Ashrae Journal - October 2008 - 41

• Sensible Heat Ratio: 1.0 • Chilled Water Flow: 17.9 gpm at 25 ft of head (1.13 L/s) at 8 m of head) • Leaving Air Temperature: 67.4°F (19.7°C) (from Equation 3) • Fan Power: 1 kWh (published manufacturer data, all losses included) • CW Pump Power: 0.34 kW (from Equation 2) The theoretical data center load of 0.75 MW would require 30 IRAH units, adding an additional 30.6 kW fan power plus 10.2 kW chilled water pump power above and beyond the 750 kW IT and lighting load. This scenario yields a total chiller load of 791 kW. Computer Room Air Handler (CRAH) Presently, common practice for reducing first-time capital expense of perimeter cooling solutions leverages the largest practical and commercially available cooling equipment. An unfortunate consequence of using large capacity boxes is a reduction in capacity resolution. An additional consideration for CRAH units is the fan placement within the air-distribution path. These fans are placed in the bottom of the CRAH unit with little or no outlet transition into the raised floor plenum. The consequence is a fan outlet system effect3 that adds a significant static effect on fans. This effect is a function of blast area, outlet area, velocity, and transition length. The outlet system effect is often overlooked and frequently may result in products as installed delivering less than anticipated airflow quantities. The theoretical CRAH depicted below has a net sensible cooling capacity of 102 kWh versus the 750 kWh for the combined IT and lighting load. In this case, full capacity without redundancy would require 7.3 CRAH units per the below specification. The IT load being considered requires eight CRAHs with an immediate over-provisioning factor of 1.09 times the base load. CRAH Unit Specifications – Static Efficiency: 0.59 – Outlet Velocity: 2,800 fpm (14 m/s) – Outlet System Effect: 0.6 in. w.c. (149 Pa) – Floor Pressure: 0.3 in. w.c. (75 Pa) – Filter Loss: 0.75 in. w.c. (187 Pa) – Coil Loss: 0.65 in. w.c. (162 Pa) (wet) – Cabinet Loss: 0.5 in. w.c. (125 Pa) • Motor Power: 11.0 kW (0.92 motor efficiency × 1.05 drive loss) • Net Sensible Cooling: 102 kW • Leaving Air Temp: 56°F (13°C) (from Equation 3) • Chilled Water Pump Power: 1.4 kW (from Equation 2) The above eight CRAH units combined would add an additional 88 kW fan power plus 11.2 kW chilled water pump power above and beyond the 750 kW IT and lighting load. This scenario yields a total chiller load of 893 kW at a sensible heat ratio of 0.95. CAHU Most applications using central air handlers will have custom air handler units designed and built for the specific project. The wide variation of design practices and component selection make it difficult to express performance data in absolute terms. The values used herein are for purpose of comparison and are believed to reasonably represent nominal values. However, some variation should be anticipated. The reader may notice that a significant contribution to the CRAH fan losses from above, outlet system effect, is missing in the below CAHU example. This is possible due to physical geometry of custom air handlers allowing better practice regarding fan placement and operation. Unfortunately, in many cases the gains from reducing and or eliminating blower outlet system effects are frequently offset by increased pressure losses in delivery system: ducting, elbows, and diffusers. The theoretical data center load of 0.75 MW will require a quantity of four CAHUs per the below specification without any redundancy. With the CAHU being custom built equipment the amount of over provisioning can be carefully controlled allowing only for the desired factor of safety. CAHU Specifications • 17,100 cfm at 0.3 in. w.c. (8070 L/s at 75 Pa) floor pressure • Sensible Cooling: 113 kWh at 75°F (24°C) DB, 45% RH, 61°F (16°C) WB • Sensible Heat Ratio: 0.95 • Chilled Water Flow: 81 gpm at 18 ft of head (5 L/s at 6 m of head) • Fans (3x) PowerShaft: 3.2 kWh each (from Equation 2) • Forward Curve 15 in. × 15 in. (38 cm × 38 cm) double inlet, double-width (DIDW) – Blast Area = 0.81 ft × 1.55 ft = 1.26 ft2 (0.25 m × 0.47 m = 0.12 m2) – Outlet Area = 1.32 ft × 1.55 ft = 2.05 ft2 (0.40 m × 0.47 m = 0.19 m2) – BA/OA = 1.26 ft2/2.05 ft2 = 0.61 (0.38 m2/0.62 m2 = 0.61) October 2008 • 34,000 cfm at 1.0 in. w.c. (16 046 L/s at 249 Pa) external static pressure • Sensible Cooling: 220 kWh at 75°F (24°C) DB, 45% RH, 61°F (16°C) WB • Sensible Heat Ratio: 0.95 • Chilled Water Flow: 158 gpm at 20 ft of head (10 L/s at 6 m of head) • Fans (2x) PowerShaft: 9.1 kW each (from Equation 2) – 76 cm backward-inclined, double-width (BIDW) – Outlet Velocity: 1,825 fpm (9 m/s) – Static Efficiency: 0.66 – Return Air Duct: 0.3 in. w.c. (75 Pa) – Filter Loss: 0.75 in. w.c. (187 Pa) – Coil Loss: 0.60 in. w.c. (149 Pa) (wet) ASHRAE Journal 41

Ashrae Journal - October 2008

Table of Contents for the Digital Edition of Ashrae Journal - October 2008

Ashrae Journal - October 2008

Contents

Commentary
Industry News
Letters
ASHRAE HQ Dedication Booklet
ASHRAE Rebuilds HQ to Reflect Mission
Feature Articles
ASHRAE's Living Lab
Meetings and Shows
Sizing Pipe Using Life-Cycle Costs
Feature Articles
Energy-Efficient Data Centers: A Close-Coupled Row Solution
The Future of 90.1
Ventilation Standard for Health Care Facilities
Grosvenor Humidity Chart
ASHRAE Research Report
ASHRAE Honor Roll
Building Sciences
Special Products
Emerging Technologies
Product Showplace
Classified Advertisting
Advertising Index
Ashrae Journal - October 2008 - Ashrae Journal - October 2008
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Contents