When I’m finished using my small compressor I release most of the air out of the tank then I open the drain valve in the bottom and tip it so the valve is straight down and let the rest of the air out, hopefully pushing any moisture out with it. My question is, is it better to leave the drain valve open or closed until the next time I use it? My thinking (which is usually pretty warped) is that if the drain valve is left open any moisture in the tank could evaporate out but room air can get in carrying moisture with it. If the drain valve is left closed, room air can’t get in but any moisture left in the tank is trapped there. So, which do you think is better?
If the compressor will be stored indoors, say a dryish basement where the temps and humidity ranges are held within a restricted range, then I would leave the valve open. If the basement conditions, especially the humidity, change dramatically throughout the year, or if outdoors, I would close the valve.
We have our heating oil tank inside the basement. We get strange looks, but our fuel is always free of moisture. Replacement air for the tank comes from the basement, and I always keep the basement below 66% humidity.
Crandell
Bob,
If you are going to store the compressor indoors, it should make no difference if the valve is open or closed. I have a large compressor/tank in the garage(insulated & heated). I always drain the moisture blow-off while the compressor is running to help move any moisture out of the tank. The blow-off is in a spring-loaded toggle. The compressor is over 30 years old(was my Dad’s) and I have had no problem with rust or air leaks.
Jim
Hummmmm, I would think that you are actually supposed to have a vent to the outside near the fill connection as per: http://inspectapedia.com/oiltanks/oilpiping.htm. Our tank is also in the basement and we have an outside vent.
The first house we lived in had an oil tank in the basement and it had a vent to the outside. It whistled as the air from the tank exited during filling and when the whistling stopped, the person filling the tank knew to stop pumping oil into it.
If its a small pancake type compressor that is easily moved by hand you could spray some WD40 in through the drain valve and move it around. When it comes time to fire it back up just leave the drain open for awhile to expel most of the WD40 and put a moisture trap/ filter on the discharge line. Also cover the air intake filter with plastic wrap. I have done this for over thirty years and never get rusty water to drain like other compressors I have seen.
The large compressors at work have to be inspected by an official of the state every other year. They have air driers, auto lubes, auto drains and still spit out rusty water. One pressure vessel was taken out of service and replaced due to rust internally. A 250 gallon pressure tank is very expensive to replace, especially when the room was built after it was in place.
Pete
Have used various portable compressors (Emglow, Hitachi, Cambel Hausfeld and Craftsman) for quite a no. of years for all sorts of nail guns. I realize that my units are seeing continuous daily use, almost abuse in some instances, due to running in extremes of wet, cold and high humidity. Moisture traps are used exclusively and tanks are drained after each daily use. I have found that after draining the valve needs to be closed. I though that if the valve was left open that the interior of the tank would totally dry out. Not really the case a few times debris, rust etc has caused the valve to leak. Opening and closing a couple times under pressure dislodged particles, etc and would seat. I feel that if left for extended periods the crud can harden on the seat and the valve will need to be replaced as it will continue to seep air. Have replaced many of these, only once due to leakage as described the other times they were worn and/ or damaged.
Maxman and Seamonster, I should have gone to take a look. My mental picture had a vent on the top of the tank, but in fact it has the outside vent with whistle. [:S] I think what I had in mind was that with the tank in a stable environment, it would not act as a bellows with shrinking and expanding volumes due to high temps. With the outside vent, some moist air will enter as the volume is drawn down due to burner usage. However, it doesn’t seem to present a problem…ever.
I have a 5 gallon compessor, a model that must be 45 years old or more, with a small motor, pulleys, a belt, and a thumper cylinder for the compressor. It has a twin-pronged bleeder valve on a small stem low on the underside of the end away from the axle and wheels. I only open it at the end of an operation while there is still some air pressure. I ensure it is loest of all parts of the tank to get the condensate, if any, to accumulate near the stem and get sucked out. I also only use it outdoors in the fall to evacuate the outside hose feeder lines to the garden and to the outdoor shower. I have never had much water ejected into my hand, which I leave under the opened valve to see what the result may be. Maybe a single drop or two, but nothing like a teaspoon or more.
Sorry for my error earlier.
Crandell
Not a problem. Just thought I’d mention it as I had this vision of someone filling your basement with oil.
I have a small Campbell-Hausfeld two gallon compressor I bought about sixteen years ago. When I’m finished using it I just pull the pin to bleed off the air pressure. I’ve never had a problem with it. It frequently sits for long periods without use.
Thanks to everyone for your advice. I usually leave the valve open between uses (and wonder why it won’t pump up pressure when I turn it on [:$] ) but I kept wondering which is better. Since I keep it in the basement which has about the same temperature and humidity as the upstairs I’ll keep leaving the valve open now (and try to remember to close it before I turn it on!). I have sometimes held my hand under the valve when I open it and occasionally get a small drop of moisture on my hand, nothing significant.
