Document wq5Kr4mmBq3b7QoxzvyEL5jwQ

706 CHAPTER 32 1952 Guide i ... 2. Total pressure of a fan is the rise of pressure from fan inlet to fan outlet. 3. Velocity pressure of a fan is the pressure corresponding to the average velocity determination from the volume of air flow at the fan outlet area. 4. Static pressure of a fan is the total pressure diminished by the fan velocity pressure. 5. Power output of a fan is expressed in horsepower and is based on fan volume and the fan total pressure. 6. Power input to a fan is expressed in horsepower and is measured horsepower deliyered to the fan shaft. . ., 7. Mechanical efficiency of a fan is the ratio of power output to power inpiit. 8. Static efficiency of a fan is the mechanical efficiency multiplied by the ratio of static pressure to the total pressure. 9. Fan outlet area is the inside area of the fan outlet. .. 10. Fan inlet area isthe inside areaiof the inlet collar. While the total pressure truly represents the actual pressure developed by the fan, the static pressure may best represent the useful pressure for overcoming resistance. In many installations, since the outlet velocity of the fan is greater than the duct: velocity, some of the velocity pressure :may be utilized by conversion to static-pressure within the system. How ever, due to the uncertainty of the flow at the points of velocity change, the amount of conversion is seldom known and therefore, most fan tables list only the static pressure as available to overcome the system resistance. Fig. 1. Names and Definitions op Types op Fans Propeller Fan A propeller, fan consists of a propeller or disc wheel within a mounting ring or plate. : Tubeaxial Fan A tubeaxial fan consists of an axial flow wheel within a cylinder. Vaneaxlal Fan A vaneaxial fan consists of an: axial flow wheel within a cyl inder, combined with a set of air guide vanes located either before or after the wheel. Centrifugal Fern A centrifugal fan consists of a fan rotor or wheel within a scroll type of housing. Fans. 707 According to the Standard Test Code3.the efficiencies may be determined by the formulas: ' Mechanical (total) Efficiency = 0.0001573 X (cfm) X total pressure (inches water) horsepower input Static Efficiency = 0.0001573 X (cfm) X static pressure (inches water) horsepower input As the static pressure is often more useful than' total pressure, static efficiency is likewise many times more useful than mechanical efficiency. However, where a high outlet velocity can be effectively utilized, the static efficiency fails to be a satisfactory measurement of performance; also when a fan operates against no resistance, the static efficiency becomes zero, and is meaningless. Under such circumstances, many engineers prefer to use mechanical efficiency. Sound developed by a fan is a characteristic which is becoming increas ingly important. Unfortunately, no method has yet been devised for accurately measuring the sound actually discharged into a . duct system. The A.S.H.V.E. Research Laboratory, in cooperation with the U. S. Navy, has a program underway seeking to find a method. Many manufacturers list the average sound (for various fan operating conditions) measured at seven stations near the fan. These stations, as specified in the N.A.F.M. Test Code,4 are located in a horizontal plane passing through, the fan'shaft, and are at a distance of one wheel diameter (but not less than 5 ft) from the fan. Such values are useful in comparing the relative sound generated by various types and sizes of fans under comparable operating conditions. FAN LAWS5 The performances of fans of all types follow certain laws which are useful in predicting the effect upon performance of changes in the conditions of operation, the duty required of the installation, or the size of the equipment due to the. space, power, or speed limitations. In the following laws, groups 1 to 6, Q = air volume and P = static, velocity or total pressure. The laws pertaining to fan size apply only to fans geometrically similar, i.e., those in which all dimensions are proportional to some linear dimension denoted as size. If the size number is also, linearly proportional, it ,may be used; otherwise, wheel diameter is commonly used as a size.criterion. 1. Variation in Fan Speed: Constant Air Density--Constant System (a) Q: (b) P: (c) Power: Varies as fan speed. Varies as square of fan speed. Varies as cube of fan speed. 2. Variation in Fan Size: Constant Tip Speed--Constant Air Density Constant Fan Proportions--Fixed Point of Rating (a) Q- Varies as square of wheel diameter. (5) P: Remains constant. i (c) RPM: Varies inversely as wheel diameter. (d) Power: Varies as square of wheel diameter.