Kindly let me know what is refrigerant in HVAC , and how is fed in to the system ?

R-22 which is being phased out in favor of r410a. R-410A is manufactured and sold under various trade names, including GENETRON AZ-20®, SUVA 410A®, Forane® 410A, and Puron®.

This is on the EPA site:
The Clean Air Act does not allow any refrigerant to be vented into the atmosphere during installation, service, or retirement of equipment. Therefore, R-22 must be recovered and recycled (for reuse in the same system), reclaimed (reprocessed to the same purity standard as new R-22), or destroyed. After 2020, the servicing of R-22-based systems will rely solely on recycled or reclaimed refrigerants. It is expected that reclamation and recycling will ensure that existing supplies of R-22 will last longer and be available to service a greater number of systems. As noted above, chemical manufacturers will no longer be able to produce, and companies will no longer be able to import, R-22 for use in new A/C equipment after 2010, but they can continue production and import of R-22 until 2020 for use in servicing existing equipment. Given this schedule, which was established in 1993, the transition away from R-22 to the use of ozone-friendly refrigerants should be smooth. For the next 10 years or more, R-22 should continue to be available for all systems that require R-22 for servicing.