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Converting from Steam to GasPUBLIC ACCESS

Some Factory Managers are Reducing Energy and Maintenance Costs by Replacing their Aging Steam Systems with Natural-Gas-Fired Heating Units.

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Mechanical Engineering 120(02), 70-73 (Feb 01, 1998) (4 pages) doi:10.1115/1.1998-FEB-2

Abstract

This article reviews how some factory managers are reducing energy and maintenance costs by replacing their aging steam systems with natural-gas-fired heating units. The benefits of lower energy bills and greater energy efficiency are obvious, but switching from steam to gas heating can also increase worker comfort, boosting morale and productivity. Such bonuses have been gained recently at a CF&I Steel L.P. works in Pueblo, Colo., and a Chrysler Corp. engine facility in Kenosha, Wisconsin. The process of switching to gas can reduce maintenance costs, because gas systems have an inherently less troublesome design than steam-distributed piping systems. A major advantage of infrared h eating systems is that they can provide the same degree of comfort at lower energy costs than conventional forced-air systems and boiler systems such as CF&I’s. Each group of five heaters in the CF&I machine shop was placed in a single thermostat system with a 24-hour timer as an additional measure to ensure efficiency.

Article

Manufacturers enthusiastically herald design improvements to their process heating systems, especially those that, reduce fuel costs such as capturing heat from a combustion stage to preheat combustion air. Just as important to cutting energy bills is the replacement of aging boiler- based steam heating systems with more-efficient natural- gas-fired heating systems. The benefits of lower energy bills and greater energy efficiency are obvious, but switching from steam to gas heating can also increase worker comfort , boosting morale and productivity. Such bonuses have been gained recently at a CF&I Steel L.P. works in Pueblo, Colo., and a Chrysler Corp. engine facility in Kenosha, Wis.

"There are two major reasons for these steam-to-gas conversions: the controllability and performance improvements that gas heating systems offer," said Michael Connor, a mechanical engineer and a principal in Talka & Connor Architects and Engineers in Alpharetta, Ga. He also chairs the Technical Committee on Industrial Air Conditioning of the American Society of Heating, Refrigerating, and Air Conditioning Engineers.

According to Connor, gas heating systems once operated on a strictly on-and-off basis, which limited their appeal to applications where that basic control was the only one needed. "However, when gas- equipment manufacturers developed gas systems with electronic controls, they enabled system operators to adjust with more precision the amount of heat they generate."

Switching to gas can also reduce maintenance costs, because gas systems have an inherently less troublesome design than steam- distributed piping systems. "Steam systems are notorious for leaking, because of air vents in traps that do not always fully close when the system heats up, causing thermal as well as chemical losses that must be made up for by burning more fuel," Connor said. "This also incurs maintenance costs to identify and repair the trap vents."

All steam systems have to be vented and drained periodically. Pipe must be insulated to keep steam at a desired temperature until it reaches the space to be heated. In addition, the water fed to steam heating systems must always be chemically treated to prevent it from scaling and corroding pipe interiors.

By contrast, a gas heating system. does not require descaling treatment, and leaks are rare and readily apparent. There are no venting or draining issues, and insulated piping is not needed, because unheated fuel is delivered to heaters located at the point of use.

Heat for Historic Steel Works

"Manufacturers and processors often switch to gas after taking a long hard look at the costs of an aging steam system," Connor said, "because age aggravates the problems inherent in steam heating systems." This was the case at CF&I's steel plant. Each year this plant produces more than 900,000 tons of rails, seamless tubes, and rod-bar products from two 150-ton electric furnaces and two continuous casters. In 1987, boiler-shop engineers were charged with improving the performance of the 30- year old steam heating system.

The CF&I engineers first 'analyzed the steam systems used to heat the Pueblo plant's 25,000-squarefoot internal machine shop, 5,035-square-foot pipe shop, and 10,750-square-foot weld shop, all housed in buildings constructed in 1910 of uninsulated metal and brick with flat metal deck roofs. They found that replacing the old pipes that carried steam into the three buildings from huge boilers in the CF&I complex would not be economical.

