The answer that I was looking for was given directly by
go_redskin. Kars and Bama also said it.
The only purpose of the compressor is to raise the temperature of the refrigerant. PERIOD
It accomplishes this by increasing pressure ( pressure increases so does temp )
A gas fired unit ( or absortion system ) uses gas to heat the refrigerant ( ammonia ), which of course also increases the pressure. These systems don't have a compressor.
Thanks for all replys,