With the year-round Children’s Center open from 6 a.m. to 6 p.m. on weekdays, and interior classrooms without windows, the demand for cooling exists 12 months per year at varying loads.

Unfortunately, high demand charges were being billed for peak kW usage, and regular kW consumption was high due to inefficient operation of the old chillers.

The challenges:

• Reduce peak kW demand charges significantly.

• Reduce regular kW consumption significantly.

• Provide smooth, efficient operation at varying loads.

• Control the existing secondary (40-ton) unit with automatic lead-lag operation.

• Use a chiller that fits on the existing concrete pad of the old, 60-ton chiller.

• Install a quiet machine since the installation is at grade level, just outside of the Children’s Center; also, the school is closely bordered by homes in a residential neighborhood. And,

• Provide a machine that will operate within the existing maintenance budget, which covers the district’s 77 academic and 23 administrative buildings.

District needs proof

The efficiency of the “ECDX,” with its evaporative condenser and screw compressor, would substantially reduce kW consumption and peak demand charges.

However, the school district has a “like” replacement program, which called for an air-cooled machine to be offered by default. Therefore, the operational savings of the ECDX over a new, air-cooled screw chiller had to be proven.

Given the school’s load profile, a comparison was generated between the Dunham-Bush ECDX-045 (evaporative-cooled chiller) and the “ACDX-050” (air-cooled chiller), each delivering 45 tons at the specified design conditions (see Table 1).

With an EER of 14.64 at the design conditions, the ECDX-045 consumed approximately 45% less kWh and peak kW than the air-cooled approach. This would result in net operating savings of about 60% over the existing, 60-ton, air-cooled reciprocating unit to be replaced.

The comparison was provided to American Chiller Services, the service contractor for the school. Armed with this information, American Chiller Services contacted Jim Hicks, plumbing-hvac foreman for Sacramento City Unified School District, who was intrigued.

“We have cooling towers on our other buildings in the district, but this was the first application of an evaporative condenser,” says Hicks. “I was out on a limb here, but Dunham-Bush provided me with the information necessary for me to understand the application, as well as the maintenance and water treatment recommendations for the condenser section of the ECDX.”

This information was factored into Hicks’ analysis; he also allowed for the cost of a $3,000 automatic water treatment system. (The district pays a flat fee for water usage, so it did not enter the equation.)

“The bottom line is that I can reap the operating savings with the ECDX, and still work within my existing maintenance budget. This unit will pay for itself in no time,” Hicks adds.

The ECDX-045 fit on the existing concrete pad from the old, air-cooled chiller. With its quiet screw compressor, it handles varying loads and the fluctuating seasonal ambient temperatures without a problem.

In fact, since the condenser performance is based on the wetbulb temperature, the ECDX-045 is able to handle the cooling load for the entire school when the ambient temperature is as high as 100ÞF. Above this temperature, the unit’s microcomputer sends a signal to the old, secondary air-cooled machine to provide supplemental cooling.

For more information on Dunham-Bush, contact the company at 101 Burgess Rd., Harrisonburg, Va. 22801; 540-434-0711; 540-434-4010 (fax); www.dunham-bush.com (website).