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  1. #53
    Join Date
    Apr 2002
    Posts
    11,808
    Originally posted by Andy Schoen
    Originally posted by airworx
    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.
    airworx, you're a bit confused a bit here. Let me help you out.

    Suction pressure is simply a function of heat load on the evaporator and your compressor/condenser capacity. If the TEV is operating properly, it will modulate refrigerant flow such that it will increase refrigerant flow into the evaporator at higher load conditions, thus raising suction pressure, and reduce refrigerant flow into the evaporatir at lower load conditions. This is due, of course, to the fact it controls superheat at the evaporator outlet. It does this function much better than a capillary tube or fixed restrictor.

    The key here is your heat load on the evaporator. It is the primary variable in determining your suction pressure.

    The TEV doesn't care about an overcharge system. It will continue to control superheat regardless. An overcharged system, however, will reduced your compressor's capacity and efficiency, which will cause higher than normal suction and discharge pressures.

    You might find the following of interest. http://www.sporlan.com/10-9.htm I wrote its original version

    It's amazing that people won't take winning tips straight from the horse's mouth
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  2. #54
    Join Date
    Jun 2001
    Location
    Michigan
    Posts
    12,077
    The friggin guy who pratically designed the TXV, wrote the book on TXV's, and the guy who practically runs the company who makes the TXV's comes on here and tells Airworx he is wrong, and then he still think he's right. What an arrogant and ignorant guy. That is insulting to me.

  3. #55
    Join Date
    Apr 2002
    Location
    Dallas, TX
    Posts
    2,987
    Originally posted by airworx
    also when you overcharge a system head does go up but on a txv system the tx bulb senses temp not pressure and as the temp drops the pressure in the bulb drops the spring and evap pressure overcome this pressure and pinch off the valve which causes the suction pressure to drop.
    I think I understand the problem here. The TEV's sensing bulb, of course, doesn't sense pressure. But the valve does. The pressure it senses is what's referred to as equalizer pressure. A bit of a misnomer, perhaps, but that's what it's called. An internally equalized TEV senses this pressure internally. An externally equalized valve senses it via a third connection on valve which is connected to the outlet of the evaporator, normally with a 1/4" OD line.

    At the TEV sensing bulb location, temperature is not being controlled.

    At the TEV sensing bulb location, only superheat is being controlled. You can't assume equalizer/suction pressure will stay the same when the bulb temperature drops. In fact, you can expect it to drop also. So the question becomes: what's causing suction pressure to drop? Low load, yes; low airflow, yes; overcharge, NOT.

    The situation with an EEV is a bit more complicated due to the fact the EEV controller may well be controlling more than just superheat. But if the controller is simply controlling superheat, the result will be the same.

    [Edited by Andy Schoen on 04-10-2005 at 06:34 PM]

  4. #56
    Join Date
    Apr 2002
    Location
    Dallas, TX
    Posts
    2,987
    Unfortunately, I only got a 72 on the a/c CMS, which is worse than the 75 I got on the refrigeration test I took a year ago.

    I did poorly on absorption systems... Nailed psychrometrics, though, as expected...

    I'm resigned now that I'll have to actually study for one of these damned tests to pass it

  5. #57
    Join Date
    Sep 2002
    Posts
    1,389

    But Wait one more time

    I have to go back to my original system of a
    Fixed orifice.
    Now I will add a system:
    5 ton compressor/condensing unit. 5 ton evaporator coil.
    3-1/2 ton load on said evaporator. Air flow at 225cfm/ton
    due to plugged coil. Evaporator temperature at 25 degrees with 10 degrees of superheat. 70 degree day.
    I, like many servicemen merely put on gauges and say, "Oh the suction is too low I need to add gas. Screw putting a suction side thermometer on". Of course I haven't done that since '66 as I recall.
    I add gas and loose the superheat and make the refrigerant temperature in the whole of the evaporator side 28 degrees, up from 25 degree equivalent pressure.
    Did I not then have the capability to create ice with any water particle that comes in contact with the evaporator tubing which is now at 28 degrees? Especially since I have reduced airflow?
    I did not gain any load by adding refrigerant. And the head won't necessarily go high enough, quickly enough to raise the suction above the 28 degrees since the condenser is capable of 5 tons.
    Need to run the graph I guess.
    Please explain to all on the thread WHY this cannot happen so everyone knows.
    Thanks, tom

  6. #58
    Join Date
    May 2004
    Location
    Los Angeles, CA
    Posts
    1,560

    Great thread................

    Dear thread,

    Good morning everyone. After reading all the above posts I can see that there has been quiet a bit of discussion deviating from Hsteiner’s original question to this thread. Since I’ve decided to step into this complicated and multifaceted thread I’ll start by giving my own answer to his original question of:

    “A co-worker insists that a overcharge on a AC unit can result in iced up condition.
    I always thought icing was caused by undercharge or low airflow so I disagreed with him.
    Who is right?”

    Based upon the lack of additional equipment information, data, before and after operating measurements, and conditions, I’ll try to answer the question in the spirit “I think” it was asked.

    Being completely accurate regarding the above question, I can only conclude that unfortunately NEITHER of you are right.

    Your co-work is NOT right because based upon a normal operating system with no other problems or deficiencies, overcharging an “air conditioning” system WILL NEVER UNDER ANY CIRCUMSTANCES cause the system to ice up period.

    You are NOT right because although an undercharged or low airflow system can cause the unit to ice up, there are many other conditions that will generate the same symptoms as well.

    My next post will comment on some of the deviated questions and comments spun off of this original question.

    Respectfully Submitted,
    John J. Dalton


  7. #59
    Join Date
    May 2004
    Location
    Los Angeles, CA
    Posts
    1,560

    Ice...yes....caused from overchargeing.....no.

    Dear Twcpipes,

    Regarding your previous post:

    “Please explain to all on the thread WHY this cannot happen so everyone knows.

    Based upon the operating conditions, measured data, “and operational problems” of your hypothetical system, I would have to conclude that ice would in fact form from anywhere downstream of the capillary tubing all the way to and including on the exterior shell of the compressor.

    Although the above referenced air conditioning system would form ice on it under the above overcharged conditions, the ice would not be a simple symptom of the overcharge, but would in fact be a product from the root cause of the system before the excess refrigerant was introduced into this system, namely the restricted evaporative airflow (225 CFM/ton as you stated).

    Respectfully Submitted,
    John J. Dalton


  8. #60
    Join Date
    Apr 2002
    Location
    Dallas, TX
    Posts
    2,987
    No doubt it is possible to overcharge a system and have insufficient airflow and create ice, all else being correct with the system. With a fixed restrictor system, the overcharge will cause floodback, and the overcharge cannot be so great as to cause suction pressures to be greater than say about 50 psig on R-22 if one expects ice to form. If a tech is mistakenly charging a system to overcome low airflow, however, he will typically charge the unit to a more reasonable suction pressure, 60+ psig, resulting in no ice formation, and greater floodback.

    A system with a TEV will behave similarly, though the TEV should prevent floodback as it would control superheat.

    As john dalton points out, however, the problem here is low airflow. The overcharge is not the cause, but simply the result of a mistaken attempt to resolve the insufficient airflow problem.

  9. #61
    Join Date
    Dec 2001
    Posts
    32
    Wow, this has become a really long and interesting thread. Thanks for all the input, hopefully everybody learned a little something new, I know I did.
    Just to clarify something John Dalton said: You are NOT right because although an undercharged or low airflow system can cause the unit to ice up, there are many other conditions that will generate the same symptoms as well.
    I am aware of that, but the first and probably easiest things to check when checking a iced up unit are filters, fan amps, pressures and subcooling.
    The point was made pretty clear throughout this thread that an overcharge on a otherwise normally operating unit will not cause icing to build, which is really what started this discussion, so I think I am a lot more right than my co-worker who apparently removed refrigerant and therby got the unit running properly.
    Unfortunately this happened to him a few years back and there is no way of verifying anything.
    thanks again

  10. #62
    Join Date
    May 2004
    Location
    Los Angeles, CA
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    1,560
    Dear Hsteiner,

    “Just to clarify something John Dalton said: You are NOT right because although an undercharged or low airflow system can cause the unit to ice up, there are many other conditions that will generate the same symptoms as well.
    I am aware of that,…..”

    Being aware of it, and stating it are apparently two different things, as evident in your original post my friend.


    “…but the first and probably easiest things to check when checking a iced up unit are filters, fan amps, pressures and subcooling.”

    When you find an iced up system, the first thing to do is shut off the compressor to deice the system, then as the system is deicing you check your evaporative motor assembly to verify it’s working and working correctly, then you can check anything mechanical in nature(filter, grills, registers, and the such). Only after the complete deice of the system can you start the system back up and obtain a complete set of operational readings to first and foremost check airflow, and then everything else.


    ”The point was made pretty clear throughout this thread that an overcharge on a otherwise normally operating unit will not cause icing to build, which is really what started this discussion, so I think I am a lot more right than my co-worker….”

    Being a little wrong or a lot wrong is like being pregnant, after all the discussion, either you are or you’re not, period.

    Thanks for the great post to get this thread started, I too believe everyone who participated here learned something new, or at least had their basic diagnostic and refrigeration skills reinforced.

    Respectfully Submitted,
    John J. Dalton

  11. #63
    Join Date
    Dec 2001
    Posts
    32
    "My Friend" John Dalton, You sound like you need to lighten up a little, don't take things so seriously and you will get a lot more out of life, ok? bye

  12. #64
    Join Date
    Apr 2005
    Location
    St. Louis
    Posts
    41
    If the system is overcharged it can flood back to the compressor. If an additional problem exists, like a dirty filter or a bad fan motor, the saturated evaporator temperature could be below the frost poing of the air. I would make sure everything else is clean and working properly before I let charge out of the system. Check superheat and subcooling.

    High head pressure would also be a symptom of an overcharged system. A lot of systems will not pump down into the condenser or receiver, without extremely high head pressure, if they are overcharged.

  13. #65
    Join Date
    Sep 2002
    Posts
    12,536
    hsteiner,

    I wouldn't say "impossible" but it seems damned unlikely doesn't it?

    How could it happen?

    A TXV will control superheat while the overchage would tend to raise head pressure and then suction presure. None of which would tend toward freezing a coil.

    A cap tube would overfeed refrigerant as head pressure and Delta P increased with the overcharge. Suction pressure would rise, superheat would fall. None of which would tend toward freezing a coil.

    Get you co-worker to explain in exact detail how it can happen. And then come back here and tell me.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

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