게시판 > 질답게시판 > 열교환기 설계 관련 문의

스테파노 Stefano

08-07-09 15:26

Q1. <본 Rating과 같이 U값이 상승(500->700)하는 것이 맞습니까? >
A1. 열교환기 설계시 계산값(500)보다 설계값(Rated Value 700)으로 했기 때문에 여유를 둔 것입니다.
이 열교환기의 경우 약 40% 정도의 여유치를 가지고 있습니다.
아마 Condenser의 경우 미응축가스나 불활성가스가 함유되면 응축능력이 크게 떨어지기 때문에 이를 감안해서 전열면적에 여유를 더욱 크게 두는 경우가 많습니다. 미응축가스를 고려한 계산치인지 확인해 보도록 하세요.

Q2. <U값의 상승은 Tube측 Velocity가 주요인입니까?( 1.07m/sec -> 1.94m/sec) >
A2. 열교환기의 U값은 Gas Side, Liquid Side, Condensing Side 순서로 영향을 크게 받습니다.
이 열교환기 Tube Side의 영향은 대체로 Reynolds No 값의 약 0.8승에 비례합니다.
응축기에 Noncondensables가 거의 없다면 Tube Side의 유량이 열교환 성능을 좌우합니다 .

Q3. <상기와 같이 U값이 상승한다면, 열교환기의 효율이 더욱더 좋다는 얘기인데, 애초에 U값을 높여서 설계하는 것이 더욱더 바람직한 것이 맞지 않습니까? >
A3. 반대로 생각하고 계십니다. 실제사용해야 할 U값보다 더 큰값의 U가 나오도록 설계해야 사용도중 오염으로 인하여 U값이 떨어져도 지탱할 수가 있습니다. 사용중 U값이 너무 떨어지면 Tube나 Shelll을 Cleaning 해야 하지요.
따라서 실제 계산값보다 반드시 더 큰 U값이 나오도록 해야 합니다 .

Q4. <3항에 대해서 U값의 상승선에 대한 기준이 있습니까? (적정 U값 선정, 적정 유속에 따른 Vibration 방지선, Fouling 등) >
A4. U값을 얼마만큼 더 크게 선정해야 하는 기준은 설계자가 판단해야 할 사항입니다. 대개 전열면적을 더 늘여 잡거나 유량 여유를 둡니다. 보통 U값 혹은 전열면적에서 10~15% 여유를 갖도록 하지만 유량변화가 큰 Reflux Condenser의 경우에는 Max Duty기준으로 하고 추가여유를 전열면적 10% 여유로 하는 것이 보통입니다.

Noncondensable의 유입이 특히 많은 Vacuum Condenser와 같은 경우 실제 U값이 크게 떨어지는 경우가 많으므로 개략계산치보다 아주 큰 여유들 두는 경우가 많습니다.

Tube의 유속이 과다하면 Erosion이 커지고 압력손실이 증가하기 때문에 Tube 내 유속은 대체로 0.5~2 m/s 범위에 둡니다. 부식성이 큰 물질의 경우 최대유속을 더욱 낮게 취합니다. Tube내 유속인 너무 낮은 경우 Laminar Flow로 흐름으로써 우선 U값이 떨어지고 Fouling이 증가하는 경우가 많기 때문에 최저 유속 이상을 유지하도록 합니다 .

Subcooling 되어 문제가 된다면 아래의 해결방법중의 하나를 택하도록 하세요.

<해결방안>
(1) C\W의 유량을 줄임 이때 최대 CW의 온도는 CW수질에 따라 달라지겠으나 일반적인 경우 45도를 넘지 않도록 합니다 .
(2) Bypass Line을 약간 열어줌 .. 위의 경우 CW온도가 45도를 초과하는 경우

(3) Inert Gas (Air 혹은 N2)를 약간 주입한 상태로 운전. Inert Gas 약간 포함되면 U값이 크게 떨어짐.

(4) Reflux Condenser의 경우라면 Subcooling 되어 Reflux 되면 Reflux 효과를 늘여주는 효과를 이미 가지고 있을 것이므로 Distillate의 성분변화여부를 관찰하면서 Reflux Ratio를 줄여서 운전.

