Air Flow formulas

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.

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