**Hi Friends,**

Generally,
fluidized bed coolers are preferred for lowering down the temperature of
calcined alumina in medium to big size Alumina refineries. But for small
capacity Alumina plant is not economical as sophisticated fully
mechanized alumina cooling facilities may not be cost effective,
thus installation of semi-mechanized equipment like Air cooler and Water cooler
are considered for the purpose.

In previous post, we have covered

**Bauxite digestion technology for gibbsite-boehmite mixed bauxite.**

**Also, we have published a very interesting article on "Impact of design defects in performance of Alumina refinery."**

In present paper,
we would broadly discuss the methodology adopted for sizing of air cooler and
water cooler including the calculations for working out the rating of their
drive motors required for their successful operation. Here, production rate of
1.00 tph Refractory grade alumina (RGA) has been considered in the sample
calculations for sizing the Air cooler and Water cooler used as Primary cooler
and Secondary cooler for lowering down the temperature of alumina to desired
level.

**Sizing of Air Cooler:**

In first step, hot
calcined alumina coming out from Rotary Kiln at around 1400

^{o}C is cooled with air in Air cooler. The generated hot air is passed in counter-current direction of alumina flow through cooler. Let the temperature of alumina drops to about 300^{o}Cand temperature of air rises from 30^{o}C to 95^{o}C. The hot air generated is fed to the Kiln for combustion of HFO.
Heat given by
alumina = 1000*0.27*(1400-300) = 297000 k.cals/hr.

Heat lost by radiation
= 25% of total heat = 0.25*297000 = 74250 k.cals/hr.

If w kg of
air is passing through the Air cooler for absorbing heat,

Then heat taken by
air = w*0.24*(95-30) = w*15.6 k.cals/hr.

Since Heat In =
Heat Out.

Thus w*15.6 + 74250
= 297000.

Therefore, w =
14279 kg/hr.

Hence, air flow
through Air cooler = 14279 kg/hr.

For design of Air
cooler, mass velocity of air through Air cooler is taken as 45,00 kg/hr.m

^{2 .}
Thus, the cross
sectional area of Air cooler = 14279 / 4500 m

^{2}= 3.17 m^{2}.
∴ (π/4)*D

^{2 }= 3.17
∴ D = 2.0 meters.

For Air coolers, L/D =4; therefore, L
= 8.0 meters.

Drive motor rating for Air cooler =
4.D

^{2 }= 4* (2)^{2 }= 16.0 kW.**Sizing of Water Cooler:**

Calcined alumina
coming out from Air cooler is further cooled in Water cooler. Here, cooling water
is sprayed over rotating shell, but the contact of water with alumina is
completely avoided. In this case, rise in temperature of water is less but
desired cooling of product alumina is easily achieved. It is assumed that final
temperature of product alumina is controlled at 90 deg.C and temperature of
water rises from 30 to 40 deg.C.

Hence heat given by
alumina = 1000*0.27*(300-90) = 56700 k.cals/hr.

Heat taken by water
= w*1.0*(40-30) = 10*w k.cal/hr.

Since heat loss =
heat gain,

Thus 10*w = 56700

Therefore, w = 5.67
m3/hr.

Thus water flow to
Water cooler

__~__6 m3/hr.
Overall heat
transfer co-efficient for water cooler = 50 k.cals/hr.m2.deg.C.

So, 56700 =
50*A*(40-30).

Therefore, A =
113.4 sq.m.

For Water cooler,
L/D = 7.

Hence, π*D*L =
113.4,

Or, π*D*7D =
113.4

Thus, D =2.27 m and
L = 15.9 m.

Drive motor rating for Water Cooler =
4.(D)

^{2}= 4*(2.27)^{2}=20.6 kW.
Trust, design
calculations for sizing of Air cooler and Water cooler required for Alumina
refinery / any other industries have been described systematically. Please put
your remarks / comments.

Regards.

**Rajendra Kunwar**

**www.ceti.co.in**

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