Auxiliary Air Tank Question

[Train]

I have a large compressor in my shop, and recently got ahold of a tank that's missing the compressor. The new tank is about a third the size of my compressor and has wheels. I bought it as a portable tank for filling tires around the farm. I made sense to me to keep it plugged into my main airline to also act as an auxiliary tank. Then I got to thinking. The compressor will go longer between runs, but it will have to run longer to pressurize both tanks to cut out pressure. So what is the actual advantage to the compressor?

 

[BORING HOP YARD]

Fewer starts and stops for one thing. I would rather have my electric motor run an extra amount of time then extra start up. I would assume it the same theory as a pump for a water well.

 

[pancho400cid]

Keep thinking. You'll eventually realize there is no advantage what-so-ever assuming you had a correctly sized accumulator to start with.

I'll probably get flamed by people who have not thought about it long enough... but the motor has a duty cycle, and the compressor pump has a duty cycle. Whoever designed your compressor had any eye on both when your accumulator was sized the first time. It's true the motor will start/stop more often, but the compressor pump cools more during the downtime.




....

 

[nadogail]

IMHO, Both Train & pancho are equally right.

 

[sberry]

I run mine continuous when doing longer jobs. It makes it easier to time to make it run steady. Ideally is to have a pump that meets the largest continuous run.
I use a 3 hp 2 stage. It's something I had for a long time and I am on rural power, no light blink, it's super easy start. I have 120 extra tanks on it, have 5 hp with 100 gallon on as a slave with the breaker off. My helper had to wait a few mins cleaning some rims, rare and irrelevant. If i wanted to buy new and wanted to do it on one unit would be 5 hp on 80. Let's a guy 6 inch da or 4 inch wire wheel as steady as he wants to work.

 

[like2wheel]

I'm with Pancho on this.

People seem to think they get more volume & can do more by adding extra storage, when it's really all about cfm.



.

 

[imjustdave]

It can help when you have a long run to have air at the end to help with big instant loads like a impact gun. on a small hose. deliver big volume quickly. but it will be short lived.

and it's handy when you need some portable air. I see nothing wrong with it it has limitations and bonuses you just have to make sure it helps in the situation your in.

 

[jubilee]

I run two tanks on the service truck ( 35 cfm total 22 gallon storage) and two tanks in garage (27 cfm total 140 gallon storage) mainly to have a “ wet” and “ dry” tank combination.

 

[The Tool Tyrant]

It's no different than a car driven 100,000 city miles vs. a car driven 100,000 freeway miles. Ask any electric motor guru and they will tell you that motor starts are harder on a motor (and capacitors if single phase) than a continuously run motor.

I'm in the 'more storage' camp. I have 360 gallons of storage on my 5 HP Quincy QR25-325 pump and takes care of a 5 man fab shop including 2 CNC machines without issue and has never left us wanting for air.

Just be certain that your 'portable' receiver is rated to the maximum pressure you'll be filling it to.

 

[The Tool Tyrant]

People seem to think they get more volume & can do more by adding extra storage, when it's really all about cfm.


.

No, you are incorrect. Another analogy for you...if you add a 100 gallon fuel tank to your truck, you WILL get many more miles between fill-ups, BUT your time filling up will take longer.
CFM= how long it takes your pump to fill your receiver.
Storage capacity= how long you can operate a tool before your compressor cycles...more storage=more usable time.

 

[TonyJ]

No, you are incorrect. Another analogy for you...if you add a 100 gallon fuel tank to your truck, you WILL get many more miles between fill-ups, BUT your time filling up will take longer.
CFM= how long it takes your pump to fill your receiver.
Storage capacity= how long you can operate a tool before your compressor cycles...more storage=more usable time.

I’m in the more storage camp too but mine is for the same but different reason as others have talked about and it could be something that many doesn’t even consider when adding or needing more storage.

If a person has a mid teen to say 20 cfm compressor then their pumps are going to be running allot to keep up and adding more storage isn’t going to be beneficial to them because of pump heat and motor heat and never really catching up but instead staying even.

Most people that will benefit from extra storage is those ppl like me that have pumps that will run circles around the operator and cycle like crazy and not run very long to even get the pump up to temperature to keep moisture out of the pump oil and as mentioned the short cycle high amps being pulled every few minutes

My problem is like many people with say a 80 gal tank I can use air and the pump will kick on and within 28 seconds it’s pumped up cut off and all while I’m still using air it kicks on again and 28 secs later kicks off and over and over again. I’ll be adding a second tank (80 gal) or replace the main tank with one around 200gal if I can find one reasonable enough that way I can use air for 2-3 Times longer and only still be having the pump run for around a minute

 

[matt_i]

The only difference I notice for incrementally more storage volume is that there is a longer delay before the compressor runs for the first time (when drawing air, starting with a "charged" setup).

But I can tell that the motor also runs a longer time to recharge the same volume.

In analogy I wait longer to start spending money but then spend more money so which is cheaper?

Its sort of a theoretical question as to what is the weaker link in a single phase system, a motor-starting pressure switch or the centrifugal start-capacitor switch on a single phase motor. I don't know. If a person could use a 3phase motor and a contactor then these issues get an order of magnitude farther away or eliminated (no start cap in 3ph).

 

[TonyJ]

The only difference I notice for incrementally more storage volume is that there is a longer delay before the compressor runs for the first time (when drawing air, starting with a "charged" setup).



But I can tell that the motor also runs a longer time to recharge the same volume.



In analogy I wait longer to start spending money but then spend more money so which is cheaper?



Its sort of a theoretical question as to what is the weaker link in a single phase system, a motor-starting pressure switch or the centrifugal start-capacitor switch on a single phase motor. I don't know. If a person could use a 3phase motor and a contactor then these issues get an order of magnitude farther away or eliminated (no start cap in 3ph).

Don’t have to have a 3phase in order to use a magnetic starter and when using one you wire the pressure switch to only cut the power to the coil in the starter so the pressure switch contacts will usually outlast the rest of the switch. But the benefit comes into play say when you have a 80 gal and it kicks on and off 10 times a hour. That’s allot of times that your power service and all your wiring is seeing high start up amps then normal run amps. If you double your storage then you should in theory cut the starts and stops in half but the longer run times filling up is allot easier on the motor and your power service

 

[like2wheel]

I'm with Pancho on this.

People seem to think they get more volume & can do more by adding extra storage, when it's really all about cfm.



.

No, you are incorrect. Another analogy for you...if you add a 100 gallon fuel tank to your truck, you WILL get many more miles between fill-ups, BUT your time filling up will take longer.
CFM= how long it takes your pump to fill your receiver.
Storage capacity= how long you can operate a tool before your compressor cycles...more storage=more usable time.

Big deal, you get a little extra usable time from the first charge, but then you're just adding that extra time on the next refill. Your gas analogy is ridiculous when it takes hours to burn the gas, but only minutes to refill the tank. If you exceed the cfm of the compressor, the refill takes longer than the consumption. Imagine if it worked that way with your vehicle.

I'll say it again: it's all about cfm.
Unless you're just buzzing a tire off on a service call, you need more cfm than you consume regardless of receiver size.

Someone did have a point about reducing the number of motor startups tho.

 

[pancho400cid]

The bottom line is the storage needs to be sized "right"... There is such a thing as too small a capacity and too large.

Too small = cycles too frequently as mentioned... Most compressors start unloaded and are at full speed in literally a second or two. Starting duty is a factor in heating but less than you might think for small shop compressors - especially 5 HP or less.

Too large = motor and pump running at high load for long periods. Many compressors have motors with "special" ratings. Those motors are not intended to run continuously at rated horsepower. They may overheat if they are forced to (especially if ambient temp is high, like up around 40 C / 104 F). They are sized with/designed with downtime between cycles to help with cooling in mind. If your storage is too big the motor may run long enough at high load to overheat. I don't mean get roasted right then and there but insulation life is basically a function of the "area" under the time-temperature curve. The cooler a motor runs, the longer it lives generally. Frequently running hot shortens the life. The rule of thumb you hear kicked around is that a 10 Deg C decrease in average lifetime temperature doubles the insulation life.

