Results 1 to 43 of 43
  1. #1
    Join Date
    Jan 2008
    Location
    Kent UK
    Age
    49
    Posts
    515
    Rep Power
    18

    Talking Superheat and Subcooling



    I was just going back through the archives and found a few threads asking about this topic.Here is an interesting document on it.



    Testing rooftop refrigeration system with a Fluke 561 Infrared Thermometer.
    Troubleshooting and servicing refrigeration and air conditioning systems can be a challenging process for both the entry level and experienced HVACR technician. Regardless of your experience, size of the equipment, or location, to troubleshoot the system it is essential that you have a solid understanding of the fundamentals of refrigeration — including the principles of superheat and subcooling. You also need to have the right tools and know-how to apply these principles to use the tool quickly and efficiently.Troubleshooting techniques often require simultaneous knowledge of temperature, pressure, voltage, and current values in a system, which means that a single-function meter won’t permit a complete analysis of the system. Frequently, multiple tools are required.
    This article provides information on troubleshooting the refrigeration system while applying the principles of superheat and subcooling to HVACR equipment. It will also teach you the proper methods to tackle some typical troubleshooting tasks using thermometers, digital multimeters, pressure/vacuum modules, and HVACR accessories. Basic refrigeration principles are provided solely to illustrate how digital thermometers, multimeters, and accessories can make servicing and maintaining HVACR systems straightforward, fast, and accurate.



    THE REFRIGERATION CYCLE

    Figure 1. The refrigeration system. In a typical refrigeration system, the compressor sends hot gas to the condenser. Then the condensed liquid passes through an expansion valve into the evaporator where it evaporates and collects heat from the area to be cooled. The gaseous refrigerant then enters the compressor where the compression process raises the pressure and temperature. From the compressor, the refrigerant is routed back to the condenser and the cycle repeats.
    Based on the principle that heat flows naturally from warmer areas to cooler areas, the refrigeration cycle consists of seven stages:1. Compression of hot gas
    2. Cooling
    3. Condensing
    4. Subcooling
    5. Expansion
    6. Evaporation
    7. Super heating
    A basic vapor compression refrigeration system consists of four primary components: a metering device (e.g., a capillary tube, fixed orifice/piston, or a thermostatic expansion valve), evaporator, compressor, and condenser. (See Figure 1.)
    Compression energy elevates the vapor pressure to a boiling point that is below the condensing mediums’ temperature. In other words, the compressor elevates the boiling point of the refrigerant to a point at which the air (or water) moving across the condenser is low enough to condense the refrigerant to a liquid. Additional passes in the condenser coil cool the liquid refrigerant below its boiling point to ensure it remains a liquid as it experiences pressure drop in its journey to the evaporator. This cooling below the boiling point is called subcooling.
    A metering device at the evaporator inlet acts as a “dam” to restrict flow and drop the refrigerant pressure to a new lower boiling point. This new boiling point is below the evaporator medium (air or water) temperature so that the air or water across the evaporator will cause the refrigerant to boil. After all of the refrigerant in the evaporator has boiled to a vapor, the vapor will pick up additional heat through extra passes in the evaporator. The amount of vapor temperature increase above the boiling temperature is called superheat.
    The compressor reduces the gas to a high pressure while simultaneously raising the temperature of the gas. The hot gas is then delivered to the condenser where it is cooled, dissipating the heat and steadily converting the gas back to a liquid state.
    Note: Liquid receivers are not typically used on refrigeration systems, which commonly rely on capillary tubes or fixed metering devices.
    When the liquid under high pressure reaches the metering device, the cycle starts over.
    While servicing most refrigeration systems, the technician will measure the temperature and pressure to determine system performance. Close monitoring of temperature and pressure to verify proper control and operation can ensure longer system life and reduce energy consumption.
    Often, measuring temperatures or pressures at key points in a system can pinpoint trouble spots. Examples of such measurements follow.



    SUPERHEAT AND ITS MEASUREMENT

    Enlarge this picture Figure 2. Superheat temperature measurement points. (Click on the illustration to view a larger image.)
    In the system’s evaporator, conversion of liquid to vapor involves adding heat to the liquid at its boiling temperature, commonly referred to as the saturation temperature. After all of the refrigerant has boiled to a vapor, any additional temperature increase above the boiling point is called superheat.Finding suction line superheat requires finding the suction pressure and two temperatures — the evaporator boiling temperature at a given pressure and the temperature of the refrigerant at the outlet of the evaporator on the suction line, commonly referred to as the superheat temperature/pressure method.
    Finding the boiling temperature is determined by using a pressure-temperature (PT) chart. On older CFC and HCFC refrigerants, and some newer ozone-friendly refrigerants such as R-134a, boiling temperature remains constant during the saturation or boiling phase provided that the pressure remains the same within the evaporator.
    On newer refrigerant blends, the temperature changes during the boiling or saturation phase. This is referred to as glide. Modern refrigerants with a temperature glide of 10°F (5°C) or higher use dew point (DP) temperature. This is the temperature of the refrigerant when the last of the liquid has boiled into a vapor. Any vapor temperature increase above the dew point temperature is called superheat. (See Figure 2.)



    Figure 3. Suction line superheat using the temperature-pressure method. Measure the pressure at the suction line service valve. Find the evaporator boiling temperature from a temperature-pressure chart using suction line pressure. Subtract this temperature from the suction line temperature measured by a digital thermometer. The difference is superheat.
    One of the best methods to determine superheat is to use a pipe clamp temperature probe and a pressure/vacuum module in conjunction with a suitable digital multimeter with type K thermocouple measurement and a mV input. The pipe clamp temperature probe allows pipe temperature measurements to be made more quickly and accurately because it clamps directly to the pipe without the need to add insulation or tape, as in the case of a bead thermocouple. The pressure/vacuum module allows accurate and quick pressure measurements.When measuring for superheat, remember to allow the system to run long enough for temperatures and pressures to stabilize while verifying normal airflow cross the evaporator. Using the pipe clamp or a Velcro pipe probe, find the suction line temperature by attaching the probe around a bare section of the pipe, at the outlet of the evaporator. Pipe temperature can be read at the inlet of the compressor on the suction line if the pipe is less than 15 feet from the evaporator and there is a minimum pressure drop between the two points. (See Figure 3.) Best results are obtained when the pipe is free of oxides or other foreign material.



    Next, attach the pressure/vacuum module to the suction line service valve (or refrigerant service port on your manifold gauge set). Make a note of the pipe temperature and pressure. This pressure reading will be that of the boiling refrigerant inside the evaporator, assuming no abnormal restrictions exist within the suction line. Using this pressure value, find the evaporator (or dew point) boiling temperature from a PT chart for the refrigerant type being used. Subtract the boiling/dew point temperature from the suction line temperature to find the superheat.The suction line temperature may also be taken by attaching a bead thermocouple to the suction line. Be careful to insulate the thermocouple and use heat-conducting compound to minimize errors due to heat loss to ambient air.



    SUBCOOLING AND ITS MEASUREMENT

    Figure 4. Subcooling. After verifying normal airflow, place the pipe clamp or Velcro probe on the liquid line. Note the temperature. Then attach the pressure/vacuum module to a port on the liquid line and measure the liquid line pressure. Determine the condensing temperature by using the temperature-pressure chart for the refrigerant type used. The difference in temperature is the subcooling value.
    In the system’s condenser, conversion of vapor to liquid involves removing heat from the refrigerant at its saturation condensing temperature. Any additional temperature decrease is called subcooling. Finding liquid line subcooling requires determining the condensing pressure and two temperatures — the condensing temperature at the measured condensing pressure and the temperature of the refrigerant at the outlet of the condenser on the liquid line. The liquid line temperature involves measuring the surface temperature of the pipe at the outlet of the condenser. (See Figure 4.)Note: Condensing temperature is derived from using the PT chart. On new refrigerant blends with high temperature glide, this is called the bubble point (BP) temperature. See Figure 2.



    To measure subcooling with a pipe clamp temperature probe, or a Velcro pipe probe, allow the system to run long enough for temperatures and pressures to stabilize. Verify normal airflow and then find the liquid line temperature by clamping the pipe clamp around the liquid line. Attach the pressure/vacuum module to a service port on the liquid line (or discharge line at the compressor if a liquid line service valve port is not available). Make a note of the liquid line temperature and pressure. Convert the liquid line pressure to temperature using a PT chart for the refrigerant type being used. The difference of the two temperatures is the subcooling value.


    TROUBLE DIAGNOSIS

    Data from superheat and subcooling measurements can be useful for determining various conditions within the HVACR system, including the amount of refrigerant charge and verifying the operating condition of the metering device. These measurements can also be used to determine the efficiency of the condenser, evaporator, and compressor.Before making conclusions from the measured data, it is important to check external conditions that influence system performance. In particular, you should inspect and verify the proper airflow in cubic feet per minute (cfm) across coil surfaces and line voltage to the compressor motor and associated electrical loads. Remember to look for obvious problems at the coil surfaces such as dirty air filters upstream of the evaporator, or leaves and outside debris restricting airflow on the condenser.



