HRAC-223: Air Conditioning Week 3

HRAC-223: Air Conditioning & Distribution Systems

• Course: Air Conditioning & Distribution Systems
• Course Code: HRAC-223
• School: Engineering Technology & Applied Science (SETAS)
• Week: 3
• Topic: Air Distribution Systems

Learning Objectives

• Characteristics of Air:
  • Air Distribution
  • Air properties
  • Air measurements
• Occupied Zone:
  • Elements of design:
    • Primary Air
    • Secondary Air
    • Stratification
• Comfort in the Occupied Zone

Air Distribution

• The aim of most air conditioning systems is to create a comfortable and high-quality indoor environment.
• A good air distribution system ensures a consistent temperature and airflow in the occupied zone, within acceptable limits.
• Insufficient ventilation, poor distribution, and improper airflow patterns can lead to stuffiness, discomfort, and drafts.

Resistance Pressure

• Air moving through ductwork faces resistance, primarily friction.
• Air movement is not linear; it's turbulent and mixes.
• Different duct materials (metal, fiberglass, flexible) have varying friction characteristics.
• Duct fittings (elbows, tees, reducers) increase resistance, requiring higher pressure or velocity for adequate airflow.

System Pressures

• Duct systems rely on three pressure types to function:
  • Static pressure: the pressure exerted by air against duct walls.
  • Velocity pressure: the pressure generated by the speed and weight of the air.
  • Total pressure: the overall pressure needed to push air through the ductwork.
  • Pt = Pv + Ps

Static Pressure

• Static pressure is the pressure exerted by air against all duct walls.
• It's both positive (+) and negative (-).

Velocity Pressure

• Velocity pressure describes pressure in the direction of airflow.
• It represents the pushing force of the moving air.

Total Pressure

• The total pressure required to move air through ductwork is the sum of velocity pressure and static pressure.

Air Properties

• Dry Bulb Temperature: The measured temperature of the air (in °F or °C).
• Dew Point Temperature: The temperature at which moisture condenses from the air.
• Relative Humidity: The percentage of water vapor in the air compared to the maximum vapor the air can hold at that temperature.
• Specific Humidity: The mass of water vapor per unit mass of air.
• Enthalpy: The heat content of the air (in BTU/lb).
• Wet Bulb Temperature: The temperature of air accounting for moisture content.
• **Specific Volume:**Cubic feet of space occupied by one pound of air.

Air Measuring Devices

• Manometers: Used to measure static, velocity, and total pressures in ducts. Also used to measure pressure drops across filter banks to indicate when replacement is needed. Types include U-tube, inclined tube, pitot tube.
• Magnehelic: Measures positive, negative, and differential pressures, similar to a manometer.
• Anemometer/Velocity Meter: Measures air velocity in feet per minute (FPM). Designed for fast response to changing air flow conditions.

Air Measurements

• Rectangular Duct:

  • CFM = Area (SQ. FT) x Velocity (FPM)
  • Area = Length x Width / 144
  • Velocity = 4005 x Square Root of Velocity Pressure (FPM)

• Round Duct:

  • CFM = Area (SQ. FT) x Velocity (FPM)
  • Area = π x Radius² / 144
  • Velocity = 4005 x Square Root of Velocity Pressure (FPM)

Air Measurement Calculations (Example)

• Calculations for CFM and velocity using known dimensions and manometer measurements (example provided)

Rules of Airflow

• Hot air rises, while cold air falls (Newton's gravitational observation).
• Free jets of air (unconstrained) generally behave independently from the source.

Occupied Zone

• The region normally occupied by people in a space typically from 6 feet above the floor, more than 3.3 feet from exterior walls or HVAC equipment, and 1 foot from internal walls.

Elements of Design

• Primary Air: Air that needs conditioning

• Secondary Air: Conditioned air

• Stratification: The tendency of air streams to remain separated; generally, maintaining warmer air above the occupied zone is preferred in heating, and cooling requires that warmer air is below the occupied zone.

• Outlets Placement: Strategically placing supply and return air vents (avoiding short-circuiting) improves ventilation efficiency.

Comfort in the Occupied Zone

• Drafts: Uncomfortable air movement from air currents caused by cold surfaces (e.g., windows).
• Filtration/Air Quality: Uncomfortable feeling due to extreme temperatures (too hot or too cold) or odors/stale air.
• Noise: Low noise coming from fans and ductwork is desirable.
• Temperature: Comfort is the absence of discomfort.
• Humidity: High humidity can cause condensation problems and affect human heat loss while lower is preferable.
• Stratification Controlling stratification is essential to occupant comfort. ASHRAE standards define acceptable temperature differences between a seated and standing person.

This quiz covers key concepts in Air Conditioning & Distribution Systems, specifically focusing on air distribution systems and their characteristics. You'll explore factors such as air properties, design elements impacting the occupied zone, and the effects of resistance pressure on airflow. Test your understanding of how to maintain comfort in indoor environments.