Boiler House Chester Electric Power Station - PECO Energy, Chester Pennsylvania
Activity in the Boiler house revolved around the primary task of providing the energy needed to convert water to steam. The physical center of this operation was the boiler, which at Chester marked Philadelphia Electric's first use of the Stirling-type boiler manufactured by Babcock & Wilcox. Pioneered by Allan Stirling in the late nineteenth century, the Stirling boiler offered "quicker steaming, greater ease of cleaning, improved fuel efficiency, and more efficient use of space," compared to other models; its design for rapid circulation of steam also reduced the likelihood of boiler explosions. The success of the Stirling Boiler Company (later Stirling Consolidated Boiler Company) attracted the attention of established boiler manufacturer Babcock & Wilcox, which purchased the company in 1906. Philadelphia Electric ordered more conventional cross-drum boilers for Schuylkill A-2 in 1913, but the greater capacity of the Stirling boilers installed at Chester (125,000 lb/hr evaporated steam v. 60,000 lb/hr for the cross-drum boiler) permitted use of fewer boilers per turbine. A bank of four boilers supplied steam to each turbine at Chester as opposed to the bank of ten needed for each turbine at Schuylkill A-2.
In addition, the Stirling boilers at Chester had integral economizers, which used exhaust heat from the furnace to increase the temperature of feed water before it entered the boiler. Economizers gained wider use during World War I as engineers sought new means of offsetting the rising cost of fuel by improving plant efficiency. Into the 1920s, boiler engineers debated the value of installing economizers instead of simply increasing the surface area of the main boiler tubing. However, Philadelphia Electric's experience with Chester consistently led the company to install boilers with economizers in its plants. The growing popularity of economizers coincided with a period of widespread adoption of surface condensing steam systems in central stations. Many early plants utilizing condensers, such as Schuylkill A-l, opted for jet or barometric types that discharged both turbine exhaust steam and cooling water as waste water. After converting steam exhausted from the turbines to water, surface condensers returned it to the boilers as part of a closed, "feed-water" system. The chief design engineer of Chester Station, W.C.L. Eglin, observed that the surface condenser was a pivotal development in maximizing steam produced by a given amount of fuel; analysis of the closed feed-water system led to the recovery of energy lost in the steam generation process through regenerative heat cycles. One method used at Chester to recover heat employed an open-type heater to raise the feed-water temperature prior to entering the boiler, thereby decreasing the amount of heat input required to generate steam. In the years following the initial phase of construction at Chester, more attention was paid to decreasing fuel consumption through improving plant efficiency, commonly measured in pounds of coal consumed per kilowatt hour.
As World War I had shown, utilities could be hard hit by sharp increases in the price of coal. The availability of coal - threatened by a coal shortage in the winter of 1917-1918 and several strikes, notably in 1919, 1922, 1946, proved another concern. Coordinating fuel pricing, procurement, distribution, and storage, for a large urban territory presented other problems. Philadelphia Electric took several steps to ensure a regular and rationalized supply of coal. First, the company purchased Petty's Island, located in the Delaware River near northeast Philadelphia, for coal storage. Second, company engineers drew up specifications for Chester's boilers that would allow the plant to burn coal, oil, or gas. This flexibility with regard to fuel proved farsighted. In 1946, a coal strike led to Philadelphia Electric to convert several of Chester's boilers to oil-firing operation, and the 1950s brought another conversion for natural gas. Third, Chief Engineer Horace Liversidge (later the utility's president) created a "Coal Bureau" to centralize control of all coal purchasing and distribution under one department in 1922. Formerly, the separate stations arranged for coal deliveries and managed stockpiles individually. The Bureau was formed as a distinct agency to coordinate company coal policy and to ease conflict that periodically arose between stations short on fuel. Philadelphia Electric workers assigned to specific stations considered Coal Bureau personnel outsiders, and little interaction occurred between the two workforces.