    USING SUPERHEAT TO TROUBLESHOOT

    Measuring liquid line temperature with a DMM and a pipe clamp.
    The superheat value can indicate various system problems including a clogged filter drier, undercharge, overcharge, faulty metering device, restricted airflow, or improper fan motor or blower direction. Suction line superheat is a good place to start diagnosis because a low reading suggests that liquid refrigerant may be reaching the compressor. In normal operation, the refrigerant entering the compressor is sufficiently superheated above the evaporator boiling temperature to ensure the compressor draws only vapor and no liquid refrigerant. On traditional HVACR systems, which utilize mechanical metering devices such as a TXV or cap tube, the superheat heating will vary between 8°F to 20°F. On newer systems, which use electronic expansion valves and solid state controllers, it is possible to see the superheat setting as low as 5°F to 10°F.A low or zero superheat reading indicates that the refrigerant did not pick up enough heat in the evaporator to completely boil into a vapor. Liquid refrigerant drawn into the compressor typically causes slugging, which can damage the compressor valves and/or internal mechanical components. Additionally, liquid refrigerant in the compressor, when mixed with oil, reduces lubrication and increases wear, causing premature failure.
    On the other hand, if the superheat reading is excessive — above 20°F to 30°F — it indicates that the refrigerant has picked up more heat than normal, or that the evaporator is being starved of refrigerant. Possible causes of this condition include a metering device that is underfeeding, improperly adjusted, or simply broken. Additional problems with high superheat could indicate a system undercharge, a refrigerant restriction, moisture in the system, a blocked filter drier, or excessive evaporator heat loads.



    USING SUBCOOLING TO TROUBLESHOOT

    Measuring temperature and suction pressure to determine superheat.
    An improper subcooling value can indicate various system problems including overcharge, undercharge, liquid line restriction, or insufficient condenser airflow (or water flow when using water-cooled condensers). The refrigerant is typically subcooled between 10°F to 20°F at the outlet of the condenser, however, some modern equipment may have subcooling values as low as 4 degrees in order to meet minimum efficiency standards.For example, a very low reading between zero to 10°F subcooling indicates that the refrigerant did not lose the normal amount of heat in its travel through the condenser. Possible causes for this condition include insufficient airflow over the condenser, metering device problems such as overfeeding, misadjustment, or being stuck too far open, or the system may be undercharged. Oftentimes, the problem is simply that the condenser coil surface needs to be cleaned thoroughly to eliminate airflow restriction.
    Excessive subcooling means the refrigerant was cooled more than normal. Possible explanations include an overcharged system, a restriction in the metering device, misadjusted (underfeeding), or faulty head pressure control during low ambient conditions.


    If it ain't broke, it doesn't have enough features yet.


    Damo

  2. #2
    Join Date
    Jan 2008
    Location
    Kent UK
    Age
    49
    Posts
    515
    Rep Power
    18

    Re: Superheat and Subcooling

    If it ain't broke, it doesn't have enough features yet.


    Damo

  3. #3
    Join Date
    Feb 2008
    Location
    Saudi Arabia
    Age
    52
    Posts
    69
    Rep Power
    17

    Re: Superheat and Subcooling

    it is great , thanks alot

  4. #4
    Join Date
    Mar 2008
    Location
    England
    Age
    54
    Posts
    273
    Rep Power
    17

    Re: Superheat and Subcooling

    Good post chunk although I work by touch n feel
    Transvestites are men who like to eat, drink, and be Mary.

  5. #5
    Join Date
    Mar 2006
    Location
    Lancashire
    Posts
    1,876
    Rep Power
    28

    Re: Superheat and Subcooling

    Thanks Chunk, a good detailed article.
    The difference between genius and stupidity is that genius has its limits.

    Marc

  6. #6
    merrimat's Avatar
    merrimat Guest

    Re: Superheat and Subcooling

    So which way is the best to correctly charge a R-22 Split unit Sub Cooling or Superheat?

  7. #7
    GXMPLX's Avatar
    GXMPLX is offline Veteran Poster I am starting to push the Mods: of RE
    Join Date
    May 2008
    Location
    weston
    Posts
    349
    Rep Power
    16

    Re: Superheat and Subcooling

    Quote Originally Posted by merrimat View Post
    So which way is the best to correctly charge a R-22 Split unit Sub Cooling or Superheat?
    Subcooling!

