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  1. #1
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    How to measure superheat and subcooling


    I am trying to establish baseline information on some of our equipment and would like it to include presure drop, superheat and subcooling. We have spolan preset the SH on all our TEV's at 10*F. How are these measurements best taken? I have read of a few different methods. What are the prefered numbers. Sorry to keep you all so busy. Ken



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    Re: How to measure superheat and subcooling

    Ken,

    You should be aware that there are two superheat's in the low side of the system. One is for evaporator superheat, the other is suction superheat at the compressor suction valve.

    To measure evaporator superheat, record the actual line temperature at the outlet of the evaporator. Hopefully, there is a pressure tap there to record the evaporator outlet pressure.

    Take the pressure reading and use a pressure-temperature chart for the refrigerant you are using in the system. Look up the pressure in the chart, then cross-reference that pressure to the corresponding temperature for the refrigerant. This is the saturation temperature for the refrigerant at that pressure.

    Subtract the saturation temperature from the actual temperature (from the evaporator outlet). This is the evaporator superheat. You should check the evaporator rating to see what is the correct superheat.

    The evaporator manufacturer should have designed the evaporator to provide the required cooling capacity at a specific superheat. That is the number you want.

    It could be 6 to 8 degrees F, so if the TXV is set for 10 degrees of superheat, you will loose a little capacity on the evaporator and compressor.

    The suction line superheat should be monitored also. High suction superheats relate directly to higher discharge temperatures.

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    Re: How to measure superheat and subcooling

    Quote Originally Posted by US Iceman
    Ken,

    You should be aware that there are two superheat's in the low side of the system. One is for evaporator superheat, the other is suction superheat at the compressor suction valve.



    I agree. If a semi or fully hermetic compressor is used, there is also a third 'invisible' superheat, and this may be academic, but it does exist; this is the heat generated by the motor that is rejected into the already superheated suction gases immediately before it is compressed. In other words the motor cooling losses.

    Even though your TXV controls a constant superheat at the evaporator exit, variations in motor power (hence rejected heat) will vary the actual superheat presented at the point of compression.

    It is difficult to assess or predict, because it is a variable function of the motor?s efficiency and actual working capacity of the system, including factors outside the designer?s control, such as the Power Factor at the motor terminals.
    Given that hermetics of both types are common and can be installed in multiples of 10, 20kW or more, motor heat can be significant in terms of condenser capacity at least.

    In cooling systems this motor heat is a tolerated loss, however as soon as it becomes a heat pump, or you use hot-gas defrost, it is a significant part of your load.

    Obviously, this does not apply to open drive compressors.

    Some systems may also have heat exchangers (called intercoolers in some quarters) that exchange heat between the liquid and suction lines. If so the thermal exchange here needs to be factored into the suction superheat gains.

    .
    ________
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    Last edited by Argus; 07-02-2011 at 09:49 AM.

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    Re: How to measure superheat and subcooling

    Quote Originally Posted by Argus
    ...there is also a third 'invisible' superheat,...In other words the motor cooling losses.
    Quite so. This is one of those areas where the manufacturers performance data does not provide much information I believe.

    You will find a similar effect in open drive compressors, where there can be some superheating of the suction gas due to heat transfer from the cylinder walls and compressor housing.

    Since the superheat does not contribute to the useful refrigeration effect I think this is picked up as part of the compressor losses.

    As Argus mentioned this is all rather academic, but does remain interesting. It is also important to read the fine print of the manufacturers performance data. You will usually find the rating basis on the tables. The items to be aware of are the amount of subcooling and return gas (suction gas) temperature.

    A lot of systems do not operate at the conditions listed in the rating tables.

    Another interesting point is a fourth superheat in refrigeration systems. This one is on the discharge of the compressors. Discharge temperature minus condensing temperature. Very useful for screw compressors and also helps to find broken discharge valves, etc, in open-drive reciprocating compressors.

