The document defines types of pressure including static pressure, velocity pressure, and total pressure. It explains that total pressure is the sum of static and velocity pressures. It also discusses pressure losses that occur in ductwork from factors like friction, fittings, and changes in velocity or flow direction.
The document defines types of pressure including static pressure, velocity pressure, and total pressure. It explains that total pressure is the sum of static and velocity pressures. It also discusses pressure losses that occur in ductwork from factors like friction, fittings, and changes in velocity or flow direction.
The document defines types of pressure including static pressure, velocity pressure, and total pressure. It explains that total pressure is the sum of static and velocity pressures. It also discusses pressure losses that occur in ductwork from factors like friction, fittings, and changes in velocity or flow direction.
The document defines types of pressure including static pressure, velocity pressure, and total pressure. It explains that total pressure is the sum of static and velocity pressures. It also discusses pressure losses that occur in ductwork from factors like friction, fittings, and changes in velocity or flow direction.
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Definition of Total Pressure,
Static Pressure, Velocity
Pressure Static Pressure, • Definition: The pressure exerted by a still liquid or gas • Measured perpendicular to the direction of flow • Overcome resistance in flow due to dynamic losses(hood, fittings, dampers) and duct friction Velocity Pressure, • Definition: The pressure created by the movement of air • Also known as dynamic pressure • Measured indirectly (By finding the difference between and ) • - = Total Pressure, • Definition: Sum of all pressures present in a reference system. According to Bernoulli principle, the total pressure consists of static pressure, velocity pressure and geodetic pressure P + + gz = constant Static Pressure Velocity Pressure Geodetic Pressure • There is usually no height difference in ducting system. Therefore, geodetic pressure can be neglected • = + Pressure for airflow in a duct Suction Side Pressure Side
= = -345 + 248 = -97 Pa 97 + 248 = 345 Pa Total Pressure below atmospheric pressure Total Pressure above atmospheric pressure Types of pressure losses in pipework & ductwork Total Pressure Loss
To obtain the fan static pressure requirement for fan selection where fan total pressure is known,
= + System total pressure static pressure velocity/dynamic pressure Factor affecting friction loss
● Length ● Roughness of material ● Fluid density ● Velocity/Velocity pressure ● Diameter Static Pressure Loss (
● Due to wall friction between the fluid and the interior surfaces of a duct/pipe ● Can be calculated using the Darcy Equation: Hydraulic Diameter (DH) Simplifies analysis of a non-circular cross-section by representing an “equivalent diameter” for calculations.
DH = 4 × A ÷ P Hydraulic Diameter = 4 × Cross-Sectional Area ÷ Perimeter Friction Loss Factor, (f) The friction factor (f) is calculated using the formula: Velocity/Dynamic Pressure Loss () • Due to the turbulent flow caused by sudden changes in flow direction or magnitude of air velocity • Occurs whenever an air stream make turns, diverges, narrows, widens, enters, exits, passes dampers, etc • Hood entry, bends, elbows, dampers, fan, etc Dynamic loss • Dynamic losses are proportional to dynamic pressure and can be calculated using the equation:
C = Fitting Loss coefficient
= Dynamic Pressure Fitting Loss coefficient • A dimensionless number which measures the number of velocity head lost at the duct transition or bend (in terms of velocity pressure) • A single system can have multiple fitting loss which contributes to the overall pressure drop • Each type of valve, fitting or component has its own specific loss coefficient • Coefficients can be found in the SMACNA fitting loss coefficient tables Example • 0.2m diameter, Smooth Radius Elbow with R/D = 1.5. The airflow through the elbow is 500cmh. The pressure loss for the elbow can be calculated as follows:
First step: Find the velocity of the Second step: Find the velocity flow pressure = = 11.76 Pa = 0.0314
V= = = 4.42m/s Example • 0.2m diameter, Smooth Radius Elbow with R/D = 1.5. The airflow through the elbow is 500cmh. The pressure loss for the elbow can be calculated as follows:
