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Mechanical Equipment – Click the questions below to go the answers.
Q. What is the difference between an air compressor, blower and fan? The Pro Says: While the industry has not standardized or differentiated in the turbomachinery classes, pressures generated and end use generally determine machine class. Compressors are high pressure units whose main function is to compress the air or gas, increase the pressure or reduce volume. Fans are generally low pressure machines whose primary function is to move gas or air through a system or process. Blowers are high pressure fans which move air above atmospheric pressure to force flow through a fixed resistance. Virtually all the machines encountered in ventilation or exhaust applications for industrial systems will be fans.
Q. How do I choose the correct fan? The Pro Says: For proper fan selection, the air or gas requirements must be identified. The basic parameters to determine fan size are capacity ( CFM – cubic feet per minute), pressure (inches- water gauge), temperature (degrees Fahrenheit), and gas density ( pounds per cubic foot). These are referred to as the design conditions. With these variables established, fan selection can be made with respect to speed and size. The physical sizing of a fan is only the start of the selection process. For any given set of design conditions many fans would be capable of satisfying particular output requirements. The optimum selection would incorporate first cost, operating costs ( power consumption), and maintenance costs (repairs). The fan supplier generally has a variety of blade types, fan arrangements, and speed options to develop an number of alternatives. Gas composition, temperature, abrasive particulate (type and amount), corrosive elements, duty cycle, power consumption, and physical fan location all need to be considered and factored in final fan choice. The experienced fan supplier will carefully consider all these elements and look at a number of alternatives before final selection. The end user or system designer must be able to identify all the performance and operating variables to ensure proper fan selection. As with all equipment, many trade offs exist. Higher speed fans are smaller which result in cheaper installation and space requirements. Higher speeds however increase sensitivity to vibration (exponential) and imbalance. Fan wear under abrasive conditions will also increase exponentially. The most efficient blade types offer the least wear resistance. Conversely, the best blade types for wear resistance exhibit the poorest efficiencies. The fan user must take the time to fully evaluate his fan requirements or request assistance from a system designer or experienced fan supplier. A poorly selected fan based on incomplete data or initial cost will certainly result in a poor installation with higher operating and maintenance costs
Q. What are fan laws and how do I apply them to my fan system? The Pro Says: Fan laws are established relationships for critical performance variables. These laws allow prediction of fan performance when individual parameters are changed. These relationships are valid only when fans are geometrically similar and possess the same operating characteristics. The basic variables include Fan Speed (N), Fan Size (D), Flow Rate (CFM), Fan Pressure (SP), Gas Density (d), and Fan Power (HP). These can be summarized mathematically to determine performance and make adjustments accordingly. Remember these only apply for geometrically similar fans. Change in Fan Size (D) Volume (CFM) varies as cube (third power) of Fan Diameter (D)
Pressure (SP) varies as square of Fan Diameter (D)
Power (HP) varies as the fifth power of Fan Diameter (D)
Change in Fan Speed (RPM) Volume (CFM) varies directly with Fan Speed (RPM)
Pressure (SP) varies as square of Fan Speed (RPM)
Power (HP) varies as cube (third power) of Fan Speed (RPM)
Change in Gas Density (d) Volume (CFM) remains constant (no change)
Pressure (SP) varies directly as Gas Density (d)
Power (HP) varies directly as Gas Density (d)
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