At Tri-North Middle School in Bloomington, IN, an uncommon chiller application is achieving high marks and offsetting rising fuel costs. By applying dedicated heat recovery technology, Tri-North saved tens of thousands of dollars in gas over the last two years, despite fuel prices rising more than 30%. A dedicated heat recovery chiller (DHRC) from Multistack, LLC, has provided the school with dramatic savings, while meeting an increased cooling load, improving comfort, and enhancing environmental friendliness.

The installation of the 100-ton DHRC was part of a 2004 renovation, which involved adding a gymnasium and auditorium to the central cooling load. The renovation also increased outside air intake to meet ASHRAE Standard 62-1999. The engineering firm of Veazey Parrott Durkin & Shoulders had to devise an efficient way of heating and cooling this additional outside air.

One challenge the engineering team faced was maintaining appropriate indoor air temperatures at 60% or less rh. In the summer, outdoor air must be cooled down to 50°F to 55° degrees in order to consistently maintain humidity below 60%. Since this air is too cool to introduce to the classroom, a certain amount of reheat is necessary. This typically means operating hot water boilers in the summer; even when outdoor air is above 80°. The DHRC helped to eliminate much of this waste.

The Design

A 100-ton Multistack DHRC was selected to replace a 70-ton air cooled chiller. The DHRC provides VAV reheat, dehumidification, space heating, and domestic water preheat, in addition to operating in series with an existing 120-ton air cooled chiller to meet space cooling loads.

The DHRC preheats domestic hot water to 120° via a dual wall plate-and-frame-heat exchanger before it is heated to its final supply temperature of 140° with a conventional gas-fired heater. A storage tank maintains the supply temperature of the hot water. Since the DHRC provides most of the heat necessary for domestic hot water, the heaters use less gas. Furthermore, overall efficiency of this design is enhanced by the fact that DHRC units operate at higher efficiencies when condenser water is at a low temperature, as is the case with domestic hot water. After the domestic load is met, the DHRC rejects any remaining condenser heat to the building’s heating loop. If no internal heating loads can use the condenser heat, it is rejected to the cooling tower.

On the cooling side, the DHRC is always loaded first because it is water cooled, and therefore more efficient than the 120-ton air cooled chiller. Much of the year the DHRC provides 100% of the school’s cooling without the second chiller.

The Learning Curve

The DHRC system, while highly effective at saving energy, presented certain challenges. For instance, the controls contractor had to master new control sequences for a system with which he was mostly unfamiliar. Control issues had to be resolved mostly by trial and error, delaying the system’s final implementation. “The good news is we can incorporate these newly developed sequences into any future DHRC installations in the school district,” said Kevin Bucy, energy management technician for the Monroe County Community School Corporation.

Bucy and his team of contractors and engineers learned other valuable lessons about DHRC systems, including the fact that they’re more sensitive to dirt and rust from old piping. With this in mind, Bucy recommended flushing existing piping systems after construction.

“We also learned that this type of system is not well suited for ‘radiation only’ heat, which was used in some smaller rooms at Tri-North. This is because the DHRC operates at a lower heating water temperature than a conventional boiler,” said Bucy. “This is only a problem during those ‘in between’ weather days when it’s cool outside but not cold enough to run the boilers. This, like the other issues we encountered, can be easily overcome by design changes in future installations.”

The Savings

Tri-North has experienced extraordinary natural gas savings since adding the DHRC. A comparison between 2003 (before) and 2005 (after) shows a difference of 4,813 MF or 48,130 average therms consumed. This translates into an annual savings of $52,461 at the 2005 rate of $1.09/therm. Since close to half of the natural gas savings are attributable to the DHRC, payback was approximately two years. This new system also dramatically reduced CO2 emissions. The decrease is the result of replacing heat from a natural gas burning boiler with heat from the DHRC. ES