LU86391A1 - ADJUSTMENT SYSTEM FOR REFRIGERATION CIRCUIT INSTALLATIONS COMPRISING A CAPILLARY RELAXATION - Google Patents

Description

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NOT

BL-3866 "System for regulating refrigerating circuits with capillary expansion"

The present invention relates to a system for adjusting refrigeration circuit installations 5 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 control systems 10 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 15 - air conditioning units - heat pumps - liquid refrigerators - still others .

From the point of view of the refrigerating 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 systems of less diffusion .

We will examine below, from the point of view of the disadvantages which they present, the refrigerating circuits with capillary tube and with thermostatic valve, with or without adjustment of the capacity by means of a bypass (gas) hot. 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.

35 refrigerator_for_us_from ^ a_or2ane_de_lamina2e à_çagillaira.

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

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This adjustment system 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 plant to the variable conditions of use of the capillary, which constitutes a regulating member of the static type, are of little importance. This non-adaptability of the capillary can alternatively cause significant overheating of the refrigerant aspirated by the compressor or liquid returns, also to the compressor. High overheating can be caused, for example, by an insufficient charge of refrigerant (see Figure 2) or by an increase in the refrigerant charge demanded by the evaporator or by a significant reduction in the condensing pressure and can seriously damage the compressor. since the cooling of the windings of the electric motor, which is almost always ensured by the refrigerant sucked into the compressors used for these applications, may no longer be sufficient.

Liquid returns can, for example, be caused by an excessive charge of refrigerant (see Figure 3) or by a decrease in the refrigerant charge required on the evaporator or by a significant increase in condensing pressure and can seriously damage the compressor since the possible presence of liquid in the compression phase can lead to the rupture of the valves 30 or to seizure of the compressor. Another problem with capillary tube installations results from the precision required for the metering of the charge, moreover rather reduced, of refrigerant. In fact, if there is any leakage - even very small - from the refri-35 manager, the installation discharges in a short time and gives rise to dangerous operating situations, such as excessively high overheating at

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-3- coinpressor already mentioned. In addition, the refrigerant charge requires a great deal of care and attention since, as it is very small, small and large variations can significantly affect the performance and the operational safety of the machine. 'installation. Refrigerator_circuits_for_dj_organes_de_lamina2e

The schematic diagram of the frigori-10 fère circuit with thermostatic valve is given in Figure 4.

As this adjustment scheme is more expensive than the capillary one, 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 controls the flow of the refrigerant passing through it so as to maintain a constant and moderate superheating of the refrigerant in the vapor state 20 at the outlet of the evaporator (see FIG. 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 25 results from the instability that this dynamic type regulating member introduces, owing to phase shifts, practically impossible to eliminate, between the effect on the system of the variation in refrigerant flow due to 1 the regulating action of the valve and the signal which controls this regulating action and which comes from the detector placed downstream of the evaporator.

This instability of the system consists, for example, of a fluctuation in the evaporation pressure 35 which gives rise to a deterioration in the performance of the installation. The superheat which must be maintained at the outlet of the evaporator must not -4-

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be excessive but never lower than a minimum value otherwise, due to the variables just described, there can at times be dangerous returns of liquid to the compressor. This means that in thermostatic valve installations a part of the evaporator exchange surface must be intended for the superheating of the vapor and that, therefore, the evaporator's exchange capacity is not fully exploited.

10 Refrigeration circuits with capacity control.

A particular, but fairly frequent case arises when the refrigerant charge required on the evaporator can vary between 0% and 100% of the maximum projected value and when, at the same time, the evaporation pressure of the refrigerant must not not be less than a predetermined value (for example 0 ° C pressure) to avoid freezing of the fluids in the evaporator, as for example when the fluid to be cooled is humid 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 near-static valve which opens a bypass between the discharge and the suction of the compressor in order to to prevent the suction pressure from falling below a preset value.

The refrigeration circuits with a capillary tube rolling member and capacity regulator 30 (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.

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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 members. 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 (see FIG. 8) in which a small container containing refrigerant in the liquid state, which simultaneously exercises the function of lung / liquid separator and of device ensuring the return compressor lubricating oil is inserted in line between the evaporator and the compressor.

The lung / separator (see FIG. 9) preferably consists of a container, for example of cylindrical shape 1, by a suction tube 2 and by a level indicator, for example a level 3 manhole fixed on the body. of the container 1, and by an inlet tube 4 of the refrigerating fluid coming from the evaporator, a tube which is preferably arranged in the lower part of the container 1.

As can be clearly seen in FIG. 10, which represents in section said lung / separator, the suction tube 2 can be constituted, by way of example, by a section of pipe in "U" shape,

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-6- the inlet end 5 is located at the upper part of the container 1 and which is provided with a calibrated passage 6 in the lower part, the reason for this consisting passage - as will be seen below -5 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, preferably consisting of a transparent window, has the function 10 of checking the height 7 of the free surface of the fluid contained in the container 1; the level indicator may have any other form of construction, even outside the container, provided that it retains the level viewing function.

Preferably, the level indicator 3 is placed higher than the inlet tube 4 so that the fluid which enters the container 1 must bubble through the liquid when the free surface of the latter is maintained in the visual field 20 of the level 3 indicator.

The capillary expansion 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 - rigene at 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 means of a variation in the sub-cooling of the liquid at the inlet of the capillary.

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 subcooling at the inlet of the capillary, that the refrigerant at the outlet

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The evaporator is saturated vapor and the lung / separator container contains a certain quantity of liquid and let us analyze what will happen in the event of an increase or decrease in the refrigerating charge requested from the evaporator.

