{HEAD ~ The curtain} or barrier is formed by a region of flowing conditioned air directed across {TAIL ~ the opening} 
{ HEAD ~ The curtain } or barrier is formed by a region of flowing conditioned air directed across { TAIL ~ the opening }
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL