lever type of valve controlling mechanisms.