air con super heat calculations

Discussion in 'All Things Boats & Boating' started by Frosty, May 22, 2012.

  1. Frosty

    Frosty Previous Member

    I have a -5 degree C on the low pressure line and 22 degrees gas temp taken near the pressure reading.

    That is 20 subtract -5

    How do you subtract -5 from 20? Some people say its 15 some say its 25.
     
  2. Stumble
    Joined: Oct 2008
    Posts: 1,896
    Likes: 71, Points: 48, Legacy Rep: 739
    Location: New Orleans

    Stumble Senior Member

    Subtracting a negative means add the integers.
     
  3. Frosty

    Frosty Previous Member

    Can you elaborate on that. I assume you are falling on the 25 side.

    20 - 5 is 15 --how can 20 subtract -5 be more than 20?

    I think Ive just cured the Greek debt crisis.
     
  4. Stumble
    Joined: Oct 2008
    Posts: 1,896
    Likes: 71, Points: 48, Legacy Rep: 739
    Location: New Orleans

    Stumble Senior Member

    Lol. Subtraction can be written as (+6)-(+3)=3. When subtracting a negative.
    (+6)-(-3)=
    Two negatives equals a positive so...
    (+6)+3=9

    For you the answer is 25.

    It may be easier to think of it as the difference between +20 degrees on one end and -5 degrees on the other. The change in temprature is 25 degrees.
     
  5. Frosty

    Frosty Previous Member


    Well yes the difference is 25 but that is the range or difference. I need to subtract -5 from 20. How the hell can it be more.:confused:

    If I have 20 dollars credit and 5 dollars debit I have 15 dollars. The difference in my credit and debit is 25 but I dont have 25 I have 15

    I just dont get this,---yes ok I must be a bit thick but I still dont get it.
     
  6. Stumble
    Joined: Oct 2008
    Posts: 1,896
    Likes: 71, Points: 48, Legacy Rep: 739
    Location: New Orleans

    Stumble Senior Member

    Another way of saying subtract is 'difference'. So in this case what is the difference between 20 and -5.
     
  7. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 148, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    Yes, it isn't easy Frosty.
    Why don't you add 273 to these values, that gives you absolute temperature (Kelvin) which is always positive.
     
  8. Frosty

    Frosty Previous Member

    Whats 273+ -5 =268
     
  9. CatBuilder

    CatBuilder Previous Member

    Yes. That's correct.
     
  10. philSweet
    Joined: May 2008
    Posts: 2,459
    Likes: 268, Points: 83, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    They want the temp of the gas inside the pipe, not room temp, Frosty.
     
  11. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 252, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    That's how Greek got into debt crisis. :p
     

  12. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 252, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Hey Frosty, I think that a visual example might help you understand how it works. Observe the length of lines in three subtractions shown:

    Additions.gif
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.