EP0205670B1 - Refrigerating or heat-pump system - Google Patents

Description

• The invention relates to a refrigerating or heat-pump system comprising an evaporator, a compressor, a condensor, and a throttle means, the compressor comprising an electric motor and a reciprocating pump, which pump comprises a cylinder in which a piston is reciprocated by the electric motor to work on a compression space above the piston, which compressor has an inlet and an outlet, which are connected to the evaporator and to the condensor, respectively by means of pipes, and further comprising a suction part, a bypass by means of which the suction part can be made to communicate with the compression space and an electromagnetically controlled valve member in the bypass.
• Such system is known from GB-A-1 158 371.
• Most refrigerating or heat-pump systems are controlled through on/off control of the compressor. To improve the efficiency of such a system throughout continuous operation, i.e. continuously operating the compressor, various control possibilities are known. An example of such a possibility is the use of speed control of the electric motor of the compressor. This control method is satisfactory but comparatively expensive. Another known control method utilizes a plurality of small compressor units, one or more compressors being rendered inoperative depending on the required capacity. This control method is economic only if the capacity of the system is sufficiently large. Another known possibility is suction-gas control, the volume of the gas taken in being reduced by keeping the suction valve open during the compression stroke or allowing a specific amount of gas to flow back from the cylinder via a bypass depending on the required capacity.
• The invention relates to a system controlled in conformity with the last-mentioned method. It is known to open or close a connection between the cylinder space and the suction part by means of a valve, as disclosed in GB-A-1 158 371. In this system the valve in the bypass line is actuated by gas pressures within the refrigerating system in a rather complicated way.
• The invention aims at improving the efficiency of the system by means of a simple and cheap control mechanism.
• According to the invention the system is characterized in that the system comprises a control mechanism controlling the electric motor and the valve member in a combined manner such that the voltage of the electric motor is reduced whenever the bypass is opened.
• It is to be mentioned here that in FR-A-2 347 819 an independent motor power control for a compressor is described, in order to reduce power consumption.
• Suction-gas control reduces the refrigerant mass being circulated, so that the evaporator temperature increases and the condensor temperature decreases, thereby reducing the work of compression. On the average cold is now produced at a higher temperature than in an on/ off controlled system. In other words, in the system according to the invention cold is produced with a higher efficiency. However, during the period in which bypass channel is open the mass to be circulated by the compressor is smaller, so that the torque to be delivered by the electric motor is reduced and the efficiency decreases. Therefore, in order to maintain the efficiency of the electric motor at the same level, the power is reduced in an efficient manner during said period. To produce the same amount of cold as in an on/off controlled system, the combined control of the system in accordance with the invention results in a net reduction in power consumption of approximately 10%.
• An on/off controlled system is to be understood to mean as a system in which during the off-period the condensor is disconnected from the evaporator. During the off-period the connection between the condensor and the evaporator should remain open (for example if the throttle means is a capillary), the power saved is even higher than the above 10%, namely of the order of 20%.
• A preferred embodiment is characterized in that the suction part comprises a damper, a suction duct and a suction chamber, the bypass is situated between the suction duct and the compression space, and the valve member is coupled to the core of an electromagnet, which valve member opens the connecting duct when the electromagnet is energized and closes said duct by means of a return spring when said electromagnet is not energized. This has the advantage that in the event of failure of the control mechanism, due to whatever cause, the return spring ensures that the valve member is set to the position in which the compressor operates at the maximum capacity, that the system reverts to normal on/off control.
• Another preferred embodiment is characterized in that the cylinder is formed with a bore in which the valve member is movable and which intersects said bypass duct, which valve member has a bore such that, upon energization of the electromagnet, this bore is situated in line with the bypass. The bore is situated at a small distance from the cylinder wall, in such a way that in the closed situation the additional clearance volume in the compression space, i.e. the volume in the bypass between the valve member and the compression space, is minimal. This has the advantage that during closure of the bypass the compression pressure exists no force on the valve member in its direction of movement. The bypass can be closed by changing the position of the valve member in the bore, either by sliding it in the said direction of the bore or by rotating it through 90° in the bore. As there is hardly any pressure differential across the valve member, the leakage path need not be exceptionally large.
• The system efficiency is improved because the flow resistance of the throttle means is increased when the bypass is opened.
• An embodiment of the invention will now be described in more detail, by way of example, with reference to the drawings.
• Fig. 1 shows the refrigerating or heat-pump system.
• Fig. 2 is a partly sectional view of the compressor.
• Fig. 3 is a cross-sectional view of the compressor taken on the line III-III in Fig. 2.
• Fig. 4 shows efficiency/torque curves of the electric motor.
• Fig. 5 shows the electric circuit of the system.
• Fig. 6 illustrates the thermal behaviour of the evaporator and the condensor.
• Fig. 7 shows a refrigerating system employing a capillary throttle resistance control.
• The system comprises anevaporator 1, acompressor 2, acondensor 3, and a throttle valve 4, which are interconnected by pipes to form a closed circuit. The compressor is mounted in a hermetically sealedhousing 5, which also accommodates an electric motor 6 and a reciprocatingpump 7. The reciprocating pump comprises acylinder 8 in which apiston 9 is reciprocated by the electric motor, acover 10, and avalve plane 11 arranged between the cover and the cylinder. The valve plane is formed with aninlet port 12 with aninlet valve 13 and anoutlet port 14 with anoutlet valve 15. Thehousing 5 of the compressor has aninlet 16 and anoutlet 17 which are connected to theevaporator 1 and thecondensor 3, respectively by means of pipes. Via the suction part comprising asuction damper 18, asuction duct 19 and asuction chamber 20 the refrigerant gas is drawn into thecompression space 21 after which it is compressed and forced into the compression chamber and thecompression damper 23 via theoutlet port 14; subsequently, the gas is fed to the condensor via theoutlet 17.
