Preliminary design of Fin tube evaporator

[teikseng]

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!

[desA]

http://www.coolit.co.za

They have a very useful evaporator design program. If you contact Bruce he'll provide you an evaluation download. It's a very useful package.

Enjoy.

[teikseng]

Thanks. Downloaded some free programs. However, it's better to get the basic right before getting those tools to help us in our works.

By the way, the objective of the fin-coil evaporator design is to cool down air flow from a axial fan. Hope if somebody can give me your inputs/advices on the steps and formulas.

[desA]

I'll bet you 1000 : 1 that no-one uses formulae here. These are all built into the software packages.

Up to you, really. Remember that you are dealing with an evaporation process. The equations for these kinds of flows are fairly complex. They will stretch you way beyond your simple heat-transfer equations listed. Calculations need to be done at around 11 or more points as the 2-phase fluid changes into vapour. It will take some time by hand.

[Aik]

teikseng, your calculation is very simple and don't consider many factors, there are complex methods of calculating evaporator capasity... You should read more special literature.

[teikseng]

desA and Aik, thanks both of you for reply.

What are the things that lack of in my calculations, besides forced convection and boiling (evaporation)?

desA : i'm trying the software that you recommended.