Loacation of makeup for cooling tower 1

[garfio]

For a new facility, the fresh water source is about 300ft from the cooling tower so I would need to run that length of pipe for the makeup. However, I could avoid that pipe if the makeup is injected in the return line.

Is there any reason why this is not a good idea? The water flow through the cooling tower would be increased by the makeup water, but it does not feel as being a big problem, considering that the temperature of the makeup water will be lower than the temperature downstream the cooling tower and the flow rate would be proportionally small.

Thanks for any comment.

 

[katmar]

In theory this will lower the efficiency of your tower. The colder return water lowers the thermal driving force, and increasing the flowrate increases the water to air ratio which is a fundamental factor in determining the operating point.

However, I suspect that in practice you will not notice any difference because of the large difference in the flowrates of the return water and the makeup water. The only way to be sure is to run the numbers, and your cooling tower supplier should have software that could calculate this for you.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[MJCronin]

garf,

I assume that you are controlling makeup flowrate by a level device in the tower.

Yes, you may save the cost of underground pipe, but you now have to get a signal to the valve supplying the makeup water.

This 300 ft distace, of course, will increase the expense of the system over the "tried and true" ball-float device commonly used in this application.

You may not have the significan savings you expect.

My opinion only

-MJC

 

[rmw]

From a strictly thermodynamic point of view I agree with Katmar regarding putting the cold make up into the return. Ignoring for a moment the logistical issues brought up by MJC, if you have access to the return line you also have access to the supply line as most often they run adjacent to each other and if the make up is that cold, it would help the users of the cooling water. Normally the supply side is at a higher pressure, so you would have to overcome a smaller additional amount of head at that point to inject into the supply line, and assuming that there weren't any chemical reasons for not introducing make up water into the supply line, you might gain a miniscule benefit in your cooling circuit.

Point being, both Katmar's observation and mine above are minor in the overall scheme of things, if faced with the need to inject at that location, other factors being equal, I would select the supply line to put the make up into.

The point is made more as a point about the thermodynamics of it all rather than the practicality. If you have to pump it 300 ft to get it to the tower, you may have enough head already, and running an instrument pair is less expensive than running pipe. IMHO.

rmw

 

[quark]

I don't see any problem, thermodynamically, as the makeup water quantity may be around 1%.

 

[garfio]

Thank you all. I have a clear idea of the factors involved now.

 

[schnipp]

I would think that if it is injected into the supply side of the cooling water loop, the lag time on the controller could be so long that control could be difficult.

 

[agf94510]

Valid observation schnipp if you are expecting a closed-loop control on the water level in the CT basin.

In reality, that level does not need to be controled by a level controller. Setting make-up rate to a fixed flowrate and allowing the (oil) skimmer blades for blow-down achieves the same result. Cycles of concentration can be adjusted by increasing or decreasing water make-up. Level is self regulating. Independent High / Low level alarms are needed advise of an unusual operation.

 

[cub3bead]

If you do it make sure you install back flow preventers if the water source is from the potable supply system. Then test them regularly.

 

[schnipp]

agf,
certainly letting the tower overflow is an easy solution. Anymore, though, the cost of water is increasing, especially treating before going to the city or river (or where-ever). That's why I've generally seen CT that try to avoid constant over flow. We didn't use cooling water in many oil situations, so I don't have experience with oil skimmers.
(I've been mainly in food plants)

 

[agf94510]

schnipp

A clarification on setting the rate is due. You can set the inlet flowrate and adjust it for cycles of concentration (based on lab results) periodically. Excess water will overflow as CT blowdown needed to maintain cycles of concentration.

As an alternate (method #2), you can set and adjust the CT blowdown abd allow a level controller to make-up water. The blowdown rate is set to maintain cycles of concentration.

Now that the process was written out, I realize that method #2 is much better in controlling level. Blowdown from a CT is a small fraction of the make-up rate.

 

[garfio]

For the method #2 (using a conductivity transmitter to control the blowdown), I've read two different positions: one saying that blowdown should be continuous (more or less constant flow), but other saying that it is better to do it intermitently, that is, once you reach the high conductivity level, you open the blowdown till it reaches the low conductivity level. One of the arguments for the latter was related to a better use of the chemical additives.

Anybody has an opinion about this?