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COATINGSPRO SEPTEMBER 2015 23 efect. Anyone who doesn't believe this should try to make a small incision in a pressurized air hose. Even right before a venturi nozzle, your average pressure gauge will read positive pressures. Once the air has lef t the the compressor and any condensate that may have accumu lated in the compressor's air tank dur ing the last per iod of nonuse. T he abi lit y of air to be sucked into the hose connections presupposes that there is an opening , regard less of size, into a line where the air is at least eight to nine times as dense as the atmosphere (14.7 psi vs. 115 to 125 psi, or 0.1 MPa vs. 0.79 to 0.86 MPa). T he velocit y inside the hose compared to the pressure is insuf f icient to overcome the static pressure of the compressor. If it were not, there wou ld be litt le force at the end of the line to propel the paint or abrasives, depend ing upon your operation. I have never seen a pressur ized hose col lapse f rom suction, but I have seen many ex plode because they were either damaged or wor n out. A: It would be an interesting academic exercise to determine the actual velocity necessar y in, for example, a 4 -in. (102 mm) inside diameter hose to cause the effect you are questioning. I believe that it would be far beyond the capacity of the compressor to produce under the normal blasting or painting scenarios. A: If you look at it f rom a theoret- ica l standpoint, what happens w ith the air in a compressed system (such as a blast setup)? T he outside air (usua l ly moist) is being sucked into the compressor. T he compressor compresses the air, at which point t wo things happen: 1) T he va lue of the air is d iminished (which, by itself, wou ld contr ibute to a r ise of relative humid- it y of that air because of less volume and the same amount of air and thus a lso moisture), and 2) Because of the compression of the air, the tempera- ture of the air a lso r ises, resu lting in g reater "water capacit y" of the air and compensating for phenomenon "1" to a cer tain extent. So now the air leaving the compressor is lifted to a certain operating pressure, for example 6 to 8 bar (600 to 800 kPa). With the latter alone, there is so much overpressure on the air hose and blast hose that it would be impossible for outside air to be sucked in, not even by a venturi Notes From the Blog Wr i te in Re ad e r In q u ir y #367