GB1562627A

GB1562627A - Refrigerating systems

Info

Publication number: GB1562627A
Application number: GB49429/76A
Authority: GB
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1975-11-28
Filing date: 1976-11-26
Publication date: 1980-03-12
Also published as: FR2333209A1; CA1046784A

Description

(54) IMPROVEMENTS IN AND RELATING TO REFRIGERATING SYSTEMS (71) We, DANFOSS A/S., a Danish Company, of DK-6430 Nordborg, Denmark, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a refrigerating system having a circuit comprising a compressor, a condenser and an evaporator.

It is known to provide a throttling device, in the form of a capillary tube between the condenser and the evaporator, to heat that tube or a conduit section immediately upstream of the tube by means of an electric heating resistor, and, thereby, to evaporate the refrigerant to produce a vapour plug which prevents discharge of refrigerant through the capillary tube. With the aid of the heating resistor, therefore, the downstream evaporator can be rendered inoperative since no refrigerant reaches it. This effect is utilised to regulate the temperature in a refrigerated compartment independently of the control of the compressor or to allow the evaporator to be defrosted.

In the known arrangement, the heating resistor has a constant heat output and is disposed outside the capillary tube or the said conduit section. However, this results in the disadvantage that, after the refrigerant has evaporated, the resistor continues to heat the refrigerant which can lead to coking of oil present in the refrigerant. Since this coking takes place in the capillary tube or in the said conduit section blockages are unavoidable.

The present invention provides a refrigerating system having a circuit comprising a compressor, a condenser and an evaporator, a throttling device connected between the condenser and the evaporator. a chamber connected between the condenser and the throttling device, and a heating element disposed in the chamber, the heating element being a P.T.C. resistor which is such that, in use, an equilibrium temperatue is attained in the chamber which although sufficiently high to evaporate refrigerant in the chamber, is below a predetermined temperature.

With this arrangement, the heating resistor is disposed in the refrigerant and therefore has the same temperature as the refrigerant. Since the heating resistor is a P.T.C. resistor, its resistance increases with a rise in temperature and its power output drops accordingly.

The P.T.C. resistor is chosen, however, so that its resistance changes from a relatively low value to a relatively high value for temperatures between the evaporating temperature of the refrigerant and the coking temperature of oil in that refrigerant. Consequently, when the P.T.C. resistor is switched on an equilibrium temperature is attained in the chamber which although sufficiently high to evaporate refrigerant in the chamber is below the predetermined temperature at which oil in the refrigerant becomes coked. There is therefore no danger of blocking the throttling device.

Such a system can be used, with advantage. as a "switch'' for the refrigerant if the throttling device is so dimensioned that it is permeable to liquid refrigerant but is practically impermeable to the refrigerant vapour produced in the chamber.

This switching effect can also be used, with advantage, if the circuit further comprises another evaporator which is connected in parallel with the aforesaid evaporator, each evaporator being associated with a respective space to be refrigerated. a thermostat being provided in each space, the thermostat in the space associated with the aforesaid evaporator controlling a switch for the P.T.C. resistor and the thermostat in the space associated with the other evaporator controlling the compressor.

The fact that the P.T.C. resistor tends to ensure a substantially uniform temperature in said chamber when it is operative also permits a very simple defrosting technique to be used which dispenses with expensive accessories such as magnetic valves for hot gas, or special heating conduits for the evaporator. This can be achieved by providing another throttling device between the condenser and the chamber, the throttling device downstream of the chamber being so dimensioned relative to the throttling device upstream of the chamber that it has a lower throttling resistance to liquid refrigerant than the upstream throttling device. In particular, the downstream throttling device offers substantially the same resistance to refrigerant vapour as both throttling devices do to liquid refrigerant.This can be achieved by having the downstream throttling device shorter in length than the upstream throttling device and/or by having its cross-section larger than that device. In this case, when the P.T.C. resistor is operative it will continuously convert liquid refrigerant to superheated refrigerant vapour in the chamber. The vapour is throttled in its flow into the evaporator and effects defrosting. With the dimensions as stated, it is even possible to ensure that, during defrosting, the pressure in the evaporator is substantially the same as the evaporator pressure during normal operation.

