Introduction
• HVAC systems – or Heating, Ventilating and AirConditioning systems – control the environment
for people and equipment in our facilities.
• They account for about 13% of the electrical
energy use in a typical residential building; and
about 20% in a typical commercial or institutional building. HVAC use in an office building might be as high as 30 – 50%.
• HVAC systems and chillers are also significant energy consumers in many manufacturing facilities.
Functions of HVAC Systems
• The purpose of a Heating, Ventilating and Air Conditioning (HVAC) system is to provide
and maintain a comfortable and safe environment within a building for the occupants or for the process being
conducted.
• Many HVAC systems were not designed with
energy efficiency as one of the design factors.
Environmental Control Factors
• An HVAC system functions to provide an
environment in which these three factors are
maintained within desired ranges. Typical design conditions are:
• 75 degrees F temperature (dry bulb)
• 40 - 60% relative humidity
• ASHRAE 62.1 – 2007/2010 Ventilation Standard
–17 CFM outside air per person, or
–CO2 less than 1000 PPM
Temperature Control Strategies
• Vary the temperature of the supply air to
the space while keeping the air flow rate
constant. This is the constant volume, variable
temperature approach.
• Vary the air flow rate while keeping the supply air temperature constant. This is the variable volume, constant temperature approach. VAV – variable air volume system.
• Vary the supply air temperature and the flow
rate, as in a variable volume reheat system.
Relative Humidity Control
• Humidification - The air is too dry and water
vapor must be added for comfort.
• Dehumidification - The air is too
wet and water vapor must be
removed for comfort.
• AC systems typically over-cool
the air to remove water vapor, and then may
have to heat the air back up - this is called
reheat, and requires additional energy.
• Either way, energy required is around 1000 Btu
per pound of water.
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