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Refrigeration TXV question

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Comments

  • Glenn Harrison_2
    Glenn Harrison_2 Member Posts: 845
    Was working on a little cooler recently...

    a prep table cooler to be exact, and found the compressor would take several tries to start. I replaced the start gear, checked the volts and amps, ohmed the comp, and everything looked OK. I then checked the pressures, and found the TXV was equalizing very slowly. I then found after a while (15 minutes or so) the pressures would equalize enough that the comp. would start on the first try.

    Now in asking this question, keep in mind I'm used to A/C TXV's that equalize quickly. Is this normal for refrigeration TXV's not to equalize, or is there something wrong with this particular TXV, or is there different types of TXV's that will equalize quickly verses slowly? It's an older Sporlan 1/4 ton with internal equalizer(?)
  • Empire_2
    Empire_2 Member Posts: 2,343
    Hello Glenn...

    Sounds to me like the system is short cycling. Even with the start accessories replaced, , they only have limited torque. You may want to increase your diff. on the (LPC). Does this system use pump down control? Also, technically I would think that the TXV should equalize somewhat quickly, but that depends on the Table temp and there I go again thinking that the diff is your problem. All compressors need time to equalize and if your TXV is externally equalized, I would think short cycling problem. One more thought,..There should be a filter screen in the inlet of the TXV...Did you check that?

    I think there are more qualified techs in here that may steer you in the right direction, but that is what I think.

    Mike T.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    TXV primer

    The answer to your question is, "It depends".

    Some TXVs allow system pressures to equalize during the off cycle, while others do not. Compressors that are used on TXV systems that do not allow system pressures to equalize (at least not very quickly) must be able to start with substantial pressure differentials across them. These compressors are usually equipped with start capacitors and associated starting relays.

    Most TXVs are avaiable with permanent bleed ports, which allow the system pressures to equalize during the off cycle. The larger the bleed port percentage, the more quickly the system pressures will equalize. The bleed port percentage typically ranges from 5% to 40%.

    Some TXVs are equipped with an RPB (Rapid Pressure Balance) feature which allows the system pressures to equalize faster during the compressor's off cycle.

    Hope this helps.
  • Glenn Harrison_2
    Glenn Harrison_2 Member Posts: 845
    That was extremely helpfull, Prof.

    So more than likely the TXV is working as designed, and the little tin can that has been replaced a few times over the years is not quite strong enough for the pressure imbalance.

    So if I want to replace the TXV to give the comp a break, should I ask for a TXV with an RPB, or is there some other term I should be using to get a rapid equalizing TXV. Also, can I get such a TXV that is internally equalized, or would I need to get an externally equalized version and add an equalizer line.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Confusing Terminology

    Contrary to what the name implies, the terms "internally equalized" abd "externally equalized" have absolutely nothing to do with the speed at which the system pressures equalize.

    There are three pressures that control the operation of the TXV: The evaporator pressure, the bulb pressure and the superheat spring pressure. The bulb pressure is the only opening pressure, while the evaporator and spring pressures combine to provide the valve's closing pressure.

    The terms "internally" and "externally" related to the point in the system where the evaporator pressure is sensed. On internally equalized valves, the evaporator pressure comes from the inlet of the evaporator coil. On externally equalized valves, the evaporator pressure comes from the outlet of the evaporator coil. This is why externally equalized valves have that extra port on the valve body and why you have to run that 1/4 inch line from the valve body to the outlet of the evaporator coil.

    When the pressure drop in an evaporator is large (over 2 psig) the amount of superheat the evaporator will maintain will be greater than desired as the evaporator saturation temperature and pressure will drop as the refrigerant flows through the coil. So, to compensate for that increase in evaporator superheat, we use externally eqaulized valves. In this case, the evaporator pressure is lower than the pressure at the inlet of the evaporator coil. This reduces the total closing pressure on the valve. If the total closing pressure is lower, the total opening pressure will also be lower (This is because valves will adjust themselves util they reach equilibrium, which is the point where the opening and closing pressure are equal). If the bulb pressure is lower, this means that the temperature at the outlet of the evaporator coil is also lower. A lower evaporator outlet temperature means that the evaporator superheat is lower.... and closer to the deisred superheat level.

    Enjoy!
  • Glenn Harrison_2
    Glenn Harrison_2 Member Posts: 845
    Professor

    Your making me realise I need to break out my school books for 15+ years ago and relearn some stuff I have obviously forgotten.

    Thank you for the refresher on TXV's
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Not a Problem

    We're constantly learning, forgeting and relearning as we go through this wondderful thing called life.

    Maybe you can join us for a training session or two when we hit the road again in the spring.

    ;)
  • Techman
    Techman Member Posts: 2,144
    TXV's

    Hi Glenn! My experince is that most refrigeration TXV's do not equalize during the off-cycle,the thermal bulb is in the refrigerated space so the TXV can't equalize the pressurers and ,simply ,most are not of the "equalizing" types.How much time between the comp shutting off and then back on ?What type of box control,temp or press control?What Refrigerant? What pressurers is the unit running at ?Enjoy Your Day!
  • Empire_2
    Empire_2 Member Posts: 2,343
    Back again......;-

    > Hi Glenn! My experince is that most refrigeration

    > TXV's do not equalize during the off-cycle,the

    > thermal bulb is in the refrigerated space so the

    > TXV can't equalize the pressurers and ,simply

    > ,most are not of the "equalizing" types.How much

    > time between the comp shutting off and then back

    > on ?What type of box control,temp or press

    > control?What Refrigerant? What pressurers is the

    > unit running at ?Enjoy Your Day!