It is particularly favourable if the compressor and P.T.C. resistor are so operatively associated that the compressor is at least temporarily operative during defrosting of the evaporator. In this way the compressor sucks off the refrigerant vapour fed into the evaporator. The low suction also ensures that no excessively high evaporator pressures occur. At the same time, the condenser is filled so that, after defrosting, the original temperature can be rapidly re-established in the refrigerated space.

This operative association may be achieved in many ways. For example, a switch for the P.T.C. resistor can also energise the compressor motor. However, the defrosting circuit can also be coupled to the compressor circuit in any other manner, either mechanically, electrically or thermally. A very simple solution is obtained if the P.T.C. resistor is operable manually or automatically, for example in response to the presence of a layer of frost on the evaporator, and the compressor is controllable by a thermostat arranged in a space to be refrigerated by the evaporator. Switching on of the P.T.C. resistor can be controlled manually. by, for example, a time clock, or automatically by, for example, a temperature sensor.In each case. the subsequent interruption in the supply of the liquid refrigerant leads to heating of the refrigerated space which, in turn, allows the compressor to start by way of the thermostat.

The or each throttling device may comprise a capillary tube. In that case, and where two throttling devices are used. if, under the same pressure conditions, one passes a liquid refrigerant through one capillary tube and a refrigerant vapour to the other, the mass of liquid passing per unit time through the said one tube can be arranged to be substantially three or four times larger than that of the vapour passing through the other tube. This produces a very good blocking effect which can for example be utilised for defrosting.

However, the consumtion of vapour can be reduced still further if the downstream throttling device is formed by an orifice. for example an axially short throttling element such as a fixed nozzle or a perforated diaphragm.

With such short throttling elements, the mass of liquid passing through per unit time can be as much as six to eight times larger than that of the vapour. Under otherwise identical conditions, therefore, only half the amount of vapour passes through as compared with a system using two capillary tubes. Consequently, an even better blocking effect is obtained, and, as a result, even less vapour flows to the evaporator. Less vapour means less energy, not only with respect to the evaporation by means of the P.T.C. resistor but also for the required subsequent condensation.

For uniformity of construction the throttling device disposed upstream of the chamber may also comprise an orifice, for example an axially short throttling element such as a fixed nozzle or perforated diaphragm.

Refrigerating systems constructed in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a circuit diagram of a refrigerating system, Figure 2 shows the characteristic curve of a P.T.C. resistor used in the system shown in Figure 1, Figure 3 is the circuit diagram of another refrigerating system, and Figure 4 is a circuit diagram of a still further refrigerating system.

Referring to the drawings, the circuit according to Figure 1 contains in its cycle a compressor 1. a condenser 2 and an evaporator 3. The latter is accommodated in a refrigerated space 4. Its temperature is monitored by a thermostat 5 which switches the compressor 1 on and off as may be required. Between the condenser 2 and evaporator 3 there is a capillary tube arrangement 6 consisting of a first capillary tube section 7, a chamber 8 and a second capillary tube section 9. The two capillary tube sections 7 and 9 are dimensioned with regard to their throttling resistance such that liquid refrigerant from the condenser 2 and under the pressure of the condenser reaches the evaporator 3 in an expanded form by an amount required for normal operation and there evaporates by absorbing heat.

In the chamber 8 there is a heating resistor in the form of a PTC resistor 10 which can be applied to mains terminals 12 by a switch 11. The switch 11 is actuated by a time block 13 which initiates a defrosting period of, for example, one hour in predetermined time intervals, e.g. every 72 hours.

The PTC resistor 10 has a characteristic curve corresponding to the diagram of Figure 2.

At low temperature, there is a flat curve section I with a comparatively low resistance R.

