The feed flow enters the vessel on the left and plug flow conditions are assumed Inside the vessel, the particles settles The design is based on “critical trajectory” model, i.e. the particle size is selected, a balance is made equating the time for the particle to settle the full basin height H, and the residence time assuming plug flow. All particles of this size will be collected Settling time (assuming plug flow) Residence time Plan area
Hence for complete removal of particles of a given size, the volume flow rate., divided by the corresponding terminal settling velocity is equal to the plan area for complete removal. If the plan area is too small, then not all of the particles of the selected size will be removed. In settling, it is always the plan area that is the important design parameter, not the cross-sectional area. All particles greater than the critical size will settle out in the available time, and the fraction of the smaller ones will removed will be proportional to the settling velocity, assuming that the feed flow is uniformly distributed thru-out the height of the vessel, and not all entering the vessel at the top.
SAMPLE QUESTIONS: 1. What would be the trajectory of particles of the same size as the critical size, but had started 3 their descent at a height less than H? Particle density = 2900 kg/m , and liquid density = 1000 3 kg/m , viscosity = 0.001 kg/m-s, Critical particle diameter = 50mm. The terminal settling velocity, -3 -5 -1 UT is: a)2.6 x 10 b.) 2.6 x 10 c.) 2.6 x 10 d.) 0.52 2. The particle Reynolds number is: a.) 0.26 b.) 129 c.) 0.129 d.) 0.00013 3. The expression of the particle residence time vertically: a.) tv = L/UT b.) tv = H/UT c.) tv = H/v d.) tv = L/UT 4. The expression of the particle residence time, horizontally: a.) th = LWH/Q, b.) th = H/Q , c.) th = L/UT d.) th = Q/LWH 5. An expression of LW is: a.) LW = UT/Q, b.) LW = Q/UT , c.)LW = HQ/UT , d.) LW = Q/HUT 6. What are the units of LW? What does it represent? 3 7. If the volume flow rate is 10m /min, the minimum settling area to remove all particles of size 2 2 2 2 50mm and bigger is : a.) 6.4 m b.) 64 m c.) 640 m d.) 6400 m