CFM vs. Tonnage 100% MUA

[gruntly]

I was taught, as a rule of thumb, CFM vs tonnage for a recycle system should be around 400cfm/ton. What would be a good cfm/ton for a 100% MUA unit design 90 to 95F?

[john dalton]

Dear Gruntly,

DX HVAC systems that recirculate their air have a range from 325 CFM/ton to over 1,000 CFM/ton depending on the application, just as DX MUA units in the 90/95 degree F intake range has an operational range between 200 and 600 CFM/ton. These ranges can be slightly higher, but should not be lower because of the potential freezing of the evaporative coil under low load conditions, and that’s even with the HGB option that some manufacture’s offer.

Hope this helps……

Respectfully Submitted,
John J. Dalton

[Carnak]

Gruntly

It will be depend on the amount of humidity in the air, but could be 200 per a little south of you.

I have a few EngA 100% O/A units down here in all of this humidity and the CFM per ton is down around 150.

This editted in. I looked at some Ottawa weather Data 87 dry bulb and only 71 wet bulb. For a make up air unit to get the supply air down to a dewpoint of 55F, you would probably be 235 CFM per ton.

For Windsor, a worst case humidity condition would be 82 dry bulb, with 75.9 wet bulb. Not the hottest Windsor gets, but the most humid. You would be about 169 CFM per ton to get that air down to a 55 dewpoint.




[Edited by Carnak on 06-16-2005 at 10:50 PM]

[chill factor]

Carnak, how did you get those values?

[gruntly]

Thank you for your post Mr. Dalton.

During the winter, I started servicing a 1 year old hotel and have a situation where I'm sure I don't have enough cooling. First, these units are 100% indirect gas fired MUA units at either end of the building equipped with split York condensing units. 4 storeys, both discharge into the corridors. One setup is just under twice the size of the other.

These condensing units are York H2CE120(10 ton) for the larger MUA and York 060 (5 ton)for the smaller MUA with respective Alco TX valves each. Both have 2 row evap coils, the larger with face area of approx 12sqft, smaller about 10sqft. Both cond units are controlled by wall stats in the conditioned space. Fan is continuous and heat is controlled by Maxitrol disharge control no space reset. Changeover is manual at the remote panel. In winter mode, these units operate well with plenty of heat at design (-5F) and lower.

I have been all over the cooling with a fine point screwdriver and have made adjustment here and there, superheat, charge, etc, but I still can't get the discharge lower than 65db,72wb, 70%rh for a 3600-4000cfm unit (474MBH out) and 69db, 67wb, 88%rh for a 2550-2800cfm unit (258.8MBH out). What the actual CFM for each is I don't know yet.

This is Barrie, Ontario and our ambient design temp is 85DB,74WB, but this design temp is flawed since the installers parked the condensing unit within 30" of the air intake and both units are dropped into a hole with walls approx 7'high. There is only 30" clearance around all the equipment. I had an ambient outside the wall of 85Fdb but the unit intake was actually 90F.

I have tempurature and humidity issues in the corridors of this hotel. I know this reads like a manual but any opinions may help.

Just to mention I take care of another hotel in town here that has a 700MBH indirect gas fired 100% MUA with a 25ton condensing unit...This building is about the same size and is quite comfortable with it's 55F discharge and staged compressors and this trane split will run flat out when it gets to 90F outside. I am making a comparison to this unit.

Thanks.

I've spent way too much time writing this, I missed the next 2 posts..Thanks for the replies.

[Edited by gruntly on 06-16-2005 at 11:28 PM]

[dan sw fl]

Originally posted by chill factor
Carnak, how did you get those values?


CARNAK did some NICE Work with a calculator
(or Spread Sheet)

160 CFM/ton = About Normal for the South.
What is NOrmal?

[Carnak]

Gruntly

Besides heat from a nearby condenser, you also have the ground heating up.

The dsicharge dewpoint is the key and to not be pumping the space full of humidity you need to have the discharge dewpoint down below 60.

65 db and 72 wb sure sounds like a typo, sure it is not 62 wb?

69 db and 67 wb sounds like something to look at. That is air with a 66 dewpoint that you are pumping into the building which is no good.

I would estimate that you would need more than double the cooling capacity as what you have to get that air down below 60. You will need deeper cooling coils as in more rows. Could be 8 rows.

If you could get accurate entering and leaving db/wb temps we could analyze it more, but with 2550 CFM, 85/74 air heated up to 90, you will need more than 12 tons to get it below 60 discharge and it sounds like you have 5 tons now.

How much air does the trane system move, around 6900 CFM?





Chill Factor

You asked how I came up with airflow per ton figures. Well I cheated and used a program. However when I was refering to Windsor and 169 CFM/ton it was based on the following.

Entering air 82F db 75.85 wb (which I rounded to 75.9) has an enthalpy of about 39.27 Btu/lb of dry air

Supply air (or fog) at 55 db and 55 wet bulb has a dewpoint of 55 and an entahlpy of about 23.18

The change in enthalpy is 39.27-23.18 = 16.09

A quick formula that approximates total cooling is

BTU = 4.5 x CFM x Enthalpy Change

or CFM =BTU/(4.5 x Enthalpy Change)

for 1 ton, 12,000 Btu/hr with an change of enthalpy of 16.09 means

CFM= 12000/(4.5X16.09)
CFM = 165.7

This is very close to the 169 my program spit out.

If you use a chart and there are slight differences in enthalpy it should not matter that much, however the change in enthalpy, 16.09, should still be very close.

I would not recommend using this method to select cooling CFMs for a building. You need to determine the sensible load in the building, in order to determine the air flow for space cooling.

[Carnak]

Originally posted by dan sw fl

Originally posted by chill factor
Carnak, how did you get those values?


CARNAK did some NICE Work with a calculator
(or Spread Sheet)

160 CFM/ton = About Normal for the South.
What is NOrmal?


I am so south I saw Arlene two days before you did.

[gruntly]

Mr. Carnak Sir,

Thank you for the information!

You are correct.

I don't know where I came up with that dbwb. I understand that wb can't surpass db.

The actual discharge from the larger unit is 70db, 68wb, 90%rh. Way too high, which is why it's so muggy in this place. I had the outlets of a 2 ton lobby HVAC sweating the other day.

We've had a little break in the weather so I have time to try and get some information together. My 1969 Ashrae Guide and Data book just couldn't do it this time. I have a good set of Manufactures Lit and your information is invaluable.

Thank you tons.

[Carnak]

You're welcome.

I think you will need a lot more cooling to get rid of the mositure, and then some re-heat so as not to freeze out the corridors.

I have done this using Engineered Air Equipment using hot gas as reheat.



[Edited by Carnak on 06-17-2005 at 10:34 AM]

[dan sw fl]

Originally posted by Carnak
I am so south I saw Arlene two days before you did.

Carnak = CUBAN

[Carnak]

Saw Arlene a day before Fidel

[Carnak]

Dan are you into ASHRAE at all, West Coast Chapter maybe?

[dan sw fl]

Originally posted by Carnak
Dan are you into ASHRAE at all, West Coast Chapter maybe? INFrequently

[hvacdoctor]

Rule of thumb only figure 150-200 cfm per ton of cooling for midwest 95db give or take. It all depends on your final leaving air temperature. Can you live with 62F or 65F LAT or do you need colder air? Do your home work and work with your vendor rep, they should know enough hopefully to help you specify the correct unit.