AU7321881A

AU7321881A - Method and apparatus for controlling operation of a thermostatic expansion valve

Info

Publication number: AU7321881A
Application number: AU73218/81A
Authority: AU
Inventors: Richard H. Alsenz
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-05-20
Filing date: 1981-05-20
Publication date: 1982-12-07
Also published as: EP0079331A4; JPS58500771A; EP0079331A1; WO1982004142A1

Description

Method and Apparatus for Controlling Operation of a Thermostatic Expansion Valve

Technical "Field

This invention relates to improvements in control¬ ling the operation of a thermostatic expansion valve used for metering refrigerant into the evaporator coil of a refrigeration system. More particularly, this invention relates to methods and apparatus for adding heat to the thermostatic expansion valve remote fluid- charged bulb for improving the operation of the valve and for correcting the superheat adjustment of the valve under adverse operating conditions.

Background Art

Thermostatic expansion valves have long been used in refrigeration flow control to control or meter the flow of liquid refrigerant into the evaporator coil. Because of its high efficiency and its ready adaptability to any type of refrigeration application, the thermosta- tic expansion valve is probably the most widely used refrigerant flow control at the present time. The operation of the thermostatic expansion valve is based on maintaining a constant degree of suction line super¬ heat at the evaporator outlet, permitting the valve to keep the evaporator filled with refrigerant under all conditions of system loading. The characteristic operation of the thermostatic expansion valve results from the interaction of three independent forces, i.e.: (1) the evaporator pressure, (2) the spring pressure of an internally adjustable spring, and (3) the pressure exerted by the saturated liquid-vapor mixture in the

OMPI valve remote fluid-charged bulb. The forces of (1) and (2) are balanced against (3). The forces generated by the fluid-charged bulb result from the heating of the remote bulb clamped to the suction line at the outlet of the evaporator coil due to the superheat of the refrigerant vapor leaving the evaporator coil.

However, many conventional thermosatic expansion valves fail at low ambient temperatures or when one or more of the following conditions exist:

1. The pressure drop across the valve decreases;

2. A pressure drop exists in the liquid refrigerant line to the valve from the condenser; or

3. The liquid refrigerant line to the valve from the condenser is heated above the condensing temperature.

Valves that fail do not fail to meter any refriger¬ ant, but meter less and less refrigerant as any of the above conditions worsen. The basic problem is that the superheat adjustment of the thermostatic expan- sion valve is a function of the capacity of the valve. Therefore, a solution to the problem is to introduce a control method which is not functionally related to the capacity of the valve. Adding heat to the remote fluid-charged bulb of the thermostatic expansion valve results in improving the control of the expansion valve. Controlling the heat applied to the bulb pro¬ vides a technique for correcting the superheat adjust¬ ment of the valve.

Control for thermostatic valves using heat is disclosed in U.S. Patent No. 2,876,629. The thermosta¬ tic valve disclosed is used to control the diversion of hot gas from the hot gas side of the condenser to the evaporator ' inlet line in a refrigeration system. This gas is diverted to the evaporator to raise the temperature of the evaporator and reduce ..the capacity of the system. This technique is often used in automo¬ bile air conditioning or other refrigeration systems where the compressor speed of the system is unrelated to refrigerator or air conditioning load requirements. The thermostatic valve has a fluid-charged bulb that is placed in the air flow path through the evaporator coil. As the air temperature rises, the bulb is heated and throttles the operation of the valve to decrease the quantity of hot gas diverted into the evaporator.

However, in automobile air conditioning systems an operator adjusted means for controlling the tempera¬ ture of the conditioned air is necessary, and an opera¬ tor adjusted means of heating the valve remote bulb is disclosed to control the operation of the valve and hence the hot gas diverted into the evaporator. How- ever, the valve is not used in the capacity of an expansion valve to meter refrigerant into the evapora¬ tor coil of the system, the control of which in a thermostatic expansion valve is dependent on the super¬ heat across the evaporator coil. The disclosed device does not control the heat applied to the bulb by means of the cooling capacity of the refrigerant itself, nor can it function to correct the superheat adjustment of a thermostatic expansion valve independent of a pressure drop across the valve.

