Motor water to air heat exchanger tubes - 70/30 or 90/10 CuNi ? 2

[edison123]

Hi engineers. Posting here after a long time.

An old heat water to air heat exchanger unit in a 2000 HP DC motor has to be replaced.

The tubes ID/OD = 15/16.7 mm

Operating water pressure is 6 kg/cm2 and the test pressure is 9 kg/cm2.

With these factors, which is a better material for the tubes, 70/30 or 90/10 CuproNickel?

I know 90/10 CuNi is thermally better (and cheaper) but will it withstand 9 kg/cm2 test pressure?

At present, I don't have the material composition data of the old heat exchanger unit.

Nice to see the veterans electricpete and skogsgurra still helping people.



Muthu

http://www.edison.co.in

 

[jbattershell]

tube material is selected based on the water conditions and the expected fouling/corrosion.

copper tubes are best for coolers if the water is not prone for high corrosion or fouling.
next step up in corrosion resistance is generally the 90/10 tubes and that should provide most users sufficient protection.
if not enough still, then 70/30 tubes are picked to provide more protection.

sometimes Navy rolled Brass is picked for corrosion resistance.
i believe you can also get 50/50 copper/nickel tubes.
if you have really bad water, then you can also select stainless tubes.

but as you point out correctly, the better the corrosion resistance, the lower the heat transfer rate. so i would only pick a material that is good enough and not go overboard on protection. if you do choose higher tube corrosion protection, then the cooler will get bigger for the application to provide more surface area (given that water flow and coolant temperature is the same for both selections)

 

[edison123]

Thanks jbattershell.

I am told the water quality is not that bad and I too would prefer 90/10 CuNi. My worry was about the test pressure of 9kg/cm2.

Muthu

http://www.edison.co.in

 

[prc]

For a liquid tube pressure of 6kg/cm2, test pressure of 9 Kg/Cm2 seems ok. Of course design pressure will be much higher.Hope you have taken ID/OD from the existing cooler. Please make sure that water velocity in tube is not too high say 2 m/sec (corrosion from erosion can occur ) or too low(< 1m/sec can result in chemical pitting corrosion)

I had to face pitting corroison and leakage with 90/10 cupronickel and clean river water as the water velocity was < 0.8 m/sec. Once velocity was increased problem disappeared.

Another good practice with copper or cupronickel tubes is to allow water to flow at rated velocity for 30-40 days immediately on commmissioning. This will create a cuprous oxide coating inside tubes which can give permanent protection from corrosion.

 

[motorspert]

Hi Edison
You don't say why the heat exchanger needs replacing. Assuming it's not a corrosion problem then either material would be ok, I personally would go for the Cu/Ni as it's readily available and relatively inert (90/10 CuNi is specified in the majority of end user specifications. The thermal coefficient is not a major consideration, the tube material thermal coeff is so much higher than air that the difference between the two tube materials is negligable. There will be fins or a spiral coil on the air side of the tube to increase the cooling area. If you want to improve the performance then increasing the surface area on the air side is key - a modern cooler will probably give you better perfomance because of the closer fin spacing.

Standard tubes wil be ok for a much higher pressure than you are looking at. The weakest part of the cooler are the headers and where the tubes are swaged into the tubeplates. The header box and tubeplates will be need to be thicker (and probably have more bolts) as you increase the pressure. The materials of the tubeplates and header also need considering. The original may have an epoxy coated steel or be made of brass or Cu/Ni. The epoxy coated steels are ok for normal applications but not recomended for sea water applications or where there are likely to be particulates in the water.