CHAPTER
8 2. The rate of boiling is governed by the heat transfer equations. 3. For multiple effect
evaporators, i.e. two or more evaporators used in series,
4. Multipleeffect evaporators use less heat than singleeffect evaporators. For an neffect evaporator, the steam requirement is approximately 1/n, but requires more heat exchange surface; the heatexchange surface required is approximately n times for the same output. From the energy viewpoint, evaporators can therefore be very much more efficient than dryers. 
5.
Condensers on an evaporator must provide sufficient cooling to condense
the water vapour from the evaporator. Condenser calculations are based
on the heat transfer equation.
6. Boilingpoint elevation in evaporators can be estimated using Duhring's rule that the ratio of the temperatures at which two solutions exert the same pressure is constant. 7. Special provisions, including short residence times and low pressures to give low boiling points, are necessary if the maximum retention of volatile constituents is important, and to handle heat sensitive materials. 1.
A singleeffect evaporator is to produce a 35% solids tomato concentrate
from a 6% solids raw juice entering at 18°C. The pressure in the evaporator
is 20 kPa(absolute) and steam is available at 100 kPa gauge. The overall
heattransfer coefficient is 440 J m^{2} s^{1 }°C^{1},
the boiling temperature of the tomato juice under the conditions in the
evaporator is 60°C, and the area of the heattransfer surface of the
evaporator is 12 m^{2}. Estimate the rate of raw juice feed that
is required to supply the evaporator. 2.
Estimate (a) the evaporating temperature in each effect, (b) the reirements
of steam, and (c) the area of heat transfer surface for a two effect evaporator.
Steam is available at 100 kPa gauge pressure and the pressure in the second
effect is 20 kPa absolute. Assume an overall heattransfer coefficient
of 600 and 450 J m^{2} s^{1} °C^{1} in
the first and second effects respectively. The evaporator is to concentrate
15,000 kg h^{1} of raw milk from 9.5 % solids to 35% solids.Assume
the sensible heat effects can be ignored, and that there is no boilingpoint
elevation. 3.
A plate evaporator is concentrating milk from 10% solids to 30% solids
at a feed rate of 1500 kg h^{1}. Heating is by steam at 200 kPa
absolute and the evaporating temperature is 75°C. (a) Calculate the
number of plates needed if the area of heating surface on each plate is
0.44 m^{2} and the overall heattransfer coefficient 650 J m^{2}
s^{1 }°C^{1}. (b) If the plates, after several hours
running become fouled by a film of thickness 0.1 mm, and of thermal conductivity
0.1 J m^{1} s^{1} °C^{1}, by how much would
you expect the capacity of the evaporator to be reduced? 4.
(a) Calculate the evaporation in each effect of a tripleeffect evaporator
concentrating a solution from 5% to 25% total solids at a total input
rate of 10,000 kg h^{1}. Steam is available at 200 kPa absolute
pressure and the pressure in the evaporation space in the final effect
is 55 kPa absolute. Heattransfer coefficients in the effects are, from
the first effect respectively, 600, 500 and 350 J m^{2} s^{1
}°C^{1}. Neglect specific heats and boilingpoint elevation.
(b) Calculate also the quantity of input steam required per kg of water
evaporated. 5.
If in Problem 4 there were boiling point elevations of 0.60, 1.50 and
4°C respectively in the effects starting from the first, what would
be the change in the requirement of input steam required per kg water
evaporated? 6.
(a) Estimate the 12°C cooling water requirement for a jet condenser
to condense the 70°C vapours from an evaporator which concentrates
4000 kg h^{1} of milk from 9% solids to 30% solids in one effect.
If the cooling water leaves at a maximumm of 25°C. (b) For the same
evaporator estimate for a surface condenser, the quantity of 12°C
cooling water needed and the necessary heattransfer area if the cooling
water leaves at a maximum of 25°C and the overall heattransfer coefficient
is 2200 J m^{2} s^{1} °C^{1} in the condenser. 7.
If in the evaporator of Problem 6, onehalf of the evaporated vapour were
mechanically recompressed with an energy expenditure of 160 kJ kg^{1},
what effect would this have on the steam economy, assuming the steam supply
was at 100 kPa absolute? 8.
A standard calandria type of evaporator with 100 tubes, each 1 m long,
is used to evaporate fruit juice with approximately the same thermal properties
as water. The pressure in the evaporator is 80 kPa absolute, and in the
steam jacket 100 kPa absolute. Take the tube diameter as 5 cm. Estimate
the rate of evaporation in the first evaporator. Assume the juice enters
at 18°C ^{}and the overall heat transfer coefficient is 440
J m^{2} s^{1.
}°C^{1}.
[ 72.5 kgh^{1} ] CHAPTER 9: CONTACTEQUILIBRIUM SEPARATION Processes Back to the top 
