Reply by clin

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Posted on My 1st Bill Since New AC Installed - WAS:100% Positive Flow AC in the Shop

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1121 posts in 1873 days

#1 posted 06-21-2017 09:44 PM

I’m not sure if what you want exists. But I ran some basic calculations to see if it makes any possible sense.

Lots of assumptions are needed. Your shop has a volume of 8,800 cu-ft. Let say the AC unit moves 1,000 cu-ft/min (CFM). So you would exchange the air about every 9 mins. This exchange rate directly affects how much cooling you need, and of course affects the filtering affect.

A 1,100 sq0ft shop is pretty big. I used a Jet room filter that moves 1,000 CFM. While making dust it keep the air pretty clean, though it will peak. My shop is 1/3 your size. So I don’t think 1,000 CFM is excessive and may not even be enough. In any case, let use that.

1,000 CFM = 28 cu-m/min Air is about 1.2 kg per cu-meter. So that’s about 34 kg of air you need to cool every minute.

Air has a specific heat of very nearly 1 kJ/(kg-K) = 1 kW-sec/(kg-K)

Let’s assume you need to cool the outside air by 18 F = 10 C = 10 K.

So we have:

34 kg/min 1 kW-sec/(kg-K) 10 K = 340 kW-sec/min.

Canceling the minutes with seconds.

340 kW-sec/min * 1 min/60 sec = 5.67 kW of cooling power.

Converting to BTU/hour (common AC capacity units)

5.67 kW x 3412 BTU/1 kW-h = 19,300 BTU/hour

Now, AC units have a quite a range of efficiency. And I’m sure there would be quite a difference between one recycling room air (air that’s just a little warm) versus drawing in hot outside air.

While SEER ratings (BTU/W-h) are based on average seasonal operation. They can be as low as 3-4 for a typical AC unit and as high as 20+ for super efficient. But lets use 10 SEER.

Power (W-hours) 19,300 BTU/hour / 10 SEER = 1,930 W = ~2 kW.

So this is just a rough estimate at how much electric power you might need to cool 1000 CFM by 18 F (10 C).

This would be continuous average power at this temperature difference. You would of course need more capacity if it is hotter outside, less when it is cooler.

But let’s say you averaged 4 hours of peak (2 kW) usage a day, that’s 8 kW-hr and at say $0.15 per kW-hr $1.2 per day.

That certainly is not excessive.

But keep in mind, this is just the energy needed to cool the incoming air to room temperature. But your shop space has all sorts of heat that needs to be removed. I.E., what a normal AC unit needs to do. So this is an estimate of the extra cost of not recycling the air.

So you might pay in the range of $40 more each month doing it this way. Lot’s of assumptions here, so I think this cold easily go 2X in either direction.

I get what your are trying to do. But here’s another idea. You’re not going to need to be replacing your air to keep it clean (effectively filter), all the time.

Why not do a conventional recycling AC system? And put in a separate exhaust fan and air intake you run only when you need to replace (clean) the air.

It’s not uncommon for people to exhaust there dust collection outside. This is effectively the same thing, you are drawing in outside air. Obviously not the most efficient thing, but those that do this say they don’t notice that much of a problem with added heating or cooling.

Of course, it all depends on how often and long you run ventilation. Moving 1,000 CFM of outdoor air is certainly a major air exchange. So if you did this a significant percentage of the time, you’re going to notice it and need more AC.

Also, there are air exchangers that do help retain some of the cool in the air. These are typically meant to ventilate a whole house at a relatively low rate. But perhaps a whole house exchanger would be a good fit for your shop. Something to at least consider.

Also, you aren’t likely going to be able to dehumidify the air very well. First off, you are drawing in outside air, and in your location I assume that is very humid air. I think a typical recycling AC system relies on the air passing over the cooling coils over and over to draw out the moisture. I’m not sure it would be possible to do that much with a single pass.

This is another reason a traditional system with separate air venting may work better. At least while not venting the shop the AC can be continually dehumidifying.

What you are proposing is unusual for residential use, and if it is done anywhere, it would be some sort of commercial application. I’m sure there must be commercial operations that need to clean the air of dust or fumes. So you might be better off checking with a commercial AC contractor.

Anyway, I hope my back of the envelope calculation provides some perspective. Your idea doesn’t sound crazy, but there may be some better ways to approach it that a pro AC guy could tell you about.

-- Clin

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