Diameter of tubes - Heat exchange 1

[adriansg95]

Hi, my name is Adrian and I'm new to this.
I have to design a 8KW water-air heat exchanger with tubes and I don't know what diameter to use. It affects to the velocity of the fluid and thus to the inner ht coef, but I'm afraid to choose a diameter too big and maybe the contact between water and the walls of the tubes won't be good. On the other hand, the bigger diameter, the bigger area of exchange and the smaller pressure drop.
1/2" (12.7mm) would be too much? Could be still greater? The length of the tubes would be around 30-60m with several bends.

Thanks in advance!
Adrian

 

[MJCronin]

Rather than reinventing the wheel, I suggest that you consider the purchase of a "tried and true" liquid HX design !!!

A smaller, parallel path, finned tube design has been in use for many many years !!! (Essentially a radiator)

Some things to consider:

- What are the acceptable in/out temperatures ?

- What are the outdoor conditions ?

- Do you want a fan to induce and ensure air flow ?... How about louvers ???

- Would you consider a used unit ?

- What materials will you consider

- How much do you have to spend ?

http://www.qats.com/Download/Qpedia_May07_Heat_Exchangers_theory_and_selection.ashx

https://www.valutechinc.com/products/heat-exchangers/air-to-water-heat-exchanger/

https://performaflexxt.com/products/water-to-air-heat-exchangers-12-sizes

- What materials are acceptable to you ?

MJCronin
Sr. Process Engineer

 

[EdStainless]

These can range in size from small radiators to large fin-fan units.
Since the air side heat transfer is the limit the tube size is all about accommodating the overall flow/pressure.
I have seen small units using 3/8" tube and large ones using 2".
Fins of some kind are nearly universal in this kind of application.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

Like MJC says I don't really know why you're designing this as opposed to just going and buying one...

I would aim initially for a residence time of between 45 to 60 seconds as a starter for 10.

Then do some analysis around those numbers.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[adriansg95]

Thank you for your answers. I guess I should have explained a bit more. The HX will be installed in an axial fan for tube (underground) ventilation. I'm thinking of several rows of tubes behind the guide vanes.
It will cool the motor of the fan in a closed cycle and it needs to be as much aerodynamic as possible (which is quite difficult). That's why a commercial HX wouldn't be ok this time.
For the calculations I've used the LMTD method.

- TEMPERATURES - Hot in: 43ºC (aprox), Hot out: 40ºC. Cold in (air): 30ºC (worst scenario).
- Water FLOW: 0.6 kg/s.
- Air goes very fast (around 40 m/s).
- FINS: I'm also considering the use of finned tubes, but I know the will decrease the aerodynamic performance of the fan.
- MATERIALS: I'm considering Cu, Al and steel. The best would be copper.

I've also thought about using hollow guide vanes through wich water would circulate (indeed it was my Master's Thesis), but it was too expensive.
Now I'm also considerating the use of 2 circuits (attached picture): one with water and another with R-134a (wich lets me increase the hot temperature from 40ºC to 60ºC (1700 kPa)). Do you like the idea?