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Vent Condenser Recovers Heat

STEVEN J. TEETS
Associate Energy Coordinator Western Electric Company, Columbus, OH
JESSE D. BOAK Contributing Editor

PROBLEM
Because nearly all of the natural gas used at Western Electric's Columbus, Ohio plant is burned as boiler fuel, concentrated efforts were made to improve boiler plant efficiency. A boiler feedwater economizer was installed in the heat stack. Also, oxygen trim, automatic blow-down, and variable speed draft controls were added, along with a high-efficiency single burner with a high turndown ratio which was retrofitted into the primary boiler.

An additional opportunity to control wasted energy was obvious by visual observation of steam escaping to atmosphere through the vent pipe from the de-aerator. A solution to this problem was sought.

The Western Electric Company's Works is the largest single-story manufacturing plant in central Ohio. The plant facilities are dedicated to the manufacture of telephone switching equipment for the world. Almost two million sq ft of floor space is used by 6,000 employees of Western Electric and Bell Laboratories to design and manufacture relays, electromechanical and electronic switching equipment, and circuit boards. The facility is involved in metalworking; plastic extrusion molding; heat treating; electroplating operations; and assembly, wiring, and testing of equipment. Also, numerous computers are used to generate the software required to operate electronic telephone support systems.

The function of the boiler facility is to provide steam at 125 psig for building heating, plating processes, heating of domestic hot water, and cafeteria use. The 125-psig steam is generated by three 65,000 lb/hr boilers and appurtenances, but normally only one boiler is required to meet the plant's load.

Also, 250-psig steam is generated to provide process requirements for the molding operations. This is generated by three 3500 lb/hr boilers and appurtenances, with only one boiler normally required to meet the molding operation's demand.  The de-aerating feedwater heater is of the tray type, mounted over a 9'-dia by 32'-overall-length storage tank. The heating and de-aerating capacity of the unit is 195,000 lb/hr at the saturation temperature of the steam supply. The storage capacity is 95,000 lb. at normal working level with 20,000 lb surge capacity between the operating level and the overflow. The normal operating temp is 225 F, corresponding to saturated steam at pressures of 3 to 7 psig.

Condensate returns to the de-aerator from the traps via condenser receivers. Part of the de-aerator steam is supplied by the flash from the boiler's continuous-blow-down system. Makeup steam is available from pressure-reducing valves off the main steam line.
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The function of the de-aerator is to heat the condensate and makeup water to approach the temperature of steam within the shell and to remove dissolved gases. In water or condensate the presence of dissolved gases (oxygen and carbon dioxide in particular) causes accelerated corrosion.

The oxygen corrosion rate increases with increasing temperature, but conversely, CO2 corrosion increases as temperatures decrease, to form carbonic acid in boilers and heat exchangers. It is the primary function of the de-aerator to prevent this corrosion by removing the dissolved gases from all sources of water supply to the boilers and heat exchangers.

SOLUTION
An investigation was made to determine the best possible method to eliminate the wasted vent steam. The solution chosen had to satisfy the company's required internal rate of return, and needed to provide for ease of installation and low maintenance.

A heat exchanger, which would function as a vent condenser, was chosen for this application. Installation was a simple procedure. A section of the vent pipe was cut out and the vertical (in line) heat exchanger was inserted. A small portion of the makeup water was diverted for use by the vent condenser; then returned as hot makeup. A separate condensate line also was added. Total installation costs were $4785.

The heat exchanger is designed specifically for heat recovery as a vent condenser. It has three shell passes and two helical tube passes. Installation is vertical in-line with a weight of 47 lb. The helical flow paths produce the centrifugal effects that enhance heat transfer and separate condensate. Thus, it develops higher heat transfer efficiency than is normally obtainable with a straight tube design of the shell-and-tube sheet type. The resulting hot water makeup is introduced into the de-aerator for use as boiler feedwater.

RESULTS
The heat-recovery process from the vent steam results in approximately 200 F makeup water, which is sent directly to the de-aerator The condensate is recovered at a temperature of 195 F and returned to the plant's condensate- return system.  The process has eliminated steam energy heat loss and, at the same time, the environmentally undesirable steam plume. With 24 hr/day, 7 day/week operations, the annual cost savings resulting from the project are $7300. The company is pleased with the results.

Reprinted from PLANT SERVICES