ENERGY OPPORTUNITY REGARDING DX COOLING This paper addresses a significant opportunity regarding HVAC systems with full mixing dampers, limited by a mixed air controller and an economiser controlled via enthalpy comparison of the outdoor air and the return air. The mechanical DX cooling, which cycles the refrigeration compressor via dry bulb temperature, is controlled from either the return air or the occupied space. A logical control system should run the refrigeration compressors for the least amount of time, while maintaining comfort level in the occupied space. This is achieved by minimizing the refrigeration compressor run time and maximizing the compressor rest time. In order to achieve this situation we must view damper economiser control from a new perspective. An enthalpy logic circuit calculates the outdoor air enthalpy and the return air enthalpy. The air stream with the lesser enthalpy is selected as the air stream passing through the evaporator coil of the refrigeration cooling system. For example: -The thermostat starts the refrigeration compressor when the room temperature rises to 76°F and stops the refrigeration compressor when the room temperature reduces to 72°F. -Return air 76°F at 40% RH contains 26.61 BTU/lb of dry air. -Return air 72°F at 51% RH contains 26.61 BTU/lb of dry air. (%RH rises as the room cools, with the same moisture content.) -Outdoor air 80°F at 15% RH contains 22.76 BTU/lb of dry air. In this example, enthalpy logic chooses to position the mixing dampers to 100% outdoor air, as this air stream has a lesser enthalpy value. (22.76 BTU < 26.61 BTU) This is the correct choice when the refrigeration compressor is running, as a lesser amount of heat exists in the outdoor air than the return air; therefore, the compressor run time will be minimized. Now consider the rest time of the refrigeration compressor, as the room temperature rises from 72°F to 76°F. Heat gain factors such as body load, light load, solar load, machine load, etc. are a constant, regardless if the fan’s mixing dampers are on 100% outdoor air or on return air with minimum outdoor air and will not alter the rate of temperature gain when the refrigeration compressor is resting or running. Keep in mind that the refrigeration compressor is controlled via dry bulb temperature. If the system is on 100% outdoor air at 80°F, the time for the room to rise from 72°F to 76°F will be less than if using the return air at 72°F to 76°F, with all other cooling loads being equal. If the refrigeration compressor is running, the dampers should be positioned to the air stream with the lesser enthalpy, which will minimize the compressor run time. If the refrigeration compressor is cycled off, when the outdoor enthalpy is less than the return air enthalpy, but the return air dry bulb temperature is less than the outdoor air temperature, select the return air with just minimum outdoor air, which will maximize the compressor rest time. If the return air enthalpy is less than the outdoor air enthalpy, but the outdoor dry bulb temperature is less than the return air dry bulb temperature, do not switch to the cooler outdoor air, when the refrigeration compressor is not running, as you may flush the building with moist air, which will cause a greater cooling load when the refrigeration compressor running. 8.119
RkJQdWJsaXNoZXIy ODY1ODQy