Hinsdale High School District 86
1998 Air Conditioning Project

In1997 the Hinsdale High School District 86 made a decision to air condition 1/3 of their school buildings so that they can have a more effective Summer Program. This decision was in concert with their decision to begin replacing mechanical equipment that was 35 to 40 years old. This project started out to be a simple infrastructure project but soon developed into a more complex project with many long term implications.

Hinsdale High School District 86 consists of two high schools, Hinsdale Central High School and Hinsdale South High School. The Hinsdale Central High School is approximately 400,000 Sq. Ft. The Hinsdale South High School is approximately 373,000 Sq. Ft.

Some history of the buildings is required before discussing the project. Dahlquist and Lutzow the Architect for the School District hired Brian Berg and Associates in 1986 to help the School District achieve their long range goals. Over the years the owner has implemented many energy saving goals beginning with the Technical Assistance Manuals prepared in the late 80's. As part of the Technical Assistance Manual it was identified that the High Schools would benefit from the use of CoGeneration to reduce their electrical demand charges. On many occasions from 1987 till today the installation of a CoGeneration plant for each of their two schools has been reviewed. Each time there was significant obstacles to implementation of CoGeneration.

Both schools have their own 4160 volt electric distribution system to unit substations. Each substation has transformers and the electrical utilization within the buildings is 208/120 volt three phase, four wire. The demand charge is not as large as it normally would be for a school this size due to the 4160 volt distribution. In addition the distributed transformers increase the costs associated with installing the CoGeneration System.

It was determined that both schools would benefit from replacing the old unit ventilators and fan coils with newer 4 pipe chilled water units. The replacement of these systems with a different type of system would have required extensive remodeling of the building. In many places it was physically impossible to install anything other than what was already there. A central chilled water plant would be designed for each school to provide chilled water to the new equipment. Area’s of the building that were not to be air conditioned would have chilled water coils installed but they would not piped to the chilled water system. The chilled water central plant would be expandable in the event that the School Board wanted to air condition the entire building.

As mentioned above the School District has always been energy conscious. Right from the beginning we discussed centrifugal chillers with cooling towers so that they could achieve air conditioning at the least operating cost. During the schematic design phase of the project centrifugal chillers and cooling towers were sized. The School District picked the area’s of each building that was to be air conditioned. At the central campus the district picked mainly north facing classrooms. At the south campus the district picked mainly west facing classrooms. The central plant for the central campus had to be 300 tons and the central plant for the south campus 400 tons. The electric service for both schools were analyzed by the Electrical Engineer and it was determined that the electric service for both schools could not support the installation of the new Electric Driven Chillers, pumps and cooling towers. We now realized that other options must be reviewed.

Three separate options for the Central Campus were reviewed in the span of one week. They were as follows:

A. Electric Driven Centrifugal Chiller. The electrical operating cost will be much higher for the Electric Driven Centrifugal Chiller. The existing electrical service lines from ComEd are too small and would have to be replaced. The electric service change out would shut down the school for a period of time and would add to the construction costs.

B. Gas Fired Engine Driven Screw Chiller. The Gas Fired Engine Chiller has an increased maintenance cost over the other chillers due to the added maintenance of the engines, Oil, Lube Filter Etc. We estimated that this option will cost approximately $28,000 less per year to operate than the all electric chiller. We estimate the simple payback to be 2.4 years without including the reduction in initial construction cost for the new electric service.

C. Gas Fired Absorption Chiller. The Gas Fired Absorption Chiller is more costly due to the increase in initial cost of the equipment and the additional stack. We estimated that this option will cost approximately $22,000 less per year to operate than the all electric chiller. This option uses double the gas than the Engine Driven Chiller. We estimate the simple payback to be 6.3 years without including the reduction in initial construction cost for the new electric service.

With a simple payback of 2.4 years the Engine Driven Chiller option was clearly the most practical. When the initial construction cost for the new electrical service is also figured the Engine Driven Chiller’s payback approaches 0 years. The electric service for the Central campus did not have to be changed due to the use of the gas engine driven chiller. The electric service at the South campus was still not capable of powering the additional pumps and cooling towers. A new 4160 volt line and unit substation was provided from the existing 4160 volt primary service to serve the new chiller building, pumps and cooling tower. The chiller building electrical distribution was 480 volt three phase and was sized to accommodate the future loads.

The chilled water and condenser water distribution systems were different for each school due to the locations that were available to install the chillers and cooling towers. The capacity for a chilled water plant to air condition the entire school at both schools is approximately 900 tons.

The Hinsdale Central High School has an old coal storage bin located below the parking lot next to the boiler room. This was used to house the new chillers. The physical properties of the structure allowed for a 300 ton chiller to be lowered through a hole in the parking lot. The chilled water system is designed for three 300 ton engine driven chillers. Only one chiller was installed during this project. The building structure did not allow for the installation of a cooling tower so space for a three cell cooling tower was found on grade. Unfortunately this was several hundred feet from the chillers. During design the condenser water piping was reviewed to route the piping from the cooling towers to the chiller room. It was found that if piping was installed to handle the initial chiller there would not be enough room to install piping for the future chillers. In addition the cost to install a set of 8" condenser water lines was not much less than the cost to install one set of 14" lines. The lines would also have to be run under the parking lot for approximately 20'. It was determined that one set of 14" lines would be installed that would handle the future load. This would eliminate disrupting the school to remove and install new condenser lines when the chiller capacity was increased. The chilled water distribution lines had similar issues. The main chilled water lines were installed as 10" to handle the future load as well as the present.

The Hinsdale South High School did not have space available to install the new chillers and the structure would not allow installing the cooling towers on the roof. Space on the site was found next to the boiler room to build a new chiller building with the cooling tower on the roof. Two factors affected the design of the chiller building. The first was that a 400 ton engine driven chiller was not available from any manufacturer. A 500 ton chiller would have to be purchased. Second the use of two chillers and corresponding cooling towers would reduce the physical size of the chiller building. The chilled water plant was designed with a 500 ton engine driven chiller, space for the chiller building electrical service, pumps and piping. The building also has space for a future 500 ton engine driven chiller. There is also space for the corresponding cooling tower on the roof above.

In both schools the chillers were located near the boiler rooms so that there would not need to be any additional gas service and the service piping would be significantly reduced. Since the boilers never run when the chillers run there was no need to increase the size of the gas service or service piping.

The High Schools actual air conditioning operation is many less hours per day than initially projected. The chilled water plants only operate during the electric company’s peak hours of operation. They rarely if ever run at night and on week ends. The additional maintenance required for an engine is not a problem because there is more than enough off time to complete maintenance.

The High School district has purchased their own gas for approximately 15 years now through the Illinois Gas Cooperative. In the first few years of operation the gas used to run the engine driven chillers was gas that was not used in the winter due to the mild winter. Last year the gas used to air condition the buildings was expensive due to the elevated price of gas.

The use of natural gas engine driven chillers is very economical for these high schools especially since they only air condition when electricity is the most expensive. This is also good for the environment since the electric used to air condition would most likely be from coal fired or the most inefficient electrical generating plants.