EP0240811A1 - Control system for installations having a refrigerating circuit with capillary tube expansion - Google Patents

Abstract

The installation, which comprises in a cycle, as usual, an evaporator (10), a compressor (12), a condenser (14), a refrigerant fluid filter (15) and an expansion capillary tube (16), is characterised in that it comprises, in a line, for control, between the evaporator (10) and the compressor (12), a small vessel (1) containing refrigerant fluid in the liquid state which fulfils simultaneously the function of lung/liquid separator and of a device which ensures a return of lubrication oil to the compressor (12). The installation can also comprise a valve (23) for controlling the capacity. <IMAGE>

Description

The present invention relates to a system for regulating refrigeration circuit installations comprising a capillary expansion, installations in which a refrigerant flows through a thermodynamic cycle which consists of evaporation, compression, condensation and expansion.

The objective is to improve the known adjustment systems both from an economic point of view and from a regulation point of view.

The fields of application of the invention can be, by way of nonlimiting example:
- compressed gas dryers
- air conditioning units
- heat pumps
- liquid refrigerators
- more besides.

• 1) un tube capillaire
• 2) une soupape thermostatique
• 3) d'autres systèmes de moindre diffusion.

From the point of view of the refrigeration circuit adjustment system, the installations can be divided according to the type of rolling element which can be:

• 1) a capillary tube
• 2) a thermostatic valve
• 3) other less widespread systems.

We will examine below, from the point of view of the disadvantages that they present, the refrigerant circuits with capillary tube and thermostatic valve, with or without adjustment of the capacity by means of a bypass (hot gas) . No detailed description will be given of the physical operating principles of the two systems, since it is considered that the reader is already aware of it.

Refrigerant circuits provided with a capillary rolling member.

The schematic diagram of the refrigerant circuit with capillary tube is given in Figure 1 attached.

Figures 2 and 3, moreover, show the same type of installation, but being in abnormal operating conditions, namely in the first case with insufficient refrigerant in the evaporator and in the second case, with an excess of refrigerant in the evaporator.

In these Figures 1 to 3, the reference 10 designates the evaporator, 11 the refrigerant (at the normal level in Figure 1, in insufficient quantity in Figure 2 and in excess in Figure 3), 12 the compressor, 13 the compressor motor, 14 the condenser, 15 a filter and 16 the expansion capillary. In FIG. 3, the reference 17 indicates returns of refrigerant in the liquid state to the compressor 12 due to the presence of an excess of said fluid at the level of the evaporator 10.

The adjustment system by capillary rolling member is generally used in low-power installations in which it is important to have a low cost and where the drawbacks arising from the imperfect adaptability of the refrigerating installation to variable conditions of use of the capillary 16, which constitutes a regulating member of the static type, are of little importance. This non-adaptability of the capillary 16 can alternatively cause significant overheating of the refrigerant aspirated by the compressor 12 or liquid returns 17, also to the compressor 12. The high overheating can be caused for example by an insufficient charge of refrigerant 11 (see Figure 2) or by an increase in refrigeration charge requested from the evaporator 10 or by a significant decrease in the condensing pressure and can seriously damage the compressor 12 since the cooling of the windings of the electric motor 13, which is almost always supplied by the refrigerant drawn into the compressors 12 used for these applications, may no longer be sufficient. The liquid returns 17 can for example be caused by an excessive charge of refrigerant fluid (see FIG. 3) or by a reduction in the fridge charge. required by the evaporator 10 or by a significant increase in the condensing pressure and can seriously damage the compressor 12 since the possible presence of liquid in the compression phase can lead to rupture of the valves or seizure of the compressor 12. Another problem with capillary tube installations 16 results from the precision required for the metering of the charge, moreover rather reduced, of refrigerant. In fact, if there is leakage - even very small - of refrigerant, the installation discharges in a short time and gives rise to dangerous operating situations, such as too high overheating at compressor 12 already mentioned. In addition, the refrigerant charge requires a lot of attention and care since, as it is very small, small and large variations can significantly affect the performance and the operational safety of the installation.

