AT520000A2 - Refrigerant circuit of a refrigeration system with an arrangement for defrosting a heat exchanger and method for operating the refrigerant circuit - Google Patents

Abstract

The invention relates to a refrigerant circuit (1a, 1b) of a refrigeration system with an arrangement for defrosting at least one heat exchanger with refrigerant. The refrigerant circuit (1a, 1b) has at least two evaporation pressure stages, wherein a first, lower evaporation pressure stage and a second, upper evaporation pressure stage each with at least one compressor (2, 4) and at least one operable as an evaporator heat exchanger (9, 16) with upstream expansion element (8, 15) are formed. The heat exchangers (9, 16) of the different evaporation pressure stages are arranged to be acted upon with refrigerant at different pressure levels. The defrosting arrangement comprises a connecting line (21) for conducting refrigerant at a constant pressure level, which extends from a branching point (19) formed at an outlet of the at least one compressor (2) of the first, lower evaporation pressure stage to one at an inlet of the extends at least one operable as an evaporator heat exchanger (9) of the second, upper evaporation pressure stage formed opening point (24a). The heat exchanger (9, 16) are arranged unidirectionally throughflow. The invention also relates to a method for operating the refrigerant circuit (1a, 1b) in a defrosting mode for defrosting at least one heat exchanger (9) of the upper evaporation pressure stage operated as evaporator in a cooling mode for heat absorption.

Description

Refrigerant circuit of a refrigeration system with an arrangement for defrosting a heat exchanger and method for operating the refrigerant circuit

The invention relates to a refrigerant circuit of a refrigeration system with an arrangement for defrosting at least one heat exchanger with refrigerant. The refrigerant circuit has at least two evaporation pressure stages, wherein a first and a second evaporation pressure stage are each formed with at least one compressor and at least one heat exchanger which can be operated as an evaporator and has an expansion element arranged upstream. The heat exchangers of the different evaporation pressure levels are arranged to be charged with refrigerant at different pressure levels.

The invention further relates to a method for operating the refrigerant circuit in a defrost mode for defrosting at least one heat exchanger operated as an evaporator in a cooling mode for absorbing heat.

In refrigeration systems known from the prior art for conditioning, in particular for cooling, air, for example in cold rooms or within refrigeration units as a cooling point, refrigerant circulates through a refrigerant circuit. The heat to be removed from the air from the cooling point to the refrigerant is transferred in a heat exchanger operated as an evaporator. The heat-absorbing refrigerant evaporates.

In the refrigerant-air heat exchanger operated as an evaporator, temperatures of the refrigerant are set which are always below the temperature of the air for heat transfer from the air to the refrigerant. Depending on the condition of the air, especially the air humidity, there is a risk that the moisture flowing through the heat exchanger and bound in the air / 33 will fail as condensate due to the cooling. If the temperatures of the surface of the heat exchanger are lower than the dew point temperature of the air, the moisture contained in the air is deposited on the surface of the heat exchanger, also referred to as an air cooler, as water. The air flowing through the heat exchanger is cooled and dehumidified.

When the temperature of the surface of the heat exchanger falls below 0 ° C, the moisture separated from the air freezes. Frost and ice form. The surface of the heat exchanger is continuously added, so that the heat transfer on the surface of the heat exchanger deteriorates with increasing icing. With increasing energy consumption, the operation of the refrigeration system, in particular the refrigerant circuit, becomes uneconomical. Icing the surface of the heat exchanger can also result in the target temperature of the air to be cooled not being able to be set within a prescribed temperature range.

In order to operate the refrigeration system, in particular the refrigerant circuit, economically and to be able to maintain the target temperature of the cooled air, a cooling cycle of the refrigeration system must be interrupted as the surface of the heat exchanger progressively freezes and a surface thawing process is initiated.

In conventional refrigeration systems, depending on the target temperature, electrical defrosting devices, for example electrical resistance heaters in the form of heating rods, serve to de-ice the surfaces of the heat exchangers operated as evaporators.

