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

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Forum topic by HorizontalMike posted 06-21-2017 08:26 PM 4045 views 1 time favorited 69 replies Add to Favorites Watch
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HorizontalMike

7625 posts in 2730 days


06-21-2017 08:26 PM

Topic tags/keywords: ac positive air flow 100

Like the title says, I am interested in installing a 100% positive flow AC in the shop. What I mean is NO RETURN AIR TO THE AC UNIT.

For all the detractors, I understand that THAT is not efficient or cost effective, so please bite the bullet on that portion. What I am trying to find is a unit that only provides NEW air (AC’d new air) into the shop. In other words, I DO NOT want to be cleaning air filters all of the time.

My shop:
  • 24’ x 30’ x12’ = or roughly 1100 sq ft for an equivalent 8ft tall shop.
  • R-10 & R 7.5 Insulation via Solar Guard in a metal shop.
  • South Texas environment, very hot summers…
  • Marginal Winters…
WHAT I WANT/DESIRE:
  • 100% New air into the shop, in order to eliminate dusty filters.
  • At least marginal cooling/dehumidifying while working in shop.

DO THEY MAKE SUCH A CRITTER?

I have read A1Jim’s post ( http://lumberjocks.com/topics/165146#reply-textbox ) and found much information from Jim and Randy, but really looking for an alternative…

SO THE BOTTOM LINE IS:
Is there an AC solution that will ONLY pump fresh air into an environment/room, without sucking in all the bad dusty air?

-- HorizontalMike -- "Woodpeckers understand..."


69 replies so far

View GR8HUNTER's profile

GR8HUNTER

2892 posts in 529 days


#1 posted 06-21-2017 09:04 PM

what about a split system Mike ….would that work for you ???? :<))

1250 square feet = 30,000 BTU

https://www.ecomfort.com/cooling/fujitsu-30000-btu-mini-splits.html

-- Tony Reinholds,Pa. REMEMBER TO ALWAYS HAVE FUN

View Rich's profile (online now)

Rich

1886 posts in 406 days


#2 posted 06-21-2017 09:06 PM

Well, like you said, it’s not efficient. You can take in air for the AC from anywhere with custom ductwork. The advantage of a closed system is that you are recirculating cooler air that’s already been dehumidified. With an open system, you’ll be cooling/dehumidifying outside air, so the cost to operate is going to be significantly higher.

I can appreciate your not wanting to be changing filters all the time, but is it possible to construct a multi-stage filter with a large, cheap fiberglass first stage filter that could be taken off and knocked clean? You could then taper down to a pleated filter to keep the AC innards clean.

-- No matter how much you push the envelope, it'll still be stationery.

View Desert_Woodworker's profile

Desert_Woodworker

1264 posts in 1031 days


#3 posted 06-21-2017 09:09 PM



what about a split system Mike ….would that work for you ???? :<))

- GR8HUNTER


This was my thought as well.

-- Desert_Woodworker

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clin

740 posts in 812 days


#4 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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clin

740 posts in 812 days


#5 posted 06-21-2017 09:48 PM



Well, like you said, it s not efficient. You can take in air for the AC from anywhere with custom ductwork. The advantage of a closed system is that you are recirculating cooler air that s already been dehumidified. With an open system, you ll be cooling/dehumidifying outside air, so the cost to operate is going to be significantly higher.

I can appreciate your not wanting to be changing filters all the time, but is it possible to construct a multi-stage filter with a large, cheap fiberglass first stage filter that could be taken off and knocked clean? You could then taper down to a pleated filter to keep the AC innards clean.

- RichTaylor

what about a split system Mike ….would that work for you ???? :<))

- GR8HUNTER

This was my thought as well.

- Desert_Woodworker

Mini-splits are great, I use one in my shop. But, he’s wanting to ventilate and not recirculate to avoid filtering.

However, by running a room filter (like a Jet), you won’t get much dust in a mini-split. But then you do have to clean the room filter. I think that’s what he is wanting to avoid.

-- Clin

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Desert_Woodworker

1264 posts in 1031 days


#6 posted 06-21-2017 10:07 PM

Very informative reply Clin- thanks

-- Desert_Woodworker

View Redoak49's profile

Redoak49

2855 posts in 1805 days


#7 posted 06-21-2017 10:15 PM

Very nice calculation…I think that the actual cost will be more as you need to add in the power to dehumidify the air.

View Joe Lyddon's profile

Joe Lyddon

9976 posts in 3868 days


#8 posted 06-22-2017 12:01 AM

It seems to me that you need a source of Fresh Air…

BUT…
If you just do not want to replace filters, why not just Leave The Filters OUT and let it circulate?

Just get the unit that will COOL the place and use it… w/o filters.