This is followed substantially above a surge temperature To by a steeper curve section II which leads to very high resistances R. The PTC resistor 10 is selected so that an evaporating temperature T, is associated with a low resistance R whereas there is a high resistance during a temperature T2 at which coking of any oil present in the refrigerant would take place. The presence of oil in the refrigerant is unavoidable: lubricating oil for the compressor being entrained by the refrigerant during operation of the compressor. On switching the PTC resistor on, i.e. when the chamber 8 is filled with liquid, the PTC resistor operates along the curve section I with a correspondingly high heat output. The refrigerant in the chamber 8 evaporates and so, the temperature of the refrigerant vapour rises, as does that of the PTC resistor, so that the heat output of the P.T.C. resistor is reduced.A condition of equilibrium is set up at the operating point A disposed on the curve section II and in every case located below the coking temperature T2. The second capillary tube section 9 is dimensioned so that a given amount of refrigerant vapour can flow from the chamber 8 into the evaporator 3. When the liquid refrigerant in the chamber 8 evaporates, the pressure conditions in the capillary tube arrangement 6 change from those during normal operation. This is because the volume of the refrigerant vapour is several times larger than the volume of the liquid refrigerant. The volume of refrigerant vapour flowing out through the second capillary tube section 9 therefore compares with a much smaller volume of the liquid refrigerant flowing in through the first capillary tube section 7. The pressure in the chamber 8 therefore rises as compared with normal operation.Whereas during normal operation the pressure drop takes place almost entirely in the first capillary tube section 7, it occurs substantially only in the second capillary tube section 9 during defrosting. As a result of the heating, the refrigerant vapour flowing out through the second capillary tube section 9 is sufficiently hot to melt the frost on the evaporator 3. In particular, the refrigerant vapour in the chamber 8 is super-heated up to the temperature of the operating point A. By switching the compressor 1 on, the refrigerant vapour is sucked out of the condenser 3 so that there can be a continuous replenishment of hot vapour.

Switching on of the compressor takes place automatically in response to switching on of the PTC resistor 10 by means of the time-clock 13. This is because when no liquid refrigerant but only hot refrigerant vapour flows into the evaporator 3, the temperature in the refrigerated space 4 rises and the thermostat 5 responds to switch on the compressor 1.

When the compressor 1 is operative but the liquid refrigerant is discharged from the condenser 2 to a reduced extent, the condenser is more intensively filled with liquid refrigerant. After defrosting, an adequate refrigeration effect is then available in order to bring the temperature of the refrigerated space 4 rapidly back to the desired value.

In the refrigerating system according to Figure 3, a compressor 14 feeds an evaporator 17 by way of a condenser 15 and capillary tube 16 and it feeds an evaporator 18, which is connected in parallel, by way of a capillary tube arrangement 21. The evaporator 17 is arranged in a first refrigerated compartment 19 of lower temperature and the evaporator 18 is disposed in a second refrigerated compartment 20 of higher temperature. The capillary tube arrangement 21 consists of a chamber 22, an upstream capillary tube section 23 and a downstream capillary tube section 23'. In the chamber 22 there is again a PTC resistor 24 which is applied to mains terminals bv a switch 25. The switch 25 is operated by a thermostat 26 when the temperature of the refrigerated compartment 20 becomes too high.

The temperature in the refrigerated compartment 19 is monitored by a thermostat 27 which controls the compressor 14 directly.

With this circuit, the capillary tube arrangement 21 serves as a switch for starting and stopping the evaporator 18. When the PTC resistor 24 is energised. the liquid refrigerant in the chamber 22 evaporates. The capillary tube section 23' is designed so that it is practically impermeable to refrigerant vapour. Consequently. liquid refrigerant is no longer fed to the evaporator 18. The entire refrigeration effect is supplied only to the refrigerated compartment 19 of lower temperature. If the temperature here drops below the set desired value, the compressor is switched off. In this way the two refrigerated compartments can be independently regulated to acquire the required temperature. Nevertheless. it is here also ensured that the capillary tube section 23 cannot be blocked by coked oil.

In an example of the circuit according to Figure 1, the refrigeration plant was designed as follows: Compressor 1 1/5 HP Refrigerant R 12 Capillary tube section 7 Length 3.0 m Internal diameter 0.8 mm capillary tube section 9 Length 2.0 m Internal diameter 1.0 mm PTC resistor 10 Cold resistance 25 chm Surge temperature To 80"C During defrosting, there was a condenser pressure of 14 atmospheres. a pressure in the chamber 8 of 10 atmospheres and a suction pressure of 1.5 atmospheres in such a plant.

