GRAPH #2 A pneumatic low selector was added to the circuit to allow the discharge temperature controller to automatically over-ride the mixed air controller; as well as control the cooling valve as per its original intent. The chiller is off during free cooling mode; therefore, the discharge controller will only have an effect on the mixed air dampers during the winter The black line illustrates the mixed air temperature, the blue line illustrates the discharge air temperature and the green line illustrates the return air relative humidity. The graph demonstrates that the change tended to correct the problem. The mixed air temperature tended to rise when the humidity sprays were active; however, the discharge air temperature still dropped an unacceptable amount when the sprays were active. We had to increase the sensitivity (lower the proportional band) of the discharge controller to allow that controller to respond with a reduced control point shift. (Lessen the drop in the discharge air temperature when the sprays were active.) The problem was that the existing sensitivity was set up to eliminate the cycling of the loop when controlling the cooling valve as per the original intent. We altered the circuit via a turn down ratio relay to effectively allow the discharge air controller to have two different sensitivities: one when controlling the cooling valve and the other when it was controlling the mixing dampers. We first used a simulator to establish the exact sensitivity with which the controller was historically running. We increased the sensitivity to the highest level to minimize the drop in discharge air temperature when the sprays were active. We allowed the discharge controller to impact on the mixed air temperature based on evaporative cooling by sending its signal directly to a low selector, which also received a signal from the mixed air controller. The discharge controller also sent a signal to the turn down ratio relay, which allowed the cooling valve loop to operate with the same sensitivity as it had before we entered the job. The turn down ratio relay sent its signal to the chilled water valve. 8.98
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