In data centers, raised floors are often used as air supply plenums to achieve balanced airflow and reduce pressure loss. In the HVAC system, the raised floor in a data center acts as a ventilation static pressure plenum. A static pressure plenum, also known as a pressure stabilization chamber, connects to the air supply outlet of the air handling unit. This connection reduces the airflow velocity in this space to nearly zero, converting dynamic pressure into static pressure. The static pressure is approximately the same at all points, ensuring that the air supply outlets deliver a uniform airflow.
What is Static Pressure Plenum?
Static pressure plenums are traditional components in HVAC systems, often used in constant temperature, humidity-controlled cleanrooms, and other environments with stringent requirements for indoor temperature, humidity, cleanliness, and even airflow distribution. Commercial HVAC systems generally operate at zero static pressure and do not require a static pressure plenum, but larger air conditioning systems (with fan power >5KW) might use them. Thus, they serve to reduce noise in the ventilation and air conditioning systems, achieve uniform static pressure output, and minimize dynamic pressure loss. Static pressure plenums are essential for reducing dynamic pressure, increasing static pressure, stabilizing airflow, and minimizing airflow vibration in air supply systems. They are typically made from galvanized steel to enhance airflow effectiveness.
(1) Four Major Functions of Raised Floor Static Pressure Plenums:
1-1. Part of the dynamic pressure is converted to static pressure, allowing air to be blown further; static pressure plenums ensure uniform static pressure output, reducing dynamic pressure loss and enhancing the overall performance of the ventilation system.
1-2. Uniform distribution of airflow; static pressure plenums ensure uniform static pressure output, reducing dynamic pressure loss and enhancing the overall performance of the ventilation system.
1-3. Noise reduction; noise can be reduced due to the decreased air velocity or by adding sound-absorbing materials (10-20dB(A) noise reduction).
1-4. System simplification; in real-world ventilation and air conditioning systems, there are often needs for various transitions in ductwork, such as square to round changes, diameter changes, right-angle turns, and multiple duct intersections. These transitions require specific fittings, which are time-consuming and material-intensive to fabricate. Using a static pressure plenum as fittings simplifies the system significantly, functioning as a universal joint.
(2) Principle of Raised Floor Static Pressure:
Static pressure is the pressure exerted by the random movement of air molecules colliding with the walls of the duct and is perpendicular to the direction of fluid motion. Its role is to overcome resistance during fluid transport. The purpose of establishing a static pressure plenum is to obtain static pressure (Total pressure = dynamic pressure + static pressure; the total pressure of the fan is constant, as airflow speed decreases, dynamic pressure decreases, and static pressure increases). In systems with multiple air ducts, to equalize the pressure in each duct, a static pressure plenum is used to convert all dynamic pressure into static pressure and then distribute it evenly to each duct. Similarly, static pressure plenums are used in return air systems to ensure thorough mixing of air before it reaches the cooling coils. Some single duct air supply systems also incorporate static pressure plenums to reduce noise, as air velocity significantly decreases inside the plenum, reducing noise levels.
(3) Design of Raised Floor Static Pressure Plenums:
The role of a static pressure plenum is to provide uniform pressure to each branch of the system. Theoretically, the static pressure inside a plenum should be equal everywhere, meaning the internal airflow speed should be zero. To achieve this, the plenum volume would need to be infinitely large, which is impractical in engineering projects. Typically, the internal airflow speed should be controlled at less than 2 m/s, or significantly reduced compared to the inlet and outlet duct speeds.
(4) Calculation Methods for Static Pressure Plenums:
- When designing a static pressure plenum, if the airflow speed is regulated, the body of the plenum would be quite large; typically, the length of the plenum should extend 400mm beyond the dimensions of the air duct, and the height should extend 400mm above the air duct height, as a rough estimate.
- Experienced engineers often use a noise reduction of 5-10 dB(A)/m for general plenums, and 10-15 dB(A)/m for complex impedance type silencers (commonly used in HVAC systems).
- If controlling the air speed to 2.5 m/s results in too large a volume, it may be slightly increased, with no specific requirements for length generally exceeding 1 meter.
- The height × depth = cross-sectional area of the static pressure plenum, and this area × 2.5 m/s = the airflow rate of the fan. How to match the height and depth is up to the designer.
- Using the airflow rate L ÷ 3 m/s, you can calculate the area of one side of your static pressure plenum. Then, based on the height of the room, if it is 4 meters and your unit is 2 meters high, subtracting approximately 0.5 meters for flexible connections and leaving 0.5 meters above, your static pressure plenum will be 1 meter high, allowing you to determine the width. Knowing two dimensions, the third dimension is easily determined, with the main consideration being whether there is enough space in the air conditioning room; if there is, make it as large as possible! The method of calculating length is similar; knowing the width, you can calculate the area by multiplying it by the length, which should also equal L/3. However, in design institutes, the face velocity is taken as 2 m/s; if there is enough space, make it larger.
- The thickness of the static pressure plenum should ideally be greater than 600mm, with a cross-sectional airflow speed of less than 2 m/s. Also, note the airflow buffer area between the inlet and outlet.
- Currently, static pressure plenums are typically made from sheet metal without sound-absorbing material; cost considerations are less critical unless using sound-absorbing materials, in which case costs must be carefully considered. Additionally, for silencers, controlling the airflow speed to less than 8 m/s ensures effective noise reduction.
- Sound-absorbing static pressure plenums are essential; sometimes it’s challenging to connect ductwork without using a plenum in machine rooms, with noise reduction being a secondary benefit. The cross-sectional airflow speed should ideally be less than 1.5 m/s, and the aspect ratio should ideally be less than 1:4.
- The function of a static pressure plenum is to convert dynamic pressure into static pressure, ensuring even distribution of airflow, often used before slot diffusers.
- The main function of a static pressure plenum is to stabilize pressure and reduce noise. According to standards, the internal wind speed of the plenum should not exceed 2.5 m/s. Based on the airflow and speed, the volume of the plenum can be determined. Static plenums are generally custom-made. The width should not be less than 500mm and is usually slightly smaller than the indoor unit.
- In raised floor air supply, two types of static pressure plenums are used: one using an interstitial space as the plenum and another using a standard static pressure plenum with several ducts connected to the air supply outlets.