Would you believe me if I told you I worked with the guy who invented it and sold it to Tyler? No foolin. As a matter of fact I service the store where it was all drempt up and is the very first concept of it's kind. Still in it's original form and running. It was actually a guy who worked for my shop and he experimented with this idea at a store, and it worked. So he took the concept and sold it's patent to tyler.
Tyler Enviroguard is a way to reduce the refrigerant charge to the minimum possible.
Your working with Enviroguard 1. An SPR valve, attached to a liquid charged bulb up on the condenser. Of the same refrigerant as is in the rack. Think of the receiver as only a holding tank. Not a flow through receiver, and not a surge receiver.
Lets first tell you that, anytime 1 or more compressors is on, the solenoid valve opens on the receiver pump out, and it pumps back into suction. It's constantly returning anything in the receiver back to the system. You'll see that the pump out solenoid is piped along with a cap tube, so it flashes gas back to the suction manifold.
Now. We "dump" liquid to the receiver anytime the subcooling value gets greater than desired. Your removing refrigerant from the drop leg and "dumping" to receiver.
To best understand it with out a true explanation of how the spr does it, is to look at enviroguard 3. If subcooling rises above 14.9 degrees, we "dump" to receiver. The second that subcooling value drops, we stop the dump. All the while were returning liquid back to the system, but at a much slower rate.
Your piping is from condenser drop leg to liquid manifold. And we have a smaller bullheaded 1/2 or 5/8' line that we use for the dump. Taking liquid out of the system, and storing in the receiver.
If I have time tnioght I will dig out the pages on version 1 and post them so you can look at it. If I start talking about it, I will screw it up.
In winter, you'll see 0% receiver, and you should see a full column of liquid at the sight glass. In summer, your gonna see depending on ambient a receiver of between 5 and 35%. The hotter the ambient, the greater the receiver level.
Just understand, the receiver does not "flow through" or "surge". We dump to it and take from it. Based on subcooling. And in this way, we can drastically reduce refrigerant charge.