air compressor speed control 2

[Herman2]

We have four (4) large centrifugal air compressors (900 - 1500 hp) that run at constant speed to supply a general 100-psig header in our factory. The air demand is somewhat variable and it takes the capacity of 2 or 3 compressors to meet the plant requirement. On occasion, we vent air to the atmosphere to maintain pressure.

It has been suggested that we consider variable speed drives and control for this equipment. Any thoughts or suggestions? Is this practical? Would it likely save enough money to justify the investment in 4160-V drives?

Thanks!

 

[pardal]

You could use 4 presostat to control each compressor, each one set at 97 98 99 100 psig.
Or you can use a analog presostat linked to a PLC.
You can select to change the way of starting each compressor
So first start 1 2 3 4 , then 4 3 2 1 , 2 3 4 1 , 3 4 1 2.
It will be a lot cheaper than variable speed for a so huge power.


Pardal

 

[quark]

Pardal's idea is very good. It seems that you don't have enough storage capacity for compressed air. In my view a switch is the best energy conservation device. If you on/off your compressor with respect to a set pressure you will get better results.

If you constantly require 100 PSIg pressure, VSDs may not be a better option. Moreover you should have a good surge control system otherwise your compressors may get damaged.

Pardal!

The pressures mentioned by you seem to be too close. I would like to have your opinion.

Regards,

 

[pardal]

Quark :

It is only a guidance, but as I do not know the up and down pressure allowed , I choose that .
Wiht a analog pressure sensor you can set up to the presicion of the sensor , even to 0.1 psig.
I do it when controlling temperatures at chillers power plants.
About the storage capacity , it will cost a lot.
Hope it help


Pardal

 

[Romefu12]

Herman2
I have experienced a similar problem in a Hydroelectric station using 3 Ingersoll Rand HP 900 Screw compressors in a similar configuration. Capacity was inadequate and cycling was also an issue.

My approach was this;
1.Complete a study of usage including CFM demand and maximum / minimum pressures needed at all times to sustain the factory.
2. Calculate cycles and operational times and determine the actual losses during heaviest usage.
3. Determine the optimal cycling of the machines for duration of operating times and starting and stopping cycles.(keep in mind the more starts and stops, the more maintenance required)
4.Compare your results to actual system design and the faults should be clear.

Some ideas:
1.With a few check valves strategically located here and there piping can be used for capacity to supplement some lost capacity to alleviate some cycling
2. Check valves to isolate certain branches of the system can alleviate some cycling.
3. Cycling air dryers unnecessarily or too often can be a source of great losses.
4. Installing smaller compressors to handle hand tools or shop tools would help alleviate some demand.
5. The key to keeping these large centrifugal compressors running reliable and efficiently is to keep them running as long as possible.Too much cycling causes moisture to corrode the entire system including the piping, tools, machines and the compressors themselves.
6. Too much cycling on these higher horsepower machines will cause starter and contacter wear that needs to be kept in check with your electrical maintenance department.
7. Speed controllers are not only expensive for these motors, they also would not allow these machines to run efiiciently because they are designed for a specific air delivery at a specific speed.
8. Changing the speed of these machines may also change the designed lubrication delivery to the machine itself which could be detrimental to the machine.
9. In your case of course there would be great value in installing a larger storeage vessel for capacity.

Hope some of this helps

John

 

[semo]

Has anyone utilized a relatively new product called Magnadrive? It is a magnetic coupler that seems to act similar to a torque converter. It couples between the pump and motor with a powerful magnet and copper plate. The distance between the magnet and plate is adjustable to provide for slip; thereby reducing the motor speed. The unit can be configured to act as a soft-start or variable speed unit.

I've seen some demonstrations of the unit; but, it does not seem to be cost effective for small motors yet. I've not had a large enough motor to be able to try it; but, it looks like a really interesting technology. Especially for retrofit.

 

[sbei]

Herman2-

Sorry for the late post.

You may have found out by now that VFD's cannot be applied to centrifugal compressors. Some other comments:

Motors of this size range are good for only 1 or 2 hot starts per hour, so cycling compressors to maintain pressure and match equipment with load is impractical - unless you can shut a machine or two off during nights or weekends.

With the size of system you probably have, once you do the math you will see it is also impractical to provide enough storage capacity to ride out the bumps in demand. Producing and storing compressed air at a higher pressure and metering it to load through a pressure reducing station is also costly and inefficient but you may find someone who will want to sell you a system to do it!!

Romefu12 has the right idea, you need to first find out how much air you are using. It may be that addition of a smaller compressor sized for night/weekend loads makes economic sense.

But your best bet for enery efficient operation of centrifugals is to invest in a compressor control system that will automatically start/stop/load/unload machines in the most efficient manner, and provide machine protections( i.e., surge control). Most major compressor manufacturers sell such systems, plus two other manufacturers that sell new and retrofit systems include Bay Controls and Compresor Controls Corp.

Happy compressing!