Sunday, March 18, 2018

Design of Air Coolers and Water Coolers for Alumina Refinery

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 1400oC 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 300oCand temperature of air rises from 30oC to 95oC. 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.m2 .
Thus, the cross sectional area of Air cooler = 14279 / 4500 m2 = 3.17 m2.
  (π/4)*D= 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= 4* (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
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.
Rajendra Kunwar

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