What are the differences between CAV (constant air volume system) and VAV (variable air volume system) ?

- 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.

CAV is constant Air Volume unit. It used serve where the temperature, peek time of different zones comes same. It is cost effective in the above situation. It is useful for conference hall, auditorium, shopping mall etc. When the return air temperature meets the set temperature machine will cut off. It will fail when temperature of each zone comes different. At that case we use VAV. VAV is variable air volume. It gives supply from one machines to different zones with different peek time. Eg: in an office zone1 is working at peek time 9am and another zone2 at 4pm. When the machine is working return air sensor senses the temperature depending up on the machines place. At morning zone1 takes time to reach set temperature, zone2 reach earlier. At the evening zone1 reach set temperature before zone2 reaches. So at every time some imbalance comes in one of these zones. At this situation we use ALD to adjust the CFM and use separate thermostat for separate zones. When temperature reaches settled temperature lover close and restrict 80% of air, then pressure built up in supply duct pressure sensor is near to the supply it sense pressure automatically and adjust drive speed to balance the duct pressure. VAV installation cost is higher than CAV so it is suggested when the requirement meets the needs of VAV