A DuPont petrochemical production plant in Orange, Texas, manufacturing ethylene copolymer resins and adipic acid for use in the manufacture of nylon and other polymers, decided to retrofit its fin fan drives. Motivated primarily by the need to streamline maintenance intervals, plant engineers retrofitted fin fan drives from friction-type V-belt drives to synchronous belt drives supplied by TB Wood's Inc., Chambersburg, PA. The retrofit has reduced maintenance, enhanced serviceability and elevated efficiency.
Choosing and Using Fin FansA fin fan is an air-cooled heat exchanger. Instead of dissipating heat into water and then transferring that heat to the air, as with shell and tube heat exchangers and wet cooling tower systems, a fin fan dissipates heat from a fluid directly into the air. Both forced and induction fans are used at the DuPont plant. Forced air fans located below the piping are used to cool quench water, and induction fans located above the piping are used for lower temperature applications.
While fin fans can be powered by a variety of sources -- steam turbines, natural gas or gasoline engines, and hydraulic motors -- DuPont's driver of choice is the electric motor. DuPont's fin fan motors typically are rated at approximately 30 hp and 1,750 rpm, and the average fan's diameter is 12 to 13'.
Fin fan heat rejection does not require a water source. Process applications requiring large amounts of cooling need not be located near a water supply. However, a fin fan's heat rejection ability is linked directly to drive system efficiency.
Previously, fan speed was generated with banded V-belts and sheaves. Eventually, this arrangement proved problematic on several fronts. According to Jay Cormier, field sales engineer for TB Wood's Inc., "A V-belt transmits power from the smooth sidewall of the belt to maintain tension on the sheaves. This makes them prone to slippage and constant adjustment and re-adjustment."
Unfortunately, belt slippage was not confined to normal operation. Slippage at startup frequently glazed the sides of the belt, thereby diminishing the belt's ability to properly grab the sheave groove. Cormier estimated that belt slippage lowered overall fin fan drive output efficiency below 85%.
Increasing EfficiencyDuPont plant operators knew there was a way to increase fin fan drive power transmission efficiency and decrease the amount of time spent adjusting belt slippage on these units.
"We chose TB Wood's synchronous belt drive," explained Doug Collier, DuPont plant engineer. "Although the device is a belt, it has the power transmission characteristics of a chain drive because it creates a positive mesh between the belt tooth and sprocket."
To conserve ground space in refinery and chemical processing applications, fin fans usually are mounted above -- and supported by -- pipe racks, and other equipment occupies the space beneath the pipe rack. This setup presented DuPont with engineering and design challenges when retrofitting because the drive shafts transmitted power up and down from the motor, instead of horizontally. The vertical power transfer configuration made shaft alignment more critical. It is important to align the belt drive so that the teeth are engaged properly with the sprocket.
In this case, however, much of the legwork was performed with design software on a computer screen rather than at the site. The TB Wood's computer belt drive design program was used to engineer an efficient, cost-effective drive. The end result was a system that transmits power via a driver sprocket and driven sprocket to create a 271 driven rpm. At the DuPont plant, a 43" center distance between shafts was required. Lighter sprockets were used to facilitate shaft installation and alignment.
Toothed Belts Exhibit Grace Under PressureThus far, the synchronous drive belts have maintained their tensile strength, high service factor and wear-resistant properties -- even under severe-duty conditions.
"The belt's tensile members are helically wound fiberglass cords that are chemically treated to ensure length stability, maximum flexibility and high tensile strength for shock loads," Cormier noted. The belt's teeth and backing are constructed from Neoprene rubber that resists heat, oil, ozone and flex fatigue.
"Because the teeth mate directly with the sprocket grooves, there is no creepage to cause speed variations," Collier said.
This mating characteristic reduces speed losses and nearly eliminates slack side tension. This attribute reduces overhung bearing loads, leading to improvements in motor and bearing life. A self-lubricating nylon cover also ensures maximum drive efficiency while providing protection against friction-related tooth wear and shear. The positive drive works well in wet conditions, which can occur when water collects around the heat exchanger.
With the fin fans in operation for more than one year, Collier and his staff have experienced a reduction in fin fan-related belt maintenance time. "We used to check and recheck belt drive tension on a regular basis -- about every two weeks per drive. So far, belt inspection intervals on fin fans are performed on a quarterly basis. That's a significant savings in time and labor for us." PCE
For more information from T.B. Wood's Inc., Chambersburg, Pa., call 717-264-7161 or visit www.tbwoods.com.