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    Re: How to measure superheat and subcooling

    Iceman and Argus, Excellent information helping to confirm some of my own observations and giving me more to concider helping me expand my thinking. Now that I have the formula to perform accurate SH readings how is it determined what the optimum SH is for a given system and how critical is a degree or two? Ken
    Last edited by kengineering; 09-01-2006 at 06:09 PM.

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    Re: How to measure superheat and subcooling

    Quote Originally Posted by kenengineering
    ...what is the optimum SH is for a given system...?
    See below...

    Quote Originally Posted by me
    The evaporator manufacturer should have designed the evaporator to provide the required cooling capacity at a specific superheat. That is the number you want.
    A close coupled system (meaning very short piping runs) should have about 10 degrees (F) of evaporator superheat.

    If the system has a stable operation (On or OFF) and no capacity control this should be OK. A couple of degrees one way or the other is not a big deal, but you should be able to nail it down if the system is stable.

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    Re: How to measure superheat and subcooling

    US Iceman and Argus made very good points as to the importance of superheat and the measurement of.

    I can added a couple of comments, if specific manufacture data not available some suggested settings are high temp 10-12*F SH, medium temp 6-8*F SH, low temp 4-6*F SH.

    You must make sure that you have at least 20*F SH 6 inches from the compressor at all times to prevent flood back. You are also limited to a maximum return gas temp but that will vary based on several factors some are suction press, head press, refrigerant type, etc.

    Return gas temps that are too high (high superheat at comp) will cause high discharge line temps that leads to oil failure then compressor failure.

    You stated that you had Sporlan set valves, if you were not specific with your details to them they will need to be set in the field if they are adjustable.

    You did not list it but I would monitor discharge line temp. It should generally not exceed 225*f on a reciprocating compressor for maximum life. The actual temps you will see are generally determined by high and low side pressures, refrigerant type, type of compressor, etc.
    Steve Wright CM
    Last edited by Steve Wright; 09-01-2006 at 07:16 PM. Reason: Added info

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    Re: How to measure superheat and subcooling

    Quote Originally Posted by Steve Wright
    ...if specific manufacture data not available some suggested settings are high temp 10-12*F SH, medium temp 6-8*F SH, low temp 4-6*F SH.
    I think these are reasonable, as long as the suction piping runs are long (like supermarkets). If the system has short runs or has capacity control, I would tend to stay with the slightly higher values of 8-10 as a minimum range.

    Having said that, and as stated by Steve, if you can control a minimum suction superheat (at the compressor) the compressor discharge temperature can be much lower, than when used with a higher suction superheat.

    The discharge temperature is a good one to get used to recording. After you develop a feel for "good" temperatures versus "bad" temperatures, it can really help nail down the best superheat (suction or discharge) for the system operation.

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    Re: How to measure superheat and subcooling

    thanku 4 all the positive input guys i enjoy reading the rite procedures. there are so many incompatant freaks in the trade that i know off! i knew about and use the evaporator s.heat method. now i have learnt about 2 more thanku.

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    Re: How to measure superheat and subcooling

    I am involved in the manufacter of two distinctly diferent types of refrigeration. Both have short suction lines ( under 10 feet/ or 3 meters) unless they are built for remote installation. Our normal display case have a fan coil and works great at 10*f of superheat. We have recently built some static coil 33-37*f deli cases for meat and cheese. They get good and cold but the static coil presures run much lower. Should I be concerned having a cut out presure of 4 psi gauge to achieve the temps required or is there a better way? Ken

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    Re: How to measure superheat and subcooling

    A zeotropic refrigerant will have a higher superheat than an azeotrope. Normally I guess you would add the glide value to the normal superheat setting ?? No one's ever given me a set value for something say R407C
    a problem shared is a problem halved

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    Re: How to measure superheat and subcooling

    The static coils have to run a little colder to produce the room temperature. On the forced convection unit coolers the fans help to keep the suction pressure a little higher.

    Should I be concerned having a cut out pressure of 4 psi gauge to achieve the temps required...
    Ken, when asking questions like this you need to provide all of the information. What refrigerant is it????