The heating rods, however, have a very high power requirement as electrical defrosting devices. In addition to the surface of the heat exchangers, the surroundings of the heating elements, such as the air to be cooled and the items to be cooled, for example the goods stored in cold rooms or in refrigeration units, are heated by the heat given off by the heating elements. The defrosting process in particular in the vicinity of the / 33

The energy transferred to the heat exchanger must be dissipated again by the refrigeration system during the cooling process following the defrosting process, which also increases the energy required for cooling.

EP 1 498 673 A1 discloses a method for operating a refrigeration system with a refrigerant circuit. The refrigerant circuit in each case has at least one heat exchanger operated as a condenser, two heat exchangers operated as an evaporator and a compressor with at least two power stages or two compressors. On the output side, the compressor or compressors are connected to the inlets of the evaporators, also referred to as refrigeration consumers, via a defrost line. Depending on the defrosting requirement of one or more refrigeration consumers, compressed, hot refrigerant is supplied to the evaporator (s) to be defrosted via the defrosting line. The compressed, hot refrigerant is expanded to the suction pressure level of the refrigerant circuit before it is introduced into the evaporator or evaporators.

WO 2013/078088 A1 discloses a refrigeration system with a refrigerant circuit with carbon dioxide as the refrigerant. The refrigerant circuit has a low-temperature stage and a normal temperature stage, each with associated compressors and evaporators. The evaporators of the low temperature stage can be operated with hot gas both in a cooling mode and in a defrosting mode. The hot gas of the refrigerant is used to defrost the evaporators and can be provided by the compressors at the low temperature level or the normal temperature level. The normal temperature stage is also designed with a separator, also referred to as a medium pressure separator. Liquid refrigerant is removed from the medium pressure separator and led to the individual evaporators. When operating in defrost mode, the hot gas is passed through a line connecting the discharge of the compressors to a distributor and is distributed from the distributor to the evaporators. In the flow direction after the evaporators, the hot gas is brought together in a collector and introduced into the medium-pressure separator.

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When operating in defrost mode, at least one of the evaporators of the low temperature stage is charged with hot gas, while the other evaporators are switched off and no refrigerant flows through them. At least one of the evaporators of the normal temperature level is operated in cooling mode.

The object of the invention is to provide an arrangement for defrosting at least one heat exchanger operated as an evaporator in a refrigerant circuit of a refrigeration system and a method for operating the refrigerant circuit during defrosting using hot gas. Only individual evaporators, in particular one temperature level, should be defrostable, while other evaporators of the same temperature level continue to be operated in a cooling mode.

The defrosting process should be made economically possible with the expenditure of minimal energy and minimal time. The temperature of the air in the cooling point is to be kept almost constant during the defrosting process and the refrigerated goods assigned to the cooling point are to be protected. Only minimal operating costs, manufacturing or installation costs and maintenance costs are to be caused.

The object is achieved by the subject matter and the method with the features of the independent claims. Further developments are specified in the dependent claims.

The object is achieved by a refrigerant circuit according to the invention of a refrigeration system with an arrangement for defrosting at least one heat exchanger with refrigerant. The refrigerant circuit has at least two evaporation pressure stages, a first, lower evaporation pressure stage with at least one compressor and at least one heat exchanger that can be operated as an evaporator with an upstream expansion element, and a second, upper evaporation pressure stage with at least one compressor and at least one heat exchanger that can be operated as an evaporator / 33 with an upstream expansion element are trained. The heat exchangers of the different evaporation pressure levels, which can be operated as evaporators, are arranged to be acted upon with refrigerant at different pressure levels.

The pressure levels of the evaporation pressure stages each relate to the evaporation pressures of the refrigerant when flowing through the heat exchangers that can be operated as evaporators.

According to the concept of the invention, the arrangement for defrosting has a connecting line for conducting refrigerant at a constant pressure level. The connecting line extends from a branch point formed at an outlet of the at least one compressor of the first, lower evaporation pressure stage to a branch point formed at an inlet of the at least one heat exchanger of the second, upper evaporation pressure stage which can be operated as an evaporator. The heat exchangers that can be operated as evaporators are each arranged to flow through unidirectionally.

The unidirectional flow, also referred to as unidirectional or monodirectional flow, is to be understood to mean that the refrigerant always crosses the heat exchanger in one direction, regardless of the operating mode of the refrigerant circuit. In this case, an outlet of the respective heat exchanger is connected to an inlet of a compressor via a refrigerant line, so that the refrigerant is sucked in directly from the compressor exiting the heat exchanger regardless of the operating mode. The heat exchangers are designed as air-refrigerant heat exchangers.

The terms inlet and outlet always refer to the direction of flow of the refrigerant through the components of the refrigerant circuit. The flow direction of the refrigerant remains constant regardless of the operating mode through all components of the refrigerant circuit.

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Instead of individual compressors, the evaporation pressure stages can in each case also be formed with compressor units comprising at least two compressors operated in parallel and which can be acted upon in parallel with refrigerant.

According to a development of the invention, a heat exchanger operated as a desuperheater for the gaseous refrigerant escaping from the at least one compressor of the first, lower evaporation pressure stage is arranged between the branching point of the arrangement for defrosting and an inlet of the at least one compressor of the second, upper evaporation pressure stage such that the Refrigerant emerging from the desuperheater is drawn in by the at least one compressor of the second, upper evaporation pressure stage.

According to a preferred embodiment of the invention, a shut-off valve is arranged in the flow direction of the refrigerant through the connecting line upstream of the outlet point of the at least one heat exchanger that can be operated as an evaporator of the second, upper evaporation pressure stage. The outlet point is advantageously formed in a refrigerant line arranged between the expansion element and the heat exchanger.

A further advantageous embodiment of the invention consists in that the connecting line is formed with a branch for integrating a refrigerant line, which extends from the branch to an outlet point formed at an inlet of the at least one heat exchanger of the first, lower evaporation pressure stage that can be operated as an evaporator. The outlet point is preferably also formed in a refrigerant line arranged between the expansion element and the heat exchanger. In the flow direction of the refrigerant through the refrigerant line, a defrost valve is advantageously arranged in front of the mouth.

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According to a development of the invention, the first, lower evaporation pressure stage and / or the second, upper evaporation pressure stage is formed from at least two heat exchangers which are operated in parallel and can be acted upon in parallel with refrigerant, an expansion element being arranged upstream of each heat exchanger.

According to a first alternative embodiment of the invention, the branch point is designed as a three-way valve.

According to a second alternative embodiment of the invention, a shut-off valve is arranged within the connecting line of the defrosting arrangement and a differential pressure valve is arranged in a suction area of the at least one compressor of the upper evaporation pressure stage. The differential pressure valve is advantageously formed between the branch point and a heat exchanger that can be operated as a desuperheater.

The refrigerant circuit also preferably has a heat exchanger, operated as a condenser / gas cooler at a high pressure level, for emitting heat, and a separator. The separator is designed as a so-called medium pressure separator and is arranged with refrigerant at a medium pressure level between the high pressure level and the pressure level of the upper evaporation pressure stage.

The object is also achieved by a method according to the invention for operating the refrigerant circuit in a defrost mode for defrosting at least one heat exchanger of an upper evaporation pressure stage operated as evaporator in a cooling mode for absorbing heat. The process has the following steps:

Detection of icing of a heat exchanger surface of the heat exchanger of the upper evaporation pressure stage operated as an evaporator in cooling mode for heat absorption,

- Closing an expansion member and the at least one heat exchanger / 33 to be defrosted

- Opening a shut-off valve associated with the at least one heat exchanger to be defrosted and

at least partially opening a connecting line at a branch point and directing at least a partial mass flow of hot compressed gas from at least one compressor of a lower evaporation pressure stage to the heat exchanger to be defrosted in the upper evaporation pressure stage,

- Passing the hot compressed gas at the pressure level of the upper evaporation stage through the at least one heat exchanger and

Suction of the refrigerant emerging from the at least one heat exchanger through at least one compressor of the upper evaporation pressure stage,

- Close the connecting line at the branch point and the defrost valve when the end of the defrost temperature on the heat exchanger.

