Cooling Load Spreadsheet 1

[bvanhiel]

I'd like to double-check our landlord. We're having extensive renovations done to our office, and we've asked that they take a closer look at the cooling. They've come back with a plan that looks pretty similar to what was there, which we felt was inadequate during Georgia summers. I think their AC guy was just looking at the square footage.

I'm an ME, but not in HVAC. I'd love to get a pointer to a spreadsheet or some demo software that would help us put our minds at ease that we won't be cooking next summer.

The rough details for those of you who love to solve problems:
Location: Atlanta, GA
Roof: 48x58ft 2" gypsum with membrane
SW Wall: 48X14ft Brick over block, interior finished
SW Windows: ~40X8ft 1/4" glass with 50% darkening film
NW Wall: 58X14ft block shared with conditioned space, interior finished
NE Wall: 48X14ft Brick over block, interior finished
SE Wall: 48X14ft Brick over block, interior finished
SE Windows: 25x8ft 1/4" glass with 50% darkening film 80% shaded.
Floor: 48x58ft slab on grade.
Occupants: about 20 people and computers

We want to make sure enough cooling gets to the SW windows. They've got 3 tons going over in their new plan, and another 5 ton unit for the rest of the space. Previously we had 2.5 tons near the windows, and it felt inadequate.

I'm not trying to get someone to do work for free, but I did want to provide all the details. A pointer to a good web reference/spreadsheet/demo would be great.

 

[IRstuff]

Seems to be more than adequate from a A/C perspective. The typical loading for southern California is about 1 ton per 500 square ft. We've got a 3-ton for a 2200 sq ft house and it'te more than sufficient.

The difference, though, is probably the humidity. Georgia is a lot more humid than SoCal. Rather than getting more tonnage, it might be better to find some way to dehumidify the air.

TTFN

FAQ731-376

 

[Golestan]

Hi,

I recommend about 250 sq-ft/ton. Units should be provided with economizer and enthalpy control option. Make sure they are taking-in 10-20% fresh air per ASHRAE std.

There is a software demo from ELITE. you may be able to use it it has a limitation on the size of the building. Easy to use. Model your building as one large room to fit this free version.
regards,

 

[bvanhiel]

A little more history:

The old buildout originally had 7.5ton total, and felt inadequate, particularly near the SW windows. It also had a suspended ceiling at 9-10 ft. The new buildout will be clear up to the deck, so we're losing some insulation value, and the new system is only .5 ton greater.

Golestan: I'll check out ELITE.

-b

 

[EmeraldCoastHVACR]

ASHRAE's "Principles of Heating Ventilating, and Air Conditioning(ISBN 1-931862-92-3) has the new RTS(Radiant Time Series) load calculation method that comes with a CD with the book. It uses MS Excel for both Residential & Commercial load calcs.

 

[gepman]

bvanhiel
You can get eQUEST v3.61 for free at

http://www.doe2.com/equest/

and it is very powerful and relatively quick to use. Thank the ratepayers in California for it. I have attached my preliminary model in it for you. It took about 10 minutes. I didn't run the analysis, that would be doing everything for you. After installing the software run the "Building Creation Wizard". It will give you a warning but just click OK. Feel free to change anything in the model that you don't think is correct.

You need to specify a) roof framing construction type b) roof insulation (I think gypsum is just drywall, you would need more insulation) c) thickness of what I assume is CMU d) filler material for CMU e) infiltration (always difficult to estimate).

 

[TXiceman]

You do not want the unit excessively large as you will not have adequate dehumidification if you are in a damp climate. You want the unit sized to the point that it will just barely hold the space or even let the temp slip a bit on a design ambient day. The unit has to run for dehumidification. If it is too large and cycles on and off a lot, it will not dehumidify the space well.

The old residential rule of thumb was 1 ton per 500 sq. ft. If this is an office environment, it will require more cooling due to the extra office equipment heat.

If you can get a copy of the ASHRAE manuals, you can do a manual load calculation.

Ken

Ken
KE5DFR

 

[msb01]

Bvanhiel,
You can also try DesignBuilder software (

http://www.designbuildersoftware.com).

It is not a free program but you can use 30 days demo version. Demo version has no limitations and I think will serve your purpose..

 

[gepman]

If anyone is REALLY interested in building energy simulation there is a comparison of twenty of the leading simulation tools at

http://www.eere.energy.gov/building...ergy_performance_simulation_programs_v1.0.pdf

Design Builder is based upon EnergyPlus which is included in the comparison.

 

[MarauderX]

As others have stated, humidity is what is going to drive the load. You should see if they are having a designer resizing the load, and if not ask for the load calculations to prove a correct sizing.
Commercial sizing is closer to 350-450 CFM/ton, or up to 1 CFM per SQFT. With the amount of dehumidifying the unit will be doing, a 10-ton unit is easy to justify. Depending on the OA requirements (20 people x 20 CFM each = 400 CFM OA) as a percent of the total CFM (400/2700 = ~15%) you might be justified in asking for 12+ tons of cooling. You will want to check against the unit's sensible load versus the latent load to see if this will really suffice.

http://www.eypmcf.com