cooling a high heat load space

[korot]

I'm faced with somewhat of a unique situation. I trying to determine the best configuration to cool a small (~75 SF) space that houses ice cream machine condensers. There are (5) machines each rejecting about 13MBH each (65 MBH total). The is an interior space and the owner wants a ductless split only as a last resort (as it will have to be a large system - $$$). The remainder of the property is served by a 5-ton DX unit, so not enough to supplement this condenser room. The space is interior but it is adjacent to an under-floor crawl space. The crawl space temperature remains in the 50F - 80F range year round. A transfer duct isn't an option due to the high heat load and limited primary AHU capacity.

I'm trying to size an exhaust system to cool the space, but I think this will result in the need for an enormous system. I'm looking to maintain 95F in the space with an estimated peak ambient of 80F in the crawl space (where the air will be exhausted). Based on these parameters, I think I would need a fan in the 4,000 CFM range (please correct me if I'm doing this wrong). In addition, I would need a make-up air system pulling and filtering OA from the crawl space. Fortunately I shouldn't need to temper the crawl space inlet air.

I have limited experience with this type of setup, so I would appreciate any input: Alternatives, critiques, corrections, etc.

Thanks

 

[DRWeig]

Your 4000 CFM is good.

I don't see a real alternative to exhausting into the crawl space and taking makeup from there too, unless you add a cooling coil. It's possible that you can get by with a smaller split system and reduce the air flow. You basically need just over 5 tons of sensible-only cooling. You could get the whole 5 tons with ventilation (4000 CFM) or you can get the whole 5 tons with a split system (maybe 2000 CFM). Or, you could design something in between. Couple tons of split system, couple tons of ventilation. Lots of choices.

Best to you,

Goober Dave

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[EnergyProfessional]

you want to ventilate from and to the crawlspace? How large is that and is it ventilated?

couldn't you just equip those ice machines with remote condensers that you put outside, like any grocery cooling system?

I realize someone else may have set this up originally, but locating the condensers inside this space was a dumb idea (sorry). I'm sure those machines are offered with remote condenser for that exact reason.

 

[korot]

HerrKaLeun: You bring up some good points - let me provide some more details. I was brought in after many decisions were already made. I had no input into the building configuration and initial equipment selections. The ice cream machines that were purchased are packaged units and are not equipped with (and don't have the option for) a remote condenser. If I had had a say in the equipment selections, I would never have allowed packaged units. Unfortunately, we are beyond that.

The crawlspace ventilation is a major concern of mine. It is not actively ventilated, but it is equipped with (2) louvers that allow some passive or natural ventilation. Seeing that I am considering a make-up air system, I didn't think pressure (+ or -) in the crawl space would be an issue. Temperature, however, worries me a bit. The crawl space is about 1,500 SF with a slab-on-grade and above-grade CMU walls.

 

[MintJulep]

By raising the crawlspace temperature to 95F you will be adding load to any other portions of the building that are above it. Do they have the extra capacity?

You will very possibly create a happy place for some form of biology. Mold, bacteria, mice, bugs, lions and tigers and bears.... Ice cream is food. Food and critters should be kept well separated.

Run ducts for supply and exhaust to and from the condenser room.

Can you go up, through the roof?

 

[korot]

The roof is not an option as this is a ground-level commercial facility below a multifamily mid-rise. Ducting the exhaust directly outside via the crawlspace is somewhat feasible (with some alterations) and is probably the best option. The crawl space intrigued me for the make-up air because of the potential to act as a natural ERV of sorts. Obviously filtration is a must in this scenario, but are there any other glaring reasons to avoid this (assuming the crawl space ventilation is adequate)?

 

[MintJulep]

Is the ice cream maker the building owner?

If not, does he have the owner's permission to use the crawlspace for any purpose at all?

 

[korot]

The ice cream maker is via the owner/contractor (same entity), so I have quite a bit of flexibility with the design. The crawl space is available for use as long as it does not affect the primary building function (which crawl space mold/temperature can impact)

 

[ChasBean1]

You’re in the right ballpark with the 4,000 cfm fan, mathematically. But now picture 4,000 cfm being exhausted from an 8’ x 9’ room… hmmm. Possibly a pressure problem and some door operating issues. Gut instinct shows this is ridiculously oversized, so trust your gut instinct and go from there.

