Upgrade - Shop Air - Quality 60-80gal compressors?

projectnut said:
I like the idea of pressure testing. I have 2 pressure washers. The smaller one is capable of 2,200 psi, and the larger is capable of 3,200 psi. The smallest one I've seen is capable of 1,200 psi. The big difference is in the GPM output. The smaller ones are usually around 1 GPM to 1.3 GPM while the largest commercial ones I've seen can put out over 10 GPM. Depending on output it could take 8 minutes, or 80 minutes to fill the tank.
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Fill the tank with water from a garden hose, then connect the pressure source. I've seen a common hand operated grease gun used to generate enough overpressure to certify a tank.

jack vines
 
There have been injuries and deaths due to tank failures, both air compressor and water heaters. Usually with compressors it is poor maintenance and accumulation of water in the tank causing corrosion, often worse with cheaper models. Problem is that these failures usually occur years later, and even with hydro testing it may not reveal a weakening in the tank. Water heater tanks are ceramic lined steel, the main failure is when the galvanic rods start to go and then the tank starts to rust from micro fractures in the lining. In most cases this creates a fracture line which seeps water. In one house we had both water heaters fail at around 10 years, measured the house water pressure line an we were at 120 PSI. Although there are pressure ratings as to burst strength, and working pressure, you are much better off with a water pressure in the 60 PSI range. Anyone using water heater as an air pressure vessel, do not expect your homeowners insurance to cover anything, wouldn't trust my life on it.

ACE Program: California Case Report 05CA010 | NIOSH | CDC

On July 23, 2005, at approximately 9:20 a.m., a 46-year-old Hispanic car wash supervisor died when an air tank exploded inside an equipment room.
www.cdc.gov www.cdc.gov
americanhomewater.com

Can Your Water Heater Explode? Warning Signs & Prevention

A hot water heater explosion can cause serious damage, physical injury, and even death. Thankfully, there are very clear preventative measures and warning signs
americanhomewater.com americanhomewater.com

So quite a bit of discussion as too size of compressor and single vs. two stage, let alone a rotary/scroll compressor. It all comes down to ones air needs, type of air equipment being used, frequency of use an cost. It was already mentioned that single stage reciprocating compressors for the most part are made to a low cost point, typically are louder and usually last maybe 2000 hours before needing to be rebuilt or tossed. Noise and frequency of run times are my biggest complaints of these compressors. If you have air equipment, they typically run at 90-100 PSI and with some air drop in lines, I have my main air regulator set to 125 PSI with 3/4" air lines and then use separate air regulators locally. A standard single stage compressor would have no reserve and turn on every time I use air. Hate the noise and frequency. A two stage compressor typically turns on at 135 PSI off at 175 PSI, so with an 80 gallon tank, the compressor cycles much less frequently and the refill time for mine is maybe 2 minutes. They are all going to pump about the same amount of air for a given "real" Hp, a 5 Hp two stage is going to be around 16-17 CFM at 175 PSI and around 20 CFM at 125 PSI. The same air pumps are often used for both 5 Hp and 7.5 Hp models, they just increase the pump speed. A rotary/scroll are quiet and usually have a reservoir tank but will be running a lot of the time, typically quite expensive but good for high volume continuous air. On a two stage compressor, the difference of having an after cooler to drop out the moisture out is significant. Haven't seen any single stage or lower priced compressors fitted with one, but do recall some home built cooling loop out of the compressor with a cyclone type water separator.

In the shop like today in 4 hours using my air collet changer, air gun for clearing chips and continuous air chip system on my mill while machining my compressor probably ran for a total of 10 minutes. Previously I had one of those Dewalt 225 PSI 15 gal that ran frequently and it drove me nuts. There are other much quieter compressors if you needs are needs are minimal and/or only have 120VAC.
 
Hot Water Heater Tanks, can reach far above the pressure of the water source as a result of the burner heating the water. It does not take much heat to increase pressure. Water Heater Tanks are fitted with a 150psi relief valve. They also suffer from stress more than air compressors as a result of heat from the burner and then cold water entering to replacing the hot water that is used. It has been long identified that a water heater is essentially a bomb that we keep in our homes. Should the thermostat malfunction it is very possible that you could exceed the safe operating pressure, which could rise above the pressure of the relief valve which have been known to fail do to mineral and sediment deposits.

