DE102021132848A1 - refrigeration cycle - Google Patents

Abstract

A refrigeration circuit (K), comprising at least the following components that follow one another in the direction of refrigerant flow: a compressor (1), a heat-emitting heat exchanger (2), a throttle element (3), and an evaporator (9), is intended to ensure particularly reliable operation and a long service life of the Enable compressor. For this purpose, the refrigeration circuit (K) comprises a heat exchanger (4) with two passages, which is designed for an exchange of thermal energy between the two passages, the first passage being connected upstream of the inlet of the compressor (1) and downstream of the first evaporator (9). , and the second passage is connected upstream of the evaporator (9) and downstream of the throttle element (3).

Description

The invention relates to a refrigeration circuit, comprising at least the following components, which follow one another in the direction of refrigerant flow: a compressor, a heat-emitting heat exchanger, a throttle element, and an evaporator.

A refrigeration cycle is a system designed to cool down equipment to a desired level, such as food refrigeration cabinets. A refrigerant that is moved in the closed circuit undergoes various changes in its physical state one after the other: the gaseous refrigerant is first compressed by a compressor. In the subsequent heat exchanger, it condenses while releasing heat. The liquid refrigerant is then expanded due to the pressure change via a throttling element, for example an expansion valve or a capillary tube. In the downstream evaporator, the refrigerant evaporates while absorbing heat at a low temperature (evaporative cooling). The cycle can now start all over again. The process must be kept going from the outside by supplying mechanical work (drive power) via the compressor.

For such a refrigeration circuit to function, it is necessary to keep the compressor in defined operating states. On the one hand, liquid must be prevented from accumulating at the compressor inlet, and on the other hand, the temperatures of the compressor must not become too high. For example, from around 140°C, coking can occur in the oil and valves can stick. This can affect the functionality and service life of the compressor.

It is therefore the object of the invention to specify a refrigeration circuit of the type mentioned at the outset, which enables the compressor to function particularly reliably and has a long service life.

This object is achieved according to the invention in that the refrigeration circuit comprises a heat exchanger with two passages, which is designed for an exchange of thermal energy between the two passages, the first passage being connected upstream of the inlet of the compressor and downstream of the first evaporator, and the second passage upstream of the evaporator and downstream of the throttle element.

The invention is based on the consideration that problems with regard to the operating conditions of the compressor arise in particular from changing outside conditions such as the outside temperature or different load states on the evaporator. At the same time, it was recognized that, in particular after the throttle element (high-pressure valve) downstream of the heat-dissipating heat exchanger, there are comparatively constant temperatures, regardless of changing external conditions. These largely constant temperatures should therefore be used to stabilize the inlet temperature of the compressor. For this purpose, a heat exchanger should be provided that enables heat exchange between the refrigerant flow from the high-pressure valve and the refrigerant flow into the compressor. This stabilizes the compressor inlet temperature.

The refrigeration circuit advantageously further comprises a medium-pressure container. The refrigerant is collected in this downstream of the throttling element, i.e. the high-pressure valve. The heat exchanger is advantageously also arranged between the throttle element and the medium-pressure tank, i.e. the inlet of the medium-pressure tank is downstream of the second passage. The liquid-side outlet of the medium-pressure tank is upstream of the evaporator.

A gas-side outlet of the medium-pressure container is advantageously connected to the inlet of the first passage via a medium-pressure valve. In the event of excessive pressure in the medium-pressure tank, the medium-pressure valve is used to drain gaseous refrigerant from the medium-pressure tank and feed it directly back to the compressor.

The heat exchanger is advantageously designed as a plate heat exchanger. Such a plate heat exchanger is formed from corrugated profiled plates, which are assembled in such a way that the refrigerant to be heated and then the heat-emitting refrigerant flows in each of the successive spaces. The plate pack is sealed to the outside and between the two refrigerant passages and is held together, for example, with clamping screws or soldered. A plate heat exchanger offers the advantage that it is very easy to expand and therefore very easy to adapt to the required heat exchange quantities. In addition, it achieves a particularly good heat exchange, since the mass flow of each passage is pressed through several small channels in the plate. The passage through the small, wave-shaped channels also promotes the mixing of the previously combined mass flows.

The heat exchanger is designed with regard to its dimensioning and heat transfer capacity between the refrigerant passages in such a way that the temperature difference at the outlets of the passages is preferably less than 2 K. This achieves a particularly stable temperature condition at the compressor inlet.

Advantageously, a deep-freeze evaporator with a downstream deep-freeze compressor is arranged parallel to the evaporator between the liquid-side outlet of the medium-pressure container and the inlet of the first passage. In parallel to the evaporator already mentioned, this enables a parallel line with different temperature levels, so that, for example, normal refrigerated cabinets and deep-frozen cabinets can be operated with one refrigeration circuit.

