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.

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

  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!

  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 ?

    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!

    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 :)

  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 ?

  5. Can someone tell me why you would use a check piston vs standard piston