According to a development of the invention, overheating of the refrigerant at the inlet into the at least one compressor of the upper evaporation pressure stage is monitored. If the overheating falls below a predefined setpoint, the connecting line on the

Branch and / or the one to be defrosted

The shut-off valve associated with the heat exchanger is closed.

According to a preferred embodiment of the invention, the method in FIG. 10 assigns a defrost mode for defrosting at least one in a cooling mode

Heat absorption as an evaporator operated heat exchanger of the lower evaporation pressure stage the following steps:

Detection of icing of a heat exchanger surface of the heat exchanger of the lower evaporation pressure stage operated as evaporator in a cooling mode for absorbing heat,

- Closing an expansion element assigned to the at least one heat exchanger to be defrosted and

- Opening a defrost valve assigned to the at least one heat exchanger / 33 to be defrosted and

at least partially opening the connecting line at the branch point and directing at least a partial mass flow of hot compressed gas from the at least one compressor of the lower evaporation pressure stage to the heat exchanger to be defrosted in each case of the lower evaporation pressure stage,

Relaxation of the compressed gas as it flows through the defrost valve to the pressure level of the lower evaporation stage,

- Passing the hot compressed gas through the at least one heat exchanger and

Suction of the refrigerant emerging from the at least one heat exchanger through at least one compressor of the lower evaporation pressure stage,

- Close the connecting line at the junction and the

Defrost valve when a defrost end temperature is reached on the heat exchanger.

When the connecting line is opened at the junction, the

Mass flow of the hot compressed gas is preferably divided into the partial mass flows in a ratio between 0 and 100%.

According to an advantageous embodiment of the invention, in the formation of the lower evaporation pressure stage and / or in the formation of the upper evaporation pressure stage, at least one of the heat exchangers is operated in defrost mode from at least two heat exchangers which can be acted upon in parallel with refrigerant, while at least one second heat exchanger is operated in cooling mode.

The heat exchangers which can be operated as evaporators of the lower evaporation pressure stage and / or of the upper evaporation pressure stage are each advantageously individually defrostable.

The arrangement according to the invention for defrosting a heat exchanger in a refrigerant circuit of a refrigeration system and the method according to the invention for operating the refrigerant circuit during defrosting have, in summary, various advantages:

individual evaporators, in particular an evaporation temperature stage or evaporation pressure stage, can be defrosted in a minimum of time, while other evaporators of the same or different temperature stage can continue to be operated in cooling mode,

- Thereby keeping the temperature of the air of the cooling point constant during the defrosting process

- minimal energy consumption for the inevitable defrosting process of the heat exchangers as well as minimal operating costs of the refrigeration system and thus quick achievement of the set climate targets and

- protecting the refrigerated goods assigned to the cold store,

- Use the climate-friendly refrigerant carbon dioxide with a neutral greenhouse potential of one, also known as GWP for "global warming potential", and

- minimal installation effort or minimal manufacturing costs and maintenance costs of the refrigeration system.

Further details, features and advantages of embodiments of the invention result from the following description of exemplary embodiments with reference to the associated drawings. Show it:

1 and 2: a refrigerant circuit with a low temperature stage and a normal temperature stage, each with associated compressors and heat exchangers operated as evaporators, and an arrangement for defrosting the heat exchangers and

Fig. 3: the operation of the refrigerant circuit in cooling mode and

Defrost mode of the heat exchangers in a log p, h diagram, especially for subcritical operation.

1 and 2 each have a refrigerant circuit 1a, 1b with a low temperature stage and a normal temperature stage with associated / 33rd

Compressors 2, 4 and heat exchangers 9, 16 operated as evaporators and an arrangement for defrosting the heat exchangers 9, 16 are shown. The refrigerant circuit 1a, 1b shown corresponds to a so-called booster circuit. The natural refrigerant carbon dioxide, abbreviated as CO2 or R744, is preferably used as the refrigerant.

The compressors 2 of the low-temperature stage, which are arranged parallel to one another within a compressor unit, draw in refrigerant with state A from the evaporators 16 of the low-temperature stage, also referred to as the lower evaporation pressure stage, and compress the refrigerant to a pressure level of the upper evaporation stage or to a pressure level above the upper evaporation level. The gaseous refrigerant at a low pressure level has overheated and is in state B at the outlet of the compressor 2.

