Cooling tower

I am thinking of puting a plc in a cooling tower to control a vfd.

Has anyone done this befor any issues I am going to assume pid controlled with a rtd input.

Does anyone want to post a exaple of a program for one that would be great.

I'm assuming you intend controlling the pumps pumping water to the tower?

Do that, and maintain a constant level in the tower (so it is always supplied with water) for maximum efficiency. For a really large tower, controlling fan speed does have an effect, but not as much as keeping the flow constant.

spidermonkey said:

I am thinking of puting a plc in a cooling tower to control a vfd.

Has anyone done this befor any issues I am going to assume pid controlled with a rtd input.

Does anyone want to post a exaple of a program for one that would be great.

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Me too...sorta.

We are replacing a refrigerant cooling tower which currently has 6 fans and five pumps with a new tower which will have 8 fans (two banks of four) and one in each bank will be on a VFD (keeping the same pumps DOL start).

I have a theory of operation in mind for the PLC code that will run the drive speed references, but being relative new to ammonia refiregeration, I am not sure how great a lag I will see in the application of fan cooling and a change in the PV (discharge pressure), and I am not sure how consistent that process dead time will be.

So I will follow this thread too.

Sorry I guess I should of worded that better.

The tower has 3 fans in the winter they are constintly turning 1 off and sometimes 2 off sothey want to put 2 of the 3 fans on vfds.

I can do it with or with out plc control but It would be nice to have the plc control the analog signal to the drive

spidermonkey said:

Sorry I guess I should of worded that better.

The tower has 3 fans in the winter they are constintly turning 1 off and sometimes 2 off sothey want to put 2 of the 3 fans on vfds.

I can do it with or with out plc control but It would be nice to have the plc control the analog signal to the drive

Click to expand...

Just my two-cents, but adding VFDs may or may not prevent cycling of the fans.

There will need to be a minimum speed setting which is fan and motor dependent and in my experience there are many times in the Winter when even that minimum speed provides too much additional cooling so the fans need to be turned off.

If I were running only three fans, I might consider putting them all VFDs (I didn't get the choice in our limited budget project to add 8 drives).

Then I would run them in unison, provide a summer/winter mode (or automatically choose based on ambient temp feedback) to control the sequencing of fans vs. pump(s). I would still have the two under VFDs run in unison, and when they are both maxed out, switch on the contactor for the third and keep it on until the VFD signal reaches it minimum (probably around 10-15 Hz depending on your fan curves), switch the DOL starter fan back off. This should reduce short cycling of contactors and virtually eliminate belt wear on the two ramped fans (are yours belt driven too?).

how would a pid look in the plc to control this tough does anyone have an example

I have a cooling tower, two banks 2 fans each. We measure incoming water flow, when flow is less than certain value we close a valve and send all the water to one fan section (without exceedind capacity). when flow is higher that a section can handle we open the valve an both section works normally. In the future maybe we change valves and use VFD when flow is low.

For an evaporative cooling tower, within a limited range, the fan speed controls the outlet temperature. Decide on a target outlet temp, make that the setpoint of a PID (actually, you could use PI or possibly even just proportional control depending on the situation.

Outlet temperature greater then target? Start the fans at min speed (if you don't have isolation, may as well save complexity and run them all together). Enable the PID. Make sure there is a min speed limit (25% is usually about right for generic motors) and let it run.

I've done a number of cooling towers (doing one now in fact). I do not recommend putting the feed pump on a VFD, just the fans. I've found that staged control gives adequate enough control for the cooling tower fans. I use four stages, 0 = OFF, 1 = 50% speed, 2 = 75% speed, 3 = 100% speed. The stages are driven by temperature and timers. Each stage has a stage up time and a stage down time.

Lots of cooling towers do use PID for the fan control. Properly tuned its a great way to do it, but a cooling tower has significant process lag and process gain varies dramatically between summer and winter so you may need different seasonal gains.

Since I prefer that the towers be "set and forget" I use the staged control method.