  8. #8
    Join Date
    Mar 2008
    Location
    US
    Posts
    74
    Rep Power
    17
    Quote Originally Posted by GXMPLX View Post
    Subcooling!
    Why not superheat? Superheat can avoid liquid into compressor.

    Good article.
    Thank you.
    Last edited by chillin out; 19-07-2008 at 03:05 PM.

  9. #9
    GXMPLX's Avatar
    GXMPLX is offline Veteran Poster I am starting to push the Mods: of RE
    Join Date
    May 2008
    Location
    weston
    Posts
    349
    Rep Power
    16

    Re: Superheat and Subcooling

    Good, Ok ... both! Can’t I make mistakes?

    Subcooling is more important. Charge until you get enough to feed your expansion device correctly.

    Then check superheat to verify that the compressor is working in safe conditions. If you have a suction accumulator you could really get by not measuring superheat (IM NOT SUGGESTING YOU DON'T, I’M JUST MAKING A POINT THAT SUBCOOLING IS MORE IMPORTANT... TO CHARGE).

    Why both because the problem by charging with subcooling is that you have to do it at maximum thermal load and your system may be overcharged for minimal loads, so you need a balance or compromise between two opposites, complying at maximum load and operating safely at minimum load.

    The correct thing would be check superheat at minimum load ... but as my wife says am at an age or repeating myself ... often... you really don't need superheat to charge.

    Why you shouldn't charge with superheat is that you don't have an exact amount of superheat and it will constantly change in contrast you can calculate exactly the amount of subcooling you need and it changes less.

    The correct amount of superheat is as low as possible without getting liquid in the compressor and not as high as to producer high discharge temperatures and that’s a wide range. Some manufactures say a minimum of 10c but it could really be as low as 0.1c as long as it is there!

    The exact amount of subcooling is given by the loss of pressure and temperature gain in the liquid line. Convert the pressure drop to equivalent saturated temperature difference using the correct refrigerant tables and add the temperature gain.

    In a working system you may measure both pressure drop and temperature gain.
    Another thing about superheat is that if your expansion valve is sized correctly and the system operates in a narrow range of temperatures and pressures the TEV will take care of superheat for you.

    So Subcooling! … no both! … or subcooling?

  10. #10
    Join Date
    Mar 2008
    Location
    US
    Posts
    74
    Rep Power
    17

    Re: Superheat and Subcooling

    Quote Originally Posted by GXMPLX View Post
    Good, Ok ... both! Can’t I make mistakes?

    Subcooling is more important. Charge until you get enough to feed your expansion device correctly.

    Then check superheat to verify that the compressor is working in safe conditions. If you have a suction accumulator you could really get by not measuring superheat (IM NOT SUGGESTING YOU DON'T, I’M JUST MAKING A POINT THAT SUBCOOLING IS MORE IMPORTANT... TO CHARGE).

    Why both because the problem by charging with subcooling is that you have to do it at maximum thermal load and your system may be overcharged for minimal loads, so you need a balance or compromise between two opposites, complying at maximum load and operating safely at minimum load.

    The correct thing would be check superheat at minimum load ... but as my wife says am at an age or repeating myself ... often... you really don't need superheat to charge.

    Why you shouldn't charge with superheat is that you don't have an exact amount of superheat and it will constantly change in contrast you can calculate exactly the amount of subcooling you need and it changes less.

    The correct amount of superheat is as low as possible without getting liquid in the compressor and not as high as to producer high discharge temperatures and that’s a wide range. Some manufactures say a minimum of 10c but it could really be as low as 0.1c as long as it is there!

    The exact amount of subcooling is given by the loss of pressure and temperature gain in the liquid line. Convert the pressure drop to equivalent saturated temperature difference using the correct refrigerant tables and add the temperature gain.

    In a working system you may measure both pressure drop and temperature gain.
    Another thing about superheat is that if your expansion valve is sized correctly and the system operates in a narrow range of temperatures and pressures the TEV will take care of superheat for you.

    So Subcooling! … no both! … or subcooling?
    Logical statementWhy both? Subcooling is enough!

    I appreciate the kind explanation by you. I found I become regular poster today. !Next I will try to answer other’s questions, like you and Iceman. Then become senior poster….