The respective states of the refrigerant can be found in the log p, h diagram shown in FIG. 3, which shows the operation of the refrigerant circuit 1a, 1b on the basis of the state points and state changes during operation in the cooling mode and in the defrosting mode of the heat exchangers 9, 16.

When the refrigerant circuit 1a, 1b, in particular the heat exchangers 9, 16 operated as evaporators, is operated in the cooling mode, the refrigerant is completely conducted to the heat exchanger 3 operated as a desuperheater, in which the refrigerant is heated to state C. The heat is transferred from the refrigerant to air, for example.

The compressors 4, which are likewise arranged parallel to one another within a compressor unit, draw in the refrigerant with the state D of the normal temperature stage, also referred to as the upper evaporation pressure stage, and compress the refrigerant to a high pressure level. The sucked-in gaseous refrigerant at the pressure level of the upper evaporation stage is overheated and is in state E at the outlet of the compressor 4.

Then the refrigerant is operated as a condenser / gas cooler / 33

Heat exchanger 5 passed, in which the refrigerant is heated to the state F and liquefied. For example, the refrigerant in turn transfers the heat to air. The refrigerant exits the heat exchanger 5 as a preferably liquid.

If the refrigerant is liquefied during subcritical or subcritical operation of the refrigerant circuit, as shown in FIG. 3, the heat exchanger 5 is referred to as a condenser. Part of the heat transfer takes place at a constant temperature. With supercritical operation or with supercritical heat emission in the heat exchanger 5, the temperature of the refrigerant steadily decreases. In this case, the heat exchanger 5 is also referred to as a gas cooler and the refrigerant as a transcritical medium. Supercritical operation can occur under certain environmental conditions or modes of operation of the refrigerant circuit, in particular with the refrigerant carbon dioxide.

The liquid refrigerant emerging from the heat exchanger 5 is expanded when flowing through the expansion element 6, which is advantageously designed as an expansion valve, from the high pressure level to a medium pressure level and in state G as a two-phase mixture of liquid and steam into a separator 7, which is also known as Medium pressure separator is introduced. The vaporous refrigerant is separated from the liquid refrigerant within the medium pressure separator 7. The medium pressure level has a pressure which lies between the high pressure level and the pressure level of the upper evaporation pressure stage.

The liquid refrigerant present at the medium pressure level is removed from the medium pressure separator 7 in state H and is passed to expansion elements 8 of the upper evaporation pressure level and to expansion elements 15 of the lower evaporation pressure level.

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Depending on the need, that is to say depending on the cooling requirement of the upper evaporation pressure stage, the refrigerant is divided into different heat exchangers 9 of the upper evaporation pressure stage which are arranged in parallel with one another and operated as evaporators. Before the inlet into the evaporators 9, the partial mass flows of the refrigerant are expanded in the expansion members 8 arranged upstream of the evaporators 9 in the flow direction of the refrigerant, from the medium pressure level to the pressure level of the upper evaporation pressure stage. The refrigerant emerging from the expansion members 8, which are in each case in the form of expansion valves, is in state K as a two-phase mixture of liquid and steam. When flowing through the evaporator 9, the liquid refrigerant is evaporated while absorbing heat, for example from the air surrounding the evaporator 9, and the gaseous refrigerant is possibly overheated. The refrigerant exits the evaporators 9 in the L state. The refrigerant mass flow divided at branch points 10 into partial mass flows of the upper evaporation pressure stage is mixed again at an outlet point 11.

In parallel with the liquid refrigerant, the gaseous refrigerant is also removed from the medium pressure separator 7. The gaseous refrigerant, which is also present at the medium pressure level, is discharged from the medium pressure separator 7 in state J and passed to an expansion element 12, in particular an expansion valve. When flowing through the expansion valve 12, the gaseous refrigerant is expanded to the pressure level of the upper evaporation pressure stage and then mixed in the state M at an orifice 13 with the proportion of the refrigerant emerging from the evaporators 9.

