Servo-controlled expansion valve for a volatile fluid

The expansion valve has an inlet connection fluidly connected to a main valve while the main valve is fluidly connected through an outlet connection to the inlet of the evaporator of a refrigeration system. The main valve has a housing that includes a main valve chamber and a valve seat for fluidly connecting the inlet connection to the main valve chamber and a valve member movable between an open position and blocking fluid flow through the valve seat from the inlet connection to the valve chamber. A pressure operated servo arrangement is disposed in the main valve chamber for controlling the movement of the valve element and opens to the pressure outlet of a pilot valve arrangement. The pilot valve arrangement is disposed in a branch path that opens to inlet connection and to the valve chamber downstream of the valve set for controlling the fluid pressure at the pressure outlet.

The invention relates to a servo-controlled expansion valve for a volatile 
fluid, particularly for use in an electronically controlled injection of 
refrigerant in the evaporator of refrigeration installations, comprising a 
main valve which is actuatable by a controlled pilot valve arrangement by 
way of a servo arrangement in which the fluid serves as pressure medium. 
In a known expansion valve (DE-PS 27 49 250, FIG. 3, which corresponds to 
U.S. Pat. No. 4,475,686 to Huelle et al), the pilot valve is controlled by 
way of a diaphragm which, in turn, bounds a chamber in which there is a 
medium having a liquid and a vapour phase. This medium is heated by an 
electric heater in the liquid, so that a controlled pressure is reached 
which opens the pilot valve against the force of a spring. When the pilot 
valve opens, liquid refrigerant flows from the inlet of the expansion 
valve through a throttle orifice in an operating chamber bounded by a 
servo piston which actuates the closure member of the main valve, and from 
there through a throttle orifice in the servo piston and through the pilot 
valve orifice to the evaporator. The differential pressure thereby created 
by the refrigerant across the servo piston sets the position of the servo 
piston and thus of the closure member of the main valve, i.e. the degree 
of opening of the main valve. 
Under certain conditions, it can happen that the refrigerant evaporates in 
the operating chamber. By reason of the compressibility of the refrigerant 
vapour, the servo piston could oscillate, leading to corresponding 
oscillations of the closure member of the main valve. The problem is 
aggravated because refrigerant vapour can also form across the servo 
piston if the temperature of the refrigerant is near the boiling point and 
a pressure drop has been brought about by the throttle orifice in the 
servo piston, so that the servo piston strikes a vapour cushion in both 
directions of movement. 
The invention is based on the problem of providing a servo-controlled 
expansion valve which has less tendency to oscillate. 
This problem is solved in an expansion valve of the aforementioned kind in 
that the servo arrangement is thermally connected to the outlet side of 
the main valve. 
Lower temperatures obtain on the outlet side of the main valve because of 
expansion. These lower temperatures cool the fluid in the servo 
arrangement, so that no vapour can form here and the fluid is present as a 
liquid. The pressure build-up and thus the control take place solely 
through this liquid, which is incompressible. This considerably reduces 
the tendency of the closure of the main valve to oscilate. 
In a preferred embodiment, the servo arrangement is disposed in a chamber 
downstream of the main valve. The chamber is traversed by the fluid that 
has passed through the main valve. Since a lower temperature obtains on 
the outlet side of the main valve, i.e. downstream of the main valve, than 
on the inlet side, the lower temperature likewise obtains in the chamber, 
which cools the servo arrangement. 
In a preferred embodiment, the servo arrangement comprises a servo cylinder 
in which a piston connected to a valve element of the main valve bounds an 
operating chamber which is subjected to a pressure controllable by the 
pilot valve arrangement. In another preferred embodiment, the servo 
arrangement comprises a diaphragm which is connected to the valve element 
of the main valve and bounds an operating chamber which is subjected to a 
pressure controllable by the pilot valve arrangement. Diaphragm is 
understood to mean any deformable bounding wall of the operating chamber. 
The operating chamber can therefore also be bounded by bellows. Since the 
servo arrangement is thermally connected to the outlet side of the main 
valve, i.e. to the cold side, the operating chamber is cooled from the 
outside. No vapour can form in the operating chamber. This avoids 
oscillations. 
