U.S. patent number 3,577,743 [Application Number 04/831,873] was granted by the patent office on 1971-05-04 for control for refrigeration systems.
This patent grant is currently assigned to Vilter Manufacturing Corporation. Invention is credited to Joseph N. Long.
United States Patent |
3,577,743 |
Long |
May 4, 1971 |
CONTROL FOR REFRIGERATION SYSTEMS
Abstract
A control system for a refrigeration system employs first and
second temperature sensors located in the system evaporator to
control the superheat condition of the system refrigerant. One
temperature sensor senses the temperature of liquid phase
refrigerant in the evaporator while the other temperature sensor
senses the temperature of gaseous phase refrigerant in the
evaporator. The temperature sensors are connected to a differential
temperature control means which regulates the system expansion
valve in accordance with a desired differential temperature or
superheat condition between the liquid and gaseous phase
refrigerant.
Inventors: |
Long; Joseph N. (Wauwatosa,
WI) |
Assignee: |
Vilter Manufacturing
Corporation (Milwaukee, WI)
|
Family
ID: |
25260067 |
Appl.
No.: |
04/831,873 |
Filed: |
June 10, 1969 |
Current U.S.
Class: |
62/212;
62/225 |
Current CPC
Class: |
F25B
41/31 (20210101); F25B 2700/21174 (20130101); F25B
2600/21 (20130101); F25B 2700/21175 (20130101); F25B
41/35 (20210101); Y02B 30/70 (20130101) |
Current International
Class: |
F25B
41/06 (20060101); G05D 23/20 (20060101); G05D
23/24 (20060101); F25b 041/00 () |
Field of
Search: |
;62/204,212,223,224,225,227 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Perlin; Meyer
Claims
I claim:
1. In a refrigeration system having a recirculating refrigerant and
an evaporator for receiving high-pressure liquid phase refrigerant
at an inlet and providing gaseous phase refrigerant at an outlet,
means for controlling the superheat condition of the gaseous phase
refrigerant in accordance with the differential in temperature
between the inlet and outlet refrigerant in the evaporator, the
control of refrigerant superheat condition being effected by
regulating the admission of liquid phase refrigerant to the
evaporator, said control means comprising an electrically operative
valve connected to the inlet of the evaporator for metering the
flow of refrigerant into the evaporator in accordance with a
controlling signal thereto, a transformer having a primary winding
connected to a power source, a secondary winding having a center
tap terminal and a pair of output terminals and a diode connected
to each of said output terminals to provide a DC current
output;
a balanced thermistor circuit connected across the center tapped
terminal and the output terminals of said secondary winding and
including (1) a first thermistor located in the evaporator for
providing a signal proportional to the actual temperature of the
liquid phase refrigerant existing in the evaporator and (2) a
selectively controllable rheostat and a second thermistor located
in the evaporator for providing a signal set by said rheostat
proportional to the gaseous phase refrigerant existing at the
outlet of the evaporator, and an amplifier connected to the output
terminals of the thermistors, the output of the amplifier connected
to said valve for controlling the superheat condition of said
gaseous phase refrigerant in accordance with the temperature
differential between said thermistors.
2. A control according to claim 1, wherein said electrically
operable valve comprises a motor driven valve.
3. A control according to claim 1, wherein said refrigerant system
includes conduit means connected to the inlet of the evaporator for
supplying high temperature refrigerant to the evaporator inlet when
the flow of low temperature refrigerant into the evaporator from
said controllable valve is reduced, and wherein said control
includes a flow regulator connected in the conduit means for
metering the flow of high temperature refrigerant into the
evaporator in accordance with a controlling signal thereto; said
differential temperature means being further connected to said flow
regulator for providing said output signal as the controlling
signal for operating said flow regulator to admit high temperature
refrigerant into the evaporator when said output signal operates
said valve to reduce the flow of low temperature refrigerant into
the evaporator.
