In one plant, design flaws in its baghouse fans were causing them to run at only 15.6-percent fan efficiency, costing the facility over $500,000 in unnecessary operating costs annually. Retrofitting the baghouse ID fans with optimized specs paid off with an eight-month return on investment.
Advice to optimize draft systems isn’t just a lot of hot air; real potential savings could be going up in smoke, and causing emissions problems in the bargain. There are six primary areas of draft systems to evaluate, and six smart tips for optimizing them.
Inadequately designed ducting can be a significant source of loss in a fuel burning system. Improvements can include installing turning vanes, maximizing length for diverging and converging flows, redesigning compound elbows, and removing obstructions in the flow path.
Improve Inlet and Outlet Conditions
Fan inlets and outlets are in a subset of ducting, but are crucial to consider for draft system optimization. Site limitations make duplicating laboratory conditions in which the fan was rated impossible, but careful design can reduce velocity distortion and “fan effects,” improving flow. Points of importance:
• Ductwork turns should be in direction of fan rotation
• Use turning vanes to make velocity profile as uniform as possible
• Long, straight duct sections on inlet and outlet
• Three equivalent duct diameters in length or more at fan discharge
“Right-Size” Fan Equipment
Oversized fans often require considerable inlet dampening, which is inefficient. If volumetric flow rate is overstated, the fan will be too wide, creating opportunity for unstable operation. If pressure is overstated, the fan will require excessive dampening. Design for too little flow rate or pressure will leave the fan short and unable to provide peak draft requirements. Often, fans can be tipped or de-tipped to enhance performance, altering the effective diameter of the impeller without changing its effective width.
Verify Proper Fan Installation
Fans must be installed and calibrated according to manufacturer’s instructions. One critical area is where the inlet cone mates to the eye of the rotating assembly. Setting the cone too far away may cause a gap and excessive re-circulation. Inserting the cone too far into the eye may cause flow separation at the inlet and a reduction in volumetric flow rate.
Use the Right Fan Control: Dampers or Variable Speed Operation
Most system fans for base-loaded applications, with little to no swing in performance requirements, are controlled by dampers on the inlet box, or by radial vane dampers in the inlet cone. On some such systems, radial dampers can reduce horsepower consumption by as much as 3 percent over the inlet box type. Where there are wide variations in process and fan operations, variable speed control may be a better option. An inverter duty electric motor with an adjustable frequency drive can provide 10-to-1 turndowns for a reasonable initial investment.
Changing blade design can give immediate payback in capacity or efficiency. Typical blade designs include radial, radial tip, forward curved, flat backward inclined, backward curved, and airfoil. Radial blades are the least efficient. Airfoil blades are the most efficient, with approximately 15-percent difference in the amount of power required to do the same amount of work.
There are potentially large operational savings with a rapid rate of return if you know where to look in a draft system. Even small investments can pay huge returns, while also reducing your carbon footprint. ProcessBarron has over 20 years of experience in the field of fuel systems, and can handle every stage of optimizing draft systems, from design and engineering to installation, maintenance and repair. Visit our Air Handling page on the web at http://www.processbarron.com/air-handling-systems.