air compressors

[Anonymous Poster]

Bird, I own both pressure pot blasters and gravity feed nonpressurized blasters, and the one common thing they share is they EAT air. It sounds like your Craftsman is a hopper feed non pressurized machine, and those usually come with a 1/4" diameter nozzle or tip. There is also a secondary air regulating tip in the gun itself, with a hole ranging from .040 to .080 diameter. BOTH of these tips wear from the abrasive passing thru the system, and efficiency drops radicly as the tips wear.
On pressure pot type units, there is a regulating/mixing valve and a tip (usually ceramic) at the end of the hose where the sand comes out. Again, both of these wear, although the ceramic tip wears a lot faster. Usually, about 4 to 6 hundred pounds of Black Beauty going thu the nozzle of a pressure pot unit will eat away sufficiently at the tip to drop efficiency sufficently to justify changing the tip. Unless the guy under the hood is getting paid by the hour, and a glutton for punishment, he will be the one wanting to change the tip.
The consumption figures you state are at 40psi of air, the CFM required will roughly quadruple when the air pressure to the gun is doubled. A lot of this is due to waste on a hopper type gun, but even in a pressure pot, the CFM rised phenominally at higher pressure.
A Lindsey 200# pressure pot machine with a 3/16" tip will require around 40cfm at 80 psi for decent cutting with #2 sand. Larger ganules of blasting media will also increase required CFM. The same machine running a 1/8" tip will consume around 25cfm at the same pressure. I can't run either size tip with a 20hp electric compressor and empty 200# of media. My 105cfm Jeager gas drive compressor spends most of it's time idling running this machine.
In terms of cutting efficiency, Black Beauty gives maximum bang for the buck. Silica sand, while it is cheaper per bag, is far more expensive to blast with. Black Beauty can be recycled, silica sand cannot with any efficiency. Take a grain of silica sand that has been run thru a blaster and hit a surphace, and compare it to an unused grain under magnification, you'll be surprized.
In any blasting situation, regardless of media, always start at the lowest pressure, and increase till you get the cutting efficiency you want. Excessive pressures create heat in the blasted surphace and damage the surphace.
Regardless of blast media, or type of sandblaster, media blasting AIN'T cheap. It's also a dangerous endevor unless the person doing it knows all the hazards. Silica sand dust can and will definitely put a hurting on your lungs, and any blast media can render you blind in an instant without the proper protective equipment.
Hi Pressure washing will often render better results at lower cost. Electrolytin rust removal is definitely more cost effective, and requires little equipment investment, and virtually NO labor.

 

[Anonymous Poster]

While most of your statement is right, I still maintain that it requires as much or more input energy (gasoline, electric, diesel) to get a 30 or whateversize gallon tank of compressed air from 125# to 175# than it takes to get that same tank from 0 to 125#. Stand next to the compressor and time the running time for yourself. If you have an amprobe, see how many amps a compressor draws filling the tank from 0 to 125# and how many amps it draws boosting the tank from 125# to 175#. Amps x volts x running time = killowatt hours, and those cost $$$$$$.
175# compressors are a sales gimmick, just like "peak horsepower". Peak horsepower is determined in a lab by hooking the motor to a dynomometer and recording the amperage just before the motor self destructs. How many times do you think a motor can sustain going to peak horsepower? Peak Horsepower on an air compressor is like Tim the Tool Man putting a souped up gocart engine on his garbage disposal.
One of the best and most honest sites for air compressor information is www.devair-compressors.com There is plenty of information there, and it's all free. A good compressor is an investment that will last a lifetime. A cheap compressor is the beginning of a nightmare, and money flushed down the toilet.

