Sears Holding Company

Chilled Water System Retro-Commissioning

Chilled Water System Retro-Commissioning - Hoffman Estates, IL

Overview

G/BA performed retro-commissioning and analysis of the entire chilled water system to identify operational problems, with particular scrutiny of low secondary supply-to-return water temperature differentials, short cycling of chillers, and excess on-line chiller capacity.

Awards

Source Award
2008 Excellence in Engineering Award, ASHRAE Illinois Chapter
2009 Technology Award, ASHRAE Region VI (first place)

Features

  • The supply air temperature setpoints for the air handling units were all found to be below their intended design values. G/BA recommended that the AHU discharge air setpoints be increased to 54°F during the cooling season.
  • G/BA discovered that the chilled water supply temperature was allowed to climb to 48°F (6°F above setpoint) before another chiller was started. G/BA recommended that the plant be operated to maintain a chilled water supply temperature of 42°F.
  • Analysis of pump performance revealed that the secondary chilled water flow rate was allowed to be greater than the primary flow rate, increasing secondary supply water temperatures. G/BA recommended implementing changes to assure that the primary flow rate always remains greater than the secondary circuit in order to increase ΔTs.
  • We discovered that an incorrect calculation had been used for determining cooling load and estimating flow rates to sequence the chillers. We recommended a corrected calculation.
  • Some chilled water flow meters, temperature sensors, and chilled water control valves were found to be broken or not-calibrated; all were fixed or replaced.
  • All recommendations were implemented by Sears' facility personnel. Material costs were less than $5,000.
  • Before G/BA's recommended adjustments, the chilled water ΔTs were typically between 6° and 8°F: approximately 50% of the design ΔT of 14°F. After recommendations were implemented, chilled water ΔTs were generally at or above the design value (e.g., ΔTs as high as 18°F have been realized).
  • Chillers can now be loaded to 100% of available cooling capacity.
  • Colder chilled water temperatures and increased chilled water ΔTs resulted in reduced fan and pump energy use. Year 2007 actual energy savings were 696,000 kWH and $64,000.