1999 Acura CL AC expansion valve location?

Where is the expansion valve located. Under the hood or under the dash. I had a couple of mechanics at Auto Zone trace the lines for me and they traced the high/low lines behind the firewall. In other words under the dash. I have had one reply from this forum and the individual suggested that maybe the evaporator may have to be removed. Any other feedback would be most appreciated. Also, to replace the expansion valve, does the whole system have to be discharged before removal/replacement?

Thanks in advance!