(5) 그래도 문제가 있다면 CW Coil의 일부를 마개로 막아둠.

Q2. <U값의 상승은 Tube측 Velocity가 주요인입니까?( 1.07m/sec -> 1.94m/sec) > 
A2. 열교환기의 U값은 Gas Side, Liquid Side, Condensing Side 순서로 영향을 크게 받습니다.  
이 열교환기 Tube Side의  영향은 대체로 Reynolds No 값의 약 0.8승에 비례합니다.    
응축기에  Noncondensables가 거의 없다면 Tube Side의 유량이 열교환 성능을 좌우합니다 .

Q3. <상기와 같이 U값이 상승한다면, 열교환기의 효율이 더욱더 좋다는 얘기인데, 애초에 U값을 높여서 설계하는 것이 더욱더 바람직한 것이 맞지 않습니까? > 
A3.  반대로 생각하고 계십니다.  실제사용해야 할 U값보다 더 큰값의 U가 나오도록 설계해야 사용도중 오염으로 인하여 U값이 떨어져도 지탱할 수가 있습니다.  사용중 U값이 너무 떨어지면 Tube나 Shelll을 Cleaning 해야 하지요. 
따라서  실제 계산값보다 반드시 더 큰 U값이 나오도록 해야 합니다 .

Q4. <3항에 대해서 U값의 상승선에 대한 기준이 있습니까? (적정 U값 선정, 적정 유속에 따른 Vibration 방지선, Fouling 등) > 
A4.  U값을 얼마만큼 더 크게 선정해야 하는 기준은 설계자가 판단해야 할 사항입니다.  대개 전열면적을 더 늘여 잡거나 유량 여유를 둡니다.  보통 U값 혹은 전열면적에서 10~15% 여유를 갖도록 하지만  유량변화가 큰 Reflux Condenser의 경우에는 Max Duty기준으로 하고 추가여유를 전열면적 10% 여유로 하는 것이 보통입니다.

Noncondensable의 유입이 특히 많은 Vacuum Condenser와 같은 경우 실제 U값이 크게 떨어지는 경우가 많으므로 개략계산치보다 아주 큰 여유들 두는 경우가  많습니다.

Tube의 유속이 과다하면 Erosion이 커지고 압력손실이 증가하기 때문에 Tube 내 유속은 대체로 0.5~2 m/s 범위에 둡니다.  부식성이 큰 물질의 경우 최대유속을 더욱 낮게 취합니다.  Tube내 유속인 너무 낮은 경우 Laminar Flow로 흐름으로써 우선 U값이 떨어지고 Fouling이 증가하는 경우가 많기 때문에 최저 유속 이상을 유지하도록 합니다 .

Subcooling 되어 문제가 된다면 아래의 해결방법중의 하나를 택하도록 하세요.

<해결방안>
(1) C\W의 유량을 줄임 이때 최대 CW의 온도는 CW수질에 따라 달라지겠으나  일반적인 경우 45도를 넘지 않도록 합니다 . 
(2) Bypass Line을 약간 열어줌 .. 위의 경우 CW온도가 45도를 초과하는 경우

(3) Inert Gas (Air 혹은 N2)를 약간 주입한 상태로 운전.   Inert Gas 약간 포함되면 U값이 크게 떨어짐.

(4) Reflux Condenser의 경우라면 Subcooling 되어 Reflux 되면 Reflux 효과를 늘여주는 효과를 이미 가지고 있을 것이므로 Distillate의 성분변화여부를 관찰하면서 Reflux Ratio를 줄여서 운전.

(5) 그래도 문제가 있다면 CW Coil의 일부를 마개로 막아둠.

romanus soongu99

08-07-10 09:59

우선 답변 주셔서 고맙습니다. 스테파노님.

말씀하신대로 본 E/X는 O/H Total Reflux Condenser입니다.
Optimization하면서, (무슨 열교환기를 거의 2배나 Oversizing해서 설계했는지?) 싶었습니다.

말씀하신대로 미응축가스를 고려한 계산치인지 다시한번 확인해 보겠습니다.

혹 본 E/X Design자료가 부실하다면, 타 O/H Condenser의 Design을 Rating 해보겠습니다.

고맙습니다.

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