4 replies to this topic

[bernath]

Dear all,

I'm currently designing a shell and tube heat exchanger. Do you know the velocity range for tubeside fluid?

The fluid is water at 3 barg and 39 deg. C. The tube material is copper and 3/4" in size.

my current tube velocity is 2.79 m/s. A little bit too high for me since i expected the velocity to be under 2.5 m/s. I can't find the recommended velocity range for this tube. As far as I know, if we use pipe with water inside, the velocity would be around 1.5 - 3m/s or 4.5 max. Can it also be applied to HX tube?

Please kindly also let me know the reference of your answer.

many thanks
regards
bernath

[ankur2061]

bernath,

I think you are overemphasizing the velocity aspect for the tube side. The dominant criteria for your tube side flow should be the tube-side allowable pressure drop which in turn will dictate the tube-side velocities.

The velocity figures you have mentioned should neither be a cause of erosion nor they should be a cause for velocity-induced tube vibrations. The velocity-indiuced tube vibrations are something which you should check out using a heat exchanger software such as HTRI or HTFS.

Hope this helps.

Regards,
Ankur.

[srfish]

Hello,

As a retired heat exchanger specialist I have seen a lot of cooling water velocities inside the tubes. Usually they are under 2.5 m/s. Personally I use a maximum of 3 m/s. No way would I use that 4.5 m/s that is reommended for pipe.

Best regards

[Art Montemayor]

Kanankiri:

I always designed my fluid velocities in process piping differently from that in condenser tubing (heat exchanger applications). But the first experience-based rule I always applied was the condition and identification of the fluids in question. The second parameter was the base material of construction. With clean process liquid fluids and carbon steel pipe, I usually applied a design velocity of 5 ft/sec (1.5 m/s). I expected that the velocity would vary about 25% more in the field – which gave me a field velocity of about 6-7 ft/sec (2.13 m/s). I never had any problems with my piping and my pressure drops always resulted in an acceptable and applicable range. Copper, Brass, and Stainless pipe were something different and I always reduced my velocities about 25 to 30% less than for carbon steel.

In the case of condenser tubing I never fixed my velocities but have always deferred that decision to an expert or fabricator that was going to guarantee the resulting performance of the heat exchanger. That doesn’t mean I didn’t make a stab at estimating them for general purposes of getting an estimated exchanger size – but I have always insisted on noting any assumptions to a heat exchange manufacturer and advising him that he/she is the expert and the design that will be accepted is the fabricator’s design that carries a performance guarantee.

I consider the subjects of fluid tube velocity and heat transfer performance so closely linked that only a true expert – one who makes heat transfer equipment his/her specialty and a day-to-day business – can really be expected to take the lead and responsibility for fixing the tube side velocities. In your case, particularly, I would point out that the copper tubes will be subject to possible erosion and rapid wear if the fluid velocities and wall thicknesses are not carefully selected. Too many times have I personally seen a situation where the heat exchanger manufacturer was told one story about the “cleanliness” of fluids and then watched how the installed heat exchangers started to get fouled, plugged, and eroded. And then the customary complaints and accusations would begin about a bad design.

I would not apply acceptable process pipe velocities to condenser tubing without knowing the other parameters involved: wall thickness, temperatures, alloy, fluid composition, etc. I would not design for an expected, steady-state flow rate of water inside a copper tube with a velocity of 10 ft/sec (3.05 m/s). But that is my judgment call, based on prior past experience.

[bernath]

@Ankur: my tube pressure drop is around 45kPa while the shell is 25kPa. The allowable pressure drop for both side is 10psi (68kPa). The pressure drop was bit little high for the tube side. I wanna make it below 40kPa.

@Art: I was using BWG 16 copper tubes with wall thickness around 1.65mm. The fluid is assumed to be pure water at temperature 40 deg.C

I will consult with the HX manufacturer then

Thanks all for the advices

regards,
bernath