Air system tech questions

First of all I am going to run 3/4 heavy copper lines & 1/2" drops, probably 4. Does it make sense to put a 3/4" water trap & coalaesing filter in the main 3/4" feed before it even makes it to the main grid that the 1/2" lines run off? And then have another filter/water setup and regulator on the 1/2" drop(s) for painting, plasma, blasting, etc? Seems like it would then be higher quality air and you could catch most of the junk before it reaches the 3/4" grid. Also on this note; Would this greatly reduce flow?

2nd question;

When I was putting the new ck valve/unloader in the tank, I noticed it had a 1/2" supply tube from the second stage to the tank. (air supply to tank manifold) The 2nd stage head threads are bushed down from 3/4 to 1/2" and so is the tank where the ck valve/unloader screws in. Its bushed from 1" on the tank to 3/4" for the ck valve to screw in. Seems to me this should be a straight 3/4" supply pipe if thats the size on the threads on the head and a 1" thread on the tank? I am just trying to figure out if this is a stock arrangement or an older quick fix. Or would it even make any difference if it was a 3/4 or 1/2" supply tube? I noticed that there are 3- 3/8" tubes with fins running from the 1st stage to the second stage. I guess, I.D wise that would equal a 3/4" feed to the tank.

What do you think about this?

Thank You,

scott#2

I see no great advantage to adding any water trap or filter prior to the main header. Water vapor in the warm air condenses into water droplets when cooled. A lot of that occurs in the tank itself. More occurs in the header as the air cools further and more moisture condenses out. Neither a trap or a filter will remove water in vapor form. Besides, the drops should be taken off the top of the header and there should be a drop (not taken off the top of the header) at the end of the header. There should be no station on this last drop - it is to collect water that has condensed in the header, flowed into and collected in that drop and removed periodically with a drain valve at the bottom. I think that should show up in one of those diagrams that you linked to in your last post. All station drops should have there own filters and regulators. Sometimes those are provided in a combined unit that has a separator, a filter and a regulator all in one assembly.

The half inch line you refer to between the compressor second stage head and the tank is very common. That is the highest pressure section and therefore the line can be smaller for the same amount of flow. It is also relatively short so the actual pressure drop will not be great. My 17cfm 2 stage has a 1/2" (5/8 OD) line there and it looks like the tapping in the head is also 1/2 NPT - that's the discharge side of the head. The line between the two stages should be a size larger because the pressure is lower there. I don't think you would see any noticeable difference, from a practical standpoint if you were to replace that 1/2" line with a 3/4" one. It would take four 3/8" lines to equal one 3/4" one since area is proportional to diameter squared (twice the diameter = 4xthe area). Those finned tube sections between the first and second stages act as an intercooler to decrease the temperature going into the second stage. That acts to decrease the total discharge temperature from the compressor and increase the overall power consumption efficiency a bit over a single stage unit. The three 3/8" lines provide almost twice the area as a single 1/2" line. That appears reasonable since the pressure is lower between the stages and a larger pipe size would be appropriate. That interstage piping is usually a single line but there is no reason why mulitple smaller lines couldn't do the job just as well.

The reason for a 3/4" header is that it is usually much longer than any other section of piping and the size should be bumped up a bit to lower the pressure drop because of that, especially for single stage systems where the end use pressure is closer to the maximum tank pressure. However, if you were to have only a single drop reasonably close to the compressor - same building wall for instance - the 3/4" would not be necessary and 1/2 would be satisfactory between the tank tapping and the station filter/regulator. If you are running a 175 psig two-stage system and don't normally use anything much over a 100 psig (as I do) then the pressure drop issues upstream of the filter/regulator are not of great importance. You should concentrate more on what happens downstream of the regulator. That's where your biggest pressure drop problems usually occur. Mostly because of the lower pressure in that section and the smaller hose sizes, plus fittings, quick couplers, etc.

Rod

scott#2

Don't sell yourself short. It's not a matter of smart/not smart. It's much more a matter of scholastic training and being professionally involved with similar matters for many years. Plus, even experts disagree. Some will tell you that you need to pitch the main header back towards the tank. I can assure you that no professional designer worth anything would ever design an air system for pay that way. Not that it wouldn't work. It's just poor practice and not as good. Whenever you have the possibility of two-phase flow, such as in an air distribution system, the headers should ways be pitched down in the direction of air flow. You can understand such thinking intuitively also. When pitched down in the direction of flow, the air flow itself assists gravity drainage of the accumulated water to low point drains. If pitched back towards the tank, the air flow is opposing and therefore helps to restrict drainage of the condensate - a poor practice, even though it will work in most cases.

Rod