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  1. #27
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    Good ansers all

    The answer that I was looking for was given directly by
    go_redskin. Kars and Bama also said it.

    The only purpose of the compressor is to raise the temperature of the refrigerant. PERIOD
    It accomplishes this by increasing pressure ( pressure increases so does temp )

    A gas fired unit ( or absortion system ) uses gas to heat the refrigerant ( ammonia ), which of course also increases the pressure. These systems don't have a compressor.

    Thanks for all replys,
    Richard

  2. #28
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    It is true that you can get a refrigeration effect by other methods like absorption or peltier effect, but in a mechanical refrigeration system (which the compressor is the heart of), the role of the compressor is much more than merely to raise the temperature of the refrigerant; to say that the ONLY function of the compressor is:
    "raise the temperature of the refrigerant. PERIOD
    It accomplishes this by increasing pressure ( pressure increases so does temp )" is over simplifying the role of the compressor.

    The compressor is also designed to move a certain pounds per minute of vapor. The circulation of refrigerant created by the pressure difference set up by the compressor is what drives heat transfer in the system.

    The compressor suction pressure establishes the boiling point of the refrigerant in the evaporator. At 40 degrees, the vapor pressure of R-22 is 83.2 PSIA. The compressor must be able to move the required pounds of refrigerant while maintaining the required PSIA in the evaporator (as well as condenser). The fact that there are different compression ratios for compressors operating in differant temperature ranges proves this.

    A high temp compressor might have a compression ratio of 4:1, medium-temp 7.5:1, low-temp 10:1. They are all operating at basically the same condensing temperature, but the evaporator temps are much different.


  3. #29
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    On call

    What you say is true, but...
    The points that you bring up are due to the fact that there IS a compressor in the system. Not that it is needed to provide refrigeration.
    Because the compressor pumps, then the amount pumped must be considered ( but only because it raises the temp by pumping up the press. ) As you would have to increase size of burners, etc for gas-fired if you increase size of system.
    And I respectfully disagree with yur statement that the compressor sets the suction pressure, it does not. The suction pressure is simply a result of how much vapor is in the system. Vapor is what creates the pressure. The amount of heat absorbed dictates the amount of vapor which dictates the suction pressure. The compressor does not maintain the suction pressure. The compressor's ratio is computed based on what suction pressure is expected ( by what refrigerant used and the BTU's absorbed )and how much that pressure has to be 'pumped up to'. to obtain the needed temperature for condensing.
    Remember, liquid cannot create pressure, only vapor can. The suction pressure is created , inside the evap coil, by the amount of vapor produced by the liquid boiling as the heat is absorbed. If not much heat is absorbed, less vapor & less pressure. If more heat is absorbed, more vapor & more pressure. The compressor just pumps up what the evap sends it.

    Respectfully,
    Richard

    By the way, I am in south Bama too.
    Who was yur Tech school instructor??

    [Edited by bornriding on 08-06-2005 at 02:11 PM]

  4. #30
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    I guess we can go around in circles (no pun here) about what is the role of a compressor. The fact is that a mechanical refrigeration system is a closed loop system. I understand that the temperature (pressure) increase has an effect on the points that I made. However, I still believe that to say that the SOLE purpose of the compressor is to raise vapor temperature for condensing (although it is probably the most important function).

    For me to agree with you on this point is to throw all my RSES manuals out the window. I will agree that we disagree on this.

    I looked at your profile after posting. It looks like we have the same stomping ground. Most of my formal HVAC technical training is through RSES and other correspondence courses. Been to Alabama Power's classes more times than I care to remember.



    [Edited by on call on 08-06-2005 at 02:49 PM]

  5. #31
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    It just pumps vapor.


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  6. #32
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    i am going to go back and read up on this when i get home. just stopped at the office for a minute and have a few calls left. it has been my understanding that the compressor has two fucntions to compress gas from alower presure to a higher. it also moves the refrigerant through the system. you get a pressure drop at the evap coil through the metering devive with alows the liquid to absorb the heat easer and super heat in to a vapor then the compresor compresses the vapor to a higher presure so when it enters the condenser coil absorbs the heat and it turns to liquid. the added heat from the compressor is the heat caused by compresion. as far as i can remamber this added heat from compresion is a reaction and not intended
    as long as the air across the condenser coil is at least 1 degree lower it will absorb the heat from the refrigerant and thus condense to a liquid because you have increased the presure. little crude but in a hurry. i will read up on this and come back
    thanks for giving me something to realy think about and go back and read up on. amazing how what we learned years ago will be inturpited diffenant over the years

  7. #33
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    The Crosley Icy ball

    is another example of a system that didnt use a compressor

    http://www.refresearch.com/m-icyball.html

    OPERATING in the museum is a Crosley Icy Ball, manufactured between 1928 and 1938.
    Powell Crosley Jr. was one of the most innovative people of his century. There was no rural electrification until after World War II. Powell Crosley Jr. manufactured this absorption type unit so people could have refrigeration in areas where there was no electricity. By regenerating this system over a kerosene burner the cabinet would be cooled to 43 or less and ice cubes could be made for a period of about 36 hours depending upon room temperature.


