Unit 40 Typical Operating Conditions

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Unit 40 - Typical Operating Conditions
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40.1. Mechanical Operating Conditions

• a. Design conditions for air conditioning
• 1. 95 degree outside air temperature
• 2. 80 degree inside air temperature
• 3. 50 humidity
• b. Systems are rated at the above conditions
• c. Standard efficiency systems condense
  refrigerant at about 125 degrees

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40.2 Relative Humidity and the Load

• a. Relative humidity increases the load on the
  system
• b. Equipment capacity varies with changes in
  humidity

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40.3 System Component Relationships Under Load
Changes

• a. Increases in outside temperature
• 1. Higher head pressure
• 2. Reduced system capacity
• b. Space temperature and humidity affects system
  capacity
• c. Refrigerant holds different amounts of heat at
  different temperatures and pressures

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40.4 Evaporator Operating Conditions

• a. Normal operating temperature is 40 degrees
• 1. 75 degree inside air temperature
• 2. 50 humidity
• 3. 70 psig for R-22
• 4. Approximate evaporator superheat is 10 degrees
• b. Increase in heat load (fixed bore metering
  device)
• 1. Inside air temperature rises to 77 degrees
• 2. Suction pressure is now 73 psig (R-22)
• 3. Evaporator superheat is now 13 degrees
• c. Field conditions are rarely ideal
• d. Common conditions are used for troubleshooting
  purposes

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40.5 High Evaporator Load and a Cool Condenser

• a. Can occur when the unit is on all day
• b. The space temperature becomes warmer than the
  outside ambient
• c. Example 75 degree outside temperature, 80
  degree inside temperature
• d. The condenser will become too efficient
• e. Liquid refrigerant will accumulate in the
  condenser
• f. The evaporator will starve and lose system
  capacity
• g. The evaporator coil may freeze
• h. Two speed condenser fans may be used
• 1. Low speed used when outside temperatures are
  low
• 2. High speed used when outside temperatures are
  high

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40.6 Grades of Equipment

• a. Three grades
• 1. Economy
• 2. Standard efficiency
• 3. High efficiency
• b. Economy and standard efficiency
• 1. Efficiencies are similar
• 2. Materials used are different
• 3. System appearance is different
• 4. Refrigerant condenses at a temperature about
  30 degrees higher than ambient
• c. High efficiency systems
• 1. More efficient than economy and standard
  grades
• 2. Operate with lower head pressures
• 3. Have larger condenser coils
• 4. Use less power
• 5. Refrigerant condenses at a temperature as low
  as 20 degrees higher than ambient

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40.7 Documentation with the Unit

• a. Pressure charts
• 1. Provides suction and discharge pressure charts
• 2. Covers a wide range of conditions
• b. Furnished with the unit in the start-up manual
• c. Conditions must be considered
• 1. Load on condenser coil
• 2. Sensible and latent heat loads on the
  evaporator coil
• d. Existing conditions are plotted on the charts

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40.8 Establishing a Reference Point on Unknown
Equipment

• a. Distinguish between standard and high
  efficiency
• b. High efficiency equipment is usually larger
• c. High efficiency systems operate with lower
  head pressures
• d. High efficiency systems have lower amperage
  ratings

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40.9 Metering Devices for High Efficiency
Equipment

• a. High efficiency systems usually use a
  thermostatic expansion valve
• b. High efficiency systems may have oversized
  evaporator coils
• c. Operating conditions near design space
  conditions for standard efficiency equipment
• 1. Suction temperature is 40 degrees
• 1. 70 psig for R-22
• 2. 37 psig for R-12
• 2. Refrigerant typically condenses at a
  temperature 30 degrees higher than outside
  ambient temperature
• 1. 95 degree outside temperature
• 2. 95 degrees 30 degrees 125 degrees
• 3. Condenser saturation temperature is 125
  degrees
• 4. 278 psig for R-22
• 5. 169 psig for R-12
• d. Space temperature higher than normal for
  standard efficiency equipment
• 1. Suction pressure higher than normal
• 1. Refrigerant normally boils at 40 degrees with
  a 75 degree space temperature
• 2. If the room temperature rises to 85 degrees,
  the refrigerant will boil at 50 degrees (84 psig
  for R-22)
• 2. Discharge temperature is determined by outside
  temperature and suction pressure
• 1. If suction pressure rises, the condensing
  temperature will rise
• 2. Head pressure will drop as the evaporator load
  drops

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40.10 Equipment Efficiency Rating

• a. EER - Energy Efficiency Ratio
• 1. EER Btu/h (input)
• Wattage (input)
• 2. The higher the EER, the higher the efficiency
• 3. Steady-state rating
• b. SEER
• 1. Seasonal Energy Efficiency Ratio
• 2. Includes start-up and shut down cycles

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40.11 Typical Electrical Operating Conditions

• a. Residential
• 1. 230 volts
• 2. Single phase
• b. Light commercial
• 1. 208/230 volts
• 2. Single or three phase
• c. 208 volts is produced from the secondary of a
  wye transformer configuration
• d. 230 volts is produced from the secondary of a
  delta configuration
• e. Three phase motors have no start assist
• f. Single phase motors usually have a start
  assist

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40.12 Matching the Unit to the Correct Power
Supply

• a. Operating voltages should be within 10 of
  nameplate ratings
• b. 208 volt nameplate has a range from 187 volts
  to 229 volts
• c. 230 volt nameplate has a range from 207 volts
  to 253 volts
• d. If the supply voltage is out of range, the
  equipment should not be started

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40.13 Starting the Equipment with the Correct Data

• a. Equipment can be started if the supply voltage
  is correct
• b. Voltage characteristics must be printed on the
  motor
• c. If the data is not accessible, contact the
  manufacturer

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40.14 Finding a Point of Reference for an Unknown
Motor Rating

• a. Electrical ratings can be improvised by
  estimating system capacity
• b. Compare the system in question to a known unit
• c. Nameplate data may not be correct if the motor
  was replaced

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40.15 Determining the Compressor Running Amperage

• a. Running load amperage is usually not provided
  on the data tag
• b. If the running load amperage is supplied it
  should not be exceeded
• c. Nominal ratings are rounded off

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40.16 Compressor Operating at Full-load Current

• a. Compressor rarely operates at full load
  amperage
• b. During high load conditions, the compressor
  may operate near full load amperage

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40.17 High Voltage, the Compressor and Current
Draw

• a. Higher supply voltages result in lower
  compressor currents
• b. Overloaded compressors may still draw low
  current if the voltage is high
• c. Nameplate currents are usually the high end of
  the operating range

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40.18 Current Draw and the Two-speed Compressor

• a. Used to achieve high seasonal efficiencies
• b. Can operate as two of four pole motors
• c. Can operate at 1800 rpm or 3600 rpm
• d. Lower speed is used for mild weather and low
  load conditions
• e. Usually controlled by electronic circuits