Refrigerant circuits provided with rolling elements with thermostatic valve.

The schematic diagram of the refrigerating circuit with thermostatic valve is given in Figure 4.

FIG. 5 also relates to this type of installation with a thermostatic valve, showing in detail the function exerted by the valve.

In these Figures 4 and 5, as above, the evaporator is designated by the reference 10, the compressor by 12, the compressor motor by 13 and the condenser by 14. The reference 18 marks the presence of a receptacle for receiving condensed fluid at the outlet of the condenser, 19 designates the thermostatic valve, 20 a detector of variation in the flow of refrigerant which will be discussed later. In FIG. 5, the reference 21 indicates the liquid in evaporation in the evaporator 10 and the reference 22 designates a zone of superheating of the gas leaving the evaporator 10 and going towards the compressor 12.

Like this thermostatic valve setting diagram that is more expensive than that with capillary, it is generally used when the latter, due to the non-adaptability to variations in operating conditions, presents the problems already described.

Relative to the capillary tube, the thermostatic valve 19 controls the flow of the refrigerant 11 which passes through it so as to maintain a constant and moderate superheating of the refrigerant in the vapor state at the outlet of the evaporator 10 (see reference 21 to the 5 to ensure the complete transformation of the refrigerant from liquid to gas and consequently to eliminate the possibility of dangerous returns of liquid to the compressor. A drawback of the thermostatic valve system results from the instability that this dynamic type regulating member introduces. , due to the phase shifts, practically impossible to eliminate, between the effect on the system of the variation in refrigerant flow due to the regulating action of the valve and the signal which controls this regulating action and which comes from the detector 20 placed in downstream of the evaporator 10.

This instability of the system consists, for example, of a fluctuation in the evaporation pressure which gives rise to a deterioration in the performance of the installation. The superheating which must be maintained at the outlet of the evaporator 10 must not be excessive but never lower than a minimum value otherwise, because of the variables which have just been described, there may be at certain times dangerous liquid returns to the compressor. This means that in thermostatic valve installations, part 22 of the evaporator exchange surface 10 must be intended for the superheating of the vapor and that, consequently, the exchange capacity of the evaporator is not fully exploited.

Capacity-controlled refrigeration circuits

A special case, but quite frequent, occurs when the refrigerant charge requested from the evaporator can vary between 0% and 100% of the maximum value projected and when, at the same time, the evaporation pressure of the refrigerant must not be lower than a determined value (for example 0 ° C gauge) to avoid freezing of the fluids in the evaporator, as for example when the fluid to cool is moist air or water. It is therefore necessary to have a regulating member capable of reducing the excess cooling capacity, a member which generally consists of a pressostatic valve (see FIG. 6, reference 23) which opens a bypass between the discharge. and the compressor suction in order to prevent the suction pressure from falling below a preset value.

Figures 6 and 7 give the diagram of installations of this type, Figure 6 for capillary tube installations (improvement to the installation of Figures 1 to 3) and Figure 7 for thermostatic valve installations (improvement to the 'installation of Figures 4 and 5).

In these figures, the common members are designated by the same reference numbers as above (10 for the evaporator, 12 for the compressor, 13 for the motor, 14 for the condenser, 15 for the filter, 16 for the capillary, 18 for the container after the condenser, 19 for the thermostatic valve and 20 for the flow variation detector). Reference 23 indicates the pressostatic or other valve for regulating the capacity and 24 the point of injection of the hot gas coming from the evaporator and from the regulating valve.

The refrigerant circuits with a capillary tube rolling member and capacity regulator (see FIG. 6) keep the pressure but not the temperature of the refrigerating fluid under control downstream of the capacity regulating valve which injects hot gas and consequently, in the event of drawbacks such as loss of refrigerant or malfunction of the valves, they are exposed to drawbacks to the compressor due to the excessively high overheating which occurs in these cases.