  • Empire_2
    Empire_2 Member Posts: 2,343
    Back again...........:-)

    > Hi Glenn! My experince is that most refrigeration

    > TXV's do not equalize during the off-cycle,the

    > thermal bulb is in the refrigerated space so the

    > TXV can't equalize the pressurers and ,simply

    > ,most are not of the "equalizing" types.How much

    > time between the comp shutting off and then back

    > on ?What type of box control,temp or press

    > control?What Refrigerant? What pressurers is the

    > unit running at ?Enjoy Your Day!



  • Empire_2
    Empire_2 Member Posts: 2,343
    Back again...........:-)

    So diff control means nothing to anyone? Short cycle is not relative here? I am not seeing what is apparently clear to everyone else, but that's OK......;-) Even with the Start assist ,....There should be time for equalization. I understand what is being talked about, but the valve is not my immediate concern right now. If set point was 43-46* and diff. was 4 to 8* how can you guy's make your opinions without the relative informational facts? No DISS intended, I just thought I was better than to miss what you are all seeing....;-( Maybe I need a refresher course......In Hawaii Eugene,.....Is that possible?....

    Mike T.
  • Techman
    Techman Member Posts: 2,144
    TXV's

    Hi Mike T.In commercial refrigeration most TXV's do not equalize the pressurers on the off cycle .A regular low pressure control would not function as a box "temp" control if that were true . I see a "start assist" (solid state types)as improper for most commercial refrigeration. This is where a "hard start kit " is required. I also want to know about the differential on the box control, press or temp.With a set point of 43 -46 psig the differental would be anywhere from 20-30psig depending on certain factors.If you are talking 43-46 degrees then that is way to high for a proper box temp of 32 1/2- 39 1/2 degrees.In this case we're talking about a small prep box.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Hmmm

    If I schedule a refrigeration basics class in Hawaii, will you sign up?
  • Empire_2
    Empire_2 Member Posts: 2,343
    If I go To Hawaii,......

    The last thing I would want to do is study anything but the ladies...;-) Buffalo, NY would get me in for a seat or 2 though... I was exaggerating when I said basics....., but I am open to anything Eugene.

    Mike T.
  • Vil
    Vil Member Posts: 1
    Commercial Air Conditioning Problem

    A commercial customer reports that an air-conditioning unit in a small office building is not cooling on the first warm day. The termostat is set at 75 degrees F and the space temperature is 82 degrees F. The air feels very humid. No air is coming from the registers. The air handler is in the attic. An examination of the condensing unit shows that the suction line has ice all the way back to the compressor. The compressor is still running. What is the likely problem and the recommended solution?
    Vil
  • Techman
    Techman Member Posts: 2,144
    Hay Vil!

    This subject might be better as it's own thread !
  • Glenn Harrison_2
    Glenn Harrison_2 Member Posts: 845
    You guys are all helping jog my memory

    This is what I get for not doing refrigeration for 9 years. I had forgotten about controlling the cabinet temp with a low pressure control.

    Anyways, this particular unit is using a Johnson controls digital controller, set for 38*. I beleive this control has a 5 minute delay built into it, to keep down short cycling. However, I have allowed this unit to set for 10 minutes twice and the comp. would not start on the first try, and frequently multiple times. I also took my gauges and equalized the pressures thru the manifold and the comp would start on the first try.

    Refrigerant was R-12, now using HotShot R-414B "universal" replacement for R-12 and R-134
  • Glenn Harrison_2
    Glenn Harrison_2 Member Posts: 845
    If you get a class near Chi town...

    I will see whatI can do to be there.
  • Glenn Harrison_2
    Glenn Harrison_2 Member Posts: 845
    No pump down...

    no pressure control. Just operated using a Johnson Control digital t-stat.

    I don't remember exactly what the pressures were, but pressures , superheat, and subcooling all looked reasonable based on the PT chart for HotShot Refrigerant
  • Techman
    Techman Member Posts: 2,144
    Glenn

    Morning! With the 5 minute time delay and you equalizing the press thru the manifold,I think you have a "tight" compressor or the start components are not correct . Does this comp have a current relay?Enjoy Your Day!
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    sounds like...

    You have a low side airflow problem, my friend. Low side air flow can be affected by any of these items:

    Dirty air filter, dirty evaporator coil, blocked return duct, loose duct liner in the return duct, defective evaporator blower, dirty blades on the blower wheel, loose belts, slipping belts, broken belts, Loose duct liner in the supply duct, closed or blocked supply registers...

    That's for starters, assuming the refrigerant charge is okay. Chances are that your evaporator coil is completely frozen, which is likely a result of one or more of the items listed above.

  • Glenn Harrison_2
    Glenn Harrison_2 Member Posts: 845
    Yes

    It does have a current relay and start capacitor, both of which were replaced with components speced by Copeland.

  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Any luck?

    Any luck?
  • scotty_3
    scotty_3 Member Posts: 6
    economizer

    i would check the minimum out door air set point.do this on newer rooftops by ajusting the potentiometerit looks like a little round thing with an arrow on it and watch the damper stroke
This discussion has been closed.

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