Lieberts 2

[CuriouslyGeorge]

In the room I am evaluating for various HVAC needs, I have two Lieberts in a computer room with a raised floor. I need to confirm my thinking that Lieberts typically are used to provide supplemental cooling to the area, but the air itself is supplied/refreshed from some main AHU, correct?

Thanks,
Curiously George

 

[HVAC68]

Not necessarily. Trust you are referring to close-control units or precision units typically used for a computer room application.

These units can either be with a chilled water coil or can be a DX unit or can be a dual-system. These are independent units and may not necessarily need a separate AHU to provide cooling.

HVAC68

 

[imok2]

Unless you are in a hermeticly sealed room you will have at minimum air at atmosphric pressure, in a computer room this air will heat up if you have any type of activity, bodies, computers etc. therefore some cooling will be necessery. In your case it sounds like it will be a computer room and you will definitely need some cooling. Remember ..Computers are more expensive to replace then people? Who said that!! By the way some large computer rooms are supplied with regular air conditioning in adition to specific ccomputer room units

 

[HVAC68]

Computer room units are generally designed for delivering higher air quantity per Ton of Refrigeration - i.e. for high sensible load applications, as is normally required for such applications.

imok2 is right. Sometimes, these type of units are used with regular air-conditioning units.

HVAC68

 

[ChrisConley]

I'm taking the expression 'supplied and refreshed' to mean ventilated (although I like refreshed better).

Traditionally any computer room I have worked with is as you described. A central AHU providing tempered fresh air to the computer room and localized units to provide the cooling for the space. Depending on your location (I'm in Canada) you might also want to plan on having the units capable of producing heat as well - in the event of a long shutdown in the winter.

Speaking of winter, if you are in a cold climate, you also need to ensure that your - condensing unit, dry glycol cooler, or whatever is rated to operate at your lowest design temperature. Many computer rooms will still require cooling when it is -40 outside and a standard condensing unit will have frozen up.

 

[CuriouslyGeorge]

Say, is it unusual for a Liebert to have two unique supply discharges off one unit? At least, that's what the drawing is showing...

 

[imok2]

[George quote]"Say, is it unusual for a Liebert to have two unique supply discharges off one unit? At least, that's what the drawing is showing...
George please don't misunderstand me I don't know how much you know about c0omputer room equip. In your first post you mentioned you had a raised floor...normally the raised floor is used to supply the required amount of air for the load and return is located on the top of the unit so i was wondering if the overhead duct work is really the return air. If that's the case it's a poor design because these units are not normally designed for ducted return and you would lose a lot of efficiency

 

[NNACons]

Any space including computer rooms requires ventilation air of sorts in addition to the required cooling. This is as per ashrae or AS1668.2 - ventilation and fresh air requirements (australia).

My take on this whole thing, is that as per HVAC68 where, "close-control units or precision units" or process cooling units, typically used for a computer room application, ventilation air must be served to the space for compliance with relavent standards and building code.

If a precision unit provides cooling to the computer room, ie Liebert / Emerson PCU or Western Industries PCU' in most cases no ventilation air or outside air mixing occurs as they are straight 100% recirculating units. In these cases a central AHU system or fresh air fan may provide necessary fresh air to space. Of course the additional outside air load would be need to be included in the space loads for sizing of the process cooling unit.

There are scenarios where computer room cooling is less critical and close tolerance control of humidity and temperature is not too important For these scenarios, many clients accept standard air conditioning of space, whether it be ducted cooling only or split type cooling units. It comes back to what will be the most suitable in terms of exising mechanical services etc.Note even in these scenarios there will be fresh air ventilation to the space.

 

[HVAC68]

Outside air can either be supplied by a separate treated fresh air unit system or can also be introduced through the close control unit. There's no hard and fast rule that the close control units have to be necessarily 100% recirculating type units.

HVAC68

 

[walkes]

The Liebert precision air conditioning units come in upflow or downflow applications. I have a client that prefers to use the raised floor only for cabling, the air is ducted directly to the server towers and the return is taken in at the front of the cooling unit.

CG it isn't unusual to have two discharges off the top of the unit but they would likely be combined to a common discharge plenum. I believe there are some drawings indicating this in Liebert's documentation.

 

[lilliput1]

Ventilation air at minimum 20 CFM of outdoor air per person or .05 CFM/SF is usually provided from the house AHU system. You have to figure the % OA supplied for the house AC to determine proper supply air. Also for reliability it may be desired to put ductwork & controls for the house AHU system to back up the computer room units in case of failure of one.

 

[ChasBean1]

NNACons, I think your post best hits the mark (nothing against all others). 100% of the Liebert systems I've dealt with are recirc only and provide virtually 100% of the cooling for these spaces. A small amount of make-up air is typically provided by a central system. I'm not sure of the required value of ventilating air. I've seen the 0.05 cfm/ft2 (that Lilliput mentions) as a requirement for corridors. I don't have the ASHRAE/BOCA tables with me now, but if I see a required minimum in the tables, I'll repost. Regards, -CB

 

[NNACons]

ChasBeans1,

Im from Sydney Australia, so im more familiar with Australian Standard requirements. Specifically AS1668.2 - mechanical ventilation for indoor air quality. Should be a derivative of ASHRAE ventilation standards (from experience).

Anyway, for a computer room where the number of occupance for the space is not known, number of occupants should be taken based on 25m2 nett floor area per person. Fresh air requirement is 10l/s per person generally, however there is provision for fresh air to be reduced to 7.5l/s per person where high efficiency filters are used.

Just a note, i agree with previous posts that fresh air to the computer room may be via a dedicated central AHU providing tempered fresh air or fresh air ducting to the space directly from outside, or even off a cooling only system which has a percentage of outside air mixture that is being fed into computer room to comply with code requirements. IE a cooling only system with 50% outside air mixture, with computer room requiring 25l/s of fresh air for one occupant might typically have 50l/s of conditioned cool air ducted to it.. (not ideal, but definitely a workaround).

 

[ChasBean1]

NNACons, you're right. I just looked up ASHRAE and the International Mech. Code (2000) and the closest ventilating requirement for these spaces are "telecommunications and data entry rooms." These simply specify 20 cfm outdoor air per person.