PREFACE Enthalpy comparison in HVAC systems presents a significant opportunity in control logic, where society may reduce electrical consumption. This applies to mechanical cooling systems with outdoor air, return air and exhaust air dampers, where the controls alter the dampers from full outdoor air to minimum ventilation, based on calculations that the outdoor air will consume more cooling energy than the return air of the building. Air at 70°F and 100% RH contains 34 BTU/lb of dry air and at 70°F and 15% RH contains 19 BTU/lb of dry air. (Environment Canada, last year, reported that Calgary experienced 100% RH as the outdoor high and 15% RH as the outdoor low during the mechanical cooling season.) These conditions present a 78.9% BTU increase from the low to the high regarding air at 70°F. Return air 75°F at 40% RH contains 26 BTU/lb of dry air, which is midpoint considering the high and low, Altering the damper position to minimum ventilation, based only on a temperature of 70°F, which occurs in many systems, would be a bad idea if the outdoor relative humidity is at the lowest recorded value and a good idea if the outdoor relative humidity is at the highest recorded value. The system must know the relative humidity level of both air sources and calculate the actual enthalpy of each to make a proper decision. This report illustrates our control circuit that calculates the enthalpy of both the return air and outdoor air. The circuit compares the two enthalpy values and selects the lower value as the air stream passing through the mechanical cooling coil. We believe that installation and set up can be accomplished by a pneumatics technician with average ability. A couple of scenarios where enthalpy logic may not apply are: -1- Some chillers will create an artificial load, internally matching a lessening load from the fan systems. The chiller’s energy consumption will not change at low load levels. -2- A unit with Freon coils controlled from the space may tend to run the compressors more frequently under certain conditions with enthalpy comparison logic. For example: The conflict is that the dampers are positioned based on enthalpy comparison and the mechanical cooling is based only on space dry bulb temperature. If the outdoor air is 80°F at 15% RH, the enthalpy is 23 BTU/lb of dry air and if the return air is 73°F and 40% RH the enthalpy will be 25 BTU/lb of dry air. The enthalpy comparator will select the outdoor as the air stream passing through the cooling coil. When the mechanical cooling is active, the outdoor 80°F at 15%RH containing 23 BTU/lb of dry air is the more efficient air stream passing through the cooling coil; however, when the mechanical cooling is not active, the outdoor air will raise the room temperature restarting the cooling in a shorter time than the cooler return air. Consider allowing enthalpy selection when the mechanical cooling is active and use the air stream with the cooler sensible dry bulb temperature during periods when the mechanical cooling is inactive. The only other pneumatic logic circuit, currently commercially available, to my knowledge, is the Johnson Control N9000 Logic Center. We tested this device’s performance and our data and Johnson Control’s specification sheet indicate that the N9000 is not a true enthalpy comparator as per pages nine and ten in this report. 8-106
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