Duct Loop Design 3

[Norm01]

I have a data centre that requires 60 tons of cooling. Based on a report the project engineer did, the design is to use a duct loop and have the diffusers discharge into the cold aisles (See attached skech). We will require each to operate in order to satisfy the cooling load. AC-1 unit is exisitng and will be converted from a front flow to a ducted upflow, while AC-2 will be a new upflow unit.
I haven't come across this type of configuration before and I am wondering how I should be calculating the static pressure required for each AC unit.
Can anyone point out any other issues that might arise with this type of system?
All comments are appreciated.

 

[Norm01]

Here is the sketch

 

[ChasBean1]

Two things:

1) The main thing is that the fans are rated at those flows at about equal pressures. This looks like it's is a low backpressure configuration. Internal static requirements plus about a half inch for external should be sufficient.

2) Each unit should have some sort of isolation damper or backdraft damper that closes when the unit is off. You don't want to push air through the idle unit and release half of what's been collected on the filter into the room.

 

[SAK9]

The intention is to have one unit serving the entire area if one breaks down.So the fan should be able to deliver air to the farthest diffuser and that should be the basis for calculating the fan pressure.

 

[SAK9]

Go through this thread to understand duct loops better:
thread403-218085

 

[BronYrAur]

just watch your static if both units are to run together. I had a situation where air flow was restricted on a 1st unit when a second kicked in. It caused refrigeration issues. See this thread:

Forgive me, but I can't seem to link this properly.

thread403-248479

 

[ChasBean1]

What if you a) kept the original unit as single discharge and ducted only the new unit, or b) split service up (see attached) - wouldn't have to worry about matched static pressures or backdraft dampers...