Free-spinning or wind milling occurs when power to ACHE fans is shut off making them free to rotate backwards. Injuries have resulted from maintenance technicians attempting to use their hands or pressing a 2x4 against the belt as a brake on back-spinning ACHE fans.
Wind milling not only poses a safety hazard to maintenance workers, but also places a strain on motor system components when they power back up.
A simple, economical solution such as the anti-rotation device makes ACHE fans safer to work on while reducing wear and tear on fan system components.
ACHE Fan System Challenges
Air-cooled heat exchangers (ACHE) are an effective means of dispelling or transferring heat from industrial processes. They are typically used in the chemical, petrochemical and oil & gas sectors. Other common ACHE users include food and beverage, power generation, pulp and paper as well as various manufacturing industries.
ACHE fans, also known as Fin-Fans are sized according to the application. A typical ACHE installation might include a large number of Fin-Fan systems operating intermittently to maintain preset temperatures within various processes.
Fin-Fans are typically driven by gears or belts powered by electric motors.
Problems Caused by Wind Milling ACHE Fan Systems
Free-spinning Fin-Fans pose two major concerns of worker safety as well as fan/motor system wear and tear.
A common but dangerous method used by maintenance workers to stop a wind milling fan is to press a 2x4 against the belt to slow it by friction. Workers have also been known to grab the belt to slow it down, only to have their fingers pinched, and sometimes lost between the belt and pulley.
The sudden reversal of motion caused by hard-starting wind milling fans delivers a shock load to the belt, which reverberates through the system affecting fan and motor shafts, bearings, mountings as well as attached equipment.
The first sign of impact is seen in the belt that starts to slip and lose efficiency. Maintaining efficiency requires frequent re-tensioning by maintenance workers who are again faced with the task of bringing the huge, free-spinning fans to a stop.
Often, to reduce the number of times a belt must be re-tensioned, maintenance technicians will set belt tension on the tighter side.
Over tensioning the belt puts a strain on motor shafts, fan shafts and bearings. Over time, these components are weakened or damaged resulting in downtime.
Preventing Hard Starts with Variable Frequency Drives
One solution to avoid 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 reduced frequency results in a soft start, placing lower mechanical stress on motor system components.
But VFDs are costly to acquire and motors with VFD controls will operate at a lower efficiency than non-controlled motors when both are delivering full rated power.
Wind milling ACHE fans need to be secured to prevent their backward rotation when power to the motor is shut off. This will allow safe maintenance on the equipment as well as prevent hard-starting shock loads that can break belts or damage equipment.
Anti-rotation devices consist of a one-way high-torque clutch, which 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 stationary position to normal operating speed.
Draftguard from Gates is an anti-rotation device designed for flexibility and to fit on nearly any ACHE drive. The device is engineered to mount to any bushing commonly used in ACHE belt drive systems.
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 such as Gates Draftguard provide an economical solution to the two major problems created by wind milling ACHE fans, specifically securing the fan drive to stop free rotation and preventing hard starts to minimise damage to the drive components from shock loading.
Engineering assistance with anti-rotation devices for ACHE fan systems is available from Gates Corporation.