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Thread: Question about Fixed Orifice

  1. #1
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    Question about Fixed Orifice

    I understand the concept of a TXV and how it is superior when it comes to controlling refrigerant flow based on demand and I understand how a fixed orifice is limited by the size of the piston, but I have a curiosity.

    If a fixed orifice is limited, does this mean that the occupants will experience fluctuation in performance? For example, on a very hot day, does this mean that the evaporator may be starved of adequate supply of refrigerant and vice versa when it's cold outside it may become flooded.

    Also since pressure and temperature are proportional, does this mean that there's sweet spot with indoor and outdoor temp that the temperature coming out of the registers will be at its coolest?


    It's just some general curiousity and unfortunately I forgot to discuss this with my hvac instructor during my time there.

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    if you keep the thermostat at 72 or whatever .... and leave it alone .... a piston works just fine

    If you turn it up several degrees when you leave the house , then back down when you get home , its not going to cool down as good as a TXV would

    As for running the unit when its cold outside .... a TXV would be better so it can squeeze down the flow
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    Quote Originally Posted by Sixty4fairlane View Post
    I understand the concept of a TXV and how it is superior when it comes to controlling refrigerant flow based on demand and I understand how a fixed orifice is limited by the size of the piston, but I have a curiosity.

    If a fixed orifice is limited, does this mean that the occupants will experience fluctuation in performance? For example, on a very hot day, does this mean that the evaporator may be starved of adequate supply of refrigerant and vice versa when it's cold outside it may become flooded.

    Also since pressure and temperature are proportional, does this mean that there's sweet spot with indoor and outdoor temp that the temperature coming out of the registers will be at its coolest?


    It's just some general curiousity and unfortunately I forgot to discuss this with my hvac instructor during my time there.
    Basically what you said is true. On days with a higher indoor load, you will have more superheat coming out of the evap. On days with less indoor load, you will have less superheat. So if the system is overcharged, the most danger is when there is the least load. A properly charged up txv system, after it stabilizes, should have the same superheat all of the time no matter what the conditions are.

    As far as when you would get the "coolest air", My guess would be when the outdoor temperature is the highest, but the indoor load is the least. Or perhaps when the air filter is the most plugged up.
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    Orifice is critical charge for one set of conditions.
    Feed based on differential pressures, ahri design conditions.
    Higher dp = lower ssh, lower dp= higher ssh.

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    Quote Originally Posted by Sixty4fairlane View Post
    I understand the concept of a TXV and how it is superior when it comes to controlling refrigerant flow based on demand and I understand how a fixed orifice is limited by the size of the piston, but I have a curiosity.

    If a fixed orifice is limited, does this mean that the occupants will experience fluctuation in performance? For example, on a very hot day, does this mean that the evaporator may be starved of adequate supply of refrigerant and vice versa when it's cold outside it may become flooded.

    Also since pressure and temperature are proportional, does this mean that there's sweet spot with indoor and outdoor temp that the temperature coming out of the registers will be at its coolest?


    It's just some general curiousity and unfortunately I forgot to discuss this with my hvac instructor during my time there.
    The TXV provides a boost in efficiency in general, but that has nothing to do with SH. The hard shut off TXV prevents off cycle refrigerant migration, reducing the amount of time required for the system to stabilize. A reduction in cycling losses in other words. The steady state efficiency can be higher on the piston system vs a TXV, depending upon the SH maintained by the TXV. Lower is better, but some TXV's will run up in the 15-20 SH range while the piston system might be running at 5 SH on a hot day, and thus more efficiently.

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    Thank you guys again and again for the very informative replies on the best HAVC site on the web. I definitely learned a couple things and reinforced what I thought I already knew.

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    As far as when you would get the "coolest air", My guess would be when the outdoor temperature is the highest, but the indoor load is the least. Or perhaps when the air filter is the most plugged up.
    If it was hot enough outside, wouldn't that affect the refrigerant condensing and some vapor would make its way back to the evaporator?

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    Did you mean to the evaporator ?

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    Quote Originally Posted by hvacrmedic View Post
    The TXV provides a boost in efficiency in general, but that has nothing to do with SH. .
    ssh will affect evaporator efficiency. Fixed orifice will vary from ~4-30 ssh, txv is typically 10* factory sp and will maintain the sp (assuming balanced port/externally equalized) over the normal ranges of operation.

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    Quote Originally Posted by ehsx View Post
    ssh will affect evaporator efficiency. Fixed orifice will vary from ~4-30 ssh, txv is typically 10* factory sp and will maintain the sp (assuming balanced port/externally equalized) over the normal ranges of operation.

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    Thanks again for this. Definitely helps.

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    Don't get too hung up on the word "fixed" in terms of performance. In this context it refers to the size of the orifice only.

    Because obviously The System is not 'fixed' - in fact; The System is quite variable. <g>

    The piston has a hole in it. The flow rate through that hole depends on the nature of the substance passing through, the inlet pressure, and the outlet pressure. Let's pretend for right now that all refrigerants are the same to eliminate them as a variable in painting this mental image / picture.

    More inlet pressure increases the flow rate. Less outlet pressure increases the flow rate.

    Less inlet pressure decreases the flow rate. Higher outlet pressure decreases the flow rate.

    Yes; the performance of a fixed orifice is somewhat limited as the orifice itself cannot alter it's size. And yes; this will result in some fluctuation in system performance. BUT very unlikely any sufficient for occupants to notice them.

    You ask: "on a very hot day, does this mean that the evaporator may be starved of adequate supply of refrigerant and vice versa when it's cold outside it may become flooded."

    Under stabilized operating conditions - not really. Yes; at initial startup with the space temperature far above comfort conditions - the suction pressure will be higher and the head pressure will still be building - so the refrigerant flow rate will be less than optimal as the pressure differential across the orifice will be limiting the flow rate.

    But in general the nature of comfort cooling is the the heat loading of the conditioned space tends to be dictated primarily by the outdoor temperature. So the fixed-orifice's flow rate adjusts to operating conditions for the most part.

    Yes; there is a "sweet spot" for performance but noting where it is without precise instrumentation would be virtually impossible. And by feeling for it based on when the air temperature coming out of the registers will be at it's coolest? Nobody is That good. <g>

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    Quote Originally Posted by Sixty4fairlane View Post
    I understand the concept of a TXV and how it is superior when it comes to controlling refrigerant flow based on demand and I understand how a fixed orifice is limited by the size of the piston, but I have a curiosity.

    If a fixed orifice is limited, does this mean that the occupants will experience fluctuation in performance? For example, on a very hot day, does this mean that the evaporator may be starved of adequate supply of refrigerant and vice versa when it's cold outside it may become flooded.

    Also since pressure and temperature are proportional, does this mean that there's sweet spot with indoor and outdoor temp that the temperature coming out of the registers will be at its coolest?


    It's just some general curiousity and unfortunately I forgot to discuss this with my hvac instructor during my time there.
    PHM
    --------
    The conventional view serves to protect us from the painful job of Thinking

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