Lecture "Chapter 7: Evaporator" content presentation: General, air cooling evaporator, liquid cooling evaporator, plate evaporator, defrosting, calculate and choose evaporator calculate and choose evaporator. Invite you to reference.
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Nội dung Text: Lecture Chapter 7: Evaporator - ThS. Nguyễn Duy Tuệ
- CHAPTER 7: EVAPORATOR
Lecturer : ThS
ThS.Nguyễn
Nguyễn Duy Tuệ
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- OBJECTIVES
In this chapter,
chapter student
st dent can:
can
- understand structure, function of some
evaporators
- calculate and choose evaporator met to the
requirements
q
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- REFERENCES
[1]. Refrigeration and Air-Conditioning
Air Conditioning - A. R.
Trott and T. Welch
[2]. Evaporator catalogue
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- CONTENT
GENERAL
AIR COOLING EVAPORATOR
LIQUID COOLING EVAPORATOR
PLATE EVAPORATOR
DEFROSTING
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- CONTENT
CALCULATE AND CHOOSE EVAPORATOR
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- GENERAL
The purpose of the evaporator is to receive low-
pressure low temperature fluid from the expansion
pressure,
valve and to bring it in close thermal contact with the
load. The refrigerant takes up its latent heat from the
load and leaves the evaporator as a dry gas.
Evaporators are classified according to their
refrigerant flow pattern and their function.
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- AIR COOLING EVAPORATOR
- Air cooling evaporators for coldrooms, blast
freezers air-conditioning…will
freezers, air conditioning will have finned pipe
coils
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- AIR COOLING EVAPORATOR
- Construction materials will be the same as for
air cooled condensers.
air-cooled condensers Aluminium fins on copper
tube are the most common for the halocarbons, with
stainless steel or aluminium tube for ammonia.
- Frost or condensed water will form on the fin
surface and must be drained away. To permit this,
fins will be vertical and the air flow horizontal, with a
drain tray provided under.
- The
Th size
i off the
th tube
t b will
ill be
b suchh that
th t the
th velocity
l it
of the boiling fluid within it will cause turbulence to
promote heat transfer.
transfer Tube diameters will vary from
9 mm to 32 mm, according to the size of coil.
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- AIR COOLING EVAPORATOR
- Fin spacings will vary from 2 mm on a compact
airconditioner to 12 mm on a low-temperature
low temperature
coldroom coil.
- Some evaporator haven
haven’tt got fin and fan; so we
call “natural evaporator”
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- LIQUID COOLING EVAPORATOR
1. Shell and tube evaporator :
Liquid cooling is mostly in shell-and-tube
shell and tube or
shell-and-coil evaporators. In the shell-and-tube
type, the liquid is usually in the pipes and the shell is
some three-quarters full of the liquid, boiling
refrigerant. A number of tubes is omitted at the top
of the shell to give space for the suction gas to
escape clear of the surface without entraining
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- LIQUID COOLING EVAPORATOR
The speed of the liquid within the tubes should
be about 1 m/s or more, more to promote internal
turbulence for good heat transfer. End cover baffles
will constrain the flow to a number of passes, as
with the shell-and-tube condenser.
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- LIQUID COOLING EVAPORATOR
Evaporators of this general type with dry
expansion circuits will have the refrigerant within the
tubes, in order to maintain a suitable continuous
velocity for oil transport, and the liquid in the shell
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- LIQUID COOLING EVAPORATOR
+ Sprayed tube liquid chiller : The spray chiller
operates with a much lower refrigerant charge than
a conventional flooded evaporator does.
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- LIQUID COOLING EVAPORATOR
- The liquid refrigerant level in the surge drum
shell is kept below the tubes and liquid is pumped to
spray nozzles which ensure that the tube surfaces
are covered with an evaporating liquid film.
- The overall heat-transfer coefficient of the
sprayed tube evaporator usually exceeds that of the
flooded type, the additional complexity of the pump
and spray assembly usually dissuaded designers
f
from choosing
h i them.
th
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- LIQUID COOLING EVAPORATOR
- The improvement in heat-transfer coefficient in
comparison to a flooded evaporator occurs because
the sprayed tube permits easy escape of the vapor
Bubbles
- In fact, there is an optimum circulation rate in
the sprayed-tube evaporator that yields the
maximum boiling coefficient. When the flow rate is
too high, the liquid film becomes thick and insulates
th tubes.
the t b Wh the
When th flow
fl rate
t is
i too
t low
l some off the
th
tube surfaces do not become wetted. A typical
circulation rate is 5 times the rate evaporated.
evaporated
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- LIQUID COOLING EVAPORATOR
For these reasons the brine can be cooled to a
temperature close to its freezing point.
point Water can be
chilled to a temperature of less than 1°C with an
evaporating temperature close to -2°C.
2 C.
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- LIQUID COOLING EVAPORATOR
2. Evaporator with open tank:
Liquid cooling evaporators may comprise a pipe
coil in an open tank, and can have flooded or dry
expansion circuitry. Flooded coils will be connected
to a combined liquid accumulator and suction
separator (usually termed the surge drum), in the
form of a horizontal or vertical drum
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- LIQUID COOLING EVAPORATOR
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- LIQUID COOLING EVAPORATOR
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- LIQUID COOLING EVAPORATOR
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