U.S. Patent No. 2,735,272 discloses a liquid level control device for establishing a desired maximum level of liquid in a container, and is particularly suited for establishing a predetermined level of liquid refri¬ gerant in a tank supplying the evaporator unit in a refrigeration system. The disclosed liquid level control device utilizes a heater coil to heat the remote bulb of a thermostatic expansion valve metering refrigerant into an evaporator header tank to maintain a desired refrigerant level in the tank. A low output heating element is used to heat a specially designed valve remote fluid-charged bulb. Heating the bulb maintains the valve open until the cold liquid refri¬ gerant level reaches the bulb location and cools the bulb at a rate faster than the low output heating element can heat the bulb. Coding the bulb closes the valve and establishes the liquid level at the location of the bulb. However, the heater element is built into the remote bulb and is not adapted for attachment externally to the bulb alone where the bulb has been clamped in a heat exchange relationship with the re¬ frigerant line.. Further, the disclosed control does not suggest correction or adjustment of the superheat of the valve measured across the evaporator coil, since the disclosed device functions only in connection with controlling the liquid level of a liquid refriger¬ ant.

Disclosure of Invention

The present invention remedies the problems of the prior art by providing in one embodiment of the invention an improved control for a thermostatic expansion valve metering refrigerant fluid into a refrigeration system evaporator coil, having heating means for applying heat to the expansion valve fluid-charged bulb that has been clamped in a direct heat exchange relationship to the refrigerant line near the outlet of the evaporator coil, and a thermal conductor mounted on the bulb in a heat exchange relationship therewith and cooperating with the heating means for transferring heat directly to the bulb at a rate less than the cooling rate of the liquid refrigerant present in the line at the bulb location.

According to another embodiment the present inven- tion, apparatus for improving the superheat control of a thermostatic expansion valve metering refrigerant into a refrigeration system evaporator coil is disclosed having a superheat setting means for establishing a required superheat, superheat determination means for measuring the superheat of the valve across the evapo¬ rator coil, differential means for comparing the measured superheat with the established superheat, and heating means cooperating with the differential means for apply¬ ing heat to the fluid-charged bulb of the valve when the measured superheat exceeds the established super¬ heat. A heating control means may also be included to control the heating means for increasing the rate of heating by the heating means when the measured super¬ heat exceeds the required superheat, and for decreasing the rate of heating by the heating means when the required superheat exceeds the measured superheat.

Accordingly, one primary feature of the present invention is to provide a means of improving the control of a thermostatic expansion valve independent of the pressure drop across the valve by adding heat to the valve fluid-charged bulb.

Another feature of the present invention is to provide a means of improving the superheat control of a thermostatic expansion valve to control the valve independent of the pressure drop across the valve by adding heat to the valve fluid-charged bulb when the measured superheat across the evaporator coil exceeds an established superheat.

CMPI Still another feature of the present invention is to provide a means of controlling the rate of application of heat to the valve fluid-charged bulb in response to the differences in the measured superheat and the established superheat.

Brief Description of the Drawings

In order that the manner in which the above-recited advantages and features of the invention are attained can be understood in detail, a more particular decrip- tion of the invention may be had by reference to speci¬ fic embodiments thereof which are illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the appended drawings illustrate only typical embodiments of the invention and therefore are not to be considered limiting of its scope, for the invention may admit to further equally effective embodiments.

In the drawings:

Figure 1 is a schematic representation of a con- ventional refrigeration system employing the present invention for adding heat to the fluid-charged bulb of the thermostatic expansion valve.

Figure 2 is a ' schematic representation of one embodiment of the invention for controlling superheat of a thermosatic expansion valve.

Figure 3 is a schematic representation of a second embodiment of the invention for controlling superheat of a thermostatic expansion valve.

Figure 4 is a graphical representation of the

OMPI superheat versus valve capacity for a thermostatic expansion valve including the effect of the superheat control according to the present invention.

Figure 5 is an enlarged fragmentary view of the mounting of the valve fluid-charged bulb on the suction line and the mounting of the heating means and thermal conductor on the bulb.

Figure 6 is a vertical cross-sectional view of the heating means, bulb and suction line taken along line 6-6 of Figure 5.