• In accordance with the invention the system is controlled by providing the compressor with abypass duct 24 between thecompression space 21 and thesuction duct 19. Alternatively, this bypass may be situated between the compression space and thesuction damper 18 or between the compression space and thesuction chamber 20. Thecylinder housing 8 is formed with abore 25 in which avalve member 26 is slidable. Thebore 25 intersects thebypass 24. Thevalve member 26 is integral with themovable core 27 of anelectromagnet 28. Thecore 27 is surrounded by acoil 29 which is included in an electrical control loop of the system. Further, the electromagnet is provided with areturn spring 30. The pin-shaped valve member 26 is formed with abore 31 which, depending on the position of the valve member, can be positioned in line or not in line with thebypass duct 24 to open or close the bypass. In the position shown in Fig. 3 theelectromagnet 26 is energized and thebore 31 is disposed in line with thebypass duct 24. During the upward movement of the piston from the lower dead centre this results in gas being forced back to the suction part 18-19-20 via thebypass 24. This continues until the piston has passed the opening of thebypass 24 in the cylinder wall. The residual gas in thecompression space 21 is then compressed to the condensor pressure. The compressor then operates at a reduced capacity. This capacity depends on the level of the opening of thebypass 24 to thecylinder 8. Advantageously, in this position the compression pressure acts on the wall of thebore 31, so that no resultant forces act on the valve member and have to be compensated by the electromagnet. When the electromagnet is not energized thereturn spring 30 urges the valve member upwards, thereby closing the bypass duct. In this position the force exerted on the valve member by the compression pressure does not act in the direction of movement, i.e. not on the spring, so that no fatigue problems will occur. In the closed position the compressor operates at maximum capacity (Fig. 2).
• The system in accordance with the invention also comprises a control mechanism which reduces the power of the electric motor in an efficient manner when thebypass 24 is connected. Fig. 4 shows two efficiency/torque curves of the electric motor. In curve I the motor power is higher than incurve 11. During the period in which the compressor operates with a closed bypass the operating range of the electric motor is situated between points A and B of curveI. Ifthe bypass 24 between thepipe 19 and thecompression space 21 is now opened, the torque Twill decrease, and hence the efficiency will decrease. The electric motor then operates for example between points C and D. The efficiency can be increased by reducing, for example, the voltage. The electric motor then operates, for example, in the range E-F of curve II, in which the efficiency is high. An efficient reduction of the power for torque control is preferably effected by means of a loss-free power controller. For example, a transformer may be used in order to reduce the voltage. However, a transformer is expensive. Thepower controller 40 shown in Fig. 5, which is also loss-free, is cheaper. Depending on the position of the switch S1 the network comprising two different resistors R₁ and R₂ (R₁ R₂), a capacitor C, a diac D, a triac T and a voltage-dependent resistor VDR controls the phase angle of the mains sinewave. The setting of the switch S1 is governed by a variable thermostat 41. At a maximum evaporator temperature (for example -3°C) the switch S1 is set to the right-hand position (full power) and at a variable minimum evaporator temperature (for example -16 to -24°C) to the left-hand position (reduced power). If the compressor operates at full power (S1 in the right-hand position) switch S2 is open and thecoil 29 of theelectromagnet 28 for the actuation of thevalve member 26 is not energized. However, in the case of reduced compressor power switch S2 is closed and the electromagnet is energized, so that thevalve member 26 opens thebypass duct 24 between thesuction duct 19 and thecompression space 21.
• Fig. 6 illustrates the thermal behaviour of the evaporator and the condensor, both for a known on/off control and for the two-position control in accordance with the invention. The plotted temperatures have been measured on the refrigerant side. The broken-line curves relate to the on/off control and the solid curves relate to the control in accordance with the invention. In the case of on/off control the condensor temperature increases from 25°C (ambient temperature) to approximately 50°C (a1) during the on-period of the compressor, while in the same period the evaporator temperature decreases from 9°C (refrigerator temperature) to -24°C (b1 In the subsequent off-period the condensor temperature again decreases to approximately 25°C (a2) and the evaporator temperature rises to approximately 9°C (b2). This control gives rise to a temperature fluctuation (c) of approximately 8°C in, for example, the refrigerating compartment (air) of a refrigerator. In the case of the novel two-position control the condensor temperature increases from approximately 32°C to approximately 45°C (d1) during the short period of full compressor capacity, theevaporator temperature decreasing from -5°C to -20°C (e1) in the same period. In the next long period with reduced compressor capacity, i.e. when the bypass between the compression space and the suction duct is open and the drive voltage has been reduced, the condensortemperature decreases to approximately 32°C (d2) and the evaporator temperature increases to approximately -5°C (e2). Consequently, the temperature fluctuations of the capacitor and the condensor are reduced substantially if the novel control method is employed. As a result of this, the temperature fluctuation (f) in, for example, the refrigerating compartment (air) of a refrigerator is also substantially smaller (approximately 3°C).
• Since the mass flow of the refrigerant is smaller in the period when thebypass 24 is open the resistance of the throttle means 4 should be higher in order to maintain the pressure differential. For a refrigerating system in which the throttle means is a capillary, Fig. 7 shows an example of how the flow resistance can be switched to either of two valves. In the circuit twocapillaries 4a and 4b are arranged in series, of the capillary 4a being bypassed by opening thevalve 42 when the compressor operates at full power. In the case of reduced-power operation the valve is closed and the two capillaries are operative. Preferably, thevalve 42 is operated electromagnetically, in Fig. 5 the location of the electromagnetic coil 33 in the circuit is indicated by a broken line, in the reduced-power mode the switch S3 is closed, coil 43 of the electromagnet is energized, and thevalve 42 is closed.
• Heat pumps generally employ a temperature- controlled expansion valve. Such an expansion valve is opened automatically to the correct extent in order to maintain the pressure differential.

Claims (4)

1. A refrigerating or heat-pump system comprising an evaporator (1), a compressor (2), a condensor (3) and a throttle means (4), the compressor comprising an electric motor (6) and a reciprocating pump (7), which pump comprises a cylinder (8), in which a piston (9) is reciprocated by the electric motor (6) in order to work on a compression space (21) above the piston, which compressor has an inlet (16) and an outlet (17), which are connected to the evaporator and to the condensor, respectively by means of pipes, and further comprising a suction part (18, 19, 20), a bypass by means of which the suction part can be made to communicate with the compression space and an electromagnetically controlled valve member (26) in the bypass, characterized in that the system comprises a control mechanism controlling the electric motor (6) and the valve member (26) in a combined manner such that the voltage of the electric motor is reduced whenever the bypass (24) is opened.

2. A refrigerating or heat pump system as claimed in Claim 1, characterized in that the suction part comprises a damper (18), a suction duct (19) and a suction chamber (20), the bypass (24) is situated between the suction duct (19) and the compression space (21), and the valve member (26) is coupled to the core (27) of an electromagnet (28), which valve member opens the bypass when the electromagnet is energized and closes said bypass by means of a return spring (30) when said electromagnet is not energized.

3. A refrigerating or heat-pump system as claimed in Claim 1 or 2, characterized in that the cylinder (8) is formed with a bore (25) in which the valve member (26) is movable, which bore intersects the bypass (24), which valve member has a bore (31) such that, upon energization of the electromagnet (28), this bore is situated in line with the bypass.

4. A refrigerating or heat-pump system as claimed in any one of the preceding claims, characterized in that the resistance of the throttle means (4) is increased when the bypass (24) is opened.

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