The evaporating temperature T1 in the chamber amounted to 40"C. The PTC resistor 10 assumed a temperature of 90"C at the operating point A. The coking temperature of T2 for the refrigerant oil is approximately 1800C.

In the refrigerating system shown in Figure 4. which operates in the same way as the system shown in Figure 1, the circuit comprises a compressor 1', a condenser 2' and an evaporator 3'. The evaporator 3' is accommodated in a refrigerated space 4' and the temperature of the latter is monitored by a thermostat 5' which switches the compressor 1' on and off when required. Between the condenser 2' and evaporator 3' there is a throttling device 6' consisting of a fixed nozzle 7', a chamber 8' and a downstream diaphragm 9'. The fixed nozzle 7' and the diaphragm 9' are dimensioned with respect to their throttling resistance so that the liquid refrigerant from the condenser 2' and under the condenser pressure reaches the evaporator 3' in an expanded amount designed for normal operation and there evaporates by absorbing heat.

In the chamber 8' there is a heating resistor in the form of a PTC resistor 10' which can be applied to mains terminals 12' by a switch 11'. The switch 11' is actuated by a time-clock 13' which initiates a defrosting period of, for example, one hour at predetermined time intervals. e.g. every 72 hours. By means of the PTC resistor. the refrigerant in the chamber 8' is heated so that it evaporates without any coking taking place.

The refrigerant vapour has a considerably larger volume than the liquid refrigerant.

Under identical pressure conditions. the mass of refrigerant flowing out of the chamber 8' per unit time is six to eight times smaller than during normal operation. The desired blocking effect is consequently obtained and this is greater than when using a capillary tube section. The vapour is sufficiently hot to melt the frost on the evaporator 3'.

A nozzle 7' or a diaphragm 9' as shown in Figure 4 may be used to replace either or both the capillary tubes 7 and 9 shown in Figure 1, or any one or more of the capillary tubes 16, 23 or 23' shown in Figure 3.

WHAT WE CLAIM IS: 1. A refrigerating system having a circuit comprising a compressor. a condenser and an evaporator. a throttling device connected between the condenser and the evaporator, a chamber connected between the condenser and the throttling device, and a heating element disposed in the chamber, the heating element being a P.T.C. resistor which is such that, in use, an equilibrium temperature is attained in the chamber which although sufficiently high to evaporate refrigerant in the chamber. is below a predetermined temperature.

2. A system as claimed in claim 1, in which the throttling device is so dimensioned that it is permeable to liquid refrigerant and is substantially impermeable to the refrigerant vapour produced in the chamber.

3. A system as claimed in claim 1 or claim 2. in which another throttling device is arranged between the condenser and the chamber.

4. A system as claimed in claim 3. in which the throttling device downstream of the chamber is so dimensioned relative to the throttling device upstream of the chamber that it has a lower throttling resistance to liquid refrigerant than the upstream throttling device.

5. A system as claimed in claim 3 or claim 4. in which the downstream throttling device offers substantially the same resistance to refrigerant vapour as both throttling devices do to

