Convert 1965 2 speed cooling tower to VFD

[dithomas]

Can anyone share with me some thoughts on the pros and cons to retro fit a 75HP 480VAC coolong tower fan with a VFD?

The motor is the original motor (I have not rewind history) and has six motor leads to the 2 speed reversible starter.

The 480VAC 3 phase system feeding the fans is a Delta - Delta ungrounded system.

What would be the energy savings?

Being an old timer with little VFD experience I remember that around 15 years ago there were concerns about retofit VFDs on cooling tower fans but do not remember the details.

Thanks in advance
Dan

 

[squeeky]

Dear Dan, unless you are controlling the water temperature with the fan there will be no energy saving. With the fan running at rated speed there is no advantage and the voltage drop accross the VSD and the cooling fans will actually show a loss.

Why is it reversable?

BAS

 

[dithomas]

Hi BAS,

Thank you for the reply and comments.

They will control fan speed based on water temp. Not being a process control guy, I would think that the time constant associated with a large volume of water would make it hard to tune the system to control the actual cold well water temp. very closely. I assume the hot well temp does not swing much. I believe the cooling towers are used to cool the water for the 10 1000 HP compressors.

The reversing is used to de-ice the tower in the winter.

Dan

 

[itsmoked]

Unless your tower runs against limits regularly, I would expect you could save some serious revenue in running costs.

The VFD has some losses that hurt you if it was to run full-on a lot of the time. You likely have a few degrees of leeway available so tuning should not be an issue.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

There are serious potentially costly risks to running VFDs from ungrounded delta power systems though. Some just plain cannot be connected to ungrounded delta systems, others allow you to do it, but require that you remove ground reference connection points inside of the VFD surge protective devices and common mode gound capacitors, which then eliminates many of the benefits those features provided. The common mode cap ground reference loss is an especially bad thing for an old non-inverter duty motor, you will likely lose the bearings and winding insulation in short order.

The first thing I would HIGHLY recommend then is the use of a delta-wye drive isolation transformer ahead of that drive so that you can feed it with a 4 wire grounded wye secondary and leave in the protective devices. Then on the output side, I would recommend a sine-wave filter, plus retrofitting the motor with shaft grounding. Lastly, many cooling towers are wired up using PVC conduit because of corrosion issues. If so, you MUST replace the motorvlead conductors and use shielded VFD cable going from the drive to the motor. All of these issues will significantly increase your installed cost, so do not make the mistake of only talking to the VFD salesmen on this, they have a vested interest in leaving out these details until later, sometimes too late even if at all.

The core concept however is sound, in fact it's now frequently offered by the cooling tower manufactures right from the factory. But aside from the above issue, is not as simple as you might think. First off, the entire concept has to be evaluated based on the temperature-day situation regarding your installation. The fact that you have a two speed system now already is a strong indicator the viability however. Still, someone should run an evaluation of whether or not there is enough of a savings delta between the VFD and the TWO SPEED operation, not the VFD vs single speed operation with dampers, because a lot of the savings is in the speed changing vs airflow restriction, which you are already doing.

That's not to say the savings are still not available to you, it's just not going to be nearly as much as compared to retrofitting in the fixed speed application. Add to this the additional cost of mitigating all of the risks from the power system and older motor use, and the payback period may be longer than the bean counters will consider acceptable.

"Will work for (the memory of) salami"

 

[rcw retired EE]

VFD's on cooling towers have some advantage at the larger motor sizes compared to the traditional two speed single winding reversing motor application. The 2S1W motor needs 6 leads sized for the high speed running current since all 6 leads are in use at high speed. (Three leads are carrying slightly less current). If you run all 6 in one conduit since the motor only has one knockout, the motor feeders have to be derated to 80%.

A VFD motor only needs 3 wires. At 100HP and above, the cost of wire and 2S1W reversing contactors can make VFD's cheaper.

A VFD can control the starting torque which is very beneficial to the gear box and drive train's longevity. It is much easier to shift speeds without having to make sure the fan has slowed down or stopped.

Basler makes a VFD driven, vertical shaft, permanent magnet motor specifically for cooling towers. It is wide and squat and is direct connected to the fan, mounted in place of the gear box. It reduces maintenance issues: no gear box oil heaters, low level switches, horizontal drive shafts and bearings or gear maintenance.

Just adding a VFD to an exisitng motor may shorten the motor life. Check with the VFD supplier for appropriate surge protection.

Energy savings might be significant if the control system can be configured to match speed to the cooling needed. Like others said, the system has a very long time constant and ambient temperature and humidity may have more of an impact than fan speed. An automatic system might end up chasing its tail. One design had plant operators manually select between a few pre-programmed speeds.

 

[dithomas]

Thank you to everyone. After reviewing the comments and some other research on the issue, we decided to leave the old large 5 cell tower as is an update the two speed starters. There is a new tower being ordered to augment the old tower and it will come from the factory with inverter duty motors and we will install the isolation transformer, vfd, and shielded motor leads as part of the new installation.

Thanks to jraef for a very good reply.

Dan