Duct Velocity Pressure, Standards and Calculations

When designing a duct system, the velocity of the air traveling inside the duct is a major factor. Allowing higher velocity will result in smaller duct size which saves cost, but there are consequences. So, in this post, I’ll explain the standards, common practices and some formulas to calculate duct velocity.

What is Duct Velocity?

Duct velocity is the velocity of the air traveling inside a duct. In duct design, velocity is a factor to consider because it affects the noise. The higher the duct velocity, the greater the noise produced.

Velocity pressure, which is the pressure exerted by the air due to its motion in a duct system is a function of duct velocity. The greater the duct velocity, the greater the velocity pressure and velocity pressure affects the pressure drop of duct fittings such as elbows (90°/45°) and transitions (enlargers/reducers).

Designing a duct system with higher velocity saves cost because the resulted duct sizes are smaller given. However, the increase in the velocity pressure may lead to higher operating cost due to greater friction loss, not to mention the potential noise issue caused by the fast moving air.

Therefore, finding the optimal duct velocity based on the applications, noise requirements, operating costs, energy efficiency and construction budget is key to a well-designed duct system.

However, in most cases, duct velocity is less of a concern because the primary factor is friction loss which the typically value is 0.1 in.wg per 100 ft (0.816 Pa/m). At such a friction loss value, the airflow must be above 6450 ft³/min (10965 m³/hr) in order for the duct velocity to start exceeding the 1500 ft/min (7.6 m/s) mark.

At such a high airflow, the concern is more on commercial and industrial HVAC systems. But, residential applications have a lower duct velocity requirement which may trigger a concern at low airflow.

Standards and Guidelines for Duct Velocity

There are several standards and guidelines on what air velocity to use when designing a duct system. Most of them are based on achieving a certain noise level requirement.

ASHRAE Handbooks

The ASHRAE Handbook of Fundamentals, Duct Design and the ASHRAE Handbook of HVAC Applications, Noise and Vibration Control outlined the recommended duct velocity for rectangular and round duct for a given duct location and noise requirement.

Here’s the recommendation:

Given that many ducts are installed above the ceiling, we can refer the duct velocity on the first row: “In shaft or above solid drywall ceiling”. But, if the ceiling is an acoustic type, then we refer the second row: “Above suspended acoustical ceiling”.

For example, if we have a normal ceiling (non-acoustical) and the noise requirement is NC35, then the duct velocity limit we should use is 2500 ft/min (12.7 m/s) for rectangular duct and 3500 ft/min (17.8 m/s) for round duct.

However, that’s for the main duct.

For branch duct, ASHRAE states that the recommended velocity should be 80% of what listed in the table and the final duct to diffuser outlet should be 50% of the listed value.

For instance, we use 2500 ft/min (12.7 m/s) to design the rectangular main duct. Then, for the branch duct we use 2000 ft/min (10.2 m/s). Finally, for the branch duct that has diffuser outlets, we use 1250 ft/min (6.4 m/s).

Nonetheless, the listed value doesn’t consider the noise produced by various type of duct fittings of various sizes. A tight fitting like a 90° elbow may required a much lower duct velocity to achieve the required NC rating.

So, in comfort cooling applications, the recommended velocity of air in ducts can be simplified to:

• Main Ducts: 700 to 900 ft/min (3.6 to 4.6 m/s) in residences, 1000 to 1300 ft/min (5.1 to 6.6 m/s) in schools, theaters, and public buildings, and 1200 to 1800 ft/min (6.1 to 9.1 m/s) in industrial buildings.
• Branch Ducts: 600 ft/min (3 m/s) in residences, 600 to 900 ft/min (3 to 4.6 m/s) in schools, theaters, and public buildings, and 800 to 1000 ft/min (4.1 to 5.1 m/s) in industrial buildings.
• Branch Risers: 500 ft/min (2.5 m/s) in residences, 600 to 700 ft/min (3 to 3.6 m/s) in schools, theaters, and public buildings, and 800 ft/min (4.1 m/s) in industrial buildings.

Maintaining these velocities helps ensure proper air distribution while keeping noise levels within acceptable limits.

ACCA Manual D

The ACCA (Air Conditioning Contractors of America) provides specific recommendations for duct velocities to ensure efficient and quiet operation of HVAC systems. According to the ACCA Manual D, the maximum recommended velocities for noise control are:

• Supply Air Ducts: Should not exceed 900 ft/min (4.572 m/s).
• Return Air Ducts: Should not exceed 700 ft/min (3.556 m/s).

These recommendations help in minimizing noise and ensuring efficient airflow within the system. Additionally, maintaining these velocities can reduce friction losses and improve the overall performance of the HVAC system.

Duct Velocity and Pressure Drop

Earlier, I mentioned that duct velocity affects the velocity pressure and velocity pressure affects the pressure drop of duct fittings. To start, we need to understand how to calculate velocity pressure.

Velocity Pressure Formula

The formula used to calculate velocity pressure is slightly different depending on whether you practice IP or SI unit. No worry, I will provide the formula for both units.

For example, given the duct velocity is 500 FPM (2.54 m/s), the velocity pressure is:

Given 0.1 in.wg is equivalent to about 25 Pa, 3.87 Pa is equivalent to 0.0155 in.wg. So, both units checked out.

Duct Fitting Pressure Drop Calculation

From velocity pressure, the conversion to the pressure drop of a specific duct fitting is easy. First, you need to identify the type of duct fitting and match it with the one stored in ASHRAE Duct Fitting Database.

For example, a 90° elbow (ASHRAE duct fitting code SR3-1) at 1400 ft³/min (2380 m³/hr) and sized at 12×10″ (300x250mm) has a coefficient of 1.17. To calculate its pressure drop, we multiply the coefficient by the velocity pressure.

But first, we need to calculate duct velocity.

How to Calculate Duct Velocity?

To calculate duct velocity, we simply divide the airflow by the duct area. Formula as follows:

So, 12×10″ (300x250mm) is equivalent to 120 in² (75000 mm²) which is equivalent to 0.833 ft² (0.075 m²). Therefore:

How to Calculate Duct Fitting Pressure Drop?

Now that we have the duct velocity, we just need to convert it into velocity pressure and then multiply the fitting coefficient to get the pressure drop of the SR3-1 fitting.

The effect of velocity on the duct fitting pressure drop is quite significant. For instance, if we increase the velocity from 1680 ft/min to 2000 ft/min, the velocity pressure becomes 0.249 in.wg and the fitting pressure drop becomes 0.291 in.wg. By increasing the velocity by about 19%, the pressure drop increased by over 40%.

In duct design, static pressure calculation is essential for us to determine the fan’s external static pressure. If you’re interested to learn how, check out my post Duct External Static Pressure Calculation.

Summary

The velocity of the air traveling inside the duct is primarily limited by how quiet we want the duct system to be. Secondly, we want to make sure the duct velocity is not too high as it’ll dramatically increase the velocity pressure and ultimately lead to high pressure drop which increases the fan power usage.

For a good duct design, it’s important to refer the recommended duct velocity for the given duct location and noise level requirement.