Constant Air Volume HVAC System

Defining the System

A Constant Air Volume (CAV) starts with air from a central source, which is then conditioned. Duct work with supply and return fans are used to circulate this air to all the occupied rooms in a building. There is a master thermostat that regulates temperature throughout the building by controlling the heating and cooling applied. There are three different variations of the CAV system, the furnace, constant air volume reheat system and the multizone system.

A furnace is normally used in a smaller building, where it eliminates the boiler and combines the heating unit with the fan blowing the air through the system. A constant air volume reheat system is similar to a normal CAV system except that as it is traveling through the ducts to be delivered to each of the rooms, it travels through a reheat coil controlled from a localized thermostat to provide more individualized temperature control. The multizone system works off the same central fan but has a separately heated system of supply ducts linked to various thermostats the control each zone.

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Notice the multiple duct tracks in the multizone system coming from the same fan room.

Typical Uses

Most CAV systems are installed in larger buildings with large open rooms like auditoriums, lecture halls, theaters and lobbies. Places where pressure control is important such as hospitals, pharmaceutical plants and electrical manufacturing plants can utilize a CAV system as well. Other buildings with fairly large spaces but with many smaller spaces like libraries, stores, shopping centers, laboratories and places of worship can try and utilize the multizone system.

Limitations of the System

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The CAV system is designed to maintain a high degree of control of air quality. Large rooms need to maintain a constant temperature throughout and this system is effective at doing that for a relatively cheap cost initial cost and easy maintenance.

It has its limitations in its lack of ability to have individualized air-conditioning control. Multizone systems work towards helping that situation, where there are several thermostats for several zones and air is sent from the centralized unit. The problem with this is that for each area that you want individualized control, you need to set-up another conditioning zone which further complicates a very simplified system. The more zones you have, the more ductwork is needed, and most buildings never provide enough space for ductwork. This system also works best in larger room settings with small windows, so it eliminates dorms, apartment complexes and other buildings with smaller rooms or large windows.

Typical Materials

The typical materials involved in a CAV system are much like other systems. A boiler with a chimney and a chilled water plant with a cooling tower help produce the hot and cold water used by the system to regulate temperature. This water is brought to a fan room by piping where the thermostat will tell the fan room what needs to be done to the air. The fan room has an intake and an exhaust to bring in fresh air and exhaust the stale air. A fan pushes the air through the supply ductwork (both vertical and horizontal) which is then diffused into the room through a supply diffuser. Air is collected through return grills and down the return ductwork to be exhausted out of the building. There is a packaged system available for smaller buildings to simplify the components other than the ducts and diffusers.

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Designing the System

For the sake of design purpose we'll use a 100,000 SF theater that is to be built. From the charts below The required cooling capacity for this system would be approximately 530 tons. The total space for a boiler room and chilled water plant would take approximately 3800 SF, while the cooling tower would require 700 SF. The expected cooling air volume would be almost 210,000 CFM. This would require 130 SF of main supply or return ducts and another 210 SF of branch supply or return ducts. The fan room would require 6000 SF with 500 SF of fresh air louvers to supply the fan room. To exhaust the air, 410 SF of exhaust air louvers would be required.

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