Pressure Dependent and Independent VAVs

Pressure Dependent and Independent VAVs

VAVs that are pressure independent or dependent are confusing and frequently misunderstood. I'm trying to explain it here.

VAVs without flow measurement are pressure dependent.

VAVs with flow measurement functionality are pressure independent. The box contains a flow measuring station, and nearly all controllers are equipped with a velocity pressure sensor.

To maintain the desired room temperature, both types of VAVs are used.

Let's look at each VAV's operation to see how they work and what advantages a pressure-independent VAV has over a pressure-dependent VAV.

Pressure Dependent VAV:

Because this VAV does not have a supply air flow measurement provision, it uses a simple single room temperature PI (proportional + integral) loop to maintain room temperature. The procedure is simple. If the room temperature rises, the damper will modulate from minimum to maximum position, delivering more air from the AHU to control the temperature.

Assume there is a VAV delivering 400 cfm in the room and the room temperature is maintained. As a result, with the damper at 45% open, its temperature loop is stable. Now, for some reason, the AHU supply fan speed has dropped by 10%. If all other conditions (all other VAV positions) remain constant, the VAV in our example will supply less air due to the decrease in speed. Assume the flow rate is reduced to 350 cfm. However, because this VAV lacks flow measurement capability, it will not recognize the 50 cfm shortfall. The room temperature will gradually rise over time as there is an imbalance between the heat rate added by the heat sources and the heat rate removed by the VAV. The heat rate added will be greater than the heat rate reduced. This will raise the temperature of the room.

As a result, the room temperature loop will become active, and the damper will be opened further from its previous 45% position. After a while, the loop will remove the error and establish a new balanced condition, where it will remain. Assume the new balanced condition is 50% open and 400 cfm. Until further disturbance occurs, the loop will remain in this state.

In the preceding example, we can see that reducing the speed causes a disturbance in the duct pressure. Room temperature disturbance is influenced by duct pressure fluctuations. As a result, it is referred to as a pressure dependent VAV. The term 'dependent' in this context refers to the fact that disturbances in duct pressure affect room temperature. The temperature loop is correcting/eliminating these disturbances.

What is important to note here is that, while the temperature loop is correcting the duct pressure disturbances, the approach here is to let the problem arise and then correct it. The duct pressure disturbances first affect the room temperature, which is then corrected by the temperature loop.

?Pressure-independent VAV :

There is a flow measurement provision in the pressure independent VAV. The supply side of the box has a flow station, and the controller has a built-in velocity pressure sensor. The velocity pressure is converted into supply flow.

Using two PI loops, it can maintain the room temperature. One is a room temperature loop, and the other is a supply flow loop. The temperature loop drives the flow loop. This means that the temperature loop generates an output ranging from 0 to 100%. The range block converts the 0–100% output into the min to max flow setpoint. The flow loop then uses this flow setpoint to maintain the flow between the minimum and maximum cooling flow and modulates damper position. This is known as a cascade arrangement. One loop is the master in this arrangement. The room temperature is a master loop because VAV's primary goal is to maintain a comfortable environment. This loop is what drives the flow loop. The flow loop is assisting the master loop in achieving its goal. As a result, the flow loop is known as a slave loop.

Consider the same hypothetical example used for pressure dependent VAV. A temp loop generates a cooling flow setpoint of 400 cfm. With the damper at 45% open, the flow loop maintains the flow at this setpoint. Now, if for some reason, the AHU supply fan speed residues by 10%. This decrease will result in duct pressure disturbance. If all other conditions (all other VAV positions) remain constant, this will reduce the flowrate in our example VAV. Assume the new flow rate is 350 cfm. When the flow rate reaches 350 cfm, the flow loop activates and begins to reduce error by opening the damper further. After a while, the flow loop will reduce total error and resume delivering 400 cfm. Assume the damper position is 50%. At this point, the flow loop will become stable.

What is important to note here is that the flow loop eliminates the disturbance in duct pressure caused by reducing the supply fan speed by 10% before it affects the room temperature. As a result, this VAV is referred to as a pressure independent VAV. Because pressure disturbances are eliminated by the flow loop, they have no effect on the room temperature. The flow loop acts as a shock absorber.

The pressure independent VAV approach is a preventive approach. It prevents problems from occurring. As a result, it is more widely used and popular than pressure-dependent VAVs.

I have never seen or worked with pressure dependent VAVs in my BMS career.

Pressure independent valves are now available and operate on the same principle. When compared to position valves, they are gaining popularity.