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  1. #1
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    converting a calculation to Si



    I have just read a post from a few years back and have noticed this
    "Most condensors operate at a close to estimated temperature of 30 degrees F over ambient.
    So if we know what the ambient is and add 30 degrees F to it we can safely estimate,

    Lets take a look at the suction pressure
    similiar as above excecpt normal operating sytems have an average coil temp of 15 degrees f
    below temp controlled area".

    But if i were to convert these temps to DegC then 30F would be -1.1DegC
    and 15f would be -9.44, surely this cant be right for si units?


    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: converting a calculation to Si

    You have two things when converting Fahrenheit to Si units.
    First is actual temperature which is converted in Celsius degree and second is temperature interval converted in Kelvin

    You need to distinguish in text what is what and corectly convert that.

    Fahrenheit temperature is converted in Celsius degree by subtracting 32 from Fahrenheit temperature and than multiplied by 5/9.
    Fahrenheit temperature interval is converted in Kelvin by multiplying Fahrenheit interval with only 5/9.

    In your example:
    30°F=30x5/9=16,6666666K
    15°F=15x5/9=8,33333333K

    For ambient temperature:

    40°F=(40-32)x5/9=8x5/9=4,44444444°C
    Last edited by nike123; 10-05-2009 at 09:51 PM.

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    Re: converting a calculation to Si

    ok so in si units, head pressure should be roughly ambient + what in DegC

    and also suction pressure should be space temp - what in DegC?

    Just as a rule of thumb.

    Ah ok just noticed your edit so the head pressure should be 16Deg C and the sp 8DegC?
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: converting a calculation to Si

    Quote Originally Posted by marc5180 View Post
    ok so in si units, head pressure should be roughly ambient + what in DegC

    and also suction pressure should be space temp - what in DegC?

    Just as a rule of thumb.

    Ah ok just noticed your edit so the head pressure should be 16Deg C and the sp 8DegC?

    Read again more carefully. It is once more edited.

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    Re: converting a calculation to Si

    If you have condenser air in temperature of 30°C than 30°F over ambient would be 46,66666°C
    If you have air in temperature at evaporator of -18°C than 15°F below ambient would be -26,3°C.

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    Re: converting a calculation to Si

    Now you've got me, where have you got 46.6 and 26.3 from? Im confues because you are using both °C and °F.
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: converting a calculation to Si

    Quote Originally Posted by marc5180 View Post
    Now you've got me, where have you got 46.6 and 26.3 from? Im confues because you are using both °C and °F.
    Quote Originally Posted by nike123 View Post
    You have two things when converting Fahrenheit to Si units.
    First is actual temperature which is converted in Celsius degree and second is temperature interval converted in Kelvin

    You need to distinguish in text what is what and corectly convert that.

    Fahrenheit temperature is converted in Celsius degree by subtracting 32 from Fahrenheit temperature and than multiplied by 5/9.
    Fahrenheit temperature interval is converted in Kelvin by multiplying Fahrenheit interval with only 5/9.

    In your example:
    30°F=30x5/9=16,6666666K
    15°F=15x5/9=8,33333333K


    For ambient temperature:

    40°F=(40-32)x5/9=8x5/9=4,44444444°C
    ..........................
    Last edited by nike123; 10-05-2009 at 09:51 PM.

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    Re: converting a calculation to Si

    Ok so for example if i had a condensor air in of 30°C then (40°F for example only) over ambient would be 52.22°C.
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: converting a calculation to Si

    I hoped there would be a simple way to check like the way the quote sounded

    Discharge pressure should = say 15°C plus the ambient
    Suction pressure should = say 10°C plus ambient (RA).
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: converting a calculation to Si

    Quote Originally Posted by marc5180 View Post
    Ok so for example if i had a condensor air in of 30°C then (40°F for example only) over ambient would be 52.22°C.
    Yes, you got it.

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    Re: converting a calculation to Si

    Quote Originally Posted by marc5180 View Post
    I hoped there would be a simple way to check like the way the quote sounded

    Discharge pressure should = say 15°C plus the ambient
    Suction pressure should = say 10°C plus ambient (RA).
    http://www.refrigeration-engineer.co...75&postcount=5

    All values here should be in Kelvins, but amount is same.
    Last edited by nike123; 10-05-2009 at 08:38 PM.

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    Re: converting a calculation to Si

    because temperature difference should be expessed in K?

    Thats just what i was after.
    Thanks Nike
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: converting a calculation to Si

    Quote Originally Posted by marc5180 View Post
    because temperature difference should be expessed in K?
    It is not mandatory:

    http://physics.nist.gov/cuu/Units/units.html

    Note on degree Celsius. The derived unit in Table 3 with the special name degree Celsius and special symbol °C deserves comment. Because of the way temperature scales used to be defined, it remains common practice to express a thermodynamic temperature, symbol T, in terms of its difference from the reference temperature T0 = 273.15 K, the ice point. This temperature difference is called a Celsius temperature, symbol t, and is defined by the quantity equation
    t= T- T0.
    The unit of Celsius temperature is the degree Celsius, symbol °C. The numerical value of a Celsius temperature t expressed in degrees Celsius is given by
    t/°C = T/K - 273.15.
    It follows from the definition of t that the degree Celsius is equal in magnitude to the kelvin, which in turn implies that the numerical value of a given temperature difference or temperature interval whose value is expressed in the unit degree Celsius (°C) is equal to the numerical value of the same difference or interval when its value is expressed in the unit kelvin (K). Thus, temperature differences or temperature intervals may be expressed in either the degree Celsius or the kelvin using the same numerical value. For example, the Celsius temperature difference t and the thermodynamic temperature difference T between the melting point of gallium and the triple point of water may be written as t = 29.7546 °C = T = 29.7546 K.

    But that is common practice:


    http://en.wikipedia.org/wiki/Kelvin#...nical_articles
    Mixed use of Kelvin and Celsius scales in technical articles

    In science and in engineering, the Celsius scale and the kelvin are often used simultaneously in the same article (e.g., “…its measured value was 0.01023 °C with an uncertainty of 70 µK…”). This practice is permissible because the degree Celsius is a special name for the kelvin for use in expressing Celsius temperatures and the magnitude of the degree Celsius is exactly equal to that of the kelvin.[5] Notwithstanding that the official endorsement provided by Resolution 3 of the 13th CGPM states, “a temperature interval may also be expressed in degrees Celsius,” the practice of simultaneously using both “°C” and “K” remains widespread throughout the scientific world as the use of SI prefixed forms of the degree Celsius (such as “µ°C” or “microdegrees Celsius”) to express a temperature interval has not been widely adopted.[6]
    Last edited by nike123; 10-05-2009 at 09:10 PM.

  14. #14
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    Re: converting a calculation to Si

    Unit conversions can be tricky if you do not follow some strict guidelines


    1. Is the temperature an an actual readin done with a thermometer, as in °F or °C.
    2. The temperature difference is expressed in the same units in the Farenheit scale or K in the Celsius scales.
    3. Always convert the number numbers to the same scale and then determine the difference. Do not use mixed units or assume the temperatures (or differences) mean the same thing.
    4. Always check your units! This is especially necessary when converting other things such as mass, volume, or length (etc).
    If all else fails, ask for help.


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    Re: converting a calculation to Si

    Quote Originally Posted by US Iceman View Post
    The temperature difference is expressed in the same units in the Farenheit scale or K in the Celsius scales.
    It was this that confused me. I know that with a temperature difference in °C then i always know to use °K but wasn't aware of °F.

    I do now though
    The difference between genius and stupidity is that genius has its limits.

    Marc

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