# Air flow and pressure relationship

### Airflow/Pressure relationship | Physics Forums

To understand the pressure relationship, recognize that the lowest pressure in a system is at the fan inlet. V. VP. = 1. THE PRINCIPLES OF AIR FLOW, AIR . “Pressure” and “flow” are common terms when discussing air compressors, but the relationship between the two is often misunderstood. Pressure, typically measured in psi, determines an air compressor’s ability to perform a certain amount of work at any given point in time. According to the Bernoulli principle, as the flow of air accelrates, it has a lower pressure than slower-moving air. Air picks up speed as it flows.

Dynamic losses occur whenever an air stream makes turns, diverges, converges, narrows, widens, enters, exits, or passes dampers, gates, orifices, coils, filters, or sound attenuators.

Velocity profiles are reorganized at these places by the development of vortexes that cause the transformation of mechanical energy into heat. The disturbance of the velocity profile starts at some distance before the air reaches a fitting.

## Airflow/Pressure relationship

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.

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.

### Basics of Air Velocity, Pressure and Flow

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. If the fluid flows down to a lower elevation, the change in elevation head will act to increase the static pressure. Conversely, if the fluid is flowing down hill from an elevation of 75 ft to 25 ft, the result would be negative and there will be a Pressure Change due to Velocity Change Fluid velocity will change if the internal flow area changes.

### Air Flow, Air Systems, Pressure, and Fan Performance

For example, if the pipe size is reduced, the velocity will increase and act to decrease the static pressure. If the flow area increases through an expansion or diffuser, the velocity will decrease and result in an increase in the static pressure.

If the pipe diameter is constant, the velocity will be constant and there will be no change in pressure due to a change in velocity. As an example, if an expansion fitting increases a 4 inch schedule 40 pipe to a 6 inch schedule 40 pipe, the inside diameter increases from 4.

## The Relationship Between Pressure and Flow in a Compressed Air System

If the flow rate through the expansion is gpm, the velocity goes from 9. The change in static pressure across the expansion due to the change in velocity is: In other words, pressure has increased by almost 0.

Pressure Change due to Head Loss Since head loss is a reduction in the total energy of the fluid, it represents a reduction in the capability of the fluid to do work. Head loss does not reduce the fluid velocity consider a constant diameter pipe with a constant mass flow rateand it will not be effect the elevation head of the fluid consider a horizontal pipe with no elevation change from inlet to outlet.