4. In a refrigeration system having a recirculating refrigerant and
an evaporator for receiving high-pressure liquid phase refrigerant
at an inlet and providing gaseous phase refrigerant at an outlet,
means for controlling the superheat condition of the gaseous phase
refrigerant in accordance with the differential in temperature
between the inlet and outlet refrigerant in the evaporator, the
control of the refrigerant superheat condition being effected by
regulating the admission of liquid phase refrigerant to the
evaporator, said control means comprising:
a valve connected to the inlet of the evaporator for metering the
flow of refrigerant into the evaporator in accordance with a
controlling signal thereto;
a first temperature responsive sensory element located in the
evaporator for providing a signal proportional to the temperature
of the liquid phase refrigerant existing in the evaporator;
a second temperature responsive sensory element located in the
evaporator for providing a signal proportional to the temperature
of the gaseous phase refrigerant existing in the evaporator;
and
differential temperature means connected to said first and second
sensory elements and responsive to said signals for providing a
signal corresponding to the difference between said first and
second signals;
said differential temperature means being connected to said valve
for controlling the same, and including electric signal means
responsive to said signals for providing an electric signal
corresponding to the difference between said first and second
signals, said valve being a pneumatically operable valve, and said
differential temperature means further including an
electropneumatic transducer connected to said signal means and
interposed between said valve and a source of pneumatic pressure,
said transducer converting the electric signal of said signal means
into a pneumatic output signal for operating said valve.
5. In a refrigeration system having a recirculating refrigerant and
an evaporator for receiving high-pressure liquid phase refrigerant
at an inlet and providing gaseous phase refrigerant at an outlet,
means for controlling the superheat condition of the gaseous phase
refrigerant in accordance with the differential in temperature
between the inlet and outlet refrigerant in the evaporator, the
control of the refrigerant superheat condition being effected by
regulating the admission of liquid phase refrigerant to the
evaporator, said control means comprising:
a valve connected to the inlet of the evaporator for metering the
the flow of refrigerant into the evaporator in accordance with a
controlling signal thereto;
a first temperature responsive sensory element located in the
evaporator for providing a signal proportional to the temperature
of the liquid phase refrigerant existing in the evaporator;
a second temperature responsive sensory element located in the
evaporator for providing a signal proportional to the temperature
of the gaseous phase refrigerant existing in the evaporator;
and
differential temperature means connected to said first and second
sensory elements and responsive to said signals for providing a
signal corresponding to the difference between said first and
second signals,
said differential temperature means being connected to said valve
for controlling the same,
said refrigerant system includes a compressor connected to the
outlet of the evaporator for compressing the gaseous phase
refrigerant leaving the evaporator, said compressor having
unloading means for unloading the compressor when the flow of
refrigerant into the evaporator is reduced, and wherein said
control includes signal responsive means coupled to said
differential temperature means and said unloading means for
operating said unloading means to unload the compressor when the
output signal operates said controllable valve to reduce the
refrigerant flow into said evaporator.
6. A control according to claim 5, wherein said temperature
responsive sensory elements provide electric signals proportional
to the refrigerant temperatures, and said differential temperature
means includes electric signal means responsive to said signal for
providing an electric signal corresponding to the difference
between said first and second signals, said valve being a
pneumatically operable valve, said differential temperature means
further including an electropneumatic transducer connected to said
signal means and interposed between said valve and a source of
pneumatic pressure with said transducer converting the electric
signal of said signal means into a pneumatic output signal for
operating said valve, and wherein said signal responsive means
comprises means responsive to the pneumatic signal of said
transducer.