The refrigerant present after mixing in the N state is mixed with the refrigerant emerging from the desuperheater 3 in the C state at the mouth 14 and sucked in as a refrigerant in the D state by the compressor unit 4.

The liquid refrigerant withdrawn from the medium pressure separator 7/33 at the medium pressure level in state H is further divided as required, that is to say depending on the cooling requirement of the lower evaporation pressure stage, into different heat exchangers 16 of the lower evaporation pressure stage which are arranged in parallel with one another and operated as evaporators. Before the inlet into the evaporator 16, the partial mass flows of the refrigerant in the evaporators 16, which are arranged upstream in the flow direction of the refrigerant, are expanded from the medium pressure level to the pressure level of the lower evaporation pressure stage. The refrigerant emerging from the expansion members 15, which are each designed as expansion valves, is present in state P as a two-phase mixture of liquid and steam. When flowing through the evaporator 16, the liquid refrigerant is evaporated while absorbing heat, for example from the air surrounding the evaporator 16, and the gaseous refrigerant may be overheated. In state A, the refrigerant exits the evaporators 16. The refrigerant mass flow divided at a branch point 17 into partial mass flows of the lower evaporation pressure stage is mixed again at an outlet point 18 and sucked in by the compressor unit 2 of the lower evaporation pressure stage. The refrigerant circuit 1a, 1b is closed.

The hot compressed gas generated during operation of the compressors 2 of the lower evaporation pressure stage, also referred to as hot gas, is introduced into the upper evaporation pressure stage and together with the suction gas from the evaporators 9 of the upper evaporation pressure stage and the throttle gas from the medium pressure separator 7 from the compressors 4 of the upper evaporation pressure stage sucked in and compressed to the high pressure level. The evaporators 9 of the upper evaporation pressure stage are also referred to as normal temperature cooling points or cooling points for normal cooling, in short NK cooling points. The high pressure level corresponds to the condensing pressure of the refrigerant.

Depending on the state of the system, the hot gas of the lower evaporation pressure stage after it has left the compressors 2 is to be cooled before mixing with the refrigerant circulating in the / 33 upper evaporation pressure stage and is passed through the desuperheater 3 for heat emission. The evaporators 16 of the lower evaporation pressure stage are also referred to as low-temperature cooling points or cooling points for deep-freezing, in short TK cooling points.

The refrigerant circuit 1a, 1b is also formed with a connecting line 21, which extends from a branch point 19 to an inlet into the heat exchangers 9 of the upper evaporation pressure stage or into the heat exchangers 16 of the lower evaporation pressure stage. The branch point 19 is formed between the outlet of the compressor unit 2 of the lower evaporation pressure stage and the desuperheater 3, so that the hot gas generated during operation of the compressors 2 of the lower evaporation pressure stage can be passed at least in part to the heat exchangers 9, 16 as required. At the branch point 19, the hot gas can be divided into a mass flow to the desuperheater 3 and a mass flow to the heat exchangers 9, 16 in a ratio between 0 and 100%.

The connecting line 21 is formed with branches 22a, 22b in order to divide the hot gas as a refrigerant mass flow and to run it parallel to the heat exchangers 9, 16 of the evaporation pressure stages. The individual flow paths of the connecting line 21 each open at the outlet points 24a, 24b in refrigerant lines, which between the expansion elements 8, 15 and the heat exchangers 9, 16 of the

Evaporation pressure levels and thus are each formed at the inlet of the heat exchangers 9, 16. Each flow path of the connecting line 21 has a defrost valve 23, 25 in order to open or close the flow path as required and to apply hot gas to the heat exchanger 9, 16 arranged in the flow direction of the refrigerant after the outlet point 24a, 24b.

The connecting line 21 with branches 22a, 22b and the defrosting valves 23, 25, which extends from the branch point 19 to the outlet points 24a, 24b / 33, are components of an arrangement for defrosting at least one of the heat exchangers 9, 16 which are operated as evaporators in cooling mode When the refrigerant circuit 1a, 1b is operating in the defrost mode, the heat exchangers 9, 16 previously operated as evaporators in the cooling mode for heat absorption are defrosted or defrosted. When the refrigerant circuit 1a, 1b is operating, in particular when operating a heat exchanger 9, 16, in the cooling mode, the defrost valves 23, 25 of the arrangement for defrosting, which are respectively assigned to the heat exchanger 9, 16, are closed.

