Innovation

Get IT Done: Calculate heat dissipation for better server room design

Tallying up the heat dissipation from servers and other hardware can help ensure your server room is built with adequate ventilation and cooling.


In the enclosed space of a server room, the heat that all those boxes generate can quickly increase the ambient temperature beyond equipment specifications. The results can be ugly: hardware failure, loss of data, and an uncomfortable working environment are all distinct possibilities.

It’s critical to keep your server room’s temperature within the listed tolerances of your hardware. Tallying up the heat dissipation from your servers and other hardware can help you ensure your server room is designed and built with adequate ventilation and cooling.

Taking inventory of your equipment
Before you can design a cool server room, you’ll need answers to these key questions:
  • ·        What kind of equipment will be in the room?
  • ·        How much heat will the equipment generate?
  • ·        How big is the room?

Equipment heat dissipation specifications, most often expressed in British Thermal Units (BTU), generally are available in either the system user’s guide or on the manufacturer’s Web site. Specs generally state how many BTU are generated in each hour by the individual machine. For example, the Dell PowerEdge 2550 (a fairly typical 2U server) by itself generates a maximum of 1,580 BTU per hour.

Of course, some IT shops still use equipment that’s no longer available on the market, or even hardware from companies no longer in business. BTU dissipation measurements might not be available for such equipment. Should you need to calculate the BTU measurement of your equipment, you can use a simple equation to approximate heat dissipation rate.

Sizing up the room
Once you get at least a rough idea of how much heat your equipment generates, how do you determine the impact on your server room’s temperature? What does one server generating 1,580 BTU per hour actually mean, in application?

As a point of reference, consider a small office with dimensions of 15 feet by 20 feet with an eight-foot ceiling, for a total volume of 2,400 cubic feet. With average insulation and in a temperate climate, this room would require between 3,900 and 7,800 BTU per hour to keep it at a comfortable temperature, depending on your preferences, seasonal variances in outside temperature, and other factors including the number and type of windows in the building. Instead of buying a furnace, that means that you could just get three Dell 2550s and run them as heaters.

Taking inventory of all of your equipment is a key component in designing server-room cooling systems. Major vendors, such as Dell or Compaq, offer software that helps you to design your racks with an eye toward heat dissipation. I’ve loaded 42U racks with 16 2U high Dell servers before. When you stand behind those racks, you don’t want to be wearing a sweater!

At the same time you’re determining heat dissipationfigures for your equipment, take a look at the “operating environment” figures. Pay close attention to these numbers, which will give you the range of temperatures and humidity in which your equipment can function. If you choose to run your equipment in an environment that is hotter or colder than specified, you run the risk of permanently damaging the hardware. You’ll notice there usually are two sets of numbers: operating environment and storage environment. The storage figures tend to be much more tolerant than the operating figures, since the equipment won’t be enduring the wear and tear of regular use while it’s in storage.

You’ve got the figures. Now what?
So you’ve methodically gone through all of your equipment and recorded the BTU output for each one, put it into a table, and come up with a grand total. At this point, you should try to forecast some future cooling needs. Just as you would consider the possibility of bandwidth expansion when you purchase a server, you should consider additional cooling needs when specifying an HVAC system. The likelihood of adding new equipment is very high, and you don’t want to have to purchase a new air conditioning unit each time you do so.

Sizing an AC unit
Although it may be tempting to rely on the building’s central air conditioning system, I’m far more comfortable assigning a server room its own unit. This approach gives you a much higher degree of control, not only in your choice of a cooling unit but also in its administration and maintenance. In many corporate buildings, the AC is automated and controlled from a central location. If your server room is under this jurisdiction, you might find it difficult to add equipment or run your full array of servers at once.

Instead, I advise simply isolating the server room and installing a dedicated AC unit. But how do you choose the proper unit? An air conditioner rated at 1 ton is roughly capable of handling 12,000 BTU per hour of capacity, as specified by the American Society of Mechanical Engineers. So, a room with equipment generating 36,000 BTU per hour would require a 3-ton air conditioner to negate the effect of that dissipation.

Let’s put that into perspective. A standard AC unit rated at 1 ton can handle only four Compaq Proliant DL580 servers. And an IT shop rarely would dedicate a single room to only four Compaq Proliant servers.

Airflow
Sizing the air conditioning unit is only your first step. Now you have to decide how you want air to flow in your new server room. Many server rooms have raised floors with 6 inches to 12 inches of space underneath to house cables and electrical outlets. These spaces also can be critical for the proper operation of air-handling equipment.

Some of the tiles in a typical raised server room floor may have small holes, rather than being solid like the others. These tiles allow air from the sub-floor to be forced into the server room, keeping it cool. These tiles are strategically placed throughout the room for maximum benefit. For example, if you have a rack full of servers all blowing hot air out the back, you might place one or two of these tiles behind the rack to deal with that heat.

When using this type of forced air system, you need to place air return vents at locations throughout the room to provide for the proper flow of air. The best place for the vents when using a raised floor is in the ceiling of the room, since hot air rises and will more likely be drawn out of the room by the elevated vents.

In addition, using this type of air handling allows you the flexibility to blow cooled air directly into a rack or cabinet, if needed. For example, if you have a storage array with 15,000-RPM SCSI disks, it will get pretty hot. Being able to blow air directly into the cabinet can help to keep this equipment functioning at peak efficiency.

Don’t forget plenum
If you use the space under the floor for air handling, you should use plenum-rated cabling to meets fire codes. Most fire codes call for plenum-rated cabling in any air-handling space, which would include the space under a raised server-room floor. While it is more expensive, plenum-rated cabling does not release noxious fumes when burned. It can save your life, or the life of the firefighter that comes in to save you. This is one area where I will not, under any circumstances, cut corners—it’s illegal, after all.

Raise the roof
Unfortunately, not every server room uses raised floors. In these cases, a system that forces air from vents in the ceiling is your only choice. This setup’s reliance on fixed cool air returns makes it much more difficult to focus airflow, especially if you have to move around equipment down the road. The air intake vents also are in the ceiling in this type of setup, which would lead to cool air being sucked back into the system before it has a chance to reach the server racks.

There are, of course, ways to maximize the air flow towards your server racks. One quick fix I often have used is to create (with the help of duct tape) a cheap cardboard or sheet metal cutout tube and attached it to the air returns. This tube allows me to direct the flow of air in whatever direction I choose.

Another quick fix involves using a flexible hose, such as a dryer hose, in the same fashion to the cool air returns. I can then direct the current of cool air to nearly any location in the room.

To hire or not to hire?
So, you thought you were just going to buy an A/C unit and put it in, eh? There’s quite a lot to think about when looking at ventilating and cooling a server room. You must consider how to compensate for the amount of heat given off by the hardware you select, as well as the way air will be forced into the room.

If you are involved in designing a server room, I highly recommend that you hire a competent contractor to help you design the power and air handling functions. Because every situation is different, it’s always best to know exactly what you are doing—the alternative may be ultimately destroying a server room full of equipment.

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