7. In a refrigeration system having a recirculating refrigerant and
an evaporator for receiving high-pressure liquid phase refrigerant
at an inlet and providing gaseous phase refrigerant at an outlet,
means for controlling the superheat condition of the gaseous phase
refrigerant in accordance with the differential in temperature
between the inlet and outlet refrigerant in the evaporator, the
control of the refrigerant superheat condition being effected by
regulating the admission of liquid phase refrigerant to the
evaporator, said control means comprising:
a valve connected to the inlet of the evaporator for metering the
flow of refrigerant into the evaporator in accordance with a
controlling signal thereto;
a first temperature responsive sensory element located in the
evaporator for providing a signal proportional to the temperature
of the liquid phase refrigerant existing in the evaporator;
a second temperature responsive sensory element located in the
evaporator for providing a signal proportional to the temperature
of the gaseous phase refrigerant existing in the evaporator;
and
differential temperature means connected to said first and second
sensory elements and responsive to said signals for providing a
signal corresponding to the difference between said first and
second signals;
said differential temperature means being connected to said valve
for controlling the same;
conduit means connected to the inlet of the evaporator for
supplying high temperature refrigerant to the evaporator inlet when
the flow of low temperature refrigerant into the evaporator from
said controllable valve is reduced; and
a flow regulator connected in the conduit means for metering the
flow of high temperature refrigerant into the evaporator in
accordance with a controlling signal thereto; said differential
temperature means being further connected to said flow regulator
for providing said output signal as the controlling signal for
operating said flow regulator to admit high temperature refrigerant
into the evaporator when said output signal operates said valve to
reduce the flow of low temperature refrigerant into the
evaporator;
said differential temperature means includes electric signal means
responsive to said signals for providing an electric signal
corresponding to the difference between said first and second
signals, and said valve and flow regulator are pneumatically
operable, and wherein said differential temperature means further
includes an electropneumatic transducer connected to said signal
means and interposed between said valve and flow regulator and a
source of pneumatic pressure, said transducer converting the
electric signal of said signal means into a pneumatic output signal
for operating said valve and flow regulator.
8. In a refrigeration system having a recirculating refrigerant and
an evaporator for receiving high-pressure liquid phase refrigerant
at an inlet and providing gaseous phase refrigerant at an outlet,
means for controlling the superheat condition of the gaseous phase
refrigerant in accordance with the differential in temperature
between the inlet and outlet refrigerant in the evaporator, the
control of refrigerant superheat condition being effected by
regulating the admission of liquid phase refrigerant to the
evaporator for supplying high temperature refrigerant to the
evaporator inlet when the flow of low temperature refrigerant into
the evaporator from said controllable valve is reduced, said
control means comprising an electrically operative valve connected
to the inlet of the evaporator for metering the flow of refrigerant
into the evaporator in accordance with a controlling signal
thereto;
a bridge circuit including (1) a first voltage divider having an
output terminal, a fixed resistance and a first thermistor located
in the evaporator for providing a signal proportional to the actual
temperature of the liquid phase refrigerant existing in the
evaporator and (2) a second voltage divider having a selectively
controllable rheostat and a second thermistor located in the
evaporator for providing a signal set by said rheostat proportional
to the gaseous phase refrigerant existing at the outlet of the
evaporator; and an amplifier connected to the output terminals of
said first and second voltage dividers and to said valve for
controlling the superheat condition of said gaseous phase
refrigerant in accordance with the temperature differential between
said first and second voltage dividers.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to automatic controls for
refrigeration systems and more particularly to such controls
employing plural sensors.
2. Description of the Prior Art
Conventional refrigeration systems employ a recirculating
refrigerant for removing heat from the low temperature side of the
system and for discharging heat at the high temperature side. A
motor driven compressor which receives low-pressure gaseous phase
refrigerant and compresses it to a high pressure provides the work
input necessary for the operation of the system.
The high-pressure gaseous phase refrigerant is supplied to a
condenser where heat is removed from the refrigerant to convert it
to a liquid. The liquid is then supplied through an expansion valve
to the evaporator where the liquid receives heat from the cooling
load, such as a chill room, which causes the refrigerant to revert
to the gaseous form. The gaseous refrigerant is then returned to
the compressor for recirculation.
The amount of heat absorbed by the refrigerant in the evaporator
includes the heat of vaporization of the refrigerant; that is, the
amount of heat which must be absorbed by a liquid at a given
temperature to convert it to a gas at the same temperature. In
addition, the gaseous refrigerant resulting from the conversion of
the liquid refrigerant may absorb additional heat, which raises its
temperature above the temperature of vaporization. The gaseous
refrigerant in such a state is said to be superheated and the
amount by which the temperature of the gas is raised above the
vaporization temperature is expressed in degrees of superheat.
In many applications of refrigeration systems, such as low
temperature applications, it is desired to prevent superheating of
the gaseous refrigerant or to regulate the superheat of the
refrigerant to a preselected magnitude. This must be accomplished
by controlling the flow of liquid refrigerant into the evaporator.
However, the conventional thermostatically controlled expansion
valve, responsive to load temperature, normally used for this
purpose is totally unsuited to controlling refrigerant superheat.