The branch point 19 of the arrangement for defrosting can be designed in various ways.

As can be seen from FIG. 1, the refrigerant circuit 1a has a three-way valve 20 formed at the branch point 19, also referred to as a defrosting main valve, which controls the mass flow of the hot compressed gas emerging from the compressor unit 2 of the lower evaporation pressure stage depending on the respective operating mode of the Refrigerant circuit 1 a controls and divides into partial mass flows in the direction of the desuperheater 3 and in the connecting line 21.

According to an alternative embodiment according to the refrigerant circuit 1b from FIG. 2, a first valve 26 and a second valve 27, in particular a pilot-controlled differential pressure valve 27, are arranged in the region of the branch point 19. The first valve 26 is preferably designed as a shut-off valve.

The defrost valves 23 of the flow paths, which extend between a branch 22a and an outlet point 24a in order to conduct hot gas to a heat exchanger 9 of the upper evaporation pressure stage, are designed in particular as shut-off valves 23, especially as solenoid valves.

When the refrigerant circuit 1a, 1b is operating in the defrost mode, a command to defrost / 33 is first issued by a higher-level control device. The three-way valve 20 of the refrigerant circuit 1a according to FIG. 1 or the valve 26 of the refrigerant circuit 1b according to FIG. 2 are at least partially opened, so that only a first partial mass flow of the hot compressed gas of the compressor unit 2 of the lower evaporation pressure stage through the desuperheater 3 into the Suction line of the compressor unit 4 of the upper evaporation pressure stage is passed, while a second partial mass flow of the hot compressed gas of the compressor unit 2 of the lower evaporation pressure stage is led to the heat exchangers 9 to be defrosted of the upper evaporation pressure stage. Depending on requirements, the three-way valve 20 or the valve 26 can also be set such that the mass flow of the hot compressed gas of the compressor unit 2 of the lower evaporation pressure stage instead of through the desuperheater 3 into the suction line of the compressor unit 4 of the upper evaporation pressure stage to the heat exchangers to be defrosted 9 of the upper evaporation pressure stage is passed.

When the refrigerant circuit 1a, 1b is operated in the defrost mode, at least a portion of the gaseous refrigerant or hot gas present in state B and compressed in state B is therefore at least part of the gas in the compressor unit 2 of the lower evaporation pressure stage or to a pressure level above the upper evaporation stage without any significant change in pressure the connecting line 21 with the branches 22a and the open shut-off valves 23 are passed to the heat exchangers 9 to be defrosted and passed through the respective at least one heat exchanger 9 at the pressure level of the upper evaporation stage. The hot gas flows through the heat exchanger 9 due to the driving pressure difference. The hot gas is heated and, if necessary, at least partially liquefied while releasing heat, and the heat transfer surface of the heat exchanger 9, which is now operated as a desuperheater / condenser, is de-iced. The refrigerant cools down and exits the heat exchanger 9 with the state Q. The heat dissipated by the refrigerant is used to melt the ice and, if necessary, to evaporate the water.

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The excellent heat transfer properties of R744 as a refrigerant make it possible to remove the ice build-up on the surface of the heat exchanger 9 without the need for any auxiliary energy in a very short time with the energy content of the hot compressed gas. The defrosting process can be carried out by melting the ice or by sublimation. In addition, an increase in the air humidity in the cooling point is possible by introducing compressed gas in the standstill phase of the heat exchanger 9.

The operation in cooling mode is interrupted for the heat exchanger (s) 9 to be defrosted for the time of the defrosting and thus the pressure gas passage. The respective expansion valve 8 is closed.

After flowing through the heat exchanger 9, the desuperheated refrigerant, possibly in the form of a two-phase mixture, is mixed at the outlet 11 with the superheated from the heat exchangers 9 of the upper evaporation pressure stage, which may continue to be operated as an evaporator. The further method steps correspond to the steps of operating the refrigerant circuit 1a, 1b or the heat exchanger 9 in the cooling mode.

