ASHRAE Journal Supplement - AMCA International In Motion - Summer 2014 - (Page 17)

construction materials. It is important to address the customer's corrosion and/or erosion concerns during this phase, as measures taken to abate these destructive processes may add mass and thus affect the design of other fan components. Wear components such as liners can be easily replaced, decreasing downtime and maintenance costs. The application should be reviewed to assure fans are designed with features that increase the life of the fan, such as coatings, wear plates and housing liners. Details and guidance on these can be found in AMCA Publication 801.3 W W W. A M C A . O R G Low-Cycle Fatigue Systems that allow fans to change speed at regular intervals require consideration of low-cycle fatigue. A fan in this system may experience several hundred speed changes per day, with the changes being sometimes greater than a 50% increase or decrease in speed. If low cycle fatigue is not considered at the design stage, the rotor could suffer premature failure. Examples of systems that require fans to change speeds at regular intervals can be found in glass plants where the glass is quenched by air. They can also be found in steel plants where pollution control systems have their speed varied to match system requirements in the electric arc furnace (EAF), casters and coke ovens. In these applications, the fan often idles at low speed. At regular intervals, usually for short periods, full capacity is 6 3 BACKPLATE Natural Frequency Considering the current widespread use of variable speed control to achieve part load operating conditions, fan manufacturers must now also consider the natural frequencies of the fan impeller as part of the design process. Natural frequency is the frequency at which a system tends to oscillate in the absence of any driving or damping force.4 Operation at or near a natural frequency will result in severe vibration that will shorten the fan's life and could cause catastrophic failure. The operational cost savings for the user and the design risks to the manufacturer can be enormous. Today, the fan might be expected to operate at any number of speeds over its lifetime. The manufacturer HOUSING SIDE PLATE Sizing Aerodynamically/Geometric Similarity The next step in the process is to size the fan aerodynamically. In simplest terms, this means applying the affinity laws to geometrically similar equipment in accordance with the customer's specified operating conditions. After finalizing the aerodynamic selection, the fan manufacturer can design the fan. Understanding rotor dynamics, stresses, pressure loads and other loads imparted to the equipment will help ensure a fan will meet its mechanical performance requirements. The information provided to the fan manufacturer, such as pressures, design temperatures, and expected operational data, plays a major role in determining the final design and cost of the fan. All custom fans designs are based on geometrically similar fans. The fan rotor is the most important and highly stressed component of the fan, and this is where custom design should begin. AMCA members go to great lengths to ensure that the fan rotor will perform safely and reliably. Some key factors the fan manufacturer must understand and analyze are (a) fan operating parameters and the margins over these values as specified by the customer; 4 (b) design temperatures; and (c) overspeed requirements. All of these factors are used to calculate stresses and fundamental frequencies of the equipment. must determine the natural frequencies of the rotor and whether they coincide with any of the anticipated forcing functions the rotor might experience. Primarily, the rotor components could be excited by imbalance at the operating speed or by misalignment at twice the operating speed or the blade pass frequency (defined as the frequency of fan impeller blades passing a single fixed object). When the fan operates at a variety of speeds, the number of avoidance bands is multiplied. If a rotor's natural frequency is close to one of these forcing frequencies, it may not be possible to change the natural frequency by altering the impeller geometry. In this case, it is important to understand the mode shape of the impeller frequency so the manufacturer can determine the likelihood of that mode shape being excited. 5 3 1 7 8 5 BLADE 1 2 1. Partial blade liner 2. Full blade liner 3. Scroll liner 4. Housing sideplate liner 5. Backplate centerplate liner 6. Cutaway centerplate 7. Replaceable nose pieces - centrifugal fans 8. Replaceable nose pieces - axial fans Figure 1. Various wear and erosion-protection accessories, as depicted in AMCA Publication 801. Image courtesy of AMCA International A M C A I N T E R NAT I O NA L inmotion Summer 2014 17 http://WWW.AMCA.ORG

Table of Contents for the Digital Edition of ASHRAE Journal Supplement - AMCA International In Motion - Summer 2014

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ASHRAE Journal Supplement - AMCA International In Motion - Summer 2014