The aging boilers and pipes were inefficient. The machine shop alone was losing 4 million Btus, because the steam systems was not a closed-loop central heating system but an arrangement where steam was piped into insulated radiators and fans located above the floors of the machine, pipe, and weld shops.

The heating also was uneven and was not comfortable to employees-a matter of some concern in Pueblo, where indoor temperatures averaged 40°F during the fall and winter. Machine-shop workers were forced to wear layers of clothes and insulated jackets to keep warm.

Having eliminated boiler replacement because of its cost, the CF&I engineers examined natural-gas-fired systems, including forced air heat, unit heaters, and radiant infrared heaters. They chose the RSTP-17 infrared heater, manufactured by the Space-Ray division of Gas-Fired Products Inc. in Charlotte, N.C., to heat the three buildings.

A major advantage of infrared h eating systems is that they can provide the same degree of comfort at lower energy costs than conventional forced-air systems and boiler systems such as CF&I's. "Facilities with forced-air and boiler systems must spend energy dollars constantly heating, circulating, and reheating cold air, because as hot air rises, it is constantly displaced by cooler air at floor level and must be recirculated," said Bob Genisol, a mechanical engineer and vice president of sales at SpaceRay. Genisol said that industrial radiant heating is normally at least 30 percent more efficient than forced air, and it can save up to 70 percent on annual fuel costs incurred by steam boilers in very large installations.

Space-Ray's infrared heating systems had proven themselves in a wide variety of industrial applications before the CF&I project. Levi Strauss & Co. saved $250,000 in gas and electricity costs over five seasons by replacing 20 forced-air heaters in its Florence, Ky., warehouse and distribution center with Space-Ray heaters. Similarly, Parkway Systems in South Amboy, N.]., cut the heating expenses of its wet-suit fabrication plant nearly in half by replacing an oil-fired heating system with Space-Ray heaters. Natural gas is piped to the Space-Ray unit , where it is regulated by a combination gas valve. A direct-spark ignition system equipped with an automatic 100-percent safety shutoff ignites the gas. Each heater contains a cast-iron burner for precision performance and complete combustion. The burner raises the temperature of the infrared emitter, a special heat-treated and calorized steel tube that maximizes heating distribution efficiency. The tube is surrounded by a polished aluminum reflector that directs the tube's radiant heat to the area being heated. A vacuum pulls the products of combustion out of the heaters so they can be vented outside of the building through a flue pipe or ductwork. One reason CF&I chose the Space-Ray heaters is their ability to provide even heat distribution. Genisol credited the heaters' U-shaped design for this. "Many other tube heaters are equipped with straight tubes that concentrate heating primarily on the burner end of the unit with little heat in the middle and at the exhaust end," he said. The greater radiant efficiency of the U-shaped tubes provides the needed radiant energy to warm the floor. "We also maintain a nearly uniform 900°F tube temperature throughout the unit," he added, "rather than 900°F at the burner end and 300°F at the exhaust end, which is common in many straight units." Other advantages CF&r saw in the Space-Ray infrared heaters included lower installation costs and simpler rearrangement within the factory, due to their individualized design. Because they can operate independently of other heaters in the system, the Space-Ray units can be thermostatically controlled, individually or in groups, giving plant engineers more precise control of heating requirements. Facilitating online diagnosis was another consideration for Space-Ray engineers, who equipped each unit with three monitoring lights to facilitate online diagnosis, even when the unit is mounted 68 feet above the factory floor. "Each light respectively alerts maintenance workers in the event of a malfunction in the ignition system or an obstruction in the air flow, and shows whether sufficient power is supplied to the unit," Genisol said. Workers in CF&I Steel's machine shop are kept comfortably warm during chilly Colorado winters by gas-fired infrared heaters (left) that replaced an aging steam-boiler system. Water Heats and Cools the Factory WHILE EXISTING INDUSTRIAL facilities are improving heating efficiency by switching from steam to natural-gasfired heating systems, some recently built factories are installing industrial heat pumps to optimize their heating and cooling operations. Mi-T-M Corp.'s pressure washer factory in Peosta, Iowa, for example, uses a radiant floor heating system. The 240,OOO-square-foot plant was built in 1995, when the business outgrew the two buildings that originally housed the equipment for manufacturing Mi-T-M's products for cleaning the exteriors of buildings and the insides of swimming pools. Mi-T-M constructs its Clean Master, Chore Master, and Work Pro washers out of reinforced V4-inch steel base plate that is powder-coated to impart extra strength. Mi-T -M management considered alternatives to the natural-gas-fired systems that were used in the older buildings. At that point, Mi-T-M's utility, Interstate Power Co. in Dubuque, Iowa, proposed a GeoExchange heat pump system designed by Terra-Therm Inc. in New Richland, Minn., to heat and cool the new building. Terra-Therm is a full-service geothermal-heat-pump distributor that designs and installs the pump systems, and provides needed accessories such as radiant flooring. The Terra-Therm engineers installed 18 industrial heat pumps made by Econar Corp. in Elk River, Minn., at the Mi-T-M facility. Each pump has a steel housing that contains an electrically powered compressor and insulated copper coils, which are in contact with each other to provide heat exchange but insulated to prevent heat loss, according to Terra-Therm chairman of the board Dick Westrum, a mechanical engineer who oversaw the installation. One coil contains the refrigerant; the other contains water that feeds the closed loop of the GeoExchange system. During winter months, the compressor pressurizes the refrigerant, raising its temperature. This thermal energy is transferred to the water coil so that it can send warm water into the building. The heated water is then sent through 20 manifolds and courses through 60,000 feet of %-inch-inside-diameter cross-linked polyethylene tubing manufactured by the Wirsbo Co. in Minneapolis. The tubing was buried inside the newly poured concrete floor of the Mi-T-M plant, so the radiant heat from the tubing system warms the factory interior. The heat pumps' performance is augmented by a 16-foot-deep, 2-acre pond that collects rainwater from the new factory's roof and parking lot. "We decided to use the pond as an economical heat exchanger for the closed-loop heating system by sinking 72 pieces of 3f4-inch polyethylene tubing, each 400 feet long, into the pond," Westrum said. Water circulated through the tubing picks up heat during the winter, typically going from 30°F to 38°F, before it is sent to the heat pumps. Compression raises it to more than 100°F, which drops to 80°F in the building's floor tubing. In the summer, the compressor's action is reversed to expand the refrigerant, vaporizing and cooling it. This cools the circulating water coil so that water at about 40°F can remove heat from the building and shed it in the pond, reducing the load on the air-conditioning units. According to Interstate Power, which has conducted extensive monitoring of the system since its completion in January 1995, the costs of the GeoExchange radiant floor system are 40 percent less than those of a natural-gas-fired system. The utility estimates payback for the system, including a$42,750 rebate, within five years.

