teikseng
10-06-2010, 09:31 AM
Hi, I'm very new in refrigeration industry. Just started learning everything about refrigeration since 1.5 months ago. I had picked up some knowledge in sizing the compressor and estimate refrigeration capacity. Currently i am going into designing a fin coil evaporator for my system. I'm trying to search through the website and some literatures that i have, however i still can't find a practical way of step-by-step to design an evaporator.
Here are the detail steps/flow that i concluded. Hopefully any expert can give me your inputs/comments:-
1. Identify process condition.- Air flow rate, Air in Temp, Air out Temp, pressure.
2. Identify type of refrigerant. E.g. R134a
3. Calculate cooling capacity:- Q = m [(h1-h2)-(W1-W2)hw2], m = air flow rate, hw2 = enthalpy of wet air at outlet
- h1, h2, W1, W2 , hw2 are taken from Psychometric chart for air at Patm.
4. Determine evaporator type -> fin tube evaporator, outline dimension and flow arrangement (parallel, counter, cross, mix)
- Fin tube is commonly used for cooling down air temperature. Similar to the one using in most of the car cooling coil.
5. Determine fins spacing -> max. 6 fins/inch for evaporator <0degC. And also determine size and # of pipe. Diameter of pipe based on type of refrigerant, cooling capacity and evaporator temperature.
- To prevent frosting, I’m going to start with minimum fins/inch.
6. Identify if it is compact heat exchanger -> Calculate total cooling surface/evaporator volume, m2/m3. If >700, it’s compact heat exchanger.
- If this is a compact heat exchanger, There will be a pressure drop .
7. Calculate LMTD -> LMTD = [(Th2 – Tc1)-(Th1-Tc2)]/ ln[(Th2-Tc1)/(Th1-Tc2)]
- Log mean temperature different (LMTD) calculation. Assuming Th is for air temperature and Tc is refrigerant temperature. Assuming there is no temperature change in refrigerant, therefore Tc1 = Tc2.
8. Calculate Mass Velocities -> G=m/Amin, m = Air flow rate, Amin = minimum cross sectional area
- G for using in calculating Reynolds number.
9. Calculate Reynolds number -> Re = (G*Dh)/µ, Dh = 4LAmin/At, µ = viscosity of air at the particular temperature, At = Frontal area
10. Calculate Total pressure drop, ∆P = (G^2)/(2ρi)[f (At/Amin)( ρi/∆Pf)+(1+σ^2)( ρi/ ρo-1)],
∆Pf = f(G^2/2ρ)(At/Amin) (frictional pressure drop), ρi = density of air in, ρo = density of air out, f = frictional factor
- In order to obtain f value, we need to have Reynolds number and refer to f vs Reynolds number graph for the specific fins-tube heat exchanger configuration. Since the data (graph) is specific to the particular heat exchanger configuration, in this case I will take the value nearest to my evaporator configuration.
-
11. Calculate heat transfer coefficient, h. Jh = (h/G*Cp)*Pr^(2/3), Jh = colburn number (referring to graph), Pr = Prandlt number (referring to air property).
12. Calculate U = 1/[(Ao/Ai)(1/ηihi)+(Ao/Rfi)/( ηiAt)+1/ηoho], assume fouling only happened inside the piping.
- For fouling factor, referring to the web. For cooling fluid, fouling factor is 0.00018. Refer to the-engineering-page.com.
- A0 value is calculated from the basic overall outline dimension and total # of fins.
- Ai value is calculated from estimated total # if tubes x inner surface area of each tube.
-
13. Used the calculated U value, Calculate A from Q = UAF *LMTD
14. Repeat above steps to re-iterate the design from steps 5.
thanks!
Here are the detail steps/flow that i concluded. Hopefully any expert can give me your inputs/comments:-
1. Identify process condition.- Air flow rate, Air in Temp, Air out Temp, pressure.
2. Identify type of refrigerant. E.g. R134a
3. Calculate cooling capacity:- Q = m [(h1-h2)-(W1-W2)hw2], m = air flow rate, hw2 = enthalpy of wet air at outlet
- h1, h2, W1, W2 , hw2 are taken from Psychometric chart for air at Patm.
4. Determine evaporator type -> fin tube evaporator, outline dimension and flow arrangement (parallel, counter, cross, mix)
- Fin tube is commonly used for cooling down air temperature. Similar to the one using in most of the car cooling coil.
5. Determine fins spacing -> max. 6 fins/inch for evaporator <0degC. And also determine size and # of pipe. Diameter of pipe based on type of refrigerant, cooling capacity and evaporator temperature.
- To prevent frosting, I’m going to start with minimum fins/inch.
6. Identify if it is compact heat exchanger -> Calculate total cooling surface/evaporator volume, m2/m3. If >700, it’s compact heat exchanger.
- If this is a compact heat exchanger, There will be a pressure drop .
7. Calculate LMTD -> LMTD = [(Th2 – Tc1)-(Th1-Tc2)]/ ln[(Th2-Tc1)/(Th1-Tc2)]
- Log mean temperature different (LMTD) calculation. Assuming Th is for air temperature and Tc is refrigerant temperature. Assuming there is no temperature change in refrigerant, therefore Tc1 = Tc2.
8. Calculate Mass Velocities -> G=m/Amin, m = Air flow rate, Amin = minimum cross sectional area
- G for using in calculating Reynolds number.
9. Calculate Reynolds number -> Re = (G*Dh)/µ, Dh = 4LAmin/At, µ = viscosity of air at the particular temperature, At = Frontal area
10. Calculate Total pressure drop, ∆P = (G^2)/(2ρi)[f (At/Amin)( ρi/∆Pf)+(1+σ^2)( ρi/ ρo-1)],
∆Pf = f(G^2/2ρ)(At/Amin) (frictional pressure drop), ρi = density of air in, ρo = density of air out, f = frictional factor
- In order to obtain f value, we need to have Reynolds number and refer to f vs Reynolds number graph for the specific fins-tube heat exchanger configuration. Since the data (graph) is specific to the particular heat exchanger configuration, in this case I will take the value nearest to my evaporator configuration.
-
11. Calculate heat transfer coefficient, h. Jh = (h/G*Cp)*Pr^(2/3), Jh = colburn number (referring to graph), Pr = Prandlt number (referring to air property).
12. Calculate U = 1/[(Ao/Ai)(1/ηihi)+(Ao/Rfi)/( ηiAt)+1/ηoho], assume fouling only happened inside the piping.
- For fouling factor, referring to the web. For cooling fluid, fouling factor is 0.00018. Refer to the-engineering-page.com.
- A0 value is calculated from the basic overall outline dimension and total # of fins.
- Ai value is calculated from estimated total # if tubes x inner surface area of each tube.
-
13. Used the calculated U value, Calculate A from Q = UAF *LMTD
14. Repeat above steps to re-iterate the design from steps 5.
thanks!