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Fundamental Series Item :HVAC Systems: How They Work file:///c~/windows/TEMP/0,2637,14506.0 15 Heating: The heating load requirement is based on design indoor and outdoor winter The design conditioned space heating load is 227,000 Btu/hr. This is the amount of I (mainly by conduction) through the walls, windows, doors, roofs, etc., in the winter. P amount of heat is required to heat the outside ventilation air based on design conditi~ To maintain the temperature and humidity in the comfort zone far the conditioned sp heating cycle is this: The supply air leaves the heating coil carrying 227,000 Btuh of I air goes through the supply air fan (SAF), down the insulated supply duct, past the rr volume dampers (MVD) which have been set for the correct amount of air for each c into the conditioned space. The supply air gives up all of its 227,000 Btuh of heat to I conditioned space to replace the.227,000 Btuh that is leaving the space through the etc. As the air gives up its heat it makes its way through the room and into the return inlets, then into the return air duct and back to the air handling unit. This AHU is loco roof and is therefore designated as a "roof top unit" (RTU). The return air goes through the return air fan (RAF), through the return air automatic temperature- controlled (ATC) dampers into the mixed air chamber and mixes with tl air (OA). The mixed air flows through the filters, through the cooling coil (which is off the heating coil. The mixed air travels through the heating coil where it picks up heat conduction through the hot water tubes in the coil. In addition to the tubes, the heater has fins attached to the tubes to facilitate the heat transfer. The supply air leaves the coil carrying its 227,000 Btuh of heat and the air cycle repeats. The water, after giving up heat to the air, leaves the coil and goes back to the oil-fire through the hot water return (HWR) pipe and into the boiler where it picks up the sar of heat that it has just given up in the coil. The water leaves the boiler, flows through water pump (HWP) and is pumped through the hat water supply (HWS) or heating h supply (HHWS) piping into the heating coil to give up its heat into the mixed air and t cycle repeats. Ventilating: In the human respiratory process, oxygen is inhaled and carbon dioxide, contaminant, is exhaled. In commercial buildings, carbon dioxide and other contamir as cigarette smoke must be continuously removed or uncomfortable or unhealthy co result. "Ventilation" is the process of supplying outside air to buildings in the proper s offset the contaminants and odors produced by people and equipment. In many situations, local building codes stipulate the amount of ventilation required ft commercial buildings and work environments to maintain good indoor air quality (IAC requirement is usually 20 cubic feet per minute of outside air for each occupant. The HVAC system supplies air to a suite in an office complex designed for 50 people. Th outside air requirement is 1,000 cfm. Air Conditioning (Cooling): For this system, the total heat given off by the people, IigF equipment in the conditioned space plus the heat entering the space through the out windows, doors, roof, etc., and the heat contained in the outside ventilation air will be approximately 195,000 Btu/hr. A ton of refrigeration is equivalent to 12,000 Btu/hr of Therefore, this HVAC system requires a chiller that can provide 16.25 tons of coolinc To maintain the proper temperature and humidity in the conditioned space, the coolie described as: The supply air (which is approximately 20 degrees (F cooler than the conditioned space) leaves the cooling coil and goes through heating coil (which is of the supply air fan, down the duct and into the conditioned space. The cool supply air heat in the conditioned space. The warmed air makes its way into the return air inlet; the return air duct and back to the air handling unit. The return air goes through the r into the mixed air chamber and mixes with the outside air. The mixed air goes throu4 and into the cooling coil. The mixed air flows through the cooling coil where it gives ~ into the chilled water tubes in the coil. This coil also has fins attached to the tubes to heat transfer. The cooled supply air leaves the cooling coil and the air cycle repeats. The water, after picking up heat from the mixed air, leaves the cooling coil and goes chilled water return (CHWR) pipe to the water chillers evaporator. The "warmed" wa into the chiller's evaporator (sometimes called the water cooler) where it gives up the