Superheat

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Tennessee Technology Center at Pulaski
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Superheat Sub-cooling

• A Technicians Guide to HVACR Diagnostics

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Introduction

• The ability to properly and accurately measure
  superheat and sub-cooling and interpret the
  results is very likely the single most important
  skill that you, as an HVACR service technician,
  can acquire
• As you study this program, keep one very
  important thing in mind .

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No Test Is Valid If TheAir Flow Is Not
Correct!!!
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Superheat

• A vapor is said to be superheated when its
  temperature is higher than its saturation
  temperature at the same pressure
• Sub-cooling occurs to a liquid when its
  temperature is below saturation for the same
  pressure

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Checking Superheat

• Allow the system to operate for 15 20 minutes to
  stabilize
• Attach an accurate thermometer to the suction
  line near the sensing bulb on TEV systems or near
  the suction service valve on fixed restrictor
  systems

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• Record the suction line temperature
• Connect a manifold gauge set to the suction
  service valve and record the suction pressure
• Use a temperature/pressure chart to obtain the
  saturation temperature for the suction gas at the
  pressure recorded
• Subtract the saturated temperature from the
  actual suction line temperature the result is
  the operating superheat

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Example 1

• Suction line temperature 55 F
• Suction pressure 68.5 psig
• 68.5 psig 40 F

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• 55 F 40 F 15 F superheat

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Lets examine some common system problems and see
how they affect superheat and sub-cooling
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High Superheat

• Possible causes and remedies for HIGH SUPERHEAT

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Excessive or HIGH superheat is an indication of
insufficient refrigerant in the indoor coil for
the heat load present

• This could be from insufficient refrigerant
  entering the coil or from an excessive heat load
  crossing the coil

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LOW REFRIGERANT CHARGE

• If there is insufficient refrigerant in the
  indoor coil, all of the refrigerant will
  evaporate in the first few passes of the coil.
• The excess sensible heat picked up by the
  refrigerant vapor causes a higher than normal
  suction gas temperature

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• Discharge pressure will be lower than normal
• Suction pressure will be lower than normal
• Superheat will be higher than normal
• Sub-cooling will be lower than normal
• Current draw will be lower than normal

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Liquid Line Restriction

• A restriction in the liquid line will not allow
  sufficient refrigerant to reach the evaporator
  coil
• This causes many symptoms similar to a low
  refrigerant charge
• Often there is a noticeable temperature change at
  the point of the restriction

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• Suction pressure will be lower than normal
• Discharge pressure will be normal to lower than
  normal
• Superheat will be high
• Sub-cooling will be high
• Current draw will be low

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Evaporator Air Flow Too High

• Excessive air flow reduces the latent capacity of
  the coil thus increasing the sensible heat load.
• This additional sensible heat results in higher
  than normal suction gas temperatures and pressures

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• Discharge pressure will be high
• Suction pressure will be high
• Superheat will be high
• Sub-cooling will be lower than normal
• Current draw will be higher than normal

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Excessive Load Conditions

• Excessive indoor coil loads will cause a higher
  than normal heat content in the air crossing the
  coil
• This excess heat will cause the refrigerant
  liquid to boil away sooner allowing the vapor to
  pick up additional superheat
• Most commonly caused by internal gains such as an
  increase in occupancy load

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• Discharge pressure will be higher than normal
• Suction pressure will be higher than normal
• Superheat will be high
• Sub-cooling will be lower than normal
• Current draw will be high

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Metering Device Not Feeding Properly

• A restriction in a capillary tube, orifice or TEV
  will reduce the amount of liquid refrigerant
  entering the evaporator.
• Symptoms are the same as for a liquid line
  restriction

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• Discharge pressure will be lower than normal
• Suction pressure will be lower than normal
• Superheat will be higher than normal
• Sub-cooling will be higher than normal
• Current draw will be lower than normal

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Low Superheat

• Possible causes and remedies for Low SUPERHEAT

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LOW SUPERHEAT

• Low superheat indicates an excess of liquid
  refrigerant in the evaporator coil
• Liquid refrigerant is very likely entering the
  compressor
• This results in reduced compressor life and
  possible imminent compressor failure

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Refrigerant Overcharge

• An overcharge forces excessive refrigerant into
  the evaporator due to increased pressure
  differential
• There is not enough heat present to completely
  vaporize the excess refrigerant
• Compressor failure is likely

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• Discharge pressure will be higher than normal
• Suction pressure will be higher than normal
• Superheat will be lower than normal
• Sub-cooling will be higher than normal
• Current draw will be higher than normal

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TEV Overfeeding

• Many symptoms similar to an overcharge
• Sensing bulb not insulated or not secured
  properly
• Improperly sized valve
• Wrong valve for the application

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• Discharge pressure will be higher than normal
• Suction pressure will be higher than normal
• Superheat will be lower than normal
• Sub-cooling will be lower than normal
• Current draw will be higher than normal

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Low Evaporator Heat Load

• Most common cause of low superheat
• Low air volume (dirty coils, filters, restricted
  duct, etc.)
• Reduces the heat available to vaporize the
  refrigerant
• Liquid refrigerant may enter the compressor

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• Discharge pressure will be lower than normal
• Suction pressure will be lower than normal
• Superheat will be lower than normal
• Sub-cooling will be higher than normal
• Current draw will be lower than normal

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Improper Metering Device

• The wrong orifice
• A capillary tube the wrong size (or that has been
  shortened)
• An improperly sized TEV
• Symptoms identical to device overfeeding

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• Discharge pressure will be higher than normal
• Suction pressure will be higher than normal
• Superheat will be lower than normal
• Sub-cooling will be lower than normal
• Current draw will be higher than normal

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Equipment Oversized

• When a system is greatly oversized there is not
  enough heat to vaporize the refrigerant present
  in the evaporator
• Symptoms are similar to a low charge, except that
  a low charge will have a high superheat and run
  excessively
• Oversized unit will likely short cycle and have a
  low superheat

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• Discharge pressure will be lower than normal
• Suction pressure will be lower than normal
• Superheat will be lower than normal
• Sub-cooling will be higher than normal
• Current draw will be lower than normal

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Condenser Air Flow

• Low condenser air flow or recycled condenser air
  will increase condensing temperature thus
  increasing condenser pressure
• Increased pressure drop across the metering
  device results in a flooded evaporator

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Caused by

• Dirty coil
• Bad motor or blade
• Shrubs, bushes or other obstructions
• Low overhangs
• Other equipment too close

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• Discharge pressure will be higher than normal
• Suction pressure will be higher than normal
• Superheat will be lower than normal
• Sub-cooling will be lower than normal
• Current draw will be higher than normal

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SUB-COOLING

• A liquid is sub-cooled when its temperature is
  below saturation at the same pressure
• Measuring sub-cooling is a good method of
  confirming your diagnosis based on other tests
• TEV systems MUST be charged by sub-cooling in the
  absence of a known charge quantity

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Measuring Sub-cooling

• Allow the system to operate for 15 20 minutes to
  stabilize
• Attach an accurate thermometer to the liquid line
  near the inlet of the metering device whenever
  possible
• The condenser outlet may be used, but will be in
  error by the amount of liquid line
  pressure/temperature losses

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• Record the liquid line temperature
• Using a gauge manifold, obtain the liquid line
  pressure
• Discharge pressure may be used, but allowances
  must be made for condenser pressure drop

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• Using a temperature/pressure chart, convert the
  pressure reading to saturation temperature
• Subtract the line temperature from the saturation
  temperature
• The difference is operating sub-cooling
• In the absence of manufacturers data, a
  sub-cooling reading of 10 degrees or more is
  usually acceptable

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Sub-cooling Losses

• Long liquid lines
• Liquid lines exposed to high ambient temperatures
  (un-insulated)
• Low condenser air flow
• Inadequate condenser size
• Long vertical lifts

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Long Liquid Lines

• Long liquid lines cause increased pressure drop
  due to friction losses
• Use the shortest lines possible
• Relocate equipment if necessary

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Liquid lines exposed to high ambient

• High ambient increases liquid line temperature
• Exposed liquid lines should be insulated
• Heat exchangers or auxiliary sub-coolers may be
  considered.

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Low condenser air flow

• Low condenser air flow reduces the condensers
  ability to reject heat
• Causes increased condensing temperature
• Clean condenser coils and clear any obstructions

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Inadequate condenser size

• A condenser coil that is too small uses all of
  the available space for condensing leaving no
  room for sub-cooling
• May be caused by the presence of NCGs
  (non-condensable gasses)

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Long vertical lifts

• The weight of the refrigerant in long vertical
  lifts causes a pressure drop
• HCFC-22 looses about ½ PSIG for every foot of
  vertical rise
• Reduce the lift, use a heat exchanger or
  artificial liquid line amplification

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Practice Exercises
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Exercise 1

• 2 ½ ton package gas unit
• 5 years old
• No previous service
• Not cooling enough

• odb 95F
• Idb 80F
• Iwb 68F
• SP 55 PSIG
• ST 36F
• DP 210 PSIG
• LT 96F
• Current low

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LOW LOAD

• Superheat is low
• Sub-cooling is low to normal
• Both pressures are low
• Current draw is low

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Exercise 2

• 2 ½ ton package heatpump
• 15 years old
• No previous service
• Not cooling enough

• odb 95F
• Idb 80F
• Iwb 68F
• SP 55 PSIG
• ST 76F
• DP 180 PSIG
• LT 96F
• Current low

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LOW CHARGE

• Superheat is high
• Sub-cooling is low
• Both pressures are low
• Current draw is low