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Thread: home work ?

  1. #1
    Join Date
    Jul 2005
    Posts
    1,035

    Confused

    home work ?

    School starts next Monday..so cant ask the instructor before then.

    I'm on a section on the refrigeration process.
    The book says that the refrigerant is 100 F high pressure on the condenser side of the metering device. IT passes through the metering device and becomes low pressure and 40 F. passes through the evaporator and picks up heat....and so on.

    MY ????? is what makes it drop 60 F just from passing through the metering device? Is it the latent heat of vaporization? The flash gas absorbs the heat? or just going from high pressure to low pressure drops the temperature?

    I am so confused.
    73% of Americans say that illegal immigration is a problem. The other 27% say, "No habla inglis!"

  2. #2
    Boyles law baby.

    http://www.grc.nasa.gov/WWW/K-12/airplane/aboyle.html

    Actually that is a crappy site, but enough you should get the idea.

    Viva la A/C!!! It's over 90 out there.

    [Edited by dave and julie on 08-09-2005 at 03:23 PM]

  3. #3
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,292
    Originally posted by doglips
    home work ?

    School starts next Monday..so cant ask the instructor before then.

    I'm on a section on the refrigeration process.
    The book says that the refrigerant is 100 F high pressure on the condenser side of the metering device. IT passes through the metering device and becomes low pressure and 40 F. passes through the evaporator and picks up heat....and so on.

    MY ????? is what makes it drop 60 F just from passing through the metering device? Is it the latent heat of vaporization? The flash gas absorbs the heat? or just going from high pressure to low pressure drops the temperature?

    I am so confused.
    Think of air in a balloon. It is at a pressure higher than the atmospheric air pressure surrounding the balloon. When you release the neck of the balloon to allow the trapped air to escape, it rushes out into the room (with companion fart sound). Once this air reaches the room, it expands due to more room for these air molecules to move around. The molecules that were in the balloon at a higher pressure will now move toward the pressure existing in the room. (Technically, the air pressure in the room will rise slightly as the higher pressure from the balloon enters the room, but it's negligible.)

    Now let's go to the metering device. You may already know that temperature and pressure are relative. Water boils at 212 degrees at sea level but at lower temperatures at higher altitudes. The liquid refrigerant upstream of the metering device is at a high pressure/high temperature (as compared to the evaporator). Once it passes through the metering device, the liquid refrigerant expands and drops in pressure. This sudden drop in pressure drops the temperature of the refrigerant to the point where it begins to boil and become a vapor, absorbing heat from the coil as it vaporizes.

    There's an involved thread up in the Residential section of this site that discusses the cause of suction pressure at length (started by member bornriding, if you're interested in reading the thread). One of the contributions to suction pressure, along with the compressor, is the difference in tube diameter between the suction and liquid sides of the system. Another is the metering device itself, imposing a restriction upon the liquid side, forming a column of liquid in the liquid line, and maintaining a condensing temperature above outdoor ambient temperature.

    And of course there's the compressor, which imposes another pressure differential from suction to discharge side.

    [Edited by shophound on 08-09-2005 at 05:25 PM]
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  4. #4
    Join Date
    Jul 2005
    Posts
    1,035

    Thumbs up

    thansk all...makes sense now.

    [Edited by doglips on 08-09-2005 at 05:36 PM]
    73% of Americans say that illegal immigration is a problem. The other 27% say, "No habla inglis!"

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