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honglo
08-29-2004, 02:27 AM
Does anyone know how to calculate the cfm of condensing units given the blade diameter and the observed negative static pressure inside the unit?

Example: An 18" blade on a condenser at (unknown) rpm will produce a -.5"wc inside the condensing unit. After cleaning, the observed static pressure is reduced to -.3"wc.
What was the increase in cfm?

Thanks,

Green Mountain
08-29-2004, 05:18 AM
Maybe I'm wrong but I would think that would be pretty hard to determine in the field.

The way I would go about it is to construct a 4 ft section of duct and place it on the discharge side of the condenser fan. Then you would have a set size of duct from which to "capture" the air being moved. Then you could take aneometer readings as you would on any duct.

honglo
08-29-2004, 07:45 PM
benncool,
Yes I have thought about that but that would mean carrying yet another bulky piece of (toolry) around. I was just hoping to streamline the act a bit.

I guess I'm looking for something like a backwards ductulator? Ever heard of such a thing?

johnl45
08-29-2004, 08:34 PM
Originally posted by honglo
benncool,
Yes I have thought about that but that would mean carrying yet another bulky piece of (toolry) around. I was just hoping to streamline the act a bit.

I guess I'm looking for something like a backwards ductulator? Ever heard of such a thing?
What would this information be usefull for?

honglo
08-30-2004, 03:49 AM
johnl45,
I was looking for another way to calculate the condensing unit effective capacity.

johnl45
08-30-2004, 11:09 AM
Originally posted by honglo
johnl45,
I was looking for another way to calculate the condensing unit effective capacity. We use 1000 to 1200 cfm per ton.

selfemployed
08-30-2004, 01:11 PM

honglo
08-30-2004, 07:46 PM
selfemployed,
Thanks for posting the chart. This might work but it would only if the static pressures as read from inside the condensing unit (negetive numbers) would corrolate directly to the positive pressures listed.
Do you think they do? In the first listing for a 12" 19deg pitch 3 wing blade at .10 giving you 560 cfm @ 1140 rpm.
Would it be the same cfm at -.10?

Wouldn't it be much easier if manufacturers put a negative press/cfm curve chart with every condenser? You know, like they do showing superheat or subcooling. That way you could see the effects of a dirty coil (or a dirty coil that looks clean) in a hartbeat!

selfemployed
08-30-2004, 08:42 PM
I would think .1 against the blade would be the same wheather + or -. Someone correct me if I'm wrong about that.

johnl45
08-30-2004, 08:49 PM
Tell me how there would be any static pressure when discharging into free air?

selfemployed
08-30-2004, 10:01 PM
Free air blades are not rated that way in the LAU book. Only by rpm, cfm & bhp. I thought honglo was talking about condenser fan applications.

selfemployed
08-30-2004, 11:07 PM

honglo
09-04-2004, 01:47 AM
I was just wondering if you could read the - static pressure inside the condenser (under the fan but after the condenser coils). Measure the fan blade diameter then simply read the cfm off a ductulator. Of course, in order to get an accurate cfm you'd want to subtract the sq ft of the motor area from the measured diameter and use the (adjusted) free surface area diameter in the cfm calculation.

Just looking for yet another way to skin a cat! :)

midhvac
09-04-2004, 09:22 AM
This is an interesting thread. I can see another use for this. Figuring out how much stress a motor is being subjected to. Condenser fan motors hate high static applications.

midhvac
09-04-2004, 09:38 AM
Unless I'm mistaken, Benncool's method would have to be used unless you have some condenser air specs from the mfgr. Unlike moving air through a solid duct, the condenser air is entering through thousands of holes in the condenser which would be very difficult to measure at the inlet, so the output of the air would be all that's measurable.

Static pressure drops across the condenser can't be used without putting a duct on top of the unit, because unlike a pressure drop in a duct, there's going to be no static pressure in the free air before or after the static pressure we're measuring, only velocity pressure.

In order to obtain a reading of volume (cfm), don't you need size? So without the duct on the top, you'd need to know the free air surface area of the grille at the top of the unit and the fpm of the air going through the grill.

I guess it would be like making a house of cards out of 5 air filters. 4 upright filters at the sides and 1 horizontal one at the top and moving air upwards through them.

[Edited by midhvac on 09-04-2004 at 09:51 AM]

honglo
09-04-2004, 01:23 PM
My first thought was similar to Bencool's also. Only what I (figured) was to put a (Lid) on the end of the duct and then measure the static pressure. It would then be a simple matter to look up the corresponding duct diameter/static pressure on a ductulator to determine cfm. The duct would have to have a (skirt) that could be tied around the top of the unit just above the condenser. It would probably look like one of those kid jump and play blow up things you see at carnivals. I was just looking for a more (compact) way to measure the cfm.
If static pressure were measured when the unit was new and then compared to readings later during service, one could determine that the condenser was causing a higher pressure and in need of cleaning. Determining actual cfm using this method is the part I can't figure out.
Maybe it can't be done!

mark beiser
09-04-2004, 01:33 PM
When I went on one of the Trane factory tours in Tyler, Texas, they had a setup in one of the labs for checking the CFM of airflow through condensors. It was a large metal box that fit over the top of the condensor and had a large duct running running out the top of it.

midhvac
09-04-2004, 02:57 PM
Originally posted by mark beiser
When I went on one of the Trane factory tours in Tyler, Texas, they had a setup in one of the labs for checking the CFM of airflow through condensors. It was a large metal box that fit over the top of the condensor and had a large duct running running out the top of it.

That would seem to be the best way to test it. You wouldn't need a lid. That way you could also check the static difference between the condenser compartment and the duct and get an esp reading, and of course you could use an air flow gauge to measure the speed and volume because you'd know the size of the duct. The only question would be effect of the increased static of the duct on the cfms.

Of course you could still have a problem with the heat transfer of the coil being impaired which may not show up in the airflow readings.

Somewhere I recall seeing some mfr's specs for condenser cfms, but can't recall where.

[Edited by midhvac on 09-04-2004 at 03:03 PM]