 

[RonL]

Franz

Probably so. But you must remember that if the output is regulated to 90 PSI it will take considerably longer for a 30 gallon tank at 175 PSI to drain down than it will for the 30 gallon tank at 135 PSI. Barring the effects of back pressure, each cycle of the compression stroke takes the same amount of energy , as you are compressing ambient pressure to the same pressure on each stroke. I suspect that compressing air to 175 PSI is slightly less efficient than compressing air to 135 PSI as you would have greater loss of energy to heat loss and you have greater back pressure. However, if your use of air is intermittent, such as an impact wrench, having a higher reserve may be usefull. A compressor that produces more CFM than the max that you use at one time, and produces it at a lower PSI, is probably more efficient. Many shops use screw compressors that produce slightly more than air than demanded at slightly more than line PSI. However, in these situations, air usage isvery predictable and steady and compressors are sized to just meet or exceed the needs.
I remember years ago I was sand blasting a building. We had a 300 pound pressurized sand blaster powered by a 85 CFM towed compressor. As the nozzle wore we would hold it further back and blast a larger area. When the compressor would no longer keep up with the sand blaster we would shut down and replace the worn out nozzle.
" Developed" horse power is pure BS. This is what turned me off of any Sears product. Any electric motor will " develope" more than its rated horsepower if it is loaded beyond its rated horsepower. It will, however, burn up in short order.
Any retailer that uses " developed " horsepower should be forced to reveal how long the motor will last while producing " developed " horsepower.

RonL

 

[centex]

Unfortunately even expensive two stage cast iron compressors use little flapper valves made of spring steel and they fatigue and break. If the compressor is not run regularly they also rust which hastens the time when they break. This seems to be the weak point in an air compressor. The spring plates are expensive but if you are lucky you can find someone who repairs them.

 

[stovepoker]

Compressed air is at best an inefficient means of providing
work. Figuring electrical, mechanical, and thermodynamic
losses, it's a pretty expensive way to move things. But
it is convenient, and it's so easy to buy all of those trinkets
that save busted knuckles. As the air is compressed, it heats
considerably. The higher the pressure, the more heat is
generated. This consumes a lot of energy. As the heat
dissipates, that energy is lost forever and never becomes
available at the airline. In the case of sandblasting, a
large volume of air being forced thru the line at high velocity,
friction losses occur in the airline. This energy is never
seen at the nozzle. For years the most efficient industrial
media blasters used a rotating radial impeller attached to
the end of a motor shaft, with the medium being fed into
the center of the disk and then flung out of the disk toward
the work. This setup consumes a fraction of the energy that a
compressed air system uses, and it operates at essentally
zero air pressure. Obviously it isn't as flexible as the
compressed air system but if I had a chance to use one for
a specific application, or maybe in a big cabinet, I wouldnt
even consider using compressed air. Keeping the pressure down will
save money. Use only as much pressure as
you need.

 

[Anonymous Poster]

Screw compressors only compare to piston compressors in that they both compress air. Gardner Denver, who did most of the developement on screw compressors, took that system to the max, including utilizing the waste heat to heat the building in winter. Unfortunately, screws don't become economicly viable below 25hp. At above 25hp, anyone not using a screw is wasting money.
Figures I've seen say 40% of the energy put into compressing air becomes heat, witch is usually wasted.
One advantage of compressed air being HOT when it leaves the compressor is that it is much easier and more efficient, to remove the contained humidity from Hot air with a simple device called a moisture seperator than it is to remove the same moisture from cool air leaving the receiver/tank.
Ritter Dental developed a water cooled moisture seperator for use on their dental power sources that takes out 75% of the contained moisture before the compressed air reaches the tank. It's not a hard device to duplicate if you know how to weld. Lindsey uses an air cooled variation of the same device on their pressure pot blasters.

 

[Bird]

I agree with everything you've said, Franz, and my little rigs are far from being "commercial" grade, but they serve the purpose for small jobs that I need to do.

 

[Molerj]

Paul, I just bought the same IR that you are talking about. Harbor Freight had it for 479.00 Plus I got 10% off because they offered a coupon to those that had bought thru the catalog in Calif. If fact, a
friend & I both bought one. I haven't had a chance to hook mine up yet, but my friend did & he is satisfied.

 

[JRP]

Many of the compressors made by Emglo use disk valves. These disk valves are much better than reed valve type. ( Emglo also sells little electric portables for roof work that use Italian compressors. These are not what I'm talking about.) I mean the units where the compressor itself is blue painted cast Iron.

Check out Emglo on ebay. I goy mine for $400. It normally sells for $800.

Jim Poux