Why is velocity important in ductwork?

Flow velocity in air ducts should be kept within certain limits to avoid noise and unacceptable friction loss and energy consumption. Low velocity design is very important for the energy efficiency of the air distribution system. Doubling the duct diameter reduces the friction loss by factor 32.

What is ductwork velocity?

Duct system is pressurized by three pressures: • Static pressure: It is the air pressure in the duct, which is used for fan selection. Velocity pressure: It is the pressure generated by the velocity and weight of the air, which is used for measuring the flow (cfm) in a system.

What is the problem with having too much of air velocity through a duct?

If it goes too high, you get too much pressure drop and possibly noise. And then there's the issue of shooting conditioned air into rooms at the proper velocity to get enough mixing of the room air.

How do you increase velocity in duct?

As duct size increases, air velocity decreases, and vice versa. So we can increase the velocity of air in a duct by making the ducts smaller and we can reduce the velocity by making the ducts bigger.

How do you increase airflow in ductwork?

Increasing airflow in ductwork can be done by cleaning the air filters and ductwork. By adding inline duct booster fans and/or air dampers significantly can improve airflow in weak areas.

Duct Traverse - Understanding Velocity Traverse

What air velocity is considered ideal for comfort?

Airflow velocity in the range of 1–2 m/s is the ideal velocity to provide thermal comfort.

Why should air velocity in branch ducts be limited to 600 feet per minute?

Why should air velocity in branch ducts be limited to 600 feet per minute? To minimize air noise.

Does static pressure change with velocity?

Pressure and velocity are inversely proportional to each other. If pressure increases, the velocity decreases to keep the algebraic sum of potential energy, kinetic energy, and pressure constant.

How do you slow air velocity in a duct?

To get the air moving slower, we need bigger ducts so there's more surface area, too. The upshot of all this is that when you're sizing ducts, you have to consider the space those ducts are in. If you put ducts in conditioned space, you can move the air as slowly as you'd like.

What is velocity pressure in HVAC?

In a moving fluid, velocity pressure (VP) is the pressure that would induce an equivalent velocity if applied to move the same fluid through an orifice, so that all pressure energy is converted into kinetic energy. It is used in air conditioning, heating and ventilating work to determine air velocity.

Does reducing duct size increase pressure?

If you reduce the opening of one register, you increase the resistance of the whole system. This increases the pressure of the whole system and reduces the volume of the whole system.

How do you calculate velocity in ductwork?

In short, air velocity in the ducts is calculated by dividing airflow by duct cross-section. Airflow is expressed as a simple number. Example: Air conditioner has a max. airflow of 600 CFM.

How is air duct velocity measured?

An anemometer, a test instrument that measures air velocity is used to determine the average airspeed in the duct. Then the average feet per minute is multiplied by the area of the duct in square feet to determine the airflow moving through the duct.

How does air velocity affect human comfort?

Indoor air velocities also affect the thermal comfort of people within spaces. The greater the air velocity, the greater the heat exchange between people in a space and the air around them.

Does duct length affect CFM?

Texas A&M did a study on the effect of flex duct not pulled tight and the results are astounding. In my article on this research, I showed from their results that a 6″ duct moving 110 cfm when pulled tight will move only about 70 cfm with 4% linear (longitudinal) compression and about 40 cfm or less at 15% compression.

Can you oversize ductwork?

Using oversized ducts will erode the system's efficiency, while undersized ducts won't carry the necessary volume of air. Balanced airflow. To maintain neutral pressure in the HVAC system and throughout the house, the duct system must deliver and return a matching volume of air.

Which causes the most duct inefficiency?

Loose or disconnected ductwork can cause the air to leak into open space and affect your indoor air quality too. According to the HVAC experts, the average home loses about 20-40% of the air that circulates through leaking ducts, causing major inefficiency in your HVAC system.

Does air velocity affect temperature?

The faster the air flow rate, the more readily the surface cools as air forming a boundary layer is replaced and the temperature measured in the area is maintained low.

What is the difference between air flow and air velocity?

Flow is a measure of air output in terms of volume per unit of time. The common units are litres per minute, cubic feet per minute (CFM), etc. Velocity refers to how fast the air is moving in distance per unit of time. The common units are feet per second, metres per second, etc.

What is the recommended FPM for return duct systems?

Return Grille Sizing

The speed of the air moving through of a return grille should typically be kept in the 300 FPM (Feet per Minute) to 500 FPM range to reduce noise through the grille.

How do you reduce static pressure in ductwork?

Dirty or high-MERV filter: Having a dirty air filter or one with too high of a MERV rating can also cause high static pressure at the return air intake. We recommend using cheap, low-MERV filters and changing them monthly to prevent this problem.