HVAC Systems Flashcards

HVAC Systems

Ventilation

  • Ventilation requirements are based on occupancy density or air changes per hour.
  • Mechanical ventilation:
    • Extract: Natural inlet, mechanical extract (negative pressure).
    • Supply: Mechanical inlet, natural extract (positive pressure, filtered air).
    • Balanced: Mechanical inlet and extract (high control, slight positive pressure).
  • Fans:
    • Axial flow: Effective against small back-pressure.
    • Centrifugal: Axial intake, tangential output, optimized for flow/pressure/quietness.
  • Sound attenuation: Achieved by lining ducts with sound-absorbing material or using silencers.
  • Air filters:
    • Dry filters: Disposable, efficient, clog quickly.
    • Wet filters: Washable, reusable, effective down to 10 µmµm.
    • Air washers: Fine sprays for humidification/pre-cooling, require eliminator plates.
    • Electrostatic filters: Up to 12 kV, effective down to 0.01 µmµm, used with pre-filters in clean areas.

Air Conditioning

  • Total control of temperature, humidity, ventilation, filtration, and air movement.
  • Seven main processes:
    • Heating: Adding thermal energy.
    • Cooling: Removing thermal energy.
    • Humidifying: Adding water vapor.
    • Dehumidifying: Removing water vapor.
    • Cleaning: Removing particulates and contaminants.
    • Air Movement: Circulating and mixing air.
    • Ventilating: Exchanging air with the outdoors.
  • Air conditioning works as a thermodynamic machine with an evaporator, condenser and compressor using a refrigerant fluid.
  • Cooling Mode: A high pressure gas from hot gas vapour cooled down to high pressure liquid through condenser. Expansion valve controls liquid creating a spray action decreases pressure for cooling.
  • Heating Mode: A reverse cycle heats room via heat pump, economical. 1Kw consumption produces 4Kw of heat

Types of HVAC Systems

  • Single-zone system: Serves one zone.

    • Examples: Window air-conditioner, mini-split system, split system, heat pumps, unit ventilator.

  • Multi-zone systems: Independent control for different zones.

    • Constant volume (reheat) system: Constant air volume, temperature controlled by heating/cooling coils.
    • VAV (Variable Air Volume) system: Varies air volume at constant temperature using dampers.

Choosing an Air-Conditioning System: Factors to Consider

  • Building design: Space for ducts, etc.
  • Location issues: Weather conditions.
  • Utilities: Availability and cost.
  • Indoor requirements and loads: Thermal, moisture, ventilation, zoning.
  • First step: Calculate system loads and establish zones.