Oval Ducts

Disadvantages of rectangular ducts are as follows:

1. They create higher pressure drop;

 2. They use more pounds of metal for the same air-flow rate as round ducts; 

 3. Their joint length is limited to the sheet widths stocked by the contractor;

 4. Their joints are more difficult to seal; 18

 5. Those with high aspect ratio can transmit excessive noise if not properly supported. 

Rectangular Ducts

Round Ducts

The duct shape that is the most efficient (offers the least resistance) in conveying moving air is a round duct, because it has the greatest cross-sectional area and a minimum contact surface. In other words, it uses less material compared to square or rectangular ducts for the same volume of air handled.

 An 18 inch diameter duct, for example, has the same air-carrying 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 a 286 sq.-in area and a perimeter of 6.2 ft. The rectangular duct thus has 32% more metal in it and would cost proportionately more. Also the insulation, supports and labor are higher for rectangular ducts of similar capacity.

Some of the advantages of round ductwork include: 

• Round shape results in lower pressure drops, thereby requiring less fan horsepower to move the air and, consequently, smaller equipment.

 • Round shape also has less surface area and requires less insulation when externally wrapped. 

• Round ducts are available in longer lengths than rectangular ducts, thereby eliminating costly field joints. Spiral lock-seams add rigidity; therefore, spiral ducts can be fabricated using lighter gauges than longitudinal seam ducts. Spiral ducts leak less and can be more easily sealed compared to rectangular ducts. 

• The acoustic performance of round and oval ducts is superior because their curved surfaces allow less breakout noise. The low-frequency sound is well contained in round ducts. 

• Round ducts can help promote healthier indoor environments. Less surface area, no corners and better air flow reduce the chance of dirt and grime accumulating inside the duct and, therefore, becoming a breeding ground for bacterial growth.

DUCT SHAPES

Round Ducts

The duct shape that is the most efficient (offers the least resistance) in conveying moving air is a round duct, because it has the greatest cross-sectional area and a minimum contact surface. In other words, it uses less material compared to square or rectangular ducts for the same volume of air handled. An 18 inch diameter duct, for example, has the same air-carrying 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 a 286 sq.-in area and a perimeter of 6.2 ft. The rectangular duct thus has 32% more metal in it and would cost proportionately more. Also the insulation, supports and labor are higher for rectangular ducts of similar capacity.

Some of the advantages of round ductwork include:

 • Round shape results in lower pressure drops, thereby requiring less fan horsepower to move the air and, consequently, smaller equipment. 

• Round shape also has less surface area and requires less insulation when externally wrapped.

 • Round ducts are available in longer lengths than rectangular ducts, thereby eliminating costly field joints. Spiral lock-seams add rigidity; therefore, spiral ducts can be fabricated using lighter gauges than longitudinal seam ducts. Spiral ducts leak less and can be more easily sealed compared to rectangular ducts.

 • The acoustic performance of round and oval ducts is superior because their curved surfaces allow less breakout noise. The low-frequency sound is well contained in round ducts.

 • Round ducts can help promote healthier indoor environments. Less surface area, no corners and better air flow reduce the chance of dirt and grime accumulating inside the duct and, therefore, becoming a breeding ground for bacterial growth.

Pressure Classification

Duct systems are also divided into three pressure classifications, matching the way supply fans are classified. 

1. Low Pressure: The term low-pressure applies to systems with fan static pressures less than 3 inches WC. Generally, duct velocities are less than 1,500 fpm.

2. Medium Pressure: The term medium pressure applies to systems with fan static pressures between 3 to 6 inches WC. Generally, duct velocities are less than or equal to 2,500 fpm.

 3. High Pressure: The term high pressure applies to systems with fan static pressures between 6 to 10 inches WC. Usually the static pressure is limited to a maximum of 7 inches WC, and duct velocities are limited to 4,000 fpm. Systems requiring pressures more than 7 inches WC are normally unwarranted and could result in very high operating costs. 

General good engineering practices are:

 1. Use of medium pressure classification for primary air ductwork (fan connections, risers, and main distribution ducts).

 2. Use of low pressure classification for secondary air ductwork (runouts/branches from main to terminal boxes and distribution devices).

Velocity Classification vs. Pressure Classification

1. Duct pressure classification influences the duct strength, deflection and air leakage. 

2. Duct velocity classification influences noise, vibration, friction losses and fan power. 

DUCT CLASSIFICATION

Ducts are classified in terms of velocity and pressure.

 Velocity Classification 

Ducts are classified into 3 basic categories:

1. Low Velocity Systems: They are characterized by air velocities up to 2000 fpm.

 2. Medium Velocity Systems: They are characterized by air velocities in the range of 2,000 to 2,500 fpm. 

3. High Velocity Systems: They are characterized by air velocities greater than 2,500 fpm.