Raised Flooring Obsolete or Not? 372
mstansberry writes "In part three of a series on the price of power in the data center, experts debate the merits of raised flooring. It's been around for years, but the original raised floors weren't designed to handle the air flow people are trying to get out them today. Some say it isn't practical to expect air to make several ninety-degree turns and actually get to where it's supposed to go. Is cooling with raised floors the most efficient option?"
No (Score:4, Informative)
Re:Air can turn on a dime. (Score:3, Informative)
The blower moving the air only has a certain amount of power. Hook it up to a duct ten feet long, and output basically equals input. Hook it up to a duct ten *miles* long -- even a perfectly airtight one -- the power you put into one end will be lost by the other end, because the air molecules lose momentum (and gain heat) as they bounce off each other and the walls of the duct.
Every time a duct turns a right angle, the molecules lose a lot of energy as they largely slam face-on into the duct work. Rounded corners improve the situation, but not perfectly so.
My HVAC designer said that as a rule of thumb, every right-angle turn in a conventional house duct was the equivalent of adding 10 linear feet, in terms of energy lost to heat.
Yes, turns (Score:3, Informative)
Now admittedly, friction isn't as important to gasses as it is to other states of matter, but it can have an effect, especially in high flow cooling.
not the best article on the subject (Score:1, Informative)
Our server room is a room in a former library, and we were still able to make raised flooring work (without many hotspots) by selecting the correct tiles to make grates and sealing others off, then have multiple return registers. Our matching room was disigned as a machine room with hot air on one side, cold on the other of each rack, voila, no hotspots. I belive the keys are positive pressure and correct locations for the grating/returns.
Thermal Dynamics... (Score:2, Informative)
Heat rises, our original designs back in 2002 for our data center called for overhead cooling using a new gel based radiator system. It would have been a great solution and caused us to go with a lower raised floor, just for cables and bracing. At the time the cost was too extreme to justify the design so we went back to traditional raised floor.
Tile placement on a raised floor is key, only allowing the cool air to be pushed up in the front of your racks and creating hot rows facing your exhaust ends into the same isle. This way the cool air is pushed up from the floor, pulled in through the rack by the server fans, and exhausted, where it can then rise to a vent.
To answer the original question, I think that using raised floors for cooling is not the most efficient solution. Top down chillers that address the heat that is rising off the servers would be better. I just don't know that the price of these solutions has reached a balance for the savings. Even with this design you need something to create a cool pad for your racks to sit on. Many times this can just be the concrete slab of the floor.
Re:I wouldn't say they're going to become obsolete (Score:3, Informative)
If cooling is not a concern, concrete slab with overhead runs is the best way. If cooling is an issue, use raised floor, for cooling only and overhead runs for cables.
Obsolete or not... (Score:3, Informative)
Very difficult to track down random machine failures to bad interior decoration choices!
Re:How about water cooling? (Score:2, Informative)
Check http://news.com.com/Photos+SGIs+Columbia+supercom
Re:Turns? OR What The Gov't Does (Score:2, Informative)
Hot & Cold Aisles (Score:1, Informative)
The more important factor to maximize the efficiency of your cooling system is to get the heat out and back to the AC before it can mix with the cold air feeding your gear.
As long as you set up the datacenter with hot aisles (the backs of the rows face each other pushing the hot air into that aisle), and have return air grills directly above this aisle to take the heat back to the AC, you should be able to feed the cool air to the 'cold' aisle from either below or above. Many older data centers that push cold air into the floor just have an AC unit with an open top (no ducting) sucking in 'warm' air from the 'upper part' of the room, but it may be 30-40 feet to the other end of the space, and that's a lot of room for mixing as the hot air makes it's way down to the unit.
We just completed a small datacanter remodel with supply and returns in the ceiling (our pre-exisiting raised floor is only 6" high and has too many wires for adequate airflow) and it works great. I have 17 racks requiring 12 tons of cooling in a room that is only 25'x25', and they key to the whole thing is the return air grills sucking the hot air out right where it is being expelled by the gear!
It's complicated, but basically, yes and no (Score:5, Informative)
If you're bored, check out TileFlow [inres.com]. It's an underfloor airflow simulator. You put in your AC units, perf tiles, floor height, baffles, you name it. It will (roughly) work out how many CFM of cold air you're going to see on a given tile. It's near-realtime (takes a second to recalculate when you make changes), so you can quickly add/remove things and see the effect. I spent some time messing with this a couple of years ago, and it's very easy to set up a situation where you have areas in your underfloow with *negative* pressure.
The article basically summed it up for me:
McFarlane said raised floors should be at least 18 inches high, and preferably 24 to 30 inches, to hold the necessary cable bundles without impeding the high volumes of air flow. But he also said those levels aren't realistic for buildings that weren't designed with that extra height.
I'd go with 24 inches MINIMUM, myself. Also, proper cable placement (ie: not just willy-nilly) goes a long way towards helping airflow issues. Like they said though, you don't always have the space.
Of course, with the introduction of a blade chassis or 4, you suddenly need one HELL of a lot more AC
Re:Why do devices need to be cooled? (Score:3, Informative)
See http://en.wikipedia.org/wiki/Reversible_computing [wikipedia.org]
Fast computing is made possible by destroying information (that's all computers do really, they destroy information). That destruction process entails an entropy cost that must be paid in heat.
