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  1. #27
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    Andy,

    It is not enough of an explanation for me. Your smart. Cause your you. Many here probably have zero idea what were thinking. In other words we must break it down and spell it out. It's important that these guys tracking this thread understand the basics of theory. I think it's extremely important for every one's knowledge.

    Theory is, basically, latent heat of evaporization. And one must understand this in order to think through and then answer to themselves with confidence that this occurence, an overcharge, by itself will not cause an evaporator to ice up, or frost up. In an Air conditioning System. Were talking AC, not refrigeration, which we can get into later.

    Of course, we could go and say low ambient, then add low airflow, low load dada dada da. But by itslef, on a design condition day, overcharge does not ice up a coil.

    But. Anyway. I wish I was as smart as you. Than I'd be not wrenching anymore. Out of curiosity. Have you ever been a feild tech before Sporlan.

  2. #28
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    Apr 2002
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    Originally posted by Dowadudda

    Theory is, basically, latent heat of evaporization. And one must understand this in order to think through and then answer to themselves with confidence that this occurence, an overcharge, by itself will not cause an evaporator to ice up, or frost up.
    That we agree. I'm simply trying to point out that an overcharge will not generally lead to lower than normal suction pressures which is ultimately why an a/c coil would ice up.

    Originally posted by Dowadudda
    Out of curiosity. Have you ever been a feild tech before Sporlan.
    Actually, I did haul around tools, among other duties, for an a/c contractor for a period of time prior to working for Sporlan. So I'm not a complete klutz with a brazing torch, wrench, and gauges . Ok, those who have witnessed my brazing technique may argue otherwise. But outside of working on my own own home or auto a/c on occasion, I don't get much opportunity to do real field work. Many of us engineers would obviously benefit from doing significant field work. But who knows, if we did this, we may find field work more interesting than being cooped up in some office.

  3. #29
    I'm not picking on anyone here, so please don't take this personally. Some here have said:

    "because an overcharge will put more liquid in evap which causes a lower line temp which will close the txv off which will lower the pressure which will lower the temp which will cause frosting."

    The vapor compression cycle doesn't work this way. An expansion valve won't feed more just because the head goes up. They open and close based on superheat; not inlet pressure (in most cases). Even so, lets assume they did for a minute. Since we would be putting more refrigerant into the evaporator, (think of a compressor as a constant volume pump, yes I know they aren't) the pressure in the evaporator has to go up. If the pressure goes up the refrigerant temp goes up according to its pressure temperature relationship. The pressure goes up because the compressor can't remove the vapor at a rate equal to what the metering device is metering in. So,... the pressure in the evap rises until a new balance point is achieved between the compressor and the flow rate.

    Think of it this way: If a unit is short charged, won't the evap pressure be lower?? Yes, because the compressor can now pump the refrigerant out of the evap faster than what the metering device is letting it in. Besides, on a good hot day doesn't the evap pressure go up because there is more load?? If there's more load don't we have to evaporate more liquid?? So, if you put more liquid in the evap, the pressure goes up and so does the saturation temp.

  4. #30
    Join Date
    Feb 2005
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    69

    how about this?

    If your overcharged, you have excessive liquid line pressure. Your now forceing high pressure liquid refrigerant into the evap. If your forceing it in the evap faster than it can boil off by the blower, won't it freeze up? Doesn't liquid refrigerant cause frost?

  5. #31
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    CR,

    Define what latent heat of evaporization is. This would explain your question.

  6. #32
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    CR,

    Define what latent heat of evaporization is. This would explain your question. Actually of you wanted to get real technical and and answer your question about liquid, and this is only if wnat to get real deep, also then explain what flash gas is.

    Flash can do this. But why?

  7. #33
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    CR,

    Define what latent heat of evaporization is. This would explain your question. Actually if you wanted to get real technical and answer your question about liquid, and this is only if you want to get real deep, also then explain what flash gas is.

    Flash can do this. But why?

  8. #34
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    Re: how about this?

    Originally posted by tech cr
    Doesn't liquid refrigerant cause frost?
    Only if your suction pressure is low enough. Ay, there's the rub. An evaporator coil being overfed will increase your suction pressure above normal. On a properly operating a/c system, suction pressures never get low enough to cause frost to form.

    Now if you have an undercharged system, low air flow, or dirty air filters, suction pressure will drop below normal.

  9. #35
    Join Date
    Feb 2005
    Posts
    69
    Wow sounds like you would like to get really technical. That's cool but last week I went on a call on a split system A/C unit. I got there and it was all froze up. I thawed it out and started it up. It had 60degrees subcooling and high suction pressure(I forget the actual pressure) after it ran for about 10 minutes it started freezing up. It was WAY overcharged. Which goes back to what I said and also to answer the question here.

    [Edited by tech cr on 04-06-2005 at 10:09 PM]

  10. #36
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    Originally posted by tech cr
    It had 60degrees subcooling and high suction pressure(I forget the actual pressure) after it ran for about 10 minutes it started freezing up. It was WAY overcharged.
    The important question here is what was your suction pressure after 10 minutes of operation. If the coil was frosting, your suction pressure was low. It is not uncommon for techs to overcharge systems attempt to compensate for low suction pressures. The real problem here is more than likely insufficient airflow/dirty filter or improperly sized expansion device.

  11. #37
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    You have to understand first what is going on inside the evaporator coil with the refrigerant to understand why that an overcharge unto itself will not cause a evaporator coil to ice up.

    I keep hitting this cause it's important, and few want to dig into it, which is why I keep hitting on it. Many many technicians lack a solid understanding of theory. And if they just took a few moments and learn up on it, their diagnostic skills would be much imporved.

    Go look up Laten Heat of Evaporization, and latent heat of condensation. This will clearly define for any of you any questions about this very subject and a whole bunch more.

    Andy is making it simple for you by just having you focus on your refrigerant temperature. But I think most won't get that, if they aint getting why it should be doing one thing versus another.

  12. #38
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  13. #39
    you guys didnt read my replay, but you still donot understand how a txv works.
    first i said aa overcharge of a refrigeration system with a txv. would cuase icing or sometime on a flotronic chiller
    with an exv an overcharge would cause the exv to frost up. i did not say a standard dx system would.


    you guys are wrong however about how a txv works.
    if you overcharge a system that has a txv and we are not talking about an orfice. but a txv will close off to keep the superheat up to its setpoint say 15 degrees. as more refrigerant is added it will have to close off more and more to keep the superheat up and that will cause the suction pressure to drop.

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