  11. #11
    Join Date
    Jun 2005
    Location
    California
    Posts
    395
    Rep Power
    19

    Re: Superheat and Subcooling

    You use sub cooling for TXV and superheat for restrictors/cap tubes and these have a critical charge because they are not very flexable with quick changing loads and therefor are prone toslugging if over charged

  12. #12
    GXMPLX's Avatar
    GXMPLX is offline Veteran Poster I am starting to push the Mods: of RE
    Join Date
    May 2008
    Location
    weston
    Posts
    349
    Rep Power
    16

    Re: Superheat and Subcooling

    Quote Originally Posted by wambat View Post
    You use sub cooling for TXV and superheat for restrictors/cap tubes and these have a critical charge because they are not very flexable with quick changing loads and therefor are prone toslugging if over charged
    No, you have to use both to make sure the cap is fed right and simultaneously measure superheat.

    You cannot use superheat only.

    Anyway I have never seen a successful field charge of these systems.

    They need extensive study of charge/load behaviour and for this reason all manufactures indicate amount to charge.

    Usually system manufacturers use compressor manufacturer's data on the amount of refrigerant in the crankcase that does not produce flooded start problems because the refrigerant will usually be in the compressor on startup.

  13. #13
    Join Date
    Jun 2008
    Location
    Serbia
    Posts
    13
    Rep Power
    0

    Re: Superheat and Subcooling

    Nice thread!

    Thanks a lot!

  14. #14
    Join Date
    Oct 2006
    Location
    CORDOBA -ARGENTINA
    Age
    75
    Posts
    69
    Rep Power
    18

    Re: Superheat and Subcooling

    Hi Chunk or Damo.

    How I shall understand the word HUNTING in the TXV troubleshooting table.
    English is not my native idiom.

  15. #15
    Join Date
    Feb 2007
    Location
    UK
    Posts
    2,479
    Rep Power
    45

    Re: Superheat and Subcooling

    Hunting=

    The TEV does not achieve a stable running condition, instead it continuously opens and closes.

    Normally when this happens you will see small variations of the suction pressure and at least 3-5 degrees "swing" of the suction temperature. (When observed over 5-10 minutes)

  16. #16
    Join Date
    Sep 2007
    Location
    Somerset
    Age
    69
    Posts
    4,758
    Rep Power
    46

    Re: Superheat and Subcooling

    Quote Originally Posted by The Viking View Post
    Hunting=

    The TEV does not achieve a stable running condition, instead it continuously opens and closes.

    Normally when this happens you will see small variations of the suction pressure and at least 3-5 degrees "swing" of the suction temperature. (When observed over 5-10 minutes)
    Hunting (Old Norse Term for providing for ones loved ones!!!)


    Grizzly

  17. #17
    Join Date
    Jan 2008
    Location
    Kent UK
    Age
    49
    Posts
    515
    Rep Power
    18

    Re: Superheat and Subcooling

    Quote Originally Posted by The Viking View Post
    Hunting=

    The TEV does not achieve a stable running condition, instead it continuously opens and closes.

    Normally when this happens you will see small variations of the suction pressure and at least 3-5 degrees "swing" of the suction temperature. (When observed over 5-10 minutes)
    Thanks Chunk
    If it ain't broke, it doesn't have enough features yet.


    Damo

  18. #18
    Join Date
    Feb 2007
    Location
    UK
    Posts
    2,479
    Rep Power
    45

    Re: Superheat and Subcooling

    Quote Originally Posted by Grizzly View Post
    Hunting (Old Norse Term for providing for ones loved ones!!!)


    Grizzly



    Actually...
    In more recent times, it is a term used by young English people searching for someone to mate with.
    [Note: The term "mate" has, in the English language, also changed meaning over the years. The old fashioned usage (as above) literarily means "to create a new DNA mixture"(apart from in some areas of UK, where it usually would mean passing on the same DNA chains to yet another generation but that's another story) .
    Recent usage of the term "mate" is referring to someone one consider to be a friend.
    This change of meaning might be due to English youngsters screwing everyone and everything, but this is still to be confirmed]

  19. #19
    Join Date
    Sep 2007
    Location
    Somerset
    Age
    69
    Posts
    4,758
    Rep Power
    46

    Re: Superheat and Subcooling

    Quote Originally Posted by The Viking View Post


    Actually...
    In more recent times, it is a term used by young English people searching for someone to mate with.
    [Note: The term "mate" has, in the English language, also changed meaning over the years. The old fashioned usage (as above) literary means "to create a new DNA mixture"(apart from in some areas of UK, where it usually would mean passing on the same DNA chains to yet another generation but that's another story) .
    Recent usage of the term "mate" is referring to someone one consider to be a friend.
    This change of meaning might be due to English youngsters screwing everyone and everything, but this is still to be confirmed]
    I don't know how to spell it but phonetically it is....
    2shay
    As we say nowadays "Nice one"!
    And no I don't want to share physical attributes!
    Grizzly
    PS do these little smiles allow us to ramble linguistically without incurring the wroth of a moderator