Such valves, when used to control superheat, suffer, among other
shortcomings, a slow response time, a wide regulating range, and a
narrow load range.
In an effort to provide better control of the gaseous refrigerant
superheat, devices responsive to the temperature of the refrigerant
itself have been placed at the outlet of the evaporator to control
the amount of liquid refrigerant entering the evaporator as a
function of the temperature of the refrigerant leaving the
evaporator. See U.S. Pat. No. 2,355,894 to Ray, which shows a
gaseous refrigerant controlled device, and U.S. Pat. No. 3,205,675
to Matthies which shows a liquid refrigerant controlled device.
While these devices represent an improvement over a thermostat,
they also exhibit one or more of the shortcomings of the latter
type of control.
SUMMARY OF THE PRESENT INVENTION
It is, therefore, the object of the present invention to provide an
improved refrigeration system control for regulating the superheat
of refrigerant in its gaseous phase existing therein.
It is a further object of the present invention to provide such a
control which is particularly suitable for use in refrigeration
systems operating at low temperatures and which permits regulation
of low superheat conditions of the system refrigerant.
It is yet another object of the present invention to provide a
control system which permits the desired amount of superheat to be
directly set, and which provides accurate, stable, and rapid
regulation of superheat conditions over a wide range of operating
conditions.
Briefly, it is the object of the present invention to provide an
improved control for regulating the superheat condition of the
refrigerant in a refrigerating system responsive to the temperature
differential between the temperature of the liquid phase
refrigerant and the temperature of the gaseous phase refrigerant
existing in the system evaporator. The control includes a valve
connected to the inlet of the evaporator for metering the flow of
liquid refrigerant into the evaporator in accordance with a
controlling signal to the valve. A first temperature responsive
sensory element is located in the inlet of the evaporator for
providing a signal proportional to the temperature of the liquid
phase refrigerant. A second temperature responsive element is
located in the outlet of the evaporator for providing a signal
proportional to the temperature of the gaseous phase refrigerant
leaving the evaporator. A differential temperature control means is
connected to the first and second sensory elements and is
responsive to their signals for providing an output signal
corresponding to the difference between the first and second
signals. This output signal is provided to the controllable valve
for controlling the supply of liquid refrigerant to the evaporator
and the superheat of the gaseous refrigerant in accordance with the
differential temperature between the temperature of the refrigerant
entering the evaporator and the temperature of the refrigerant
leaving the evaporator.
The output signal of the differential temperature control means may
be utilized to operate other auxiliary apparatus in the
refrigeration system such as a compressor unloading means or a hot
gas supply valve recirculating gaseous refrigerant to the inlet of
the evaporator.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic illustration of a refrigeration system
employing the control of the present invention;
FIG. 2 is a schematic view of portions of the control showing
another embodiment of the present invention;
FIG. 3 is a schematic diagram of portions of the control of the
present invention showing the use of the control to operate
compressor unloading means; and
FIG. 4 is a partial schematic diagram of the control of the present
invention, showing use of the control in regulating gaseous
refrigerant recirculation to the evaporator.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning now to the drawings, there is shown in FIG. 1 thereof a
refrigeration system of conventional construction designated
generally by the numeral 10 and the control 8 of the present
invention. Refrigeration system 10 includes an evaporator 12,
comprised of a labyrinthine conduit or coil 14 which receives low
temperature refrigerant in a liquid phase at inlet 16 and
discharges refrigerant in a high temperature gaseous phase at
outlet 18. The refrigerant is converted from low temperature to
high temperature by the absorption of heat from the space
surrounding coil 14, for example, chill room 20.
The external power necessary for the operation of refrigeration
system 10 is provided by motor driven compressor 22. Compressor 22
takes the gaseous refrigerant discharge from evaporator 12 in
conduit 26 and compresses it to a high-pressure gaseous
refrigerant. The high-pressure gaseous refrigerant is supplied to
condenser 28 in conduit 30 where the refrigerant is condensed to a
liquid phase as by the giving off of heat to the circulating water
in water jacket 32. The liquid refrigerant in condenser 28 may be
supplied to receiver 34 by conduit 36 for storage until needed by
refrigeration system 10.