Is the 13 MBH rejection based on nameplate info or based on real heat rejection? Real might be on the order of a fifth of nameplate. A very important consideration.

 

[korot]

The 4,000 CFM fan is the exact reason I'm searching for a second opinion - this space can't handle a fan that size (even with a make-up air system). It would be an absolute wind tunnel in that space.

You bring up a great point about the heat rejection rating. 13 MBH is the rated heat rejection, so I'll dig into the equipment specs to see what I can find. I'd imagine you're correct in that the actual output is considerably less.

 

[willard3]

There will also be some diversity in loading, ie, it won't be 100% all the time.

 

[317069]

I am trying to figure the situation.
an interior space contain ice cream machines, rejecting 65000 BTU/hr.
this space is adjacent to under floor crawl space, how come?
is this interior space under floor too or above the crawl space,
crawl space usually is less than 8 feet in height( is it a crawl space or basement).
you want to keep the space with 95F (why),and the crawl space at 80F,
if you exhaust 95F, 4000 cfm to the crawl space then how the crawl space will reject this amount and heat to outside

 

[chicopee]

Two problems that I thing will arise introducing and rejecting air fro and to the crawl space are 1) the smell of that crawl space and 2) air will be recirculated until the crawl space heats up enough as to be useless about cooling the condenser room.

 

[korot]

I guess I should follow up and clarify at this point. I call it a 'crawl space,' but its not a true crawl space by definition. Its a naturally ventilated space under the property. Its roughly 3 ft in height. I've thrown out the idea of exhausting directly into crawl space out of fear of mold. After surveying the area I think I can manage running duct work through the crawl space to the exterior. I am hesitant bringing in make-up air from the outside because this space will need to be cooled/ventilated during the winter. Because of limited funds, pre-conditioning of this make-up air will be difficult. I liked the idea of pulling in make-up air from the crawl space as it would provide 'free' tempered air. The space is equipped with louvers open directly to the outside. Obviously there are some air quality issues to overcome.

To follow up on other design comments: I ran a quick simulation of the crawl space in order to determine a design temperature. A typical CDD isn't applicable here since the space is partially sealed from the outside and it is on=grade. The simulation showed a peak annual temperature of around 77F. I used 80F to be conservative. 95F was recommended by the owner. I'm working on contacting the ice cream machine manufacturer to check temperature ranges.

 

[317069]

- How your crawl space will provide a free tempered air?

 

[chicopee]

"... I am hesitant bringing in make-up air from the outside because this space will need to be cooled/ventilated during the winter..." I don't get that statement as you want the 75 sq.ft. room to be ventilated in the winter. Two ducts in the crawl space should not be that expensive but make sure that the duct openings within the room are as far apart as possible for an effective ventilation system. What will be expensive will be the fan and its housing within one of the ducts.

 

[korot]

The space housing the condensers will need to be cooled during the winter months, thus I'll need to run the exhaust fan and bring in makeup air. Bringing in make-up air at freezing temperatures may present problems for the equipment in the space as these are packaged units not intended for outdoor (or outdoor-like) conditions. I'm thinking the crawl space may have the ability to provide tempered make-up air. My question at this point is what issues may I have bringing in make-up air from the crawl space (other than air quality)?

I'm leaning towards using a hybrid system with a small ductless split and an exhaust fan system. This will allow for a significantly reduced fan size and allow for a reduced cost since a smaller system will be used.

Thank everyone for all of your help.

 

[NegativeZero]

all you need to do is keep the room from getting to hot above design of what the condensors are rated for. So I wouldn't worry about bringing the temp down to much. if you kept the room at 80* you will be fine. So the worst case scenario is all machines running when its 95* outside. if your moving 4000 cfm in that small space and that fast. I don't see a problem. Plus, you will not get a split wall mounted ac that big to control the temp should the fan fail. I wouldn't give that split system a year anyway, because it will get dirty and fail. The condensate pumps on them are not great so expect to mount a bigger one or hope you can run a gravity drain somewhere. and in most cases the fan will run 99% of the time anyway so having the ac will do nothing. you could add a stat on the wall, so during the cold months the fan could shut off when there is no load. and if your really set on cooling the room with an AC. check out Movicool AC units. I have used these in computers rooms and other tight places and they worked great.

 

[chicopee]

Install a thermostat to energize and de-energize the fan in order to regulate the temperature during winters.