Efficient HVAC Systems

Heating, Ventilation and Air Conditioning systems (HVAC) controls the indoor climate by adding or extracting heat and adding or removing mass (e.g. water vapour and dust). To combat summer heat and winter cold, heating, ventilation and air conditioning systems (HVAC) provide solutions for poor indoor environment quality to meet the comfort levels required by its occupants.

Physiological comfort factors include:

  • Dry bulb temperature (DBT, tdb): Temperature of air as registered by ordinary thermometer
  • Mean radiant temperature (MRT): Temperature registered by a thermometer whose bulb is covered by a wetted wick and exposed to a current of rapidly moving air.
  • Relative humidity (RH): Ratio of the actual water vapour pressure of the air to the saturated water vapour pressure of the air at the same temperature
  • Humidity Ratio (aka Moisture Content/ Mixing Ratio): Ratio of the mass of water vapour to the mass of dry air
  • Air movement
  • Odour concentration
  • Airborne dust
  • Noise
Summary of comfort conditions (McMullan 2007)

Summary of comfort conditions (McMullan 2007)

 

Factors influencing thermal comfort include:

  • Human heat loss: Convection, evaporation, radiation
  • Insulation factor: Clothing
  • Physiological factors: Activity, age, health
  • Thermal comfort factors: Air temperature, air motion, surface temperature, radiant heat (human metabolism generates heat, we maintain average body core temperature ~36.7C by exchanging heat with the environment [Radiation (40%), Convection (20%), & Evaporation by sweating and breathing (40%)], ASHRAE, 2005), draught (undesired cooling of human body caused by air movement ASHRAE, 2001), outdoor environment (solar radiation, humidity, wind, pollution, etc)

HVAC system significantly impact energy requirements and indoor environmental quality of buildings. HVAC systems must provide thermal comfort as well as acceptable indoor air quality. Indoor air environment is influenced by many parameters in a complex web of interactions in a dynamic system.

Indoor Air Quality Interactions

Indoor Air Quality Interactions

The main physical transfers are: Heat, Mass and Momentum (air flow). There is also continuous interaction between the outside and indoor environment.

Ventilating (all year round) is the supply or removal of air in any space to provide high indoor air quality (i.e. to control temperature, replenish oxygen, or remove moisture, odours, smoke, heat, dust, airborne bacteria, and carbon dioxide). Ventilation is for odour control and excessive moisture, introducing outside air, to keep interior building air circulating, and to prevent stagnation of the interior air. Ventilation includes both the exchange of air to the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings. Methods for ventilating a building may be divided into mechanical/forced and/or natural types. It’s purpose is to add or remove heat and/or humidity from occupied spaces; and supply fresh air to meet health requirements. AS 1669 Part 2 defines minimum levels of ventilation in offices: generally 10 L/s per person.

Levels of CO2 & associated air quality level

Levels of CO2 & associated air quality level

Air Quality levels (ASHRAE 2005)

Carbon dioxide concentration can be taken as an approximate surrogate for all other contaminants, at least those related to human activity (e.g. odour)

Fresh Air

Fresh Air (ASHRAE 1989)

Fresh Air Requirements (ASHRAE 1989) 

Fresh air can be either outdoor air or treated indoor air with minimal contaminant levels. In some countries, outdoor air is quite contaminated and unsuitable for use in buildings.

Air change rate (ASHRAE 2005)   

The removal of stale and polluted air from the inside of the building to the outside and replace it by an equivalence of fresh air. The Air Change Rate is the air change per hour which represents the number of times per hour that an enclosure’s total volume of air is exchanged with indoor air.

Air change rate (ASHRAE 2005)

Air change rate (ASHRAE 2005)

Passive and Active ventilation

Active (fan forced) /Mechanical supply of air. Such as ceiling fan, floor, displacement along walls.

Passive / Natural ventilationSuch as operable windows and thermal chimneys. Cross wind plan width of approximately 5 times the height of the ceiling (10 – 14m). Benefits include:

  • lower construction costs (if any)
  • reduced energy consumption (no fans, no chillers, no boilers)
  • lower operating costs
  • simpler control systems
  • enhanced user satisfaction (control of local environment)
  • increased fresh air quantities, at times

Limitations: Variable results due to the reliance on wind speeds which varies with height. Outdoor air pollutants can easily enter the building, smoke and fire spreads.

 

Heating and humidification (in winter) and the air conditioning and dehumidification (in summer) provide the the closely controlled indoor environment necessary for the comfort, working efficiency and well being a building’s occupants.

HVAC Systems include the heating and cooling equipment (e.g. Boiler, Chiller, Heat pump), Air handling unit (AHU) and the Distribution and supply of heat and coolness (steam pipe, chill water pipe, air duct).

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System components

  • Chiller (condenser, evaporator, compressor, thermostatic expansion valve)
  • Fan (centrifugal/axial)
  • Filter
  • Damper

 

Internal design conditions

Dry Bulb temperature

  • 19 – 21C in winter
  • 24-26C in summer

Relative humidity ~50%

 

Loads

  • The heating load on a building is that rate at which heat must be added to the building in order to maintain a constant temperature
  • The cooling load on a building is that rate at which heat must be extracted by the cooling equipment in order to maintain a constant temperature

 

Types of HVAC Systems

Types of HVAC Systems. Adapted from Jagemar 1995

Types of HVAC Systems. Adapted from Jagemar 1995

 

Standard industry air-conditioning systems (large scale)

Constant Air Volume (CAV) System: Constant volume of air suppled to the space, varying temperature depending on load

Variable Air Volume (VAV) System: Varies the amount of air supplied to the building depending on the load. Fans and pumps have a variable speed drive so as to vary their duty depending on the load (Reduce duties results in lower energy consumption, more efficient than CAV)

Variable Refrigerant Volume (VRV) System: Unlike CAV systems, which supply a constant airflow at a variable temperature, VAV systems vary the airflow at a constant temperature. It differs from VAV systems, which work by varying the air flow to the conditioned space based on variation in room loads. The advantages of VAV systems over constant-volume systems include more precise temperature control, reduced compressor wear, lower energy consumption by system fans, less fan noise, and additional passive dehumidification.

Displacement System: Supply air at a low velocity (~0.2m/s) at low level into an occupied space.Cooler air supply forms a layer at low level which slowly rises to replace the air present in the room. Exhaust is removed at high level. It conditions only the occupied space rather than mixing the whole occupied volume. Low velocity means reduced fan power. Smaller chiller size required which reduces energy consumption and plant space required.

Chilled Beams or Ceilings:  There are three types of chilled beam systems: chilled ceilings, passive chilled beams, and active chilled beams. Chilled water flows through tubes laid in the ceiling or factory assembled beams to carry off the room-cooling load. Cooling transferred to occupant through radiation exchange which then cools the body. Air is still supplied to meet fresh air requirements and remove to latent load. Reduction in air volumes results in low noise levels, reduction in plant space requirements and lower energy consumption.

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System selection depends on the following 

  • Number of zones
  • Loads
  • Chill water or direct expansion cooling
  • Central or local air handling units
  • Constant or varibale air volume supply
  • Acceptable noise and air pollutant levels
  • Energy sources & Energy efficiency
  • Cost: Initial and Operation & Maintenance

 

Relative Energy Efficiency of HVAC Systems

Relative Energy Efficiency of Air-Conditioning Systems

Relative Energy Efficiency of Air-Conditioning Systems

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