 

[TonyJ]

I got my second tank hooked up last night and it greatly extended its off time and naturally doubled its run time to shut off but it being a 45cfm pump it still only took 59 seconds to shut off (140-175psi). I’m now running 2 80 gal tanks and hopefully one day I’ll find a 160 to 200 gal and I’ll go back to having one tank

 

[The Tool Tyrant]

"GREATLY EXTENDED" Exactly my point, these folks that have not experienced this have no idea what a difference adding storage capacity adds to your 'USEABLE AIR' supply.

Of course, if your use exceeds your pump output, at some point you'll run out of 'USEABLE AIR' but unless you're WFO (sandblasting) for an extended period of time, having the extra storage capacity allows the pump to refill while your use is idle.

I'm quite sure that if my QR25-325 had just it's OEM 80 gallon receiver, it would leave us 'wanting for air' at times of heavy use, but as I've stated, with 360 gallons of reserve, we have NEVER had to wait for the pump to catch up. This is fact, not an assumption.

 

[sberry]

Yes, refill while idle, let's a guy time manage to run continuous. Can resume with the pump running.

 

[imjustdave]

CFM needed to run device
X
Length of time needed.

If your pump can always meet this need then you don't need storage or much of it. Example screw compressor.

but most of us have intermittent demands and most single phase compressors are lacking true CFM delivery.

So having storage is nice, and will make a smaller compressor a little nicer.

Distance from the storage and line size also play a big role. Anyone who has used a 3/4 line VS 3/8 line knows that the cost is stupid but well worth it when you air gun needs a little more CFM to turn those nuts off.

 

[KenC]

One huge storage capacity advantage is having cool air available, especially when painting. It's an extreme example, but at one time I had a large propane tank behind the shop that wasn't in use and a small compressor.
Using that compressor to paint resulted in water in the air, as it never was allowed to cool and condense in the tank. I hooked it up to the big tank and that fixed the issue. Starting with a full tank at 125psi, I could spray an entire car without the pump running! Like I said, extreme example, but illustrates the point. Cool air is good. Any water vapor present condenses in the tank where it can be drained, not expelled through your tools or paint gun.

 

[strutaeng]

I'm with Pancho on this.

People seem to think they get more volume & can do more by adding extra storage, when it's really all about cfm.



.

I'm on this camp as well. Theoretically, if you have an infinitely large tank, you don't need a pump, right? But practically, adding a 30 gallon tank doesn't do anything, unless you are running the compressor continuously, which at that point the tank size is almost irrelevant.

For media blasting, needle scaler, die grinder, spray painting, DA sanding, "maybe" there's an advantage. Certainly not for filling tires and nailing.

You can argue both ways I suppose.

I've got a 30 gallon 7.5 HP with a Curtis ES-100 pump. Puts out something like 24CFM. I made it a while ago, but hardly ever use it, unless I'm using the HVLP gun.

I plumbed a pressure switch and a pilot valve so I can run it start-stop or continuous-run. For such a small tank and very short recovery time, start-stops would be an issue with electric motors, that's why I ran the continuous-run, but this is obviously an odd duck. I never have actually used the start-stop, as I have smaller compressors.

I recall about 10 years ago, it seemed that like EVERYBODY thought an air compressor with a huge tank was the way to go. Seemed like folks were paying attention to the "HP" rating on the tank and the size of the tank itself back then, not much attention to the CFM rating...until your pump wasn't able to keep up... Didn't Sears or some else have a 60 gallon model with a little 120V pump found on the 20/30 gallon units? Still had the same CFM rating as the little guys.

Duty cycles are typically 50% on homeowner units, but are higher for industrial units, if anyone cares. The only compressors that I can think of have a large tank with low CFM are the HVAC control compressors, but those things are designed to be run 24/7/365 forever! 30 gallon duplex with alternator setup. They are actually kind of cool.

