Alternate Air Distribution

Several years back, we developed a substantial analysis, encompassing several hundred pages of computer runs, as well as construction cost estimates from a large nationwide mechanical contractor to determine the most cost effective means to provide air conditioning to a building. Back in the old days, when chillers routinely operated at 0.90 kW input per ton of cooling output, other firms had determined that 55°F supply air temperatures provided the best blend of chiller plant and air distribution system efficiencies.

Today, with the advent of 0.45 kW per ton constant speed chillers, and variable speed chillers that run in the 0.30 to 0.35 kW per ton range for most of the year, as well as the use of Thermal Energy Storage during peak hours of the day, the fixed 55°F supply air temperature requirement is no longer valid for most buildings.

On most retrofit projects, we typically incorporate a continually resettable supply air temperature software routine that minimizes the chiller plant and air distribution system energy. An integral part of this routine is the recognition that chiller efficiency changes almost linearly with load and temperature changes, while variable speed drive equipped fan energy responds in a nearly cubic relationship with load and temperature differential.

Where TES is involved, we typically size the storage capacity slightly larger than would typically be selected by other firms, and then use this extra cooling capacity to reduce the supply air temperature of the AHU’s.  A 10% reduction in fan volume made possible by supplying colder air typically results in fan energy savings in excess of 20%.  On most projects we are able to reduce fan volume by 20% to 25% on peak load days, thus making the TES system that much more cost effective.

Low Temperature Air

Underfloor Air

Mixed Mode Ventilation