We have concluded the TXV is too large. It is not common to use a TXV on such a small system. So far, we have not found a suitable replacement. However, a properly sized cap-tube or fixed orifice would probably do the trick. We welcome any suggestions.
I will install a cap-tube (problem solve) . "It would not hurt to do away with the TXV because it's a small unit, but if you insist that you must use a TXV pls go Danfoss".
judging from the frost right at the outlet of the txv and the slow pressure change but fast temp change I suspect there is a restriction either in the txv body or at the inlet of the txv that is restricting flow into the txv contributing to the fairly stable pressure yet not providing enough flow when the txv opens which causes the valve to overshoot the desired suction line temp.
a system in a space that small would never have an externally eq txv because the evaporator would not be large enough to have a distributor (3 or more lines into different runs of the evap coil). the ext eq line and spring pressure in the txv are going to have much more mechanical force on the pin than the bellows can produce causing it to close fast but be slow to respond to drop in suction temp
A friend had a small fridge with this problem. He called for service because the new compressor "didn't sound right" and he didn't want to have it blow too. The tech removed all the gas and replaced it. He said the previous tech who replaced the compressor probably didn't vacuum air from the system before adding the gas.
vlt the suction gauge shows u what the saturation temp and pressure is in evaporator.. its at saturation there.. once its superheated and becomes unsaturated the temp pressure chart no longer correlates.. u can adjust what the evaporator operates at by adding or removing refrigerant
Any updates on this problem? Ive been having problems with 4 units where i work, same units but the txv's hunt like crazy. Ive checked and adjusted the charge (410) with subcooling, insulated the bulb, adjusted bulb position and still nothing.
I don't understand how the thermometer is showing rapid changes in superheat but yet the suction line guage is not fluctuating rapidly, I don't get it - if the temperature in the suction line is fluctuating heaps then the suction pressure should be shown to be fluctuating heaps not minor amounts like shown?
VLT - Yes, normally this is true. In our case, we placed a temp clamp immediately downstream of the TXV and another downstream of the evaporator. Then we read the temp differential on a fluke 52. Since there is a mixture of liquid and vapor immediately after the TXV, it will indicate (at/near) saturation temperature. Therefore the differential temp will indicate actual super heat without needing to interpolate it from a PT chart. Since the vapor in the evaporator acts like a big spring (think shock absorber) its response is a little delayed compared to this method of directly reading temps.
My txv's are hunting as well because trane sent us oversized txv's because they didn't have the exact size. Engineered that way and now im dealing with it. It sucks.
some refrigeration cycles pull almost a vacuum, but it depends on the cross charge in side the sensing bulb, what exactly are you trying to accomplish?
Honestly, that was my first thought however unlikely! ! (I was kind of surprised!)...remember it is a total system. The total volume of the evaporator of course ,size of the suction line. And Just as important the Thermostatic expansion valve MUST feed the low pressure saturated mist into the Evaporator at the same rate at which the vapor pump (compressor) is pulling the superheated vapor from the Evaporator. Excellent Video! The craftsmanship looked to be top shelf as well.
Fathma - Give the system about 20 minutes of run time to allow the pressures/temps to stabilize. You will need a good clean pipe surface on the suction line downstream of the evaporator. Place a temperature clamp on it and wrap some insulation around it to ensure accurate readings. Then using the refrigerant pressure gauges, take a suction pressure reading. Compare the "saturated" temperature (found on the P/T chart) with the actual temperature reading. The difference will be the *F of superheat. Suction temperature should remain higher than saturation temperature, usually not less than 12*F and not more than 20 *F. Always consult manufactures documentation before adjusting. On the high side, use the same method but place temp clamp immediately after the condenser. Compare the head pressure to a P/T chart. The difference should indicate amount of sub-cooling.
Ron, without having access to pressures it is hard to be accurate. There is one possible option that can get you close. If it is possible to get a temp reading between the metering device and the evaporator, this will usually correspond to saturation temperature. The number read can be subtracted from the temperature leaving the evaporator to derive the superheat temperature. This only works when there is a mixture of liquid and vapor immediately after he metering device. In some cases, like very low charge or even over charge, the temp may start to drift away from corresponding saturation temp. So, use this method with caution and due diligence.
My co-worker just called me with the same problem after replacing a compressor on a sandwich display cooler. My guess is oversized txv. The last compressor only lasted 10 months. Checked: Air flow is good, amps almost at rated FLA which seems high for a new compressor, hot gas is not as hot as it should be, low head pressure and very low suction pressure, frost back to the compressor, and superheat is hunting the same as in the video, txv bulb mounted well. Any suggestions on this one?
I take it you are not targeting cooler temps below 32*f, therefore frost (on suction line) at the compressor would indeed seem bad. Are you sure there is good air flow and heat exchange at the evaporator. If the evaporator is not absorbing heat, then the whole system will have low pressure. Here are some other thoughts... The main point as you indicated, is low suction superheat. Liquid could be returning in the suction line which would kill the compressor over time (or immediately in extreme cases). It will also cause high amp draw on the compressor. Low suction pressure at part load is one reason evap coils frost up, this will aggravate all the symptoms you describe. How is the refrigerant charge? Are you getting proper sub-cooling on the condenser? Also, you will want to have a liquid line hx as described at :54 seconds on the video. If you don't already have one, then install one asap. It will at least provide a safety net for the compressor.
this seems to be set up like mini cooler so im guessing refrigeration , not airconditioning , most refrigeration has a suction of lets say 15 psig that should put more load on it and stop the hunting .. more refrigerant or smaller txv.
We have concluded the TXV is too large. It is not common to use a TXV on such a small system. So far, we have not found a suitable replacement. However, a properly sized cap-tube or fixed orifice would probably do the trick. We welcome any suggestions.
I will install a cap-tube (problem solve) . "It would not hurt to do away with the TXV because it's a small unit, but if you insist that you must use a TXV pls go Danfoss".
If it’s a constant load, an AEV Would work.
judging from the frost right at the outlet of the txv and the slow pressure change but fast temp change I suspect there is a restriction either in the txv body or at the inlet of the txv that is restricting flow into the txv contributing to the fairly stable pressure yet not providing enough flow when the txv opens which causes the valve to overshoot the desired suction line temp.
a system in a space that small would never have an externally eq txv because the evaporator would not be large enough to have a distributor (3 or more lines into different runs of the evap coil). the ext eq line and spring pressure in the txv are going to have much more mechanical force on the pin than the bellows can produce causing it to close fast but be slow to respond to drop in suction temp
A friend had a small fridge with this problem. He called for service because the new compressor "didn't sound right" and he didn't want to have it blow too. The tech removed all the gas and replaced it. He said the previous tech who replaced the compressor probably didn't vacuum air from the system before adding the gas.
vlt the suction gauge shows u what the saturation temp and pressure is in evaporator.. its at saturation there.. once its superheated and becomes unsaturated the temp pressure chart no longer correlates.. u can adjust what the evaporator operates at by adding or removing refrigerant
I had the exact same thing happen to a 410a piston unit. What should I have done?
Any updates on this problem? Ive been having problems with 4 units where i work, same units but the txv's hunt like crazy. Ive checked and adjusted the charge (410) with subcooling, insulated the bulb, adjusted bulb position and still nothing.
I don't understand how the thermometer is showing rapid changes in superheat but yet the suction line guage is not fluctuating rapidly, I don't get it - if the temperature in the suction line is fluctuating heaps then the suction pressure should be shown to be fluctuating heaps not minor amounts like shown?
VLT - Yes, normally this is true. In our case, we placed a temp clamp immediately downstream of the TXV and another downstream of the evaporator. Then we read the temp differential on a fluke 52. Since there is a mixture of liquid and vapor immediately after the TXV, it will indicate (at/near) saturation temperature. Therefore the differential temp will indicate actual super heat without needing to interpolate it from a PT chart. Since the vapor in the evaporator acts like a big spring (think shock absorber) its response is a little delayed compared to this method of directly reading temps.
Bulb in wrong position picking up liquid on after evap
have you adjusted spring on txv? you can adjust spring tension to help txv dial in superheat. hope this help
My txv's are hunting as well because trane sent us oversized txv's because they didn't have the exact size. Engineered that way and now im dealing with it. It sucks.
some refrigeration cycles pull almost a vacuum, but it depends on the cross charge in side the sensing bulb, what exactly are you trying to accomplish?
Honestly, that was my first thought however unlikely! ! (I was kind of surprised!)...remember it is a total system. The total volume of the evaporator of course ,size of the suction line. And Just as important the Thermostatic expansion valve MUST feed the low pressure saturated mist into the Evaporator at the same rate at which the vapor pump (compressor) is pulling the superheated vapor from the Evaporator. Excellent Video! The craftsmanship looked to be top shelf as well.
use a TXV a size smaller so that when it does open, you dont see the big swings in temp.
i need help ,can somebody explain me how can i measure superheat if the system have a electronic expansion vavle
Fathma - Give the system about 20 minutes of run time to allow the pressures/temps to stabilize. You will need a good clean pipe surface on the suction line downstream of the evaporator. Place a temperature clamp on it and wrap some insulation around it to ensure accurate readings. Then using the refrigerant pressure gauges, take a suction pressure reading. Compare the "saturated" temperature (found on the P/T chart) with the actual temperature reading. The difference will be the *F of superheat. Suction temperature should remain higher than saturation temperature, usually not less than 12*F and not more than 20 *F. Always consult manufactures documentation before adjusting.
On the high side, use the same method but place temp clamp immediately after the condenser. Compare the head pressure to a P/T chart. The difference should indicate amount of sub-cooling.
Lance Reeves how can you do this on a sealed system with no service port? Thank you
Ron, without having access to pressures it is hard to be accurate. There is one possible option that can get you close. If it is possible to get a temp reading between the metering device and the evaporator, this will usually correspond to saturation temperature. The number read can be subtracted from the temperature leaving the evaporator to derive the superheat temperature. This only works when there is a mixture of liquid and vapor immediately after he metering device. In some cases, like very low charge or even over charge, the temp may start to drift away from corresponding saturation temp. So, use this method with caution and due diligence.
Lance Reeves thank you.
Same way
My co-worker just called me with the same problem after replacing a compressor on a sandwich display cooler. My guess is oversized txv. The last compressor only lasted 10 months.
Checked: Air flow is good, amps almost at rated FLA which seems high for a new compressor, hot gas is not as hot as it should be, low head pressure and very low suction pressure, frost back to the compressor, and superheat is hunting the same as in the video, txv bulb mounted well. Any suggestions on this one?
I take it you are not targeting cooler temps below 32*f, therefore frost (on suction line) at the compressor would indeed seem bad. Are you sure there is good air flow and heat exchange at the evaporator. If the evaporator is not absorbing heat, then the whole system will have low pressure. Here are some other thoughts... The main point as you indicated, is low suction superheat. Liquid could be returning in the suction line which would kill the compressor over time (or immediately in extreme cases). It will also cause high amp draw on the compressor. Low suction pressure at part load is one reason evap coils frost up, this will aggravate all the symptoms you describe. How is the refrigerant charge? Are you getting proper sub-cooling on the condenser? Also, you will want to have a liquid line hx as described at :54 seconds on the video. If you don't already have one, then install one asap. It will at least provide a safety net for the compressor.
this seems to be set up like mini cooler so im guessing refrigeration , not airconditioning , most refrigeration has a suction of lets say 15 psig that should put more load on it and stop the hunting .. more refrigerant or smaller txv.
no load is the problem..load willll give your 5 degrees
Agreed.
Richard Gatewood oil it is on the evaporator. Flush amigo. More than one time I've encountered this problem.
Richard Gatewood
Adjust txv or add a P-trap at evaporator? Ive seen smaller systems work fine with txv. Interesting.
Move the bulb out of the box and further down the suction line until hunting stops.
Thanks, we tried that and the problem got worse.
Clogged equalizer, will hunt because it can NOT equalize !
gezzz NO SUPER HEAT ON TXV people
thanks
Yes you can check superheat on a txv