Lectures "Chapter 8: Expansion valve" content presentation: Thermostatic expansion valve, electronic expansion valve, low pressure float valve, high pressure float valves, capillary tubes and Restrictors,... Invite you to reference.
Lecturer : ThS.Nguyễn Duy Tuệ Lecturer : ThS Nguyễn Duy Tuệ
INTRODUCTION
THERMOSTATIC EXPANSION VALVE
ELECTRONIC EXPANSION VALVE
LOW PRESSURE FLOAT VALVE
HIGH PRESSURE FLOAT VALVE HIGH PRESSURE FLOAT VALVE
CAPILLARY TUBES AND RESTRICTORS
OTHER LEVEL CONTROL
CHOOSING THERMOSTATIC EXPANSION VALVE
l th fl - The purpose of f i t t f f
the expansion valve is to control the flow of refrigerant from the high-pressure th hi h condensing side of the system into the low-pressure evaporator. evaporator
- In most cases,
g ,
the pressure reduction is achieved through a variable flow orifice, either modulating or two-position. Expansion valves may be classified according to the method of control.
1. Reason why we have to use thermostatic
l expansion valve ? ?
there is no risk of there is no risk of
p g
region. The amount of superheat
i Direct expansion circuits must be designed liquid liquid and installed so that and installed so that refrigerant returning to the compressor. To ensure this state, heat exchange surface in the evaporator , the dry saturated gas into the is used to heat superheat is usually of the order of 5 K. -> Useful superheat
2. Operation principle : t ti Th b i th h i l
- The basic thermostatic expansion valve has a detector and power element, charged with the same refrigerant as in the circuit. refrigerant as in the circuit
g p g p y
- The pressure ps generated in the phial by the superheated gas passes through the capillary tube p to the top of the diaphragm. An adjustable spring provides the balance of ps ;pe at the diaphragm, and the valve stem is attached at the centre.
- Should the superheat fall for any reason, there
will be a risk of liquid reaching the compressor. will be a risk of liquid reaching the compressor
ps>pe+pspring
: Valve open
pspring
ps pe
ps t ill If
f i d th ill b li increases,
the load on the evaporator
refrigerant will evaporate earlier and there will be
t
more superheat at the phial position. Then p s will
increase and open the valve wider to meet the new
increase and open the valve wider to meet the new
demand. http://www.ra.danfoss.com/TechnicalInfo/Approvals/Files
/RAPIDFiles/01/Asset/TE55-Ver/TE55-Ver.10.03.25-.02-
2D-L.swf
2D L swf http://www.ra.danfoss.com/TechnicalInfo/Approvals/Files
/RAPIDFiles/01/Asset/TE55-Ver/TE55-Ver.10.03.25-.02-
2D-L.swf
2D L swf Dependence of ts-ps at the bult t f i ithi i d b it h The power element can be limit charged so that
all
the refrigerant within it has vaporized by a
th
ll
predetermined temperature (commonly 0°C). Above
this point, the pressure within it will cease of follow
this point
the pressure within it will cease of follow
the boiling point curve but will follow the gas laws as
shown in : -> the valve will remain closed. Detector pressure for limit charged valve: t t fi ti hi h t - This is done to limit the evaporator pressure
when first starting a warm system, which might
i ht
h
overload the drive motor. This is termed limit
charging or maximum operating pressure -> MOP
charging or maximum operating pressure -> MOP
valve - Such valves must be installed so that the phial is p the coldest part. When soldering, we have to wrap the valve’s body by a wet cloth
b d b
th t l i l i li th Th small drops
p can
g coil
y j 3. External equalizer :
- The simple thermostatic expansion valve relies
ti
t
l
the diaphragm being
on the pressure under
approximately the same as that at the coil outlet,
approximately the same as that at the coil outlet
and
be
pressure
accommodated by adjustments to the spring setting.