Advantageously, the pilot valve arrangement has in series between the inlet 
and outlet of the expansion valve a fixed and a controlled variable 
throttle between which the pressure controllable by the pilot valve 
arrangement can be derived. In one embodiment, the variable throttle may 
be disposed upstream of the fixed throttle and in another embodiment the 
fixed throttle upstream of the variable throttle. By changing the degree 
of opening of the variable throttle, which may be formed by a controllable 
valve, the pressure can be set to a large range of values between the 
inlet and outlet pressures. 
In an alternative embodiment, the pilot valve arrangement has in series 
between the inlet and outlet of the expansion valve two controlled 
variable throttles between which the pressure controllable by the pilot 
valve arrangement can be derived. This embodiment of the pilot valve 
arrangement is more expensive to construct but the control pressure 
produced by the pilot valve arrangement can thereby be set to practically 
every value between the inlet and outlet pressures of the expansion valve. 
In a third alternative, the pilot valve arrangement is in the form of a 
controlled three-way valve communicating with the inlet and outlet of the 
expansion valve and the operating chamber of the servo arrangement. The 
inlet thus communicates with the fluid, such as the refrigerant, in front 
of the expansion valve where there is a higher temperature than at the 
outlet of the expansion valve, to which one outlet of the three-way valve 
is connected. The second outlet of the three-way valve is connected to the 
operating chamber of the servo arrangement. 
One thereby likewise achieves favourable temperature influencing of the 
three-way valve, so that here, too, no vapour can form substantially 
because of the throttling effect of the outlet leading to the operating 
chamber. 
Advantageously, the pilot valve arrangement is electrically controllable. 
For this purpose, the variable throttles may be in the form of 
electrically or electromagnetically actuatable valves. Similarly, the 
three-way valve may have one or two electrically actuatable valves at its 
inlets or outlets. To achieve a throttling effect, the valves may also be 
opened and closed in cycles. Direct electric control is rapid and can be 
easily effected with the aid of known control means. 
Preferably, the chamber and the outlet of the main valve are in a metal 
housing. Since there is a lower temperature on the outlet side of the main 
valve and metal is a good thermal conductor, this ensures that the chamber 
is cooled directly by the fluid on the outlet side. Of course the inlet of 
the main valve must also somehow open into the housing. However, by means 
of a suitable conduit system, one can ensure that the temperature 
influence by the outlet is greater. 
It is in this case preferable for the pilot valve arrangement in the 
housing is disposed at the housing parts bounding the chamber. This 
ensures that the pilot valve arrangement is cooled not only by the fluid 
around it but also by the cold flow through the metal housing.

An expansion valve 20 comprises an inlet connection 1 which is connected to 
the outlet of the condenser 41 of a refrigeration installation, and an 
outlet connection 2 for a volatile liquid which is connected to the inlet 
of the evaporator 40 of the system, separated by a main valve 21 which is 
bridged by a branch path 3. The branch path 3 has a branch inlet 4 
branching off from the inlet connection 1. The branch path allows the 
liquid to flow to the outlet connection 2 through its branch outlet 5. A 
pilot valve arrangement 6 is disposed in the branch path 3. 
There are various constructions for the pilot valve arrangement, as will be 
explained in conjunction with FIGS. 3 and 4. Two throttle points are 
provided in series between the branch inlet 4 and branch outlet 5. In FIG. 
3a, these are a fixed throttle 7 and a variable adjustable throttle 8 
which can, for example, be formed by a magnetic valve. Between the two 
throttling points, a control pressure P.sub.S can be derived at a control 
pressure outlet 12. This pressure is adjustable between the condenser 
pressure P.sub.K at the branch inlet 4 and the evaporator pressure P.sub.V 
at the branch outlet 5. If the variable throttle 8 is closed, the pressure 
P.sub.S at the control pressure outlet is equal to the pressure at the 
branch inlet. On the other hand, if the adjustable throttle 8 is opened 
completely, the pressure P.sub.S at the control pressure outlet 12 depends 
on the amount of fluid flowing through. 