    If you are controlling the compressor off of a low-pressure switch, you may have to run the system until you get the space temperature desired. At that point, you have to decide how much differential you need to keep the space from warming up too much, or from short cycling on the low-pressure switch.

    Once you get a low-pressure switch set properly, it should work very well. The one thing you will not have is a low pressure cutout in case the refrigerant leaks out. You could add an additional low-pressure switch for a safety cut-out.

    On the superheat... I think 10 degrees F is a good starting point, but you should also verify the compressor suction superheat during pulldown and a couple of cycles to make sure it is stable.
    Last edited by US Iceman; 10-01-2006 at 03:15 AM. Reason: correcting quote

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    Re: How to measure superheat and subcooling

    ...unless head pressure is stabilized year round...
    If you are done laughing out loud, you might consider that the display cases being built are more than likely installed in a conditioned space so the head pressure would in all likelihood be constant.

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    Re: How to measure superheat and subcooling

    When I started in refrigeration, I learned the job from an older tech (then +/-55 years old) or let me sya...an experienced tech.
    When we did an install in a restaurant or a hotel for a smaller unit and like in many places, the compressors were placed in the cellar, he never installed a thermostat, only a LP switch.
    He regulated the LP switch always with a freon bottle and his manifold before connecting it to the circuit.
    Once the start point was set at 2C, he locked it with a little bit of leak lock so that they shouldn't turn on this knob.

    I often still see nowadays on older installations 2 thermostats: the room thermostat shuts off the evaporator fan and a second with the bulb fitted between the fins shuts off the compressor (set at start 2C and a DT of +/- 8K)
    The second thermostat serves as a defrost thermostat.

    It is a relay simple setup - often used in the past - and the evaporators never frost.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: How to measure superheat and subcooling

    Hello Ken,

    You mention the piping runs being short unless they are built for remote installation, so I would imagine these have condensers inside the space. To achieve the temperatures you are talking about, you are probably needing to evaporate at at least minus 10 degrees C, maybe lower. I assume that you are using electric defrost, on a timer system with a pump down system off the thermostat, which many european manufacturers sometimes do. I used to do alot of work with dairy cabinets, usually I have not seen a HP control, due to the low evaporating temperatures used, because the pressures used in the evaporator to do the job are lower than the point at which ice build up will form. HP control is generally used for situations where you don't necessarily utilise a defrost, for example cellar coolers,and IT rooms a/c systems where you are running at much higher evaporating temperatures. If you are using a remote condenser/receiver outside then depending on where in the country you are you may need to install a check valve to stop migration of refrigerant to the condenser (especially if lower than the compressor). Normally two thermistors can be used to control- one for cabinet temp, and one in the coil for defrost (which I think Peter mentioned) Set a 5 minute delay on the evaporator fans follwing a defrost cycle to ensure that the coil comes down to temp before hot air etc gets blown all over the place. If you do that, and build on the advice from others in the thread you will be fine.

    Best Wishes
    a problem shared is a problem halved

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    Re: How to measure superheat and subcooling

    Quote Originally Posted by Peter_1
    He regulated the LP switch always with a freon bottle and his manifold before connecting it to the circuit.
    This is a very good idea to use. Any pressure switch should be checked for the intended operation. You can use the refrigerant pressure from a bottle (I would not recommend this in today's legal world). However, you can obtain the same result by operating the compressor down to a lower suction pressure to make sure the switch operates at the desired pressure.

    Otherwise, you can plan on setting there to watch the switch operation.

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    Re: How to measure superheat and subcooling

    When I come home from work I often think how lucky I am to have chosen a trade which is allways challengeing. US Iceman I will most certainly give more specifics in the future. Thanks for all the suggestions . Ken

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    Re: How to measure superheat and subcooling

    That is correct.

    There are still plenty of antique systems still operating around here and many are only controlled by pressure controls.

    Most of the low pressure switches i set for temp control usually need a slight adjustment on the diff to achieve the desired effect.