Best Mode for Carrying out the Invention

Referring now to Figure 1, a simplified schematic representation of a typical refrigeration system 10 is shown having a compressor 12 and condenser coil 14 for condensing the compressed refrigerating fluid from a vapor state to a liquid state. A conventional thermo¬ static expansion valve 18 receives the condensed liquid refrigerant from the condenser coil through tubing 16. The expansion valve 18 controls or meters the flow of liquid refrigerant into the evaporator coil through tubing 20 where the cold liquid refriger¬ ant is vaporized by heat exchange with the refrigera¬ tion load (not shown) . The refrigerant in the vapor state returns through suction line tubing 24 to the compressor 12 for compression and recycling in the closed loop refrigeration system.

The expansion valve 18 is a conventional thermosta¬ tic expansion valve which functions to maintain a con¬ stant degree of superheated refrigerant vapor in the suction line 24 as the vapor exits the evaporator coil 22. Such a thermostatic expansion valve 18 typi-

CMFI cally includes a remote fluid-charged bulb 26 which is attached in a thermal heat exchange relationship to the suction line 24 near the outlet from evaporator coil 22 by means of a clamp or strap 25... The bulb 26 is heated to the same temperature as the refrigerant in suction line 24 which vaporizes some of the liquid in the bulb 26. The pressure exerted by the saturated liquid-vapor mixture in the remote bulb 26 is communi¬ cated to the expansion valve 18 through a capillary tube 27. The characteristic operation of the thermo¬ static expansion valve results from the interaction of three forces: (1) the evaporator pressure and (2) the pressure exerted by an internal and adjustable valve spring acting against (3) the pressure exerted by the liquid-vapor mixture of the remote bulb. The superheat setting of valve 18 determines how much of the evapo¬ rator coil 22 surface will be utilized. A low superheat setting utilizes a greater percentage of the evaporator coil 21 surface.

In Figure 1, the superheat control 30 is shown for adding heat to the remote bulb 26 as will be here¬ inafter explained in greater detail. In its broadest form, the present invention provides an improved con¬ trol for a thermostatic expansion valve 18 metering refrigerant fluid into an evaporator coil 22 by provid- i ng a heating means (control circuit 38 and heater 40) for applying heat to the fluid charged bulb 26 clamped in a direct heat exchange relationship to the suction line 24 near the outlet of the evaporator coil 22. The heating means includes a heater 40 which may con¬ veniently be a resistive heater coil housed in a suitable houseing or container 44, such as a case of epoxy resin or the like. A clip 42 of thermally con¬ ducting material cooperates with the heater 40 and is adapted for mounting directly on the remote bulb 26 for applying heat directly to the bulb. The heat applied to bulb 26 causes the expansion valve 18 to open for increasing the quantity of refrig¬ erant metered into the evaporator coil 22. However, the thermal conducting clip 42 limits the transfer of heat from heater 40 to the bulb 26 at a rate less than the coolirig rate of the liquid refrigerant present in the line 24 at the bulb location. The rapid cooling of bulb 26 by the liquid refrigerant in suction line 24 will rapidly overcome the heating effect applied by the heating means through thermal conductor 42 and rapidly cause the valve 18 to decrease the quantity of refrigerant metered into the evaporation coil 22. De¬ creasing the quantity of refrigerant admitted into evaporator coil 22 prevents further liquid refrigerant from reaching the remote bulb ' 26 location on suction line 24. The rate of cooling of bulb 26 is decreased until the quantity of heat applied by the heating means again effects the liquidvapor mixture pressure exerted by bulb 26 on valve 18 to open the valve and increase the quantity of refrigerant metered into evap¬ orator coil 22.