The aforesaid elements form components of conventional
refrigeration systems operating in accordance with the general
principles of a Carnot cycle and further description is not deemed
necessary.
THE CONTROL
The Control Valve
The liquid refrigerant in receiver 34 is supplied via conduit 38 to
expansion or metering valve 40 which meters the flow of refrigerant
to evaporator 12 in accordance with a controlling signal to the
valve. Valve 40 may be responsive to controlling signals provided
in various forms. An electrically operated valve is shown in FIG.
1.
Valve 40 includes a housing 42 having an inlet connected to
receiver conduit 38 and outlet connected to evaporator inlet 16 via
conduit 44. A valve stem 46 extends from housing 42. Housing 42
includes a means for regulating the flow of refrigerant between the
inlet and outlet thereof in response to mechanical movement of
valve stem 46. A vane, valve disc and seat, or other well known
structure may be used for this purpose.
In the embodiment of the invention shown in FIG. 1, the mechanical
movement to valve stem 46 is in the form of rotary motion. For this
purpose, motor 48 is mounted on housing 42 by means of bracket 50.
The output shaft of motor 48 is connected to valve stem 46 so that
by rotating motor 48 valve stem 46 will be rotated to change the
position of a vane in valve housing 42 or the relationship of a
valve disc to a valve seat in housing 42.
Motor 48 is energized by a controlling signal provided in
conductors 52 and 54. The direction of rotation may be controlled
by the relative polarity of the voltage in conductors 52 and 54. A
feedback signal corresponding to the actual positioning of the
elements of valve 42 by motor 48 is provided by potentiometer 56
mounted on valve stem 46 and operable by the rotation of valve stem
46.
It will be appreciated that motor 48 may be in the form of a
solenoid motor which provides mechanical movement to valve stem 46
in the form of raising or lowering the valve stem, if desired.
TEMPERATURE SENSORS
Temperature sensors are located in evaporator 12 for measuring the
temperature of both the liquid and gaseous phase refrigerant in the
evaporator. Specifically, a first temperature sensor 60 is located
in the portion of evaporator 12 containing refrigerant in the
liquid phase. Temperature sensor is illustratively shown in FIG. 1
as located at the inlet 16 of evaporator 12 and for this purpose
temperature sensor 60 may be inserted or embedded in coil 14. While
many types of sensors may be employed, the presently preferred
embodiment of the invention utilizes resistive elements, the
resistance of which varies as a function of its ambient
temperature. Such resistive elements are commonly termed
thermistors. A second temperature sensor 62 is located in the
portion of refrigeration system 10 containing the gaseous phase
refrigerant so as to measure its temperature. Temperature sensor 62
is illustratively shown as embedded on conduit 26 at the outlet 18
of evaporator 12. Sensor 62 may also comprise a thermistor.
CONTROL MEANS
Temperature sensors 60 and 62 are connected to differential
temperature control means 70 via conductors 64 and 66,
respectively. While the resistive elements of temperature sensors
60 and 62 are actually located in evaporator 12, as shown in FIG. 1
and described above, their connection to control means 70 by
conductors 64 and 66 incorporates these elements in the control
means. Therefore, to facilitate the analysis of the construction
and operation of control means 70, the resistive elements 60a and
62a of temperature sensors 60 and 62 respectively, are shown as
located in control means 70 by dotted lines. Control means 70
provides an output signal proportional to the difference between
the temperature sensed by sensor 60 and by sensor 62. Control means
70 may typically include a power supply 72, an electric bridge 74,
and an amplifier 76.
Power supply 72 comprises a transformer 78, the primary winding of
which is connected to an available source of alternating current
and the secondary winding of which is center tapped and contains
diodes 80 and 82. The rectified output of the secondary winding of
transformer 78 is filtered by capacitor 84 and resistor 86 to
provide a direct current in conductors 88 and 90 to remaining
portions of control means 70.
Electric bridge 74 of the Wheatstone type, has its input terminals
94 and 96 connected across conductors 88 and 90. Bridge 74 includes
a pair of voltage dividers, each comprised of a pair of center
tapped resistive elements, extending between the input terminals 94
and 96. The center tap of each voltage divider comprises an output
terminal 98 and 100 of bridge 74 and the magnitude and polarity of
the voltage existing between the output terminals is an indication
of the relative resistance of the various resistive elements in the
voltage dividers.
