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.

A significant requirement of the plant (Photo A) concerned the production of steam. The design engineer for the heart hospital advised the design engineer for the plant that 70,000 lb of steam per hour (PPH) would be required for heating, humidification, domestic-water heating, and instrument sterilization. To provide additional steam redundancy, the installation of a 100,000-PPH boiler was authorized.

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.