In another embodiment, control 30 includes appa¬ ratus for improving the superheat control of a thermo¬ static expansion valve 18 metering refrigerant into an evaporator coil 22, including a circuit 38 for receiving superheat temperature information from temperature sens¬ ing devices 32 and 34, attached to the inlet line 20 of evaporator 22 and to the suction line 24 adjacent remote bulb 26 and the outlet of evaporator 22, re- spectively. Devices 32 and 34 may conveniently be attached to the refrigerant lines 20 and 24 by means of clips or straps 36. Temperature information from temperature sensing probes 32 and 34 is applied to control circuit 38 through conductors 33 and 35, re- spectively. Control circuit 38 utilizes the superheat temperature information received from probes 32 and 34 to control a heating means 40 mounted in a heat exchange relationship to remote bulb 26 for heating the bulb. Such heating permits controlling the superheat sensed by bulb 26 for correcting the superheat adjustment of valve 18 'without changing the mechanical superheat setting of the thermostatic expansion valve.

As will now be described with reference to Figures 5 and 6, heating means 40 may be mounted directly on the bulb 26 by means of a thermally conducting member 42. The clip or thermal conductor member 42 may con¬ veniently be a metal clip that surrounds the heater coil 40 and has a pair of descending spring-clip legs 42* . The upper portion of member 42 and heating coil 40 may conveniently be cast into an epoxy resin block or body 44 for providing a degree of insulation for coil 40 and member 42 and a means for storing heat. The spring-clip legs 42* are clipped over the body of bulb 26 to provide a direct heat exchange relationship with bulb 26, but not with the refrigeration suction line 24 to which the bulb is shown clamped by clamp 25. Accordingly, the clip 42 may be constructed of any suitable thermally conducting material but sized to limit the heat transfer from heater 40 to bulb 26 to a rate less than the cooling rate of- the refrigerant present in suction line 24 at the bulb 26 location.

Referring now to Figure 2, one embodiment of the superheat control 30 and the control circuit 38 is shown in greater detail. The temperature probes 32 and 34 may be any appropriate temperature responsive sensing means that will detect temperature and generate an electrical signal representative thereof, such as a temperature responsive diode or a thermistor. In Figure 2, the temperature sensing devices 32 and 34 are shown as temperature responsive diodes (32 and 34) having their cathodes connected in parallel to ground potential through conductor^" 46. The anode of the inlet diode 32 is connected through conductor 33, biasing resistor 50 and conductor 52 to a source of positive Voltage (+V) . The anode of the outlet diode 34 is connected through conductor 35, resistors 48 and 49 and conductor 52 to a source of positive voltage (+V) in parallel with diode 32. The resistor 49 acts as a biasing resistor for diode 34, and resistor 48 is a resistor that functions as a superheat setting means for establishing a required superheat for the valve 18. Resistors 48 and 49 comprise a voltage level establishing means or a voltage divider network.

The anode of diode 32 is also connected through conductors 33 and 55 as one input to a differential amplifier 56. The anode of diode 34 is also connected through conductor 35, resistor 48 and conductor 53 as a second input to amplifier 56. The output of amplifier 56 is applied through conductor 57 to an integrating circuit 58. A variable voltage signal output from integrator 58 is applied through conductor 59 to a voltage controlled oscillator (VCO) 60. The variable voltage input signal to oscillator 60 causes the oscillator to generate a variable frequency output signal the frequency of which increases or decreases in a functional relationship to the increase or decrease in voltage of the received integrator signal.

The oscillator output signals are applied through conductor 61 as an input to a conventional pulse gener¬ ating circuit 62. Pulse circuit 62 receives the var¬ iable frequency output signals from oscillator 60 and generates in response thereto a series of voltage pulses for application to the heating means 40 through

OMPI conductors 41(43). The frequency of the pulse signals from generator 62 is functionally related to the fre¬ quency of the variable frequency signals received from oscillator 60. Heating means 40 may be., any desired thermal energy source such as a resistive heating element as hereinabove described. Of course, other suitable thermal energy sources may be utilized.

