Originally Posted by
Matt Day
I’m in the same boat, looking for a bigger compressor. Could someone walk us through what else is needed? Dryers, water separators, auto-drains, regulators, etc. I’d like to know how many and where they should be placed. What type of piping should be used? I know there are alternatives to copper. And what type of air fittings? I know there are million standard, hi-flo, etc and they aren’t all compatible.
Matt
Any air system, regardless of size, has the same requirements. You want clean, moisture free, air at the outlet. Depending on how much demand on the system there is, and how often it is actually used, will go a large way to determining exactly what devices will be needed, where they will be placed, and how many.
If you were to be running a spray booth 6-7 hours a day, versus you just need the occasional air, as the OP does ,to blow down his equipment and then the odd HVLP use, your requirements are vastly different.
When the compressor is running it is taking in the ambient air, at what ever moisture content and temperature it is, at the inlet of the compressor. As the compressor "compresses", the temperature is raised, sometimes pretty high, over 200 degF. and the air now being compressed has a moisture content for a given volume greater than it was at ambient temperature and pressure.
Out of the compressor the newly compressed, high moisture content air goes into the receiver tank. While the receiver tank does have a "reserve" volume associated with it, it's truer purposes is to allow the now compressed, high temperature, air a place to expand and drop the pressure. When this happens, the moisture in the compressed air, entering the receiver tank, has to come out, for the given volume as it expands. This is physics. The moisture content will ultimately be dictated by the ambient temperature of the tank and the pressure inside the tank. The removal of moisture in the air, via this process, is the first stage of "mechanical separation" to remove moisture from air. It is for this reason that the receiver tank needs to be "blown down" at periodic intervals to rid the tank of the moisture, now in the form of actual water, that is accumlating at the bottom of the tank. An " Auto Drain" can be used, or it can be manually performed, by opening the small petcock valve at the bottom of the tank. The more demand on a system, the more often this needs to be done.
If you were to be doing it manually you would observe that moisture will continue to come out the bottom of the tank for days following a compressor cycling, even if the air was not even used. As the ambient temperature of the tank decreases, moisture will continue to come out, until the moisture content of the air, inside the receiver tank reaches equilibrium with it's environment. This is why sizing the tank on a compressor is important. Not so much for the "reserve volume of say 60 gallons to 80 gallons, but the cubic feet of the tank internal. Lower usage, would equate to a smaller required tank, larger usage, the tank needs to be bigger.
The piping for an air system does a few things. Obviously it delivers to the air to point of use, but it also controls the pressure and flow of air within the system. The larger the piping, the more flow, but less pressure, the smaller the piping, the inverse. Pipe sizing will change in a system of any size to accommodate both of these requirements. The piping will also mechanically separate even more moisture from air. As the air moved through the piping, the pressure drops and volumes can expand.
A small shop piping system should be setup using a "loop". Two branch headers will ultimately meet in the "middle". Along these two branch headers will be the "drops" for point of use. The branch headers in a large system, will be much larger than a small system for flow requirements, and in the small shop setup they could actually be the same size. This "loop" performs a few functions. Firstly it mitigates the pressure drop across the system from many single point of use and the air receiver tank. It also reduces the penalty associated with any elbows, or tee's installed in the system to oppose flow.
The "drops", the lines coming off the branch header, are where you will access the system. Typically, there will be a Tee a few inches from the bottom of the air outlet to act as a condensation drip leg. You can install valves at these points, or not. It's up to you and how the regulators are positioned in the system.
Filters and regulators can be installed in a few different strategies. One is at the point of use, and the other is for the whole system. I like to have them both.
You first regulator in a system needs to be 25' of piping or greater away from the receiver tank outlet. The air at this regulator will first go through a "coalescing" filter. This filter removes some moisture,form the air, but is primarily to rid the system of oils, in the form of vapor. It removes moisture also.
Why the 25' pf piping? This is enough to give the air coming out of the receiver tank enough volume to allow even more moisture to me mechanically separated as it expands through the piping.
You will often see these sections of piping connected to the receiver tank and that form a" labyrinth" up the wall, behind a compressor. Generally there will be two or three legs, with drip legs and valves at the bottom of each leg. People will often say that these legs "cool" the air, but that's not really the case. In use, an individual molecule of air is moving way to fast to be cooled by such a short section of piping. What is happening here is, again, mechanical separation of moisture.