The refrigeration circuits with laminating member with thermostatic valve and capacity regulator (see FIG. 7) keep the temperature of the refrigerating fluid drawn in by the compressor under control, provided that the injection of hot gas occurs upstream of the detector of the thermostatic valve, but they can present, at partial loads, that is to say when the capacity regulating valve intervenes, fairly intense evaporation pressure fluctuations determined by the interaction between the two regulating organs. In practice, these interactions are very difficult to eliminate since both the thermostatic valve and the pressostatic valve are controlled by the evaporation pressure and that the intervention of any of their control functions influences this pressure.

The present invention consists of a capillary tube adjustment system making it possible to obviate the drawbacks of the existing systems indicated above.

It relates to a system for regulating refrigeration circuit installations comprising a capillary expansion, installations in which a refrigerant flows through a thermodynamic cycle which consists of evaporation, compression, condensation and expansion, said system being characterized by fact that it comprises, in line, between the evaporator and the compressor, a small container containing refrigerant in the liquid state which simultaneously exercises the function of lung / liquid separator and of device ensuring a return of oil from compressor lubrication.

According to a preferred embodiment, the small lung / liquid separator container has at its lower part a vapor inlet tube coming from the evaporator, at its upper part a suction tube extending into the container by a section of "U" -tube, the second branch of which stops a short distance from the upper part of the container and fitted in the bottom bend with a calibrated passage used to sample of a small quantity of liquid, and a liquid level indicator fixed on the body of the container, at a height located between the two tubes making it possible to ascertain the level of the liquid which is contained in the container and in which the incoming steam should bubble. It goes without saying that this level indicator can have any form of construction, even external to the container, provided that it is able to exercise its function.

The expansion capillary tube and the capacity of the lung / separator are dimensioned in such a way that whatever the operating condition (i.e. at any evaporation and condensation pressure) the refrigerant to the evaporator outlet is always in the saturated vapor state (never superheated).

In this way, the adjustment of the capacity of the capillary to the various operating conditions of the installation is obtained by varying the sub-cooling of the liquid at the inlet of the capillary.

Let us consider for example a stationary regime installation (that is to say characterized by a constancy over time of the characteristic parameters of the installation) in which there is a certain degree of sub-cooling at the inlet of the capillary, that the refrigerant at the outlet of the evaporator is saturated vapor and the lung / separator container contains a certain amount of liquid and analyze what will happen in the event of an increase or decrease in the refrigerant charge requested from the 'evaporator.

1) Increase in the refrigeration charge requested:

At the same time as an increase in the evaporation pressure, it happens that the refrigerant at the outlet of the evaporator comes out superheated but that by mixing with the liquid which is inside the lung container / separator it returns to the state of saturated vapor at the expense of a certain evaporation of this this.

The level in the container drops due to migration of refrigerant from it to the condenser which, due to the partial flooding of the condenser, causes an increase in subcooling and condensing pressure. In turn, these increases cause an increase in the capacity of the capillary and therefore an increase in the flow rate of the refrigerant. The final situation will therefore be as follows:
- increase in evaporation pressure
- increase in condensing pressure
- increased sub-cooling of the liquid at the capillary inlet
- decrease in level in the lung / separator container
- always saturated steam at the outlet of the evaporator.

2) Reduction in the refrigeration charge requested:

It is exactly the opposite of what has been described above, that is to say that the final adjustment of the capillary capacity will be obtained because of a partial emptying of the condenser and consequently of a decrease. of the sub-cooling of the liquid at its entry.

At high condensing pressures, it can happen that we are in a situation in which the condenser is completely emptied of liquid which is consequently saturated at the inlet of the capillary. In this case, a further reduction in the capacity of the capillary is obtained at the expense of a certain amount of uncondensed saturated vapor which enters the capillary at the same time as the liquid. It is obtained that small percentages of vapor relative to the liquid cause large variations in the capillary passage capacity, whereby the amount of uncondensed vapor is always very small and is in any case compensated for by the fact that the complete emptying of the condenser on the part of the liquid, and therefore the absence of subcooling allows the best use of the entire surface of the condenser.

The capacity of the container / lung is established in relation to the variations in volume of the coolant as a result of the variation in the operating conditions. This variation in volume must be less than the capacity of said container and the free surface of the liquid refrigerant in the container must always be between the refrigerant outlet section, to prevent liquid from returning to the compressor, and the return device. of oil or the fluid inlet section coming from the evaporator, so that the oil return device is always immersed in the liquid and that there is always a certain amount of liquid above the entrance section.

