Air Flow Formulas

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:

()()MATx − %100 RAxRAT OAT = O=Outside A=Air R=Return M=Mixed %OA T=Temperature

()− MATRAT %OA = x 100 ()− OATRAT

()()%OAxOAT + %RAxRAT MAT = 100

()()MATx − %100 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 V ∆×= 2)( oHinchesPCCFM ∆ 2)( oHinchesP 2 ⎛CFM⎞ ∆ 2)( oHinchesP = ⎜ ⎟ ⎝ 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 ⎠