The process of separating compost into different grain sizes by sifting surface is called waste sifting. Compost trommel screen is widely used compost screening machinery, it is the use of rotary cylindrical sieve body will compost according to the granularity of the classification of machinery. The screen surface is generally woven net or punching thin plate, working cylindrical screen body inclined installation. The compost to be sifted turns spirally with the motion of the screen body. Material with smaller size than the sieve is sifted down, while material remaining on the screen body is discharged from the bottom of the screen body. Usually the cylindrical screen body is supported on the supporting roller on the frame through the ring body, as shown in Figure 1.
Although the use of compost trommel screen has been very common, but the domestic research on this kind of machinery is little, the design of its parameters through experience. In this paper, the main design parameters will be derived by studying the motion characteristics of the material in the screening process.
Compost trommel screen design parameters are divided into geometric parameters, motion parameters and
Dynamic parameters. The geometric parameters include the length of the screen body L, the diameter of the screen body d, the installation Angle 0, the diameter of the screen hole d, the motion parameter is the speed of the screen body n, the dynamic parameter is the driving power of the screen body P.
Usually the known conditions of the design of the trommel screen are:
(1) Productivity, that is, the amount of compost handled by the trommel screen per unit time, is generally measured by volume; (2) Screening efficiency n, that is, the ratio of the actual measured amount of material under the screen to the theoretical amount of material under the screen n=c/eX100%, where c is the proportion of the actual material under the screen to the feeding amount, e is the proportion of the content less than the sieve size in the feed; (3) Screen size, it is the limit of the sieve and the boundary of the sieve, sieve size should be considered the specific use of the trommel screen, but also should consider the composition of the waste screening and the requirements of the sieve or sieve; (4) The physical characteristics of the screened material, the material characteristics that affect the screening efficiency mainly include the unit weight of the material, the shape of the material and the friction characteristics of the material.
2 The movement of materials in the compost trommel screen The movement of materials in the trommel screen can be decomposed into the linear movement along the axis of the screen body and the plane movement perpendicular to the axis of the screen body. The linear motion along the axis of the screen body is generated by the inclined installation of the screen body, and its speed is the speed of the material passing through the screen body.
The movement of the material in the plane perpendicular to the axis of the screen body is closely related to the rotating speed of the screen body.
When the screen body speed is low, the material with the rotation of the compost trommel screen body began to offset, when the inclination exceeds the natural Angle of rest began to slide (or roll), this movement is called falling movement; With the increase of the speed of the screen body, the material is brought to a certain height after parabolic drop, this movement is called dropping motion, dropping motion is conducive to the screening; If the speed of the screen body exceeds a certain critical value, the material is no longer separated
Screen surface and do centrifugal movement, at this time the material can not be screened, the critical value of the screen body speed is called the critical speed.
w=LOY(9-8cosα)8sin'u tar
Can be used to analyze the movement law of the material. When the material arrives at point A and leaves the screen surface for dropping motion, the normal component N of the material's gravity G is equal to the centrifugal force c, that is, mv/R=Gcos. G=mg, v=ⅡRn/30 can be obtained by substituting G=mg, V = ⅱrn /30 into the above equation to obtain 30&cos zn=(1). When the material reaches point Z, the speed is the critical speed
The Z speed of the compost trommel screen is always lower than the critical speed and the throwing motion. Therefore, the motion of the material in vertical XG straight to the axis plane of the screen body is a combination of circular motion and falling motion. In Fig.2, the material leaves the screen surface at point BA for throwing. Fig.2 The material falls in the compost trommel screen, and then moves in A circular motion with the screen body to point A after reaching point B. Take point A as the origin to establish a plane coordinate system, and the trajectory equation of circular motion is:
When the screen body is installed tilted, the actual material movement track becomes an irregular spiral, the spiral pitch △1 is approximately:
△I=lya-yalta rf=4Rsinwcosutarf(0 is the installation Angle), the time required for the material to complete a cycle movement (walking a pitch) r=ci(180-2z)+120sim.cox L3nJInci is the correction factor considering the sliding factor of acceleration section. So the average velocity of the material along the axis of the screen surface can be expressed as V=△l/t(2).
3 Determination of the main parameters of the trommel screen 3.1 Screening efficiency and productivity in the design of the compost trommel screen should first determine its main parameters to meet the requirements
Screen in the screening of specified material to meet the design requirements (generally productivity and screening efficiency). Usually the compost trommel screen other parameters are determined, its productivity and screening efficiency into a nonlinear inverse proportional relationship.
Usually we always define the material flow Qo at the feed as the productivity of the trommel screen. Obviously increase the material layer cross-sectional area qo(or generally said layer thickness) at the feed, can improve productivity, but when the other parameters of the trommel screen are unchanged, screening efficiency will be significantly reduced, so to obtain the same screening efficiency, must change other parameters, the simplest is to increase the length of the screen body. Taking into account the structure of the trommel screen, generally take the length of the screen is 3~5 times the diameter of the drum.
Usually we can determine the screening capacity of the screen unit area under the given screening efficiency through the test to estimate the productivity of the trommel screen.
3.2 trommel screen speed The trommel screen speed n is an important design parameter. Due to the existence of centrifugal force in the rotary movement of the material, the value of the screen body speed n is generally less than its critical speed ne, usually in order to obtain a better screening effect, should make the material in the screen body to make a larger flip. Can calculate the material in the screen body to obtain the maximum drop conditions, that is, in Figure 2 make LYC-y Bl=(Rsi no cosx) /2+4Rsinocos maximum, let lyc~yal'=0, α=54.7, at this time the trommel screen speed n, can be calculated by equation (1) n, The test shows that the rotary speed of compost trommel screen is generally 30%~60% of the critical speed is ideal, and the value is slightly lower than the speed n required by the material to obtain the maximum drop.
3.3 the retention period of material in the sieve sieve content inside the screen time for t = L/V, type of L for the length of the compost trommel screen, V for materials in the sieve along the axial movement of the average velocity can be calculated by type (2), the type of O (installation Angle) usually take.
3.4 Useful power compost trommel screen Useful power N is an important parameter, can be derived to calculate the formula w=LOY(9-8cosα)8sin'u tar
