Evaporation Flashcards
an important unit operation commonly employed to remove ________
from dilute liquid foods to obtain _____________ liquid products
EVAPORATION
Water
concentrated
removal of water from foods provides __________ ________
and assists in reducing transportation and storage costs
microbiological stability
How is evaporation differs from dehydration?
the final product remains in liquid
state
How is evaporation differs distillation
the vapors produced in
the evaporator are not further divided into fractions
provides the means to transfer heat
from low-pressure steam to the product
non-contact heat exchanger
he product inside the evaporation chamber is kept under ____________ →causes the temperature difference between ______ and the _________ to increase, and the product boils at relatively_____ temperature, thus minimizing _____ damage →the vapors produced are conveyed through a _________ to a vacuum system →the steam condenses inside the heat exchanger and the condensate is ________ ( singleeffect evaporator: the vapors produced are discarded; multiple- effect
evaporator: vapors are reused as the heating medium in another
evaporator chamber)
vacuum
steam
product
low
heat
condenser
discarded
the_________ of the liquid food have a profound effect on the performance of the evaporation process
in designing evaporation systems, it is important to keep in perspective the preceding __________of liquid food
characteristics
concentrated liquid results in _________ heat transfer
reduced
the __________ _______ rises as the liquid concentrates, resulting in a smaller differential of temperature between the heating medium
and the product → causes reduced rate of heat transfer
boiling point
food products are noted for their __________
________thus evaporation
must involve reducing the temperature for boiling as well as the
time of heating, to avoid excessive product degradation
heat sensitivity
_________(make dirty) of the heat-exchange surface can seriously
reduce the rate of heat transfer
fouling
increase in boiling point over that of pure water, at a given
pressure
Boiling Point Elevation
a simple method to estimate boiling-point elevation is the use of
_______________ →states that a ______relationship exists between the boiling point temperature of the_________ and the boiling-point temperature of ________at the same______________ (hold over moderate
temperature ranges which is quite acceptable)
Duhring’s rule
linear
solution
water
pressure
Types of Evaporators
❖Batch-type Pan Evaporator
❖Natural Circulation Evaporators
❖Rising-Film Evaporator
❖Falling-Film Evaporator
❖Rising/Falling-Film Evaporator
❖Forced-Circulation Evaporator
❖Agitated Thin-Film Evaporator
one of the simplest and perhaps the oldest type of evaporator used in the food industry
Batch-Type Pan Evaporator
the product is heated in a _____________→ the heating vessel may be open to the _______ or connected to a ______ and vacuum
→permits boiling the product at temperatures lower than the ________ at atmospheric pressure, thus reducing
the thermal damage to heat-sensitive products
steam jacketed spherical vessel
atmosphere
condenser
heat transfer area per unit volume is_______
residence time is long , up to several hours
small
Batch-Type Pan Evaporator
heating of the product occurs mainly due to natural convection, resulting in smaller convective heat-transfer
coefficients
poor heat-transfer=reduce the processing capacities
Batch-Type Pan Evaporator
short vertical tubes, typically 1-2 m long and 50-100 mm in diameter, are arranged inside the steam chest
Natural Circulation Evaporators
✓the whole calandria (tubes & steam chest) is located in the bottom of the vessel
Natural Circulation Evaporators
the product, when heated, rises through these tubes by natural circulation while steam condenses outside the tubes
but the evaporation takes place inside the tubes, and the product is concentrated
the concentrated
liquid falls back to the
base of the vessel
through a _________
a _________ heat
exchanger can be
provided outside the
main evaporation
vessel to preheat the
liquid feed.
central
annular section
shell-and-tube
✓involves a noncontact heat exchanger where liquid food is circulated at high rates
Forced-Circulation Evaporator
in Forced-Circulation Evaporator a hydrostatic head, above the top of the tubes,
eliminates any_______ of the liquid
boiling
In Forced-Circulation Evaporator inside the separator, absolute pressure is kept slightly
lower than that in the tube bundle → the liquid entering the separator______ to form a vapor
✓the temperature difference across the heating surface in the heat exchanger is usually______
flashes
3-5ºC
