PERIOD #1 PERIOD #2 PERIOD #3 PERIOD #4 PERIOD #5 FREE COOLING 40 BTU/# DRY AIR NO FREE COOLING NO FREE COOLING NO FREE COOLING 28.8 BTU/# DRY AIR 25.5 BTU/# DRY AIR 25 BTU/# DRY AIR 21 BTU/# DRY AIR FREE COOLING FREE COOLING OUTSIDE AIR ENTHALPY RETURN AIR ENTHALPY N9000 SIGNAL APS ENTHALPY CIRCUIT SIGNAL JOHNSON CONTROL N9000 AND APS ENTHALPY PERFORMANCES The above graph compares the performance of the Johnson Control N9000 Enthalpy Logic Module and the APS enthalpy comparison circuit. The return air enthalpy and outdoor enthalpy values were determined via the APS enthalpy comparison circuit illustrated on pages 8-111 and 8-113 in this report. The accuracy relative to the standard psychrometric chart on page 8-107 is presented on page 8-110. As illustrated above the outdoor enthalpy was lower than the return air enthalpy during the whole test. The APS enthalpy comparison circuit correctly determined that the system should remain on free cooling from outdoor air. The Johnson Control N9000 Enthalpy Logic Module determined that free cooling was appropriate for two periods and no free cooling was appropriate for three periods. The APS enthalpy circuit compares enthalpy of the return air and the outdoor and these are the only factors determining the switching point. The Johnson Control N9000 Logic Module literature states “ When outside air enthalpy is greater than return air enthalpy or when the outside air temperature is greater than the return air temperature, the N9000 provides a maximum output signal of 10 PSIG (70 kPa) or greater. Note: if the outside air temperature is greater than return air temperature, the output signal is at maximum and will not be reduced by a change in the humidity signal.” 8-115
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