I am wondering if I can use a shop air compressor (120psi) to achieve sonic flow? I am building a vortex tube (for spot cooling) and need a high velocity jet to form the vortex. What is the minimum pressure diff. required to achieve sonic flow of compressed air? Does varying the diameter of the orfice change the velocity if pressure is held constant? If someone could help me here or point me towards an equation that will answer my question that would be awsome.