It is directly proportional to the velocity of the air. The term highpressure or highvelocity ductwork includes ductwork systems and plenums from the fan discharge to the final highvelocity mixing boxes, or other final pressurereducing devices or any air supply system served by a fan operating with a static pressure range of. By multiplying air velocity by the cross section area of a duct, you can determine. Rules of duct design acca manual d residential duct systems. First we choose a blower that will give us the total air flow we need. A formal pattern of sensing points across the duct cross section is recommended.
Doubling the duct diameter reduces the friction loss by factor 32. Bernoullis equation relates a moving fluids pressure, density, speed, and height from point 1. The fundamentals of duct system design mcgill airflow. Multiplying air velocity by the area of a duct allows you to determine the air volume flowing past a point in the duct per unit of time. The series 480 does not come with a scale reading in inches w. The fan outlet is roughly 25x18 and encounters an abrupt transition to 52x26. What is the recommended air flow rate for air conditioning systems. Velocity 2000 3500 ftmin 10 18 ms maximum velocity. Air velocity in a ventilation duct can be calculated in imperial units as. The quantity of air moved per unit time is represented by the equation q av, where q is the velocity of air or its flow rate, a is the crosssectional area across which the air under study is flowing, and v is the linear velocity of the air, i. This is the most widely used method of sizing lower pressure, lower velocity duct systems. While all flows are compressible, flows are usually treated as being incompressible when the mach number the ratio of the speed of the flow to the speed of sound is less than 0. For each new calculation a unique conversion scale will be generated for the range of flow velocities and volumetric. This gauge measures both velocity pressure pv and velocity fpm for standard air conditions 0.
Volume flow is generally measured in cubic feet per minute cfm. Use the mercury manometer measurement to calculate the static and stagnation pressures of the flowing air. At sea level, this pressure is equal to approximately 14. To obtain the average total velocity in ducts of 4 diameter or larger, a series of velocity pressure readings must be taken at points of equal area. For a skydiver with high speed and a large body, turbulence is a factor. The chart below can be used to estimate velocities in air ducts. For incompressible flow, the dynamic pressure of a fluid is the difference between its total pressure and static pressure. The higher the pressure difference, the greater the air velocity and. Acca manual tair distribution basics for residential and. It is included as an accessory with each dwyer pitot tube. A given fluid flow has a finite pool of energy, and that pool comes from the energy stored as pressure and the kinetic energy of the moving fluid. When supply air is delivered into a room, an equal amount of air must. The output is an electronic analog signal that can be used by a controller, indicator or data logger.
Finally, when the damper is closed fully, airflow stops and no velocity pressure exists in the ductwork. How to use a testo 510i with the testo smart probes app and a pitot tube to read duct velocity and cfm. Since a single pitot tube is greatly affected by variations of the. Flow velocity in air ducts should be kept within certain limits to avoid noise and unacceptable friction loss and energy consumption. The crosssectional area of the duct is multiplied by the average air velocity to find the volume per time or flow rate. Why pressure decreases with increase in velocity physics. Now were going to take that and use it to figure out how to make all the parts work together properly. Calculations the pitot tube is the primary standard used to calibrate all other air.
Air flow, air systems, pressure, and fan performance. Static pressure is a measure of the static energy of the air flowing in a duct system. This is because friction against duct walls causes the air velocity to be lower near the sides than the velocity in. Lowvelocity ductwork design is very important for energy efficiency in air distribution systems. Smacna hvac systems duct design manual, fourth edition december 2006, table. Acca manual q low pressure, low velocity duct system design for correct fan performance.
Velocity pressure article about velocity pressure by the. Smacnas hvac air duct leakage test manual 1985 to verify the intent of. The pressure on the back side of the vane equals the pressure. The total static pressure is several factors added together. They are entry loss, dirty filter loss, static pressure of the worst branch duct, static pressure of the main duct, and static pressure of the return duct. Before further discussion, an explanation of convergent ducts, divergent ducts, and.
A manometer is used to measure the pressure in a tank. Air pressure on the duct wall is considered static. Data collection, accurately and precisely measuring air velocity across ducts, has been a difficult task. The two fundamental concepts, which govern the flow of air in ducts, are the. The pitotstatic tube consists of two concentric tubes, the central one with an open end pointing upstream of the fluid flow and the other closed at the end but with small holes drilled at right angles to the direction of flow. A simple slide rule, it provides for all the factors needed to calculate air velocity quickly and accurately. Static pressure the pressure at which an equal amount of air is supplied to and. Often the equivalent length of a fitting can be equal to or longer than. Carvill, in mechanical engineers data handbook, 1993. The air velocity in natural or free flow in air ducts has been shown to vary across the duct as well as in the flow direction with small numerical values 25, 47, 181. A skydiver will reach a terminal velocity when the air drag equals their weight. It can be thought of as the fluids kinetic energy per unit volume. The induced airflow rate needs to be determined for the analysis of any natural flow systems, which normally. Air conveyed by a duct system imposes both static and dynamic velocity pressures on the ducts structure.
For air to flow in a duct system, a pressure differential must exist. All of the usable fan energy is now being converted to static pressure. It is the air pressure in the duct, which is used for fan selection. Highvelocity hv duct systems are characterized by air velocities in the range of 2500 to 3500 fpm. A skydiver will reach a terminal velocity when the air. Performance characteristics of fans, duct systems, airside devices, and duct. It is common to keep main duct velocity above 20 ms 3940 ftmin.
