This drawing illustrates a common approach to preheating fresh air. A preheat coil is added with control valves (V1) and (V2). Temperature controller (TC3) opens V2 completely when the fresh air drops to 38°F preventing the preheat coil from freezing. TC2 modulates V1, maintaining the preheat discharge temperature at 13°C (55°F). The circuit exists in many buildings and does provide preheated outside air; however, the design tends to waste energy. With a return air temperature of 21°C (70°F) and the preheat discharge temperature of 13°C (55°F) the mixed air temperature will be 18°C (65°F) if 33.3% is preheat air. TC1 is commonly set for 13°C (55°F); therefore, automatically brings in cooling via damper (D1) solely to compensate for the excessive heat introduced through the preheat coil. Often V2 opening forces the preheat discharge temperature to rise above 13°C (55°), causing greater energy loss. We developed a circuit, which tends to correct this matter. There are a few other means of preheating fresh air. This is the one we have seen most often. Fresh air has two functions, regarding HVAC systems. The first relates to a fixed quantity of outside air, which must always enter the system during occupied periods addressing air quality, exhaust air replacement and building pressurization: the second is free cooling when the outside air is suitable for cooling purposes. Free cooling is normally limited to a minimum fan supply temperature of about 13°C (55°F), preventing draft complaints in the space. In this example the minimum ventilation requirement is 33.3% of the fan’s total volume. The ratio of mixed air causes the mixed air temperature to drop to the minimum desired temperature of 13°C (55°F) when the outside air temperature drops to 8.6°C (47.5°F). When the outside air temperature drops to the design temperature of –23°C (-10°F) the mixed air temperature will drop to 6°C (43°F). At design outside air temperature (-10°F in this example), the 9,000 CFM of mixed air requires 116,640 BTU’s to bring its temperature from 8.6°C to 13°C. (43°F to 55°F) (CFM X 1.08 X T = 9,000 CFM X 1.08 X 12F° = 116,640 BTU) A preheat coil is required preventing the mixed air temperature from dropping below 13°C (55°F). 5.41 OUTSIDE AIR PREHEAT WITH MIXED AIR SYSTEMS 0 CFM NC 21˚C 6,000 CFM RETURN FAN 6,000 CFM 0 CFM NC 3,000 CFM (33% MIN.) -23˚C DESIGN TEMP. NC NO 9,000 CFM SUPPLY FAN REHEAT COIL (TYPICAL) THERMOSTAT (TYPICAL) REHEAT VALVE (TYPICAL) MAIN AIR SOLENOID VALVE DM1 DM2 DM3 DM4 6˚C MIX NC NO NC NC MAIN AIR 0 TO 6,000 CFM VARIABLE 0 TO 6,000 CFM VARIABLE 6,000 CFM ETURN FAN 9,000 CFM SUPPLY FAN REHEAT COIL TYPICAL THERMOSTAT TYPICAL REHEAT VALVE TYPICAL SOLENOID VALVE TC1 TC2 V1 V2 DM4 DM2 DM3 DM1 3,000 CFM TC3 V3 NO TC4
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