HVAC CONTROLS (See section 18.9)

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HVAC CONTROLS (See section 18.9)

• Thermostat (additional information)
• A thermostat is the example of a sensing device.
  Sensing devices of an HVAC control system sense
  air or water temperature, humidity, and steam
  pressure or air pressure, and convert the
  sensation into a transmittable signal.
• A thermostat signals the heating/cooling
  equipment when to start and stop and how much to
  modulate. The temperature difference between when
  the equipment is shut off and when it is called
  to restart is referred to as the "dead band."

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HVAC CONTROLS

• When the room temperature falls below a specified
  set point for heating minus the thermostat dead
  band, the thermostat will call for heating.
• If the room temperature rises above a
  user-specified set point for cooling plus the
  dead band value, the thermostat will call for
  cooling.

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HVAC CONTROLS

• Thermostats must be located where they can sense
  the average temperature of the space to be
  conditioned.
• They should not be located where there is a
  possibility of exposure to conditions that are
  not representative of the whole space.
• such as in direct sunlight, in a cold draft, on
  an outside wall, or near a heating or cooling
  surface.

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HVAC CONTROLS

• If the thermostat is desired to be concealed for
  certain reasons, a variation of the standard wall
  thermostat can be located in the return air duct.
• Thermostats are also available with
  remote-sensing wires which may be wrapped
  inconspicuously around an art object.

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HVAC CONTROLS

• Self-contained microprocessor-based thermostats
  can be programmed for a wide range of daily
  setback cycles that can change for each day of
  the week. These devices are simple to operate and
  can control a number of different zones with
  separate sensors.
• Dampers (additional information)
• Circulating and controlling of air extensively
  involve dampers.

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HVAC CONTROLS

• Single blade They are mostly manually operated
  and installed with a locking device, called a
  locking quadrant, by means of which they may be
  fixed in the desired position. May also be
  automatic and may either be center or edge
  pivoted.
• Barometric Usually made with fine-tune
  adjustment features and equipped with adjustable
  counterweights. Often used in the smoke pipe of a
  furnace or boiler to stabilize draft conditions
  and permit close adjustment of fuel burners.
• Multiblade Used for control of large volumes of
  air moving at high or low velocities in large
  duct systems. They are of two types parallel
  blade and opposed blade. In parallel blade type,
  all blades turn in same direction. In opposed
  blade type, alternate blade turns in opposite
  direction.

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NOISE AND VIBRATION (Additional information)

• Sound becomes noise when it is objectionable to
  the occupants of a building. Without proper
  precautions, sounds generated by HVAC systems may
  become irritating noise.
• Sound is a by-product of energy supplied to the
  moving components of HVAC equipment. During the
  transformation of electrical or combustion energy
  into useful work, energy losses appear as heat,
  vibration, and sound. The items contributing the
  most sound are pumps, fans, compressors, and room
  air diffusers.

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NOISE AND VIBRATION

• The HVAC industry has established noise criteria
  (NC) values for evaluating the acceptability of
  sound levels. NC values for different types of
  buildings range from 30 to 40 decibels.
• A decibel is a unit of comparative sound
  measurement (a whispered conversation at a
  distance of 6 ft. from the ear, for example, has
  a sound pressure level of 30 decibels).
• The noise level and vibration transmission can be
  partially controlled by mechanical isolation,
  shields, baffles, and acoustical liners.
• Massive concrete pads and vibration isolators are
  placed between the equipment and the building to
  avoid transmission of vibration and noise through
  the structure. Equipment rooms can be
  acoustically isolated if necessary.

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NOISE AND VIBRATION

• Rectangular ducts, particularly those with high
  aspect ratio, can transmit excessive noise if not
  properly supported.
• Round ducts are stronger and have better
  aerodynamic characteristics, and therefore
  experience fewer noise problems.
• Registers and diffusers should be selected to
  minimize noise output.
• Equipment spaces should be separated as far as
  possible from spaces with demands for low
  background sound levels.
• Use of flexible fabric connections is recommended
  at points where the ductwork connects an
  equipment subject to vibration.

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MISCELLENEOUS (Additional information)

• Aspect ratio
• It is the width to height ratio of an air supply
  device (e.g. a supply duct). A rectangular duct
  with an aspect ratio closer to one has lower
  frictional resistance.
• Aspiration ratio
• As supply air leaves a supply register, it has a
  certain volume of flow rate expressed in cfm
  this is called primary air.
• As soon as this air leaves the outlet, it begins
  to attract air already existing in the room this
  room air, called the secondary air, joins the
  primary air and is carried along with it.
• The moving air stream has now a much greater
  volume by the time it reaches the end of its
  throw.
• This total volume, divided by the primary air
  volume is called the aspiration ratio.
• A high aspiration ratio is good, because it means
  that a greater quantity of air is kept in motion,
  with less chance of stagnation in parts of the
  room and with less chance of temperature
  stratification within the room.