By performing square root extraction and scaling to the velocity pressure signal, the resulting output signal is linear to velocity fpm or volume cfm. The measured difference between the total and static pressure is the velocity pressure. It is recommended that a manometer with a dual scale be used with pitot tube measurements. Low velocity design is very important for the energy efficiency of the air distribution system. Insitu fan differential pressure rise for a series vavfanpowered terminal unit with scr control athe semiimplicit method for pressurelinked equations algorithm is used to solve the problem of velocity pressure couple. The air velocity in the small duct is over 2100 fpm, and the velocity pressure is about 19 in. With that face velocity and those dimensions, the air flow rate is 1,367 cubic feet per minute cfm.
Air velocity can be calculated with the calculator below. The atmospheric pressure of air is a static pressure 14. Static pressure 101 static pressure is the amount of pressure a fan has to push and pull against to move air through a duct system. In part 1 of this duct design series, i discussed the basic physics of moving air in ducts. There are more complicated ways to figure the entry loss of your system, but we find it usually equals a loss of 1.
Then we design a duct system that will deliver the proper amount of air to each room. For example, when a fan is moving air through a duct system, two types of pressure are encountered. And, poor data collection procedures produce the errors in duct balancing. The goal is to achieve 100% effective duct length for optimal static regain. This is because friction against duct walls causes the air velocity to be lower near the sides than the velocity in the center, creating a parabolic velocity profile. If the velocity of air through an 8 x 20 duct is 700 fpm, how many cfm of air is being carried by the duct. The drag force then is approximately proportional to the square of the velocity. If the flow encounters a constriction, the velocity must increase due to mass conservation. Duct traversing for average air velocity and air volume. Available pressure the difference between the static pressure and the pressure against which the.
The central tube pressure is equal to the static pressure plus the velocity. To save time when converting velocity pressure into air velocity, the dwyer air velocity calculator may be used. Source book for efficient air duct systems in europe. In air conditioning, heating and ventilating work, it is helpful to understand the techniques used to determine air velocity. Manual q is your onestop reference book on the lowpressure, lowvelocity systems used in small and midsize commercial buildings. The main disadvantage of this method is that there is no equalization of pressure drops in duct branches unless the system has a symmetrical layout. Ordinary heating, ventilating, and air conditioning duct systems read air pressures at 0. An instrument used to measure the actual air velocity is the. All you need to know is the fluids speed and height at those two points. A total pressure probe, aligned into the airflow, senses the duct velocity pressure and the static pressure, which equals the total pressure.
Air flow measurement how to measure air velocity in a duct. Because bernoullis equation relates pressure, fluid speed, and height, you can use this important physics equation to find the difference in fluid pressure between two points. It makes sense that you need to know how air flows in ducts and what factors affect the flow. The total pressure loss in an air duct distribution system and. The static pressure is responsible for much of the force.
Relationship between velocity pressure and static pressure. A static pressure probe aligned at a right angle to the airflow senses only the static pressure. Principle of duct design in hvac systems cedengineering. Duct traversing for average air velocity and air volume the velocity of an air stream in a duct is not uniform over the cross section of the duct. This presentation about the hvac system according to duct sizing methods and filter types. They used 492 feet per minute fpm as the face velocity because thats what the ashrae filter standard 52. Learn vocabulary, terms, and more with flashcards, games, and other study tools.
The difference between the total pressure reading and the static pressure reading is the velocity pressure. This pressure is similar to that found by blowing up a balloon. Duct design 2 available static pressure energy vanguard. The velocity of an air stream in a duct is not uniform over the cross section of the duct. When measuring static pressure, the unit of measurement. However, dynamic velocity pressure introduces a rapidly pulsating load. The static pressure in a duct is the total pressure less the velocity pressure. Static pressure is exerted equally on all sides of a duct system. Air velocity is measured by sensing the pressure that is produced through the movement of the air. Compressible flow or gas dynamics is the branch of fluid mechanics that deals with flows having significant changes in fluid density. From a table, i determined that the fitting loss is equal to 45% of the velocity pressure. However, we do have differential pressure gages or manometers than can be used to measure the differential pressure in an inches w. Finally, velocity pressure was measured in the discharge duct and primary air duct.
Conversely, if a conversion from pressure to velocity is needed, a convergent duct is used. How is hvac air flow rate, air velocity, or cfm cubic feet per. That is, energy must be imparted to the system by a fan or air handling device to raise the pressure above or below atmospheric pressure. Reading duct velocity pressure, cfm and static with a. Use bernoullis equation to calculate pressure difference. The free area of most registers and grills is 65% of the total face area. The velocity in a 12 inch duct with air flow cfm can be calculated as. This tool will calculate the volumetric flow rate in any units from the specified velocity of a substance flowing through a defined crosssectional area. In the case of a 4,100 cfm duct, the nominal diameter is. The atmospheric pressure of air is a static pressure. How to increase air conditioning or heating return air flow to improve system performance return air adequacy on heating and air conditioning duct systems. Flow of air in pipes equation and calculator engineers.
The default values are for air flow 400 cfm 680 m3h, duct size 8 in 200 mm and velocity 1150 fpm 5. To achieve 100% effective duct length, a straight run from the blower or a turn, etc. The static pressure is responsible for much of the force on the duct walls. By multiplying air velocity by the cross section area of a duct, you can determine the air volume flowing past a point in the duct per unit of time. This design methodology sizes the supply duct system for a constant pressure loss per unit length.
Air flow, air systems, pressure, and fan performance captiveaire. In this field, air velocity distance traveled per unit of time is usually expressed in feet per minute fpm. Hvac air flow measurement, averaging pitot tube by paragon. Acca manual q low pressure, low velocity duct system.