Diagnosing Improper Fixed Orifice Sizes

  I have found that this topic often goes unnoticed, or can be a guessing game out there in the field.  Diagnosing improper fixed orifice sizes is actually a fairly simple, cut and dry procedure.  First, I would argue for efficiency reasons, as well as ease of proper charging, you should just field install a TXV.  Of course, when you are on the job site you don't want to spend precious time attempting to adjust refrigerant charge with the incorrect orifice. You would never be able to get the Superheat and Subcooling within proper parameters.  The next best thing to field installing the TXV is actually installing the correct orifice to match the condensing unit - which is why they are shipped accordingly.
  So, what happens if you don't have the correct orifice installed?  Sure, you could attempt to adjust the Superheat to the target, but it will never reach the desired goal.  If you did not know what the target should be, use this simple formula that most to all Superheat Charts are based on, using the Condenser Ambient Temperature and Return Air Wet Bulb (taken at the unit, not in the space).

Target Superheat Formula


Low Superheat & Low Subcooling

If you have an over-sized orifice, you will be overfeeding or flooding the evaporator.  If you did not verify that there was some sort of reasonable subcooling value, than you would expect that the unit was overcharged based on the low superheat.  Taking a look at the first picture of the digital gauges, you can see that the Superheat and Subcooling are both low, indicating that adjusting the charge will not only be impossible, but could cause the refrigerant to flash off prior to the metering device since the subcooling will also go down as refrigerant is removed.  When charging to the proper Superheat, you should have at least 5F, but not more than 25F subcooling with a fixed metering device. This indicates the proper size orifice and that the refrigerant will remain a liquid until meeting the orifice.


High Superheat & High Subcooling
  If you have an undersized orifice, you will be starving the evaporator.  Without looking at the subcooling value, you would believe the unit is undercharged due to the higher than target superheat.  If you can see on the second picture, we have high superheat and high subcooling, noting that the refrigerant charge is not the problem.  This is generally either a restriction, or an undersized orifice which in it's own sense is in fact a restriction.  If you were to start adding refrigerant, the subcooling would continue to rise, with little to no change in the Superheat.  Once the head pressure reaches a point that  the scroll compressor cannot handle, somewhere in the range of 20-30F of subcooling, the head pressure will start fluctuating drastically and you will see 75 - 100# swings in some cases.  Do not mistake this for non-condensables!  This will happen with significantly overcharged TXV's too.

  I guess the moral of the story here is to install the correct orifice for the condenser being installed, this way you can avoid this diagnosis from the start.  As a technician, if it came time to verify the orifice size, I would take no chances and just install a TXV.  I would hate to go through the process of pumping down the system, changing the orifice, vacuuming the system, and trying to adjust the charge just to find out that the new orifice was still an issue.  Plus, by installing a TXV you will for the most part add an additional 1 SEER to the system!

6 comments:

  1. What happens when Installer fails to install and orifice in the line. Will this damage the ac system.

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  2. Yes, you will definitely compromise the durability of the compressor as you will see little to no superheat and slugging. If the compressor has not yet failed, than you are lucky!

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  3. Hello All,
    I have recently moved to an apartment with an A/C which the compressor is on the roof and the evaporator is in bathroom ceiling.
    my problem is when the compressor starts for about 15 second (when the pressure is building up) the system creates a very loud noise and then the noise drops to almost a normal level. Then again when the compressor stops i have about 1 minutes of noise until the pressure is balanced.
    its good to mention that before evaporator, there is a capillary distributor (1 to 6) and also it has a fixed orifice nozzle ( and not a orifice tube ) . capillary has been recently changed and also the orifice. so there is nothing wrong with parts. i believe it is a system design and the orifice is not the right one for the system size. it sounds like cavitation noise after the orifice. The orifice was engraved with 92 (perhaps in inch) but first I am not sure what is the right size for it based on system and also I am not sure whether we should increase or decrease the orifice size and what would be the effects on the system performance and cooling capacity. Can you help me in this regards ?

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    Replies
    1. Hi Mr. Alavi, thanks for reading my blog! You are correct in your assumption regarding noise with the fixed orifice. Unfortunately, it will not matter which orifice you choose, you will likely always get said noise. Unless you use a TXV and a pump down system. To check the correct orifice size, you can likely find the size labeled on the condenser nameplate. If you change the orifice size, it could lead to improper refrigerant charge operation and altering of the superheat. Low superheat could cause liquid slugging of the compressor, and high superheat will eventually burn out a compressor. I typically find noise issues like what you are describing due to very short line set lengths. If less than 15', this could create some velocity issues that would cause noise at the metering device, as well as an undersized line set.
      Good luck with your system, I hope this information was useful!
      Chris

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    2. Hi Chris,
      thanks for putting time and replying.
      the orifice size in engraved with 92 and it is quite a big orifice. also the length or the pipes are quite enough and it is about 50 meters. unfortunately there is no lable on the unit showing the size of orifice.
      I have decided to change the orifice and replace it with a TXV. do you have any suggestion for the correct TXV size or model ? teh compressor has a cooling capacity of 50.9 Mbh or 14.9 KW.
      I am trying to go for a Danfoss TXV but I need to be sure what is the right TXV size.
      Any help would be highly appreciate :)
      Cheers,
      Farzam

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  4. Hey Christopher,i work for an apt complex which is over 35yrs old i have eva coils with capillary tubes,i'm having an issue with those new condensers that use the scroll compressors within months i can hear a hissing noise form the compressor and it's gone but if i install a new coil with a fix orifice it works fine but i don't have that issue with the reciprocating comp with coil using capillary.
    Any idea what causes that ?

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