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- for CAV system AHU contains a constant speed fan , dampers adjust for only one time the flow stay constant.
- For VAV system AHU contains a variable speed fan , balance dampers adjust for one time , inside the conditioning space VAV unit will be install and it is simply a variable position damper (there is more than one type) the position of that damper is related to a thermostat which take the space temperature , finally there is a pressure differential sensor to modify RPM of the AHU fan.
thermostat gives a signal to the VAV actuator , damper moves to close or open according temperature , according to pressure increase or decrease the differential pressure sensor gives a signal to VSD of the fan , fan motor decrease or increase the RPM which affect the CFM
constant air volume or CAV is the system where the AHU has fan with constant speed all the time. This is because it is mostly serving the zone which will be either ON or totally off all the time and no other option.
VAV will be used where the fan is with VFD Drive and the load will be modulating and it will modulate the speed of fan so the load on AHU will be 0% to 100% and it will be adjusted by the speed of fan. Normally this type of system have many zones and controlled by VAV equipments.
VAV systems vary the airflow at a constant temperature. CAV systems supply a constant airflow at a variable temperature. The advantages of VAV systems CAV systems include more precise temperature control, reduced compressor wear, lower energy consumption by system fans, less fan noise, and additional passive dehumidification.
In a CAV system air delivery is constant or fixed.They have mechanical manual dampers.These systems can only cater to a fixed load which is a disadvantage.Whereas,in a VAV system,air delivery is regulated based on the load by means of an automatic damper(s).This has an electronic sense and act loop.The air conditioned room has a sensor which communicates with the automatic damper based on the load.The set temperature on the thermoastat is maintained by varying the amount of air flow inside the conditioned room.