Considering bridge 74 in detail, conductor 88 is connected to input
terminal 94. A fixed resistor 102 of a preselected magnitude is
connected between input terminal 94 and output terminal 98. The
resistive element 62a of temperature sensor 62 is connected between
output terminal 98 and input terminal 96. Input terminal 96 is
connected to conductor 90. Resistor 102 and thermistor 62a form one
of the voltage dividers of bridge 74. As resistor 102 is of fixed
resistive magnitude, the voltage provided at output terminal 98
will be a function of the resistance of thermistor 62a, which in
turn is a function of the temperature of the gaseous refrigerant at
outlet 18 of evaporator 12.
The second voltage divider of bridge 74 is formed by thermistor 60a
of temperature sensor 60, which is connected between input terminal
94 and output terminal 100 and rheostat 104, which is connected
between output terminal 100 and input terminal 96. The resistive
portion of rheostat 104 may have the same resistance as resistor
102. The voltage provided at output terminal 100 will be a function
of the resistance of thermistor 60a, which in turn is a function of
the temperature of the liquid phase refrigerant entering inlet 16
of evaporator 12. The magnitude of the voltage provided at output
terminal 100 by thermistor 60a may be adjusted by adjusting the
position of wiper 104a of rheostat 104. The position of wiper 104a
may be controlled by knob 105 and dial 107. Knob 105 and dial 107
form a means by which the degree of superheat of the gaseous
refrigerant leaving evaporator 12 may be directly set, as
hereinafter described.
The output signal of bridge 74 consists of the differential voltage
existing between output terminals 98 and 100. This differential
voltage is provided in conductors 106 and 108 connected to output
terminals 98 and 100, respectively, to the input terminals of
amplifier 76, where the signal is amplified, and provided to
conductors 52 and 54 as the controlling signal to motor 48 of valve
40. The feedback signal from potentiometer 56 is also provided to
the input of amplifier 76 via conductor 110.
A commercially available control means 70 of the type described
above which may be used in the control of the present invention is
one made and sold by the Electronic Construction Corp., Milwaukee,
Wis., under the trade name Delta T Sensor.
OPERATION
In operation, control means 70 is energized by transformer 78 to
provide a voltage across the input terminals 94 and 96 of bridge
74. The resistance of thermistor 62a in temperature sensor 62
assumes a value which is a function of the temperature of the
gaseous phase refrigerant leaving the evaporator 12 at outlet 18.
Thermistor 60a in temperature sensor 60 assumes a value which is a
function of the temperature of the liquid refrigerant entering
evaporator 12 at inlet 16. Neglecting for the moment the effects of
adjustments to rheostat 104, the voltage at output terminals 98
will be a function of the resistance of thermistor 62a and the
temperature of the gaseous refrigerant, while the voltage at output
terminal 100 will be a function of the resistance of resistor 60a
in the temperature of the liquid refrigerant, so that the
difference in temperature between the gaseous phase and liquid
phase refrigerant appears as a difference in the voltage between
output terminals 98 and 100 and as a voltage signal in conductors
106 and 108 to amplifier 76.
A control signal generated by amplifier 76 to motor 48, responsive
to the input signal in conductors 106 and 108, causes motor 48 to
operate valve 42 in a manner to reduce the temperature difference
between the gaseous phase and liquid phase refrigerant. For
example, if the temperature of the gaseous refrigerant leaving
evaporator 12 is excessive, due to an excessive cooling load on the
evaporator, the polarity and magnitude of the control signal to
motor 48 is such as to cause motor 48 to rotate in a direction to
open valve 40 to allow more refrigerant to enter evaporator 12 to
handle the increase cooling load. The increased liquid refrigerant
available in evaporator 12, reduces the temperature of the gaseous
refrigerant leaving the evaporator and the control signal of
amplifier 76 is reduced. In this manner, the temperature of the
gaseous refrigerant is regulated with reference to the temperature
of the liquid refrigerant.
