A room thermostat operates a damper or other throttling device that controls the airflow to the zone in responding the load. Early VAV system were limited to systems where the cooling load did not decrease below approximately 75 percent of design, because the reduction in air supply volume resulted in adequate air circulating and distributing the development of improved terminal air units, capable of throttling air volume considerably while maintains adequate room air circulation has extended the range of VAV system to application with large variations in loads.
Because the volume of supply air is increased in response to the room sensible cooling load the room humidity will usually raise at part load if the latent load remains constant (zone zl) for application with a high sensible heat ratio the effect is negligible. A combination of VAV with the reheat results n an energy efficiency system. Yet also provides good humidity control even arrangement supply air volume is reduced to a minimum of perhaps 50-60 percent. If the sensible load falls further, a reheat coil in the VAV terminal unit is activated in this way better humidity control is obtained.
Duct and fan heat grains are not in the psychometric hart to simply the explanation latter in the chapter system types will be discussed from the view point of the energy consumption.
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