Temperature and Altitude Affect Fan Selection - New York Blower

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If Example 2 were such a case, the fan would require 30.0 BHPwhen operating at 70°F., and 15.0 BHP when the oven hadwarmed to 600°F. Very often a damper is furnished with the fanso that, during the warming-up period, the fan can be damperedto reduce the horsepower. Without the damper, a 30 HP motorwould be needed.Confusion can be avoided if the SP is specified at the temperatureit was calculated. In Example 2, the specifications should readeither:11,000 CFM and 6" SP at 600°F., or11,000 CFM for operation at 600°F. and 12" SP at 70°F.Table 1 gives correction factors used to convert from a nonstandarddensity to a standard density of 70°F. air. These factorsare merely the ratios of absolute temperatures. Multiply theactual static pressure by the specific temperature/altitude factorso standard catalog rating tables can be used. Divide the brakehorsepower from the catalog rating table by thetemperature/altitude factor to get BHP at conditions.AirTemperature°F.Table 1 - Corrections for TemperatureFactorAirTemperature°F.Factor-50 0.77 275 1.39-25 0.82 300 1.430 0.87 325 1.48+20 0.91 350 1.5340 0.94 375 1.5860 0.98 400 1.6270 1.00 450 1.7280 1.02 500 1.81100 1.06 550 1.91120 1.09 600 2.00140 1.13 650 2.09160 1.17 700 2.19180 1.21 750 2.28200 1.25 800 2.38225 1.29 900 2.56250 1.34 1000 2.76AltitudeFeet AboveSea LevelTable 2 - Corrections for AltitudeFactorAltitudeFeet AboveSea LevelFactor0 1.00 5000 1.20500 1.02 5500 1.221000 1.04 6000 1.251500 1.06 6500 1.272000 1.08 7000 1.302500 1.10 7500 1.323000 1.12 8000 1.353500 1.14 8500 1.374000 1.16 9000 1.404500 1.18 10000 1.45HOW TO CALCULATE ACTUAL FAN PERFORMANCEAT OTHER THAN SEA LEVELA fan operating at an altitude above sea level is similar to a fanoperating at air temperatures higher than 70°F.; it handles airless dense than standard. Table 2 gives the ratio of standard airdensity at sea level to densities of 70°F. air at other altitudes.Example 3. Required: 5800 CFM at 6" SP at 5000 ft. altitude.70°F. air at sea level weighs 1.20 times as much as 70°F. air at5000 Ft. Therefore, at sea level, the SP is 1.2 x 6 = 7.20" SP.The fan would need to be selected for 5800 CFM at 7.2" SP at70°F. .075 density.When both heat and altitude are combined, the density of the airis modified by each, independently, so that the correction factorscan be multiplied together.Example 4. Required: 5800 CFM at 6" SP at 5000 ft. altitude at600°F. Air at 70°F. at sea level weighs 2.00 x 1.20 = 2.40 timesas much as 600°F. air at 5000 ft. altitude. At sea level and 70°F.,SP = 2.40 x 6 = 14.4" SP. Select a fan for 5800 CFM at 14.4"SP. Divide the brake horsepower in the rating table by 2.40 toobtain horsepower at 600°F. and 5000 ft. If the fan is to startcold, it will still be at 5000 ft. altitude. Therefore, to get the“cold” horsepower requirement, divide by 1.20, the altitudefactor only.DENSITY CHANGES FROM OTHER THAN HEAT ANDALTITUDEFan densities may vary from standard for other reasons than heatand altitude. Moisture, gas, or mixtures of gases (other than air)are a few possibilities. In these cases, it is necessary to obtainthe actual density of the airstream gas by some other referencematerial. A similar factor, as shown in Table 1, is then createdusing the standard density of air .075 lb. per cubic foot divided bythe new density..075 lb./ft. 3Factor = special gas densityACFM-SCFM DEFINITIONThe terms ACFM and SCFM are often used in design work andcannot be used interchangeably.SCFM is Standard Cubic Feet per Minute corrected to standarddensity conditions. To determine the SCFM of the volume usedin Example 2, which was 11,000 CFM at 600°F., we wouldmultiply the CFM by the density ratios..03711000 x .075= 5500 SCFMThis indicates that if the weight of air at 600°F. were corrected tostandard conditions its volume would be reduced to 5500 CFMACFM stands for Actual Cubic Feet per Minute. It is the volumeof gas flowing through a system and is not dependent upondensity.The terms ACFM and SCFM are often used in system designwork where both quantities need to be known. It should beremembered, however, that since a fan handles the same volumeof air at any density, ACFM should be used when specifyingand selecting a fan.Page 2
FAN SAFE SPEED AND TEMPERATUREWhenever a fan is used to move air at temperatures substantiallyabove or below 70°F., care must be taken to ensure that the safespeeds of wheel and shaft are not exceeded, and that bearingtemperature and lubrication are satisfactory.The maximum safe speed of a particular fan must be determinedby calculations or actual tests. Safe speed depends entirely uponthe wheel and shaft assembly’s ability to withstand thecentrifugal forces created by its own weight. Highertemperatures can affect the wheel and shaft assembly’s ability towithstand these forces and therefore must be considered.Most metals become weaker at higher temperatures. Thisweakness is measurable in terms of yield and creep strength. Itcan be translated into formulas that accurately determine the safespeed of a wheel and shaft assembly in relation to its testedmaximum speed at standard conditions. Manufacturers providesafe speed reductions in their catalogs based on the alloy thatwas used to manufacture the wheel and/or shaft.Some metals withstand heat better than others. Certain grades ofstainless steel can be substituted to increase temperature limits.On the other hand, fan wheels constructed of aluminum shouldnever be operated above 200°F.For information regarding fiberglass reinforced plastic fanequipment, consult the appropriate product bulletin.Table 3 gives an indication of the speed derate factors for severaldifferent alloys. These are listed for reference purposes only.For a specific fan, consult the appropriate product bulletin.Table 3 - RPM Derate Factors By MaterialTemperatureStainless SteelMild Steel Aluminum°F.304L 316L 34770 1.0 1.0 1.0 1.0 1.0200 .97 .97 .88 .95 .95300 .95 -- .82 .92 .93400 .94 -- .78 .89 .90500 .93 -- .75 .86 .90600 .92 -- .73 .84 .90800 .80 -- -- .79 .861000 -- -- -- .75 .83The limiting temperature on any fan is the highest temperaturethat any component of the fan assembly will reach during anyoperating cycle. A fan in a process oven application may handleair several hundred degrees above the highest temperature theoven reaches, especially during start-up. On such applications, atemperature indicator should be located in the fan inlet tocontrol the heat source and to keep the fan within its maximumsafe temperature. This is particularly true where burners arelocated on the inlet side of the fan. In all cases, the fan shouldremain in operation until the air is cooled to 180°F. or less toprevent “heat soaking” of the fan shaft which could cause sagging.Bearings must be kept cool; otherwise standard lubricants losetheir effectiveness and bearing failures are likely. For axial fans,where the bearings are located in the airstream, care must betaken to ensure proper lubrication. Special fan and bearingdesigns, as well as high temperature lubricants, are available toextend the operating range to higher temperatures.Arrangement 4 centrifugal fans, where the fan wheel is mountedon the motor shaft, should not be used above 180°F., unlessspecial provisions are made (i.e., a shaft cooler or heat shield) tokeep heat radiated from the housing from increasing motorbearing and winding temperatures.When fan bearings are located outside of the airstream, as inArrangement 1, 8, and 9 centrifugal fans, higher airstreamtemperatures are possible. Table 4 lists some typical maximumrecommended operating temperatures for fans using ball orroller bearings.A conventional fan using standard bearings and standard lubricantcan normally be operated to a maximum of approximately300°F. With the addition of a shaft cooler (Figure 3), thistemperature limitation can be extended to 650°F. The shaftcooler has the effect of absorbing and dissipating heat from theshaft while circulating air over the inboard bearing.Table 4 - Maximum Fan Inlet TemperaturesArrangement 1 and 8 (Overhung Wheel)Standard ConstructionWith Shaft CoolerWith Shaft Cooler and Heat GapWith Shaft Cooler, Heat Gap,Stainless Wheel, and Alloy Shaft300°F.650°F.800°F.1000°F.Arrangement 3 (Wheel Suspended Between Bearings)Standard ConstructionArrangement 4 (Wheel on Motor Shaft)Standard ConstructionEnclosed Bearing Fans (Axial Fans)Arrangement 4Arrangement 9With Special V-Belts with 2.0 S.F.Arrangement 9 Duct FanWith Heat-Fan ConstructionArrangement 3Arrangement 4Plenum FansFigure 3 – Shaft Color200°F.180°F.105°F.120°F.200°F.600°F.105°F.105°F.Page 3
Page 1: ENGINEERING LETTER 4The New York Bl
Page 5 and 6: Figure 7 shows a system in which di

Temperature and Altitude Affect Fan Selection - New York Blower

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