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CREDIT FOR THE GRAPH BELOW
The following graph contains simple arithmetic calculations based on known information and relationships between watts per square foot and the HVAC cooling equipment. These calcuations were produced by mechanical engineer Bill Mann (the legendary "Dr. Air"), President of Data Processing Air Conditioning. Mr. Mann has designed the mechanical systems for many of the largest data centers and web hosting sites in America. For the best in mechanical system design, Mr. Mann can be reached at 310-378-9988.
A DESCRIPTION OF WHAT THE GRAPH PRODUCES
The following graph outlines "sensible" tons of air based on wattages per square foot for electronic equipment (including aisle space). Sensible tons are "net" tons of air produced by an HVAC system. (For example, a 15 ton Computer Room Unit (CRU) may produce only 11 tons of sensible air). Note that the calculations below do not include lights, shell load, UPS loss and HVAC Equipment itself. However, it is quite satisfactory for electrical and mechanical program requirements.

NOW FOR SOME KNOWN VALUES
1 watt = 3.41 BTUs
12,000 BTUs = 1 Ton of Sensible Air

THE MAGIC GRAPH
WATTS/
SQ.FT.		    BTUs
PER
WATT		    BTUs
PER
SQ.FT.		    DIVIDE
12,000
BY BTUs
PER SQ.FT.		    SQ.FT.
PER TON
OF AIR		    SENSIBLE
TONS OF
AIR PER
1,000 SQ.FT.
40    3.41    136.4    87.98    87.98    11.37
45    3.41    153.45    78.20    78.20    12.79
50    3.41    170.5    70.38    70.38    14.21
55    3.41    187.55    63.98    63.98    15.63
60    3.41    204.6    58.65    58.65    17.05
65    3.41    221.65    54.14    54.14    18.47
70    3.41    238.7    50.27    50.27    19.89
Note that as watts/sq.ft. and sensible tons of air go up, the square feet per ton of air goes down. This means more space for environmental systems and less available for business equipment.

TYPICAL WATTS PER SQUARE FOOT
Watts
Per Sq. Ft.		 Comments
1. Legacy Data Centers (i.e. Large IBM, Unisys, etc.) 35-45
2. Mixed Legacy and Servers (6-8 Servers Per Rack) 45-50
3. Server Rooms with 6-8 Servers Per Rack (12-30 Racks) 45-50 1.8 kW - 2.5 kW per Rack
4. Server Rooms with 8-12 Servers Per Rack (20-35 Racks 50-60 2.5 kW - 3.0 kW per Rack
5. Server Rooms with 12-18 Servers Per Rack (40-80 Racks) 55-70 3.0 kW - 4.0 kW per Rack
6. Server Rooms with 20-40 Servers Per Rack (Over 60 Racks) 70-100 Over 4.0 kW per Rack
The following table is provided for you to make your own high-level power per square foot and HVAC calculations. Fill in values for questions 1 and 2 below and then click on "Calculate" below. A new section will appear with full calculations.
1. How many watts per square foot are estimated?      
2. How many square feet in your computer room?  
Total Watts   0
Total BTUs   0
Total Sensible Tons of Air   0.00
FINAL NOTE

The calculations above do not reflect the additional electrical and HVAC systems that may be added to the systems totals for redundancy.