The oil return device necessary for the lubrication of the compressor serves the fact that due to the complete evaporation of the refrigerant in the evaporator there occurs a complete separation between the oil, initially mixed with the refrigerating fluid to liquid state, and vapor. When in fact, in systems with only a capillary or a thermostatic valve, the oil return is ensured by the mechanical drive due to the high speed of the vapor in the suction pipe, in this case, due to the low speed of the vapor in the lung / separator container, which must ensure the separation of the entrained droplets of liquid, it is necessary to withdraw (for example by exploiting the Venturi effect in a U-shaped tube) a small amount of liquid containing the oil not entrained in solution.

The level indicator is used to control the amount of refrigerant present and makes the charging phase of the refrigerant in the installation elementary. In fact, as soon as we know the level that the fluid must reach under the preselected charging conditions, it suffices to charge the refrigerant until that it reaches the prefixed level without any type of control. When the refrigerant charge requested from the evaporator can vary from 0% to 100%, an additional device can be provided for regulating the capacity of the pressostatic valve type.

In this case, the gas taken from the compressor discharge is mixed with the refrigerant at the outlet of the evaporator before the lung / separator container in order to eliminate the superheating of this gas by bubbling through the liquid contained therein, and therefore always ensure the presence of saturated steam at the compressor intake.

Compared to capillary installations of which it retains the qualities of low cost, operating stability and reliability, the present invention has the following advantages:
- adaptability to wide variations in operating conditions,
- possibility of application to installations having refrigerant charges which may be significant, thanks to an adequate dimensioning of the volume of the container / separator, without imbalance due to variations in speed,
- functionality, even in large cooling capacity installations which generally require relatively large refrigerant charges,
- consistency of the suction temperature of the refrigerant to the compressor, which always corresponds to that of the saturated steam.
- insensitivity to small refrigerant leaks due to the capacity of fluid contained in the lung / separator,
- extreme ease of charging the refrigerant thanks to the level indicator,
- suppression of the superheating of the hot gas injected by means of a possible capacity regulation valve in installations with variable refrigerating charge and at operating temperature on the evaporator minimal.

Compared to installations fitted with a thermostatic rolling valve, the present invention is characterized by the advantages mentioned below:
- reduced cost due to the absence of the thermostatic valve itself,
- operating stability due to the absence of dynamic type regulations,
- maximum use of the heat exchange surface of the evaporator since, unlike what occurs for the thermostatic valve, it is not necessary to overheat the refrigerant which therefore always works in the saturation state,
- minimum torque at the compressor motor thrust, since the capillary quickly balances the suction and discharge pressures of the refrigerant circuit,
- absence of instability of the operating conditions, even in the case of using a capacity regulation valve by injection of hot gas, thanks to the functional stability of the capillary,
- efficient cooling of the electric motor of the compressor thanks to the small withdrawal of liquid which ensures the return of the oil.

• La figure 8 donne un schéma général d'une instal­lation suivant l'invention.
• La figure 9 représente une vue latérale, agran­die, du récipient poumon/séparateur qui constitue une caractéristique de l'invention.
• La figure 10 montre une coupe de ce même réci­pient.
• La figure 11 donne un schéma général d'une installation analogue à celle de la figure 8, mais pour­vue en supplément d'une soupape régulatrice de la capa­cité.

In what follows, the invention is explained with reference to Figures 8 to 11 which illustrate an embodiment given only by way of example.

• FIG. 8 gives a general diagram of an installation according to the invention.
• Figure 9 shows an enlarged side view of the lung / separator container which is a feature of the invention.
• Figure 10 shows a section of this same container.
• Figure 11 gives a general diagram of an installation similar to that of Figure 8, but additionally provided with a capacity regulating valve.

As in Figures 1 to 7, reference 10 indicates the evaporator, the number 12 the compressor, 13 the compressor motor, 14 the condenser, 15 the filter 16 the expansion capillary and 23 (figure 11) the capacity regulating valve.