The recirculated water from a pond loop cools the powder-coating area in Mi-T-M's pressure washer factory, even in winter, when it is needed because of heat from the curing oven.

Positioning for Optimum Efficiency

The installation at the CF&I plant began with a heat-loss analysis to determine where and how many infrared heaters were needed. "This was a challenge because the buildings were uninsulated and the original building plans were so old ," Genisol said. "They were the first blueprints on silk that I had ever seen." The Space-Ray engineers relied on their proprietary CompuHeat software to perform heat-loss analysis, a fuel cost estimate, payback analysis, and a 10-year life-cycle analysis.

Based on these findings, Space-Ray recommended the installation of 35 of its 175,OOO-Btu RSTP-17 units at the Pueblo site in September 1988. Because the three metal buildings were not insulated, the tube heaters were vented to the outside of the buildings to prevent condensation from forming inside.

In each shop, the Space-Ray heaters were installed to suit the work environment. For example, in the machine shop, where 25 heaters were installed, most employees work around stationary equipment such as lathes; this allowed the heaters to be positioned to provide the optimum heating efficiency. Space-Ray work crews mounted the h eaters on chains and installed them 1 foot from the ceiling of the machine shop. They positioned the RSTP-17s horizontally or at a 30-degree angle, depending on the work position.

was placed in a single thermostat system with a 24-hour timer as an additional measure to ensure efficiency. This system maintains temperatures at 68°F from 7 A.M. to 6 P.M. Monday to Friday. Night and weekend temperatures are kept at 58°F The system starts up at 5 A.M. to ensure that the shop will be warm enough in time for when the first worker arrives.

