Vertical U-tube HX tube side velocity

[Engineerworx]

Hello All,

I am trying to figure out velocity in a vertical U-tube heat exchanger. the tubes are 3/4" OD, 0.065 wall thickness, and number of passes = 2.

I have a volumetric flow rate and I would like to calculate the minimum and maximum velocity in the tube side. Also what would be the flow rate in the pipe at its maximum velocity?

Thanks

 

[ash9144]

Divide tube vol flow by number of tubes to get vol per tube. A U tube HEX automatically has 2 passes by design.

Divide vol per tube flow by area of pipe this gives velocity in each tube. Assuming clean service, balance flow, etc

 

[Engineerworx]

Thanks ash9144,

I was using the following formula:

Velocity = (Flow Rate * Vf * Passes) / No. of tubes

Velocity, ft/sec
Flow rate , gal/min
Vf = velocity factor
for standard tubing,
1/4" = 9.66
3/8" = 4.02
5/8" = 1.47

it doesnt have the velocity factor for the 3/4" tube. can anyone help with this data?

 

[StoneCold]

Why don't you start over from basic priniples.
Take the flow rate in the pipe. (xx gpm) Convert that to gallons per second. Then divide that flow rate by the number of complete u-tubes. That will give you gallons per second per tube. Convert gal/sec to cubic inches/second. Then divide that by the cross-sectional area of one tube.
Now you have the velocity in the tube in inches/second. You should be able to convert that to any units that make you happy.


Regards
StoneCold

 

[djack77494]

I don't know what the "velocity factor" is all about. You say that you have the volumetric flowrate. The cross-sectional area of a single tube * number of tubes / number of passes will give the flow area. Velocity is then the volumetric flowrate / flow area or (length^3/time / length^2) = length/time units.
Doug

 

[Engineerworx]

Doug,

Its simple enough....I was trying to compare the basic principles method with the velocity factor method! thats all

thanks

 

[djack77494]

I've never heard of the "velocity factor" method and I don't understand it in the context of the above entries. As I stated, the velocity is volumetric flowrate divided by cross-sectional area. You state that you have the former; the latter is readily obtained from the system's geometry. Forgive me, but I'm highly suspicious of anything more complicated than this.
Doug

 

[CJKruger]

ash9144, while a U-tube typically has 2 tube passes, any number of even passes are possible.