Small Store AC Sizing?

[Fett]

Hello Gents,

I have a small storefront that is having cooling issues. The units run all day long and never shut off. The only way to keep the space cool is to run the units all night long to get the space cooled down with hopes that they maintain it during the day.

It is 5,500 Sq. Ft. with 12 Ft. Ceiling (T-Bar Tiled).

It has two 6 ton Carrier WeatherMakers (R-22 HeatPumps) that are 6 years old.

The walls are R-6 insulation and the ceiling is R-6 also.

It has a south facing window (with double doors) dual pane lightly tinted of 180 Sq. Ft.

Our current outdoor ambient is 116F (46-47C).

I have already replaced the OFM's on both units and a compressor and IFM on one of them. The owner no longer wants to repair the units but wants to upgrade to something more capable for the space.

As far as math goes the two units are only capable of moving 144,000 BTUs/Hour.
I figure that this space is @ or around 66,000 cubic feet of airspace. Does 6 BTU's per cubic foot of airspace sound about right? That would put me at 396,000 BTUs/Hour.
People always calculate for just square footage and that just doesn't seem right to me. Any help would be appreciated.

[Magoo]

Hi Fett. +46 ambient is blistering, is that normal. My old Carrier info says 200 sq/ft per ton refrig at the high end. Equates to 330 K btus, can you reduce /stop fresh air make up. Possible run night time mode lower temp and use structure as a heat sink during day.

[Fett]

Yes, we are slightly above our average ambient temperature this year by a few degrees. Right now there is zero fresh air make-up. I used to run it at 15% but in July and August we get monsoons and the humidity makes everything worse. I run the night time temp at about 68F and then at 6am it adjusts up to 74F but by about 2pm its over 80F in my conditioned space. The structure of the building is 6 inch concrete walls framed with 4 inch aluminum studs covered with 3/4" sheetrock. The roof is steel I beam with plywood ontop and 4 inches of 1/2lb foam.

[Magoo]

You seem to be between a rock and a hard spot, OK system is under sized. Possibly designed for winter heating, a common problem. seriously short with summer cooling. Even with high ambients and high space temps., some gain can be achieved by increasing air flow rates and increasing sensible cooling factor. Is there shading on the front of shop, reducing solar gain through glass is huge.

[Fett]

The front of the shop has two concrete walls that extend about 8 feet out on either side of the glass entrance so the windows are shaded in the morning and in the late late afternoon but mid-day are exposed. I have opened all my manual dampers fully so the supplies are unrestricted.

[Magoo]

Hi Fett. I could suggest reflective tinting on exposed glass, but be careful as glass surface temp increases and glass expands and can crack, not a good result. Another option is an indoor air curtain giving separation between indoor and out door temps over public access door/s, reduces temp and humidity migration. Indoor lights and particularly spot lights generate huge amounts of heat, LED's are minimal heat generators. cheers magoo

[FreezerGeezer]

I've read about some pretty impressive results from painting the roof (& as much else of the sun-exposed outdoor structure as practical) with a paint designed to maximise solar heat reflection. Unfortunately I can't recall the name / brand, but a quick search shows a few. Dulux infraCOOL is first on the list.

Are you able to plant shade trees around the building? That can also make a heck of a difference.