RETURN AIR TEMPERATURE RETURN AIR RELATIVE HUMIDITY OUTSIDE AIR TEMPERATURE OUTSIDE AIR RELATIVE HUMIDITY RETURN AIR ENTHALPY OUTSIDE AIR ENTHALPY N9000 SIGNAL PERIOD #1 PERIOD #2 PERIOD #3 PERIOD #4 PERIOD #5 40 BTU/# DRY AIR 99% RH 79⁰F 81⁰F 68⁰F 72⁰F 75⁰F 73⁰F NO FREE COOLING NO FREE COOLING NO FREE COOLING 50% RH 50% RH 28.5 BTU/# DRY AIR 25.5 BTU/# DRY AIR 24% RH 24.75 BTU/# DRY AIR 24% RH 21 BTU/# DRY AIR FREE COOLING FREE COOLING ENTHALPY LOGIC MODULE N9000 PERFORMANCE GRAPH The Johnson Control N-9000 Enthalpy Logic Centre’s specification sheet states: -1- “The N9000 imposes the lowest cooling load on the mechanical cooling equipment.” -2- “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 or greater. This signal returns the outside air damper to its minimum position. Note: If the outside air temperature is greater than the return air temperature, the output signal is at maximum and will not be reduced by a change in the humidity signal. When outside enthalpy is less than return air enthalpy and the outside air temperature is less than the return air temperature, the N9000 provides a 0 PSIG output signal. This signal places the dampers under control of the system controller to obtain free cooling.” -3- “By enthalpy and sensible heat comparison of both outside and return air conditions, the N9000 provides the most efficient use of free cooling and thus true economizer operation.” These quotes are contradictory, as they state the N9000 will select the air stream with the lesser cooling load. They also state the N9000 will switch based on dry bulb temperature and humidity will have no effect. At times the N9000 positions the dampers solely on a sensible heat comparison and ignores the humidity level, as per quote number two; therefore, ignores the enthalpy levels. The performance graph above illustrates the N9000’s signal under varying humidity, thermal and enthalpy levels. The enthalpy levels were obtained via the enthalpy circuit which is the subject of this paper. During the five periods of testing, the outside air enthalpy was always lower than the return air enthalpy; however, the N9000 determined that free cooling was appropriate for two periods and no free cooling was appropriate for three periods. A true enthalpy controller would have kept the system on fresh during all five periods, as the outdoor enthalpy was lower than the return air during the five periods. We suggest that existing N9000 installations be altered to the use the circuit presented in this paper, if enthalpy comparison is a valid feature in those buildings. 8-114
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