QUESTION $1$

The process for producing cassava flour $($shown in the figure below$)$ involves mixing wet,

mashed cassava with dried cassava flakes in a $90$:$10$ weight ratio and passing the mixture

through a granulator before drying using a spray dryer. The wet, mashed cassava contains

$82\%$ water wet basis and the dried cassava flakes contain $3\%$ water wet basis. There are no

losses from the material entering and leaving the granulator. The moisture content of the

final product $($cassava flour$)$ is negligible.

Calculate the following:

$1\mathrm{.}1$ The amount of dried cassava flakes $($X kg$)$ entering the mixer and the amount of water

$($Z kg$)$ that must be removed by the spray dryer for every $100$ kg of cassava flour

produced. $(8)$

$1\mathrm{.}2$ The moisture content $($in $\%$ m$/$m wet basis$)$ to the nearest whole number of the

granulated paste fed to the spray dryer. $(4)$

$[12$ MARKS$]$

QUESTION $2$

A cold store is to be erected to maintain an internal temperature of $18$oC with a surrounding

air temperature of $25$oC$.$ It is to be constructed of concrete blocks $20$ cm thick and then $15$ cm

of polystyrene foam. The surface coefficients of heat transfer are $10$ J$/$m$2$soC for the external

air and $6$ J$/$m$2$soC for the internal air and the store is $40$ x $20$ x $7$ m high. Thermal conductivity

of concrete is $0\mathrm{.}87$ J$/$msoC and for polystyrene foam $0\mathrm{.}036$ J$/$msoC$.$ Ignoring the heat loss

through the floor, determine the rate of heat transfer in kW $($to the nearest whole number$)$

into the cold store from its surrounding air.

q $=$ U A $$T

$1/$U $=1/$hi $+$ z$1/$k$1+$ z$2/$k$2+1/$ho

$[10$ MARKS$]$

MIXER GRANULATOR SPRAY DRYER

Wet, mashed cassava $($Y kg$)=9$Xkg

$82\%$ water

Dried cassava flakes $($X kg$)$

$3\%$ water

Water $($Z kg$)$

Cassava flour $(100$ kg$)$

$0\%$ water

Granulated

paste

Page $3$ of $5$

QUESTION $3$

Apple juice at $25$oC with $10\%$ total solids is being concentrated to $50\%$ total solids in a single$-$

effect evaporator. The product moisture evaporates at $65$oC$,$ while saturated steam is being

supplied at $100$oC$.$ Condensate is released at $100$oC$.$ The feed material enters the evaporator

at a rate of $200$ kg$/$h$.$ A small amount of energy, $2500$ kJ$/$h$,$ leaves the evaporator as losses to

the surrounding environment. The overall heat transfer coefficient is $943$ W$/$m$2$oC$.$ The

specific heat of the feed is $4$ kJ$/$kgoC and of concentrated product is $3\mathrm{.}2$ kJ$/$kgoC$.$ Calculate:

$3\mathrm{.}1$ The steam economy $($mv$/$ms$)$ for the evaporator. $(12)$

$3\mathrm{.}2$ The area of heat transfer for the evaporator in m$2$ to two decimal places. $(5)$

$\{$Rate of heat transfer q $=$ UA$($Ts $$ T$1)=$ ms$($Hvs $$ Hcs$)\}$

$[17$ MARKS$]$

QUESTION $4$

$1000$ kg$/$min of air, originally at $62$oC dry bulb temperature and $35$oC wet bulb temperature is

heated to $98$oC dry bulb temperature and then used in drying by passing consecutively over $2$

shelves containing diced apples in a tray dryer. The following conditions are applicable to the

dryer:

$$ After the first drying cycle in the first shelf, the air regains its original dry bulb

temperature $(62$oC$).$

$$ After the first drying cycle, the air is reheated to $98$oC dry bulb temperature and used

for the second drying cycle in the second shelf.

$$ After the second drying cycle in the second shelf, the air regains its original dry bulb

temperature $(62$oC$).$

$$ After the second drying cycle, the air is mixed with another stream of air $($named X$)$ to

produce a resultant air stream which has a dry bulb temperature of $72$oC and moisture

content of $64$ g$/$kg dry air. The weight of air stream X is half that of the air obtained

after the second drying cycle.

$4\mathrm{.}1$ On a copy of the psychrometric chart, show and clearly label the following:

$$ The two air heating processes $(2)$

$$ The two drying cycles $(2)$

$$ The position of air stream X $(4)$

$4\mathrm{.}2$ Determine the total rate of removal of water in kg$/$s $($to $2$ decimal places$)$ from the

diced apples in the tray dryer. $(7)$

$4\mathrm{.}3$ Estimate the moisture content and the specific volume of air stream X$.(4)$

$[19$ MARKS$]$