Until 1992, the Minnesota State University, Mankato (MSU) campus was air-conditioned by individual building units. Each unit operated independently based on temperature set points for each specific building or air-conditioning zone. However in 1992, the University decided to install the first stage of its central chilling system. This system localized the cooling functions in one location through the use of several larger chillers and replaced the many individual units. As described in the Facility Overview, chillers are refrigeration units which cool water to be distributed to remote buildings for cooling purposes.

The purpose of the central chilling system is to supply chilled water to almost all of the campus buildings for air conditioning. An array of piping within the chiller plant can be seen in the picture. Each of the chillers is capable of producing chilled water at 42°F. Once the chilled water is used in the air handlers within each building, the water will return to the chilling facility at roughly 54°F. The water will once again be chilled and sent back out on this continuous loop.

Centrifugal Chillers Walkthrough Video

The decision to install any central chilling system is based on many factors including maintenance issues, efficiency, and lifespan. A major consideration for MSU was that buildings

 

 

equipped with their own air-conditioning units also required an individual cooling tower to reject the heat to the environment. As described in the Cooling Tower section the water in these devices must be chemically tested every day. With a central unit, most of this inspection and maintenance work was eliminated and can now be performed on only a few units at the main plant.

A second consideration was that the efficiency of a central unit was much higher than the individual units. An individual air  conditioning unit might have an efficiency of roughly 1.5 kW/ton, whereas a centrifugal chiller has efficiency around 0.5 kW/ton (a ton is how much energy it takes to freeze 2000 lbs of water at 32 degrees Fahrenheit). This means a smaller individual unit can require up to three times the amount of electrical power as a centrifugal chiller to maintain the same cooling load. The centrifugal chillers are quite large as can be seen in the picture above.

A deciding consideration was that in 1992 many of the building units were approaching the end of their lifespan. Without the central plant these units would have needed to be replaced.  As any good thermal design engineer will tell you, the best time to upgrade is when you have to purchase new equipment anyway. In other words, this was an opportune moment economically to make the changeover.