    Doesnt take long to watch it reach cut out. Beats coming back to move the diff slightly at a later date.

    It also amazes me how many times you find a system controlled by t/stat that hasn't had the pressure control set up to control the icing of the coil.It seems some people would rather install a defrost timer instead of turning a screw.

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    Re: How to measure superheat and subcooling

    I was with US Iceman,
    Liquid subcool and superheat calculated as;
    a)Liquid subcool- Condenser temp minus
    liquid line temperature.
    b)Superheat- suction line temperature
    minus evaporator temperature.

    Cheers

    rudi
    mustardseed:) :) :)

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    Re: How to measure superheat and subcooling

    how to set the super heat

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    Re: How to measure superheat and subcooling

    Quote Originally Posted by rpbuenaflor View Post
    I was with US Iceman,
    Liquid subcool and superheat calculated as;
    a)Liquid subcool- Condenser temp minus
    liquid line temperature.
    b)Superheat- suction line temperature
    minus evaporator temperature.

    Cheers

    rudi
    Hi Rudi
    I am new to A/C but I thought S/H is calculated by using The P/T chart, Temp reading,and pressure reading on the return pipe from the evaporator. Can you use pipe temperature reading only?
    Regards Eamonn

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    Re: How to measure superheat and subcooling

    One of the best places I have used to get pressure on an evaporator with no acess port has been the equaliser line. Both the external equaliser line and the bulb are located at the same point so no pressure drop is involved. I undo the 1/4" flare connection on the TXV connect it to my gauge manifold open the low side valve and connect the flexible line to the equaliser port so pressure and temperature are read at the same point. Now superheat can be calculated and adjustments made. No good if a soldered equaliser line is fitted but perfect on 90% of plant.

    On measuring superheat and subcooling all I remember is superheat is the sensible heat of a gas and subcooling is the sensible heat of a liquid.

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    Re: How to measure superheat and subcooling

    working principles of a Screw compressor?

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    Re: How to measure superheat and subcooling

    It turns...mostly.. and it slides sometimes...and it makes noise
    Good first post you made.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: How to measure superheat and subcooling

    this has been a big help, is it possible to measure evap superheat where there is no shrader valve on the suction line and the expansion valve equalizing line has been brazed in place? any help will be appreciated

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    Re: How to measure superheat and subcooling

    Quote Originally Posted by jmully View Post
    this has been a big help, is it possible to measure evap superheat where there is no shrader valve on the suction line and the expansion valve equalizing line has been brazed in place? any help will be appreciated



    No. Not with any accuracy because you cannot access to measure what you want to measure.

    You may take the surface temperature of the pipe at the exit point of the evaporator, then measure the suction pressure at the compressor service valve if this is available to you, adjusting the pressure reading for your estimated pressure loss in the suction line.
    By convention, on suction lines of normal size and length, this is estimated as the equivalent of 2 degrees F or 1 degree C - make an adjsut ment to your calculation.

    As you cannot measure it, you don't know and will have to estimate the pressure losses in the evaporator.

    However if you have a Zeotropic blend (i.e. a blend with a glide) there is a further complication isofar as you will have to estimate the mean superheat for the entire evaporator.

    Finally, this holds true only for an open type compressor

    The total superheat of the suction gases in a hermetic system includes both the SH of the evaporator and the compressor motor. This is what the compressor has to deal with at the point of compression and is an important element in the efficiency of heat pumps.




    .

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    Re: How to measure superheat and subcooling

    please let me know how to read suction gas super heat

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    Re: How to measure superheat and subcooling

    Quote Originally Posted by rubi123 View Post
    please let me know how to read suction gas super heat
    Read the posts above yours, start at number 2.
    Brian - Torquay, Devon, UK
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    They're starting to take it as a challenge...

