Thermodynamics - Steady Flow Devices - Heat Exchanger

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[@quincy3718]

For Refrigerant-134a - Outlet enthalpy h2= 100.865 kj/kg (interpolate) - Inlet enthalpy h1= 303.85 kj/kg (it actually superheated) so you go to supaheated table and read h @ P=1MPa and T=70°c He forgot to mention in the video, maybe assuming we already know how to get these values.

[@emergencysocks]

Hello, thank you for clarifying. I now see where I went wrong. However I just wanted to ask, if you can reply, what values does he use to interpolate for h, with a known temperature of 35ºC? The only two values he could use from the superheated tables are 40ºC and 39.37ºC which is T saturated, and T = 35ºC does not fall within those two values. At T = 35ºC, it is not superheated. Do I go back to the saturated tables instead, or am I doing something wrong?

[@alvinlin8140]

Why didn’t he use pressure to find hf instead? And why is h1 and h2 equal to hf in the first place? Thanks

[@alvinlin8140]

And how did you know h1 was superheated?

[@quincy3718]

@@alvinlin8140 He mentioned it in the video, it a heat exchanger.. it a pipe with water flowing on top of it to COOL IT DOWN, so logically you only cool something that is hot (in this case SUPERHEATED) also compressed liquid usually means sub-cooled liquid (COLD) So if the refrigerant was initially cold there’s no need for water to cool it down, hence why it SUPERHEATED at inlet.. hope I’m making sense.

[@alvinlin8140]

@@quincy3718oh I see so, also why didn’t he use pressure to find hf instead at the beginning? And why was he using hf as h1? (And thus for h2 also since h1=h2). I’m really stuck on that part thanks!

[@ColonialDagger]

For anyone wondering how he got 29.1 kg/min in his flow rate calculation at 10:00, he accidentally swapped the outlet enthalpies around. The correct equation should be: (6 kg/min)(303.87 kJ/kg) + ṁ(62.982 kJ/kg) = (6 kg/min)(100.88 kJ/kg) + ṁ(104.83 kJ/kg) The resulting answer is ṁ = 29.1 kg/min. If you use the incorrect equation shown in the video, you'll get ṁ = 31.5 kg/min, which is wrong.

[@BloolpyGaming]

R-134a is enters as superheated vapor and leaves as a compressed liquid. The numbers are correct though. Thank you!

[@duakhan5882]

Yes, that is correct

[@duffyduck1373]

how do we know that it leaves as a compressed liquid?

[@emreyuksel8517]

@@duffyduck1373 Because at Table A-11 at 36 or 34 degree we have less pressures than 1 MPa it indicates that we have a compress liquid form of ref-134a

[@raymondtrann]

@@Ko0okieeZ We are given the P and T of R134A at the inlet. Look up these values on the saturated tables A-11 and/or A-12, and we can see that the state is super heated.

[@bazen9627]

I just want to pass this course and never think about that again inşallah

[@engineeringdeciphered]

🙌 good luck!!!

[@LusandaThabethe]

Did you pass?😅

[@sunmichoi6888]

Thank you so much!!!!!!!!!

[@abdulkadermechhawi635]

Hello. please i want to know how did you estimate the enthalpy and if i use another values our calculation should be true ?

[@aliozaltn5928]

which tables did you look at for refrigerants because in A5 its different than what you told

[@engineeringdeciphered]

refrigerants are tables A-11 through A-13

[@Sumee21051987]

@@engineeringdeciphered Your hf for water @ 15 ° C and 25 ° C are accurate from Table A-4. But in Table A-11, @ 70° C, hf is 156.15 kJ/kg.

[@Jtruongg]

@@Sumee21051987 Yea i think he made a mistake, the ref enters as a superheated and you get that from superheated tables, then to get the enthalpy out, you have you interpolate from the sat temp table of ref 134 between 34 and 36 degrees

[@amzar_khan]

how to know whether to use pressure table or temp table for hf 35deg??

[@samkeloinnocent8316]

Hello bro can you help me understand what you did here isn't the case that the refrigerant at 70 degrees celcius is a superheated because its pressure is less than the saturation pressure and therefore we will use hg, from table A11 I am getting hg as 280.46kJ/kg where did you get 303.87 and why it?

[@engineeringdeciphered]

I think I misspoke in this video. I think you’re right, it’s superheated at the inlet. So you need to go to the superheated table A-13 to get the correct value to h at that temp and pressure. But I think 303.87 is the correct value. Does that work out?

[@samkeloinnocent8316]

@@engineeringdeciphered oh yes I see my mistake was taking hg from A11 which is the saturated refrigerant table forgetting that if it is superheated I need to use superheated table. Thank u so much for your response and for you videos I wish you to have many views, likes and subscribers you so helpful.

[@shaquilsebastian1363]

Where can I get the Tables?

[@engineeringdeciphered]

www.dropbox.com/s/tuqy5e8657ysoda/Property%20Tables%20-%20Appendices%201%20and%202.pdf?dl=0

[@shaquilsebastian1363]

@@engineeringdeciphered Thank you.