I started working on my 30 hp Quincy rotary screw compressor the other day and made a short video of it.
This is one of my first exploratory runs of this compressor. I have never owned a rotary screw type of compressor before and I was unfamiliar with what to expect.
The concept is different then the reciprocal compressors I am used to. With a recip the storage tank is vital and you fill that to as high a pressure as reasonable. Typically 175. Then depending on the compressor setup it either shuts off or idles. These recip compressors are not made to produce max air 24/7. They are also not suppose to be started more then 6 times per hour. That is what the electric motor makers state. The purpose is to keep the heat build up from the starts down.
Right away we can see a flaw in the setup here when max air is needed most of the time. Take a look at these variables. Let’s say most of your machines need 100 psi to operate. You have purchase a compressor that has a pulley ratio on it designed to produce air at the 175 psi level. This means the compressor turns at a lower speed to over come the resistance of 175 psi. But ask yourself this; how much time does your compressor spend pumping air at 175? See the flaw? Very little of the compressors life is spent at the max pressure. Once your shop gets up to production either two things will happen. The air demand will be more then the compressor can deliver and it will run continuously at around 120 to 140, or less, until production slows due to lack of air. Or it can meet the air demand and it will shut off, but only for a couple of minutes, then turn back on. Remember the six starts per hour rule? Is your maintenance guy going to sit back there and count how many times it goes on and off ?
So that means that on those high demand days you would set the compressor to load/unload. There should be a switch or valve on your compressor if you have this feature. This means you avoid the constant start stop which can hurt the motor, and also jack your electric bill out of sight with demand charges.(Google that if you don’t know about demand meters.) So now your compressor is running full time. It loads and unloads. But it is not loading at 174 psi and unloading at 175. It is down at around 120 to 130 psi. That is where the load unload settings are. So now if you are on the edge of having enough air to power your machine or sandblaster, then you are not in the ideal setup. You could get more air if you turned the compressor faster. It would need less horse power at the lower pressure thereby the same hp motor could turn it a bit faster. This is similar to single stage compressors that deliver more air at the lower pressure. Because the pressure is lower they can be turned faster, or more realistically have a slightly larger displacement per hp.
So now on to the rotary screw compressor. These seemed to come on big in popularity in the 1960’s. The difference is they work best fully loaded and can run like that at max load 24/7. So let’s say you have a large shop and all your machines work fine at 100 psi. You can actually connect the rotary screw up to your shop air pipe system and get away with out any tank or receiver. The rotary would feed 100 psi into your air lines all day long. There are no pulsations that have to be smoothed out, it is just a solid 100 psi pressure. Now what happens when you don’t need the amount of air that the rotary puts out. It modulates the air. It will block off the intake in a proportional amount, restricting the compressor output. This works pretty seamlessly. You would not even notice. It is not that the compressor stops pumping at 100 psi and resumes at 80 or 90, it is a seamless modulation that starts at 100 and keeps it at 100. No matter how much air you are using the compressor will always keep 100 psi in the lines. Unless of course you use more then it can deliver. This sounds perfect and it is except for one little detail. Energy use. Back in the 60’s energy use wasn’t a big factor but that has all changed. Now the epa or other government agencies don’t want all the energy waste. You see the problem is that when the rotary screw compressor modulates it doesn’t cut the energy as much as you would expect. If the rotary is only producing 40% of the air it could supply it is still using 80% of the energy. The reciprocating compressors use only about 25% of the energy when they are idle. But with the reciprocating compressor you get wide swings in the air pressure. Ten to 20 or 30 pounds difference between high and low. This is why you need big air tank receivers to store air at a higher pressure then you really need. If you don’t do that all the machines and people running them would experience fluctuations that would be annoying and effect the quality of the work. With the rotary the plant gets 100 psi with no fluctuations and everyone is happy.
To combat the waste in energy now they have variable speed rotaries which are suppose to save electricity by modulating the compressor through the speed of the electric motor. But from what I have read there are some drawbacks to that and this is what they are advising now. Figure out your plants air requirements and buy a regular plain rotary to supply 80 percent of that and then buy a variable screw for the last 20 percent. This makes the variable screw a smaller compressor and it is modulating the last portion of your air requirements. The article I read stated that the variable screw uses a bit more energy at max load then the plain jane ones and they also cost more. So use a big standard one and a smaller variable.
All this is interesting but I realize this is not that important to the person running one of these on single phase. I would figure that this person all ready has smaller air compressor, probably a single phase five horse unit. They have gotten by with that for years but now would like more air to accomplish some specific task. So the question they are asking is how to get this more air with out three phase power. If they are sandblasting then a common solution is a diesel unit. Sandblasting is one of the most intensive uses of air that a home shop is going to encounter. There is nothing wrong with diesel but you have the upfront expense and the continuing expense of maintenance and the operating cost of the fuel. Running a large electric compressor eliminates the diesel fumes, noise, and fuel costs. Plus electric power is there, in all the quantity you need, you don’t have to make a run up to the fuel station for more during the middle of a job. So which is better? I would have to say, now that I have gotten use to the rotary screw compressor, that is the one I would use. I would try to get the nozzle sized right so that the compressor would run at max load continuously. I would not be worried about the additional energy use when the compressor idles. Look at it this way; you start blasting and you are charging money to do this or it is worth money to you to do this. The rotary screw gives you more air and that allows you to cover more area. The only time this compressor is going to spend time idling is when you re position parts or refill the media tank. That is inconsequential in the grand scheme of things. At ten cents an hour for electricity here in Idaho the cost of running the 30 is about $2.76 an hour. Hopefully you are charging more then that to sandblast something.
So where does a big reciprocating compressor fit in? I can see that if you have a shop and there is a machine or a couple of machines that use a large quantity of air at various times during the day then the reciprocating one would be ideal. It could be turned on in the morning and as long as you are vigilant about leaks it may not run for a few hours. But when you use the bigger air hungry machines it is there and ready to go. Maybe it runs for a few times here and there through out the day. It can work as a back up for your 5 hp that you normally run. And if you were going to do something that was air intensive then just flip a switch and put it on the load/unload setting while you are using that amount of air. Go back to shut off mod when done. Maintenance is simpler with the recip compressor. Three to four quarts of oil and a change of the intake air filters is all you need. The rotary is going to need $125 worth of oil and some expensive $37 filters. Plus the usual intake air filter.