Velocity of Air Through Ducts

An estimate of the linear velocity of air through ductwork is a common exercise for process engineers. The calculation requires information on the static pressure drop and volumetric flowrate at any given temperature and pressure. 

A simplified estimate can be made by first converting the flow at actual conditions to the flow at standard conditions (i.e., at 70 F and 1 atm). The calculation basis for the linear velocity assumes a roughness coefficient of 0.0005 and a kinematic viscosity for air of 1.62 x lO fF/sec. From the ideal gas law, the following expression is developed  

From the standard cubic feet per minute estimate, the linear velocity is as follows  

Where V is the hnear velocity in fpm and h is the static pressure drop in units of inches of water per 100 ft of pipe. 

---

Calculate the mass flow rate of air through the duct, the average velocity, the ratio of the average to the maximum velocity and the Reynolds number. Comment on these results. 

Pressure drop Flow of air through the fluid-bed processor is created by the blower or a fan located downstream from the process chamber. This fan imparts motion and pressure to air using a paddle-wheel action. The moving air acquires a force or pressure component in its direction of motion because of its weight and inertia. This force is called velocity pressure and is measured in inches or millimeters of water column. In operating duct systems, a second pressure that is independent of air velocity or movement is always present. Known as static pressure, it acts equally in all directions. In exhaust systems, such as fluid-bed processors, a negative static pressure exists on the inlet side of the fan. Total pressure is thus a combination of static and velocity pressures. Blower size is determined by calculating... 

As the air or gas flows through the blower system (piping/ ducts, filters, etc.), the movement causes friction between the flowing air/gas. This friction translates into resistance to flow, whether on the inlet (suction side) or outlet (discharge side) of the system in which the blower is a part and that creates the pressure drop (see Chapter 2, V. 1, 3 Ed., of this series) which the blower must overcome in order for the air/gas to move or flow. This resistance to flow becomes greater as the velocity of flow increases, and more energy or power is required to perform the required flow movement at the required pressures. 

Air at 323 K and 152 kN/m2 pressure flows through a duct of circular cross-section, diameter 0.5 m. In order to measure the flow rate of air, the velocity profile across a diameter of the duct is measured using a Pitot-static tube connected to a water manometer inclined at an angle of cos-1 0.1 to the vertical. The following... 

The air-intake used to induce air from the flight-altitude atmosphere plays an important role in determining the overall efficiency of ducted rockets. The air pressure built up by the shock wave determines the pressure in the ramburner. The temperature of the compressed air is also increased by the heating effect of the shock wave. The fuel-rich gaseous products formed in the gas generator burn with the pressurized and shock-wave heated air in the ramburner. The nozzle attached to the rear-end of the ramburner increases the flow velocity of the combustion products through an adiabatic expansion process. This adiabatic expansion process is equivalent to the expansion process of a rocket nozzle described in Section 1.2. 

The third factor that is important in determining the detection limit is the conversion efficiency of the kinetics. A conversion efficiency of 1.0 requires that the airstream have a velocity substantially less than 200 m/s because uniform mixing of NO is very difficult. At the same time, collisions of the sample airstream with wall surfaces in slower inlet systems may cause a chemical loss of CIO and BrO, because they are both reactive with wall surfaces. The solution to this problem was suggested by Soderman (83). Soderman s novel design consists of two nested ducts in which the air speed is decreased from 200 m/s to 60 m/s in a 14-cm-diameter outer duct that protrudes 60 cm in front of the left wing pod and is reduced to 20 m/s inside a smaller 5-cm-square duct in which the measurements are made. The entrance to the smaller measurement duct is 60 cm downstream of the entrance to the outer duct, and the NO injector tubes, the two CIO detection axes, and the one BrO axis are 25 cm, 37.5 cm, 55 cm, and 72.5 cm downstream of the entrance of the measurement duct. Ninety percent of the air that enters the outer duct bypasses the measurement duct through additional duct work, and only the center 10% of the airstream is captured and sampled by the measurement duct. These two flows are recombined downstream of the instrument and are vented out the side of the wing pod that houses the instrument. 

Shallow beds are easier to maintain in stable fluidization and of course exert a smaller load on the air blower. Pressure drop in the air distributor is approximately 1 psi and that through the bed equals the weight of the bed per unit cross section. Some pressure drop data are shown in Table 9.14. The cross section is determined by the gas velocity needed for fluidization as will be described. It is usual to allow 3-6 ft of clear height between the top of the bed and the air exhaust duct. Fines that are entrained are collected in a cyclone and blended with the main stream since they are very dry... 

The operation of this sensor requires the blowing of an air (or some other gas) jet through a nozzle and detecting the deflection of this jet caused by the velocity of the process gas in the duct. This deflection causes an increase in pressure at the downstream receiver port and a decrease at the upstream one. Therefore, the process flow is related to the pressure difference between the two receiver ports (Figure 3.66). When there is no flow in the process pipe or duct, the jet is centered between these receiver ports, and the differential pressure is zero. As the process gases start flowing, the jet is deflected, and this deflection is converted into flow. 

Air flows through a plane duct, i.e., effectively between two large plates, with a width of 2 w. The flow can be assumed to be the same at all sections across the duct. The velocity distribution in the duct is approximately given by u - 12[(w - y)/w]1/5 m/s, y being the distance horn die center-plane to the point at which u is the velocity and the temperature distribution in the flow is approximately given by 60 - 40[(w — y)/w],/5eC. Find the mean temperature in the flow. 

Example 4.2 An aerosol comprised of 1.0-p.m-diameter spheres flows through a 16-in-diameter duct with a velocity of 3500 ft/min. Determine the Reynolds number of the air flowing in the duct and of the particles in the air. 

Calculate the Reynolds number for air flowing through a 10-in-diameter pipe at a velocity of 10 cm/s. Is the flow through the duct laminar or turbulent ... 

---