UNIX System Administration Handbook - Evi Nemeth [444]
We have found that it’s a good idea to double-check the cooling load estimates provided by the HVAC folks, especially when installing a system for a machine room. You’ll definitely need an HVAC engineer to help you with calculations for the cooling load that your roof, walls, and windows (don’t forget solar load) contribute to your environment. HVAC engineers usually have a lot of experience with those components and should be able to give you an accurate estimate. The part you need to check up on is the internal heat load for your machine room.
You will need to determine the heat load contributed by the following components:
• Roof, walls, and windows (see your HVAC engineer for this estimate)
• Electronic gear
• Light fixtures
• Operators (people)
Electronic gear
You can estimate the heat load produced by your servers (and other electronic gear) by determining their power consumption. Direct measurement power consumption is by far the best method to obtain this information. Your friendly neighborhood electrician can often help you with this. Alternatively, most equipment is labeled with its maximum power consumption in watts, though typical consumption tends to be less than the maximum. You can convert this figure to the standard heat unit, BTUH, by multiplying by 3.412 BTUH/watt. For example, if we wanted to build a machine room that would house 25 servers rated at 450 watts each, the calculation would be:
Light fixtures
As with electronic gear, you can estimate light fixture heat load based on power consumption. Typical office light fixtures contain four 40-watt fluorescent tubes. If your new machine room had six of these fixtures, the calculation would be:
Operators
At one time or another, humans will need to enter the machine room to service something. Allow 300 BTUH for each occupant. If you want to allow for four humans in the machine room at the same time:
Total heat load
Once you have calculated the heat load for each component, add them up to determine your total heat load. For our example, we will assume that our HVAC engineer estimated the load from the roof, walls, and windows to be 20,000 BTUH.
20,000
BTUH for roof, walls, and windows
38,385
BTUH for servers and other electronic gear
3,276
BTUH for light fixtures
1,200
BTUH for operators
62,861
BTUH total
Cooling system capacity is typically expressed in tons. You can convert BTUH to tons by dividing by 12,000 BTUH/ton. You should also allow at least a 50% slop factor to account for errors and future growth:
See how your estimate matches up with the one provided by your HVAC folks.
Temperature monitoring
If you are supporting a mission-critical computing environment, it’s a good idea to monitor the temperature (and other environmental factors, such as noise and power) in the machine room even when you are not there. It can be very disappointing to arrive on Monday morning and find a pool of melted plastic on your machine room floor. Fortunately, automated machine room monitors can watch the goods while you are away. We use and recommend the Phonetics Sensaphone product family. These inexpensive boxes monitor environmental variables such as temperature, noise, and power and telephone you (or your pager) when a problem is detected. You can reach Phonetics in Aston, PA at (610) 558-2700 or visit them on the web at www.sensaphone.com.
24.8 Power
Computer hardware would like to see nice, stable, clean power. In a machine room, this means a power conditioner, an expensive box that filters out spikes and can be adjusted to provide the correct voltage levels and phases. In offices, surge protectors