If your solution to stopping a back-spinning air-cooled heat exchanger (ACHE) fan is to apply your hand to the belt as a brake, think twice! Maintenance technicians who try this method injure fingers. Pressing a 2×4 against the belt is another non-conventional, and dangerous, way to slow a free-spinning ACHE fan.
Free-spinning (or ?wind milling?) occurs when ACHE fans are powered off, becoming free to rotate backwards. In this condition, fans pose a safety hazard to maintenance workers and place a strain on motor system components when they power back up. There is an economical solution to this problem. It?s called an anti-rotation device, and it makes ACHE fans safer to work on while reducing wear and tear on fan system components.
ACHE FAN SYSTEM CHALLENGES
ACHE fans dispel or transfer heat from industrial processes. They are typically used in the chemical, petrochemical and oil and gas sectors and various other manufacturing industries.
ACHE fans (also known as ?Fin-Fans?) vary in size depending on the application. In refinery operations, Fin-Fan units may be nine metres off the ground with fan blades ranging from two metres to five metres in diameter. A typical ACHE installation might include large numbers of Fin-Fan systems operating intermittently to maintain pre-set temperatures within various processes.
Fin-Fans are driven by gears or belts, either V-belts or synchronous belts, powered by electric motors. Indeed, synchronous (timing) belt drives have been replacing V-belt drives to increase efficiency.
WIND MILLING PROBLEMS
On an idle belt-driven ACHE, the force of air moving over or beneath the blades allows the belt to rotate backwards in the sheave or sprocket. This ?wind milling? effect might be caused by other fans in the system or by environmental factors such as wind. Wind milling Fin-Fans pose two major concerns for worker safety and fan/motor system wear and tear.
Worker Safety
Performing maintenance in the hot, cramped space around an ACHE unit nine metres above the ground with a freely rotating shaft poses a threat to the safety of technicians.
A common method used by maintenance workers to stop a wind milling fan is to press a 2×4 against the belt to slow it by friction. This method is fraught with danger. Workers have also been known to grab the belt to slow it down, only to have their fingers pinched, and even lost, between the belt and pulley.
The issue of worker safety is compounded when maintenance crews re-tension belt drives or replace damaged equipment.
System Wear and Tear.
Of and by itself, a wind milling fan poses no threat to the system. However, when the fan is powered up, the sudden reversal of motion is like shifting a forward moving car from drive into reverse gear. The belt receives a shock load that reverberates through the system?fan and motor shafts, bearings, mountings and attached equipment.
On V-belt drives, hard starts glaze the walls of the belt and pulley, causing the belt to slip and lose efficiency. Maintaining efficiency requires frequent re-tensioning?not easy for maintenance workers who must expose themselves to the elements and wrangle huge, free-spinning fans to a stop. Often, to reduce the number of times a belt must be re-tensioned, they will set belt tension on the tight side. Over tensioning the belt strains motor shafts, fan shafts and bearings. Over time, these components are damaged.
Ultimately, the belts, motors or bearings fail. When the drive stops, so does the cooling of the associated process. The operation must be shut down until repairs are made. This downtime is costly, particularly for facilities that operate 24 hours per day, and the failure may occur at times when maintenance crews are not readily available.
PREVENTING HARD STARTS
A solution to avoiding shock load when starting up a wind milling ACHE fan is to install a variable frequency drive (VFD). A VFD regulates motor speed electronically by changing the frequency of the alternating current supplied to the motor.
Starting a motor with reduced voltage and frequency results in a ?soft start?, placing less mechanical stress on motor system components. However, VFDs are costly to acquire and will operate at a lower efficiency than non-controlled motors when both are delivering full rated power.
While VFDs may ease wear and tear on belt drive systems, they are costly, require additional wiring and do not solve the safety issue of wind milling ACHE fans.
ANTI-ROTATION DEVICES
Clearly, a method is needed to secure wind milling ACHE fans and prevent their backward rotation while power to the motor is shut off. This method is needed both to allow safe maintenance on the equipment, and to prevent hard starting shock loads that can break belts or damage equipment. The answer is the anti-rotation device.
Anti-rotation devices comprise a one-way clutch that allows the ACHE fan shaft to rotate in one direction only. When the motor is shut off and the fan system slows to a halt, the clutch prevents the ACHE fan shaft from rotating backwards when air currents circulate over the fan blades. When the motor is turned back on, the system accelerates from a standstill to normal operating speed.
Gates designed Draftguard, an anti-rotation device, to be flexible and to fit on any ACHE drive. The core of the product is a high torque, one-way clutch. The device is engineered to mount to any bushing commonly used in ACHE belt drive systems, including QD and Taper-Lock bushings or directly to the shaft.
The torque arm mounts to the frame to prevent accidental rotation of the ACHE fan shaft during maintenance. The device adapts to clockwise or counter-clockwise rotation, and fits within most fan cages or guards without modification. Bearings are greased for life, so the device requires no maintenance.
Anti-rotation devices like the Draftguard provide an economical solution to the two major problems created by wind milling ACHE fans. From a safety standpoint, they secure fan drives and prevent them from rotating freely when not receiving power, allowing maintenance technicians access to the fan cage without risk of injury. They also prevent hard starts by allowing fan drives to power up from a neutral, standstill position, minimising damage to drive components.
Engineering assistance with anti-rotation devices for ACHE fan systems is available from Gates Corporation.
Source: Gates Australia