1) Increase in the refrigeration charge requested:

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

The level in the container drops due to migration of refrigerant therefrom to the condenser which, due to 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 the evaporation pressure - increase in the condensation pressure 25 - increase in the sub-cooling of the liquid at the inlet of the capillary - decrease in the level in the lung / separator container - always saturated steam at the outlet of the evaporator-30.

2) __ Diminution_de_la_charge_f rigor if èr e_demand.ee:

The exact opposite of what has been described above occurs, i.e. the final adjustment of the capillary capacity will be obtained due to partial emptying of the condenser and therefore of a reduction of the sub-cooling of the liquid at its entry.

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At high condensing pressures, it may happen that a situation arises in which the condenser is completely emptied of liquid which is therefore 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 complete emptying of the condenser on the part of the liquid, and therefore the absence of sub-cooling, makes it possible to make the best use of the entire surface of the condenser.

The capacity of the container / lung is established in relation to changes in the volume of the refrigerant as a result of the change in operating conditions. In accordance with FIG. 10, this variation in volume must be less than the capacity of the container 1 and the free surface 7 of the liquid refrigerant 25 must always be between the outlet section of the refrigerant 5, to prevent returns of liquid to the compressor, and the oil return device 6 or the inlet section 4, so that the oil return device 6 is always immersed in the liquid 30 and that there is always a certain amount of liquid above it. of the input section 4.

The oil return device necessary for the lubrication of the compressor serves the reason 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. in the liquid state, and vapor. Whereas

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-9- in fact, in systems having 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, 5 in this case, due to the low speed of the vapor in the container 1, 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" tube) a small quantity of 10 liquid containing the oil not entrained in solution.

The level indicator 3 is used to control the amount of refrigerant present and makes elementary the charging phase of the refrigerant in the installation. In fact, as soon as the level to be reached by the fluid under the preselected charging conditions is known, it is sufficient to charge the refrigerant until it reaches the prefixed level without any type of control. When the refrigerating charge requested from the evaporator can vary from 20% to 100% and a device for regulating the capacity of the type of the pressure switch is required, the reference diagram is that of FIG. 11.

In this case, the gas taken off at the outlet of the compressor is mixed with the refrigerant at the outlet of the evaporator before the lung / separator container in order to eliminate the overheating of this gas by bubbling through the liquid which is contained therein, and therefore always ensuring 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,

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-10- - possibility of application to installations with potentially large refrigerant charges, thanks to an adequate dimensioning of the volume of the container / separator, without imbalance due to variations in speed, - functionality, even in significant cooling capacity which generally requires relatively large charges of refrigerant, 10 - constant suction temperature of the refrigerant to the compressor, which always corresponds to that of saturated steam, - insensitivity to small refrigerant leaks thanks to the capacity of fluid contained in the lung / separator, - extreme ease of charging the refrigerant thanks to the level indicator, - suppression of the overheating of the hot gas injected by means of a possible capacity regulation valve in installations with load variable refrigerator with operating temperature at · the minimum evaporator.

Compared with installations provided 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 30 regulations of the dynamic type, - maximum exploitation of the heat exchange surface of the evaporator since, contrary to what occurs for the thermostatic valve, it is not necessary to overheat the fluid 35 refrigerant which therefore always works in the saturation state,

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-11- - minimum torque at the thrust of the compressor motor, since the capillary quickly balances the suction and discharge pressures of the refrigerant circuit, - absence of instability in the operating conditions, even in the case of use of 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.

Claims (7)

1. Regulating system for refrigerating circuit installations comprising a capillary expansion, installations in which a refrigerating fluid flows through a thermodynamic cycle which consists of evaporation, compression, condensation and expansion, characterized in that said system comprises, in line, between the evaporator and the compressor, a small container containing refrigerant fluid in the liquid state which simultaneously exercises the function of lung / liquid separator and of device ensuring a return of lubricating oil to the compressor. 2. Adjustment system according to revendi cation 15, characterized in that the small lung container / liquid separator has at its lower part a vapor inlet tube from the evaporator, at its upper part a tube of suction extending into the container by a section 20 of U-shaped tube, the second branch of which stops a short distance from the upper part of the container and provided in the bottom bend with a calibrated passage used for sampling '' a small quantity of liquid, and a liquid level indicator fixed on the body of the container, at a height situated between the two tubes making it possible to ascertain the level of the liquid which is contained in the container and in which the vapor incoming must dabble. 3. Adjustment system according to one of claims 1 or 2, characterized in that the capillary expansion tube and the capacity of the lung container / liquid separator are dimensioned so that whatever the operating conditions (pressures ) the refrigerant 35 at the outlet of the evaporator is always in the saturated vapor state. V -13- 4. Adjustment system according to claim 3, characterized in that the capacity of the lung container / liquid separator is established in relation to the variations in volume of the coolant as a result of the variation in the operating conditions, taking account of that this variation in volume must be less than the capacity of said container and that the free surface of the liquid refrigerant in the container must always be between the outlet section of the "U" tube of the refrigerant, to prevent liquid returns to the compressor, and the oil return device or the fluid inlet section from the evaporator, so that the oil return device is always immersed in the liquid and there is always a flap of liquid above the inlet section. 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 through the calibrated passage provided therein. 6. Adjustment system as claimed in claim 5, characterized in that the withdrawal of a small amount of liquid from the lung / separator container is effected by exploiting the Venturi effect in a "U" tube. 7. Adjustment system according to claim 4, characterized by a capacity regulating valve mounted between the compressor and the outlet of the evaporator to the lung / separator container.