Re:Where else? (Score:4, Informative)
http://www.canford.co.uk/commerce/resources/catde
or maybe even one of these...
http://www.canford.co.uk/commerce/resources/catde
Raised flooring is useful for several reasons. (Score:5, Informative)
Raised flooring also provides significant storage for those large eletrical "whips" where 30A (in most US DCs any how) circuits are terminated as well as a place to hide miles of copper and fiber cable (preferably not too close to the electrical whips). Where else would you put this stuff? With high density switches and servers, we certainly aren't seeing less cable needed in the data centers. Cabinets that used to hold five or six servers now hold 40 or more. Each of these needs power (typically redundant) and network connectivity (again, typically redundant), so we actually have more cables to hide than ever before.
Cabinets are built with raised flooring in mind. Manufactureres expect your cabling will probably feed up through the floor into the bottom of the cabinet. Sure, there is some space in the top of the cabinets, but nothing like the wide open bottom!
Anyhow, there you have the ideas of someone who is quickly becoming a dinosaur (again) in the industry.
Have you looked at LED efficiency (Score:3, Informative)
LEDs are certainly better than flashlight bulbs.
But when a white LED delivers 15-19 lumens per watt, its about the same as a 100W incandescent and five times worse than a fluorescent. LEDs appear bright because they put out a fairly focused beam - not because they put out lots of light.
Re:How about water cooling? (Score:3, Informative)
Water (non-pure... which it will be as soon as it hits your computer) conducts electricity.
Antifreeze is not better and conducts electricity.
The liquid you're looking for is fluorinert, but the price is one the order of hundreds of dollars per gallon.
When you consider the price, you'll see why many people just use water and high-quality plumbing. Why use $500 of flourinert to protect a $500 computer? If your plumbing fails it's still going to overheat, so the tradeoff is really between the cost of the computer times the likelihood of the pipes failing vs the cost of the flourinert and the likelihood of the pipes not failing.
Since your pipes are more likely to not fail than fail (unless you just totally suck at life), and the flourinert is going to cost more than your computer, it just doesn't make sense.
Re:More about bad rack design (Score:3, Informative)
I used to do commercial HVAC work, and everybody in the business does the opposite from what you describe. The ducts are largest near the air handler, and they are smallest at the end of the line. Typically, the main trunk of the duct gets smaller in diameter after each branch comes off of it and goes to a diffuser.
One issue with raised floors, especially very large ones, is that the "ducting" or the floor is the same diameter across the whole room, and the machines that are furthest from the air handlers get the least cooling. Also, the floor is not (to my knowledge) insulated in any way, so even the air going through it will raise the temperature of the air all along the path.
But if the total area of the inlets to the modules is more than the incoming duct area, the modules furthest from the duct (i.e. the ones at the top) will be starved of air.
True to a point. Its also compounded by the fact that cold air sinks and hot air rises. I work with one completely populated rack of 1U machines. They have good airflow through them, there is no real difference in heat inside of the boxes from top to bottom.
Sucking air out at the top will only work if the air flow is so great it creates a significant pressure drop across the servers, which leads to noise problems, is inefficient, and may adversely affect local cooling inside the server.
I specifically ordered new tops for our racks with 2 fans in them when we got some hot computers. I don't remember how much it cooled down the racks, but it was like 5 to 10 degrees. These are solid racks with 3 8" fans in the rear as well. They work pretty good.
Re:I got a totally impracticable solution right he (Score:3, Informative)
Almost the right idea (Score:3, Informative)
Why? You must keep in mind, you're not trying to pump "cold" air in, you're trying to take heat out, and as Mother Nature knows, heat rises. So why not harness the natural convection of heat, allow it to flow up to the ceiling, and have some "perf" ceiling tiles and use the space over the ceiling tiles as your return plenum. Thus, you end up with a positive pressure beneath the raised floor, your heat load in the data room, and your negative pressure over the ceiling tiles leading back to your CRAC units.
I assure you it works fabulously in our 2 year old data center at a major financial company. The other advantage to raised floor is, you don't have to worry about water being overhead. No one wants a condensor water, chilled water, or glycol pipe bursting over a row of server racks. But, put your power whips in liquid-tite conduit, use cable racks for the CAT5 and there's no problem if you have a leak. The leak detection ropes pick it up and you can contain it before it becomes a problem.
Re:It's complicated, but basically, yes and no (Score:3, Informative)
I'd go with 24 inches MINIMUM, myself.
Not bad, at about 1" per year is typical. Might last a career.
A layer each for:
Oh, and don't forget power, 2 phase and 3 phase, 240v and 120v. And those silly traceiver boxes and modems.
Floors end up being garbage pits...
It's complicated, but do your fluid dynamics (Score:2, Informative)
We've speculated that if we were to start all over again we'd skip the raised floor and do a bi-level drop ceiling with one level being cold air distribution to cold rows and the other level being the hot air return. Let cold air fall and warm air rise, and augment it all with XDO's from Liebert [liebert.com].
Re:sub-floor (Score:3, Informative)