Four steam-to-water heat exchangers provided the facility's heat and hot water. A Boston utility company supplied steam for both cooling and heating. Within the building, hot water was supplied to three separate zones including air-handling units, induction units, and perimeter radiators, as well as a domestic hot water storage tank.
The mechanical room is located on the tenth floor with the chillers located in a penthouse on top of the tenth floor roof and the cooling towers located on the roof outside the penthouse. Space in the penthouse was limited when allowing for tube pull. The floor was also only a 3-in.-thick concrete slab with limited baring capacity.
Evaluating optionsThe current owner of the facility is the Commonwealth of Massachusetts, Department of Mental Health. The agency saw the need to replace one of the 25-year-old absorbers and in the process wanted to increase efficiency, reduce maintenance, and become less dependent on the local steam utility.
An evaluation of available chillers and site requirements, as well as a desire to reduce operational costs, led to the selection of a Broad Spectrum natural-gas-fired absorption chiller/heater. Jake Carr of Power Specialties, a representative of Broad, said initial cost studies revealed the advantages of gas-fired chilling equipment. The unit acts as a chiller and heater - eliminating the need for a supplemental boiler - and its low electric usage required no additional electric infrastructure in the building.
The unit acts as a chiller during the summer and is sized for the full 528 tons of cooling. Due to its versatility in design, it was also provided with four separate water-to-water heat exchangers which interconnect with each of the existing hot water zones. The total combined heating load for winter operation is 12,000 MBtuh.
The Boston Gas Company, now called Keyspan, offered a rebate program that covered some of the construction costs since the hospital was committed to installing a gas-driven piece of equipment and eliminating the need for utility company steam. The unit allowed the building to be heated and cooled from a single piece of equipment and was readily adaptable to the existing pumps and cooling tower.
Worth the WeightEnterprise Equipment (Weymouth, MA) was selected as the installing contractor with total project responsibility for evacuation of lithium bromide, demolition of the existing chiller, and installation of the new unit.
The new unit weighs 35 tons while the old unit weighed only 12 tons and, due to the penthouse location, considerable supplementary steel and bracing was necessary. This would not normally be a consideration for basement or grade applications. However, even with the additional cost of steel, this method of cooling and heating was preferable to using two separate systems.
The challenges of rigging and penthouse access were overcome by cutting an 8-in. wide by 12-in.-high hole in the sidewall. The absorber was shipped in two sections with the main shell separated from the high-stage generator (HSG) to facilitate placement. The rigging took place on the ground, and within four days the new absorber was installed and ready for piping. The welding and connection of the main shell and HSG was done under the supervision of Broad technical staff who also checked out the installation and commissioned the unit.
The Broad control system included standard monitoring and control points as well as being capable of being tied into the building automation system. Power Flame (Parsons, KS) supplied the burner and gas train. The burner management system and flame safeguard controls are housed in the burner-mounted control panel. The system incorporates a remote wall-mounted touch screen and monitoring panel that allows the operators ease of changing setpoints and program data from the machine location or remotely via phone if desired.
Start-up was limited to three days in order to evaluate, adjust, and test all system parameters. The entire installation took three months of construction and two months of installation. The unit was delivered on time within 10 weeks of receiving approval and since its installation in September 1999, it has been operational.
Chief operator Joe Sealy says the high-tech control system and remote communication capability make operation easy and reduce service calls. Increased efficiency and switching fuels has resulted in a dramatic reduction in the hospital's utility costs. The hospital's steam bill for 1997 was just under $500,000 while the gas bill for 2000 was just over $100,000 - a 70% decrease.