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AIRFLOW IN A SYSTEM

Learn about the elements of an air system, physical properties of air, effects of components on airflow, and more in this detailed technical seminar. Gain insights on pressure, conservation of energy, and air system design parameters.

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AIRFLOW IN A SYSTEM

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  1. AIRFLOW IN A SYSTEM Presented by: Bill Howarth, Illinois Blower, Inc. AMCA International Technical Seminar 2009

  2. The Air Movement and Control Association International (AMCA), has met the standards and requirements of the Registered Continuing Education Providers Program. Credit earned on completion of this program will be reported to the RCEPP. A certificate of completion will be issued to each participant. As such, it does not include content that may be deemed or construed to be an approval or endorsement by NCEES or RCEPP.

  3. Describe the elements of an air system • Know the physical properties of air • Describe the effects of system components on airflow • Understand the concept of pressure • Understand how the conservation of energy relates to airflow • Understand an air systems operating point Learning Objectives

  4. MOVING AIR Air Air Air at “A” Air at “B”

  5. AIR SYSTEM Air Air System Air at “B” Air at “A”

  6. Properties of Air Conservation of Energy Friction And Friction Losses Fan Characteristics AIR SYSTEM DESIGN PARAMETERS

  7. PROPERTIES OF AIR Standard Air Density Pressure Temperature

  8. STANDARD AIR The Reference Gas for Air System Design

  9. Ingredients: 1.105 X 1025 Molecules of Nitrogen (N2) 1.480 X 1023 Molecules of Oxygen (O2) 6.558 X 1021 Molecules of Argon (A) 2.190 X 1020 Molecules of Carbon Dioxide (CO2) Pinch of other trace gases RECIPE FORSTANDARD AIR

  10. Mix Well in a sealed box one foot on a side and one foot deep. Heat to 68F. Warning! if you are in a vacuum, it will take 2117 pounds of force to hold the lid on the box. RECIPE FORSTANDARD AIR

  11. If you followed the instructions properly, the container will have gained in weight by 0.075 lb. The density of standard air is: RECIPE FORSTANDARD AIR

  12. STANDARD AIR DEVIATIONS • Due To: • Change In Pressure • Change in Temperature • Addition of other Component(s), such as Water

  13. STANDARD AIR • Pressure

  14. PRESSURE • 1 Cubic Foot • at 68F. • Air molecules are in continuous random motion. The average impact of the molecules against the sides of a container result in the phenomenon known as pressure.

  15. PRESSURE • 1 Cubic Foot • at 68F. • Forcing the same number of molecules to occupy a smaller volume (compressing the air) will increase the frequency of the molecular impacts, which is an increase in pressure.

  16. PRESSURE • 1 Cubic Foot • at 600F. • Increasing energy raises the random motion and the temperature. Pressure also increases. But; a cubic foot of air at 600F and 14.7 lb/in2has fewer molecules - It is less dense.

  17. BAROMETRIC PRESSURE • The weight of our atmosphere compresses air to a pressure of 14.7 lb/in2 or 29.92 in. Hg (average at sea level with 50% relative humidity). AIR

  18. PRESSURE • Absolute Pressure • Any Pressure referenced to absolute zero pressure. • Barometric Pressure is an absolute pressure.

  19. AIR DENSITY • Density at a given temperature and barometric pressure:

  20. EFFECT OF HUMIDITY • The addition of water vapor to air will decrease the density of the air.

  21. PSYCHROMETRIC CHART

  22. GAGE PRESSURE BarometricPressure • Gauge Pressure is a Differential Pressure. 1 in. wg Water

  23. BarometricPressure Air 1 in. wg Static Pressure Water STATIC PRESSURE • Fan Static Pressure is a gage pressure, indicating compression of the air.

  24. BarometricPressure Air 1 in. wg Velocity Pressure Water VELOCITY PRESSURE • Velocity pressure is a measurement of the energy needed to accelerate air to a given velocity.

  25. TOTAL PRESSURE • Total Pressure= • Static Pressure + Velocity Pressure • or

  26. ACFM vs. SCFM • Actual Cubic Feet Per Minute (ACFM) • Standard Cubic Feet Per Minute (SCFM) • ACFM  SCFM • 1 Cubic Foot • at 600F. • 1 Cubic Foot • at 600F. • 1 Cubic Foot • at 68F.

  27. CONSERVATION OF ENERGY

  28. BERNOULLI'S LAW • For ducted airflow which is: • Constant with time • Incompressible • Without friction • (If we neither add nor subtract energy, energy is constant.)

  29. Area 2 Airflow TP1 SP1 Pitot Tube FLOW THROUGH A NOZZLE

  30. BERNOULLI'S LAW • May be used in system calculations wherever friction can be ignored. • Do NOT use for: • Abrupt Expansion • Abrupt Contraction

  31. FRICTIONANDFRICTION LOSSES

  32. TOTAL PRESSUREIN AN AIR SYSTEM Duct Loss Total Pressure Duct Length • Total Pressure declinesas duct length increases.

  33. FRICTION LOSS • Caused by non-uniform velocities across the ductwork, coupled with the viscosity of air. • Always results in the conversion of Total Pressure to Heat • Turbulence (irregular or chaotic air flow) will amplify the friction loss.

  34. R D LOSS FACTORS FOR ROUND ELBOWS

  35. D 100 LOSS FACTORS FOR STRAIGHT DUCTS

  36. SYSTEM LOSSES • Duct Friction Chart • Based on standard air, 0.075 lbm/ft3 . • This chart based on galvanized ducts with Beaded slip joints every 48” (=0.0003). • Other charts available.

  37. LOSSES IN A REALAIR SYSTEM • Add losses for each component. • Add a safety factor to all for the impact of one component connected directly to the next. Example: 

  38. AIR SYSTEMS • Basis for development of an Air System • Ventilation Rate • Air Changes/Hour • Face Velocity • Exhaust Requirements • References: • Fan Application Manual • ASHRAE Handbooks • Industrial Ventilation Guide

  39. AIR SYSTEM • Convert Ventilation Rate in to Flow Rate (CFM) • Develop a detailed duct system layout.

  40. Do: Calculate: Actual Cubic Feet Per Minute Static Pressure Requirement Air Density Include all entrance and discharge points Pay careful attention to fan entry and exit conditions Don’t: Simplify component losses Abruptly change velocity through the air system Neglect System Effects on the fan Inlet and Outlet Density AIR SYSTEMS

  41. System Resistance Curve System Losses Plotted System Loss Pressure CFM SYSTEM CURVE

  42. THE FAN’S JOB • The purpose of a fan is to supply an air system with energy (in the form of pressure) necessary to maintain airflow.

  43. FANCHARACTERISTICS

  44. FANS • There are many types of fans. • For each type, there may be many sizes. • All fans have one thing in common: Accurate prediction of aerodynamic performance requires a test.

  45. THE AERODYNAMIC PERFORMANCE TEST • At: • Constant Speed, • Known Density Power Pressure Airflow

  46. THE FAN LAWS • Are used to calculate fan performance at: • Other Speeds • Other Densities • Other Fan Sizes

  47. THE FAN LAWS • First Law:

  48. THE FAN LAWS • Second Law:

  49. THE FAN LAWS • Third Law:

  50. Pressure Power Airflow THE FAN LAWS • Changes in Speed

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