The mechanical system used to chill water for central cooling is comprised of three main parts. Chillers remove heat from the water. Cooling towers reject the waste heat to the outside air. And, pumps “push” the chilled water out to campus buildings through an underground network of pipes.
The Plant currently operates two steam-driven chillers and two electric chillers.
The steam-driven chillers (both installed in 2007) each have an air-conditioning capacity of 1,365 tons of cooling.[One ton of cooling is equivalent to the amount of power required to melt one ton (2000 lbs) of ice (and thus produce one ton of chilled water) in a 24-hour period.] These chillers are unique because they use steam energy (instead of electricity) to drive the turbine which cools the water. Because the steam boilers in the Central Plant need to be in standby mode—even in summer months—there is excess steam capacity available that the chillers can put to use.
The mechanical energy used to drive the refrigeration process in the chillers is produced using steam-driven turbines that are incorporated into each chiller. Steam, which is produced by the boilers in the Central Plant for the purpose of heating water used on campus, is channeled into a loop of steam-supply pipes in the chilling system. High-energy pressurized steam entering this loop from the boilers is “worked” as it rotates the blades of a turbine. The steam molecules collide with the surface of these blades and transfer their energy to spin the turbine. The exhaust, lower energy, lower pressurized steam is then passed through a steam condenser on top of the Chiller. This allows the steam to transfer away a little more energy so that it can condense to liquid form and re-enter the plant feed water supply system for the boilers. The spinning turbine on each chiller powers a compressor which drives the vapor compression cycle explained above.
In using steam instead of electricity for cooling, the steam-driven chillers avoid putting extra strain on the regional energy grid during peak electricity demand times in the hot summer months.
Each chiller has several components that allow it to do this job. These components work together to perform continuous rounds of a vapor-compression cycle, during which a chemical substance called a refrigerant is allowed to evaporate and condense so that it can take up and release energy from the water. Inside the chiller, pipes carry the water to be chilled through an evaporator where heat is transferred between the water and the refrigerant. This heat transfer process lowers the temperature of the water to about 42 degrees F, at which point it is cold enough to make the journey to the buildings. Aside from the evaporator, the other working components in each chiller include a refrigerant condenser, a compressor, a steam turbine, and a steam condenser. These components function to support the work of the evaporator, and interface with the other parts of the mechanical cooling system, like the cooling towers and pumps.
The electric chillers were installed later on: The first one in 2017 was part of the major Central Plant expansion project described in this journal article and the second one was installed in 2022.