With any thermal oxidizer system, the fan, along with the VFD is the "heart" the drives the system.   Quality fans are typically robust.  In most common industrial applications, with proper periodic maintenance (PM), they operate with high reliability and few problems. 

Clearly, the first step in fan performance in an oxidizer system is proper selection; every fan has an individual performance curve. Fans selection is based upon the operating conditions; specifically, total air volume of the process, temperature, and calculations of and capacity required by the system, including ΔP across the entire system and its ductwork. The fan will move air based on its design and the power, measured in brake horsepower (BHP), required to operate. Brake Horsepower is calculated from the ACFM (Actual Cubic Feet/Minute), static pressure, a compressibility factor, and the fan's own static efficiency. Care should be taken to account for and adjust to any anomalies in an existing system and to avoid similar challenges in new systems. A critical factor is to manage air load distribution at the inlet cone to ensure there is even airflow and distribution without introduction of an eccentric or spinning flow. This can be avoided through duct design and potentially mitigated by in-duct modifications including turning vanes and straighteners.  Using fan curves provided by and in constant consultation with the fan manufacturers, NESTEC selects the optimum fan required by the conditions.  Additionally, and importantly, is the sizing of the motor. The BHP available from the motor should not only be adequate, but have enough available remaining capacity to ensure it can operate within its most efficient range.  If 99 horses are required, it is certainly best to upgrade from a 100 horse motor to a 125 horse motor. Today, the use of a variable frequency drive (VFD) is virtually universal.   Any potential capital savings achieved by using a damper controlled system are quickly lost to increased operating costs.  Additionally, the VFD provides an additional level of operational control and flexibility. 

Finally, in most oxidizer systems, the fan can be placed for forced-draft (FD) or induced draft (ID).  FD fans “push” the air through the oxidizer, ID fans “pull” the air.  Generally speaking, FD fans provide some advantages as they are smaller and are not impacted by oxidizer exit temperatures.  ID fans are approximately 25% larger in RTO systems (due to increased temperature), but this provides some protection from condensable materials which might get on the fan blades, affecting balance and particulate.  The potential negative is exposure to much higher temperatures. 

Fan Installation and Operation

Installing the fan in the field requires a level and stable base or pad.  If the fan is mounted on a roof platform, fan vibration and harmonics must be accounted for and attenuated by rechecking fan rotation and ensuring strong fan mounting.  To obtain the maximum fan life, users should adhere to the the manufacturer’s operation and maintenance guidelines. 

In summary the common key points are:

• Ensure the fan remains clean for both consistent airflow and balance.
• If vibration is an issue:
  • Ensure no liquid or particulate is on OR in the blade (if a hollow airfoil).
  • Assess the blade for abrasion if the fan is exposed to particulate.
  • Inspect the belts, drive, wheel alignment, and bearings; also ensure the fan shaft is straight.
  • Confirm all fasteners are tight, including the mounting bolts and fan set screws.
• Noise is commonly caused by belts or bearings.  Both should be checked and adjusted as necessary.
• Bearings are obviously critical and should be monitored and maintained. Both the recommended grease and the recommended amount should be used. 

Assessing your process to determine the correct fan, designing the system for efficiency, and maintaining the fan according to recommendations can ensure a long operational and economical fan life.  NESTEC, Inc. can help you achieve this.  Call us at 610-323-7670 or email us at  jnester@nestecinc.com to learn more.