AirFlowFormulas.pdf

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.