Cooling tower: the use or not of VFD pumps 4

[DownesB]

Hello (new to the site)

I am a recent MS graduate in ME. I am working on a cooling tower application.

The cooling tower is for a steam turbine. Every degree that we can lower the water temperature means we can produce more kilowatts. But every motor used in the process is less killowatts net. The application is in the design phase and is in the Southeastern part of the united states (high wet bulb temps in the summer).

I am looking at the possiblity of installing VFDs on our pumps for this reason:
If we can reduce the energy needed by the cirulating pumps, we can reduce the ancilary load. During the summer the wet bulb temperature is very high and if the water can only be cooled to a certain point anyway then we can reduce flow. Is this common? Any steam turbine / cooling tower experts out there?

Thank you for your time and consideration.

 

[rmw]

Under these extreme summer conditions reduced CW flow will have a detrimental effect on your condenser performance.

Look at your condenser curves and look at the effects of various flow rates, or ask your condenser OEM for a set (family) of condenser curves at varying flows like a plant on a river with a variable level (and hence, variable condenser CW flow) would have.

Or, google HEI and find their website, and order a condenser software that will let you model the condenser water flow at the lowest attainable summertime temperature, and you will be able to see your back pressure rise, penalizing you by way more kilowatts than you are saving with vfd motors on your pumps.

It is actually in the wintertime, when you have more than enough cooling capacity that you would benefit from reducing your CW flow. But, you generally don't need the KW as bad then, either.

rmw

 

[prochem]

DownesB,

Check the following thread for the discussion.You can install VFDs on your pumps but not without considering all aspects of your individual case.

thread407-90212
thread237-94051

 

[MJCronin]

Downer,

A common configuration for pumps on "Cooling Tower Water" circulation uses multiple pumps with two speed motors. This, of course, depends on what your design flow is for the cooling water ( by the way, this system is commonly called "service water")

The pumps can be of horizontal axis, double suction design (placed outside and below the tower basin) or of vertical axis design

The amount of control in the flow that you need can be met with multiple pumps without the VFDs and increased capital cost. IMHO, the capital money saved may be better (thermally) spent in a larger/deeper cooling tower sump

I would post this question in the "power plant" forum of this website

MJC

 

[timbones]

In my experience, you are much better off conserving the power on the cooling tower fans during the cooler months(unless of course your tower is natural draft). The fans consume much more power than the pumps. Even in this instance, it may be much more economical to go with two speed fans.

 

[EdStainless]

If you are talking about the main cooling water pumps, and reducing the water velocity through the condenser, don't do it.
You take a hit because of less flow.
You take a hit because of resduced velocity.
And you will have more fouling issues at lower velocities.
Water velocity through the condenser tubes is your friend. Keep it as high as you can (8ft/s is good, 10 is nice depending in the materials) and look else where for reductions in parasitic losses.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[CARF]

http://www.cti.org/

Cooling Technology Institute

+ some demo software to download

 

[davefitz]

There are several other practical issues that need to be adressed before one reduces the circ water flowrate thru a condenser.

a) if the condenser tubes are stainless steel, they become liable to experience MIC ( microbiologic induced corrosion ) if the water velocity drops below 6 fps

b) There could be developed tube to tube flow unbalances if the condenser tube circuitry is not specifically designed to have a "natural circulation " characteristic to the DP vs W curves.