Lessons at the Millennium

Engineered Systems, March, 2000 by Richard Bartuska

At an Illinois school with a brand-new name, the pairing of a classic two-pipe system with modern controls technology earns high marks for short-term affordability and long-term efficiency.

Chicagoland's hot housing market and rising population are driving its area schools into an expansion mode. The southwest suburban village of Tinley Park had been feeling the pinch in recent years as enrollment at the village's seven schools approached capacity. In August 1998 Tinley Park's Kirby School District 140 broke ground in the construction of a new 80,000-sq-ft elementary school to relieve overcrowding.

"Lots of growth coupled with the trend toward smaller class size meant we needed more rooms," said Bob Prost, supervisor of operations and maintenance for District 140. The new school, Millennium Elementary, is designed to hold 800 students in grades K-5. It opened last fall and already serves more than 500 students.

The design of Millennium School combines a classic dual-temp loop with a sophisticated control system. The approach was to install sound, yet not extravagant, hvac equipment while investing in a high-end control system that would foster energy savings down the road.

"It's economics," said Prost, referring to the primary factor in the district's design decisions.

Consulting engineer R.L. Millies & Associates Inc. of Munster, IN, designed the fourwing Millennium Elementary School to suit the district's budget without sacrificing air conditioning capacity. Millennium is constructed with unit ventilators in the classrooms and multizone air-handling units (AHUs) for the common areas. Construction took place from August 1998 to August 1999. The fast schedule was a challenge, but it enabled District 140 to take advantage of available state money, approximately 70% of the facility's $8 million budget. Team Mechanical Inc. (Wheeling, IL) installed the hvac system, and Johnson Controls (Milwaukee) provided the building automation system. CMS Inc. (Lockport, IL) was the general contractor for the project, and the architect was Healy, Snyder, Bender and Associates (Naperville, IL).

A TWO-PIPE SYSTEM FOR SAVINGS

A two-pipe dual temperature system was selected for Millennium School instead of the more expensive, and more conventional four-pipe system. Some might argue that the two-pipe hvac system, in which the same supply and return pipes are used for both hot and chilled water, is a thing of the past. The installation at Millennium School, however, demonstrates that a two-pipe system may be a perfectly suitable option when construction and operating costs are a concern.

"When paired with sophisticated controls, Millennium's two-pipe system comes close to doing the work of a fourpipe system and with substantially lower costs," said Wayne Orlowski, Team Mechanical Inc.'s project manager.

With a two-pipe system, Millennium School does sacrifice some flexibility in switching between heating and cooling when unseasonably warm or cold days strike. The upside, however, is cheaper construction. Installation costs alone for a two-pipe system are approximately 30% less than for a four-pipe system of the same size. Plus, the district saves on annual energy costs, since a two-pipe arrangement requires less fuel.

To further promote efficiency and cost savings, the system at Millennium School contains no glycol. Glycol, although it prevents pipes from freezing in the winter, reduces the efficiency of a system. Because Millennium's chiller is distributing pure water rather than a glycol solution, it operates at peak efficiency but does need to be drained every fall. A set of isolation valves closes off the outside chilled water pipes, and the outside piping is heat-traced as well for further freeze protection.

In another dollar-conscious decision, a single 225-ton chiller was chosen over two smaller chillers. The selected Carrier (Syracuse, NY) chiller provided the desired redundancy as it has two separate refrigeration circuits and multiple compressors.

PRIMARY-SECONDARY SYSTEM

Millennium School is equipped with a primary-secondary pumping system. Small pumps circulate water through either the chiller or the boilers on a constant-flow basis. These small pumps inject tempered water into the larger secondary loop that extends to all of the hvac equipment in the building. Large pumps circulate water in the secondary loop, but only the amount required to meet the needs of the areas served. The speed of these large secondary pumps is controlled by variable frequency drives (vfd) that slow the pumps down when their full capacity is not needed to heat or cool the building.

The building automation system monitors the conditions throughout the facility and commands the secondary pumps, through their vfd's, to deliver only the amount of heated or chilled water necessary to meet the needs of the building.

The primary-secondary system did require that the school spend extra money at the time of installation, but with the expectation that the money will be made up in future energy savings since primary-secondary systems require less energy to operate.