AC Evaporator coil--Piston or TXV?

[wendel]

I have read where a TXV needs a solid flow of R-22 at the valve to work properly. Doesn't a piston/orifice metering device also need a solid flow of R-22 to work properly?

[Mr Bill]

Originally posted by wendel
solid flow of R-22

Are you asking liquid or gas? I never heard of it called solid flow but maybe some do, You need to be getting liquid to the metering device if that helps.

[wendel]

My bad terminology. Do both a TXV and piston/orifice require liquid flow to operate properly?

[hvac r us 2]

Originally posted by wendel
My bad terminology. Do both a TXV and piston/orifice require liquid flow to operate properly?

Both require a full column of liquid to the inlet of the metering device for proper operation.

Hope this helps

[roosterman]

I think the pston is more consistent,but this little piston can get stuck open or closed,the txv is better for eficientcie,better metering

[docholiday]

Originally posted by williesanchez69
I think the pston is more consistent,but this little piston can get stuck open or closed,the txv is better for eficientcie,better metering

Allow me to elaborate. A piston has one size opening, thats it. It's flow is strictly dependent on the liquid pressure and can provide flow on a properly charged system from as little as 5 degrees superheat to 30 degrees superheat depending on the load. The TXV has a needle valve that is operated by spring pressure, evaporator pressure and bulb pressure. It maintains a constant superheat level. A very simple machine that works well.

In both cases there is a moving part but frankly true failures with either are far and few between. More often than not, they are replaced or condemned on a guess.

FYI, I suspect you will see pistons go by the wayside when 13 SEER minimum comes around. Just curious but is anyone aware of a 13 SEER system with pistons?

[wendel]

Thanks for the helpful reply, doc. One question though. Doesn't a TXV also have an orifice at the end? So when it's fully open, there's a flow rate determined by that valve's orifice. Isn't that why there are different sized TXVs for different tonnage condensers? Won't a TXV for a 5 ton condenser will have a larger orifice that one for a 2 ton condenser?

[Shophound]

Originally posted by wendel
Thanks for the helpful reply, doc. One question though. Doesn't a TXV also have an orifice at the end? So when it's fully open, there's a flow rate determined by that valve's orifice. Isn't that why there are different sized TXVs for different tonnage condensers? Won't a TXV for a 5 ton condenser will have a larger orifice that one for a 2 ton condenser?

One might call the opening in which a TXV "needle" sits within an orifice. And yes, once the needle is fully removed from the orifice, the TXV would essentially act as a piston metering device. The difference is the TXV would only operate like that under full to extreme load conditions.

A piston runs like that all the time. In fact, as far as superheat is concerned, a piston metering device is like a stopped watch; it has the correct time only twice a day. A piston will give a system the correct superheat only under ideal conditions, which could actually occur as little as twice a day as outdoor temperatures rise and fall. During the night, the evap will tend to run a bit flooded, and during the high heat of the day, it'll run a bit starved. Neither is ideal for efficiency and longivity of the compressor.

[wendel]

Thanks for the explanation, Shophound. I think you corroborated the HVAC company's position. If the evaporator is oversized, the superheat will, most of the time, be greater than 10 degrees and will be full open giving a full flow of R-22. Therefore the TXV would be open and the flow limiting device would be the TXV's orifice.

[batesx01]

TO clear up things the TXV has to have Out vap superheat of 12-15 deg to ensure there is no liquid refigerant after the TXV(this will cause the hunting not good) so dependant on the line set length and pressure drop you can be running a 25 deg sh @ comp. a Pisotn you can slightly over saturate the evaporator to lower the sh @ compressor.However the comment the piston is optimed in a couple of times a day is correct to some extent becouse at the ablient change so will the sh setting but it still can be just a efficient as a piston. But to answer the question YES bot need a liquid seal to opeteat correctly piston to keep it seated forward and the TWV to keep pressure on the spring so the o/e does not change.

[casturbo]

Originally posted by docholiday


....The TXV has a needle valve that is operated by spring pressure, evaporator pressure and bulb pressure. It maintains a constant superheat level. A very simple machine that works well.



So what happens when the TXV closes off somewhat (reduced A/H air flow for example), doesn't the liquid high pressure skyrocket? Then, compressor load must increase alot too, right?

[batesx01]

Casturbo. Yes when the TXV starts to close then the Suction Pressure drops and the Head pressure increases so the LBS/Hr of refrigerant, capacity, and Efficency all decreases.

[docholiday]

The HP doesnt increase, the subcooling does. And a proper txv should not close under most conditions unless you are lacking airflow as mentioned. It merely throttles the refrigerant flow. They dont sit there and open and close, thats called hunting and usually means an oversized valve for the system.

[batesx01]

BOth increace and yes it should not Hunt but he asked the question Reduce airflow. ALso a block in the equilization tube, Sensing liquid on the bulb, Bad location of the bulb,low sub cooling, Ect. all can cause Hunting situation

[beenthere]

Originally posted by wendel
Thanks for the explanation, Shophound. I think you corroborated the HVAC company's position. If the evaporator is oversized, the superheat will, most of the time, be greater than 10 degrees and will be full open giving a full flow of R-22. Therefore the TXV would be open and the flow limiting device would be the TXV's orifice.

Define over sized.

Many high seer units use an indoor coil rated at a higher btu then the outdoor unit. And the txv will still maintain proper sh.

What is your hvac company telling you?

[wendel]

Thanks for the reply, beenthere. The company is telling me that because the evaporator is 4 tons and the condenser is and TXV are only 3 tons that the coil load will be such that the TXV will be wide open 90% of the time and therefore be acting like a piston with one constant flow 90% of the time.

[beenthere]

When the system first starts the valve may be fully open, but as the coil gets colder it will throttle down.
The lower the humidity in your house the more it will close down also.

We install York/Luxaire, and they use 4 ton indoor coils on their 3 ton 15 seer units, and the txv's throttle down as the latent heat is removed, and I believe most manufactures do just about the same as far as the indoor coil being 1/2 to 1 ton larger then the outdoor unit.

Your contractor may need to send their guys to more classes.

[rob10]

Good compressors die an early death from using a piston instead of a txv. Too many techs shy away from txv's because they simply don't understand them.

[billygoat22]

We sell Gibson, too, and if I recall correctly all their units have pistons (even 15seer), 60a breakers in all a/hs regardless of wattage, 3/8" liquid lines for all condensors, same defrost board for all ht pumps.
I figure they're trying to stupid proof the units as well as simplify inventory control.