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. In an example of the circuit according to Figure 1, the refrigeration plant was designed as follows: Compressor 1 1/5 HP Refrigerant R 12 Capillary tube section 7 Length 3.0 m Internal diameter 0.8 mm capillary tube section 9 Length 2.0 m Internal diameter 1.0 mm PTC resistor 10 Cold resistance 25 chm Surge temperature To 80"C During defrosting, there was a condenser pressure of 14 atmospheres. a pressure in the chamber 8 of 10 atmospheres and a suction pressure of 1.5 atmospheres in such a plant. The evaporating temperature T1 in the chamber amounted to 40"C. The PTC resistor 10 assumed a temperature of 90"C at the operating point A. The coking temperature of T2 for the refrigerant oil is approximately 1800C. In the refrigerating system shown in Figure 4. which operates in the same way as the system shown in Figure 1, the circuit comprises a compressor 1', a condenser 2' and an evaporator 3'. The evaporator 3' is accommodated in a refrigerated space 4' and the temperature of the latter is monitored by a thermostat 5' which switches the compressor 1' on and off when required. Between the condenser 2' and evaporator 3' there is a throttling device 6' consisting of a fixed nozzle 7', a chamber 8' and a downstream diaphragm 9'. The fixed nozzle 7' and the diaphragm 9' are dimensioned with respect to their throttling resistance so that the liquid refrigerant from the condenser 2' and under the condenser pressure reaches the evaporator 3' in an expanded amount designed for normal operation and there evaporates by absorbing heat. In the chamber 8' there is a heating resistor in the form of a PTC resistor 10' which can be applied to mains terminals 12' by a switch 11'. The switch 11' is actuated by a time-clock 13' which initiates a defrosting period of, for example, one hour at predetermined time intervals. e.g. every 72 hours. By means of the PTC resistor. the refrigerant in the chamber 8' is heated so that it evaporates without any coking taking place. The refrigerant vapour has a considerably larger volume than the liquid refrigerant. Under identical pressure conditions. the mass of refrigerant flowing out of the chamber 8' per unit time is six to eight times smaller than during normal operation. The desired blocking effect is consequently obtained and this is greater than when using a capillary tube section. The vapour is sufficiently hot to melt the frost on the evaporator 3'. A nozzle 7' or a diaphragm 9' as shown in Figure 4 may be used to replace either or both the capillary tubes 7 and 9 shown in Figure 1, or any one or more of the capillary tubes 16, 23 or 23' shown in Figure 3. WHAT WE CLAIM IS:

1. A refrigerating system having a circuit comprising a compressor. a condenser and an evaporator. a throttling device connected between the condenser and the evaporator, a chamber connected between the condenser and the throttling device, and a heating element disposed in the chamber, the heating element being a P.T.C. resistor which is such that, in use, an equilibrium temperature is attained in the chamber which although sufficiently high to evaporate refrigerant in the chamber. is below a predetermined temperature.

liquid refrigerant.

6. A system as claimed in claim 4 or claim 5, in which the compressor and P.T.C.

resistor are so operatively associated that the compressor is at least temporarily operative during defrosting of the evaporator.

7. A system as claimed in claim 6, in which the P.T.C. resistor is operable manually.

8. A system as claimed in claim 6, in which the P.T.C. resistor is operable automatically.

9. A system as claimed in claim 8, in which the P.T.C. resistor is operable in response to the presence of frost on the evaporator and the compressor is controllable by a thermostat arranged in a space to be refrigerated by the evaporator.

10. A system as claimed in any one of claims 1 to 9, in which the throttling device or at least one of the throttling devices comprises a capillary tube.

11. A system as claimed in any one of claims 1 to 9, in which the throttling device or at least one of the throttling devices comprises an orifice.

12. A system as claimed in any one of claims 3 to 9, in which one throttling device comprises a capillary tube and the other throttling device comprises an orifice.

13. A system as claimed in claim 11 or claim 12, in which the orifice is constituted by a perforated diaphragm.

14. A system as claimed in claim 11 or claim 12, in which the orifice is constituted by a nozzle.

15. A system as claimed in any one of claims 1 to 13, in which the circuit further comprises another evaporator which is connected in parallel with the aforesaid evaporator, each evaporator being associated with a respective space to be refrigerated, a thermostat being provided in each space, the thermostat in the space associated with the aforesaid evaporator controlling a switch for the P.T.C. resistor and the thermostat in the space associated with the other evaporator controlling the compressor.

16. A refrigerating system substantially as hereinbefore described, with reference to, and as illustrated by, Figure 1 of the accompanying drawings.

17. A refrigerating system substantially as hereinbefore described, with reference to, and as illustrated by, Figure 3 of the accompanying drawings.

18. A refrigerating system substantially as hereinbefore described, with reference to, and as illustrated by, Figure 4 of the accompanying drawings.