Crandell:
My understanding of the potential problem with rust in a fuel oil tank is that the condensation will collect in the bottom of the tank. The fuel oil, being lighter than water, will stay on top. That creates a possibility (likelyhood?) that the bottom of the tank will rust out. Being a fellow Canadian you are likely aware that our home insurance companies are quite proactive in forcing homeowners to replace aging tanks. The damage done by a fuel leak inside the house can be bad enough but if your tank is outside and it leaks, you are responsible for all the related environmental cleanup costs which, for a big spill, can run into the tens of thousands of dollars. Even an inside spill that goes down the floor drain can wreak havoc if the oil ends up in the storm sewers.
As far as compressor condensation goes, I have a moisture filter on my small pancake unit and I have never seen any moisture in it. However, there is an obvious sign of moisture present inside the tank when I open the release valve. It will blow for a few seconds and then freeze up, stopping the flow of air temporarily until the frost thaws, and then it will blow…freeze up…blow… and so on. Obviously there has to be moisture present for frost to form. My practise is to leave the valve open after the pressure is released because the reduction in temperature caused by decompression will chill the tank and that will lead to condensation staying on the inside of the tank even after decompression. Makes sense to me anyhow to let it dry out through evaporation.
Dave
I have a 7 gallon compressor I keep in the kitchen {where else in a trailer with little storage?}.
I keep the valve open when not in use. I figured the same thought…it could dry out between usings then.
If I am wrong for doing it that way, i see I am not alone!
The only problem I do have is as mentioned, when I turn it on, i foget to tighten the valve!
It can stand unsued for several months and is now running on a few years of inactive use!
[8-|]
Most of the water is generated by the heating and the cooling of the compressed air by the compressor itself. Larger the compressor and tank, more water made, more extreme temps the compressor sits in, more water. More frequently it runs, more water. I doubt if a water fllter between the compressor and tank would make a difference. They are designed to be used after the tank to keep water out of the air you are using. Smaller home air compressors under usual use, rarely build up enough water to create any issues. Unless you use them extremely, as in construction, where the thing is just running all the time, heating up and cooling down, you should be ok with just opening the valve after use and closing after the water, if any, quits coming out even if there is still pressure in the tank. If you are going to let the compressor sit for a while or you just feel better draining it completely, you may still close the valve when done and suffer no ill effects. Again, generally. There are exceptions, but I doubt they apply to you or most of us.
My leaving the valve open or closed depends on when the moisture in the tank stops. If the moisture stops and the air is still under pressure and coming out pretty good, the escaping air is usually sufficient to “air dry” the moisture out of the bottom of the tank as it blows by, so then I will close the valve. If I’ve used it a lot, and the temps are extreme (no temp controlled garage for me) and on the rare occasion water still comes out after the tank is depressurized, I will leave the valve open. Does that make a difference? oh I don’t know. I guess water would evaporate out as quick as it would in a cup. Main reason is to let the water drain out and then only if I don’t have time to pressurize the tank and drain again which is what I usually do on my 34 year old compressor. But that usually only happens once in a blue moon. If you choose to leave the tank open, it’s not gonna hurt anything either.
My portable one I drain the air and leave the valve open for a few days then close it, if I remember to do it. My big one in the garage I drain the air and then close it back up. I’ve done it both ways for 15+ years with no problems with either compressor.
The amount of air that can exchange through a small drain is negligible and any moisture that goes in with it, will eventually condense and drain out. So, leave the drain open when your compressor is sitting idle.
I’m of Jeffrey’s ilk. My system has a 50’ hose from the basement to the garage and has a slow leak somewhere, so the pressure/air drains in about 24 hrs, so I don’t even bother with the air/water release. Every now and then I check the water drain, but I’ve never had anything but air come out of it. The moisture trap at the airbrush has never shown any moisture. My system is about 20 years old.
It is pretty common for condensation to occur when compressing air; so, I think draining the tank down is a good idea. If your not getting condensate, that’s a good thing. However, watching for it does make sense. Because the process of compressing air does produce condensates, most compressor companies expect moisture to be produced and use coatings on the inside of the tank to counteract corrosion.
My understanding of the water/rust problem is not that the tank will rust through, but that the rust particles may get picked up when the compressor is in use.
In my youth I owned a '68 MGB (I still miss that car) and being British everything rusted on it including the gas tank. Despite the use of a fuel filter very fine grains of rust would end up in the bottom of the carburator bowels. It didn’t seem to cause any problems but if the same stuff was pumped into your airbrush unpleasant things might happen.
Dave