The Cleveland Clinic is one of the highest-rated cardiac-surgery hospitals in the world and has been ranked No. 1 in the country by U.S. News & World Report for 12 years running. Covering approximately 40 square city blocks, it is frequented by international dignitaries, including royalty. In 2005, it logged more than 2.9 million outpatient visits, with every state and more than 80 countries represented.
In 1999, the Clinic's board of directors approved the design and construction of a new 1-million-sq-ft heart hospital. As building design commenced and cost-reduction possibilities were evaluated, the use of a separate, free-standing central utility plant was approved.
Steam Production and Delivery
Boiler-plant design proceeded with the selection of a D-type water-tube boiler capable of delivering 100,000 lb of saturated steam per hour at 150 psig. A welded-membrane water-cooled wall design with water-cooled, gas-tight furnace areas was utilized for optimum emissions performance.
The boiler design featured an integrated 121-MMBtuh burner. This high-efficiency, low-nitrous-oxide (NOx) burner was custom-designed to ensure it and the boiler would run as a seamlessly integrated package. Air and fuel are monitored and controlled independently to ensure a precise air-to-fuel ratio. A modulating flue-gas-recirculation (FGR) damper is utilized to maintain the 30-ppm NOx-emissions requirement. The FGR damper allows the boiler to run at different load levels while still meeting the required emission levels, unlike typical systems, which are designed to meet required emission levels at one load level.
A steam pre-heater was included to heat inlet combustion air and avoid the possibility of water vapor condensing out of flue gas. A 28,550-cfm, 150-hp combustion air blower delivers air from outside the building through the pre-heater and directly into the boiler combustion chamber.
Ensuring the continued operation of the boiler in the event of a loss of natural gas to the plant required a dual-fuel burner system, with No. 2 oil serving as the secondary fuel. This required a means of quick oil ignition, as well as a means of oil atomization. Oil is injected into the firebox under pressure and atomized into droplets between 10 and 100 micrometers (microns) in size. Upon boiler startup, this atomization is done with compressed air; after initial firing and commencement of steam production, it is done with steam. This oil/air (or steam) mixture is ignited with a propane ignition burner. The process greatly improves fuel-oil heat release and burning efficiency.
The steam that is produced is distributed to the heart hospital and to the campus steam loop for utilization in approximately 15 buildings. Because the system is required to deliver steam for sterilization year-round, a minimum boiler base load is maintained during summer. The maintenance of boiler loading above a minimum level helps to maintain maximum boiler operational efficiency. At greatly reduced boiler loads, boiler-jacket heat losses are what they would be if the boiler were at full fire because the temperature required for steam production is the same. In addition, stack losses remain relatively constant when boiler operation is below a certain minimum point. Therefore, maintenance of boiler steam delivery between 25 and 75 percent of full load is the most efficient point for boiler operation. To ensure the existence and maintenance of this base load, a 1,440-ton, single-effect steam absorption chiller is used. The use of the chiller during summer requires the delivery of 24,000 lb of steam per hour at peak chilled-water delivery. This steam delivery is 24 percent of the boiler delivery capability, enabling the steam-production facilities to maintain the required base load.
Boiler-Feedwater Treatment and Delivery
The return of condensed steam to the boiler, in combination with the addition of makeup water, requires the provision of proper water treatment. Even though the Cleveland Clinic's maintenance routines do an excellent job of minimizing the loss of condensate—returning, on average, 80 percent of delivered steam as condensate—it is virtually impossible to eliminate condensate loss. Furthermore, the steam supplied for humidification and sterilization is not returned to the condensate-return system. The combination of these losses requires a significant amount of fresh-water makeup to the boiler-feed system (Figure 1).
Even though the makeup water, which is obtained from the Cleveland municipal water system, is supplied at a relatively high quality, it can present a highly corrosive condition within the boiler. The treatment of fresh water for the campus boilers consists of the initial treatment of city water through a water softener and dealkalizer. The water softener removes calcium and magnesium ions, which increase scale buildup, from the water and replaces them with sodium.
In a fashion similar to the operation of the water softener, the dealkalizer reduces the pH of water by replacing alkalinity ions with chloride ions. Excessive alkalinity causes foaming and, thus, excessive solids carryover to the steam delivered to the system. These solids typically are removed from the boiler drum by blowing down the water at the water/steam interface within the steam drum. If the alkalinity is reduced, the amount of blowdown can be reduced (increasing the chloride-ion concentration in the boiler), which further reduces the amount of makeup water required. If the amount of makeup water required is reduced, the amount of energy required to heat the makeup water to the steaming point will be reduced.
Treated fresh water is mixed with condensate-return water, which contains natural carbon dioxide (CO2). If this CO2 were left in the boiler feedwater, carbonic acid would form, resulting in a lower pH. The carbonic acid not only would cause severe corrosion of the condensate-return piping, but be carried over in the steam and corrode the supply and return piping throughout the system. Furthermore, the cold-water supply would contain a significant amount of free oxygen, which would exacerbate the corrosive properties of the liquid in the boiler. While CO2 and free oxygen can be removed chemically, mechanical removal, called deaerification, is much more efficient.