The lung / separator container placed at the outlet of the evaporator 10 is designated by the reference 1. By way of example given here, it may be of cylindrical shape (see FIGS. 9 and 10) and be provided with a tube d suction 2, of a level indicator 3 (for example a porthole-shaped sight fixed on the body of the container 1) and by an inlet tube 4 of the refrigerating tube coming from the evaporator 10, which tube is arranged preferably in the lower part of the container 1. In this embodiment (see FIG. 10), the suction tube 2 is constituted, for example, by a section of pipe in "U" shape, the end of which inlet 5 is located at the upper part of the container 1 and which is provided with a calibrated passage 6 in the lower part, passage serving (as we have seen) to ensure adequate return of oil to the compressor, the second branch of the U-shaped tube stopping close to the top of the container.

The level indicator 3, which here has the shape of a transparent porthole, is placed higher than the inlet tube 4 so that the fluid which enters the container 1 must bubble through the liquid as far as the free surface of the latter is maintained in the visual field of the level 3 indicator.

FIG. 11 gives the diagram of an installation according to the invention which can be used more particularly in cases where the refrigerating charge requested from the evaporator can vary within significant limits, or even from 0% to 100%. It is then necessary to provide, in addition, a body for regulating capacity. In the embodiment of Figure 11, this member consists of a valve 23, which is here of the pressostatic type, placed in bypass between the discharge of the compressor 12 and the outlet of the evaporator 10 in order, as mentioned above, to always ensure the presence of saturated steam at the suction of the compressor.

Claims (7)

1. - System for regulating refrigerating circuit installations comprising a capillary expansion, installations in which a refrigerating fluid traverses a thermodynamic cycle which consists of evaporation, compression, condensation and expansion, characterized in that said system comprises, in line, between the evaporator (10) and the compressor (12), a small container (1) containing refrigerant in the liquid state which simultaneously exercises the function of lung / liquid separator and of device ensuring a return d lubricating oil to the compressor. 2. - adjustment system according to claim 1, characterized in that the small container (1) lung / liquid separator has at its lower part an inlet tube (4) of the steam coming from the evaporator (10) , at its upper part a suction tube (2) extending into the container (1) by a section of tube in "U" whose second branch (5) stops a short distance from the upper part of the container and provided in the bottom bend with a calibrated passage (6) used for the withdrawal of a small quantity of liquid, and a liquid level indicator (3) fixed on the body of the container, at a height located between the two tubes making it possible to ascertain the level of the liquid which is contained in the container and in which the incoming steam must bubble. 3. - Adjustment system according to one of claims 1 or 2, characterized in that the capillary expansion tube (16) and the capacity of the container (1) lung / liquid separator are dimensioned so that whatever the operating conditions (pressures) the refrigerant at the outlet of the evaporator (10) is always in the state of saturated vapor. 4. - adjustment system according to claim 3, characterized in that the capacity of the lung container / liquid separator (1) is established in relation with the variations in volume of the refrigerant as a result of the variation in the operating conditions, taking into account that this variation in volume must be less than the capacity of said container (1) and that the free surface of the liquid refrigerant in the container (1) must always be between the outlet section of the U-shaped tube of the refrigerant, to prevent liquid from returning to the compressor (12), and the oil return device (6) or the inlet section (4) of the fluid coming from the evaporator (10), so that the oil return device is always immersed in the liquid and that there is always a flap of liquid above the entry section (4). 5. - Adjustment system according to any one of the preceding claims, characterized in that the return of oil to the compressor is ensured by drawing off a small quantity of liquid containing oil in solution from the small lung / separator container liquid (1) via the calibrated passage (6) provided therein. 6. - Adjustment system according to claim 5, characterized in that the withdrawal of a small amount of liquid from the lung / separator container (1) is carried out by exploiting the Venturi effect in a "U" tube. 7. - Adjustment system according to claim 4, characterized by a capacity regulating valve (23) mounted between the compressor (12) and the outlet of the evaporator (10) to the lung / separator container (1).

Images