confusing control circuit

[FEISTY]

Converting indoor walk-in refrig. condensing system to outdoor rooftop location. R404 medium temp unit with sister back-up unit. Several companies have turned old system into bowl of spagetti nightmare. Need to start over. Problem is best way to create switch-over control circuit in event of Sys"A" failure. The sister unit MUST cycle on within 1 minute of failure. Temp rise will be trigger. Requires total isolation of Sys"A" when Sys"B" engages. Looking for most reliable way to shut down and start up. I am confused as to how to engineer valving parts using common single pipe run. Any suggestions are greatly appreciated. Involves medical facility and guaranteed cooling in required. Thanks for all advice. Have a good day!!! Jeffrey5151

[Gary]

I'm thinking piping and wiring diagrams would be very useful. In order to know the best way to switch it over, we need to know exactly what you have. Are the two units currently connected... physically or electrically?

[Gary]

Is there some reason you need to use a single pipe run and shared evaporator as opposed to having two completely separate systems?

Setting aside the obvious oil equalization problems, if the main system has a burn-out, the contaminants will be shared by the back-up system.

[malcoa]

Hi jeffrey, i havnt been around for a while but i hope this gives you some ideas,
You need to be looking at a temp controller with an alarm output such as a eurotherm, the controller will run unit A as normal, a high level alarm will bring on unit b via a seperate relay, each unit runs via its own contactor, the control side of the contactors will need to be electrically interlocked so that A will not energise when B is energised and vice versa (making use of auxillary normally open and closed contacts on the contactors or perhaps by the additional relay), additional auxillary contacts will energise and de-energise any solenoid valves that you may use as change over valves for the pipework, you might need to incorperate a timer between the valve changover and the compressor startup but i'm not sure it would be required since i'm not that knowledgeable in the refrige field so this post might be open to critisism