  20. #20
    Join Date
    Sep 2007
    Location
    Split Croatia
    Age
    57
    Posts
    6,175
    Blog Entries
    6
    Rep Power
    36

    Re: Superheat and Subcooling



    By amount of smiles and posts that are off-topic, it is obvious that is Friday evening.

  21. #21
    Para-air's Avatar
    Para-air Guest

    Re: Superheat and Subcooling

    Really nice,feeling better,thanks!

  22. #22
    Join Date
    Nov 2006
    Location
    At home
    Posts
    181
    Rep Power
    18

    Re: Superheat and Subcooling

    thanks this is great article.

  23. #23
    Join Date
    Jun 2008
    Location
    Hong Kong
    Age
    62
    Posts
    35
    Rep Power
    0

    Re: Superheat and Subcooling

    Is the oil leve at the compressor's oil glass an indication of liquid refrigerant enters the compressor?

    When a brand new compressor came into my factory, I marked the oil level on the oil glass. After charging of refrigerant and runing, I found in many case the oil level were higher than the initial mark even though the superheat is high enough. Therefore, my questions are:

    1) My systems had accumulator, how do I know it works properly?

    2) Are there other factors that cause rise in oil level?

    3) Should I increase superheat if the oil level of compressor oil glass is near the top visible line?

    4) What if the system has high superheat and high oil level?

    Looking for advice.

    Dsltung

  24. #24
    Join Date
    Jul 2009
    Location
    west midlands
    Age
    70
    Posts
    8
    Rep Power
    0

    Re: Superheat and Subcooling

    Hi chunk
    excellent artical:-
    1) To calulate superheat do you need wet bulb and dry bulb temperatures? If you do how do you calculate it?
    2) If you are calculating superheat ,you measure the temp at the exit of the evaporator, take a pressure reading at the same position? use /P/T chart and subtract the two temps,it this correct?
    3) the P/t chart is only accurate if the gas is in saturated condition?
    4) air on and air off at the evaporator and the condensor what can you use these temp changes for?
    Regrads Eamonn

  25. #25
    Join Date
    Jul 2009
    Location
    west midlands
    Age
    70
    Posts
    8
    Rep Power
    0

    Re: Superheat and Subcooling

    Hi Chunk
    How do you know what the boiling point is on the P/T chart?
    "Find the evaporator boiling temperature from a temperature-pressure chart using suction line pressure"
    Regards Eamonn

  26. #26
    Fresh P's Avatar
    Fresh P Guest

    Re: Superheat and Subcooling

    i know this may seem basic to alot of you highly experianced engineers but i am a trainee just gettin started in the field of a/c and refrigeration and if possible would like someone to explain the following to me: suction superheat, discharge superheat? and how i would measure them? kind regards

  27. #27
    TRASH101's Avatar
    TRASH101 is offline Veteran Poster I am starting to push the Mods: of RE
    Join Date
    Mar 2007
    Location
    england
    Posts
    397
    Rep Power
    18

    Re: Superheat and Subcooling

    welcome to the forum Fresh P

    There is plenty of information on superheat on this forum if you use the search on the forum toolbar.

    Briefly though

    superheat = amount of energy in a vapour above its boiling point measured in degrees of temp.

    suction sh is ambiguous so I'll describe it as

    evap sh - measured directly after the evap
    suction sh - measured at the suction line near to the comp.
    discharge sh - measured near the discharge of the comp.

    To measure you must know the pressure, the boiling point of that refrigerant at that pressure and the actual temp. of the refrigerant at that point.

    then sh = actual temp - refrigerant temp.

    if the answer is less than 0 then you have sub cooling.
    Quidvis Recte Factum Quamvis Humile Praeclarum.

    Latine dictum, sit altum videtur.

    Si hoc comprehendere potes, gratias age magistro Latinae.

  28. #28
    TRASH101's Avatar
    TRASH101 is offline Veteran Poster I am starting to push the Mods: of RE
    Join Date
    Mar 2007
    Location
    england
    Posts
    397
    Rep Power
    18

    Re: Superheat and Subcooling

    welcome Eamonn

    1) please read above post (wet bulb and dry bulb are generally used for describing moisture in the air but the pressure temp. could be classed as a wet bulb temp. and the actual temp the dry bulb)

    2) yes

    3) correct because super heat and sub cooling are relative to the saturated condition i.e. boiling point.

