tube exchanger

[mir]

hi guys... every now and then I have to go over my thermodynamics and then come back:

I have to size the length of a tube exchanger where:

1. power = 150 kw (the exchanger is dimensioned on the maintenance of temperature) = 540000000 [J/h ]2. liquid1 to be heated: m1=27 mc/h from t0° to 55°
3. liquid2 heating: m2=? mc/h from 75° to 69°
4. liquid1 and liquid2 are equal and are water (to simplify the accounts)
5. exchanger with w=100 [W/m2 K]use the classic formula: p = m cp dt and I find t0

T0 = 55°c- 540000000 [J/h ] / (27 * 1000 [kg/h] * 4186 [J/Kg K]) = 50.24 °C

calculates now the flow m2 = 540000000 [J/h ] / (4186 [J/Kg K] * (75-69) = 21500 kg/h = 21.5 mc/h

area exchanger = pot / (w * dtl)

where dtl is the logarithmic average for countercurrent flows= (69- 50.24) - (75-55) / ln (69-50.4)/(75-55)) = 19.37

area exchanger = 150000 [W] / (100 [W/m2 K] * 19.37 [K]) = 77.44 sqm

Does it seem consistent as I wrote?

thanks for the help
♪

 

[paulpaul]

I didn't check all the numbers but it seems to me that reasoning is consistent. of the remainder of the preservation of energy and the average logarithmic delta are the two cornerstones of the heat calculation of the exchangers: from here you start to consider everything else.
If I can give you advice, instead of converting kw to j/h (!) convert the kg/h to kg/s: it is coherent and more intuitive units of the j/h...
and write m2 (as you did in the global coefficient of exchange) and not square meters...

 

[mir]

strict but just
Thank you.

 

[mir]

hmmm I have a doubt

area exchanger = 150000 [W] / (100 [W/m2 K] * 19.37 [K]) = 77.44 sqm

or

area exchanger = 150000 [W] / (100 [W/m2 K] * (19.37+273.15) [K]= 5.13 mq

 

[paulpaul]

hmmm I have a doubt

area exchanger = 150000 [W] / (100 [W/m2 K] * 19.37 [K]) = 77.44 sqm

or

area exchanger = 150000 [W] / (100 [W/m2 K] * (19.37+273.15) [K]= 5.13 mq

if you have temperature differences (as in the average logarithmic dt) it is indifferent to use i °c or k, since the additive constant 273,15 is eliminated, since it is never to be added to a temperature difference but to its own temperature. so the first formula is right, the second has no physical meaning!
absolute temperature (and pressure) should be used in thermodynamic calculations where these quantities do not enter as differences: For example, in the equation of state pv = mrzt both p and t must be absolute.

 

[mir]

I agree with you but I have to make a correction to what I wrote:

area exchanger = 150000 [W] / (100 [W/m2 K] * 19.37 [°c] = 77.44 sqm

so they wouldn't return me the units of measurement...

 

[bip]

the temperature difference is always 19.37, whether you indicate it in centigrade degrees or in kelvin, here is that the units of measurement return perfectly.


area exchanger = 150000 [W] / (100 [W/m2 K] * 19.37 [K]) = 77.44 m2

ps: I did not verify the calculations