In operation (see Figures 1 and 2) , the temperature responsive diodes 32 and 34 detect the refrigerant temperature at their respective locations on the refrigerant lines 20 (inlet) and 24 (outlet) of the evaporator coil 22. Since the refrigerant vapor will be superheated as it leaves evaporator coil 22 through suction line 24, and the refrigerant will be in its liquid (cold) state as it enters coil 22 through line 20, the difference in voltage at the anodes of diodes 32 and 34 will be an approximate measure of the super¬ heat of the refrigerant at the outlet of the evaporator coil 22. Diodes 32 and 34 respond to temperature by increasing conduction as the temperature rises, there¬ fore, as the temperature increases the voltage across the diodes decreases. Since the evaporator outlet temperature sensed by diode 34 will generally always be higher than the evaporator inlet temperature sensed by diode 32 the voltage signal level produced at the anode of diode 32, and applied to amplifier 56 via conductor 55, will also generally always be higher than the voltage signal level appearing at the anode of diode 34. However, when voltage setting means 48 is set for a required superheat setting, the series addition of the anode voltage appearing on conductor 35 and the voltage drop on the leg of resistor 48 combine to produce a third voltage signal applied as the second signal input to amplifier 56 through con- ductor 53.

OMPI The differential amplifier 56 compares the voltage signals applied as inputs and generates an output signal (Vy) representative of the differences in the input signals (i.e., the superheat). The V_τ output signal will be positive or negative depending on whether the irst ^'voltage signal from diode 32 (inlet) applied through conductor 55 to amplifier 56 exceeds or is less than the combined third signal from diode 34 (outlet) and voltage divider 48 applied through conductor 53 to amplifier 56. When the voltage signal on input 55 to amplifier 56 exceeds the voltage signal on input 53, the measured superheat exceeds the required super¬ heat. On the other hand, when the voltage signal on input 53 to amplifier 56 exceeds the voltage signal on input 55, the required superheat exceeds the measured superheat.

The voltage signals V<p applied to integrator 58 are integrated and an integrator output voltage signal (Vj) is generated for application as an input to VCO 60. The voltage of the integrator output signal increases or decreases in a functional relationship to the differences beween the voltage inputs to amplifier 56 as hereinabove described. Accordingly, V_τ will increase in a functional relationship to the differences in the input signals to amplifier 56 when the measured superheat exceeds the required superheat, and will decrease in a functional relationship to the differences in the input signals to amplifier 56 when the required superheat exceeds the measured superheat. The integra- tor variable voltage signal Vj is applied to the VCO 60 as hereinabove described to produce output signals the frequency (fj) of which are functionally related to the voltage level of the input V_j signals. The vari¬ able frequency voltage pulses generated by pulse gene- rator 62, as hereinabove described, are applied to heating means 40 for increasing or decreasing the rate of heating by the heater 40.

If the measured superheat across the evaporator coil 22 exceeds the established required superheat, the frequency of the voltage pulses from pulse genera¬ tor 62 will increase, thus increasing the rate of heating by heater 40 and adding heat to the remote bulb 26. Increased heating of bulb 26 will cause the valve 18 to open further to meter an increased quantity of cold liquid refrigerant into the evaporator coil 22, thus tending to decrease the measured superheat until it equals or is lower than the required superheat. When the required superheat exceeds the measured super¬ heat across the evaporator coil 22, the frequency of the voltage pulses from pulse generator 62 will de¬ crease, thus decreasing the rate of heating by heater 40 and applying less heat to the remote bulb 26. Applying less heat to bulb 26 will cause throttling of the valve 18 to meter a decreased quantity of cold liquid refrigerant into the evaporator coil 22, thus tending to increase the measured superheat until it equals or exceeds the required superheat.

In Figure 3, a second simplified embodiment of the control circuit 38 is shown. The temperature sensing means 32 and 34 are arranged in electrical parallel with the voltage divider 48 and 49 disposed in series with diode 34 as hereinabove previously described. Since the function of the temperature sens¬ ing devices 32 and 34 and the voltage divider 48 and 49 are identical to the functions hereinabove described, no further description will be given here. The voltage signals from the temperature measuring circuit are applied as identical inputs to those hereinabove des-

OMPI cribed through conductors 53 and 55 as inputs to a comparator circuit 66.

