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Join Date: Jan 2007
Posts: 38

# Sizing an air conditioning unit

02/20/2007 1:30 PM

Consider this area:

2857 square feet

33316 cubic feet

Space is for offices

24 persons

I have done a cooling calculation on the area and have come up with 8 tons.

How do I figure out the capacity (cfm) that I will need? I want this unit for year round air conditioning so I will need cooling in the summer and in the winter we will have to heat the air coming in from out side but not the area. How do I figure out my fresh air?

I am sort of stuck, can anyone provide me a push in the right direction?

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Active Contributor

Join Date: Feb 2007
Posts: 11
#1

### Re: Sizing an air conditioning unit

02/20/2007 4:55 PM

ASHRAE Fundamentals Handbook would be a good investment for the practicing engineer, espically if you are dealing with HVAC equipment sizing and calculations. In Ch 8.14 of the Handbook, there is Fig. 7 which relates face velocity in fps to percentage of people dissatisfied with the conditions, citing three common trends for ambient temperature. For a given temp of 73.5 F, 10% of workers are dissatisfied when there is a face velocity of 0.5 fps. There needs to be consideration for where each of the supply diffusers are located in relation to the people in the enviroment. A good rule of thumb, what I have found, is that between 50 and 100 cfm per person is typical for office settings. I hope you found this helpful. As far as your fresh air goes, it is up to the person footing the bill to run the unit. As it is dependent on the relative humidity of the fresh air compared to the current return air and desired RH index. Again the ASHRAE handbooks would be a good investment, tell your boss that your company needs to buy them for you so you can do your job properly.

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Join Date: Jul 2006
Location: East Anglia, UK
Posts: 2003
#2

### Re: Sizing an air conditioning unit

02/21/2007 2:53 AM

I had to do a similiar thing, I found this a good contact -

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Join Date: Dec 2005
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#3

### Re: Sizing an air conditioning unit

02/21/2007 3:31 AM

I suggest you take dskktb's advice and obtain copies of the ASHRAE manuals, especially the first one "Fundamentals".

From your post,the unknowns are design temperatures, summer - dry bulb / wet bulb, and winter design temperature. I'm sure you have these already, otherwise you would have difficulty arriving at the 8 ton load. This would be the internal load (without outside air load), and would be the total load, which consists of sensible heat load plus latent heat load.

The CFM is determined by the sensible heat load, as follows: CFM = 1.08 x sens load / (rm air dry bulb temp - dry bulb temp lvg the coil). You will find that in most cases the average lvg dry bulb temp for comfort conditions will be around 55 deg F. The next step is to determine the sensible heat. Average sensible heat ratio for an office building is around 85%, this means that the sensible heat is would be around 85,000 btu/hr (8 tons rounded up to 100,000 btu's). Using the above formula, the cfm required would be 4590 cfm supply air at 55 deg F to maintain 75 deg F rm temp.

However, you must add the outside air load. To determine the amount required, ASHRAE guidelines provide a series of values depending on type and length of occupancy, smoking,etc. In addition, you need the O.A. design db / wb. Using 10% O.A. as an example, and 95 deg F db, the O.A. sens load would be 8500 btu/hr. Adding this to the rm sens, total sens would be 93,500 btu's. 10% O.A. will add 10% to the air lvg the room, return air, thus the air entering the coil will be 77 deg db. To find the correct lvg coil db temp = 1.08 x 93,500 / 4590 = 22 deg dt. So, in this example the lvg coil db would be 77-22 = 55.

The heating coil required for winter preheat of O.A. needs to raise the O.A. temp to room temp, 75 degF. If O.A. winter design is 0 degF, then the heat required would be 495 X 1.08 X (75-0) = 40,095 btu/hr. This coil would be in the O.A. duct and controlled to maintain 75 F to be mixed with return air, also at 75. OK!!

Now, have at it.........I haven't addressed the Latent load, but I'll let you figure that out yourself.........

g scott

Commentator

Join Date: May 2005
Posts: 56
#4

### Re: Sizing an air conditioning unit

02/21/2007 9:13 AM

For ventilation air it would be a good idea to get a copy of ASHRAE Standard 62.1 on ventilation and indoor air quality. This will tell you cfm per person needed for the function of the facility.

Anonymous Poster
#5

### Re: Sizing an air conditioning unit

02/21/2007 9:47 AM

IAQ ASHRAE 62.2 might recommend 10-15 CFM per person of tempered outside air.

Member

Join Date: Dec 2006
Location: Northern California
Posts: 6
#6

### Re: Sizing an air conditioning unit

02/22/2007 12:41 AM

Patyo,

Before investing in this I would look at Radiant Foil Barrier to see if this material could be used to make your envelope more efficient, then in place of using a single FAU and condenser unit i would look into using a unit called the cirrus spelling I'm not to sure of. It is a neat unit,the condenser has the ability to feed two different FAU units which lets you do dual zones for areas that are not in use or to compensate for loss in distance. Another neat feature is it has a water cooled condenser that lets it operate more efficiently if you can not find it locally let me know and I can send you a number of a company that can help. They recently put a 5 ton unit on a 2800 sq-ft house that should be comparable to your project.

The next invention in solar driven devices needs to be a AC/DC condenser unit. :)

Jason

Guru

Join Date: Mar 2007
Location: Etherville
Posts: 12334
#7

### Re: Sizing an air conditioning unit

03/18/2007 4:59 AM

It looks as though you have a height of about 11 1/2 foot (?) , which is a bit over the top for office needs . Can you alter this with a false ceiling to change the scale of the problem , or would doing so make things worse (the resulting 2 air volumes could be dealt with separately).

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