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dokundot
10-12-2016, 04:29 PM
Hi ALL,

Question: Is it possible to connect a dual circuit condensing unit to a single circuit AHU coil?

Background: My firm is contracted to replace an old Carrier 39NXV AHU with Dual coils matched to 2 condensing units 38AH (operating as one duty/one standby). That is each dual circuit condensing unit worked with one of the AHU coils at any particular time. So the old AHU has each of its independent coils split to receive the four lines from each condensing units.

Now, we gave Carrier this details and they selected a new 39HQ AHU (each independent coil is 211kw) from Holland and a new condensing unit 38APD090 (from the U.S). The AHU just arrived site but we observed that each of the independent AHU coil has only one liquid and suction line as against the four lines that will come from the dual circuit 38APD. The 38APD is yet to arrive site.

Now we have contacted Carrier as to how this installation is to be carried. Are we to merge two suction lines and liquid lines from the condensing unit to the coil from the AHU (has only one LL and SL)? We are awaiting their response.

Is there anyone who has encountered this scenario? and is there a solution?

I will be glad of your insights.

Thanks.

Brian_UK
10-12-2016, 06:22 PM
The four lines of the 38APD are two liquid and two suction, perfect for two AHU coils.

dokundot
10-12-2016, 07:43 PM
The four lines of the 38APD are two liquid and two suction, perfect for two AHU coils.

Yes, I exactly as one would have expected.

However, like I explained the two (independent) coils in the AHU (each 211kw) are meant for two independent 38APD condensing units.

Carrier supplied an AHU with two independent coils as ordered but each independent coil having only 1 LL and 1 SL (as against the expected 2SL and 2LL). So the 2LL and 2SL from each 38APD has only 1LL and SL to connect to.

Have you any ideas as to solving this?

Thanks

Brian_UK
10-12-2016, 10:47 PM
So you have two, 2, 38APD units ordered then?

dokundot
11-12-2016, 01:22 AM
So you have two, 2, 38APD units ordered then?

Yes, 2 38APD ordered to run as duty and standby.

narkom
13-12-2016, 04:35 PM
For control and redundancy, many refrigeration systems include two or more refrigeration circuits.
Each circuit must be kept separate and designed as if it were a single system. In some cases, a single
refrigeration circuit serves multiple evaporators, but multiple refrigeration circuits should never be
connected to a single evaporator. A common mistake is to install a two circuit condensing units
with a single circuit evaporator coil. In that case an oil distribution problem will arise.
It would be interesting to read Carrier response.

dokundot
13-12-2016, 06:15 PM
Hi ALL,

I do not think a solution exist for my conundrum. However, glad to inform that Carrier has realized their mistake and are proposing to send us a matching dual coil as replacement for the AHU.

Thanks ALL.

dokundot
13-12-2016, 06:20 PM
Narkom,

They have realized their folly. They will send us replacement dual circuit coils.

I think rather than use AHU builder for the selection, someone in Carrier was using doing a manual match up of the AHU to the condensing unit.

Your statement "In some cases, a single refrigeration circuit serves multiple evaporators, but multiple refrigeration circuits should never be connected to a single evaporator. A common mistake is to install a two circuit condensing units with a single circuit evaporator coil" is exactly what McQuay Piping Guide says on the issue.

Thanks.