Reducing the Cost of Data Center Cooling
Figure 2 describes a water-cooled system with the heat exchanger in series with the chiller, which is correct. Unfortunately, many systems are designed with the heat exchanger in parallel with the chiller, resulting in a much smaller number of winter hours of operation.
James B. Rishel PE
Dir of Mechanical Operations
Dear Mr. Rishel:
Mr. Rishel raises some interesting points. In my article I used 1000 kW as a simple, round number, but I could have used 17,500 kW with the corresponding savings for each system. All four free-cooling systems described in the article can be used up to the 5,000-ton system Mr. Rishel describes, but other factors come into play with larger installations.
Larger, water-cooled centrifugal compressors have a kW/ton of 0.5 to 0.6, while high-efficiency air-cooled screw compressor chillers using R-134a can now achieve 1.05 to 1.1 kW per ton. However, in most cases an evaporative cooling tower supplies cooling water to the condenser of a water-cooled chiller. Therefore the cost of the water-cooled chiller power must include the cooling tower fan motor(s), spray pump motors, and/or condenser water circulating pumps, plus the cost of makeup water, which closes or eliminates the gap between the two designs.
Air-cooled free cooling chillers occupy more outside space, while water cooled chillers occupy more expensive indoor space and require space for plate-and-frame heat exchangers, and piping and valves for a winter free-cooling system.
Water availability is a major risk factor for mission-critical cooling. A 5,000-ton chiller installation rejects approximately 6,700 tons of heat (with added power input from the compressors). A cooling tower requires about 2 gallons per minute (gpm) of makeup water per million Btu/h. As 6,700 tons = 80 million Btu/h, this amounts to 160 gpm (230,000 gallons/day) not including blow-down losses. Water treatment must be provided for this makeup volume.
As I mentioned in the article sidebar, a close study is needed to determine the best possible design for any large installation, considering total power cost, free cooling hours achieved, reliability, simplicity, and risk factors.
Figure 4 on p. 29 of the November/December issue was mislabeled. The figure shows a direct air-side economizer system.