2 stage air compressor

The reason you want more pressure is that air is molecules, if you compress the air higher, you have packed more molecules together in the same space compared to ambient (or uncompressed) air. This highly compressed air, combined with a large tank, allows you to draw it off thru a regulator and have

1) a steady regulated pressure (as opposed to air pressure varying up and down with the on and off pressures of the compressor)

and 2) a long time between cycles of the pump. It takes lots of current to start a motor and much less to keep it running. You want as few starts as possible, and you achieve this with the large tank and the high pressure, reduced to a working pressure. This is due to a greater ratio of expansion in the higher pressure air vs a lower pressure compressor.

If you do not reduce the pressure down to working pressure (100 psi or so) you will get a varying pressure as you comsume the air, from a high of 175, where the pump shuts off, down to a low of about 145, where the pump kicks back in. In addition, you will not lengthen the cycle time as you are not taking advantage of having the high tank pressure vs lower working pressure.

This is different from a water system or a hydraulic system because the fluid (air) is compressable, while in liquid hydraulic systems (be they water or oil) the fluid is not compressable (in the sense that the molecules can be appreciably squeezed closer together, and that is why hydraulic systems, such as well systems or closed center hydraulic systems have to have air over fluid accumulators to work properly.

Its kinda like stuffing 10 lbs in the 5 lb sack (single stage pump) vs stuffing 15 lbs in the 5 lb sack (two stage pump)

I Shopped long and hard looking at 7.5 horse two stage compressors. I wound up with a Home Depot/Husky (made by Cambell-Hausfeld and identical except for color, to one they market) I looked at IR, Champion, Emglo, and Quincy to name a few, and decided that most sleeve bearing, non lubricated pumps were similar performing and for the low hrs/yr use I would put on it, it would perform as well as a much more expensive pressure lubricated ball/roller bearing pump.

I bought with the intention of running a blasting cabinet, so with 25+ CFM of airflow, I have more than enough. Even with steady blasting, the compressor will be able to catch up and shut off and rest (cool) a moment before it kicks in again, due to the greater margin between what I demand in air and what the compressor produces, compared with say, a 5 horse model.

Note that not all two stages are created equal. Home Depot for instance sells a 7.5 RUNNING horsepower compressor, 25+ CFM airflow. they also have a 7.0 hp two stage on the same or similar 80 gal tank. Whats the diff.???

The 7.0 horse power is STARTING horsepower.... that compressor has a RUNNING hp of only about 2.5 hp

For comparison mine has 21 STARTING hp and 7.5 RUNNING hp, about three times as much. Both have the same volume tank (80 gal) and so you get the same volume off the tank between shut off and start up. The difference lies in how long the pump has to run to pump it back up, alot longer.

I found all 7.5 hp compressors were Running hp (apples to apples) and that most but not all compressors below that (depends on if new or years old) will claim Starting hp (it looks bigger, better in marketing's eyes) and a few will claim running hp (which is really what a motor is all about). Real old compressors generally list the running hp. Apples to oranges.

Word to the wise, do not waste your time on any compressor with an oil free pump. They are noisy, and do not last long. Just plain CHEAP. You get what you pay for, not much.

I hope this helps, I sure there is a better explaniation on the web somewhere (www.howthingswork.com ???)

Charles

I'll try. A two stage compressor is quite similar to two separate compressors operating in series if you can visualize such an arrangement. The air being discharged from the first stage goes in the input to the second stage.

A single stage compressor does the compression all in one shot. At 125 psig, the compression ratio is about 9.5:1. This high ratio leads to very high temperatures (in excess of 400 degF). That puts a realistic limit to the maximum pressure available from a single stage unit. You will be unlikely to find a single stage compressor operating much over 125 psig although some can go to 150 psig but that is very high.

By contrast, a two stage compressor's first stage discharge pressure is about 40 psig maximum (less than 4:1 compression ratio). This air typically will be put through a finned section that acts as an air cooler (called intercooler) that cools the air prior to entering the second stage. Air enters the second stage at a maximum of about 40 psig and discharges at up to about 175 psig (also less than 4:1 compression ratio). Because of the lower compression ratios and the interstage cooling of the air, a two stage compressor is more efficient, power-wise than a single stage for compressing air to pressures commonly used. It also can develop higher pressures while keeping maximum temperatures down lower than a single stage unit. Many two stage units also do not have the siamesed cylinders so typical of single stage ones. That fact promotes better external cylinder cooling with cooling fins fully around each cylinder.

I am not sure what you are calling an unloader. Engine driven compressors or compressors than run constantly, even w/o a demand for air have an unloading device(s) to permit the compressor to run with out delivering any air at all. Another type of "unloader" is used on most compressors and is usually part of the pressure switch that turns the compressor on and off as tank pressure changes. This type of "unloader" vents off a small amount of air existing between the compressor head and the check valve at the tank inlet. This permits the compressor to start up next time w/o a back pressure during the very first revolutions. It makes it easier to start. In my opinion, you should always have this type of "unloader switch".

Rod

Charles