1、 Resistance comparison
When we calculate the ventilation duct, we usually can not do without the four elements of flow (air volume), unit friction resistance, Reynolds number and friction resistance coefficient.
In calculation, the Reynolds formula is simplified as
Where v is the kinematic viscosity coefficient (m2/s)
ν Is the wind speed (m/s)
D is the inner diameter of air duct (m)
It can also be seen from this formula that when the kinematic viscosity coefficient and pipe diameter are fixed, the larger the Reynolds number is, ν The smaller the value, the less power.
According to the experiment of the British scholar Renault, the generation of laminar flow and rate flow is related to the volume section size, characteristic velocity (average velocity) of the fluid flowing through and the viscosity coefficient and density of the fluid. See the dimensionless quantity - Ro similarity criterion number formula:
Ro -- dimensionless quantity (number of similarity criteria)
U -- mean flow velocity.
D - is the feature dimension.
P - is the density of liquid.
U - is the viscosity coefficient of the liquid.
ν-- Is the characteristic velocity.
According to the formula, we can verify the similarity criterion Ro for rectangular and elliptical pipes with a sectional area of 1 square meter. Suppose that the average velocity u of water flowing through the pipe is 5 m/s, and the characteristic velocity ν 1.13 × 10-6m2/s. When the section shape is rectangular, the number of similarity criteria is Ro1; When the section shape is elliptical, the number of similarity criteria is Ro2. Substitute the known conditions into the dimensionless formula, Ro1=5 × 1/(1.13 × 10-6)>Ro2=0.5 × zero point five × 1.28/(1.13 × 1)
We know that the premise of calculating their wet perimeter is to calculate their perimeter. As the name implies, the wet perimeter is also their wet perimeter, that is, perimeter.
The calculation formula of ellipse perimeter is
Perimeter P=3.14 × [1.5（a+b）-（a × b）1/2]
Where, a is the long radius of the ellipse and b is the short radius of the ellipse. The formula for calculating the sectional area of the ellipse is:
S=3.14 × 0.5A × 0.5B
Where, A is the major diameter of the ellipse and B is the minor diameter of the ellipse
According to the product catalog, the long diameter A and short diameter B of oval air duct with a sectional area of one square meter are 2000mm and 600mm respectively
Then: around it
P=3.14 × [1.5 × (1+0.3)-(1 × 0.3)1/2]=4(M)
Calculate the wetted perimeter of rectangular air duct with a sectional area of one square meter
Let its side moment dimension be (A × B)2000 × five hundred
That is, its wet perimeter
It can be seen that the wetted perimeter of oval duct is 20% smaller than that of rectangular duct, that is to say, the circumference of oval duct is only 80% of that of rectangular duct for ducts with the same cross-sectional area. If rectangular duct with the same cross-sectional area is changed again, and the set length moment is 2500mm and short moment is 400mm, its wetted perimeter is 5.8m, which is 31% larger than that of oval duct. Therefore, the wet perimeter of the air conditioning duct with elliptical section shape is small.
Jinan ventilation pipe processing http://www.jnqsg.com For more than ten years in the industry, due to the increase of the wet perimeter, that is, the perimeter increases, which means that more materials are needed, thus increasing the cost of investment.