Furthermore, an injection valve is advantageously arranged parallel to the evaporator between the liquid-side outlet of the medium-pressure container and the inlet of the first passage. As additional protection for the compressor, this can also be used to reduce the pressure gas temperature after the compressor by injecting refrigerant directly before the compressor or before the heat exchanger, bypassing the evaporator. As a rule, it should only be used if the suction gas conditioning by the heat exchanger is not sufficient due to special circumstances.

For this purpose, a temperature sensor is advantageously arranged at the inlet and/or at the outlet of the first passage. These make it possible to precisely determine the temperature upstream and downstream of the heat exchanger and thus in particular at the compressor inlet. If the temperatures are too high, additional measures can then be taken as required, such as opening the injection valve described above.

The refrigeration cycle is advantageously designed to operate with carbon dioxide as the refrigerant and/or the heat exchanger of the refrigeration cycle is a gas cooler. In the case of such refrigeration circuits, which are designed for operation with carbon dioxide, the heat exchanger described offers particular advantages, since precisely here the temperatures at the compressor can be particularly high.

The refrigeration cycle is advantageously designed for the stationary cooling of refrigerated cabinets or cold rooms.

The advantages achieved with the invention consist in particular in the fact that the arrangement of a heat exchanger, which transfers heat from the suction gas flow to the compressor into the refrigerant flow expanded in the high-pressure valve, causes the suction gas flow to the compressor to be conditioned to a stable temperature and in this way the compressor is optimized in terms of service life and function.

• 1 einen Kältemittelkreislauf,
• 2 ein Druck-Enthalpie-Diagramm für einen Kältemittelkreislauf aus dem Stand der Technik, und
• 3 ein Druck-Enthalpie-Diagramm für den Kältemittelkreislauf aus 1.

The invention is explained in more detail with reference to drawings. Show in it:

• 1 a refrigerant circuit,
• 2 a pressure-enthalpy diagram for a refrigerant circuit from the prior art, and
• 3 a pressure-enthalpy diagram for the refrigerant circuit 1 .

1 shows a refrigeration circuit K, which is used for stationary cooling of freezers and freezers, for example in supermarkets. The refrigeration circuit K is described below based on a compressor 1 . The compressed refrigerant in the compressor 1 - in the exemplary embodiment carbon dioxide (designation according to DIN 8960: R-744) - is passed through a refrigerant line into a heat-emitting heat exchanger 2 designed as a gas cooler in the exemplary embodiment, in which the compressed refrigerant is cooled and liquefied. From there it flows via a throttle element 3 designed as a high-pressure valve into a medium-pressure container 5. In the refrigerant line between the compressor 1 and the heat-emitting heat exchanger 2, further components, in particular an oil separator, are usually arranged. However, these are not shown for reasons of clarity.

From the medium-pressure container 5, the refrigerant flows from an outlet on the liquid side via an injection valve 8 into an evaporator 9. Here the refrigerant expands and absorbs heat, so that the desired cooling effect is achieved. From the evaporator 9, the refrigerant, which is now in gaseous form again, flows back into the compressor 1, where it is compressed and the cycle begins again.

In order to be able to release a possibly too high pressure in the medium-pressure container 5 depending on the operating state, the medium-pressure container 5 is connected to the inlet of the compressor at a gas-side outlet via a medium-pressure valve.

A parallel deep-freeze line is provided in parallel with the injection valve 8 and the evaporator 9 . This also branches off from the liquid-side outlet of the medium-pressure container 5 and comprises an injection valve 10, a deep-freeze evaporator 11 and a deep-freeze compressor 12 in the flow direction of the refrigerant.

Again parallel to the injection valve 8 and evaporator 9 as well as to the deep-freeze line just described, the liquid-side outlet of the medium-pressure container 5 is connected to the inlet of the compressor 1 via an injection valve 7 . This serves to reduce temperatures that may be too high in the compressor by injecting liquid refrigerant as required. Too high temperatures in the compressor 1 can lead to damage.

To avoid this, the refrigeration circuit K of 1 a heat exchanger 4. This is designed as a plate heat exchanger and has two passages that are in thermal contact. The first passage is the inlet of the compressor 1 immediately upstream, ie it is the merging of the refrigerant lines downstream of the evaporator 9, freezer compressor 12, injection valve 7 and medium-pressure valve 6 downstream. The second passage of the heat exchanger 4 is arranged between the throttle element 3 and the inlet of the medium-pressure container 5 .