  8. #34
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    For all those that think there is more than one reason for the compressor - more than raising temp - then please explain how a gas-fired system can operate without one?
    What moves the refrigerant in an absorption system???

    Thank YOu CT, that was very interesting. Would love a closer look at the ball.

    Thia leads me to one more quiz type question, which I will ask on a different thread.

  9. #35
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    Pressure differencial.

    Created by hydrogen, on atleast some continous cycle absorbers.

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  10. #36
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    OK, there are many forms of refrigeration. 1 ton is defined as 2,000 pounds of ice melting over 24 hours. The latent change of ice to water is where we get our 12,000 BTUH/ton standard. Ice was used to cool ice boxes in the early 1800s. That IS a form of refrigeration.

    By the 1900s there were 3 pratical refrigeration systems: adsorption, vapor compression, and dense air system.

    The operation of the adsorption refrigeration relies on different boiling points and vapor pressures of various solutions such as ammonia and water and use heat to boil off part of the solution.

    The Icy Ball did provide refrigeration, but how?

    It was shaped like a bent dumbell. One ball was placed over a kerosene heater, the other in the cabinet to be cooled. Heat was applied to the outside ball which evaporated the liquid in the dumball. The heat was turned off, and heat was extracted from the inside cabinet to drive the refrigerant back to the outside ball. This was refrigeration. Heck any liquid can be used as a refrigerant if it absorbs heat at a low temperature and pressure and releases it at a higher temperature and pressure, including water.

    What we are talking here is mechanical refrigeration utilizing the vapor compression system. Yes, refrigeration like the other two systems, but the system for controlling the flow of refrigerant is different. Just because those systems didn't have a compressor doesn't prove anything. Those systems didn't have a compressor because they aren't using the mechanical vapor compression method.

    As I have previously stated:

    1. The compressor raises the pressure of the vapor for condensing in the condensor
    2. It provides for the flow of refrigerant through the vapor compression process
    3. Regulates pressure in the evaporator by the compressor suction pressure.

    So to post what you said about suction pressure:

    "The suction pressure is simply a result of how much vapor is in the system. Vapor is what creates the pressure. The amount of heat absorbed dictates the amount of vapor which dictates the suction pressure. The compressor does not maintain the suction pressure. The compressor's ratio is computed based on what suction pressure is expected ( by what refrigerant used and the BTU's absorbed )and how much that pressure has to be 'pumped up to'. to obtain the needed temperature for condensing.
    Remember, liquid cannot create pressure, only vapor can. The suction pressure is created , inside the evap coil, by the amount of vapor produced by the liquid boiling as the heat is absorbed. If not much heat is absorbed, less vapor & less pressure. If more heat is absorbed, more vapor & more pressure. The compressor just pumps up what the evap sends it".

    I partially agree with this statement, but we have to remember that we are talking about saturated pressures here. The pressure will not be affected by the amount of heat as long as the mixture is saturated. Less heat does mean less vapor. But at saturation, the pressure WILL NOT CHANGE. Saturation pressure is the same for a refrigerant with 1% vapor 99% liquid, as it is for 99% vapor and 1% liquid; because of this fact we can determine superheat and subooling using the saturated pressure of the refrigerant. If the temperature changed for saturated pressures, we could not use the subcooling/superheat calculations we do now.

    The compressor suction determines the flash temperature (pressure) in the evaporator. Want a lower temperature with R-22? Lower the pressure that the liquid is boiling at and you get a lower temperature. Instead of 40 deg (68 PSI) go down to 22 deg (45 PSI).

    I'm tired, and my head hurts. This is definately the most I've ever posted at one time. There are probably 10 new posts by the time I post this. Good stuff, though.







    [Edited by on call on 08-07-2005 at 02:30 PM]

  11. #37
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    ct thank you that was very interesting. wish there was more about it.
    oncall i wish i could but it in writing as good as you
    since the presure and temp are related as the compresor pulls in the low presure vapor and compreses it into a higher presure and pushes it in to the condenser it does increase the temp but this is a reation from compresion. i still cant anything that says raising the temp is the sole purpose of the compressor. i have read in sevaral books
    the compresor pulls in low presure superheated gas and compreses it in to high presure supeaheat gas as it pushes it into the condenser.

  12. #38
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    The purpose of the compressor is NOT to raise the temperature of the refrigerant. It happens to be a thermodynamic byproduct of raising pressure. If you look at a geothermal system with a desuperheater, the hot refrigerant is cooled by a heat exchanger before the refrigerant goes into the condenser, and the process works the same.

    paul

  13. #39
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    Originally posted by bornriding
    What is the 'one' purpose of having a compressor in a refrigeration system??? In other words, what is the compressors designed function.

    I used to ask my class this question, on a test, after taking the basic refrig. course.

    Richard
    To create a differential in pressure.

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