ENERGY CONSERVATION BENEFIT OF ENTHALPY COMPARISON IN HVAC Energy conservation benefit regarding enthalpy comparison is significant. If owners, operators and service technicians have a true practical understanding of their importance in maintaining the control logic, they will better fill their respective roles of good management, proper monitoring and precise calibration. The psychrometric chart on page four high lights the thermal characteristics of the return air and outdoor air illustrated in the sample drawing on page three. The orange and green lines follow two psychrometric characteristics of the example return air. The orange line overlays the enthalpy line starting at 76° F at 40% RH running up to the enthalpy value of 26.61 BTU per pound of dry air. The green line overlays the moisture content line running to the value of 53.5 grains of moisture per pound of dry air. The pink and brown lines follow two psychrometric characteristics of the example outdoor air. The pink line overlays the enthalpy line starting at 80° F at 15% RH running up to the enthalpy value of 22.76 BTU per pound of dry air. The brown line overlays the moisture content line running to the value of 22.5 grains of moisture per pound of dry air Each pound of evaporated water produces 970 BTU’s of heat, as it condenses from gas to liquid. This latent heat of evaporization often causes cooler, moist air to consume more cooling energy than warmer dry air. Enthalpy comparison loops tend to minimize the condensing of moisture at the cooling coil. On page four the saturation temperature figures following the 100% humidity line toward the left side of the chart present the temperatures (dew point) where the air is not capable of holding more moisture than the grains of moisture per pound of dry air scale figure you see if you draw a line straight across the graph to the far right. The target supply air temperature for cooling is often 55° F. As the return air cools to 55° F, it becomes saturated at 60°F, containing 78 grains of moisture per pound of dry air. At 55° F the air is only capable of holding 64 grains of moisture per pound of dry air; therefore, 14 grains of moisture must be condensed out of the air as it cools from 60° F to 55° F. The latent heat of evaporization, as it condenses, plus the sensible heat reduction of the air, must be addressed by the cooling system. As the 80° F outdoor air, at 15% RH, cools to 55°F, the relative humidity level rises to only 95.5% RH; therefore, no condensing takes place, allowing only sensible heat as the cooling load. Based on 20,000 CFM, when the refrigeration compressor is running, the cooling load, using return air completely, is 7,560 BTU’s sensible heat plus 2,906 BTU’s latent heat = 10,466 BTU’s total. Based on 20,000 CFM, when the refrigeration compressor is running, the cooling load, using outdoor air completely, is 9,000 BTU’s sensible heat and no latent heat = 9,000 BTU’s total. The cooling energy required by DX mechanical systems not comparing enthalpy, under the sample conditions, is 16.3% over a system comparing return air and the outdoor air enthalpy. Regarding the example figures, when the refrigeration compressor is resting, enthalpy control of the dampers will add an extra 15.7% more sensible heat to the air supply, shortening the refrigeration compressor rest time. The net result will be more running time for the refrigeration compressor with absolutely no temperature difference in the space served. 8.122
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