oldred said:
Apparently you seem surprised that the two stage runs cooler? It does (due to that tube which removes a LOT of heat!) and this has always been one of it's selling points although it's main advantage is in volume. Basically it works like this, given a certain amount of power and a fixed ratio you can produce high volume at low pressure or you could produce high pressure at low volume but you can not produce both high volume and pressure at the same time, V @ P = HP. Think of a car with a certain amount of HP and only one gear, depending on which gear ratio it could either accelerate rapidly to a low top speed or with a ratio change it could accelerate less rapidly to a higher speed. Now by using a two speed transmission that same amount of power could be used to both accelerate rapidly AND reach a higher speed. In the case of the two stage compressor vs a single stage with the same rating for each, say 15 CFM@90 PSI, the two stage would produce somewhat more volume at the lower pressures, thus the CFM rating@90PSI would be the same but the two stage would reach it sooner. On the two stage set-ups with a lower cut-out speed (lots of two stage outfits are set up to cut out at around 135 CFM) there is a heck of a lot of difference in recharge time if the pump/motor is geared to reach maximum torque at that cut out setting like the single stage would be.
The total volumetric area of the cylinder per revolution x revolutions per minute /efficiency of cylinder and valve seal determines the output.
If one cylinder displaces .03 cu ft , and then shoves it into a cylinder of .02 cu ft. then the pressure of that single compressed cycled will increase, but the total volume of influent is still .03 cu ft.
It has now been compressed to a higher pressure which occupies less total volume in your supply tank . Nowhere in this process did the compressor magically increase the cu.ft. of air it inhaled in that one cycle.
Now multiply that one cycle by number of cycles per minute and you have the CubicFeetMinute the compressor is rated at.
If a single stage dual cylinder compressor operating at 85% efficiency and has a total displacement per revolution of .008cu.ft and compresses that volume to 90 psi , then it would be fair to say it is rated at .408 cu.ft.per minute.
Multiply that number by rpm's and total output is determined.
If a 2 stage compressor with a first stage cylinder has a volume of .008 cu ft. and its ring and valve seal efficiency is 85% then the output of a single intake cylce will be .008 cu ft, but its pressure is increased by the action of the much smaller secondary cylinder that is not a volume increaser but is a pressure increaser.
The influent of a single revolution is the same as with the single stage example above.
the 2 stage compressor is more efficient at producing high pressure due to the compress and re-compress action of each cycle. The total volume is only determined by the above mentioned factors.
As far as being more efficient from an energy standpoint that is not so. it takes a given number of watts to produce pressure at volume, or volume at pressure , which ever way you wish to look at it.
Shops run a 2 stage compressor to get hi volume storage at minimal cost. Storage tanks cost $, and take up a foot print that also cost $.
Body shops are a minimal cost operation,therefore they run the cheapest option available, not necessarily the best.
A true industry using compressed air will run a multiple stage turbine which has far greater capacity, but the $$ to acquire are far more.
I only question the heat statement as I have not done any tests of compressors to see what the effluent temperature is. Some single stage compressors also run fin tube and have lower operating temps than a non cooled compressor.
Anyone serious about dry air will install a refrigerated air cooler, no matter which type of compressor they have.
I have installed, maintained and rebuilt industrial air compressors for 30 years. Joy(Now Cameron),Sullair, and Fuller are the primary units I am trained on. Have also maintained Ingersol-Rand and Curtis industrial recprocating(piston) type compressors.
Cameron multiple stage centrifical compressor:
1500-2200hp 6500-11800cfm 80-150psi
How about this little boy for a body shop?
Its only a single stage 84 cfm@175 psi 25 hp.
Small foot print, large volume, no oil in the air, why doesn't the local shop have it? its not because their compressor is better, its because it is cheaper.
http://www.sullair.com/corp/details/0,,CLI1_DIV61_ETI8582,00.html
A little guide to types of air compressors:
Bottom line, buy what ever works for you and your budget. If I had the money some of you guys have I would have a
Curtis SE7.5TD in my hobby shop :mwink: