Chapter 16 Refrigeration

1
Chapter 16Refrigeration Air Conditioning
Technology 5th EditionAdvanced Automatic
Controls
2
Control Applications

• Original system controls were manually activated
  and operated
• Thermostats and other simple controls were added
  to reduce human contact with the equipment
• Modern automatic control systems require very
  little human attention

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Control Applications

• Large buildings can be maintained by a small
  staff of technicians
• Sophisticated building systems are often used
• Many staff members do not know how to operate
  these systems
• There are often areas that are cooler or warmer
  than desired
• Controls must be understood
• Technicians need to be educated
• Should be able to calibrate controls
• Need to be able to interpret blueprints
• Need to interpret the intentions of the system
  designer

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Types of Control Systems

• First control systems started and stopped main
  system components
• Newer controls can modulate (start, stop, or
  adjust) the flow or operation of system
  components
• Three basic components of a control system
• The sensor measures changes in conditions
• The controller sends an output signal to the
  device being controlled
• The controlled device reacts to the controller
  signal to modulate fluid or electrical current
  flow
• Control systems are often divided into two
  control loops

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Pneumatic Controls

• Mechanical systems
• Use air to transmit signals between the sensor
  and the controlled device
• Electronic can be integrated into the pneumatic
  systems
• Advantages of pneumatic systems
• Simple to install and understand
• Safe around combustible materials
• Versatile and reliable
• Lower installation cost

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Pneumatic Controls

• Require a clean and dry permanent air supply
• Air pressure (100 to 250 psig) generated by an
  air compressor
• Regulators reduces pressure to working pressure
  (20 to 30 psig)

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Cleaning and Drying Control Air

• Pre-filter to the compressor
• Moisture is removed from the air
• Local water supply and coil
• Chilled water coil
• Desiccant
• Refrigerated drier (most common)
• High Density filters
• Final filtering stage
• Removes oil vapor and remaining liquid

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Control Components

• Control air used to position-controlled devices
• The pressure on control valves is equal to the
  air pressure multiplied by the cross sectional of
  the diaphragm
• Air lines are typically ¼ or 3/8
• Typically controlled components are valves,
  dampers, pumps, compressors, and fans
• Valves can be normally open or normally closed
  devices

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Control Components

• Some heating systems have normally open valves
  (reverse acting)
• Pilot positioners
• Use a small volume of air to control a larger
  volume of air
• Operates an air cylinder that control the valve
• Fluid flow can be modulated
• 100 flow through the device
• 0 flow through the device
• Partially open to permit intermediate flow
  percentage

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Control Components

• Based on sensitivity or gain
• Pressure and temperature at fully open position
• Pressure and temperature at fully closed position
• Sensitivity change in pressure / Change in
  temperature
• High sensitivity control will hunt
• Low sensitivity Noticeable difference in space
  temperature
• Pressure gauges used to measure pressure at the
  controls
• Gauges have quick connects
• Different gauges for different manufacturers

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Control Components

• Controls adjusted with Allen wrenches
• Can be used to control electric switches or
  circuits
• Pneumatic electric (PE) device
• Similar in operation to a pressure switch
• Electrical devices can be used to control
  pneumatic devices
• Electric-pneumatic (EP) devices

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Direct Digital Controls

• Referred to as DDC
• Operate at very low voltages
• Can be used to open, close, or modulate valves,
  dampers, motor speeds, etc.
• Systems with DDC can be controlled from remote
  locations via computers and phone lines
  (Internet)
• Corrective actions can be taken from these remote
  locations

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Direct Digital Controls

• Temperature, humidity, motor speed, and other
  system parameters can be observed
• Can be used for
• Energy management (load shedding)
• Night/weekend set-back
• Lighting circuit
• Smoke detectors
• Other building functions

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Residential Electronic Controls

• Electronics used on residential systems for many
  years
• Oil burner control boards
• Room thermostats
• Entire board is evaluated, not the individual
  board components
• Use manufacturers diagnostic systems
• Boards are often treated with switches
• Boards often have diagnostic codes for
  troubleshooting

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Residential Electronic Controls

• Thermistors are often used
• Temperature-sensitive devices
• Resistance changes with changes in temperature
• Temperature difference point in a system can be
  measured
• Functions of electronic thermostats
• Time delay for compressor starting
• Cycle timer for heating systems
• Stage timer for two-stage heating systems
• Heating/cooling set-points
• Automatic changeover between heating and cooling
  modes
• Power checks
• System error codes
• Smart recovery

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Residential Electronic Controls

• Often equipped with DIP switches
• Allows control to be configured for the equipment
  
• Allows user to customize system operation
• Electronics in heat pump systems
• Heating or cooling operation
• High or low speed for multi-speed compressors
• Status lights indicate mode of operation
• Discrepancies need to be evaluated
• Chart on equipment provides status light codes