condensing unit air discharge

[mohvac75]

I am supervising a commercial building with Dx-split A/C system. Condensing units due to architectural considerations are installed inside service shaft dedicated for this purpose which has louvers to exhaust air discharge and introduce fresh air for condenser.
The contractor has raised the concern that due the pattern of air discharge (v-shape) the hot air discharged from the units might accumulate around the unit and cause tripping.
We were asked to make exhaust duct for discharged air to make sure that all air is expelled outside and only fresh air from outside is inhaled.
The problem that the manufacturer did not disclose any information regarding static pressure available from the fan or air flow pattern characteristics, so that we can make an operational design of the required duct.
Any suggestions regarding the above problem.
This is very essintial and urgent.

[john dalton]

Dear Mohvac75,

Unfortunately most condensing units have no provision for external discharge ducting to be installed on the condenser’s outlet, although a small number do. Lets start with the Make and Model number of the condensing unit and the size and configuration of these “condenser shafts” and then we can start to evaluate what we are dealing with friend.

Respectfully Submitted,
John J. Dalton

[mohvac75]

Dear John

thanks for your prompt reply. The condensing units make is CARRIER, model Nos. 38CKC060 and 38ARZ008 & 012. The head above the units (to the units in next level) is about 10 ft.
The shape of the shaft is a triangle with two equal sides of 14.6ft in length and base length of 20.5ft. The louvers are to be located near the head of the triangle covering half distance of each equal side.

I hope the information given above are clear.

[marauderx]

For 10 feet? Something tells me this contractor hasn't installed many of these things...unless I'm not understanding this correctly. How big is the 'triangle' area the condensers are in?

An outdoor condenser plume can reach 10 feet upward and draw down mixed air easily. Why the need for the specific shafts? If anything provide a section of duct out of the condenser airstream from the top of the triangle to a foot above the bottom. The last thing I would do is enclose a duct around each of the three condensers - it's only 10'!

[john dalton]

Dear Mohvac75,

Do I understand you right that within the 149.7 square feet bottom of this shaft you are going to install three(3) Carrier condensing units with a total condensing discharge capacity of 16,400 CFM? Although all three of these Carrier condensing units discharge their condensing air straight up, each one of the units have between a 171 to 223 feet per minute face inlet velocity back through the condensing coil surface.

Even giving an exact equal spacing between the condensing units in this relatively small footprint, the air change rate in this area will approach 11 times per minute. None of these condensing units were designed to have discharge housings installed above their 0.25 HP condensing motors, therefore they will lose “some” overall operating capacity if you attempt to do so, but given the relatively small space and the large CFM discharge amount, I would have to concur with the installing HVAC contractor that he should at least voice a concern overall this design and the potential of re-circulated condensing air that may follow such an installation.

I would also suggest that even with the lose of some “small” portion of the overall operating capacity of this equipment that this equipment be installed WITHOUT the discharge housings and be monitored during the warmer weather to determine if discharge housings would actually be required.

If these discharge housings are required, then simply install discharge shafts that equal the single 29 inch round discharge from the 38CK060 unit, and two each of the 22 inch round discharges for each of the 38ARZ008 and 38ARZ012 units. These shafts could be supported from their bases on the condensing units themselves, while having cable guild wire at the tops of the shafts as they approach the top of the equipment shaft.

Anyway, that’s my option and take upon the subject.

Respectfully Submitted,
John J. Dalton