文档介绍：Designing Air Flow Systems
A theoretical and practical guide to the basics of designing air flow systems.
Air Flow
Types of Flow
Types of Pressure Losses or Resistance to Flow
Total Pressure, Velocity Pressure, and Static Pressure
Air Systems
Fan Laws
Air Density
System Constant
Pressure Losses of an Air System
Sections in Series
Sections in Parallel
System Effect
Fan Performance Specification
Fan Total Pressure
Fan Static Pressure
Pressure Calculations
Methodology
Assumptions and Corrections
Problem # 1 – An Exhaust System
Problem # 2 – A Change to the System’s Air Flow Rate
Problem # 3 – A Supply System
Appendix 1 – Equations
Appendix 2 – ASHRAE Fittings
Appendix 3 – Bullhead Tee Curves
1. Air Flow
Flow of air or any other fluid is caused by a pressure differential between two points. Flow will originate from an area of high energy, or pressure, and proceed to area(s) of lower energy or pressure.
FLOW
P0
P1
P1>P0
Duct air moves according to three fundamental laws of physics: conservation of mass, conservation of energy, and conservation of momentum.
Conservation of mass simply states that an air mass is neither created nor destroyed. From this principle it follows that the amount of air ing into a junction in a ductwork system is equal to the amount of air mass leaving the junction, or the sum of air masses at each junction is equal to zero. In most cases the air in a duct is assumed to be pressible, an assumption that overlooks the change of air density that occurs as a result of pressure loss and flow in the ductwork. In ductwork, the law of conservation of mass means a duct size can be recalculated for a new air velocity using the simple equation:
V2 = (V1 * A1)/A2
Where V is velocity and A is Area
The law of energy conservation states that energy cannot disappear; it is only converted from one form to another. This is the basis of one of the main expression of aerodynamics, the Bernoulli equation. Bernoulli's equation in