Cooling Tower Fan Controls 1

[gilbert123]

Cooling towers fans at our facility are manually controled. We are looking into automating this process. We do not plan to upgrade the cooling tower fan motor, which is just on/off. Is there any advice? Can this be done through chiller output signal or do we need to install separate controls. Is there any reading material out there either on the net or books someone could recommend. If we do need controls what are the possible manufactures recommended. Thanks guys

 

[PacificSteve]

Gilbert,

Marley sells a controller / sequencer that worked well on a 2 tower project I did. It has capability to control, I think, up to 6 towers. I recommend a purpose built device rather than trying to patch together components or run through EMS.

Regards,

Pacific Steve

 

[ChasBean1]

Gilbert - I don't have direct experience in installation of a condenser water control system as does Steve, but I do test and commission them. I'm not really sure what would be too difficult in installing a variable frequency drive for each fan motor (sample manufacturer, ABB), taking an analog signal from the condenser water supply temperature and controlling fan speed based on that. I don't know if you have a three-way valve system also that bypasses the cooling towers during low load, but the three way would first control to maintain setpoint (e.g., 70°F). The cooling tower fans could then start at minimum speed at a certain point above setpoint (say, 72°F), and continue to increase speed to maximum at a given high temperature (e.g., 80°F). An A/D point would be 72°F - fans start on low speed, 72° gives 4 milliamp (or 1 volt) signal to the drive, 80°F gives 20 milliamp (or 5 volt) signal to the drive.

I don't have information regarding cost savings or payback period for implementing VFD control for cooling tower fans - perhaps someone else can shed light on that?

Anyway, best of luck. -CB

 

[PacificSteve]

ChasBean1,

You are correct on idea that it is not hard to install VFD to control tower fan, however, motor must be "inverter ready", at least an EE or PE motor. I have done VFD on tower, and in retrospect, was higher cost than it deserves. Tower temperature does not need to be controlled tightly, so sequencing of fans on or off (especially good with a 2 speed motor, and using a judicious deadband, say 3 degrees or higher to prevent excessive cycling) works fine. A counterintuitive event happens at night...the fans work harder since the WB approaches DB, i.e. RH heads towards 100% and the evaporative cooling doesn't work so well. In this case, you can literally "spin your wheels" trying to get to a low condenser water setpoint, and it is just not worth it. A real smart system would recognize this area of operation, and target a higher setpoint or at least just let the fans lope along at half speed (.5^3 power energy usage).

I hope this is more helpful than confusing.

PacificSteve

 

[quark]

PacificSteve is correct. A simple temperature controller which starts and stops a fan will be a good option for 3 deg. operating range. Switch is the best energy saving device invented ever.

Regards,

 

[SAK9]

Steve ,

You are right.I have not come across any cooling tower installation with VFD on fan motor.Fan sequencing seems to be the way to go.

 

[KenRad]

A quick note on VFDs:

For applications which require tight control of tower water temperature, a VFD on the fan provides the best option. Obviously, most HVAC and process applications will tolerate fairly wide swings in temperature, so VFD control is usually overkill.

I did an energy optimization scheme for a couple of 700 ton chillers where I used a reset of condenser water temperature to minimize overall system energy consumption. The basic premise is that the kw/ton on the chiller drops as you lower the condenser water temperature; in low wet bulb weather, when you can achieve low condenser water temps "cheaply" (ie, with a low fan speed), you can achieve significant energy savings by doing so. My reset schedule varied the condenser water setpoint between 60F and 85F, and I used VFDs on the tower fans for this. I could have used two-speed fans for this application, but I would have had a difficult time at the low end of the range, where overshoot would have caused problems with the chiller.

I would encourage anyone who needs relatively tight control of tower water temperature to at least take a look at VFDs; their prices have dropped in recent years, and in my experience, they are very reliable. In some cases, the cost can be partially offset by the elimination of the two-speed starter and its controls.

 

[gilbert123]

Thank you all for your help. I really appreciate it.

Gilbert

 

[Chillermec]

There are a few things that you may want to consider. If your chiller is relatively new with micro controls, it may have the capability of operating the tower fan as required. If it is a large tower with say, 10-12 fans that may not be an option due to output limitations of the chiller controller. If you have an old tower the motor insulation may not be up to adding a VFD. I would recommend VFD if possible due to the fact that stable condenser entering water control makes for much more stable superheat control on the chiller. This also tends to lessen wear and tear on the tower fan drive components due to the limited cycling. A little more info would be helpful to determine the "best" way to go......

 

[Chillermec]

There are a few things that you may want to consider. If your chiller is relatively new with micro controls, it may have the capability of operating the tower fan as required. If it is a large tower with say, 10-12 fans that may not be an option due to output limitations of the chiller controller. If you have an old tower the motor insulation may not be up to adding a VFD. I would recommend VFD if possible due to the fact that stable condenser entering water control makes for much more stable superheat control on the chiller. This also tends to lessen wear and tear on the tower fan drive components due to the limited cycling. A little more info in the way of # of tower fans, chiller controls, would be helpful to determine the "best" way to go......