If it is desired to maintain the gaseous refrigerant leaving
evaporator 12 at a condition of zero superheat, that is, at a
temperature which is the same as the liquid refrigerant entering
evaporator 12, valve 40 is operated to supply sufficient liquid
refrigerant to evaporator 12 to reduce the temperature of the
gaseous refrigerant leaving evaporator 12 to its saturated
temperature. At this point, the temperature sensed by temperature
sensors 60 and 62 will be identical and the differential
temperature signal provided by control means 70 will be zero. The
refrigeration system is thus maintained in a condition of zero
superheat.
Under many conditions, it is desired to maintain a preselected
amount of superheat in the gaseous refrigerant leaving evaporator
12. That is, it is desired to heat the gaseous refrigerant by a
preselected amount above the temperature of the liquid refrigerant
entering evaporator 12. Rheostat 104 is incorporated in bridge 74
for this purpose. By adjusting the position of wiper 104a, the
voltage at output terminal 100 may be varied independently of the
resistance of thermistors 60a and 62a. The variation in the voltage
at output terminal 100 varies the voltage signal in conductors 106
and 108 to amplifier 76 and the control signal to valve motor 48.
This causes motor 48 to operate valve 40 to adjust the temperature
of the gaseous refrigerant leaving evaporator 12 at outlet 16 so
that the gaseous refrigerant has the desired degree of superheat.
For this purpose, dial 107, which operates in conjunction with knob
105 to vary the position of wiper 104a of potentiometer 104 may be
calibrated directly in degrees of superheat.
DESCRIPTION OF OTHER PREFERRED EMBODIMENTS
FIG. 2 shows an alternative embodiment of the control of the
present invention. In the embodiment of FIG. 2, a pneumatically
actuated valve 401 is employed to meter the flow of refrigerant to
inlet 16 of evaporator 12. Valve 401 may be a diaphragm operated
valve in which pneumatic pressure supplied to inlet 200 raises or
lowers diaphragm 202 to operate valve 401. The control 801 includes
an electropneumatic transducer 204 which converts the electric
signal in conductors 52 and 54, generated by control means 70, to a
pneumatic signal having corresponding control pressure variations.
A typical electropneumatic transducer 204 which may be used in
control 801 is shown in FIG. 2.
Electropneumatic transducer 204 includes solenoid 206, the coil 207
of which receives the electric signal in conductors 52 and 54. The
position of plunger 208 of solenoid 206 is a function of the
energization of coil 207 provided by the electrical signal from
control means 70. Cavity 210 in the body of transducer 204 contains
a diaphragm 212 positioned across the cavity and containing
cylindrical valve seat 214. A spacer 216 having a plurality of
vents 218 is positioned between plunger 208 and valve seat 214 to
couple the former to the latter. A spring 220 biases valve seat 214
against the force exerted by spacer 216.
An exhaust port 222 connects the upper portion of cavity 210 to the
surrounding atmosphere. An output port 224 is located in the lower
portion of cavity 210 to provide the output pressure to airline 223
and to pneumatically operated valve 401. A valve stem 226
positioned in cavity 210 has a truncated conical portion 228 which
closes valve seat 214 when the valve stem is in the upper position
and the valve seat is in the lower position. A second conical
portion 230 closes valve seat 232 between cavity 210 and chamber
234. A spring 236 positioned in chamber 234 biases valve stem 226
upward. Supply airline 238 supplies inlet air to chamber 234 from
air supply source 240.
A commercially available electropneumatic transducer which may be
employed in control system 801 as transducer 204 is made and sold
by the Fisher Governor Company, Marshalltown, Iowa under the model
designation type 546.
In the operation of control 801, temperature sensors 60 and 62 and
control means 70 function in the same manner as described above to
provide an output signal in conductors 52 and 54 responsive to the
temperature sensing of thermistors 60a and 62a.
The output signal in conductors 52 and 54 drives transducer 204.
Assume in an exemplary instance, the output signal of amplifier 76
is of a polarity to indicate that additional refrigerant is desired
in evaporator 12. The signal in conductors 52 and 54 energizes
solenoid coil 207 to drive plunger 208 downward into cavity 210.