As long as the required superheat __ exceeds the measured superheat (voltage input 53 exceeds voltage input 55) ^'no output signal is generated by the compara¬ tor 66, and thus heating means 40 is turned off and no heat is added to the remote bulb 26. However, when the measured superheat exceeds the required superheat (voltage input 55 exceeds voltage input 53) the compar- ator 66 will generate an output voltage level applied through conductors 41(43) to heater 40. Heating means 40 then applies heat to the remote bulb 26 for control¬ ling the operation of valve 18 as hereinabove described for the first embodiment of the invention (Figure 2). In some applications, it may be desirable to add a thermal mass 70 interposed in a heat exchange relation¬ ship between heating means 40 and the valve remote fluid charged bulb 26 for stabilizing the rate at which the heating means 40 applies heat to the bulb 26. Thermal mass 70 acts as a thermal integrator to stabilize the heating rate of bulb 26, rather than having a cut-on (heating) or cut-off (no heating) mode of operation.

In a conventional thermostatic valve 18 the valve capacity vs. the superheat setting is shown in Figure 4 as curve 80 for a predetermined condenser line pressure and evaporator pressure. A preferred mechanical super¬ heat setting of 10° (point 82) may provide approximate¬ ly 55% capacity of the valve. However, if the conden- ser line pressure changes or the evaporator pressure changes, or other external conditions cause a change^' in the pressure drop across the valve, the superheat of the valve will also change. If the pressure drop P decreases, then the capacity vs. superheat curve 80 is shifted to 80', and the 10° superheat setting will only yield a partially fed evaporator coil due to a reduction in capacity to 25% as shown at 84. The capacity (C) of the valve can be expressed as follows:

^* C = f(SH)

where:

C = capacity of valve; f = functional relationship; and

SH = superheat of the valve

This implies that the superheat (SH) is functionally related to the capacity (C) .

Accordingly, external conditions that change the superheat of the valve also change the operating capacity of the valve to meter refrigerant admitted into the evaporator coil 22. Therefore, a method of changing the superheat of the valve that is not functionally related to the mechanical valve capacity is needed. The above described methods and circuits for adding heat to the valve fluid-charged bulb 26 solves the problem.

As shown in Figure 4, curve 80 is the capacity of the valve 18 vs. the valve superheat with a 10° superheat setting shown as point 82, corresponding to a 55% valve capacity as hereinabove described. By adding heat to the remote bulb 26, as hereinabove described, the curve 80 can be shifted to curve 86 (dotted line curve) and the required superheat may be lowered to 5° of superheat as shown at point 82' while retaining the same 55% valve capacity as previously described. The shift in superheat between the superheat setting (point 82) and

OMPI the required superheat (point 82') is T represented at 88.

Numerous variations and modifications, may be made in the methods and structures herein described without departing from the present invention. Accordingly, it should be clearly understood that the forms of the invention herein described and shown in the figures of the accompanying drawings are illustrative only and are not intended to limit the scope of the invention.

OMPI

Claims

1. A method of improving the control of a thermostatic expansion valve metering refrigerant into .a refrigera¬ tion system evaporator coil, comprising the steps of:

clamping the expansion valve fluid-charged bulb in a direct heat exchange relationship to the refrigerant line near the outlet of the evaporator coil,

supplying heat from a selected source, and

limiting the heat exchange transfer between said heating source and said bulb through a thermal conductor coop¬ erating with said selected source of heating and mounted in a heat exchange relationship with said bulb for transferring heat to said bulb at a rate less than the cooling rate when liquid refrigerant is present in the line at said bulb location.

2. The method as claimed in clia 1, wherein said supplying heat step further includes the step of storing heat supplied by said heating source for stabilizing the heat transfer from said- heating source to said bulb through said thermal conductor.

3. A method of improving the superheat control of a thermostatic expansion valve metering refrigerant into a refrigeration system evaporator coil, comprising the steps of:

establishing a required superheat,

measuring the superheat of the valve across the evapora¬ tor coil,

πτ>'

OMPI co paring the measured superheat with the established superheat, and ^•

applying heat to the fluid-charged bulb .of the valve when the measured superheat exceeds the established superheat.^'

4. The method as claimed in claim 3, wherein said applying heat step further comprises:

applying heat to the fluid-charged bulb of the valve at an increasing rate when the measured superheat exceeds the established superheat, and

applying heat to the fluid-charged bulb of the valve at a decreasing rate when the established superheat exceeds the measured superheat.

