cooling water piping velocity

[prasad123x]

Does anyone know of a recommended minimum piping velocity for a closed loop fresh cooling water circuit (ie. minimum velocity to prevent sedimentation and other problems)? I can't seem to find any recommendations on minimum fresh cooling water velocities (but have found several recommendations on maximum cooling water velocities).

Thanks.

 

[TD2K]

I've commonly used 3 ft/sec in piping systems as a rule of thumb to avoid this. Velocities through heat exchanger tubes are somewhat higher, 5 ft/sec or more to help prevent scaling.

A lot of it depends just what is in the water that you are trying to prevent settling out obviously so a one size fits all number really doesn't exist.

 

[SmartEngineer]

From my experience, the usual recommended velocity range for CW is 0.3 to 2.4 m/sec and for seawater (assume it is dosed with chlorine) is 1.5 min and 4 m/s max.

Hope this would help

cheers

SmartEngineer

 

[MJCronin]

prasad....

I am having trouble understanding how a "closed loop" cooling system would develop sediment. The typical systems that i have encountered have involved a corrosion preventative in the closed loop....

Could you explain ?

-MJC

 

[DocMax]

Hi prasad..

I'm interested to your question and I want to know exactely what kind of circuit do you mean, this is a closed cooling circuit with a plate heat exchangers? Is a pressurized cooling circuit? What is the quality of the water in the circuit?
What are the users that you want to cool?

Could you explain?
Thanks
Max

 

[WebWizz]

Have a look at the line sizing routines (utilities) at

http://www.engineeringpage.com

Gives tips and does calculations for you

 

[prq123]

MJC/Doc,
It is a closed loop fresh cooling water circuit. Plate/frame exchangers are used to cool the fresh cooling water against sea water. It is a pressurized system (about 5 bar pressure) used in cooling mostly hydrocarbon streams. The concern was primarily about corrosion products from the carbon steel piping settling due to low velocities. In our specs there is minimum velocity criteria for tubes inside cooling water heat exchanger, but there is no criteria for minimum velocity in the pipes.

Our contractor has now decreased the line sizes to give a more reasonable velocity. Previously some lines had velocities around 0.05 m/s.

 

[SmartEngineer]

From my experience, the usual recommended velocity range for CW is 0.3 to 2.4 m/sec and for seawater (assume it is dosed with chlorine) is 1.5 min and 4 m/s max.

Hope this would help

cheers

SmartEngineer

 

[cub3bead]

The loop is closed in the sense that it recirulates the cooling water within the system. Sediment can accumulate in the system because the cooling tower is one giant air washer.

I have advocated the use of side stream filters for many years to help control the suspended solids within the cooling loop.

I have always used 3 fps as a minimum flowing velocity and then do a life cycle cost analysis to determine the design velocity based on the cost of power and the installed piping system. The last time around it was about 10 fps.

 

[DocMax]

Prq 123

In my opinion if you have problems in corrosion you should feed in the water a corrosion inibitor.
Can you give me the water analysis? what is the langelier saturation index in the circulating water?

If the problem are the suspended solids you have to use a side stream filter in the cooling tower basin. What is the total flow rate in the cooling tower?

For the velocity in the pipe i think 2.2 - 2.5 m/s is right.
I suggest to you to controll the water quality in the circuit and contact a local water chemical supplier for ex Nalco o Betz. They suggest to you the right chemical conditioning program.

Cheers and have a nice job

 

[bewdley]

Basically I would follow the advice of DocMax. If you feel solid settling is an issue, I would be scared of the fouling of the PHE rather than the (crevice) corrosion of the carbon steel pipe.

A point that was not considered here is the pumping energy. Roughly, at 3 m/s you have 9 times the pressure drop you get at 1 m/s, so give a thought to the money you donate to utilities. Surely a few occasional ppm of corrosion inhibitor plus some quid spent in pipe diametre are outweighted by the energy expenditure.

 

[SmartEngineer]

Sorry for making a question with this post but it is related to the same subject:
Which one velocity limit or pressure drop is the criteria of sizing of cooling water piping?

Cheers
SmartEngineer

 

[Harold53]

prasad123x
Instead thinking of a flow velocity, it seems a little more reasonable to think of specific head losses. Good practices recommend Sizing discharge lines for 2 - 2.5 psi/100ft and suction lines for 0.5 psi/100 ft. However, you are quite right about the sedimentation problem and for cooling water normally you can get in troubble when going about 1.2 ft/s in round pipes and 1.5 ft/s in open square channels. So the main channel from the tower to the pumps should be designed for a low velocity to achieve sedimentation in the channel before entering to the pumps.

Regards