Flexible Duct Pressure Drop Calculator
| Airflow (CFM) | Duct Length (feet) | Pressure Drop (inches of water) |
|---|---|---|
| 50 | 25 | |
| 100 | 50 | |
| 150 | 75 | |
| 200 | 100 | |
| 250 | 125 |
FAQs
What is the pressure loss per 100 feet of ducts? The pressure loss per 100 feet of ducts depends on various factors such as duct size, shape, airflow velocity, and friction. It can be calculated using duct pressure loss charts or using duct pressure loss equations.
How do you calculate pressure drop in ductwork? Pressure drop in ductwork can be calculated using the Darcy-Weisbach equation or the Colebrook-White equation, which takes into account factors such as duct size, length, surface roughness, and airflow velocity.
What is the max distance for flex duct? The maximum distance for flex duct varies depending on the duct diameter, type, and local building codes. Generally, it is recommended to limit the length of flexible duct runs to around 25 to 30 feet.
How much CFM is lost through ducting? The amount of CFM lost through ducting depends on the duct system design, including duct size, length, and fittings. Proper duct design and installation can minimize CFM losses.
What is the rule of thumb for duct static pressure? The rule of thumb for duct static pressure is to design the system to have a static pressure between 0.1 to 0.3 inches of water column per 100 feet of duct.
What is the rule of thumb for HVAC duct sizing? The rule of thumb for HVAC duct sizing is to size the duct based on the required airflow rate, ensuring the velocity does not exceed 600 to 900 feet per minute (FPM) for supply ducts and 700 to 1000 FPM for return ducts.
What is the formula for pressure drop in valve? The pressure drop across a valve can be calculated using the valve flow coefficient (Cv) and the flow rate. The formula is ΔP = (Cv * ρ * Q^2) / (2 * A^2), where ΔP is the pressure drop, ρ is the fluid density, Q is the flow rate, and A is the valve area.
What are the pressure losses in ducts? The pressure losses in ducts are caused by friction between the air and the duct surfaces, as well as by fittings and bends in the duct system.
How do you calculate CFM from pressure drop? CFM (Cubic Feet per Minute) can be calculated from pressure drop using the fan laws, which relate CFM, pressure, and fan performance. Alternatively, CFM can be measured directly using airflow measurement tools.
What are the rules for flex duct? The rules for flex duct installation include limiting the length of runs, avoiding sharp bends, proper support, and securing connections with approved clamps or tapes.
What is the disadvantage of flex duct? The disadvantages of flex duct include higher pressure drop, potential for kinking or sagging, and higher susceptibility to damage compared to rigid ducts.
What is the two foot rule for ductwork? The two-foot rule for ductwork refers to the practice of keeping the distance between any bends or fittings in the duct at least two times the duct diameter to reduce pressure losses.
Can I do 600 CFM with 6 in ducting? The ability to achieve 600 CFM with 6-inch ducting depends on the system’s static pressure, duct length, and other design factors. Proper duct sizing is essential to achieve the desired airflow.
Does increasing duct size increase CFM? Increasing duct size can increase the CFM capacity if the system’s static pressure remains constant or decreases. It allows for lower friction losses, resulting in higher airflow.
How do you calculate flexible duct size? Flexible duct size can be calculated based on the required airflow rate, available static pressure, and the length of the duct run. Proper duct sizing charts or software can assist in the calculation.
What is the recommended velocity for flexible duct? The recommended velocity for flexible duct typically ranges from 700 to 900 feet per minute (FPM) for supply ducts and 600 to 800 FPM for return ducts.
How much static pressure is too much? Static pressure exceeding 0.5 inches of water column is generally considered high and can lead to reduced airflow and efficiency issues in the HVAC system.
What is considered high-pressure ductwork? Ductwork with static pressure exceeding 0.5 inches of water column is considered high-pressure ductwork.
Does duct length affect CFM? Yes, duct length can affect CFM due to pressure drop and friction losses along the length of the duct.
How many CFM per square foot? The recommended CFM per square foot varies depending on the application. For residential HVAC systems, the common rule of thumb is 1 CFM per square foot of living space.
What is the formula for calculating duct size? The formula for calculating duct size involves determining the required airflow rate (CFM) and selecting the appropriate duct size that maintains the recommended velocity and pressure drop.
Does pressure drop increase with flow rate? Yes, pressure drop typically increases with higher flow rates due to increased friction losses and higher velocities.
Does pressure drop reduce flow rate? Yes, pressure drop reduces the flow rate of the fluid or air passing through the system.
What is the maximum allowable pressure drop in valve? The maximum allowable pressure drop in a valve depends on the specific application and the system’s requirements. Generally, it is recommended to keep the pressure drop within acceptable limits to maintain proper system performance.
What is acceptable pressure loss? Acceptable pressure loss depends on the system requirements and the application. In HVAC systems, pressure losses of 0.1 to 0.3 inches of water column per 100 feet of duct are typically acceptable.
What is the difference between pressure loss and pressure drop? Pressure loss and pressure drop are often used interchangeably and refer to the reduction in pressure from one point to another in a fluid or air system.
What happens to pressure when pipe size decreases? When the pipe size decreases, the fluid or air velocity increases, leading to higher pressure losses due to increased friction.
What is the CFM at 90 PSI? The CFM at 90 PSI depends on the specific compressor and its performance rating.
How do you convert pressure drop to flow rate? Pressure drop can be converted to flow rate using the fan laws, which relate pressure drop, flow rate, and fan performance.
How do you convert pressure to air flow rate? Pressure to airflow rate conversion depends on the system’s characteristics and requires knowledge of fan performance or airflow measurement data.
How many CFM does a flex duct have? The CFM capacity of a flex duct depends on its size, length, and the system’s static pressure.
Should flex duct be pulled tight? Flex duct should not be pulled too tight as it can restrict airflow and lead to increased pressure losses.
Should flex duct be hung? Yes, flex duct should be hung or supported adequately to prevent sagging and kinking.
Does Flex duct reduce airflow? Flex duct can reduce airflow due to its higher resistance and pressure drop compared to rigid ducts.
What is the alternative to flex ductwork? Rigid ductwork, such as sheet metal or duct board, is an alternative to flex ductwork.
How often should flexible ductwork be replaced? Flexible ductwork should be replaced when it shows signs of damage, wear, or deterioration. Proper maintenance and inspection can help determine the replacement interval.
Does reducing duct size increase pressure? Yes, reducing duct size can increase static pressure due to increased air velocity and friction losses.
Is it better to push or pull air through a duct? It is generally better to push air through a duct as pulling air can cause the duct to collapse or become dislodged.
Is oversizing ductwork bad? Oversizing ductwork can lead to reduced system efficiency, increased noise, and improper air distribution.
Is 900 CFM too much? The suitability of 900 CFM depends on the specific application, system design, and requirements.
How many CFM do I need for 1000 square feet? The required CFM for 1000 square feet depends on factors like occupancy, cooling/heating load, and system type. The rule of thumb is typically around 1 CFM per square foot.
How many CFM do I need for 50 square feet? The required CFM for 50 square feet depends on the cooling/heating load and system type. The rule of thumb is typically around 1 CFM per square foot.
How many CFM do I need for 120 square feet? The required CFM for 120 square feet depends on the cooling/heating load and system type. The rule of thumb is typically around 1 CFM per square foot.
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