CHAPTER
6
2. For jacketed pans:
3. For sterilization of cans: 
(b) F value is the thermal death time at 121°C. For Clostridium botulinum, it is about 2.8 min. (c) z is the temperature difference corresponding to a tenfold change in the thermal death time (d) t_{121} = t_{T} x 10^{(121T)/z} or 4. The coefficient of performance of refrigeration plant is: (heat energy extracted in evaporator)/(heat equivalent of theoretical energy input to compressor). 5. Freezing times can be calculated from:
where for a slab P = 1/2 and R = 1/8 and for a sphere P = 1/6 and R = 1/24. An improved approximation is to substitute DH over the whole range, for l. In addition for brick shapes a multiplier of around 1.2 is needed.
1.
A stream of milk is being cooled by water in a counter flow heat exchanger.
If the milk flowing at a rate of 2 kg s^{1}, is to be cooled
from 50°C to 10°C, estimate the rate of flow of the water if it
is found to rise 22°C in temperature. Calculate the log mean temperature
difference across the heat exchanger, if the water enters the exchanger
at 5 °C.
2. A flow of 9.2 kg s^{1} of milk is to be heated from 65°C
to 150°C in a heat exchanger, using 16.7 kg s^{1} of water
entering at 95°C. If the overall heattransfer coefficient is 1300
J m^{2} s^{1} °C^{1}, calculate the area
of heat exchanger required if the flows are (a) parallel and (b) counter
flow. 3.
In the heat exchanger of worked Example
6.2 it is desired to cool the water by a further 3°C. Estimate
the increase in the flow rate of the brine that would be necessary to
achieve this. Assume that: the surface heat transfer rate on the brine
side is proportional to v^{0.8}, the surface coefficients
under the conditions of Example 6.2 are equal on both sides of the heattransfer
surface and they control the overall heattransfer coefficient. 4.
A counter flow regenerative heat exchanger is to be incorporated into
a pasteurization plant for milk, with a heatexchange area of 23 m^{2}
and an estimated overall heattransfer coefficient of 950 J m^{2}
s^{1} °C^{1}. Regenerative flow implies that the
milk passes from the heat exchanger through further heating and processing
and then proceeds back through the same heat exchanger so that the outgoing
hot stream transfers heat to the incoming cold stream. Calculate the temperature
at which the incoming colder milk leaves the exchanger if it enters at
10°C and if the hot milk enters the exchanger at 72°C. 5.
Olive oil is to be heated in a hemispherical steamjacketed pan, which
is 0.85 m in diameter. If the pan is filled with oil at room temperature
(21°C), and steam at a pressure of 200 kPa above atmospheric is admitted
to the jacket, which covers the whole of the surface of the hemisphere,
estimate the time required for the oil to heat to 115°C. Assume an
overall heattransfer coefficient of 550 J m^{2} s^{1}
°C^{1} and no heat losses to the surroundings. 6.
The milk pasteurizing plant, using the programme calculated in worked
Example 6.6, was found in practice to
have a 1°C error in its thermometers so that temperatures thought
to be 65°C were in fact 64°C and so on. Under these circumstances
what would the holding time at the highest temperature (a true 65°C)
need to be? 7.
The contents of the can of pumpkin, whose heating curve was to be calculated
in Problem 9 of Chapter 5, has to
be processed to give the equivalent at the centre of the can of a 10^{12}
reduction in the spore count of C. botulinum. Assuming a z
value of 10°C and that a 10^{12} reduction is effected after
2.5 min at 121°C, calculate the holding time that would be needed
at 115°C. Take the effect of the heating curve previously calculated
into consideration but ignore any cooling effects. 8.
A cold store is to be erected to maintain an internal temperature of 18°C
with a surrounding air temperature of 25°C. It is to be constructed
of concrete blocks 20 cm thick and then 15 cm of polystyrene foam. The
external surface coefficient of heat transfer is 10 J m^{2} s^{1}
°C^{1} and the internal one is 6 J m^{2} s^{1}
°C^{1}, and the store is 40 x 20 x 7 m high. Determine the
refrigeration load due to building heat gains from its surrounding air.
Assume that ceiling and floor loss rates per m^{2} are onehalf
of those for the walls. Determine also the distance from the inside face
of the walls of the 0°C plane, assuming that the concrete blocks are
on the outside. 9.
For a refrigeration system with a coefficient of performance of 2.8, if
you measure the power of the driving motor and find it to be producing
8.3 horsepower, estimate the refrigeration capacity available at the evaporator,
the tons of refrigeration extracted per kW of electricity consumed, and
the rate of heat extraction in the condenser. Assume the mechanical and
electrical efficiency of the drive to be 74%. 10.
A refrigeration plant using ammonia as refrigerant is evaporating at 30°C
and condensing at 38°C, and extracting 25 tons of refrigeration at
the evaporator. For this plant, assuming a theoretical cycle, calculate
the: 11.
It is wished to consider the possibility of chilling the apples of worked
Example 6.10 in chilled water instead
of in air. If water is available at 1°C and is to be pumped past the
apples at 0.5 m s^{1} estimate the time needed for the chilling
process. 12.
Estimate the time needed to freeze a meat sausage, initially at 15°C,
in an air blast whose velocity across the sausage is 3 m s^{1}
and temperature is 18°C. The sausage can be described as a finite
cylinder 2 cm in diameter and 15 cm long. 13.
If the velocity of the air blast in the previous example were doubled,
what would be the new freezing time? Management then decide to pack the
sausages in individual tightfitting cardboard wraps. What would be the
maximum thickness of the cardboard permissible if the freezing time using
the higher velocity of 6 m s^{1} were to be no more than it had
been originally in the 3 m s^{1} air blast. 14. If you found
by measurements that a roughly spherical thin plastic bag, measuring
30 cm in diameter, full of wet fish fillets, froze in a 30°C air
blast in 16 h, what would you estimate to be the surface heattransfer
coefficient from the air to the surface of the bag? CHAPTER 7: DRYING Back to the top 
