Sizing an evaporator

[gth724a]

Hi,

I am a student working on a project, and I need help sizing an evaporator from a strictly conceptual view. I have the temperature that I wish to maintain the coldspace at, the evaporator refrigerant temperature, and Im going to assume my capacity is 3hp (does this seem like a standard capacity?)

Anyways, I still have 2 unknowns, U and A. (Using Q=UA(T2-T1), or should I use the NTU method?)

I'm not sure how to get U, I know if I select the tube materials/geometry, determine the air and refrigerant flow rates, I can calculate U for a smooth tube. But how do I adjust U for finned tubes?

I'm assuming that once I get U, I can solve for the area needed. But does is the area affected by the fact that im using finned tubes? or can i just use the area of smooth tubes to size it?

I guess what I'm asking are equations are used to determine the overall heat transfer coefficient. I'll select an operating point, so I'll have the enthalpies, temps, flow rates, etc. But i don't know how to get U. I was going to use copper tubes and an air flow rate between 350~600 CFM. But I only have equations for smooth tubes, and I really dont know much about enhanced tubes.

Please advise. Any input would be greatly appreciated. Thanks

[sterl]

Almost all the methods involve an estimated fin effectiveness and the ratio of prime area (tube) to secondary area (fin, less tube holes). How densely you can pack fins, and your air velocities, depends on what your frost concerns; dehumidifiaction; filtration and moisture carryover concerns might be. Note that at air conditioning type conditions, your load may be as much latent (moisture removal) as it is sensible.

Actually, the finned tube side of an evaporator for a given duty is not that difficult to derive and model for a clean, dry fin; gets a little more involved once a film of water is factored in because both cleanliness and gravity will play a large role.

Also the refrigerant circuiting is a major player, this usually developed by the manufacturers for each refrigerant and anticipated temperature difference as a specific loading per tube to keep the 2-phase flow from going plug, sewer or otherwise getting lazy....Annular aerosol is the "extent" of the 2-phase mechanisms where the best of the heat transfer happens. If your temperatures are pretty ordinary, and your tube sizes are below 20mm, the normal refrigerants in a dry expansion evaporator will function in that regime until the quality gets to about 85%...

The basic modelling and math, though is all over the Web; involves some boundary layer calculus...But here's an older, open link that goes at it pretty fundamentally:

http://irc.nrc-cnrc.gc.ca/pubs/fulltext/nrcc18834/nrcc18834.pdf

Good stuff starts on Page 3.

[US Iceman]

I think you can find a lot of the information you are looking for in this link. Be prepared for a lot of math and time spent pulling this all together.;)

http://www.wlv.com/products/databook/db3/DataBookIII.pdf

If you can get to a point where you have a reasonable inside film coefficient for the boiling refrigerant, assume that is constant. Otherwise you will drive yourself crazy trying to worry about coil circuiting and 2-phase flow regimes and 2-phase heat transfer.

Then if you build a simple model for fin efficiency with the coil bundle, tube pitch (spacing and orientation of tubes both vertically and horizontally), fin spacing (distance between fins), fin root resistance, etc you can begin to develop an outside film coefficient for the air side of the exchanger (dry of course). Condensation and frost formation add to the complexity of modeling this.

I hope you have the time to spend on this, because it will take a fair amount...

[desA]

^ That's an excellent reference source. Thank you for that. :)

[amitsaxena]

I could not download, is there any issue with the like or plz send it to me through private mez.

thanks

[desA]

You will need to open the first part, save it. Then consecutively open each section & save them individually. It's a long book - but an excellent resource. Thome is one of the leaders in his field.

[amitsaxena]

Sorry, Its not happening as its showing a mez that the file is not available.
Thanks and help me out.

[icecube51]

no,its not working for me eather.
looks great do.
ice