According to the hotel’s property operations manager, Ian Murray, the hotel recently re-evaluated its air conditioning chilled water plant with an eye to ensuring continued system reliability and operating economy. The hotel was built in 1962 and was operating on its original chiller plant. That plant had included two 1,100-ton steam turbine-driven centrifugal chillers and one 400-ton electric centrifugal machine. Steam for the two large machines was supplied by New York’s district steam system.
A Variety of Challenges“The steam machines were great in their time,” said Murray, “but we suspected they were near the end of their useful lives and wanted to looked at all the options.” The hotel retained the consulting firm of Jaros, Baum and Bolles (JBB) to evaluate the present system and suggest chiller replacement options. The consultant looked at not only replacement chiller choices, but also at the physical planning requirements of a plant upgrade.
The hotel’s mechanical room is located in a deep subbasement. Although the mechanical room is quite roomy, access to it with large equipment is very challenging. Any equipment moved in or out requires extensive planning. The ultimate recommendation of the engineer, and the choice of the owner, was to replace the steam turbine machines with new, high-efficiency centrifugal chillers.
The economic analysis revealed that the steam turbine option was not as attractive as it had been when the hotel was built. Up until 1995, Consolidated Edison had offered rebates as high as $500 per ton for new steam refrigeration installations. In 1995, possibly influenced by electric deregulation on the horizon, the practice of offering these rebates for steam installations halted. For the first time in nearly 10 years, the economic choice between electric and steam-based refrigeration equipment had to stand on its own. The analysis by JBB revealed that electric centrifugal machines could save significantly in building operating costs for this 24-hour-a-day operation. During offpeak (evening and weekend) hours, the hotel is able to take advantage of low rates to operate at a cost of less than $.03/ton/hour vs. nearly $.10/ton/hour for steam. The Sheraton accepted bids for high-efficiency chiller alternatives in addition to the .62 kW/ton-based-bid. Alternatives for machines with .565 and .528 kW/ton were analyzed against the additional first cost of these higher efficiency machines. In the end, the .528 kW/ton was accepted. This alternative promised a payback of less than 3.5 years. The Sheraton selected this longer-term outlook against the .565 kW/ton option, which would have had a payback of less than three years.
A Fitting SolutionThe equipment chosen to replace the two large steam units was a pair of 1,100-ton Trane (LaCrosse, WI, www.trane.com) CenTraVac® two-stage chillers. According to Murray, the hotel made the decision to order the high-efficiency centrifugal chillers on the basis of long-term operating savings. Murray noted, “These are major pieces of equipment you’re going to have for a long time. You have to look at efficiency.”
Murray points out another important requirement. “We absolutely insisted that we had to have a continuous stream of chilled water. We don’t have the luxury of shutting down the plant. In the hotel business, guest comfort is paramount.” For that reason, the job had to be designed for a time of year other than the peak cooling months. And the first of the new chillers had to be on-line before the last old unit was taken off-line.” In the end, the work was completed between November 1996 and June 1997.
Units with separable shells and compressor doweling helped to simplify unit disassembly and reassembly in the mechanical room. The replacement chiller components would have to be lowered into the mechanical room through a 7- by 12-ft opening cut in the concrete floor above. The old chiller equipment had to be removed the same way. The project obviously required extensive rigging work on both floors.
The hotel environment had other implications for the conversion process. The Sheraton had to install new electrical feeders and rearrange others to provide service to the new chillers. The challenge again was to do the conversion in a way that would be invisible to guests. “To do that,” said Murray, “took some planning. We needed to avoid interrupting service to any guest rooms. And we were successful.” And as far as the hotel guests were concerned, it never happened.