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Balanced Flow - On Static and Dynamic Pressure

In this article, we discuss how tipping the balance between static and dynamic pressure leads to airflow. Learn the difference between these two pressure types and how they affect your HVAC system.

A balanced HVAC system will manage temperatures and air pressures for an entire building while drawing the least amount of power. It will keep every room warm in the winter and cool in the summer without breaking the bank. The system is constantly balancing hot and cold air across your building to keep every room at the right temperature. To achieve this, HVAC systems must tip the balance between static and dynamic pressure.

Consider a fixed object placed in an airstream. The air flowing through cannot lift the object out of the way, so it will pass around the object. As air passes around, the object will create resistance that lowers the air’s pressure and its velocity. This reduction is called a pressure differential, or “pressure drop”. In the HVAC industry, we often use pressure drop to determine the air performance of a louver or damper. Static pressure is the resistance caused by the obstruction. More objects in the air stream will add more resistance to airflow. As you add equipment to the duct, like control dampers, you will also increase the static pressure in that duct. If the static pressure is equal to the force used to push the air through, then the air is at a standstill.

Louvers and dampers will increase the static pressure in the system.

Louvers and dampers are important to your HVAC system, but each one will cause static pressure.

Adding obstacles or increasing air velocity will increase static pressure, including control dampers. If you have too much pressure in the duct, then your dampers may not open or close when necessary. Every damper will have a maximum rated static pressure, or the maximum static pressure allowed in the duct for the damper to safely operate. If static pressure exceeds this limit, then the damper is no longer guaranteed to open and close when needed. You risk damaging the damper.

Why not remove equipment from the duct to reduce static pressure? You can't remove necessary equipment from your HVAC system, like louvers and dampers. You will need those dampers to direct and balance airflow. This is where the balance comes into play. You can increase dynamic pressure in the duct to overcome the static pressure.

Dynamic pressure is the pressure exerted to move air. In our previous example, we discussed air passing around an object. If static pressure is the resistance created by the object, then dynamic pressure is the force pushing the air towards and around the object. In an HVAC system, the fans control the flow. System fans create dynamic pressure and drive air through the ductwork. The goal is to maintain the necessary dynamic pressure through the duct system, to get conditioned air to where it is needed.

System fans will generate the dynamic pressure necessary to keep air moving.

System fans will drive air through the HVAC system. To overtake static pressure, the fans will need to spin faster to generate more dynamic pressure.

HVAC systems transfer air from one point to another. This means drawing air into the system, conditioning it to the right temperature, and passing it through ductwork to the room that needs it. To keep air moving through the system, the dynamic pressure of the air flow must exceed the static pressure throughout the system. You need to tip the balance between static and dynamic pressure.

Static and dynamic pressures will vary at different stages of the HVAC system. Different ducts will serve different purposes and will be configured in different ways, but they will all need to pass air through to the right place. A length of ductwork with a control damper installed will require an increase in dynamic pressure to maintain the necessary airflow rate, even with its blades in the fully open position. Increasing fan speed will increase dynamic pressure, but will also require more power. The costs of this small increase will add up as you add more sections of ductwork to the equation.

An efficient HVAC system aims to keep dynamic pressure over static pressure across all ductwork, while using the least amount of energy. Understand your system's requirements and minimize static pressure where you can. Make sure the system can generate the necessary dynamic pressure to keep air moving. Minimize resistance wherever you can by choosing louvers and dampers with low pressure drop.

Always remember to fulfill important requirements first. For example, you may need a louver with high pressure drop to protect your system from wind driven rain. Your building will require a fire damper within certain barriers instead of a standard control damper. The choice between air performance and project requirements will often be a balancing act. Always make sure you meet the project's requirements.

As a rule, more free area translates to lower pressure drop which means better air performance. Make sure your dampers can operate within the duct’s intended static pressure. These simple steps can keep air flowing through your HVAC system while using minimal power.

Have you ever experienced issues with static pressure? Were control dampers the culprit? Share your thoughts and stories in the Comments section. MCDLG wants to hear from you! 

By tipping the balance, you can keep air flowing through your ductwork

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