Variable air volume. In this article, we are going to look at the VAV system. VAV stands for Variable Air Volume. VAV systems are very common in offices. This is because they are much more energy efficient than the CAV (constant air volume) model.
Scroll down for YouTube Video Tutorial on Variable Air Volume
VAV allows zoning within the building. This is ideal in most situations, as each room in a building will experience a slightly different heating/cooling load. VAV systems allow each room to control the amount of cooling, and optionally heating, individually. Each room is fed by the same main CTA and the same ducts. Different parts of the building may have dedicated AHUs and ductwork, for example, the south and north sides of a building or selected floors based on functionality.
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| VAV-layout- jcool |
In the illustration above, we have modeled a basic VAV system for an office.
The AHU is located in the mechanical engine room. Two ducts come out of it, the impulsion and the return. The two ducts cross the building, taking the shortest path, to supply air conditioning to the rooms. The supply duct supplies the air and the return duct draws in the dirty exhaust air.
In a VAV system, the AHU supplies air at approximately 13 degrees Celsius (55 degrees Fahrenheit). This air circulates through the main supply duct and is distributed throughout the room. After the air has provided heating/cooling, it will be drawn into the return duct and back to the main air handling unit.
So the main difference between a VAV system and a CAV system is the VAV terminal. This is located right on the branch that leaves the main conduit. Each VAV box serves a certain area (room or group of rooms). The VAV box is connected via a cable to a thermostat located in the room or zone. The thermostat will indicate on the VAV terminal whether to increase or decrease the flow to meet the cooling demand.
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| VAV-Terminal-jcool |
The VAV box will look like this.
You can see it's a pretty basic box. Nothing too complex happens inside them. We have the controller located mounted on the actuator. The actuator controls and turns the shaft that is connected to the shock. The damper can modulate its position to control the airflow and therefore the amount of air entering the junction box and area.
The air usually comes in at about 13 degrees Celsius or 55 degrees Fahrenheit, but this temperature can be adjusted via BMS. Sometimes it's not hot enough, so we can find an electric heater inside that will turn on to warm the air in its path. You don't have to have this inside, but it's pretty common. They don't have to be electric either, they can also be powered by the building's LTHW hot water system or it can also come from, for example, a heat pump or something like that. But for simplicity, I just left it as an electric heater.
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| VAV-Air-flow-sensor jcool |
At the back of the VAV, at the entrance, there are some tubes that form a cross. It's an air flow sensor. Measures the change in pressure across the device. From this you can calculate the average air velocity and therefore the flow rate at the VAV terminal. Basically there are some small holes in the front and also in the rear of the tubes. If you imagine supply air coming in and hitting the front face, it will impact that surface and cause pressure. So the front will have a high pressure side and the rear will be a low pressure point. You can find the differential pressure across the entire fixture, and if you average the duct size and calibrate it, you can calculate the airflow entering that box and area.
The damper never closes completely, or at least it doesn't usually, and that's because a certain amount of cold air is always needed to get into the space. If you suddenly obtain a gain of hair plus height on the piece and that the temperature of the piece begins to mount au-dessus du point de consigne souhaité, register it will begin to be heard and it will allow more air to enter in the space. Remember that the air is around 13 degrees Celsius, 55 degrees Fahrenheit, which will start to cool the room down and force all that hot air into the return duct.
In the middle of summer, when all the solar heat gain is coming in through the windows and there might be a meeting room full of people, there's a lot of heat being generated there, so the cash registers will be 100% open and this will allow that in like as much cold air as possible. During the winter, or when there are less people in the room, the position of the damper is closer to the closed position, remember not quite, and this modulates the cooling.
Since a building's ventilation system is connected to multiple VAV terminals, the VAV terminals will vary their load throughout the day as the dampers open and close. The AHU fans will force air into the supply duct, which will pressurize the duct. When many VAV gates open, the pressure in the duct drops and when many VAV gates close, the pressure rises.
To combat this we will find a pressure sensor in the main conduit towards the furthest point of the system. These will be connected to a variable speed drive VSD (VFD) or a variable frequency drive. The VSD/VFD will connect to the supply and return fans. The fan speed will increase or decrease in response to duct pressure. This will ensure efficient operation.
You can also get fan-assisted VAV boxes. They are essentially the same, except the unit has an internal fan that draws air from the space. This is useful because the fan will maintain a constant volume through the box. If the damper is closed, less air will enter the space, but the fan will ensure a constant exchange of air in the room. When the dampers are open, fresh air mixes with the recirculated air in the room.
Additionally, the fan-assisted VAV will prevent cold air from directly entering the space and personnel. It could make the person sick.
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