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Hi DS, The fact that a compressor is two stage rather than one has nothing to do with reserve capacity or with thicker receivers. It just so happens that the two stage units are USUALLY supplied with larger storage tanks and have larger maximum pressure capability (e.g. 175 psig is a common maximum for two stage units whereas 135 psig is about the maximum for single stage ones). The reason two stage units can go to higher pressures has to do with temperature. The theoretical discharge temperature for a single stage unit operating at 135 psig (compression ratio of about 10:1) is about 560 deg F! A two stage unit operating at 175 psig (compression ratio of about 3.6:1 per stage with perfect intercooling between stages) has a theoretical discharge temperature of about 300 deg F. If that same two stage unit were operating at 135 psig (the same as the single stage) the compression ratio per stage would be about 3.2:1 and the theoretical discharge temperature would be about 280 deg F. Half that of the single stage! The big benefit of the two stage is, therefore, lower operating temperatures resulting in less problems with downstream moisture. In many instances they are also rated for more or less continuous duty because of the lower temperature whereas the single stage units are not, as far as I know. An extra benefit of the lower temperatures is better efficiency regarding power consumption. For example 5 hp will deliver more air using a two stage compressor that it would driving a single stage unit. The advantage in storing air at 175 instead of 135 is that it provides a longer consumption time between compressor starts that you can utilize when using air exceeding your compressor capacity. Don't forget the sometimes significant drop in pressure between your storage tank and your tool throttle. It can easily be 20-40 psig, depending on what you have installed. You may NEED 120 psig or so at the tank in order to get 90 psig at your end use point. If that is the case, you may only be able to maintain the high pressure at your tool for only a short period (until your tank pressure drops below 120 in this example). That can be pretty quick if your maximum is only 135 psig to start with.  Rod
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