Coal delivery also figured prominently in the site selection and final layout of Chester Station. Designers placed the boiler house along the river to facilitate coal loading and ash removal. The main disadvantage of this arrangement was the increased distance between turbines and river water, necessitating longer intake and discharge tunnels for the circulating water system. At Chester, engineers attempted to solve this problem and improve water access by locating the plant well into the river.
This strategy had several ramifications. On one hand, it increased coal storage space behind the plant. On the other, it precluded construction of the tilted conveyor system used to transfer coal from barges to storage bunkers at Schuylkill and elsewhere. Gone, too, was the conventional coal-loading dock. Instead, a pair of "coaling channels" flanked Chester Station. After entering the channels, coal barges were secured to a motorized pulley system controlled by hoist operators in the coal towers. From a seat high above the water, the operator maneuvered the barge and lifted coal with a grab bucket to the tower's upper floor. Because most plants used belt-conveyor systems rather than hoists to raise coal to bunkers, some unusual factor may have driven the decision to adopt an alternative arrangement at Chester. The close proximity of boiler house to river, the use of coaling channels, and the tower-hoist system could have been intended to provide a common access point to coal in the event that another power plant was added inland from Chester (a possibility anticipated in the company minutes and a Coal Bureau manual). However, the design proved expensive. Later stations maintained Chester's boiler-side-to-water orientation while reverting to more conventional coal delivery systems.
The coal towers served as points of entry for the gravity-feed distribution system. After passing through sorters and crushers, coal was guided by hopper into cable-driven cars that followed a set of tracks to one of two main bunkers. The distribution system was designed to maintain a constant supply of fuel in the event that one tower hoist failed. Below the bunkers, feed pipes distributed coal to mechanical stokers in the firing aisles of the boiler house. When the installation of two larger, high-pressure boilers in 1939 boosted the plant's coal consumption, Philadelphia Electric replaced the tracks serving the downstream tower with a belted conveyor system that fed the bunker over the new boilers. The original system continued to operate in conjunction with the upstream coal tower.
Chester's original boilers were equipped with Taylor underfeed stokers that mechanically fed the furnace with coal. Efficient combustion, often gauged by the quantity of smoke in the exhaust, depended on careful control of air and fuel. Earlier hand-fired boilers depended on the skill of the fireman for smokeless combustion, a task replaced by mechanical stokers in larger central stations between 1900 and 1915. Uniform control of the feed-rate enabled engineers to test boiler performance by monitoring the supply of coal and air. As new methods of improving boiler performance were introduced, testing became an increasingly vital means of measuring plant efficiency. Philadelphia Electric implemented a systematic program to weigh the benefits of the latest auxiliary boiler technologies as part of the second major phase of construction at Chester between 1923 and 1924.
During its initial five years of operation, Chester ran at only half of its intended capacity. Most of Philadelphia Electric's post-World War I construction funds were allocated for completing the Delaware Station, and the post-war recession dampened demand for electricity. By 1923, company revenue had increased sufficiently for managers to advance plans for bringing Chester Station up to full capacity. Eight more Stirling boilers, two turbogenerators, and the concomitant switch gear were needed. This upgrade coincided with an interesting period of experimentation with innovative technologies in the boiler house, including air preheaters and water-cooled boiler walls.
Air preheaters transferred heat from exhaust gases to air entering the boiler furnace, reducing both the temperature of stack emissions and the heat input needed for combustion. Although air preheaters appeared to be a promising technology, their effectiveness could not yet be measured with any precision. Because boiler-house conditions often varied widely between utilities and even among stations, weighing such variables as initial cost, impact on boiler maintenance, and overall effect on boiler efficiency had proved problematic. With "great difficulty," Philadelphia Electric equipped several of the 1924 boilers with different types and sizes of air preheaters in an effort to determine the best combination. Two boilers, numbers 7 and 8, had particularly unusual configurations. Neither had an economizer (standard on all other boilers); an experimental Ljungstrom rotary preheater was fitted to No. 7, and No. 8 had a tubular preheater nearly twice the size of those installed on the other boilers.
Boilers 7 and 8 continued to serve as trial units in later years, as Philadelphia Electric experimented with several other innovations and modifications such as Bailey water walls on these units. The water walls at Chester were retro-fitted into the refractory brick walls of the furnace interior to reduce wall temperatures and provide further heating area for feed water. Water walls proved a successful experiment and later became a crucial part of boiler design as boiler manufacturers turned to pulverized coal to achieve higher volumetric combustion rates. A Babcock & Wilcox steam manual noted the importance of utilities' early experiments with water walls: the advantages gained by firing pulverized coal "could not have been fully exploited without the use of water-cooled furnaces" that prevented the rapid deterioration of the refractory brick caused by slag (molten ash) build-up at higher temperatures. Other utilities likely found Philadelphia Electric's published comparisons of air preheater performance and Bailey water walls informative, as the reports identified problems and advantages uncovered over several years of testing.
Efforts to improve boiler efficiency through modification and the addition of auxiliary equipment were characteristic of progress in plant engineering during the prosperous 1920s. But even as the U.S. economy struggled through the Great Depression, Philadelphia Electric placed new boilers in service that generated steam temperatures and pressure unattainable at Chester Station. The plant's relatively low efficiency compared to other parts of the system had significant ramifications for workers. Philadelphia Electric shut down Chester for two extended periods during the 1930s, canceled the employee wage dividend program, Christmas bonus, and laid off workers. Although the plant continued to contribute to Philadelphia Electric's total system capacity during the Depression, in just over ten years the primary generating technologies at Chester were fast becoming out of date.
Rather than condemn Chester as obsolete, Philadelphia Electric turned to a technique, developed in 1925, known as superposing or "topping." Superposition recouped the large initial investments made in older technologies by coupling newer high-pressure systems to the existing low-pressure units. Steam entered Chester's original low-pressure turbines, units 1 through 4, at approximately 230 psi and 600 degrees Fahrenheit. Essentially, the topping turbine received steam at a higher pressure and was designed to exhaust it directly into the lower-pressure turbines at the required inlet conditions. By this method, older units could be made a useful part of a system that gained advantages from the higher pressures and temperatures permitted by newer designs and materials; the modification gave the plant "a new lease of life, and protect the older investment from the insidious disease of obsolescence."
Philadelphia Electric initiated a superposition program to upgrade the aging Chester Station just prior to World War II. In conjunction with the topping unit, two massive, open-pass, pulverized-coal-fired, high-pressure boilers were installed, replacing the station's four oldest boilers. This new equipment brought steam up to the temperature and pressure that the topping turbine required. In Philadelphia Electric's Current News, company reporters noted the project's progress and photographers showed the installation of the two steam drums; "the largest ever built by the well-known firm of Babcock & Wilcox." Several less spectacular changes also accompanied the large-scale refitting. In order to supply the new boilers with large quantities of coal dust, Philadelphia Electric added the aforementioned coal conveyor system and a pulverizer. These World War II-era modifications were the last significant changes made to the steam production system at Chester.
Conversion to open-pass, pulverized-coal-type boilers followed a path taken by many utilities with large central stations, as had the adoption of the Stirling boilers in the original plant design. If the primary boiler technologies used at Chester were fairly typical of the industry, the efforts of Philadelphia Electric engineers to improve the economy and efficiency of the boiler operations were bolder. Through centralizing and streamlining coal distribution, systematically testing feed rates, coal chemistry and combustion temperatures, and experimenting with auxiliary technologies to utilize waste heat, Philadelphia Electric engineers devised their own means of reducing a utility's problematic vulnerability to the price of fuel.
In addition to fuel costs, which constituted a significant percentage of operating expenses, boiler technology was closely tied to the progress of steam turbine technology. Because boiler output was limited by the capacity of turbines to withstand high temperature and pressure steam, boilers and turbines developed symbiotically. As new materials and designs increased that capacity, boiler manufacturers introduced models that produced steam with commensurately higher temperatures, pressures, and flow rates. By World War II, the growth of boilers, both in sheer size and in performance, paralleled that of steam turbines. And that was no small feat. Turbines exhibited meteoric gains in output and capacity during these years; their evolution influenced the history of power industry in the United States.