The main hot fluid MS pipe line is 40 mm. I want to cool the fluid by passing the fluid through 3 copper tubes of 12 mm dia (brazed to the main fluid line) and water over the copper tubes and then again brazed similar on the other end. As the cross section is different, velocity in the copper tubes will be a bit more. Is this variation in the cross section induces any appreciable pressure drop?
I suggest you look at the following Mathematica demonstration:
http://demonstrations.wolfram.com/FrictionalPressureDropInAPipe/
This should help you answer your question once you have specified the Reynolds number for the two scenarios you have: one 40mm diameter pipe, 3 12 mm diameter pipes. But keep in mind that you will have some pressure drop associated with entrance and exit regions where the copper tubes are brazed to the main fluid line.
You need to look at the cross-sectional area of the two.
Pi /4 x 40 x 40 = 1257 mm^2
Pi/4 x 3 x 12 x 12 = 339.3 mm^2
The ratio is 1257 / 339.3 = 3.7 :1
You will have a velocity increase in the three smaller tubes,
assuming a constant volume of flow, and 100% of the volume
goes from 40 mm through the 12 mm pipes.
You might want 12 each 12 mm pipes to keep the velocities
similar.
The objective is to increase the surface area of contact with the cooling
medium, thus increasing the rate of cooling, while minimizing
the other bad effects, like turbulence, and increased velocity,
or increased pressures.
@ Clark and Salem - If there are just 3 tubes of 12 mm - what are the practical problems ? Whether the pump driving the fluid gets overloaded ?
Or it is not a general design principle ?
Thanks
The Velocity Increases, the pressure increases, the residence time
decreases, and the ability to cool efficiently decreases because the
fluid is not in the 12 mm tubes long enough, or, you need to make
the 12 mm tubes very long.
Also if Pressure increases, then temperature increases.
PV = nRT ( Pressure, Volume, Temperature relationship )
@ Clark - Thank you for the input - If I make the tubes very long by making spiral type heat exchanger - things will be OK ?
In the US there are floor heaters where hot water passes thru the 1/2 Inch
copper tubes that have 2 inch x 2 inch radiating fins already attached.
This could reduce the required length. You could also use a fan, and let
air do the cooling. I doubt India needs more warm air, unless you
are up in the Mountains.
@ Clark - Seems Interesting - I am looking for the same what you have said - I need a small clarification - Whether the copper fins are brazed to the copper tube - having a small air gap (or) formed - means no air gap between fin and tube ?
The fins are aluminum, and they are stamped out of a sheet of metal
such that the hole curves inward and increases the surface area of
contact with the copper pipe. They are then pressed onto the pipe with
an interference fit, so they stay in place, and have good heat transfer.
Yes, there will be a pressure drop as per the basic principles and you can very well work it out for the specific configuration you have described.
The C.S. are ratio worked out by Michael Clark has considered just one tube whereas actually there are 3 tubes and the fluid in the 40 mm pipe will be distributed in 3 12 mm tubes. Hence the tube area to be considered in calculating the area ratio should be corrected accordingly.
- Badges
- Science topic
- Similar topics
- Engineering
- Thermal Engineering