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Thread: Theory Fundamentals

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    Theory Fundamentals

    Had a guy a few periods under me ask a question from school which made me think. Question is: In a system using CFC-12, evaporator temperature is 20F ; condensing temperature is 110F. If refrigerant leaves the evaporator as a saturated vapor, and leaves the condenser as a saturated liquid, the condenser will remove _______ BTU from each pound of refrigerant.

    Now I understand the main point of a refrigeration system is to boil off refrigerant at a specific temperature in the evaporator (at whichever temperature is useful). This is done by pressurizing the substance so it boils at say 40F (at atmospheric pressure, most refrigerants have very low boiling temperatures). And as the gas boils, it picks up a certain amount of heat (enthalpy).

    My question: is the condensing pressure dictated by the required enthalpy of the refrigerant so that we can get rid of the heat picked up in the evaporator and the heat of compression?

    My answer: 85.12 BTU per pound

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    Math ahhhh. Its more less dictated as can you condense it... I leave load the sales guys and out put to the manufacturer.

    As a tech I figure out why its not within specs if that's the case. But as I see it its a dance of keeping pressures high enough to get a good spray and low enough to condence.under various outdoor conditions ofcoarse

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    Geez, I always thought that the refrig leaves the evap as a SH vapor and the liquid leaves the cond as a SC liquid. But, now, you and your classmate have me wondering?!?!??

    If "enthalpy" another word for "latent heat", then yes.

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    Quote Originally Posted by TechmanTerry View Post
    Geez, I always thought that the refrig leaves the evap as a SH vapor and the liquid leaves the cond as a SC liquid. But, now, you and your classmate have me wondering?!?!??
    TMT, the question had me wondering, too!


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    I am wodering why a class is discussing R12.
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    Yeah, it does, or should. I think the point of the question was to understand how enthalpy changes in relation to pressure/temperature. So they just made it easier by saying it was at saturation.

    The refrigerant must have a higher enthalpy in the condenser because it will be releasing more heat than it picked up in the evaporator. This is achieved by bringing it to a certain pressure in the condenser.

    Is this how engineers decide which compressor to use for a system?

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    WHAT?

    I thought "enthalpy" was "total heat" ,while "latent heat" was "hidden heat" and was reserved for the "boiling process" only. 212* water to 212* steam = 970btu/lb.= no rise of sensible heat temp.
    Well,a TXV "should" be called a SH Controller, but it isn't.

    The cond unit releases more heat than the evap picks up is due to the 5 different SH that gets picked up along the way.

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    Quote Originally Posted by nutradesman View Post
    .

    My question: is the condensing pressure dictated by the required enthalpy of the refrigerant so that we can get rid of the heat picked up in the evaporator and the heat of compression?

    My answer: 85.12 BTU per pound
    Condensing pressure is dictated by the temperature of the cooling medium to which the heat is being rejected to, there is a minimum pressure required to properly feed the refrigerant through the metering device and into the evaporator. The correct answer to the question is 46 BTU/LB.

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    JHC, you guys make me think!!!!!!!!!!!!!!!

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    Me too, and it gives me a head ach. (:-
    I haven't had to think for years.
    Quote Originally Posted by TechmanTerry View Post
    JHC, you guys make me think!!!!!!!!!!!!!!!

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    Quote Originally Posted by valdelocc View Post
    Condensing pressure is dictated by the temperature of the cooling medium to which the heat is being rejected to, there is a minimum pressure required to properly feed the refrigerant through the metering device and into the evaporator. The correct answer to the question is 46 BTU/LB.
    Gotta show your work to get credit!


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    first i forgot the specific heat of r-12
    second i forgot how to use a pressure/enthalpy chart

    work on those and you'll have your answer
    my boss thinks its possible to repeal the laws of physics

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    Quote Originally Posted by rcstl View Post
    Gotta show your work to get credit!
    Just simple subtraction between vapor and liquid enthalpy at given temperatures.

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    Quote Originally Posted by nutradesman View Post


    Is this how engineers decide which compressor to use for a system?
    In a nutshell,compressor selection is based among other factors on refrigerant mass flow and yes enthalpy is the driving factor, saturation temps are use in the calculations because most of the refrigerant delta enthalpy comes from latent change of state and sensible heat brings next to nothing in terms of BTU/LB.

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    The condensing pressure should be such that the condensing temperature should be above the current ambient temperature in order to be able to reject the heat collected at the evap and the heat of compression. By how much depends on the design of the system.

    As far as an exercise in reading pressure enthalpy charts it should not really matter what refrigerant is used - however I found it hard to find a pressure enthalpy chart for r12 in imperial units in any of my books or online. Most of the ones I saw online were for SI units (KJ/Kg). I finally found a hard to read one in the RSES SAM. The exact value will depends on your entry point into the saturation curve which is dictated by the amount of flash gas once the refrigerant exists the expansion device. very crudely the amount I get is about 55 Btu/lb (because the chart I have is really hard to read) based on a 70% liquid/30% gas at the input to the evap.

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    The exact values at saturation are 80 btu/lb vapor at 20F and 33.91 btu/lb liquid at 110F, the difference is 46.09 btu/lb and thats also called the NRE, there isn't really any gray areas.

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    Don't the percentage lines under the saturation curve dictate the heat capacity of the refrigerant? If you move horizontally while under the saturation curve in the pressure enthalpy chart you will see a chance in Btu/lb content based on percentage of vapor/liquid. Or am I missing something?

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    Don't look to deep into the question. It says if you have only vapor leaving evap (no sh) how much heat do yo need to remove from the refrigerant to make it liquid(no sc) leaving the condenser at temp of 110. Which would be the latent heat of R-12 per pound.
    There is only one truly right way to do something, but there are thousands of wrong ways to varying degrees to do it.
    So the question is: If you don't do it right, then how wrong is it going to be???

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    Quote Originally Posted by Robber View Post
    Don't look to deep into the question. It says if you have only vapor leaving evap (no sh) how much heat do yo need to remove from the refrigerant to make it liquid(no sc) leaving the condenser at temp of 110. Which would be the latent heat of R-12 per pound.
    You are correct I just re-read the question - if you look at it that way then there is only one answer.

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    The answer to your question is quite simple. The temperature of the condenser needs to be higher than the air [or whatever] that it is rejecting the heat to.

    And the temperature of the evap needs to be colder than the air [or whatever] it is absorbing the heat from.


    Quote Originally Posted by nutradesman View Post
    Had a guy a few periods under me ask a question from school which made me think. Question is: In a system using CFC-12, evaporator temperature is 20F ; condensing temperature is 110F. If refrigerant leaves the evaporator as a saturated vapor, and leaves the condenser as a saturated liquid, the condenser will remove _______ BTU from each pound of refrigerant.

    Now I understand the main point of a refrigeration system is to boil off refrigerant at a specific temperature in the evaporator (at whichever temperature is useful). This is done by pressurizing the substance so it boils at say 40F (at atmospheric pressure, most refrigerants have very low boiling temperatures). And as the gas boils, it picks up a certain amount of heat (enthalpy).

    My question: is the condensing pressure dictated by the required enthalpy of the refrigerant so that we can get rid of the heat picked up in the evaporator and the heat of compression?

    My answer: 85.12 BTU per pound

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