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The Variable Air Volume (VAV) system makes it possible for the users to control the volume of conditioned air that arrives in any defined space or zone. The conditioned air mixes with the room air and cools or heats the zone independent of the zones in the building. This is achieved by incorporating a system dampers, air valves, and fans into the delivery subsystems. When a loads for a given space is reduced then the system will respond by turning off a fan or closing a damper such that air is diverted away from that space and is sent only the those rooms that need cooling or heating. This results in a more efficient system that requires less power to operate and saves money.
Explanation of Subsystems:
There are six popular VAV systems. They are:
The shutoff simply is comprised of a VAV system which has an air valve that
opens and closes. The users can simply open or shut off the air flow
traveling through the ductwork allowing for the desired temperature to be
achieved in a given space.
This type of VAV system is used primarily to cool a space which does not normally require heating. An example of this might be a space with lots of windows providing a surplus of solar heat. Instead of locating the heating equipment inside of the space hot water, steam, or an electric coil can be run through the VAV outlet terminals and reheat the air before it enters a space. This option is chosen primarily when cost is the primary criteria being taken under consideration.
The parallel fan powered VAV terminal system is commonly used to condition the air for a space because it takes into consideration minimum ventilation and air changes requirements. The conditioned air first passes through a fan which accelerates the air velocity to a rate needed to provide minimum CFM for a space. The second local fan then provides an additional boost to the air velocity if more cold air is called for to cool a space. If the second fan is turned off then the system will automatically operate at the minimum required air delivery rate. In this situation the second fan does not block the incoming air from the primary fan as it travels through the ductwork and into the room. The fans work together to service a given space and velocity is the key factor for the success of this system. It is important to make sure that the design takes into account a fluctuating air velocity into the space.
The Series fan powered terminal unit system is also a popular VAV system. It is similar the the Parallel Fan VAV system except that all of the air passes through both of the fans. The secondary fan is sized to handle 100 percent of the airflow and is turned on only when additional air flow is called for. This ensures that the air enters the space at a constant flow with only the temperature varying. As a room cools the dampers will close and allowing for more room air to be recirculated and less and less incoming cool air to be introduced into a space. These systems can be commonly found in bathrooms, entrance ways, hallways, and conference rooms.
A Dual Duct terminal unit has two air valves which connect into a VAV box.
One of the ducts transports warm air while the other transports cool air into a
space. The air mixes in the VAV box and is delivered to a space.
This system allows for a variable temperature and air volume. In many
cases this system works well however it is more expensive up front because more
ductwork needs to be installed into the system.
When the initial cost is the deciding factor the changeover/bypass systems may be the cheapest option. It is called the changeover/bypass system because it changes over between heating and cooling operation and uses a bypass loop to allow constant volume. A monitoring system controls the
The VAV system could be considered inappropriate for a building with only one zone required or needed. In that case a CAV system would be a better option because you would not need to condition air in multiple zone and it is cheaper.
Types of materials:
The ducts are normally constructed out of aluminum or stainless steel sheets.
Types of VAV Systems
Source: Basic Numerical Parameters for VAV System
VAV Sizes and Capacity’s
Links to Relevant Resources:
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