Mechanical removal is possible because as the temperature of the water/condensate mixture rises, the solubility of the CO2, as well as that of any dissolved oxygen, is reduced. A deaerating feedwater heater (Photo B) removes these gasses and, thus, considerably reduces the corrosive nature of the feedwater. In the heater, feedwater is sprayed into a steam atmosphere. The spraying is done in such a manner that the water has the maximum possible surface area so that the temperature rise is as rapid as possible. As the water reaches the saturation temperature, non-condensable gasses are released and vented to the atmosphere.
Treated feedwater is transferred from the deaerating feedwater heater to the boiler by the main feedwater pumps, which deliver 200 gal. of water per minute at a pressure of 150 psig to the boiler drum.
Further boiler-system-efficiency improvement is accomplished with a stack economizer (Photo C). This "coil" is constructed of non-corrosive materials and positioned in the boiler flue. Feedwater is circulated through the economizer, where its temperature is increased by 56 F, on its way to the boiler. This increase in makeup-water supply temperature results in an approximately 4.75-percent improvement in the efficiency of the steam-production process.
Current air-pollution standards typically are not a concern with heating boilers in Northeast Ohio, which is considered a "non-attainment zone." U.S. Environmental Protection Agency (EPA) emission requirements for gas-fired boilers, which are permitted under Title V of the Clean Air Act, are met if the "best available technology" is utilized in burner control. The Cleveland Clinic, however, operates five boilers—two at 135,000 PPH, two at 70,000 PPH, and the new boiler at 100,000 PPH. The total production of 510,000 lb of steam per hour puts the Clinic in the category of large producer, which requires maintenance of an annual emissions limit, established as a certain number of tons of NOx discharged per year. It was determined that total plant emissions would remain below the EPA-dictated limit if the new boiler were operated with average NOx emissions of 30 ppm. Construction of the boiler in conjunction with the operation of the burner controls enabled the boiler/burner manufacturers to provide a guarantee that NOx emissions would not exceed 30 ppm.
A NOx-monitoring system provides a continuous record of emissions for annual verification of compliance with EPA requirements and confirmation of boiler operation within manufacturer-guaranteed limits. If the flue-gas NOx levelwere to exceed 30 ppm, the system would send an alarm to the continuously occupied operator station. This would let the operator know that permissible limits had been exceeded, giving him or her the opportunity to determine if it would be preferable to tweak the burner manually or to shut down the boiler until the reason for the excessive emission levels were known.
Construction of the boiler plant was complicated by the large physical size of the new boiler and associated equipment. Approximately the size of a railroad car and weighing 110,500 lb, the boiler had to be delivered by rail (Photo D). Prior to installation, it was stored on a rail siding approximately 3 miles from the construction site. Before the boiler could be installed, the surge tank (Photo E) and deaerating feedwater heater had to be put in place.
The surge tank and deaerating feedwater heater are located at the lowest level of the boiler plant, beneath the new boiler. They had to be dropped through the first floor onto their concrete pads, with the concrete floor then poured in place. Once offloaded from the railcar and placed on a flatbed trailer, the boiler had to be transported through downtown city streets. This required a full-time police escort and significant planning with regard to traffic lights and other suspended wiring, which had to be protected or moved out of the path of the boiler. Furthermore, normal downtown traffic had to be stopped and/or rerouted to avoid conflict. Upon arrival at the hospital campus, the boiler was lifted off of the trailer and over a pedestrian bridge and placed on a specially constructed structural steel platform, from which it was slid into the plant (Photo F).
The successful completion of the Cleveland Clinic's new steam-production facility was the result of significant commitment by, and coordination among, the members of the design, construction, manufacturing, expediting, and facilities teams. Through the hard work and dedication of many people working very long hours, the Cleveland Clinic was able to obtain the steam capacity necessary to supply the new heart hospital on time and within budget. Furthermore, the successful operation of the new steam-production system provides one small part of the reason for the Cleveland Clinic's continued status as one of the world's leading hospitals.
About the Authors
A longtime member of HPAC Engineering's Editorial Advisory Board, Dennis J. Wessel, PE, LEED AP, is senior vice president of, and director of marketing for, Cleveland-based Karpinski Engineering Inc. An American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Fellow and Distinguished Service Award recipient, he chairs the ASHRAE technical committee on large-building air-conditioning systems, is a member and former chair of the ASHRAE technical committee on tall buildings, and is a member of the ASHRAE Program Committee.
R. Wayne Thomas is a senior project engineer for Karpinski Engineering Inc. A member of ASHRAE and the American Society of Plumbing Engineers, he has 30 years of experience in commercial, institutional, and industrial design.
Frank A. Eisenhower, PE, LEED AP, is a mechanical associate for Karpinski Engineering Inc. A 1994 graduate of The Pennsylvania State University's architectural-engineering program, he has more than 12 years of HVAC experience, with emphasis on infrastructure projects within the health-care industry. In 2000, he received a master-of-business-administration degree from Case Western Reserve University.