The best preventative maintenance that you can do is to drain and flush the tank once a year. Although that is rarely done and I am as guilty as the next guy. But all those minerals and sediment that separate from the water during the constant heating and cooling build up in the bottom where the heat is applied essentially causing that area to be heated far more than it was designed. That sediment displaces the water where the heat is applied resulting in a thick mud void of circulating water and most of the cooling to the surface of the tank. This is what generally causes a water heater to burst just above this area as the differential in temperature puts major stresses on the metal and welds.

I would never use a water heater for an air tank. You don't know the condition of the tank, how it was cared for or if it suffered from overheat which weakens the metal.

Just my $0.02 worth.
 
mksj said:

ACE Program: California Case Report 05CA010 | NIOSH | CDC

On July 23, 2005, at approximately 9:20 a.m., a 46-year-old Hispanic car wash supervisor died when an air tank exploded inside an equipment room.
www.cdc.gov www.cdc.gov
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Not an applicable case.

From the case:
An evaluation performed by an independent failure analysis laboratory, paid for by the insurance carrier for the compressor manufacturer, found products of combustion in the air tank and a rupture pattern of the tank walls that was consistent with an explosion rather than rupture.
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Bold added.

If you read the case the cause of the combustion was due to an improper lubricant that caused combustible vapor to develop inside the tank.

Regardless, it is still an example of a tank exploding, however the cause was not corrosion weakening the tank to the point of failure as is my concern.
 
I did read the full report NIOSH report, I recall when the case when it first reported along with a few others, it is more to demonstrate the impact of a tank failure no matter what the cause. As with many catastrophic failures there are often a number of contributing factors. Through a process of failure analysis we try to reduce risk, but if a pressurized vessel fails it can cause significant damage, and many individuals ignore routine maintenance as well as draining the tanks regularly.
 
The kinda "cheap" two stage compressor from TSC has served me well. I run mine almost daily in my home shop. The compressors are made in India I think. 80 gallon / 220 volt. Put a ball valve on the tank drain and dump the pipe in a bucket. I have never rusted out a tank yet. Its usually the pressure switches and motors that fail first. So far I haven't killed a compressor on the 80 gallon version, I have lost compressors on smaller 20 gallon cheapos. I change the oil a couple times a year. They need proper wiring too, I always overkill the wire size and use a dedicated circuit breaker. Don't let those compressors recieve low voltage or share stove, dryer or whatever, good way to kill the motor way early.
 
That is the exact one I have been looking at. The one at my local store has a compressor made in China on it.

Joe
 
projectnut said:
I used a cheap single stage compressor for years, and had no problems when the required pressure was below 90 psi. The thing worked hard, and didn't sound all that great, but it did the job for over 25 years. It's major limitation was the maximum pressure it could put out.

When airing up the tires on the trucks or motorhome it just wasn't up to the job. The maximum pressure the pump would put out was 125 psi. While it was high enough to do most of the vehicles it struggled for those that needed 110 psi to 125 psi. If the tire(s) needed to be topped off (especially in cold weather) it took forever. Since the differential between what the compressor could put out and the pressure needed was minimal the tank had to be charged to the maximum.

Once the pressure was near the differential point air flow was reduced to a minimum. The tank pressure had to go down to 80 psi (highest the switch could be set for) before the compressor would restart. I had to constantly bleed off air to get it down to the start pressure. Then I had to wait for the tank to build to full pressure before I could continue to fill the tires.

One downside of a single stage compressor is no pressure relief (unloader) valve between the head and the tank. The compressor has to start at whatever pressure is in the tank. It's difficult for the motor to start the pump when pressures are above 80 psi, so that's generally the highest setting available to restart it.

On a 2 stage pump the unloader eliminates that problem allowing the pump to be started with no head pressure. Thus the minimum kick in pressure can be set higher.

Here's a tutorial from Quincy as when to best use single and 2 stage compressors

Single Stage vs. Two Stage Air Compressors | Quincy Compressor

As for automatic tank drains there are several on the market ranging in price from $65.00 to over $200.00. I have no experience with any of them so I can't comment on their quality or reliability
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You can fit a simple home made unloader to a single stage, just a simple solenoid valve fitted in the discharge pipe between the compressor and the tank, you will need to make sure the non return valve at the tank entry is sealing well. that is open with power off and close with power on. Even though the valve will close as soon as the pressure. switch closes the line is empty of pressure, so the compressor can build up speed before there is any significant pressure in the line. This will also ensure that your compressor stops on cut out with little to no pressure.
 
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