OP never mentioned the compressor in question, aux. tank or what he's using it for, so we are just guessing here, LOL

 

[sberry]

I use a small pump as the main and large tanks. If its charged can use quiote a little before it even comes on but I like the longer run when sanding when I do it and really should have a gauge or clock so I can resume just prior to shut off while the tank is full and pump running.
If done on occasion can get the feel oif it, can pick up at any point but resume before cut off is ideal. Dont need any special controls. All operated in normal fashion.
I did a day or 2 sand last winter and meant to walk over and turn the demand on but the wait is all so minor and a guy really needs to stop and reposition, get new disk its not that big a deal to be underpowered a little. A lot would be too much, I am about 25% short.
With 5 hp a guy wouldnt have to manage much but the cycle is fast. My full cycle is 4 minutes. A 5 is ideal, while a 7.5 is more it will cycle. It would run steady under 2 continuous units but that is rather rare.
This is a point I was trying to make to the screw wanna type. If I was a 1 or 2 man show a 5 would be where I was to start and to simplify might add a second when the time came. While a 10K$ comp is better than a 2500$ one doesnt make it sized right. It even states this in earlier user guides/sales stuff.
3 hp mechanics used to be common but once 100A service became common the systems could tolerate a bit better comps for minor more money and the 3 was short for body work.
If I recall the op said he had a large comp with a fairly large tank and was wondering about efficiency and I say hook the fugger up and see how it works. One of the benefits of having a larger unit is you dont need to have all kinds of extra, auxiliary shate to make up for poor performance to start with. Done need to make a 4 banger race to so what a v8 does easy and there are a couple other analogies but ****,,,

 

[sberry]

But,,, you dont have an infinitely large tank and it has to be pumped no matter what size it is,,, tank doesnt make any air, simply allows for more stored energy and a longer time available to store it back. Just allows for smaller peaks and faster demand for short time.
Got a bud with a similar shop has 3 phase all he wants, had a single 10 hp unit with about 120 gallons, does not want to fuss with anything complicated and its not so monster it slams on for every chore. Got rather simple inch mains, couple stations with a couple branches added.

 

[sberry]

But,,, you dont have an infinitely large tank and it has to be pumped no matter what size it is,,, tank doesnt make any air, simply allows for more stored energy and a longer time available to store it back. Just allows for smaller peaks and faster demand for short time.
Got a bud with a similar shop has 3 phase all he wants, had a single 10 hp unit with about 120 gallons, does not want to fuss with anything complicated and its not so monster it slams on for every chore. Got rather simple inch mains, couple stations with a couple branches added.
Services trucks but not a truck service center. 90% + of the tool use like the rest of us, 1/2 impact, some sanders. Filling tires.
I remember the trucker here was wanting a comp,,, was fussing with some conversion ideas due to power but he could easily use 5 hp. While he has more and bigger than most people a tire R&R or air fill within a cycle of a 5/80 and may even fill as good or better than a larger unit, it will come on and be pushing it higher in the pressure range depending on reg pressure. Even the 3 hp 2 stage fills hi pressure truck tires well.

 

[sberry]

Had a bud fill a tire here the other day,,, said,, mine wont even do this and yours didnt even come on.
Technology has changes some things, this remains fairly constant. You can do work and can do the work with smaller units and battery has replaced some need. Some big powerful demand in impact, saves a lot of money too. But the bench mark is 5 hp 2 stage. Welding used to be 50A, now the majority of it can be done 30 but 50 is still a bench for existing equipment. 60A used to be the line and still is to some extent but 90 or 100 is considered full service for small garage/shop with basic equipment, mainly air comp and welder really.

 

[sberry]

Even lots of weld machine can be done with 100 for single or 2 user. All my real tools other than air, pressure wash and welders are 120V. Whoops, hoist 240.

 

[Dbovill21]

I have a 29 gallon compressor that puts out 7.5 c FM two 20 gallon compressors that put out 11.5 if I tie all three compressors into a 80 gal tank would I be able to maintain like 23 c f m to run a small sandblaster?

 

[PoorUB]

To the OP. If you main compressor is large enough to keep up with your demand the benifit is questionable. Sure the compressor will run less cycles but I doubt anyone will know the difference in the long run.

If the compressor is way to small you get a long run time on your air tool before you run out of air, but the recharge time *****.

I worked at a machine shop and we had two 10 HP compressors. We also had rarely used machine that when added to the rest of the demand the two compressors would not keep up. They added a second 80 gallon tank, and told the person operating that machine to not start it is the tank pressure was below a certain point. It was a secondary machine and who ever was running it had a primary job to do, so the wait was not a big deal.