Configuring 100% Back up

[NewmanRef]

Gents,

I hope you are all well!

I have been asked to design a refrigeration system to connect to an AHU supplying a food production factory. The client wants 100% back up should the system fail. The AHU has only 1 DX coil but I need to connect 2x 66kw multi compressor packs to it. 1 pack will do the duty whilst healthy and the 2nd pack is to be solely for 100% back up should the 1st pack fail. I am wondering if any of you guys have any ideas as to how we could configure this?

Obviously we have just one DX Coil and so just 1 refrigerant circuit, to which we need to connect both packs. I am thinking along the lines of motorised ball valves at the pack end to shut off whichever is not in use. My concern is with the refrigerant charge and how much refrigerant could potentially be trapped in the pack which is shut down, leaving the circuit short of gas. If we do not isolate the pack which is not in use then obviously we are likely to fill it up with oil from the other one?!

Any ideas please would be very much appreciated!!!

Thank you in advance!

[frank]

Gents,

I hope you are all well!

I have been asked to design a refrigeration system to connect to an AHU supplying a food production factory. The client wants 100% back up should the system fail. The AHU has only 1 DX coil but I need to connect 2x 66kw multi compressor packs to it. 1 pack will do the duty whilst healthy and the 2nd pack is to be solely for 100% back up should the 1st pack fail. I am wondering if any of you guys have any ideas as to how we could configure this?

Obviously we have just one DX Coil and so just 1 refrigerant circuit, to which we need to connect both packs. I am thinking along the lines of motorised ball valves at the pack end to shut off whichever is not in use. My concern is with the refrigerant charge and how much refrigerant could potentially be trapped in the pack which is shut down, leaving the circuit short of gas. If we do not isolate the pack which is not in use then obviously we are likely to fill it up with oil from the other one?!

Any ideas please would be very much appreciated!!!

Thank you in advance!

You could always change the DX coil in the AHU for a water/glycol coil then use 2x plate heat exchanges, 1 for each pack so that they are separate refrigeration circuits, and use a shunt pump and sol valves between the PHE's and the AHU coil.
I would also add a buffer vessel.

[Ian_Eb]

Surely you will still have a common point of failure in the single coil. Wouldn't it all fail in the event of a coil failure?

I would say it isn't 100% backup unless there were two complete systems

[hyperion]

Evaporator coil failure is less likely than a refrigerant leak or expansion valve failure, which would affect either of the systems.
Suggest as others that you have a twin circuit evaporator coil, thereby eliminating the common failure factor.

[martinw58]

you need two seperate systems to have 100% backup

[cadwaladr]

you need two seperate systems to have 100% backup

Agreed two separate systems is the only way,I built an freezer room recently with two systems the only thing that's common is the incoming power supply to the whole building .

[hookster]

Simpler idea would be to use a single larger condenser pack with step / stage control and multiple compressors. As you only have a single refrigerant circuit in your initial idea you are not going to cover this with both the designs but cost and simplicity is better.