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    Re: How to measure superheat and subcooling

    [IMG]file:///C:/Users/lisa/AppData/Local/Temp/moz-screenshot.png[/IMG] [IMG]file:///C:/Users/lisa/AppData/Local/Temp/moz-screenshot-1.png[/IMG][IMG]file:///C:/Users/lisa/AppData/Local/Temp/moz-screenshot-2.png[/IMG]http://www.uri.com/is-bin/intershop....eview/B16S.jpg


    Valve is soldered on to copper tube providing a permanent access fitting.
    Valve body can be removed and replaced

    I solder them on live systems they work well.

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    Re: How to measure superheat and subcooling

    Quote Originally Posted by tonyhavcr View Post
    [IMG]file:///C:/Users/lisa/AppData/Local/Temp/moz-screenshot.png[/IMG] [IMG]file:///C:/Users/lisa/AppData/Local/Temp/moz-screenshot-1.png[/IMG][IMG]file:///C:/Users/lisa/AppData/Local/Temp/moz-screenshot-2.png[/IMG]http://www.uri.com/is-bin/intershop....eview/B16S.jpg


    Valve is soldered on to copper tube providing a permanent access fitting.
    Valve body can be removed and replaced

    I solder them on live systems they work well.



    Not sure if anyone got what I was trying to say with this post these valves will give you a permanent access fitting when needed in less then 10 mins.
    there is one for ea. line size to check superheat or add a lp or hp
    you can soft solder them on or braze. I soft solder them to keep from burning the oil.

    let me know what you think
    Attached Images Attached Images

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    Re: How to measure superheat and subcooling

    Quote Originally Posted by tonyhavcr View Post
    Not sure if anyone got what I was trying to say with this post these valves will give you a permanent access fitting when needed in less then 10 mins.
    there is one for ea. line size to check superheat or add a lp or hp
    you can soft solder them on or braze. I soft solder them to keep from burning the oil.

    let me know what you think

    I`m not sure if I`m reading this right but are you saying you solder these to a pressurised system. Just think of the consequences if the pipe burns through

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    Re: How to measure superheat and subcooling

    Quote Originally Posted by glenn1340 View Post
    I`m not sure if I`m reading this right but are you saying you solder these to a pressurised system. Just think of the consequences if the pipe burns through

    will not burn through copper I'm not using ox acct here
    just soft solder but funny

    they are made for this just look @ them Google it
    Melting Point: 1083.0 C (1356.15 K, 1981.4 F)
    he behavior of tin-lead solder is shown by the diagram in figure 6-8. This diagram shows that 100% lead melts at 621F and 100% tin melts at 450F. Solders that contain 19.5% to 97.5% tin remain a solid until they exceed 360F. The eutectic composition for tin-lead solder is about 63% tin and 37% lead. (Eutectic means the point in an alloy system that all the parts melt at the same temperature.) A 63/37 solder becomes completely liquid at 361F. Other compositions do not. Instead, they remain in the pasty stage until the temperature increases to the melting point of the other alloy. For instance, 50/50 solder has a solid temperature of 361F and a liquid temperature range of 417F.

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    Re: How to measure superheat and subcooling

    An important point to remember when setting evaporator superheat is too keep in mind the evaporator T.D.as the superheat will need to be below this for the valve to operate correctly. E.G.If your room temp is
    2C and your evap temp is -2C you cannot operate at 5.6KTD (10F)

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    Plz Help

    Hello Gentleman's, i need a help planing to buy reefer containers, 10 unit. I'm looking for professional who is able to do a good inspection on this containers. Location: Port Newark NJ. if you can help me please drop me a line, Thanks

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    Re: How to measure superheat and subcooling

    i'm glad i'm having chicken tonight.
    not relevant but it doesn't seem to matter!
    thanks for the informative discussion. the relevant parts of this post should be put into a Basics Archive.
    Peter please stop making me laugh i can't concentrate on the cooking.

