What Is a Return Air Grille?

The grid is the most important auxiliary equipment in the sewage pumping station. The grid is generally composed of a set of parallel grids, which are placed obliquely at the inlet of the pumping station's sump. Its tilt angle is 60 ° ~ 80 °. A workbench should be set behind the grille. The workbench should generally be 0.5m higher than the highest water level upstream of the grille. For the grid for manual slag removal, the length of the workbench in the direction of water flow is not less than 1.2m, for the grid for mechanical slag removal, the length is not less than 1.5m, and the width of the aisle on both sides is not less than 0.7m. ^[1]

Pollutants in sewage generally exist in three forms:

According to the shape, the grille can be divided into two types: flat and curved grille. The flat grille consists of a grid and a frame. Curved grille can be divided into fixed curved grille and rotary drum grille. According to the net spacing of the grid grid, it can be divided into three types: coarse grid (50 ~ 100mm), middle grid (10 ~ 40mm), and fine grid (1.5 ~ 10mm). Both flat grille and curved grille can be made into thick, medium and fine. As the grid is an important facility for physical treatment, newly designed sewage treatment plants generally use two coarse and medium grids, and even three coarse, medium and fine grids. According to the slag cleaning method, the grille can be divided into two types: manual slag cleaning and mechanical slag cleaning. The artificial slag cleaning grid is suitable for small sewage treatment plants. When the amount of grid slag is greater than 0.2m ³ / d, in order to improve workers' labor and sanitary conditions, mechanical slag cleaning grids should be used. ^[1]

A grill must be installed before the sewage treatment system or the pump. The number of suspended matter and floating matter (collectively referred to as grid slag) that can be trapped by the grid varies greatly depending on the gap width between the selected grids and the nature of the sewage. The width of the gap between the grids of the grid should meet the following requirements: 16 ~ 25mm when mechanical removal is used before the sewage treatment system, and 25 ~ 40mm when manual removal is used; before the pump, it should be determined according to the requirements of the pump. For example, when the width of the gap between the grids of the pump grid is not greater than 20mm, the grid can no longer be provided in front of the sewage treatment system. The amount of grid slag is related to the width of the gap between the grids: when the gap width between the grids is 16 ~ 25mm, the amount of grid slag is 0.10 ~ 0.05m 3/10 ³ m ³ sewage; when the gap width between the grids is When it is 25 ~ 40mm, the amount of grid slag is 0.03 ~ 0.01m 3/10 ³ m ^{3 of} sewage. A workbench should be set behind the grille. The workbench should generally be 0.5m higher than the highest water level upstream of the grille. For the grid for manual slag removal, the length of the workbench in the direction of water flow is not less than 1.2m, for the grid for mechanical slag removal, the length is not less than 1.5m, and the width of the aisle on both sides is not less than 0.7m. ^[1]

The design content of the grid includes size calculation, hydraulic calculation, grid slag calculation, and selection of slag cleaning machinery. Figure 1 is a simplified diagram of grid calculations. Among them, 1 is a grid, and 2 is an operating platform.

Figure 1 Schematic diagram of grid calculation

(1) Grid groove width

Where Bgrid width, m;

sgrid width, m, generally s = 0.01m;

ethe net distance between grids, m, coarse grid e = 50 ~ 100mm, middle grid e = 10 ~ 40mm, fine grid e = 1.5 ~ 10mm

nthe number of intervals between the bars;

Q _max design maximum flow, m ³ / s;

Grid setting inclination angle, degree;

hwater depth in front of the grid, m;

vcrossing grid velocity, m / s, preferably 0.6 ~ 1.0m / s;

-Experience coefficient.

(2) Head loss across the grid

Where h ₁ head loss across the grid, m;

h ₀ Calculate head loss, m;

ggravitational acceleration, 9.81m / s ² ;

k-factor, the multiple of the increase of head loss after the grid is blocked by pollutants, generally k = 3;

resistance coefficient, related to the shape of the cross section of the grid,

.

For a rectangular section, = 2.42. In order to avoid causing gushing water in front of the grid, the fall of h ₁ at the bottom of the grid behind the grid is used as compensation, see Figure 1.

(3) Total height of grid slot

Where Hthe total height of the grid slot, m;

hwater depth in front of the grid, m;

h2Super high channel in front of the grid, m, generally 0.3m.

(4) Total length of grid slot

Where Lthe total length of the grid slot, m;

H ₁ height of grid front groove, m;

L ₁ the length of the gradually widened inlet channel, m;

B ₁ width of inlet channel, m;

₁ Inlet opening angle, generally adopt 20 °;

L ₂ The length of the tapered channel where the grid channel is connected to the outlet channel.

(5) Calculation of daily grid slag amount

Where Wthe daily amount of grid slag, m ³ / d;

W ₁ The amount of grid slag per 10m sewage (m 3/10 ³ m ³ sewage), take 0.1 ~ 0.01, small value for coarse grid, large value for fine grid, and median value for medium grid;

Q _max maximum design flow, m ³ / s;

KThe total coefficient of variation of sewage.