Minisplit AC in Small Server Room 1

[AxisCat]

Hi All,

Let me start by making you aware I am a HVAC contractor but have lots of years in the field. I am selecting a system for a small server room, A couple of normal size racks and all the stuff you would normally see. I am awaiting a full audit of the equipment from their IT person so I can run the cooling load.

In the meantime I am playing around with a nominal 3-ton unit. If this was a really serious room I would be using a unit specifically designed for the task, but around here these small rooms all seem to get mini-splits which is what I am looking at using.

I plugged in 68 DB and 57 WB (50% RH) as my entering air conditions in my design software. IT guys like to keep things cold. To my surprise the software complained about low inlet temperatures on the air handler. After playing around a bit I discovered 59 deg. WB is the minimum regardless of the DB. I can only assume that when running low DB and WB we could approach freezing conditions on the evap coil.

But like I mentioned there are tons of mini's out there doing these small computer rooms and they don't seem to have any issues. But as we all know 99% of the cooling load is going to be sensible so how can these units not get into trouble by drying the air out below 50% RH?

What do you all think?

 

[Alistair_Heaton]

on phone of course it's meant to be SPARC.

Also used to be irix SG current. But didn't see them again after I left the fea world.

SG extreme graphics on ANSYS and Pro Engineer was a sight to be seen.

 

[dbill74]

Pumping the cold air directly into the racks would be a nice optimization. I will have to talk to them about that option. I intend to do ceiling cassettes as they have better air distribution than the wall mounts, but that is me thinking from the perspective of human comfort. In my small pond I only do a couple projects like this a year and most are very small compared to what you all are describing. Personally I hate wall mount units, they are pain to get the condensate to work and ever more of a pain to service.

Use caution when using ceiling mounted cassette, especially in the small closet sized server rooms. Despite Ductless split systems being bad about removing humidity and these rooms not having a lot of humidity to be removed, you can generate some condensate still. You do not want that over the server racks. Servers and water do not mix, having condensate drip onto the servers, whether from the coils themselves or sweating casing is a recipe for a lawsuit. Either go with wall mounted or have a means to protect the servers.

Pumping air into the racks sounds questionable, unless the racks are specifically designed for it. The closest I've ever done to something like this is pumping chilled water into the racks; mind you this was a client request and the racks were special made with sections dedicated to piping chilled water and cooling adjacent sections. From your OP, it sounds like you have a small room with just a handful of serves in a couple racks, nothing big or fancy needed.

Does having dry air not cause problems with the IT equipment?

Yes it does. If the air gets too dry, 20-25% or lower IIRC, servers and electronics can have problems with static electric discharges.
In small rooms and in geographic locations with moderate to high humidity levels, this is rarely if ever a concern.

Also, "cold" is a relative term and feeling cold isn't just about the dry bulb temperature, the wet bulb temperature and corresponding RH has a lot to do with it too.

Generally speaking, having a split system in a small server room/closet is mostly for the ability to provide cooling when the building is unoccupied and the primary HVAC is in an "unoccupied" mode to conserve energy.

 

[AxisCat]

Excellent point about cassettes directly overhead the server cabinets dBill. I was already rethinking things a bit. I was wanting 4 tons units and so far I see most high wall air handlers stop at 3 ton. This might give you some scale to my project, there will be a lay-in ceiling at around 10':

 

[AxisCat]

The server cabinets will actually be open front and back and I was thinking distributing the air along the front of the cabinets. Thoughts?? This thread has been an excellent guide for me and I appreciate everyone's help.

 

[HVAC-Novice]

You probably looked into this... but for any DX solution you have to consider refrigerant line length limits. Sounds easy to just put an evaporator unit into a closet (after you figured out where to pump condensate). But in reasonably large buildings you find out the nearest possible condenser location is far away.

And yes to the above, don't put anything that ever could have water inside or outside above the racks.

 

[AxisCat]

Yep, the condenser is 50' straight up on the roof.

 

[dbill74]

That is getting to be borderline needing floor mounted CRAC unit. From my experience, I'd say you're looking at closer to 10+ tons.
Depending on how critical the servers are, having n+1 redundancy (minimum 2 CRAC units) may be needed, talk with users to confirm.

What kind of flooring is there? If raised floor, have CRAC unit blow down to under flow (maybe some ducting to the racks) and return air from the top of the CRAC units. Space depending of course, need to coordinate with server provider and architect.

Lengths of refrigeration lines on Dx equipment is something to keep in mind, but even the smallest units have limits of 100-125 ft, but can extend that with larger lines.

Another reason to avoid ceiling mounted units is maintenance access. Can maintenance get in with a 8 ft ladder to change filters?

 

[AxisCat]

I am still waiting on the equipment audit from their IT department. They said they wanted (2) 3-tons, I said that might be a bit small so was bumping up to (2) 4-tons with a space for a 3rd in the future.

*typo.. they were thinking (2) 3-tons

 

[AxisCat]

I am competing against (2) other contractors both quoted (2) 3-ton splits so a raised floor didn't enter into our conversation. It always nice to be the last contractor to bid something, save the best for last kinda thing

 

[AxisCat]

Staying with the minisplit idea for a moment I could do this. (2) 5-ton VRFs with alternating high wall air handlers.

 

[HVAC-Novice]

If you are designing, you should determine the load. Their equipment likely has data on the design cooling load for each and they can tell you if they plan x% expansion.
I don't know those IT folks, but they likely aren't HVAC engineers qualified to do a load calc. But I could be wrong. If it is too warm, they will blame/sue you regardless of if they said 3 tons.

Whoever stamps and submits plans is the one responsible. So that person should size.

 

[Alistair_Heaton]

That, the beauty of those HPA cassettes that I posted the video of

https://www.youtube.com/watch?v=LBiRrsOaqXo

They can be anywhere you like just duct into them. They can be vertical screwed to the wall in a different room.

The condensate can be pumped via a syphon where ever you like lower.

 

[AxisCat]

Pretty slick they are multi position mount. I have not had any luck at all finding a distributer in the US for any Panasonic HVAC products. I sent out a couple of emails for info and haven't received any replies back yet. Still working on that side of things.

 

[Alistair_Heaton]

I had the same issue where I am in Europe. I could get the domestic single phase stuff easy enough. But I needed a 3 phase 400V. Everyone wanted to install 3 single phase units.