Too simple? LOL

-- Have Fun! Joe Lyddon - Alta Loma, CA USA - Home: http://www.WoodworkStuff.net ... My Small Gallery: http://www.ncwoodworker.net/pp/showgallery.php?ppuser=1389&cat=500"

View HorizontalMike's profile

HorizontalMike

7625 posts in 2730 days


#9 posted 06-22-2017 12:14 AM

I have an HF DC-System with a Wynn filter. I move the 20’ intake hose from tool-to-tool as needed. Things like lathe and 1/4-sheet sanders don’t get the DC (sweep and suck). Here in South Texas Summer, the heat is high and dry for the most part. My biggest problem is dripping sweat on all my cast iron, literally. If it is only 80-90F then I can often get away with opening the 16’ x 8’ garage door and running 5 x fans in a circular in/out pattern.

The fans actually create a bigger dust mess in the long run. I am trying to find a more “pleasant” way to spend time in the shop during these really hot months. In the Winter, I can usually get by with a 1500W base-board heater. BTW, insulation is R10 roof, R7.5 walls, using Solar Guard on my metal building. IMO, spending more on upgrading insulation would be too tedious since the shop is already “full” of stuff.

So bottom line is trying to make things comfortable for the few hours, at a time, that I spend in the shop. The more comfortable it is, the more I can spend time there… ;-) A dry 80F would be nice…

-- HorizontalMike -- "Woodpeckers understand..."

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HorizontalMike

7625 posts in 2730 days


#10 posted 06-22-2017 12:29 AM



...[snip]...
I can appreciate your not wanting to be changing filters all the time, but is it possible to construct a multi-stage filter with a large, cheap fiberglass first stage filter that could be taken off and knocked clean? You could then taper down to a pleated filter to keep the AC innards clean.
- RichTaylor

Am wondering about doing that, though real estate is very tight and not sure where/how to do that. What ever gets done, I will need to have access to it at ground level if changing filters etc. FWIW, me, ladders and deer on MCs don’t get along very well any more (+50 fractures), so closer the to ground level the better. BTW, HorizontalMike wouldn’t mind “earning” a different name eventually…;-)

-- HorizontalMike -- "Woodpeckers understand..."

View jonah's profile

jonah

1386 posts in 3115 days


#11 posted 06-22-2017 02:10 AM

Why not get a system with a washable/reusable filter? You’d save tons of money in operation, and you could buy two filters. You use one until it’s dirty, then put the other in and clean the first one. Some of them even go in the dishwasher.

Either that or a mini split system and a ceiling mounted air cleaner.

View clin's profile

clin

740 posts in 812 days


#12 posted 06-22-2017 05:21 AM



Why not get a system with a washable/reusable filter? You d save tons of money in operation, and you could buy two filters. You use one until it s dirty, then put the other in and clean the first one. Some of them even go in the dishwasher.

Either that or a mini split system and a ceiling mounted air cleaner.

- jonah

This is exactly what I have and it works well. As is commonly done, my room filter hangs from the ceiling, and therefore doesn’t consume any floor space. Changing filters does require using a ladder. However, I find it works pretty well to just use the shop vac to vacuum the dust off the filter. That can be done standing on the floor.

Not saying you’d never have to climb a ladder or step stool to replace the filter, but it wouldn’t have to be all that often.


BTW, insulation is R10 roof, R7.5 walls, using Solar Guard on my metal building. IMO, spending more on upgrading insulation would be too tedious since the shop is already “full” of stuff.

- HorizontalMike

Insulation really should always be the first step. You have about 1/3 what you should have. Is your ceiling really that inaccessible that you can’t upgrade the insulation?

Is your humidity low enough for an evaporative cooler? If so, that would fit your original requirement. These are specifically designed to continuously draw in outside air and exhaust the structure.

But, I’m guessing you’d already be doing this if it were an option in your area. And you already mentioned dehumidifying, so again, I’m guess you really aren’t that dry.

Bottom line, if people use evaporative (swamp) coolers in your area, that’s a perfect option for what you want. Very cheap to install, can move a huge amount of air, much cheaper than refrigerated AC to run. They just don’t work well or at all if the humidity isn’t low.

-- Clin

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clin

740 posts in 812 days


#13 posted 06-22-2017 05:30 AM

I found this table for evaporative cooler use. As you can see, if you have outside temps of 100 F or so, you need humidity in the range of 20% or less for it to work very well.

-- Clin

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hotbyte

989 posts in 2792 days


#14 posted 06-22-2017 11:10 AM

Don’t mini splits have filters?

View mer500's profile

mer500

7 posts in 155 days


#15 posted 06-22-2017 11:46 AM

Great Thread

showing 1 through 15 of 69 replies

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