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.
if you keep the thermostat at 72 or whatever …. and leave it alone …. a piston works just fineIf 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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Originally Posted by Sixty4fairlane
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.
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.