5. An improved control for a thermostatic expansion valve metering refrigerant into a refrigeration system evaporator coil, comprising:

heating means for applying heat to the expansion valve fluid-charged bulb that has been clamped in a direct heat exchange relationship to the refrigerant line near the outlet of the evaporator coil, and

a thermal conductor mounted on said bulb in a heat exchange relationship therewith and cooperating with said heating means for transferring heat to said bulb at a rate less than the cooling rate when liquid refrig- erant is present in the line at said bulb location.

6. The improved control as claimed in claim 5, further including a thermal mass connected to said thermal conductor for stabilizing the rate at which said heating eans applies heat to said thermal conductor for appli¬ cation to the expansion valve fluid-charged bulb.

7.- Apparatus for improving the control of a. thermostatic expansion valve metering refrigerant into a refrigera- tion system evaporator coil, comprising:

superheat setting means for establishing a required superheat,

superheat determination means for measuring the super¬ heat of the valve across the evaporator coil,

differential means for comparing the measured superheat with the established superheat, and

heating means cooperating with said differential means for applying heat to the fluid-charged bulb of the valve when the measured superheat exceeds the estab- lished superheat.

8. The apparatus as claimed in claim 7, wherein said superheat setting means comprises a voltage level estab¬ lishing means for establishing a preselected voltage level representative of said required superheat.

9. The apparatus as claimed in claim 8, wherein said superheat determination means comprises:

a first temperature responsive ^' means attached in a heat-exchange relationship to the refrigerant line at the inlet to the evaporator coil for generating a first voltage signal representative of the temperature of the refrigerant in said line, and

CMPI a second temperature responsive means attached in a heat-exchange relationship to the refrigerant line at the outlet of the evaporator coil for generating a second voltage signal representative of., the tempera- ture of the refrigerant in said line, said voltage level establishing means being disposed in an electrical series relationship with said second temperature res-

^'ponsive means for adding said preselected voltage level and said second voltage signal to form a third voltage signal representative of the sum of said voltages.

10. The apparatus as claimed in claim 9, wherein said differential means comprises a comparator circuit for re¬ ceiving said first and third voltage signals and gene¬ rating an output signal for application to said heating means when said first voltage signal exceeds said third voltage signal.

11. The apparatus as claimed in claim 10, wherein said heating means further includes a thermal mass inter¬ posed between said heating means and said valve fluid- charged bulb and in a heat exchange relationship there¬ between for stabilizing the rate at which said heating means applies heat to said expansion valve fluid- charged bulb.

12. The apparatus as claimed in claims 7 or 11, further including a thermally conductive member for mounting said heating means directly to said valve fluid-charged bulb for conducting heat from said heating means to said bulb.

13. The apparatus as claimed in claim 9, wherein said differential means comprises: a differential amplifier circuit for receiving said first and third voltage signals and generating an output signal representative of the difference be¬ tween said first and third voltage signals., and

heater control means receiving said output signals from said differential amplifier circuit for gene¬ rating heater control signals for application to said heating means for increasing the rate of heating by said heating means when said first voltage signal exceeds said third voltage signal and decreasing the rate of heating by said heating means when said third voltage signal exceeds said first voltage signal.

14. The apparatus as claimed in claim 13, wherein said heater control means comprises:

an integrating circuit for receiving said output sig¬ nals from said differential amplifier circuit for integrating and signals and generating output signals the voltage of which increases when said received difference signal is representative of said first voltage signal exceeding said third voltage signal and generating output signals the voltage of which decreases when said received difference signal is representative of said third voltage signal exceeding said first voltage signal,

a voltage controlled oscillator receiving said output voltage signals from said integrating circuit for generating variable fequency output signals the fre¬ quency of which increase or decrease in a functional relationship to said increase or decrease in voltage of said received integrating circuit signals, and

a pulse generating circuit receiving said variable

OMPI frequency output signals from said oscillator for generating in response thereto a series of voltage pulses for application to said heating means the fre¬ quency of which is functionally related to. the frequen- 5 cy of said received variable frequency output signals.

15. The apparatus as claimed in claim 9, wherein said first and second temperature responsive means com¬ prise temperature responsive diodes.

■"" 15. The apparatus as claimed in claim 8, wherein said 0 voltage level establishing means comprises a voltage divider network.