In FIG. 3b, the sequence of fixed and variable throttle is reversed. In 
this case, behind the branch inlet 4 there is first a variable throttle 8' 
and, downstream thereof, a fixed throttle 7'. If the variable throttle 8' 
is closed, the evaporator pressure P.sub.V obtains at the control pressure 
outlet 12. If the variable throttle 8' is opened, the pressure P.sub.S at 
the control pressure outlet 12 depends on the amount of fluid flowing 
through. 
In FIG. 3c, both throttles 9, 10 are variable. One can thereby ensure that 
the pressure at the control pressure outlet 12 in the valve bottom wall 43 
of the housing 34 can assume the value of the pressure P.sub.K at the 
branch inlet 4 as well as the pressure P.sub.V at the branch outlet 5. 
Both throttles, which may be electrically actuatable valves, can be 
operated independently of each other. 
FIG. 3d shows a fourth embodiment in which the pilot valve arrangement 
consists substantially of a three-way valve 11. The function of this 
three-way valve corresponds to the function of one of those shown in FIGS. 
3a to 3c, depending on its construction. It could also be the case that, 
without a pressure drop at its inlet, the three-way valve divides the 
inlet pressure amongst the control outlet 12 and branch outlet 5. 
FIG. 4 illustrates a standard symbol for all the pilot valve arrangements 
of FIG. 3, the control pressure P.sub.S at the control pressure outlet 12 
setting itself between the value P.sub.K at the branch inlet 4 and the 
value P.sub.V at the branch outlet 5 as a result of a signal at one 
control inlet 13, for example an electric connection. This symbol is 
employed in FIGS. 1 and 2 in order to illustrate the pilot valve 
arrangement. 
The main valve 21 of the expansion valve 20 contains in a housing 34 a 
valve seat 22 against which a closure member 23 is movable. When the 
closure member 23 lies against the valve seat 22, the main valve 21 is 
closed. The movement of the closure member 23 is controlled by a servo 
arrangement 24 by way of a tappet 25. 
The servo arrangement 24 according to FIG. 1 comprises bellows 26 bounding 
an operating chamber 27. The bellows are compressed under the force of a 
spring 28 supported against an abutment 38 which is fixed with respect to 
the housing, whereby the closure member 23 moves to the open position of 
the main valve 21. The operating chamber 27 is impinged by the control 
pressure P.sub.S from the control pressure outlet 12 of the pilot valve 
arrangement 6. The control pressure P.sub.S thus acts against the force of 
spring 28 to bring the main valve 21 to the closed position. The servo 
arrangement 24 is disposed in a chamber 33 located on the outlet side of 
the main valve 21, i.e. traversed by expanded and thus cooled liquid. The 
chamber 33 is in direct communication with the outlet connection 2. This 
ensures that the fluid that has passed through the main valve 21 also 
flows around the servo arrangement before it leaves the expansion valve 20 
through the outlet connection 2. Since the fluid on the outlet side of the 
main valve 21, i.e. in the chamber 33, has a lower temperature than at the 
inlet connection 1, no vapour can form in the operating chamber 27 which 
is likewise filled with fluid by way of the pilot valve arrangement 6. The 
fluid in the operating chamber 27 is, through external cooling, held at 
substantially the same temperature as the fluid in the chamber 33. At this 
temperature, however, the fluid is in its liquid phase. Since the liquid 
is incompressible, no oscillations can arise that might become 
disturbingly noticeable as oscillations of the closure member 23. 
