Refrigerating or heat-pump system

Abstract

 The invention relates to a refrigerating or heat-pump system in which the compressor 2 has a clearance pocket 21 which can be made to communicate with the compression space 19 by controlling a valve member (24). In order to improve the efficiency of the system it is proposed to control the valve member 24 electromagnetically and to reduce the power of the electric motor of the compressor when the clearance pocket 21 is connected.

Description

[0001] 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 comprises a cylinder in which a piston is reciprocated by the elctric 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 a clearance pocket, which can be made to communicate with the compression space by controlling a valve member between the compression space and the clearance pocket.

[0002] Most refrigerating or heat-pump systems are controlled through on/off control of the compressor. To improve the efficiency of such a system through 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 utits, 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 clearance-volume control, an additional clearance volume, a so-called clearance pocket, being added to the normally drailable compression space depending on the required capacity.

[0003] 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 compression space and the clearance pocket by means of a pneumatically or manually controlled valve.

[0004] The invention aims at improving the efficiency of the system by means of a simple and cheap control mechanism.

[0005] According to the invention the system is characterized in that the valve member is controlled electro magnetically and the system comprises a control mechanism which reduces the power of the electric motor when the clearance pocket is connected.

[0006] Clearance-volume 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 the clearance pocket is made to communicate with the compression space, 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 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%.

[0007] A preferred embodiment is characterized in that the compressor is provided with a cover in which the clearance pocket is situated, which clearance pocket is connected to the compression space by means of a connecting duct, 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.

[0008] Another preferred embodiment is characterized in that the compressor is provided with a valve plate which is situated between the cover and the cylinder, which valve plate is formed with a bore in which the valve member is slidable, and which intersects said connecting duct, which valve member has a transverse bore such that, upon energization of the electromagnet, this bore is moved into line with the connecting duct. This has the advantage that upon energization of the electromagnet the compression pressure does not subject the valve member to a varying face.

[0009] The system efficiency is improved if the flow resistance of the throttle means is increased when the clearance pocket is connected.

[0010] 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.

[0011] The system comprises an evaporator 1, a compressor 2, a condensor 3, and a throttle valve 4, which are interconnected by pipes to form a closed circuit. The compressor is mounted in a hermetically sealed housing 3, which also accommodates an electric motor 6 and a reciprocating pump 7. The reciprocating pump comprises a cylinder 8 in which a pistor 9 is reciprocated by the electric motor, a cover 10, and a valve plate 11 arranged between the cover and the cylinder. The valve plate is formed with an inlet port 12 with an inlet valve 13 and outlet ports 14 with outlet valves 15. The housing 5 of the compressor has an inlet 16 and an outlet 17 which are connected to the evaporator 1 and the condensor 3, respectively by means of pipes. Via the suction chamber 18 and the inlet port 12 the refrigerant gas is drawn into the compression space 19 after which it is compressed and forced back into the system via the outlet port 14 and the delivery chamber 20.

[0012] In accordance with the invention the system is controlled by providing the reciprocating pump 7 with a clearance pocket 21. The valve plate 11 is formed with a connecting duct 22 which connects the clearance pocket 21 to the compression space 19. The valve plate 11 is formed with a bore 23 in which a valve member 24 is slidable. The bore 23 intersects the connecting duct 22. The valve member 24 is integral with the movable core 25 of an electromagnet 26. The core 25 is surrounded by a coil 27 which is included in an electrical control loop of the system. Further, the electromagnet is provided with a return spring 28. The pin-shaped valve member 24 is formed with a bore 29 which, depending on the position of the valve member, can be positioned in line with the connecting duct 22 or not to open or close the connecting duct. In the position shown in Fig. 2 the electromagnet 26 is energized and the bore 29 is disposed in line with the connecting duct 22, so that the clearance pocket 21 communicates with the compression space 19. The compressor then operates at a reduced capacity. Advantageously, in this position the compression pressure acts uniformly on the wall of the bore 29 of the valve member 24, so that no resultant forces act on the valve member and have to be compensated by the force of the electromagnet. When the electromagnet is not energized the return spring 28 urges the valve member to the left, thereby closing the connecting duct. In this position the compressor operates at maximum capacity (Fig. 3). Preferably, the clearance pocket 21 is insulated to preclude additional loss of heat.

[0013] The system in accordance with the invention also comprises a control mechanism which reduces the power of the electric motor when the clearance pocket 21 is connected. Fig. 4 shows two efficiency/torque curves of the electric motor. In curve I the motor power is higher than in curve II. During the period in which the compressor operates without clearance pocket the operating range of the electric motor is situated between points A and B of curve I. If the clearance pocket 21 is now put into communication with the compression space 19 the torque T will 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 the motor power. The electric motor then operates in the range E-F of curve II, in which the efficiency is high. 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. The power controller 30 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 (^R_l < R₂), a capacitor C, a diac D, a triac T and a voltage-dependent resistor VDR controls the phase angle of the mains sine-wave. The setting of the switch S1 is governed by a variable thermostat 31. 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 the coil 27 of the electromagnet 26 for the actuation of the valve member 24 is not energized. However, in the case of reduced compressor power switch S2 is closed and the electromagnet is energized, so that the valve member 24 opens the connecting duct 22 between the clearance pocket 21 and the compression space 19.

[0014] 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 (al) during the on-period of the compressor, while in the same period the evaporator temperature decreases from 9°C (refrigerator temperature) to -24°C (bl). 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 32⁰C to approximately 45°C(d1) during the short period of full compressor capacity, the evaporator 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 clearance pocket has been connected and the drive voltage has been reduced, the condensor temperature decreases to approximately 32⁰C (d2) and the evaporator temperature increases to approximately to -5°C (e2). Consequently, the temperature fluctuations of the evaporator 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).

[0015] Since the mass flow of the refrigerant is smaller in the period when the clearance pocket 21 is operative the flow 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 two capillaries 4a and 4b are arranged in series, the capillary 4a being bypassed by opening the valve 32 when the compressor operates at full power. In the case of reduced-power operating the valve is closed and the two capillaries are operative. Preferably, the valve 32 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 33 of the electromagnet is energized, and the valve 32 is closed.

[0016] Heat pumps generally employ a temperature-controlled expansion valve. The expansion valve is then opened automatically to the correct extent in order to maintain the pressure differential.

Claims

1. 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 comprises a cylinder, in which a piston is reciprocated by the electric motor in order 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 a clearance pocket, which can be made to communicate with the compression space by controlling a valve member between the compression space and the clearance pocket, characterized in that the valve member is controlled electromagnetically and the system comprises a control mechanism which reduces the power of the electric motor when the clearance pocket is connected.

2. A refrigerating or heat-pump system as claimed in Claim 1, characterized in that the compressor is provided with a cover in which the clearance pocket is situated, which clearance pocket is connected to the compression space by means of a connecting duct, 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.

3. A refrigeration or heat-pump system as claimed in Claim 1 or 2, characterized in that the compressor is provided with a valve plate which is situated between the cover and the cylinder, which valve plate is formed with a bore in which the valve member is slidable, and which intersects said connecting duct, which valve member has a transverse bore such that, upon energization of the electromagnet, this bore is moved into line with the connecting duct.

4. A refrigerating or heat-pump system as claimed in any one of the preceding Claims, characterized in that the flow resistance of the throttle means is increased when the clearance pocket is connected.

Drawing