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Air Flow formulas CFM = Duct area sq ft x Velocity (Mercury) Standard Air= 70F @ 29.92” HG 1 cubic foot of standard air = 0.075 pounds 13.3 cubic feet of standard air = 1 pound FAN LAWS: Remember RPM is interchangeable for CFM Note: new is the same as 1 and old is the same as 2 Fan Law #1 ⎛ CFMnew ⎞ ⎛ RPMnew ⎞ ⎟ ⎜ ⎟=⎜ ⎝ CFMold ⎠ ⎝ RPMold ⎠ Formulas for problem solving ⎛ RPMnew ⎞ CFM new = CFM old X ⎜ ⎟ ⎝ RPMold ⎠ ⎛ CFMnew ⎞ RPM new = RPM old X ⎜ ⎟ ⎝ CFMold ⎠ Fan Law #2 ⎛ CFMnew ⎞ ⎟⎟ ⎜⎜ ⎝ CFMold ⎠ 2 = SPnew ⎛ CFMnew ⎞ Or ⎜ ⎟ SPold ⎝ CFMold ⎠ = SPnew SPold Formulas for problem solving CFM new = CFM old x SPnew ⎛ CFMnew ⎞ SP new = ⎜ ⎟ SPold ⎝ CFMold ⎠ 2 Fan Law #3 3 BHPnew CFMnew ⎛ CFMnew ⎞ or = ⎟ = ⎜ BHPold CFMold ⎝ CFMold ⎠ 3 BHPnew BHPold Formulas for problem solving CFM new = CFM old X 3 BHPnew BHPold ⎛ CFMnew ⎞ BHP new = BHP old x ⎜ ⎟ ⎝ CFMold ⎠ 3 Volume calculations: Air Changes Per Hour = CFMx60 Cubicfeet CFM = Room Volume x Calculating Outside Air quantities: AirChangesperhour 60 (MATx100) − (% RAxRAT ) OAT = %OA O=Outside A=Air R=Return M=Mixed T=Temperature (RAT − MAT ) (RAT − OAT ) %OA = MAT = x 100 (%OAxOAT ) + (% RAxRAT ) RAT = 100 (MATx100) − (%OAxOAT ) % RA Velocity and Velocity with density correction: For Standard Air (70F @ 29.92"): ⎛ V ⎞ ⎟ VP = ⎜ ⎝ 4005 ⎠ V = 4005 × VP 530 BAR × Den = 0.075 × 460 + T 29.92 For Other Than Standard Air: 2 VP Den V = 1096.7 × Airflow stations flow: CFM = CV × ∆P(inches) H 2o Cv = 2 CFM ∆P(inches) H 2o 2 ⎛ CFM⎞ ∆P(inches)H 2o = ⎜ ⎟ ⎝ Cv ⎠ Air Thermal calculations: sp.ht = 0.24 13.3 cu.ft. = 1 lb. 0.075 lbs. = 1 cu.ft. .075 lbs. x 60 min. = 4.5 lbs per hour 4.5 x 0.24 = 1.08 BTUH = M × sp. ht .× ∆T M = Mass Flow Rate in Lbs. per Hour. F. BTUH ( Sensible) 108 . × ∆T ( DryBulb) BTUH ∆T = 108 . × CFM CFM = sp.ht. = Specific Heat in BTU/Lb. per Degree BTUH = CFM × 108 . × ∆T × BTUH (Total ) = CFM × ∆h × 4.5 × Den 0.075 Den 0.075 h = Enthalpy in BTU per Lb. BTUH = MBH 1000 Fan calculations: FanBHP = Static fan efficiency: FanTipSpeedFPM = RPM × RPM = Ts( FPM ) × 12 Circ.(in.) BL = 2C + ( 157 . × ( D + d )) + Circ.(in.) 12 CFM × SP 6356 × SE Circ.(in.) = ( D − d)2 Ts( FPM ) × 12 RPM FanRPM = MotorRPM × 4C d D D MotorRPM = FanRPM × d BL = Belt Length (in.) C = Distance between shaft centers (in.) (in.) d = Motor Sheave dia. (in.) D = Fan sheave dia. ExsistingFanSheaveDia. Max. (min.) Fan sheave Dia. = 3 Max.BHP ExsistingEstimatedBHP Max. Motor sheave = Existing Motor sheave Dia. X 3 Max.BHP ExsistingEstimatedBHP ⎛ RPMnew ⎞ New Motor Sheave size Diameter new = Dia.old x ⎜ ⎟ ⎝ RPMold ⎠ ⎛ RPMold ⎞ New Fan Sheave size Diameter new = Dia.old x ⎜ ⎟ ⎝ RPMnew ⎠ © 2005, National Energy Management Institute Committee. All rights reserved.