QUOTE(bhautomation @ Jun 15 2007, 05:38 AM) [snapback]55564[/snapback]
Hi all!!
I have two Screw Grasso Compressors and i must control them by a SLC500 system. The capacity control logic of two compressors used proportional and Integral (P&I). But i don't know exactly the operating principle of Screw compressor - PI control so I hope everybody who have experience of working with screw compressor can help me how to control them or show me the operating princible document of screw compressor.
Thanks you very much!
(Sorry about my English!!
)
Forget PID's. They don't really have much of a role with an air compressor except in a very low end kind of way. There are a lot more details to consider if you are trying to control a screw compressor because every single screw compressor automatically comes with at least 2 control options (on/off and load/unload), and can offer up to 5.
There are 6 possible ways to control a screw compressor, regardless of the manufacturer:
1. You can turn it on and off. I mean actually stop/start the motor.
2. You can run a VFD control. This is not always the most efficient (if you run in the 50%+ load range constantly, the most efficient method is a turn valve). There are also a lot of ugly details to using a VFD with compressors. You will want to be talking to the manufacturer if you decide to do this.
3. You can load and unload the compressor. This means that the motor is still running but the compressor air inlet is completely shut off, dropping the output to zero. Power draw drops down to about 25% of full load. BUT, it isn't instant like it is with other compressors. It takes time for all the oil to drain back into the sump and while this is happening, the compressor still draws a load. Generally, this can take about 20-30 seconds but with some compressors it could take minutes.
4. You can adjust the turn valve or poppet valve (which is usually adjusted via pressure sensor). This is a mechanical device mounted underneath the screws. It opens up the casing on the side of the screws, bypassing some air flow back to the inlet. In effect, it adjusts the volumetric compression ratio of the compressor.
5. You can throttle the inlet via an inlet valve.
6. This isn't really controlling the compressor, but you can put in a "demand-side" control valve. This is a pilot operated control valve installed between the distribution piping and usually the receiver tank or filter or dryer or whatever is just upstream of the distribution piping. It sets the distribution pressure to a constant. Then the compressor can go up and down according to the pressure in the receiver tank. It decouples air supply from the air users.
In terms of efficiency in terms of production, simple inlet damper throttling works from 60%-100% of full load output. Turn valves give you 50-100%. VFD's generally work for 30-100%. Since a VFD always hits you with a small (about 5%) hit in terms of efficiency on it's own, if you are running in the upper demand range (50%+), it turns out that turn valves are the most efficient, followed by inlet dampers once you are above about 70-80% of full load. Once you exceed the lower limit, it is better to unload or shut down. In terms of shutting down, the startup time and how often you are cycling the motor is a factor. If your receiver tank isn't big enough or your air demand swings are very large, unloading would be a better choice. With unloading, that nasty delay on unload (loading is almost instant) is always a consideration.
The reason that I'm telling you all this is that at a MINIMUM every screw compressor I've seen comes with a load/unload function. At a MINIMUM you can usually interface to the starter circuit in some way and achieve on/off control. Most screw compressors offer one or more of the other options, or they can be added for various amounts of money. And which approach you take depends on your air demand. Compressors that have one or more of these options usually have the electronics to run some of it themselves, and if they have any of the throttling features, they are optimized to mix-and-match. For instance, I have a compressor that has an inlet damper, an unload valve, and a turn valve. Depending on the current demand, it will vary the output using any or all of them.
All of this gets a heck of a lot simpler with a demand-side control valve. In that case, if you are trying to achieve say 90 PSI air, then you can set the compressor up to run at say 95-105 PSI in the receiver in the most efficient way possible and the downstream equipment will always see 90 PSI.
If you have multiple compressors, this gets even more complicated. In this case you can mix-and-match with larger and smaller units and staging them all, along with rotating them if you want to even out the maintenance cycles, and you can mix-and-match plant air pressures as well. The chance of making a mistake in terms of how efficiently you can run the system also escalates.
PID control doesn't really apply to screw compressors. The distribution piping itself along with the receiver tank if they are set up properly provide a certain amount of "dampening"...they smooth out any surges and also provide you with a cushion so that you can vary the output. Almost all compressed air systems control via pressure. The "cycle" of an air compressor is hopefully several minutes long (assuming that you don't have other problems). Most industrial equipment these days specs out 90 PSI air at a given CFM. If you give it higher air pressure, all the tool will do is waste more air. Similarly, if the air pressure goes up in the presence of a leak, it will also leak more air. So the first thing to do is figure out just how low you can operate and run your compressed air at that point. In the plant I work at, we've found that 70 PSI is generally the cutoff which is a lot better than 90-100 PSI.
The second thing to do is to measure your air demand. This isn't hard to do. There is a formula called the "pump up equation" which is one way to find out what it is experimentally. You can also stick a manometer in the inlet valve area of the compressor and directly measure air flow that way. The most accurate way is using air flow meters which depend on the thermal cooling effect but these are fairly expensive. You need to know you actual air demand and how it cycles because this determines how you set up a compressor to operate (on/off, load/unload, turn valve throttling, inlet throttling, VFD throttling). You also need to know which options your compressor can support.
No matter what you will do, you will generally find yourself in a situation where you want to run a mixture of both on/off or load/unload controls, AND some sort of throttling control. In either situation, you should know the minimum air pressure required. The goal for energy efficiency reasons is to run as close to that limit as possible (every PSI increase costs 1/2% increase in total energy load in air compressors). With even outright load/unload control, you should be able to run with a 10 PSI range. In other words, go to load at the minimum PSI. Unload at 10 PSI above that. If you have throttling (almost any variety), you can reduce this range down to about 2 PSI.
In any case, did I mention PID? It's simply unnecessary with air compressors. The air distribution piping and the receiver provide enough of a dampening effect that PID control is not really effective or useful. You can simply ramp up/down depending on the pressure signal alone.
Air compressors aren't like controlling motors, especially screw compressors since screw compressors have so many bells and whistles, and compressed air in general has lots of options to consider. Before you go any further, I'd strongly suggest you enlist either a compressed air guru or the screw compressor vendor, or start doing a lot of studying. In the U.S., the Department of Energy has had a program running for about 10 years where they have been running training, supplying contact information, and such for industry because every horsepower of energy in compressed air at 100 PSI takes 8 HP worth of electricity to produce! It is by far the most expensive utility in a plant, even exceeding natural gas or propane. It's called the "Compressed Air Challenge". Look it up and you'll find a lot of information.