With regard to its dimensioning, the heat exchanger 4 is designed such that there is a maximum temperature difference of 2 K between the outlets of the passages. Since the temperature of the refrigerant expanded in the throttle element 3 is comparatively stable, regardless of the outside conditions, a stable temperature at the inlet of the compressor 1 is also achieved in this way. Temperature sensors 7.1 and 7.2 are provided in the first passage to the compressor 1 before and after the heat exchanger 4 to check the temperatures and, if necessary, to cause an additional opening of the injection valve 7.

The influence of the additional heat exchanger 4 is shown using the pressure-enthalpy diagrams in 2 and 3 explained. These each show the pressure on the ordinate in bar (10 - 120 bar) and the enthalpy on the abscissa in kJ/kg (80 - 600 kJ/kg). The diagrams each show for the in the refrigeration circuit K 1 used refrigerant R-744 (carbon dioxide) the critical point, the boiling line and the dew line, as well as the isotherms. The isovapores are also shown within the boiling and dew line.

In both diagrams, the cycle process described above is plotted in the pressure-enthalpy diagram. while showing 2 the cyclic process without the heat exchanger 4, 3 the cyclic process with heat exchanger 4. The changes in state (represented by lines) in the cyclic process are each identified by the reference symbols from FIG 1 Mistake.

The compression process in compressor 1 is shown in the diagram 2 indicated by the upper right diagonal line. The relaxation of the coolant in the throttle element 3 (high-pressure valve), however, is characterized by the upper left vertical line (Isenthalpe).

The heat exchanger 4 described above now establishes a thermal connection between the refrigerant downstream of the throttle element 3 and upstream of the compressor 1 . The effect of the heat exchanger 4 is shown in the diagram 3 shown. The change in state of both refrigerant flows in the two passages of the heat exchanger 4 takes place isobaric. In the passage downstream of the throttle element 3, the enthalpy increases. The temperature does not change. In the passage upstream of the compressor 1, the enthalpy is reduced at the inlet of the compressor 1. Since the refrigerant is in the gas phase here, the temperature is also reduced as a result.

This effect shifts the entire state change in compressor 1. As can be seen from the diagrams, the end temperature at the outlet of compressor 1 drops from approx. 145 °C to approx. 100 °C. This prevents coking in the oil and valves from sticking.

Reference List

1

compressor

2

heat-emitting heat exchanger

3

throttle organ

4

heat exchanger

5

medium pressure vessel

6

medium pressure valve

7

injector

7.1

temperature sensor

7.2

temperature sensor

8th

injector

9

Evaporator

10

injector

11

Freezer evaporator

12

freezer compressor

K

refrigeration cycle

Claims (10)

Refrigeration circuit (K), comprising at least the following components that follow one another in the direction of refrigerant flow: - a compressor (1), - a heat-emitting heat exchanger (2), - a throttle element (3), and - an evaporator (9), characterized in that the refrigeration circuit (K) comprises a heat exchanger (4) with two passages, which is designed for an exchange of thermal energy between the two passages, the first passage being upstream of the inlet of the compressor (1) and downstream of the evaporator (9), and the second passage to the evaporator (9) upstream and downstream of the throttle element (3). Refrigeration circuit (K) according to the preceding claim, further comprising a medium-pressure container (5), the inlet of which is downstream of the second passage and the liquid-side outlet of which is upstream of the evaporator (9). Refrigeration circuit (K) according to the preceding claim, in which a gas-side outlet of the medium-pressure container (5) is connected to the inlet of the first passage via a medium-pressure valve (6). Refrigeration circuit (K) according to one of the preceding claims, in which the heat exchanger (4) is designed as a plate heat exchanger. Refrigeration circuit (K) according to one of the preceding claims, in which the heat exchanger (4) is designed in such a way that the temperature difference at the outlets of the passages is less than 2K. Refrigeration circuit (K) according to one of the preceding claims, in which a deep-freeze evaporator (11) with a downstream deep-freeze compressor (12) is arranged between the liquid-side outlet of the medium-pressure container (5) and the inlet of the first passage parallel to the evaporator (9). . Refrigeration circuit (K) according to one of the preceding claims, in which an injection valve (7) is arranged between the liquid-side outlet of the medium-pressure container (5) and the inlet of the first passage parallel to the evaporator (9). Refrigeration circuit (K) according to one of the preceding claims, in which a temperature sensor (7.1, 7.2) is arranged at the inlet and/or at the outlet of the first passage. Refrigeration circuit (K) according to one of the preceding claims, which is designed for operation with carbon dioxide as refrigerant and/or in which the heat-emitting heat exchanger (2) is a gas cooler. Refrigeration circuit (K) according to one of the preceding claims, designed for the stationary cooling of refrigerated furniture.

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