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  • Tom Greenleaf
    replied
    ? Where is this thread going? Helium takes a wild pressure isn't useful for mobile A/C.

    I'm not looking it up for trivia it's not a reasonable refrigerant nor condensable gas it's what is current and works??

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  • Cornbinder89
    replied
    No, the reason I mentioned phase change is because high side pressure is irrelevant to the refrigeration process, The high side has to be in liquid form, and the less heat contained in the liquid the better the system will work.
    Other than that, the high side pressure is just the condensing temperature of the refrigerant. It doesn't have to be 2x plus 15%, in fact it is better if it is far lower than that.
    It is also why you can't accurately charge by pressure. You can make some approximate guesses based on assumptions of the efficiency of the components at moving heat, but they are nothing more than approximation. Charging by weight is the accurate way to do it. Pressures are just a "window" into how the system is operating, and not an absolute on anything.
    Not sure why you brought in the quote about staged systems and cyro on an auto A/C forum?
    This isn't the place to "baffle with big quotes" , most who come here are just looking to fix their car, or have run into a problem they don't understand. Many do not fully understand how it works, and need some guidance, but we try and stick to what applies to auto a/c.

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  • bbrickson
    replied
    I will agree with that but it seems no one can see the +15% after the X 2. Unless the vehicle is custom the manufacturer is required to have a charge weight sticker in the engine compartment. Charging by weight is the correct way to recharge automotive A/C. It is the correct amount for the various conditions the vehicle be driven in.



    I was wondering why you mentioned change of state here and it must be in regards to a different question that I answered.

    That being that the change of state has nothing to do with the static pressure of an A/C system.

    An A/C system is a container. If a 4 LB system has 4 LBS of refrigerant in it and an identical system had just 1 oz of liquid refrigerant in it and they were both in the same 2 car garage overnight, in the morning they would have exactly the same pressure in them. They would be the same temperature with the same pressure. That is why the static pressure is only useful to determine if the contacts of a low pressure switch should be closed.




    This excerpt from the 4 cryogenic principles explains how refrigeration works.




    The second physical process with cryogenic applications is evaporative cooling, which occurs because atoms or molecules have less energy when they are in the liquid state than when they are in the vapor, or gaseous, state. When a liquid evaporates, atoms or molecules at the surface acquire enough energy from the surrounding liquid to enter the gaseous state. The remaining liquid has relatively less energy, so its temperature drops. Thus, the temperature of a liquid can be lowered by encouraging the process of evaporation. The process is used in cryogenics to reduce the temperature of liquids by continuously pumping away the atoms or molecules as they leave the liquid, allowing the evaporation process to cool the remaining liquid to the desired temperature. Once the desired temperature is reached, pumping continues at a reduced level in order to maintain the lower temperature. This method can be used to reduce the temperature of any liquid. For example, it can be used to reduce the temperature of liquid nitrogen to its freezing point, or to lower the temperature of liquid helium to approximately 1K (-458°F [-272°C]).




    The third process makes use of the Joule-Thompson effect, and provides a method for cooling gases. The Joule-Thompson effect involves cooling a pressurized gas by rapidly expanding its volume, or, equivalently, creating a sudden drop in pressure. The effect was discovered in 1852 by James P. Joule and William Thompson, and was crucial to the successful liquefaction of hydrogen and helium.

    A valve with a small orifice (called a Joule-Thompson valve) is often used to produce the effect. High pressure gas on one side of the valve drops very suddenly, to a much lower pressure and temperature, as it passes through the orifice. In practice, the Joule-Thompson effect is used in conjunction with the process of heat conduction. For example, when Kamerlingh Onnes first liquefied helium, he did so by cooling the gas through conduction to successively lower temperatures, bringing it into contact with three successively colder liquids: oxygen, nitrogen, and hydrogen. Finally, he used a Joule-Thompson valve to expand the cold gas, and produce a mixture of gas and liquid droplets.




    Today, the two effects together comprise the common refrigeration process. First, a gas is pressurized and cooled to an intermediate temperature by contact with a colder gas or liquid. Then, the gas is expanded, and its temperature drops still further. Ordinary household refrigerators and air conditioners work on this principle, using freon, which has a relatively high boiling point. Cryogenic refrigerators work on the same principle but use cryogenic gases such as helium, and repeat the process in stages, each stage having a successively colder gas until the desired temperature is reached.
    Last edited by bbrickson; 2 weeks ago.

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  • Cornbinder89
    replied
    There really is no "rule" rather there are accepted compromises for engineering limitations.
    From a purely refrigeration standpoint, you would want the high side pressure to be that of the refrigerants condensing pressure at ambient temp. In other words the refrigerant has given off all its heat to the condensing medium. In practice that is not possible, and the greater the difference between the cooled and cooling medium, the greater the amount of heat transferred for a given area.
    Given space limits, your "rule" is considered acceptable but is by no means ideal. The further you get away from ideal, the poorer the system will preform. Any heat left in the refrigerant when it leave the condenser is recycled into the evaporator and reduces what heat can be absorbed there.
    While 2x the ambient may give acceptable cooling it is by no means a desired temp/pressure.
    It is the phase change that allows for the heat to be absorbed, not the pressure change, otherwise you could just use compressed air for the refrigerant.

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  • bbrickson
    replied
    Do you see how the standard figure of 2 X the ambient temperature + 15% came to only 1.4 PSI off? It isn’t for me as in I have owned an automotive A/C shop for 30 years and have 4 employees? I will never learn how to do A/C? Is that what you are saying?

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  • Tom Greenleaf
    replied
    HIgh side if fans are on with 134a should be 2.5 times temp F. of incoming air thru a grille in front of it.

    2 would be low for some reason but might work.

    Have to know temps and where plus at what RPM or this isn't for you,





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  • bbrickson
    replied
    While you are adding you open the low side valve. The low side reading increases because you are measuring the pressure in the cylinder while open. The cylinder has to have higher pressure or it would not add. The correct way to charge is by weight but without a scale 82F is a good temperature to use gauges. The high pressure should come out to 2 x the ambient temperature plus 15% or 188.6 PSI on the high side and 27 low is good. You probably shouldn’t have added any.

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  • muscle spasm
    replied
    IR surface temp guns should be outlawed for AC work. Better than nothing yes, BUT. The better ones can adjust the reflectivity rate for the IR. Meaning you can tweak the setting and get incorrect temp readings. Different things like aluminum, human skin and steel reflect heat at different rates. Blah blah blah......

    Long story short, get a K probe and a simple multi-meter. Tape the K probe on the line and be done with all the "guessing" from IR guns.

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  • Tom Greenleaf
    replied
    Ah jeez Wink I'm here too is where I started. YOU STILL REFUSE TO SAY WHAT THE CAR IS? It's a 2013 Mustang so said elsewhere or we get noplace at the speed of light.
    Now you say you found a leak you wouldn't see a torn "O" ring so if nothing else did it hold a full vacuum for your altitude? You should have done that and then send it out for proper charge it matters totally.

    BTW - temp is taken a foot or so in front of grille not the forecast or thermo on home or around you. It's plain luck this even has a high side port most vehicles quit that as you charge thru low side check temps accurately also tell of what pressure is where you don't really need gauges just touch and feel or best infrared thermo at lines are they proper where they should be with given conditions. Engine RPM should be held at about 1,500 for testing so you know. Just tests the charge amount can't be known with that either just that it's within performance ranges.

    Have or get a thermo for inside, center vent output @ a real 80F should be in the mid 40sF - at the raised idle speed or driving along even better fans no longer needed by over 35 +/- MPH in many vehicles not sure for this exact one may or will stay on.

    Please allow others here to chime in this site is A/C specific has some super pros at the biz. Site may also not tell you a reply was made just check back if not? I can't fix that about this joint,

    Last edited by Tom Greenleaf; 04-20-2022, 06:34 AM.

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  • winkawak
    started a topic Need help with recharging

    Need help with recharging

    Hi my initial manifold reading was 27 psi low and 190 psi high with outside ambient around 82F. When i try to add refrigerant low pressure side would increase then drop down to 27 again but high pressure side increased and is at a stable 245. Later i came to find out my low pressure hose inner o ring was teared so i dont know if this is the cause. Will bad low pressure hose effect high pressure? Also why is my low pressure side not increasing?
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