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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
The term VAV is commonly used for air terminals which give variable air flow by its modulating damper. Increase or decrease signal is given to the damper actuator by the VAV controller.The controller take the controlled point and set point input from its wall mounted module.
And the CAV could be an AHU, FAHU and an air terminal which give constant flow of air. An AHU can also give variable flow if it is fitted with VFD.
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https://www.bayt.com/en/specialties/q/277261/what-are-the-differences-between-cav-constant-air-volume-system-and-vav-variable-air-volume-system/?feed=answers
VAV is a variable air volume by the term it means , it is used to varies the supply of air volume in a condition spaces through ductwork , whereas CAV constant air volume used to supply constant air volume into the condition spaces through ductwork. both are centralised system and has lower energy consumption too.
Variable Air Volume (VAV) and Constant Air Volume (CAV) units are increasingly used for the energy savings they can provide. As well as offering a reliable, cost-effective ventilation solution they can also offer heating and cooling.
VAV systems vary the amount of air supplied to a given area, offering lower energy consumption of central air handling plant. Local reheat coils can be supplied for zone heating requirements. CAV systems deliver a constant flow of air into a specified area offering ventilation and temperature control. Attenuation can be integrated or separate from either system. ( for your kind information i am not a master for your question just to share for you
Constant air volume (CAV) systems supply a constant airflow at a variable temperature,VAV systems vary the airflow at a constant temperature.
VAV is temperature based air distribution system (air flow changes as per the temperature of the space) however CAV is independent from the temperature of the space as it gives constant air flow to the space. VAV is used for office areas and CAV is used for public areas (toilets etc...)
In VAV we use automatic dampers which control air flow as per fluctuating loads of space.
· CAV systems are small and serve a single thermal zone.
· A CAV unit operates the fan and compressor at full capacity until the temperature drops to a specified setting, then the compressor turns off. The on/off cycling is not efficient at keeping the space at a constant temperature.
· VAV Systems were developed to meet the varying heating and cooling needs of different building zones. For example, a zone can be either a single room or a cluster of rooms all sharing the same heat gain, and heat loss characteristics.
· Offers more precise temperature control as the fan speed varies depending on the temperature in the space. The compressor regulates the refrigerant flow to maintain a constant air temperature.
· The HVAC industry regards pressure independent VAV systems as the best HVAC system design available. This is a result of improvements in the terminal unit.
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