As each one of the legs has to be connected, with 90 degree elbows, top and bottom. These legs form a "resistance" to air flow. It's measured in hundredth's of an inch of water, but it is there. Each elbow has a pressure drop across it. The air pressure going into the elbow, will be higher than the air pressure exiting the elbow. At these points, moisture has to come out of the air due to the change in pressure, and those drip legs will collect it. That's why you need a valve the bottom of each drip leg to remove the accumulated moisture. You will be surprised at just how much moisture, two or three legs of piping will be removed. These legs though should be of a larger diameter. You want flow, at a reduced pressure, to aid in mechanically separating the moisture from the air.
So, in my "garage system". I have a compressor and receiver tank, that then go in to the "labyrinth", and them into a 100' coils of 3/4" tubing along the wall and then to the first filter/regulator. I probably have 150' of piping between the compressor and the first regulator, even though they are only about 15' away from each other across the garage floor.
I also have a blow down valve just prior to the the first filter. My compressor is rated for 17cfm, the filter and regulator I have installed are rated for over 70cfm. This regulator is my system regulator.I t's set for 125psi. I have no losses, or penalties, from this regulator and filter.
I have two points of service in the shop. Each has it's own desiccant filter and regulator attached to it. These are adjustable, but the CFM rating is still higher than the compressor. There is also another at the front of the garage, between the two doors for air tool use. This meets my needs. I get dry air out of each point of service. The regulators in the shop are adjustable. This would be a "basic" system.
I do not use a lot of air continuously, which is why I can have such a basic system. I may go weeks without using any air at all. Then get into some car work and that compressor will cycle multiple times per day, and sometimes hour. Because I have these long "dead times", I can mechanically separate the majority of air from my system and some simple desiccant cartridge filters will suffice. I have a shutoff valve at the receiver tank outlet and at the exit of the "labyrinth" of piping, going into the main header.
When not in use, the tank outlet is shut, and the system branches are depressurized. The receiver tank is not depressurized, and this is a source of debate. Many people will depressurize their entire system when not in use, sometimes each night. I personally do not understand this, from anything other than a safety point of view. Having to re-pressurize that receiver tank only starts the whole process of mechanical moisture separation over again, basically starting out with a saturated system. All of the air is now moisture laden again and any air in the system now has moisture, in the form of vapor, moving through it. The system is in essence "saturated". Filters are not going to remove vapor, they can't. This is why I don't depressurize the receiver tank. It's already done it's job, why make it do it again, especially just prior to use?
Now, if you believe that painting and spraying finishes is in your future, you're going to need some more equipment. At the point of use you want a dedicated filter/regulator and you want a nice one. Some folks will cool the air here with an air cooler. Some will leave a valve cracked to educate air and spit it out, some will put 100' of copper tubing in a bucket of ice to cool the air in the system piping, and as long as the point of use does not exceed the compressor rating, of the volume of air in the tank and piping will be sufficient. If the com requirements are exceeded for a given length of tim the system as whole will move toward the point of saturation, and given long enough demand times, eventually moisture is going to be present at the sprayer. Tsunami makes some systems that accommodate this, but they are wicked expensive, and if the system does become saturated, even they won't work.
Given all of this the system needs to be sized to accommodate the use. It's a balance and sometimes just a pure guess, but there are some fundamentals that have to be followed, because the physics is not going to change.
As for materials; There are so many choices. I like copper, but it can be very expensive these days.I don't have any issues with plastic tubing, but if you live in a area where you get hard freezes, it may not be the best material choice. For a period of time I had a second receiver tank, remotely, connected with garden hose material rated for 200psi. I was blasting and grinding the frame of a truck. I think that garden hose is now in use at the barn??
BTW
Following what used to be known as the "Addy protocol", so that you know I do have some experience in this field. I have been installing, repairing and maintaining large capacity, industrial facility, air systems, for over 35 years in nuclear power plants. I do wear the Brooks Brothers "outfits" to work now, But I spent many years "banging air".
Last edited by Mike Cutler; 08-11-2018 at 9:02 AM.
"The first thing you need to know, will likely be the last thing you learn." (Unknown)
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I did change the intake filter to quiet it down.
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