Taking temperatures

[marc5180]

Hi all, once again i've been reading some books and im confused by the whole different temperatures. I always measure superheat in DegC but i've read and seen many posts stating for example on one on one particular system that superheat should be 20DegF, which equates to -7DegC. I though superheat should be around the 5-10DegC mark which equates to 41-50DegF. Am i missing something. Also i see posts from people using K. Now i know that K is kelvin and what it is but i dont understand how they convert the degf to kelvin?
For example i shall post a line from a book that i am reading;
If the difference between the 2 temperatures is more than 30degF (16degK) dt, the condensor is not getting enough airflow.

[The Viking]

Ah, the lovely topic of backwards people VS the future..

(Imperial versus Metric)

You are both right and wrong my dear fellow.

20F does equal -6.67C but this is absolute temperature and this can not be applied to temperature difference.
Let me give you an example:
If you on the gauge read 28F then it roughly equal -2C
If you on the pipe then measure 48F (20F superheat as in the example you gave) that would equal about 8.8C....Or ~10C superheat.
Therefore, if you want to compare different scales, you have to do the translation BEFORE you enter them in to any calculations.

BTW. The difference between Kelvin and Celsius is that C=0 when water freezes and K=0 at the absolute zero, their "stepping" is the same. i.e. 1K difference = 1C difference and the "standard" is to measure in C but to give temperature differences in K...

[US Iceman]

Temperature differences should only be calculated after you have done the conversions. So, °F - °F = °F temperature difference. The same applies to subtracting °C, except the convention is to use °K to indicate temperature difference, instead of someone thinking the temperature is actually °C.

The actual superheat settings should be uniform per the application, if I remember it correctly. You have a couple of things to consider; evaporator superheat, which the evaporator manufacturer will have used for designing their coils (hint; too much superheat will loose evaporator capacity), and the minimum superheat the compressor manufacturer wants to ensure dry gas entering the compressor.

...but this is absolute temperature...

Kelvin and Rankine are the absolute temperature scales, but i know what you were trying to say.

Ah, the lovely topic of backwards people VS the future..

Oh crap, here we go again.

[marc5180]

Ok i understand it a little better i think but in my first post i gave the paragraph
"If the difference between the 2 temperatures is more than 30degF (16degK) dt, the condensor is not getting enough airflow."
then how does 16C equal 30F? 30F is -1.11C

[The Viking]

Ok i understand it a little better i think but in my first post i gave the paragraph
"If the difference between the 2 temperatures is more than 30degF (16degK) dt, the condensor is not getting enough airflow."
then how does 16C equal 30F? 30F is -1.11C

OK,
30F might equalise -1.11C but that's irrelevant.

What you must remember is that all conversions between different "scales" has do be done BEFORE any calculations are made. And the value 30F above is the difference between two measurements
(difference = a calculation has been made).

In the example you have given, you have to visualise what it is representing.....

Discharge temperature of 180F = ~82C
Liquid line temperature of 150F = ~65.6C

Imperial difference = 30F
Metric difference = 16.4C

And as we are talking temperature difference C=K



EDIT:
Sorry, just re-read the paragraph, the example should have been:

Ambient temperature of 70F = ~21C
Air discharge temp of 100F = ~37.8C

Imperial difference = 30F
Metric difference = 16.8C

[stuartwking]

Thanks for the enlightenment gents, Was scratching my head to start with but thanks for explaining it so well,. cheers,.

[SteinarN]

Ok i understand it a little better i think but in my first post i gave the paragraph
"If the difference between the 2 temperatures is more than 30degF (16degK) dt, the condensor is not getting enough airflow."
then how does 16C equal 30F? 30F is -1.11C

Lets say we have two different temperatures, some water at 8C and some water at 10C. We then have a difference of 2C. None of the temperatures is actually 2C, but the difference is 2C (10-8=2). OK?

If we take the same temperatures measured in Fahrenheit we get the low/high water at 46,4F and 50F. The difference is 3,6F (50-46,4=3,6). OK?

We se that 2C and 3,6F doesnt tell us how warm or cold the water is, it only tell us the difference of the warm and cold water. The "lenght" of 2C equals the "lenght" of 3,6F. If we lay the C scale together with the F scale we would see that each one C is as "long" as 1,8F. If we count ten C's those C's is as "long" as eighten F's.

We have to separate between difference and actual temperatures. If we count, lets say, ten C's we have a difference of 10C, but not nessecary an actual temperature of 10C. Maybe we was counting that ten C of difference from the number 100C (boiling temp of water) and up to 110C. Thats a difference of 10C.

[marc5180]

I've been trying to work it out as you described, it took a little bit of thinking through your examples helped. thanks for explaing so well (again)

[US Iceman]

Another way to look at this is take the temperature difference in °C and divide it by 1.8.

If I borrow the numbers used by Mr. Viking then we have:

Condensing temp of 100°F = (100-32)/1.8 = 37.7°C
Ambient temperature of 70°F = (70-32)/1.8 = 21.1°C


Imperial difference = 100-70 = 30°F
Metric difference = 37.7-21.1 = 16.6°C = 16.6°K

Or, if you take 30°F/1.8 = 16.6°C

Or, if you take 16.6°C X 1.8 = 29.9°C (which is close to 30°C because of a rounding of decimal points).

[marc5180]

Another way to look at this is take the temperature difference in °C and divide it by 1.8.

If I borrow the numbers used by Mr. Viking then we have:

Condensing temp of 100°F = (100-32)/1.8 = 37.7°C
Ambient temperature of 70°F = (70-32)/1.8 = 21.1°C


Imperial difference = 100-70 = 30°F
Metric difference = 37.7-21.1 = 16.6°C = 16.6°K

Or, if you take 30°F/1.8 = 16.6°C

Or, if you take 16.6°C X 1.8 = 29.9°C (which is close to 30°C because of a rounding of decimal points).

Clever Clogs

[US Iceman]

Sometimes it makes sense if you get lucky!

[Brian_UK]

I suppose you can ignore the different name for the temperature reading as long as it is the same type.

ie. If someone tells you that the superheat should be say 10° C or 20°F then you really don't need to know what the temperature is. You only need to subtract one temperature reading from another one to get the answer.

Assuming modern electronic temperature measuring devices all have a °C/°F button you should be able to get your result whichever language handbook you are using.

It's a bit like listening to the UK weather forecast, we always get temperature forecasts in °C and °F, but it's still going to be hot or cold whichever one you use. I can't mentally convert from °C to °F to see if the day is going to warm enough for me, I don't need to.

If it's going to be +/_ 10°C outside then I know that it won't be too bad - what is it in °F? I don't care - it won't make it any warmer or colder will it?

So, finally they say, don't make life too complicated for yourself. Keep the numbers simple and life will relax

[Josip]

Hi, Marc5180

Hi all, once again i've been reading some books and im confused by the whole different temperatures. I always measure superheat in DegC but i've read and seen many posts stating for example on one on one particular system that superheat should be 20DegF, which equates to -7DegC. I though superheat should be around the 5-10DegC mark which equates to 41-50DegF. Am i missing something. Also i see posts from people using K. Now i know that K is kelvin and what it is but i dont understand how they convert the degf to kelvin?
For example i shall post a line from a book that i am reading;
If the difference between the 2 temperatures is more than 30degF (16degK) dt, the condensor is not getting enough airflow.

All this can confuse a little, but it is not that difficult....superheat and subcooling we have to express by convention in Kelvin/Rankin i.e. absolute temperature scales without to mention degree, it is practical because those temperatures and values are always positive numbers

....there is no negative temperature in Kelvin or Rankin, calculating with negative numbers can confuse sometimes
... (0 K = 0 R = - 273,16*C = - 460*F exactly 459,67*F)

Kelvin scale has equal divisions as Celsius scale but with different starting point.... 1*C = 1K

Celsius scale (sometimes centigrade) means there are 100 degrees/divisions from water freezing point (0*C=273,16K=32*F) to water boiling point (100*C=373,16K=212*F) .... not to forget at sea level atmospheric pressure

Fahrenheit scale has equal division as Rankin but also with different starting points....1*F = 1R

The same scale expressed in Fahrenheit degrees has 180 degrees/divisions from water freezing point (32*F=492R=0*C=273,16K) to water boiling point (212*F=672R=100*C=373,16K) ....also at sea level atmospheric pressure...

So, having temperature in *F (minus 32) and divide it with 1.8 we have *C i.e. 1*F = 5/9*C...

having temperature in *C multiple it with 1.8 (plus 32) we have *F i.e. 1*C = 9/5*F

Hope this is of some help

Best regards, Josip