g
- If evaportor have high pressure drop due to
distributor, large coil, there will be the large pressure
difference between inlet and outlet coil -> Have
equalizing connection Thermostatic expansion valve with external
equalizer
li 4. What is the hunting of expansion valve :
Excessive
E hunting
h ti the
th of
f i thermostatic
th
ti t expansion valve means that: - The evaporator surface has an irregular
- The evaporator surface has an irregular
refrigerant feed with a resulting slight loss of heat
transfer effectiveness. - a time lag between the change of valve position and the effect at the evaporator outlet - Over-sized valves and incorrect phial position can also give rise to hunting. + Solutions:
l
Th hi t d th ld h h l - The phial should always he located on the
l
horizontal outlet, as close to the evaporator as
possible and not on the underside of the pipe.
possible and not on the underside of the pipe - Electronic expansion valves are now becoming increasingly popular for many applications. g y p p y pp + Installation diagram: need solenoide valve before thermostatic expansion valve
before thermostatic expansion valve
http://refrigerationandairconditioning.danfoss.us/kno
wledge center/multimedia/refrigeration system
wledge-center/multimedia/refrigeration-system-
overview/ 1. Benefits of electronic expansion valve: l i t ff fi l The electronic expansion valve offers a finer
Th
i
degree of control and system protection. The
benefits can be summarized as:
benefits can be summarized as: capacities . - Precise flow control over a wide range of
p
- Rapid response to load changes .
- Better control at low superheats so that less
evaporator surface is required for superheat. More
surface for evaporation results in higher evaporating
temperature and better efficiency.
temperature and better efficiency ibilit ff t t l i - Electrical connection between components
offers greater flexibility in system layout, which is
hi h i
t
fl
important for compact systems. - The valve can close when the system shuts
- The valve can close when the system shuts
down, which eliminates the need for an additional
shut off solenoid valve. With l th ifi t l i i 2. Kinds of electronic expansion valve :
- With electronic expansion valve, the orifice
size is varied by : a stepper motor, and a pulse
width modulated (PWM) type.
width modulated (PWM) type - In each case a controller p is used in
conjunction with the valve. The controller is pre-confi
j
the refrigerant and valve type and it
gured for
receives the information from sensors, for example,
pressure and temperature at the evaporator outlet.
This enables the superheat to be determined. The
output signal
to the valve initiates the orifice
to the valve initiates the orifice
output signal
adjustment. 2. Kinds of electronic expansion valve :
- With electronic expansion valve, the orifice
With electronic expansion valve the orifice
size is varied by : a stepper motor, and a pulse
width modulated (PWM) type.
width modulated (PWM) type. - In each case a controller t th d t tl t t is used in
conjunction with the valve. The controller is pre-confi
gured for
the refrigerant and valve type and it
receives the information from sensors, for example,
pressure and temperature at the evaporator outlet.
t
This enables the superheat to be determined. The
to the valve initiates the orifice
output signal
to the valve initiates the orifice
output signal
adjustment. + Installation diagram for electronic expansion valve :
valve : http://www.ra.danfoss.com/TechnicalInfo/Approvals/Files/RAPIDFiles/01/
Asset/ETS6-Ver/ETS6-Ver.20.08.12-.02-2D-L.swf l th In the case of
hi b t
ti the PWM valve it
i d l i is the
relationship between the opening and closing which
hi h
determines the capacity of the valve. The valve is
either open or closed and each time interval of a few
either open or closed and each time interval of a few
seconds will
include an opening period depending
on the signal. g http://www.ra.danfoss.com/TechnicalInfo/Approvals/
Files/RAPIDFiles/01/Asset/AKV10-Ver/AKV10-
Files/RAPIDFiles/01/Asset/AKV10-Ver/AKV10-
Ver.10.01.27-.02-2D-L.swf or partial condition i hi h h temperature, p p floating
load
Under
condensing pressure, which happens at
low
l
t
d
the condensing pressure
ambient
decreases. Thermostatic expansion valves tend to
decreases Thermostatic expansion valves tend to
hunt, but systems with electronic components
operate at partial
y
load in exactly the same and
stable manner as at full load. 1. Description :
- Flooded evaporators require a constant liquid
Flooded evaporators require a constant liquid
level, so that the tubes remain wetted. A simple float
valve suffices (applied), but must be located with the
valve suffices (applied), but must be located with the
float outside the evaporator shell in the mechanism. The float
b h t f t - Since the surface of the boiling liquid is agitated
(stir) and the constant movement would cause
is
excessive wear
therefore contained within a separate chamber,
th
ithi
d
i
coupled with balance lines to the shell 2. Figure of low pressure float valve: p p - Such a valve is a metering device and may
not provide positive shut-off when the compressor is
not provide positive shut-off when the compressor is
stopped. Under these circumstances, refrigerant will
continue to leak into the evaporator until pressures
have equalized, and the liquid level might rise too
close to the suction outlet. To provide this shut-off, a
solenoid valve is needed in the liquid line. d d i h li id li id l l i - Since the low-pressure float needs a solenoid
valve for tight closure, this valve can be used as an
valve for tight closure this valve can be used as an
on–off control
in conjunction with a pre-set orifice
and controlled by a float switch
and controlled by a float switch + Figure of float switch : p 1. Description :
A float valve can be fitted which will pass any
A float valve can be fitted which will pass any
to the
drained liquid from the condenser direct
evaporator. The action is the same as that of a
is at condenser
steam trap. The float chamber
pressure and the control is termed a high-pressure
flfloat 2. Installation diagram: In this diagram, the first upper coil superheat
the vapour, the second one boil off the remaining
the second one boil off the remaining
the vapour
liquid g y , The variable orifice of the expansion valve can
be replaced, in small systems, by a long thin tube.
