The cooling coil needs to drop the energy of the air to 23.2 Btu/lb before it enters the room. With a conventional economizer, the control system compares the 77°F return temperature and the 85°F outside air temperature and closes the outside air dampers to minimum. This results in each pound of recycled air requiring the removal of 8.4 Btu of energy.
With an economizer that checks the enthalpy, the total enthalpies would be compared. In this case, the 28.8 Btu/lb would be compared to the 31.6 Btu/lb and the outside air dampers would be set to full open. The supply air now only needs to remove 5.6 Btu for each pound of air.
Using an enthalpy economizer for this situation has reduced the cooling energy demand by 50%.