Need advice for computer room cooling

Hi I'm just a layman and a newbie at HVAC. I am looking for second or third opinions on the advice that we're getting from a local HVAC engineer.
We have a computer room that needs more cooling capacity. Currently there are 2 cooling units, about 5 tons each. This has been adequate, until one of them fails. Also we are adding a lot of computer and network equipment and expect to add a lot more. We expect to need 25 - 30 tons at the end of 5 years from now.
The computer room has a raised floor with only cabling underneath and no room for anything else. There is a drop ceiling, which contains the supply and return ducts. The air cooling is done in a room in the floor above. Apparently there is no more room above the drop ceiling for more ducting. The approach we've been using so far is the cold aisle/hot aisle method, where you dump cold air in the aisle between two rows of computer racks, suck the cold air through the computer equipment and dump the warm air in the aisles behind the racks, where the return ducting sucks up the warm air. Seems to work fine.
We've hired a local HVAC consulting engineer but I'm not sure he's had much computer room experience. We don't want to put additional cooling units inside the computer room itself. His advice was to add a mechanical room to our building right next to the computer room (ground level), put the cooling units in the mechanical room, and punch holes through the computer room's outside concrete wall for the ducting. There would be 2 Lieberts of 25 tons each. The supply ducting would be two semi-circular rings that run just under the drop ceiling and right against the walls. There would be no return ducting, just two big return vents built right into the outside wall back to the cooling units.
So it looks to me like he's just going to let the new cooling units cool the room air as a whole, instead of dumping the cold air in the cold aisles. He seems to know what he's doing but I don't know if this approach is OK. I guess he's depending on the air to just mix around but is that good enough? I know he's really overloaded with work so I'm just a little nervous that he may not be putting enough thought into this, but I don't know enough to question his approach.
Any thoughts or advice?
Thanks. Armin
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snipped-for-privacy@hydro.mb.ca wrote:

Well Armin, it sounds as though he is not given much opportunity due to the limited space constraints that he has to work with, if I were suspect of the design I would ask him to show me another setup like the one that you describe, I can tell you from my experience that there are a lot data centers out there that have air patterns similar to what he is describing.
Fat Eddy www.hvactalkforum.com
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Fat Eddy wrote:

Thanks! OK, that's useful information "there are a lot of data centers out there that have air patterns similar...". I still don't know why he wouldn't run the supply ducting right over the cold aisles. Maybe he's thinking it's to our advantage to keep the ducting to the sides, for when racks are moved or power distribution unit expansions are installed. I think there's enough room but maybe I need to have a closer look.
Thanks again! It's a load off just to know it's "within normal". Armin
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snipped-for-privacy@hydro.mb.ca wrote:

ok, so 50 tons of air in a new mechanical room is going to replace the existing 10 tons and their associated ducts above the t-bar ceiling.
So if the ductwork above the t-bar is going away, why not install the supply ducting for the new lieberts in this area?
I've had an IT manager tell me he's lost more than one RAID drive because the supply air was located in the hot aisle......seems like those damn computer fans are working against the normal air currents one would see in a typical supply & return air situation.
Smart money will keep the cold aisile- warm aisile approach and concern itself with cooling the equipment and not the room. That was the premise of the whole raised floor approach in the first place. Then it was determined the equipment at the tops of the racks werent getting as much cooling as the lower equipment, so now they're back to the overhead approach.
If I were you, I'd tell the consulting engineer to sharpen his pencil and figure out a way to put the cold air into the cold aisles and the returns in the warm aisles.
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How many parallel rows of racks are there? Would the wall mounted return vents be sucking heat from a two-deep or ten-deep array of racks? If the rack array is 'shallow', cooling the room is fine; if the rack next to the return wall is getting the heat from 9 racks in front of it, you could be in trouble. And what is the shape of the room? Squareish, wide and narrow, skinny and deep? Are the supply ducts to have down vents or horizontal vents or a mix?
I suppose the bottom line is: did the HVAC engineer do the calcs to show that his plan would work?
snipped-for-privacy@hydro.mb.ca wrote:

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Armin, most computer rooms I've seen supply air under the floor which has several panels with holes in them...supply air. The Lieberts take in return air from the top. Also most have at least 1 unit that will take-over if one fails. I have an account with no redundancy and the room goes into high temp alarm when doing routine service. If one breaks down...rare for a liebert.. the @%$# hits the fan! I don't know why 50 tons to replace 10 tons?? You do have a bit of a situation with your set-up. If you only are looking for backup how about ductless splits. you can get them to hang on wall or in the ceiling if you have room.
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snipped-for-privacy@earthlink.net wrote:

Lieberts can take in air either way, depends how you spec it when you buy it, the way this guy is dioing it is not a problem ,, preferably you would like to see it coming up from the floor behind the receivers, but it doesn't really matter as long as you are circulating air through the entire area.
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