be replaced in small systems by a long thin tube
This is a non-modulating device and has certain
limitations, but will give reasonably effective control
over a wide range of conditions if correctly selected + Capillary adjustment : - Tube bores of 0.8–2 mm with lengths of 1–4 m
are common. The capillary tube is only fitted on
are common The capillary tube is only fitted on
factory-built and tested equipment, with exact
refrigerant charges. g g
- Mass flow is a function of pressure difference and the degree of liquid subcooling on entry. ill Th b i d l l i - The capillary tube is used almost exclusively
l
in small air conditioning systems and is self
Increasing
Increasing
regulating within certain parameters.
regulating within certain parameters
ambient
temperature -> increasing load and the
condensing pressure rise more refrigerant flow.
condensing pressure rise->more refrigerant flow. - Tube bores of 0.8–2 mm with lengths of 1–4 m
are common. The capillary tube is only fitted on
are common The capillary tube is only fitted on
factory-built and tested equipment, with exact
refrigerant charges. g g
- Mass flow is a function of pressure difference and the degree of liquid subcooling on entry. ill Th b i d l l i - The capillary tube is used almost exclusively
l
in small air conditioning systems and is self
Increasing
Increasing
regulating within certain parameters.
regulating within certain parameters
ambient
temperature -> increasing load and the
condensing pressure rise more refrigerant flow.
condensing pressure rise->more refrigerant flow. Figure shows how the device is applied in a reversible air conditioner.
reversible air conditioner is placed at - If a small heater element the
required liquid level of a flooded evaporator,
required liquid level of a flooded evaporator
together with a heat-sensing element, then the latter
will detect a greater temperature if liquid refrigerant
will detect a greater temperature if liquid refrigerant
is not present. This signal can be used to operate a
solenoid valve. h d b th l d t t t l - The thermostatic or electronic expansion valve
can also be used to maintain a liquid level. The phial
and a heater element are both clamped to a bulb at
b lb t
the required liquid level. If liquid is not present, the
heater warms the phial to a superheat condition and
heater warms the phial to a superheat condition and
the valve opens to admit more liquid. We have to follow manufacturer catalogue for
is
valve. Following this
valve
is
Following this expansion
choosing
expansion
choosing
Danfoss’s catalogue: + Lưu ý : Trong ví dụ trên ta chưa kể đến hệ số
hiệu chỉnh do độ quá lạnh của môi chất trước khi
hiệu chỉnh do độ quá lạnh của môi chất trước khi
vào VTL. Độ quá lạnh lỏng càng lớn thì NSL tăng do
sự hóa hơi đột ngột sau van giảm
gộ ự g ộ12/2015
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LOW PRESSURE
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HIGH PRESSURE
FLOAT VALVE
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CẤP DỊCH BẰNG VAN PHAO
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FLOAT VALVE
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CAPILLARY TUBES AND
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OTHER LEVEL CONTROLLS
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CHOOSING THERMOSTATIC
S O
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