Air Distribution Ductwork

Air conditioning and heating systems rely on ductwork to channel conditioned air to the desired
locations. The airflow through supply ducts is driven by the fan in the Air Handling Unit (AHU)
and the motor powering it.
Key Considerations in Duct Design
1. Cost Trade-offs:
o Initial Investment vs. Energy Costs: Larger ducts require a higher initial
investment but can reduce long-term fan energy costs.
2. Design Factors:
o Space Availability: Consideration of available installation space is crucial.
o Noise Levels: Duct design can impact the noise produced by the system.
o Capacity for Expansion: Future expansion needs should be anticipated in the
design.
o Aesthetics: The appearance of ductwork is also a design consideration.
3. Standards and Guidelines:
o The design should adhere to the ASHRAE Guide, SMACNA Manuals, and the
Associated Air Balance Council Manual to meet the required air conditioning
load.
Ductwork Categories
Duct systems can be categorized based on their shape:
Round Ducts
 Efficiency: Round ducts are more efficient than rectangular ones, providing maximum
air-carrying capacity with minimal pressure loss.
 Cross-Sectional Area: For example, an 18-inch diameter round duct has the same air
capacity as a 26” x 11” rectangular duct. The round duct has a cross-sectional area of
254.5 sq in and a perimeter of 4.7 ft, while the rectangular duct has 286 sq in area and a
perimeter of 6.2 ft.
 Material Cost: Round ducts require less material, resulting in lower costs for insulation,
supports, and labor.
Rectangular Ducts
  Space Limitations: They are often used in spaces with height restrictions. For instance, a
rectangular duct can fit in a 14-inch clear height space when a round duct cannot.
 Aspect Ratio Considerations:
o Maintain an aspect ratio close to 1:1 to minimize frictional resistance and
material use.
o Avoid aspect ratios greater than 3:1, as they increase both installed and
operating costs.
Velocity Classification of Duct Systems
Duct systems can be classified based on the air velocities within them, affecting design,
efficiency, and cost.
Low-Velocity Systems
 Velocity Range:
o Designed for air velocities of 1500 feet per minute (fpm) or less for comfort air
conditioning.
o Up to 2500 fpm for industrial applications.
 Design Characteristics:
o Typically larger duct sizes.
o Doubling the duct diameter can reduce friction loss by a factor of 32 and
decrease noise levels.
 Space and Cost Considerations:
o Occupy more space and have higher initial costs.
o Facility owners may hesitate to allocate space for larger ducts; however,
increasing duct size by even one standard increment can lead to significant
energy savings.
High-Velocity Systems
 Velocity Range:
o Operate at duct velocities up to 6000 fpm.
 Design Characteristics:
o Smaller duct sizes for a given air quantity.
o Require less space for installation.
  Cost Implications:
o Higher initial costs and greater operating costs due to increased resistance
(pressure drop), leading to higher fan horsepower requirements.
Calculation of Velocity
The velocity of air in a duct can be calculated using the formula:

Pressure Classification of Duct Systems

Duct systems are categorized into three pressure classifications, which correspond to the type
of supply fans used. These classifications account for total pressure, including losses through the
air source unit, ductwork, and air terminals.
Pressure Classifications
1. Low Pressure: Up to 4.0 inches of water gauge (in-wg)
o Associated with Class I Fan.
2. Medium Pressure: From 4.0 to 6.0 in-wg
o Associated with Class II Fan.
3. High Pressure: From 6.0 to 12.0 in-wg
o Associated with Class III Fan.
Engineering Best Practices
 Primary Air Ductwork: Should be classified as medium pressure (for fan connections and
main distribution ducts).
 Secondary Air Ductwork: Should be classified as low pressure (for run-outs, branch
ducts, and distribution devices).
Duct Frictional Resistance
Frictional resistance in duct systems affects the movement of supply air and varies based on air
velocity:
 Frictional resistance is proportional to the square of the velocity ratio. For example, if
one duct carries air at 1000 fpm and another at 2000 fpm, the frictional resistance of the
second duct will be four times greater.
  The power required to overcome frictional resistance increases with the cube of the
velocity ratio. For instance, if the second duct is twice the velocity of the first
(2000/1000), the power required will be eight times greater.
Pressure Measurement
The fan in the AHU must generate pressure to overcome the frictional resistance in the
ductwork, measured in inches of water. The total pressure in a duct system is the sum of:
 Static Pressure: The outward push of air against duct surfaces due to the fan's
compressive force.
 Velocity Pressure: The thrust of supply air resulting from its velocity.
Duct Equivalent Length (TEL)
To account for additional frictional resistance from fittings and accessories in the ductwork, the
concept of equivalent length is used:
 Total Equivalent Length (TEL):

o Where LLL is the actual measured length of the duct, and CCC is a coefficient
reflecting the complexity of the duct system (0.4 for simple systems, 1 for
complex systems).
Duct Sizing Methods
When designing duct systems, two primary methods are commonly used for sizing ducts: the
Velocity Method and the Drop Method. Each method has specific applications and formulas to
ensure effective airflow and pressure management.
1. Velocity Method
The Velocity Method focuses on determining the duct size based on the desired velocity of
supply air. The formula used is:

Frictional Allowance:
 Round Ducts: 1.0
 Square Ducts: 1.10
 Rectangular Ducts: 1.25
2. Drop Method
The Drop Method involves calculating the static pressure drop (friction loss) in the ductwork.
This method helps to ensure proper duct sizing based on the pressure requirements:
 Static Pressure Drop: Typically, for a low-velocity duct system, a pressure drop of 0.1
inches per 100 feet of total equivalent duct length is a common standard.
3. Regain Method
The Regain Method addresses changes in air velocity throughout the duct system. It involves
calculating static regain to adjust duct sizes, ensuring consistent static pressure and air quantity
at each outlet.
Duct Testing Guidelines
 Pressure Ratings:
o For systems with 3 inches of water gauge (WG) and lower: Test at 1.5 times the
pressure rating.
o For systems with 3 inches WG and higher: Also test at 1.5 times the pressure
rating.