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Thread: Air compressor questions

  1. #1
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    Air compressor questions

    I just had the motor on my 20 year old compressor go DOA. It's a 20 Gal tank with an approx. 1 hp motor and max PSI of 135. The motor is no longer available through the manufacturer. I replaced it with a Dewalt 15 gal compressor with a 1.6 hp motor with max PSI of 225. Both comps are rated at approx 5 SCFM at 90 psi. The price at the blue box store right now made it not worth trying to source my own motor for the old one. I would have liked to go with a full size upright, but the funds are not available right now.

    My first question is, as PSI increases does the actual volume of free air being held increase in a 1 to 1 ratio? Meaning if a tank is holding 200 psi would it have taken in twice the ambient air as a same size tank holding 100 psi given the ambient conditions were identical?

    My second question is, the tank on my old compressor appears to be good from the outside, however I have not done an internal inspection. I thought about removing the motor and turning it into a portable storage tank to take on the go. Would it be of any benefit to add it after the compressor to increase the storage of my shop system? Obviously I would have to regulate the PSI down to 135 before the tank. The lines in my shop are only rated at 150 PSI so I have to regulate the new comp down anyway. The old comp was adequate for what I was doing, however I would like to start spraying in the near future.

    Thanks
    Ben
    I've got lots of practice at making firewood!

  2. #2
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    Ben
    The answer to the first question is no. At least the way you asked it. I think you're actually asking if there is more "available" air in the tank, and the answer is yes. The volume of the compressed gas reduces, but the available storage volume stays the same. Ergo, there is more air "available" in the tank. I don't know that it doubles though, as the temperature inside the tank during compression will not remain constant. It can't.
    Boyle's Law describes the relationship between the pressure and volume of an enclosed gas when Temperature remains constant. ... Since pressure x volume remains constant, for example, doubling the pressure on an enclosed gas will reduce its volume to 1/2 its previous size.

    The answer for your second question is absolutely yes.
    Add your old tank in series with your new tank,and before the first system regulator, as a second receiver tank. This will provide you two large volumes for mechanical moisture separation. It will also give you" reserve volume", but it's truer value would be removing moisture from your system.
    Mechanical moisture separation occurs whenever the pressure drop in an expanding gas. The first tank gets hot compressed air from the compressor. As that air enters the first tank it expands. It has to give up an amount of moisture to expand. That is why moisture condenses in the tank. Every time there is an orifice restriction, or an elbow, there will be a differential pressure across them. This removes moisture. Adding the second receiver tank will perform this same function, but on a larger scale.
    Bottom line; Your old compressor tank is very valuable to you. Install it as a permanent component of your system.
    Last edited by Mike Cutler; 07-11-2019 at 8:47 PM.
    "The first thing you need to know, will likely be the last thing you learn." (Unknown)

  3. #3
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    For what it's worth, I recently learned that the local fire extinguisher inspection company can test other kinds of pressure tanks, including SCUBA and air compressor tanks.

  4. #4
    With all respect, I have to disagree with some of the previous comments. Rather than invoking Boyle's law, what gives a better answer to Ben's first question is the ideal gas equation: PV=nRT. n is the number of moles (amount) of gas, R is a constant, and T is the gas temperature. The volume of the tank (and the air inside) does not change- 20 gal. for this compressor. The temp of the air will increase, since the work required to compress the air is converted to heat (conservation of energy), but in a short time the tank will lose the heat to the surroundings. At that point, since V, R, and T are constant, the air pressure is directly proportional to n, the amount of air in the tank. So, a 20 gal tank at 200 psi will contain twice the amount of air as one at 100 psi.

    As far as moisture condensation, strictly speaking it's not correct that air "has to give up an amount of moisture to expand". Rather, it's the reverse. The air will expand as it leaves the tank, regardless of the moisture content. However, as it expands, it cools, and since cold air cannot contain as much water vapor as warm air, water vapor will condense. Sorry for the rant- it's the chemistry teacher in me rearing its ugly head.


  5. #5
    If you decide to use you existing tank for additional capacity be sure to leave the safety valve in place. You may also want to consider mounting your old tank at the far end or middle of your system.

  6. #6
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    Would it be of any benefit to add it after the compressor to increase the storage of my shop system?
    Some - but - remember as you add capacity, you also add run time to your pump.
    That DeWalt only has a 50% duty cycle - it should only be running 1/2 hour out of every hour. ( I looked it up online in the manual - they say it can run continuously, but, you should limit the run time to 50 -75% for longest pump life - which more or less translates to - "we cover our butts when we tell you what you want to hear"...



    Personally - I wouldn't go to the expense or trouble of turning the old tank into a portable. I bought a portable tank & it just collects dust. If I need or want just a small amount of air for something, I picked up one of those cheap 1 to 3 gal tank compressors they sell for about $40/$50.
    My granddad always said, :As one door closes, another opens".
    Wonderful man, terrible cabinet maker...

  7. #7
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    Quote Originally Posted by Rich Engelhardt View Post

    Personally - I wouldn't go to the expense or trouble of turning the old tank into a portable. I bought a portable tank & it just collects dust. If I need or want just a small amount of air for something, I picked up one of those cheap 1 to 3 gal tank compressors they sell for about $40/$50.
    Unless the reason Ben needs the air is a quarter mile from the nearest electric outlet. The tank may not be big enough but then again it may.

  8. #8
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    John

    Thank you for the more complete, correct ,explanation. You stated it much better than I what is happening. We are both in agreement, you just explained it better than I. I tried to keep it simple. Looks as if I failed.
    I have been repairing, maintaining, installing, and setting up industrial air compressors/systems for over 30 years. There is a lot more to it than people may realize.I read alot of incorrect information regarding the subject.
    When you referred to the changing of pressure as "cooling", that was a "little different" for me to hear it expressed that way, but that is what is happening. It is also what I refer to as "mechanical separation" (Using the physical design of a system to remove as much excess moisture as possible before using dryers, dessicant filters, moisture separators, and A/C units.)
    Once again, Thank you for the more succinct explanation.
    "The first thing you need to know, will likely be the last thing you learn." (Unknown)

  9. #9
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    Ohio
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    Thanks for your answers. I didn't take temp into consideration when I initially ran it through my head.
    I've got lots of practice at making firewood!

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