This causes spacer 216 to depress valve seat 214 and valve stem
226. The depression of valve stem 226 increases the air flow
through valve seat 232 from chamber 234, connected to air supply
line 238, to cavity 210, connected to airline 223. This increases
the air pressure in airline 223 and in cavity 210. The increased
air pressure in cavity 210 causes diaphragm 208 to flex upwardly,
raising valve seat 214 and valve stem 226 against the downward
force exerted by solenoid plunger 208. The opposing forces on
diaphragm 212 assume a state of balance so that the air pressure in
airline 223 is a function of the energization of solenoid 206 by
amplifier 76. The air pressure in airline 223 operates diaphragm
202 to open valve 401 and admit more refrigerant to evaporator
12.
In the event the output signal of amplifier 76 is of the opposite
polarity, solenoid 206 is energized to withdraw solenoid plunger
208 from cavity 210. This permits valve seat 214 to be raised off
valve stem 226 by means of spacer 216 and allows the air flowing
from chamber 234 into chamber 210 to escape through valve seat 214
and out exhaust port 222, reducing the air pressure in cavity 210
and in airline 223 and closing valve 104. The reduced pressure in
cavity 210 causes diaphragm 212 to flex downward, restricting the
flow of air through valve seat 214 so that the opposing forces of a
diaphragm again balance when air pressure in airline 223 has
reached a value proportional to the magnitude of the output signal
of amplifier 76.
Control means 70 may be employed to operate the apparatus in
addition to metering valve 40. For example, it is often desired to
reduce the volume of gaseous refrigerant compressed by compressor
22 as the rate at which liquid refrigerant is metered to evaporator
12 is reduced, so as to maintain proper operating conditions in
evaporator 12. The reduction in gaseous refrigerant compressed by
compressor 22 is obtained by an unloading mechanism 250 (FIG. 3)
interposed between gaseous refrigerant inlet line 26 and compressor
22 which permits compressor 22 to operate without compressing
gaseous refrigerant.
To coordinate the operation of unloading mechanism 250 with the
operation of the valve 40, a sensory element is placed in the
output of amplifier 76 which is responsive to the amplifier output
signal. An exemplary embodiment of the above described arrangement
is shown in FIG. 3 in which pneumatically operated relay 252 is
connected in output airline 223 of electropneumatic transducer 204.
Relay 252 is operable by the air pressure in airline 223 so that
when the pressure in the airline is such as to cause valve 401 to
reduce the flow of refrigerant to evaporator 12, relay 252 is
energized to cause unloading mechanism 250 to unload compressor 22
and reduce the inlet volume.
In a similar manner, an electrically operated relay, inserted
directly in the output of amplifier 76, may be used to operate
unloading mechanism 250 to unload compressor 22.
To maintain stable operating conditions in evaporator 12,
particularly when the cooling load on the evaporator is light, it
is often desired to recirculate gaseous refrigerant from the outlet
16 of evaporator 12 to the inlet of evaporator 12. This techniques
is aptly called "false loading" as it increases the effective
cooling load on evaporator 12 without altering the temperature
maintained in chill room 20. An additional valve 402 may be
connected to the output of control means 70. FIG. 4 shows the use
of such an additional valve in the pneumatically operated
embodiment of the invention shown in FIG. 2. Valve 402 is
interposed in conduit 260 which receives gaseous refrigerant from
the outlet of the condenser and provides it to inlet 16 of the
evaporator. Valve 402 is connected to airline 223 so as to be
operable by the air pressure in such line generated by
electropneumatic transducer 204. However, valve 402 is constructed
so that its operating mode is opposite to that of valve 401. That
is, the pneumatic signal in airline 223 which causes valve 401 to
close, causes valve 402 to open, and vice versa. This may be easily
accomplished by placing the inlet of airline 223 on the other side
of diaphragm 202, as shown in FIG. 4.
In operation, when the control signal to electropneumatic
transducer 204 from amplifier 76 is such as to cause valve 401 to
reduce the flow of refrigerant to evaporator 12 responsive to a
reduced loading condition in the evaporator, the same signal is
used to cause valve 402 to open to admit gaseous refrigerant to the
inlet 16 of evaporator 12 via conduit 44 to increase the load on
the evaporator and maintain stable operating conditions
therein.
It will be appreciated that other modifications and changes may be
made to the control the present invention and it is desired to
include all such modifications and alterations as come within the
true scope and spirit of the claims below.
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