ENERGY OPPORTUNITY EXAMPLE, REGARDING DX MECHANICAL COOLING SCENARIO “A” RETURN FAN 100% RETURN AIR CYCLES EXHAUST 76°F @ 40% RH TO 72°F @ 51% RH ENTHALPY 26.61 BTU/LB DRY AIR 0% RETURN AIR SUPPLY FAN 100% SUPPLY AIR OUT DOOR AIR 55°F 80°F @ 15% RH DX COOLING ON ENTHALPY 22.76 BTU/LB DRY AIR ACTIVE RUN TIME 3 MINUTES Scenario “A” illustrates a fan system, when the DX cooling compressor is active. Enthalpy comparison determines the heat content of the return air and outdoor air. The system dampers are on full outdoor air, as the total heat content of the outdoor air is approximately 14.5% less than the total heat content of the return air. Selecting the air stream with the lesser heat content causes the refrigeration compressor run time to be minimized. SCENARIO “B” RETURN FAN (CONVENTIONAL LOGIC) 100% RETURN AIR CYCLES EXHAUST 76°F @ 40% RH TO 72°F @ 51% RH ENTHALPY 26.61 BTU/LB DRY AIR 0% RETURN AIR SUPPLY FAN 100% SUPPLY AIR OUT DOOR AIR 80°F 80°F @ 15% RH DX COOLING OFF ENTHALPY 22.76 BTU/LB DRY AIR REST TIME 4 MINUTES Scenario “B” illustrates the fan system, when the DX cooling compressor is not active. Enthalpy comparison determines the heat content of the return air and outdoor air. The dampers are on full fresh air, as the outdoor air contains 14.5% less total heat than the return air. SCENARIO “C” RETURN FAN (CONSERVATION LOGIC) 0% RETURN AIR CYCLES EXHAUST 76°F @ 40% RH TO 72°F @ 51% RH ENTHALPY 26.61 BTU/LB DRY AIR 100% RETURN AIR SUPPLY FAN 15% MINIMUM SUPPLY AIR OUT DOOR AIR 73.2°F TO 76.6°F 80°F @ 15% RH DX COOLING OFF (RETURN AIR, PLUS 15% OUTDOOR AIR) ENTHALPY 22.76 BTU/LB DRY AIR REST TIME 7 MINUTES Scenario “C” illustrates the fan system when DX cooling compressor is not active. Cycling the mechanical cooling is based on dry bulb temperature of the space; therefore, the rest time of the mechanical cooling will be greater if the supply air is at the lower dry bulb temperature of the return air than the higher outdoor air. SUMMARY The mixing dampers should be positioned based on return air and outdoor air enthalpy comparison when the mechanical cooling is active, lessening the run time of the mechanical cooling. The mixing dampers should revert back to minimum ventilation, when the mechanical cooling is at rest, if the return air temperature is less than the outdoor air temperature, increasing the rest time of the mechanical cooling. 8.120
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