Air Supply

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AIR SUPPLY

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Surface Area Formulas

Round Duct — Circumference
C = π × D
π ≈ 3.14. Convert inches to feet: divide by 12. SA = C (ft) × Length (ft)
Rectangular Duct — Perimeter
P = 2 × (W + H)
Convert to feet first. SA = P (ft) × Length (ft)
Oval Duct — Perimeter
P = π × b + 2 × (a - b)
a = major axis, b = minor axis. Same as a rectangle with semicircle ends.
90° Elbow — Surface Area
SA = C × (π × r / 2)
r = centerline radius of the elbow. Same principle: perimeter × arc length.

TAB Core Formulas

CFM — Airflow
CFM = Area × Velocity
Area in ft², Velocity in FPM. Divide area in in² by 144 to get ft².
Velocity from VP
V = 4005 × √VP
VP = velocity pressure (in. w.g.). Result in FPM.
Percent of Design
% = (Actual ÷ Design) × 100
Acceptable range: ±10% of design CFM.
Fan Law 1 — CFM vs RPM
CFM₂ = CFM₁ × (RPM₂ ÷ RPM₁)
CFM changes linearly with RPM.
Fan Law 2 — SP vs RPM
SP₂ = SP₁ × (RPM₂ ÷ RPM₁)²
Static pressure changes as the square of RPM ratio.
Fan Law 3 — BHP vs RPM
BHP₂ = BHP₁ × (RPM₂ ÷ RPM₁)³
Brake horsepower changes as the cube of RPM ratio.
Sensible Heat
BTUH = 1.085 × CFM × ΔT
ΔT = temperature difference (°F). 1.085 = air constant at sea level.
Hydronic Heat
GPM = BTUH ÷ (500 × ΔT)
500 = water constant (8.33 lb/gal × 60 min/hr).

Leakage Rate Calculation

Allowable Leakage CFM
L = F × SA ÷ 100
F = leakage factor from chart (CFM/100 ft²), SA = total surface area in ft².
Leakage as % of System CFM
% = (Leakage CFM ÷ System CFM) × 100
Used to verify the system is within acceptable leakage limits.

TAB Calculators

Enter values and press Calculate. Each result shows the formula with your numbers substituted in.

1. Allowable Leakage CFM
2. Register / Grille Airflow
3. Velocity Pressure
4. Fan Laws
5. VFD — Speed & Fan Laws

Slide to a frequency. Enter nameplate values to see how CFM, SP, and HP change under the fan laws.

0 Hz 60 Hz 60 Hz
Speed
100%
of nameplate
Actual RPM
--
RPM
CFM
--
proportional to speed
Static Pressure
--
proportional to speed²
Horsepower
--
proportional to speed³
Energy Savings
0%
vs full speed
6. Motor Voltage Imbalance

NEMA limit: 1% max imbalance. Above 2% requires derating. Enter all three line-to-line voltages.

7. Motor Amp Load

How hard is the motor working? Compare actual amps to nameplate FLA.

8. Synchronous Speed

The theoretical no-load speed of a motor. Actual speed is slightly less due to slip.

5. Duct Sizing

F Chart Calculator

F = CL × P0.65 — Pick a leakage class and test pressure, then enter duct area to find allowable leakage.

Leakage Class (CL)
Test Pressure (in-wg)
--
F (CFM/100 sq ft)
Select CL and pressure above

Seal Classes

A
Most Restrictive
Seal ALL transverse joints AND longitudinal seams and all penetrations. Required on high-pressure systems and where specified.
B
Moderate
Seal ALL transverse joints. Seams do not need to be sealed unless specified. Most common for medium-pressure systems.
C
Least Restrictive
No sealing required unless otherwise specified. Used for low-pressure systems where leakage is acceptable.

Pressure Classes

Pressure ClassInches w.g.PascalsOperating Pressure
1/2" wg1/2"125 PaUp to 1/2" wg
1" wg1"250 PaOver 1/2" to 1" wg
2" wg2"500 PaOver 1" to 2" wg
3" wg3"750 PaOver 2" to 3" wg
4" wg4"1000 PaOver 3" to 4" wg
6" wg6"1500 PaOver 4" to 6" wg
10" wg10"2500 PaOver 6" to 10" wg

Leakage Classification — Default Values

Duct TypeDefault Leakage Class
Rectangular Duct16
Round Duct8
Oval Duct12

Common Sealants

TypeBest ForNotes
MasticLongitudinal seams, joints, large gapsFlexible, long-lasting. Do not apply over dirt or oil.
Foil TapeJoints, connectionsMust be pressure class rated. Not all foil tape is approved.
Liquid SealantPenetrations, connectionsCheck manufacturer specs for temperature limits.
Duct Wrap + MasticExposed ductwork requiring appearanceUsed in schools, airports, public spaces.

Basic Test Kit Components

ComponentPurpose
Test FanPressurizes the duct section to the specified test pressure
Orifice TubeMeasures airflow through the system; has orifice plate + two pressure taps
Two Manometers / MagnehelicsOne measures SP in duct; one measures ΔP across orifice plate
Calibration ChartConverts ΔP reading to actual CFM leakage rate for the specific orifice tube
TubingConnects pressure taps to manometers
Prints F reference grid, classification charts, and your unit notes.
Duct Leakage Testing — F Chart Reference
F = CL × P^0.65  |  L (allowable CFM) = (SA / 100) × F
F = CL × P0.65    L = (SA ÷ 100) × F
Duct Leakage Testing — Classification Quick Reference
Seal classes, pressure classes, default leakage class values
Seal Classes
ClassNameRequirement
AMost RestrictiveSeal ALL transverse joints, longitudinal seams, and penetrations
BModerateSeal ALL transverse joints. Seams unsealed unless specified.
CLeast RestrictiveNo sealing required unless specified. Low-pressure systems.
Default Leakage Class Values
Duct TypeDefault CL
Rectangular16
Round8
Oval12
Pressure Classes
ClassIn. w.g.Operating Pressure Range
1/2"0.5Up to 1/2" wg
1"1.0Over 1/2" to 1" wg
2"2.0Over 1" to 2" wg
3"3.0Over 2" to 3" wg
4"4.0Over 3" to 4" wg
6"6.0Over 4" to 6" wg
10"10.0Over 6" to 10" wg
Common Sealants
TypeBest ForNotes
MasticSeams, joints, gapsFlexible. Clean surface — no dirt or oil.
Foil TapeJoints, connectionsMust be pressure class rated.
Liquid SealantPenetrationsCheck temperature limits.
Study Notes
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