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    Re: How to measure superheat and subcooling

    Just a comment to " tonyhvacr " about soft soldering a speed valve on a refrig. system. Here in Chicago , code requires braze on any connection to a refrigeration system. If an inspector spots that solder, you're tagged and they pick the rest of the system apart. Relates to our code requirements for PRV valves on all refrig. systems mounted as close to compressor discharge as possible. It is an easy write-up violation these days. Inspectors look for " fake " PRV connections soldered on surface of piping. They carry pocket gauge and test to see if valve was pierced. Just a word of warning. Have a good day!!
    Don't leave it like you found it.....Leave it like it should be !!!

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    Re: How to measure superheat and subcooling

    Quote Originally Posted by iceman007 View Post
    A zeotropic refrigerant will have a higher superheat than an azeotrope. Normally I guess you would add the glide value to the normal superheat setting ?? No one's ever given me a set value for something say R407C
    http://www.asercom.org/files/ASERCOM...essQuality.pdf
    Last edited by nike123; 17-02-2010 at 08:16 AM.
    Now, when I am officialy citizen off EU, I am looking for decent job! For any job offer please check my profile!

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    Re: How to measure superheat and subcooling

    Thank you Feisty for posting that I can't beleive people said as little as they did about tonyhvacr soft solder technic. It is code for a reason!!!!!!!!!! Soft solders can not withstand the pressures and temps related to HVAC/R systems. Yeah its no wonder he has to use soft solder. One short cut leads to another.

    Quote Originally Posted by FEISTY View Post
    Just a comment to " tonyhvacr " about soft soldering a speed valve on a refrig. system. Here in Chicago , code requires braze on any connection to a refrigeration system. If an inspector spots that solder, you're tagged and they pick the rest of the system apart. Relates to our code requirements for PRV valves on all refrig. systems mounted as close to compressor discharge as possible. It is an easy write-up violation these days. Inspectors look for " fake " PRV connections soldered on surface of piping. They carry pocket gauge and test to see if valve was pierced. Just a word of warning. Have a good day!!

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    Re: How to measure superheat and subcooling

    Although brazed joints offer higher joint strength in general, the annealing of the tube and fitting that results from the higher heat used in the brazing process can cause the rated pressure of the system to be less than that of a soldered joint. This fact should be considered in choosing which joining process to use.

    Than why brazing in refrigeration is prefered? Short answer is halocarbon-based refrigerants are incompatible with lead-based solders. They will eventually leak.
    Last edited by nike123; 03-12-2010 at 11:50 PM.
    Now, when I am officialy citizen off EU, I am looking for decent job! For any job offer please check my profile!

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    Thumbs up Re: How to measure superheat and subcooling

    ...if specific manufacture data not available some suggested settings are high temp 10-12*F SH, medium temp 6-8*F SH, low temp 4-6*F SH.
    I think these are reasonable, as long as the suction piping runs are long (like supermarkets). If the system has short runs or has capacity control, I would tend to stay with the slightly higher values of 8-10 as a minimum range.

    Having said that, and as stated by Steve, if you can control a minimum suction superheat (at the compressor) the compressor discharge temperature can be much lower, than when used with a higher suction superheat.

    The discharge temperature is a good one to get used to recording. After you develop a feel for "good" temperatures versus "bad" temperatures, it can really help nail down the best superheat (suction or discharge) for the system operation.

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    Smile Re: How to measure superheat and subcooling

    I can added a couple of comments, if specific manufacture data not available some suggested settings are high temp 10-12*F SH, medium temp 6-8*F SH, low temp 4-6*F SH.

    You must make sure that you have at least 20*F SH 6 inches from the compressor at all times to prevent flood back. You are also limited to a maximum return gas temp but that will vary based on several factors some are suction press, head press, refrigerant type, etc.

    Return gas temps that are too high (high superheat at comp) will cause high discharge line temps that leads to oil failure then compressor failure.

    You stated that you had Sporlan set valves, if you were not specific with your details to them they will need to be set in the field if they are adjustable.

    You did not list it but I would monitor discharge line temp. It should generally not exceed 225*f on a reciprocating compressor for maximum life. The actual temps you will see are generally determined by high and low side pressures, refrigerant type, type of compressor, etc.
    Steve Wright CM
    Thanks**

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