    4) calculating system capacity/performance when used in conjunction with air volume. (also requires the specific heat multiplier for accurate result in your case for air)
    Quidvis Recte Factum Quamvis Humile Praeclarum.

    Latine dictum, sit altum videtur.

    Si hoc comprehendere potes, gratias age magistro Latinae.

  29. #29
    Join Date
    Apr 2001
    Location
    New Port Richey, Florida - USA
    Age
    78
    Posts
    5,072
    Rep Power
    34

    Re: Superheat and Subcooling

    Check out this thread to get you started:

    http://www.refrigeration-engineer.co...ad.php?t=19701

  30. #30
    Join Date
    Jul 2009
    Location
    west midlands
    Age
    70
    Posts
    8
    Rep Power
    0

    Re: Superheat and Subcooling

    Hi Trash
    You explained super heat in an excelent way.
    1)Now once you have got superheat worked out, how do you know what it should be with out manufaccturers information.
    2) on a P/T chart what pressure/ temp does R134 boil at in the evaporator and in the condensor?
    Regards

  31. #31
    TRASH101's Avatar
    TRASH101 is offline Veteran Poster I am starting to push the Mods: of RE
    Join Date
    Mar 2007
    Location
    england
    Posts
    397
    Rep Power
    18

    Re: Superheat and Subcooling

    Hi Eamonn

    1) Guesswork - it depends on a lot things that revolve around the main thing of....how do we get rid of the heat. In england and similar climates 40 C is a magic no. for a lot of condensors and for evaps it depends on the application e.g. aircon can be from -5 to +10 C as a guide.

    2) Don't really do much with 134a but I think it was roughly 10 barg @ 40C and 2 barg @ -10 C
    Quidvis Recte Factum Quamvis Humile Praeclarum.

    Latine dictum, sit altum videtur.

    Si hoc comprehendere potes, gratias age magistro Latinae.

  32. #32
    Join Date
    Jul 2009
    Location
    west midlands
    Age
    70
    Posts
    8
    Rep Power
    0

    Re: Superheat and Subcooling

    Hi Trash
    "Guesswork" I know you are more professional than that? I don't believe A/C engineers guess superheat, every one says how important it is now you are saying guess it. There must be another way?

  33. #33
    Join Date
    Jul 2009
    Location
    west midlands
    Age
    70
    Posts
    8
    Rep Power
    0

    Re: Superheat and Subcooling

    Hi Gary "excellent articals on superheat"
    1) When you have worked out your superheat or subcool, how do you know what value it should be? no guessing please? only profesional answer.
    2) Air on and air off across the evap and cond subtract one from the other gives the difference in temp. what do you do with the temp difference now?
    3) I have to repair a high Pressure compressed air cooling system.it uses Refrig- 407C. and they have used a hot gas bypass valve. I believe it has something to do with varying loads how do I check it is set up corectly?
    4) the problem is a broken pipe which I have to repair using nitrogen purge during welding. Do i have to change the filter after repair because the filter is welded in place as well.
    5) is it illegal to use hoses without ball valves on the ends?
    Regarads Eamonn

  34. #34
    Join Date
    May 2008
    Location
    Belgium
    Age
    66
    Posts
    802
    Rep Power
    16

    Re: Superheat and Subcooling

    Quote Originally Posted by eamonn493 View Post
    Hi Gary "excellent articals on superheat"
    1) When you have worked out your superheat or subcool, how do you know what value it should be? no guessing please? only profesional answer.
    2) Air on and air off across the evap and cond subtract one from the other gives the difference in temp. what do you do with the temp difference now?
    3) I have to repair a high Pressure compressed air cooling system.it uses Refrig- 407C. and they have used a hot gas bypass valve. I believe it has something to do with varying loads how do I check it is set up corectly?
    4) the problem is a broken pipe which I have to repair using nitrogen purge during welding. Do i have to change the filter after repair because the filter is welded in place as well.
    5) is it illegal to use hoses without ball valves on the ends?
    Regarads Eamonn
    hey Eamonn,
    1#the superheat has to be between 6 and 10K, the subcool between 5 and 8K.
    2# it tells you if the condenser is clean or needs to be cleaned.
    3#don't touch if you din'd toucht it before, once it is out of balance it is very difficult to get it back in good working ordre. it is one of the most difficult part in the plant to handle.many books has being writtin about those things.
    4#if you have to change the filter,try to put some valve's fore and aft the filter. works better that way.
    5#some country's ..yes, whit valve's works better for mounting or dismounting the manifold.

