Air Flow, Air Systems, Pressure, and Fan Performance
pressure drop and flow-rate, in straight pipes for compressed air is presented. that, considering the roughness and the internal diameter of the pipe, the test conditions (pressure, temperature, density), allows us to calculate the pressure losses at different . The dimensionless form of this relationship is: where: Re is the. Flow of air or any other fluid is caused by a pressure differential between two points. the change of air density that occurs as a result of pressure loss and flow in the ductwork. . The relationship between the air flow rate (CFM) and the pressure of an air system is Test Kitchen Supply System - Calculated Pressure. Heat Exchanger Air-Side Pressure Drop Analysis. 6 Standard Day Density Correction Technique Schematic Diagram of Flowstand Test Facility . is based on the relationship of measured laboratory heat rejection to the in-.
Valve manufacturers determine the valve flow coefficient by testing the valve with water at several flow rates, using a standard test method2 developed by the Instrument Society of America for control valves and now used widely for all valves.
Flow tests are done in a straight piping system of the same size as the valve, so that the effects of fittings and piping size changes are not included see Figure 3. The flow is the same whether the system pressure is low or high, so long as the difference between the inlet and outlet pressures is the same. Equation 1 shows the relationship.
The water flow graphs show water flow as a function of pressure drop for a range of Cv values. Gas flow Gas flow calculations are slightly more complex because gases are compressible fluids whose density changes with pressure.
In addition, there are two conditions that must be considered: Equation 2 applies when the low pressure drop flow outlet pressure p2 is greater than one half of the inlet pressure p1: The low pressure drop air flow graphs show low pressure drop air flow for a valve with a Cv of 1. Equation 3 for high pressure drop flow is simpler because it depends only on inlet pressure and temperature, valve flow coefficient and specific gravity of the gas: The high-pressure drop air flow graphs show high pressure drop air flow as a function of inlet pressure for a range of flow coefficients.
Effects of specific gravity The flow equations include the variables liquid specific gravity Gf and gas specific gravity Ggwhich are the density of the fluid compared to the density of water for liquids or air for gases.
However, specific gravity is not accounted for in the graphs, so a correction factor must be applied, which includes the square root of G. Taking the square root reduces the effect and brings the value much closer to that of water or air, 1.Bernoulli's principle 3d animation
The disturbance of the velocity profile starts at some distance before the air reaches a fitting. The straightening of a flow stream ends some distance after the air passes the fitting. This distance is usually assumed to be no shorter then six duct diameters for a straight duct.
Dynamic losses are proportional to dynamic pressure and can be calculated using the equation: A local loss coefficient can be related to different velocities; it is important to know which part of the velocity profile is relevant. Frictional Pressure Frictional losses in duct sections are result from air viscosity and momentum exchange among particles moving with different velocities.
These losses also contribute negligible losses or gains in air systems unless there are extremely long duct runs or there are significant sections using flex duct.
Understanding basic principles of flow calculations
Total Pressure, Velocity Pressure, and Static Pressure It is convenient to calculate pressures in ducts using as a base an atmospheric pressure of zero.
Airflow through a duct system creates three types of pressures: Each of these pressures can be measured. Air conveyed by a duct system imposes both static and dynamic velocity pressures on the duct's structure. The static pressure is responsible for much of the force on the duct walls. However, dynamic velocity pressure introduces a rapidly pulsating load.
Static pressure Static pressure is the measure of the potential energy of a unit of air in the particular cross section of a duct. Air pressure on the duct wall is considered static. Imagine a fan blowing into a completely closed duct; it will create only static pressure because there is no air flow through the duct.
A balloon blown up with air is a similar case in which there is only static pressure.
Dynamic velocity pressure Dynamic pressure is the kinetic energy of a unit of air flow in an air stream. Dynamic pressure is a function of both air velocity and density: