This graph illustrates the relationship of the requirement for cooling by the warmest room to the actual mixed air temperature. (The mixed air temperature is the cold deck for winter operation.) The mixed air temperature based on original design is also illustrated. On drawing two you see that control loop controlling the mixed air does not communicate with the space cooling requirements. The loop simply produces the degree of cooling required for the maximum anticipated cooling requirement of the occupied space. There is no compensation for load variations in the occupied space. Drawing three shows the communication between the zone with the greatest requirement for cooling and the cooling functions on the fan system. The graph shows that the warmest zone’s cooling requirements increase during the day when the outside air temperatures are higher and heat gains from lights, bodies, solar gain, etc. are present. When the warmest zone signal is at 12 PSIG or below there is no requirement for cooling; therefore, the mixed air loop provides only enough fresh air to satisfy the ventilation demands, addressing air quality requirements. When the warmest zone’s signal exceeds 12 PSIG the system “knows” that zone has its cold deck damper completely open and its hot deck damper completely closed. With this assurance that the demanding zone will not mix hot deck air with the requested cooling, the cold deck is gradually cooled as the warmest zone’s signal approaches 15 PSIG. The mixed air temperature is limited to a minimum of 55°F. The average actual mixed air temperature of the total points logged, on the illustrated graph is 65°F. The original design maintained the mixed air at a constant 55°F. Based on a fan volume of 15,000 CFM the original design would have used 162,000 BTU/HR more than the new conservation circuit. 55°F DESIGN MIXED AIR TEMPERATURE DAY DAY DAY DAY DAY NIGHT NIGHT NIGHT NIGHT NIGHT 8.87
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