Eight heaters were installed in the weld shop by means of angle brackets and chain that suspended them 14 feet from the 30-foot ceiling. Thermostatic controls in both buildings maintain daytime and nighttime temperatures. Two units were installed in the pipe shop. During the first two heating seasons from 1988 to 1990, according to CF&I's calculations, the company saved more than $230,000. Payback for the infrared system was accomplished in less than one year. The heating season at the Pueblo steel works begins in October and runs through April, with average winter temperatures of about 30°F, but it is not uncommon for the mercury to drop below 10°F Instead of bundling up, CF&I workers in the infrared-heated shops now work in comfortable clothing. Warm, Fresh Air Auburn Hills, Mich.-based Chrysler decided to replace the steam-boiler heating system at its Kenosha engine facility in 1994. The idea was to improve ventilation of the structure and upgrade air quality as well as take advantage of the greater efficiency and lower maintenance burden of gas heating compared with steam heating. The facility machines and assembles the 2.5-liter four-cylinder and 4.0-1iter six-cylinder engines for the Jeep line as well as a 2.7-liter V-6 engine for the new Dodge Intrepid and Chrysler Concorde. This heating retrofit would be the second for the Kenosha site in the last 10 years. Chrysler had already saved$1 million in 1991 by replacing four 120,000-pound-per-hour boilers with four modern Clayton steam generators, each capable of producing 17,000 pounds of steam per hour. Despite this improvement, the new steam system could not furnish the desired air-quality improvements, nor could it provide positive pressurization or added air exchanges. In addition, like all steam systems, the distribution system at Kenosha required costly high maintenance on all supply and return piping systems.

Chrysler plant engineers determined that direct-fired air-handling units offered the highest thermal efficiency of all gas-fired systems, and they selected the industrial heating and ventilating equipment designed and manufactured by Weather-Rite Inc. in Minneapolis. This equipment offered the benefits of increased quantities of outside air and filtration for recirculated plant air. More outside air and filtration provides the workers with a cleaner, healthier work environment.

A major challenge of the Kenosha installation was the varying structural designs of the buildings that comprise the facility, which covers 1.3 million square feet. "We wanted to take advantage of the economics of scale offered by Weather-Rite, but we had to minimize the unit-weight impact on the older building structures," said Leon Fasano, a mechanical engineer and plant engineering supervisor at Kenosha. Fasano said the answer was to reinforce the trusses to support the 10,000-pound roof top units.

A total of 93 Weather-Rite air handlers were installed at the facility from 1995 to 1997. Each unit provides 30,000 cubic feet of air per minute. Damper systems within the unit mix filtered and heated outside air with filtered return air and discharge the mixture into the building to maintain a slightly positive pressure. Individual units are controlled by a programmed process controller on the unit that monitors outside-air, return, and space temperatures. The unit controllers are linked to a local network processor controlling the individual building units and exhaust fans. The network processors are linked to an engineering personal computer with Johnson Metasys software, enabling Chrysler's Kenosha engineers to monitor unit status and change set points, as well as turn units and exhaust fans on and off.

Within the building, the air is discharged and returned at the 14-foot level to keep fresh and filtered air at the worker level. The cycle loops are kept to no more than 75 feet between supply and return, ensuring adequate air exchange at the lower level without creating drafts. Excess stagnant air is purged by limited exhaust at the ceiling level. During the summer, the units deliver 100-percent outside air with no return, and their exhaust operates at full capacity, lowering the temperature in the space and providing the workers with fresh air and some cooling.

"These changeovers from steam to gas will continue as long as gas rates remain low, and as long as aging steam plants are being replaced," Connor said. "Virtually always, if a facility has a large floor space, it will be converted." However, there will still be plenty of work for steam heating systems in many manufacturing and processing facilities, he noted, "either where there is a process requirement for steam, such as with pulp and paper, or where there are small points of use, for example a lab building that requires individual room reheat control."

Burners within the Weather-Rite air handlers are designed to contact only outside air, to prevent contaminant buildup inside the buildings they heat.

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