L-23Gravity settling chamber

Air Pollution and Control

(Elective-I)

Gravity settling chamber

• Gravity settling chamber is used toremove large and abrasive particlesgreater than 50 µ from a gas stream.

• This is a simple particulatecollection device using the principle ofgravity to settle the particulate matter in agas stream passing through its longchamber. 2Prof S S Jahagirdar, NKOCET

• The primary requirement of such a devicewould be a chamber in which the carriergas velocity is reduced so as to allow theparticulate matter to settle out of themoving gas stream under the action ofgravity.

• This particulate matter is then collected atthe bottom of the chamber.

• The chamber is cleaned manually todispose the waste.

3Prof S S Jahagirdar, NKOCET

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Gas outlet

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Gravity settling chamber with trays

7Prof S S Jahagirdar, NKOCET

• The gas velocities in the settling chamber must be sufficiently low for the particles to settle due to gravitational force.

• Literature indicates that gas velocity less than about 3 m/s is needed to prevent re-entrainment of the settled particles. The gas velocity of less than 0.3 m/s will produce good results.

8Prof S S Jahagirdar, NKOCET

Design Parameters:-For 100 % efficiency

--------------Eq -i

Where,

H= height of settling chamber

L = Length of settling chamber

Vt = Velocity of settling

VH = velocity of flow Prof S S Jahagirdar, NKOCET 9

t H

H LtV V

From equation I Settling velocity may be given by

-------------Eq – ii

W= width of chamber

(As Q = area of inlet x velocity = W.H.VH )

For streamline flow settling velocity is given by

------------- Eq- iii

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HV H QVt

L LW

2. .

18.

p p

t

g dV

• Equating equations ii and iii,

therefore, we get,

-------Eq iv

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2. .

18.

p pHg dV H

L

min

18. .

. .H

p

p

H Vd

g L

dpminis min size of particle that can be removed

with 100 % efficiency

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min

18. .2

. .H

p

p

H Vd

g L

For better and conservative results multiplying factor 2 can be taken

Efficiency of settling chamber is given by

min

2

p

p

d

p

d

d

• Another equation for calculation of efficiency is

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2. .1 exp

18. . .

p p

H

g d L

V H

Settling chamber with trays

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Settling trays can be used to improve removal efficiency.

If, n = number of trays, then equation iv becomes

min

18. .

. . .H

p

p

H Vd

n g L

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Advantages1. Low initial cost

2. Simple construction

3. Low maintenance cost

4. Low pressure drop

5. Dry and continuous disposal of collected solids

6. Can be constructed of any material

7. Temperature and pressure limitations are imposed by type of material used

Prof S S Jahagirdar, NKOCET

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Disadvantages

1. Large space requirement

2. Only comparatively large particles can be collected

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Applications of settling chamber• Settling chambers are mostly used as

pre-cleaners. They are sometimes used in the process industries, particularly in the food and metallurgical industries as the first step in dust control.

• Use of settling chambers as pre-cleaners can also reduce the maintenance cost of high efficiency control equipment, which is more subject to abrasive deterioration. 18

Prof S S Jahagirdar, NKOCET

Problem (Peavy -Rowe)

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Assume viscosity of air = 2.1 x 10-5 kg/m.s

Solution :-

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Objective questionsQ1. Gravity settling chamber removes _____________

particles.

Q2. In gravity settling chamber, the gas velocity of less than _______ m/s will produce good results.

Q3. If trays are used in settling chamber efficiency _______________.

Q4. True or false?

1.Settling chamber can be connected with other control equipments in series to increase efficiency.

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2. Settling chamber can remove less than 1 µ particles with 90% efficiency.

3. Increase in flow velocity increases efficiency of settling chamber.

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Theory questions

Q1. Explain working of gravity settling chamber with neat sketch. Also give advantages and disadvantages.

Q2. Write a note on ‘Design of gravity settling chamber’.

23Prof S S Jahagirdar, NKOCET