    thats my opinion,

    Ice

  35. #35
    jordan cumbee's Avatar
    jordan cumbee Guest

    Re: Superheat and Subcooling

    Post deleted, multi post and in wrong thread; original moved.
    Last edited by Brian_UK; 17-02-2010 at 08:05 PM. Reason: Wrong thread

  36. #36
    Join Date
    Sep 2007
    Location
    Split Croatia
    Age
    57
    Posts
    6,175
    Blog Entries
    6
    Rep Power
    36

    Re: Superheat and Subcooling

    Quote Originally Posted by jordan cumbee View Post
    what would be the problem if you had reg head pressure,low suction pressure,reg subcooling,and low superheat
    What would be problem if you start new thread with your questions, since these are not directly related with subject of this thread.
    And, what do you mean with term "reg"?

  37. #37
    Brian_UK's Avatar
    Brian_UK is offline Moderator I am starting to push the Mods: of RE Site Moderator : and general nice guy
    Join Date
    Mar 2001
    Location
    Dorset
    Age
    76
    Posts
    11,192
    Rep Power
    60

    Re: Superheat and Subcooling

    ^^ Fair comment Nike, post moved.
    Brian - Newton Abbot, Devon, UK
    Retired March 2015

  38. #38
    Steve0's Avatar
    Steve0 Guest

    Re: Superheat and Subcooling

    Chunk,

    Very nice info! I really appreciate your time and dedication put into it. great site here, I am a first time poster and hope to have many more because I need help accasionly so talk to you soon...

  39. #39
    Join Date
    Jun 2010
    Location
    Maldives
    Posts
    14
    Rep Power
    0

    Re: Superheat and Subcooling

    Very useful information specially from chunk and Ice cube. Thanks a lot

  40. #40
    Join Date
    Oct 2010
    Location
    Australia
    Posts
    9
    Rep Power
    0

    Re: Superheat and Subcooling

    Hey, interesting read.

    I have bought myself the fieldpiece ssx34 which automatically works out subcool and superheat also the diagnostic psychrometer, which tells you what your super heat and sub cool should be.

    I have a client i look after that has loads of apac commerical air conditioning units. I have got all the coils, drains filters clean

    I now want to performance test and check with the new meters

    I really have never been shown properly, and the college text books are useless.

    i appreciate all the information and i cant wait to become a better fridge mech.

  41. #41
    Join Date
    Oct 2010
    Location
    MALTA
    Posts
    9
    Rep Power
    0

    Re: Superheat and Subcooling

    Hi Chunk,thank you for all informations you gave us!.Reading posts in this forum i'm improving my Refrigeration's knowledge..and my english!.

  42. #42
    Join Date
    Sep 2007
    Location
    Split Croatia
    Age
    57
    Posts
    6,175
    Blog Entries
    6
    Rep Power
    36

    Re: Superheat and Subcooling

    Quote Originally Posted by Egle76 View Post
    Hi Chunk,thank you for all informations you gave us!.Reading posts in this forum i'm improving my Refrigeration's knowledge..and my english!.
    Same with me my friend! Keep coming here and contribute.

  43. #43
    boonch's Avatar
    boonch Guest

    Re: Superheat and Subcooling

    Someone may help me clarify the following.

    Generally, reading temperature using pipe clamp probe assuming the same as refrigerant temperature. And calculating superheat and subcool accordingly

    Would be it much error in assumption because of heat exchange between refrigerant and surrounding air thru the pipe surface ?

Similar Threads

  1. How to measure superheat and subcooling
    By kengineering in forum Technical Discussions
    Replies: 40
    Last Post: 18-01-2011, 10:03 AM
  2. Help on superheat and subcooling
    By hvac student in forum Trouble Shooting
    Replies: 8
    Last Post: 08-08-2008, 02:25 PM
  3. Benji
    By benji in forum Trouble Shooting
    Replies: 29
    Last Post: 30-01-2008, 02:25 PM
  4. Superheat and subcooling
    By marc5180 in forum Training
    Replies: 27
    Last Post: 08-10-2007, 09:21 PM
  5. setting up a unit using superheat
    By marc5180 in forum Trouble Shooting
    Replies: 5
    Last Post: 26-06-2007, 11:25 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •