The static pressure is controlled by increasing or decreasing the speed of the blower. As your CFM increases, the static pressure will decrease. Modulating supply fans typically controlled by a VFD are best used in a system for regulating the static pressure. This system is known as a
Static pressure is the resistance to airflow and signifies the amount of pressure a fan has to pull and push. When static pressure increases, airflow decreases, and when airflow increases, static pressure decreases.
Basically, the airflow of a fan is proportional to the rotational speed, and the static pressure is proportional to the square of the rotational speed. In other words, doubling the rotational speed doubles the airflow and quadruples the static pressure.
In compressed air terms, pressure delivers the force, yet horsepower delivers the flow. These two measures are inversely related: as pressure (PSI) increases, the flow rate (CFM) decreases, and vice versa.
For example, 100 CFM at 100 PSIG. You do have some flexibility. But it comes at a cost, reflecting the relationship between flow and pressure: when you increase the pressure, you decrease the flow. And vice versa.
If the measured ESP is greater than 0.5” WC, or if the measured ESP is beyond the maximum allowable of the blower performance curve this MAY indicate a restrictive system due to undersized duct, dirty components and/or closed branch ducts.
Static pressure can't change without a reason. It changes only as response to a dynamic pressure change. For example as in my given picture,- mass conservation of water says that volumetric flow rate cm3/1s should be same at each cross section of pipe.
Pressure that's too high or low is usually caused by improperly sized equipment. An oversized furnace can be too powerful and cause high static pressure inside your ductwork. An undersized furnace won't be able to move enough air through the system to keep you comfortable.
Fan Law 2 tells us that the change in total static pressure of the ventilation system will increase by the square of the change in propeller speed of the fan. If the propeller speed is increased by 10%, the total static pressure will increase 21%.
CFM represents the volume of air we move per minute, so static pressure is the pressure this movement exerts on our ventilation ducts. To illustrate this phenomenon, let's imagine we're blowing into a small straw. Our cheeks swell because too much air wants to pass through the straw at the same time.
For every horsepower, a compressor delivers 4-5 cfm, at 100 psi pressure. In other words - a 1 horsepower compressor will output around 4 to 5 cfm at 100 psi pressure. A 10 HP unit will output around 40 to 50 cfm at 100 psi.
Static Pressure From CFM Formula
To calculate the static pressure from CFM, divide the CFM by the area, divide the result by 4005, square this result, then finally, subtract this from the total pressure.
On the other hand, if your CFM is too high for your room size, it can cause ventilation problems. Too much air results in too little ventilation, which in turn causes high humidity. The excess moisture in the air can then lead to mold and mildew, as well as other problems.
Explanation: Static pressure is directly proportional to the square of the flow rate. Therefore, the relation between static pressure and airflow rate is Non-Linear.
An ideal system operates at 0.5 in WC. However, if your static pressure is below 0.9 in WC, you are still within the ideal range. The higher the value, the stronger your static pressure will be and the weaker the airflow.
Static pressure is typically measured in units of force per unit area, such as pascals (Pa) or pounds per square inch (psi). It is influenced by factors such as fluid density, height of the fluid column (in cases where gravity is involved), and the geometry of the system.
Adding obstacles or increasing air velocity will increase static pressure, including control dampers.
Three causes of high static pressure in ductwork are: Air Filter is Clogged or Too Restrictive. The Air Returns are Undersized. Indoor Coil is Dirty or Too Small.
So, we can see that the static friction force increases as the applied force increases, but there is some limit to how large it can become. This limit, or maximum value, depends on the same factors as the kinetic friction force: the magnitude of the normal force exerted between the two surfaces.
The airflow is typically highest in the air vent closest to the unit, but higher static pressure will mean reduced airflow as the air travels further away from the unit. This can lead to uneven temperatures and discomfort in the space.
The tool to measure static pressure vs airflow is known as a manometer. To use the manometer, you have to drill into both the supply and return of the HVAC system in question. The static pressure equation is simple: you must add together the measurement from the return and the supply.
According to Pascal's Law, “The external static pressure applied on a confined liquid is distributed or transmitted evenly throughout the liquid in all directions”. The static pressure acts at right angles to any surface in contact with the fluid.
In choosing a static pressure sensor, the HVAC controller should be operating in the upper half of the sensor's normal operating range. The normal range for building static pressure is from 0.02 in. to 0.06 in.