EXECUTIVE SUMMARY This paper addresses a means of reducing the total annual running hours of mechanical cooling compressors in DX applications, while maintaining exactly the same dry bulb temperature in the occupied space. The subject systems have outdoor, exhaust and return air dampers with an enthalpy comparison economizer circuit. During warm weather, typically these systems position the dampers at either 100% outdoor air or primarily return air with minimum outdoor air, addressing ventilation requirements. The circuit selects the air stream with the lesser total heat content (enthalpy) to pass through the refrigeration coil. This is the most efficient means of mechanical cooling, when the refrigeration compressor is running and tends to reduce the run time of the refrigeration compressor. If the particular unit runs properly with potentially low load conditions, this is the correct operation of the DX mechanical cooling, minimizing compressor run time. With the compressor run time minimized, we now have to consider the duration of the rest time, when the refrigeration compressor is inactive, but the fan is running. The magnitude of both the run time and the rest time determine the annual run time of the refrigeration compressor. When the controlled space cools to the bottom of the cooling dry bulb differential range, the refrigeration compressor shuts down, the fan continues running, providing the required ventilation, respecting building codes. The dampers conventionally maintain their position based on the air stream with the lesser enthalpy. The cycling of the refrigeration compressor is based on dry bulb temperature of the occupied space, not enthalpy. The cooler the supply air is to the controlled space, while the refrigeration compressor is inactive, the longer the rest time of the refrigeration compressor. Consider an outdoor condition of 80°F, with lesser enthalpy than a return air condition of 73°F. Conventional systems, based on enthalpy control of the dampers, maintain an 80°F supply air, while the refrigeration compressor is inactive; however, this new circuit switches to a 73°F supply air, when the refrigeration compressor is inactive. Supply air of 73°F causes a longer rest time, prior restarting the refrigeration compressor, than the 80°F supply air. When the outdoor air enthalpy is less than the return air enthalpy, the dry bulb temperature of the return air is less than the dry bulb temperature of the outdoor air and the refrigeration compressor is inactive, switch the dampers to return air. This will maximize the rest time of the refrigeration compressor. Combining enthalpy logic when the refrigeration compressor is active with dry bulb logic when the refrigeration compressor is inactive, minimizes the annual run time of the refrigeration compressor, while maintaining identical dry bulb temperature conditions in the occupied space. 8.118
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