For a still better thermal coupling of the servo arrangement to the cold 
fluid at the outlet side of the expansion valve, the housing 34 is made of 
metal. The servo arrangement 24 is secured to the metal housing. It will 
be known that metal is a good thermal conductor, so that the housing 34 
and thus the servo arrangement 24 will not be able to store heat. Instead, 
the heat is dissipated immediately. Naturally, the relatively warm fluid 
must be fed to the expansion valve 20 by way of an inlet connector 35. The 
inlet connector 35 should therefore be thermally uncoupled from the 
housing 34, for example by an interposed thermal insulator (not shown). On 
the other hand, an outlet connector 36 forming the outlet connection 2 may 
be made in one piece with the metallic housing 34 because the outlet 
connector 36 is cooled by the fluid on the outlet side of the expansion 
valve 20. From a constructional point of view, the conduit system can be 
made so that the metal housing comes into contact with the cooler fluid on 
the outlet side of the expansion valve 20 over a larger area than with the 
warmer fluid on the inlet side. This ensures that a cooling effect is 
exercised on the servo arrangement 24 not only by way of the chamber 33 
but also by way of the metal housing 34. Although the servo arrangement 24 
is illustrated as bellows in the present example, the operating chamber 
may also be surrounded by a solid body, for example a cylinder closed at 
the end by a diaphragm. The closure member 23 of the main valve 21 has to 
execute only relatively small movements which can also be produced by a 
diaphragm. 
FIG. 2 shows a different example of a servo arrangement. Parts 
corresponding to those in FIG. 1 have been provided with the same 
reference numerals. The servo arrangement 24' comprises a cylinder 29 
which, together with a piston 30, bounds an operating chamber 37. The 
piston 30 is connected to the tappet 25 of the closure member 23. The 
piston 30 works against the force of a spring 31 which is supported 
against an abutment 32 fixed with respect to the cylinder. The operating 
chamber 37 of the servo arrangement 24' communicates with the control 
pressure outlet 12 of the pilot valve arrangement 6. Fluid entering the 
pilot valve arrangement 6 through the branch inlet 4 enters the branch 
outlet 5 and also through the control pressure outlet 12 the operating 
chamber 37. This fluid is in the liquid phase but near its boiling point. 
The throttling effect of the pilot valve arrangement 6 could therefore 
cause it to vaporise. However, since the cylinder 29 is arranged in the 
chamber 33 which is traversed by the cooler fluid, the fluid in the 
operating chamber 38 is also cooled so that the temperature drops to far 
below the boiling point. The danger of forming vapour is therefore 
eliminated. The operating chamber 37 therefore remains filled with fluid 
in the liquid phase, whereby oscillations are avoided. 
FIG. 5 shows a pressure-enthalpy diagram illustrating the function of the 
illustrated servo-controlled expansion valve. The curve E represents the 
relationship between enthalpy and pressure, the liquid being at boiling 
point. Below the curve E, the refrigerant is present as saturated vapour. 
Along the arrow A, compression of the saturated refrigerant vapour takes 
place from a pressure P.sub.V to a higher pressure P.sub.K. At a constant 
pressure P.sub.K, condensation takes place along the arrow B up to the 
point I which represents the condition of the refrigerant at the outlet of 
the condenser and thus at the inlet 1 of the expansion valve 20. From the 
point I, the expansion valve 20 brings about expansion of the refrigerant 
to the point D along the arrow C, the pressure dropping from the condenser 
pressure P.sub.K to the evaporator pressure P.sub.V. The enthalpy will be 
reduced correspondingly. The point IV corresponds to the condition of the 
refrigerant in the servo arrangement 24, 24' having a pressure P.sub.S and 
an enthalpy corresponding to the point V. Since this point lies above the 
limit between the liquid phase and gaseous of the refrigerant the 
refrigerant in the servo arrangement 24, 24' will always be in the liquid 
phase. At a constant refrigerant pressure P.sub.V after the point V, 
heating takes place by the absorption of heat from the surroundings in the 
evaporator along the arrow D, whereby the circuit is closed. It will be 
evident that, if the refrigerant in the servo arrangement is kept cooled, 
the formation of vapour can here be supressed with certainly, thereby 
producing a "stiff" regulating system. 
The pressure P.sub.S set by the pilot valve arrangement 6 between the two 
throttling points is determined by the following equation: 
##EQU1## 
In the throttling point adjacent to the outlet 5, e.g. the second throttle 
6, 7' or 10, the fluid is throttled from point IV (P.sub.S) to point V 
(P.sub.V), which takes place without the formation of vapour because the 
servo arrangement 24, 24' as well as the associated conduits and the 
bellows 26 or cylinder 29 are thermally coupled to the lower temperature.