Dear:
I understand your questios is ...how to implement an 8 channell A/D converter at the input of a 8051.?

OK, you could use an 8 channell analog switch (like two 4066's) and a ADC08831 (serial input) A/D as converter chip, I think it's enought for your system (air conditioning plant).
At the end you probably coud use an TLC5628 chip for D/A. It's a serial input and 8 channell analog out D/A chip.
Look for datasheets at any place.
Good luck.