Air Flow formulas CFM = Duct area sq ft x Velocity Standard Air= 70F @ 29.92” HG (Mercury) 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 ⎞ ⎛ CFMnew ⎞ CFM new = CFM old X ⎜ ⎟ RPM new = RPM old X ⎜ ⎟ ⎝ RPMold ⎠ ⎝ CFMold ⎠ Fan Law #2 2 ⎛ CFMnew ⎞ SPnew ⎛ CFMnew ⎞ SPnew ⎜ ⎟ = Or ⎜ ⎟ = ⎝ CFMold ⎠ SPold ⎝ CFMold ⎠ SPold Formulas for problem solving 2 SPnew ⎛ CFMnew ⎞ CFM new = CFM old x SP new = ⎜ ⎟ SPold ⎝ CFMold ⎠ Fan Law #3 3 ⎛ CFMnew ⎞ BHPnew CFMnew BHPnew ⎜ ⎟ = or = 3 ⎝ CFMold ⎠ BHPold CFMold BHPold Formulas for problem solving 3 BHPnew ⎛ CFMnew ⎞ CFM new = CFM old X 3 BHP new = BHP old x ⎜ ⎟ BHPold ⎝ CFMold ⎠ Volume calculations: CFMx60 AirChangesperhour Air Changes Per Hour = CFM = Room Volume x Cubicfeet 60 Calculating Outside Air quantities: ()()MATx100− %RAxRAT OAT = O=Outside A=Air R=Return M=Mixed %OA T=Temperature ()RAT− MAT %OA = x 100 ()RAT− OAT ()()%OAxOAT + %RAxRAT MAT = 100 ()()MATx100− %OAxOAT RAT = %RA Velocity and Velocity with density correction: For Standard Air (70F @ 29.92"): For Other Than Standard Air: 2 ⎛ V ⎞ VP VP = ⎜ ⎟ V =×1096. 7 V =×4005 VP ⎝4005⎠ Den 530 BAR Den =×0. 075 × 460+ T 29. 92 Airflow stations flow: CFM Cv = 2 CFM= CV × ∆ P( inches ) H 2 o ∆P( inches ) H 2 o 2 ⎛CFM⎞ ∆P( inchesH ) 2 o = ⎜ ⎟ ⎝ 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 sph..t ×∆T M = Mass Flow Rate in Lbs. per Hour. sp.ht. = Specific Heat in BTU/Lb. per Degree F. BTUH() Sensible Den CFM = BTUH=××× CFM108. ∆ T 108.(× ∆TDryBulb ) 0. 075 BTUH ∆T = 108. × CFM Den BTUH() Total=××× CFM∆ h 45 . 0. 075 h = Enthalpy in BTU per Lb. BTUH = MBH 1000 Fan calculations: CFM× SP FanBHP = Static fan efficiency: 6356× SE Circ.( in .) Ts()FPM ×12 FanTipSpeedFPM=× RPM Circ.( in .) = 12 RPM Ts()FPM ×12 RPM = Circ.( in .) ()Dd− 2 d BL=+2157 C(). ×++() D d FanRPM=× MotorRPM 4C D D MotorRPM=× FanRPM d BL = Belt Length (in.) C = Distance between shaft centers (in.) D = Fan sheave dia. (in.) d = Motor Sheave dia. (in.) ExsistingFanSheaveDia. Max. (min.) Fan sheave Dia. = Max.BHP 3 ExsistingEstimatedBHP Max.BHP Max. Motor sheave = Existing Motor sheave Dia. X 3 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. .