Also when operating the refrigerant circuit 1a, 1b in the defrost mode, it must be ensured that only superheated and therefore gaseous refrigerant is drawn in by the compressor unit 4 in order to avoid liquid hammer which can damage or destroy the compressor 4.

In particular, when several heat exchangers 9 of the upper evaporation pressure stage are defrosted at the same time, mixing of the refrigerant mass flows emerging from the heat exchangers 9 at the outlet point 11 could result in refrigerant in the two-phase mixture and / 33 not being overheated.

It is to be prevented that at least partially liquid refrigerant despite mixing with the suction gas from the heat exchangers 9 operated as evaporators in the upper evaporation stage at the outlet point 11 and with the throttle gas from the separator 7 at the outlet point 13 and, if appropriate, the at least partially heated or unconditioned hot gas from the compressor unit 2 of the lower evaporation stage at the mouth 14 reaches the compressor unit 4 of the upper evaporation stage. It must therefore be ensured that a sufficient amount of hot gas with the condition B is passed through the desuperheater 3 in order to be mixed at the mouth 14 with the refrigerant flowing in from the mouth 13 and thus to draw the refrigerant in the superheated condition D from the compressor unit 4 , It must be ensured that a sufficiently large proportion of the refrigerant mass flow is passed through the desuperheater 3, the heat to be transferred in the desuperheater 3 also being influenced, for example by controlling a fan. If no heat is to be transferred in the desuperheater 3, the air supply to the desuperheater 3 must therefore be stopped. The distribution of the refrigerant mass flow at the branch point 19 is to be controlled accordingly.

The overheating of the refrigerant at the inlet of the compressor unit 4 of the upper evaporation pressure stage, also referred to as intake superheating, is always monitored. If the intake superheating falls below a predetermined target value, either the connecting line 21 in the region of the branch point 19 or the defrost valve 23 of the respective evaporator 9 are closed. The connecting line 21 of the refrigerant circuit 1 a according to FIG. 1 is closed by means of the three-way valve 20 and the connecting line 21 of the refrigerant circuit 1 b according to FIG. 2 is closed by means of the shut-off valve 26. This effectively prevents the compressor unit 4 from drawing in and damaging refrigerant with a liquid fraction, that is to say refrigerant present as a two-phase mixture.

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When the end of defrost temperature at the respective heat exchanger 9 is reached, the connecting line 21 is closed first and the defrost valve 23 of the heat exchanger 9 is closed with a time delay. After a predetermined period of time has elapsed, for example for the water to drip off the surface of the heat exchanger 9, the heat exchanger 9 is again operated in the cooling mode and thus as an evaporator, and the cooling process at the cooling point is continued.

The operation of the refrigerant circuit 1a, 1b in the defrost mode takes place with minimal energy consumption. For example, an evaporator in a cold room with a temperature of + 1 ° C must be defrosted at least three times a day. When defrosting with an electrical defrosting device, approximately 9.8 kW / day of electrical energy must be used. To defrost the same evaporator using hot gas, the consumption of electrical energy is reduced by up to 88%.

A heat exchanger of a heat pump operated as an evaporator must be de-iced several times a day at outside temperatures of less than + 1 ° C., the power consumption of the electrical defrosting device being approximately 16.8 kW. To defrost the same evaporator using hot gas, the consumption of electrical energy is reduced by around 78%.

The defrost valves 25 of the flow paths, which extend between a branch 22b and an orifice point 24b in order to conduct hot gas to a heat exchanger 16 of the lower evaporation pressure stage, are in particular designed as solenoid valves with an expansion function.

When the refrigerant circuit 1a, 1b is operated in the defrost mode of the heat exchangers 16 of the lower evaporation pressure stage, at least part of that in the compressor unit 2 of the lower evaporation pressure stage is compressed to a pressure level of the upper evaporation stage or to a pressure level above the upper evaporation stage and in / 33

Condition B present gaseous refrigerant or hot gas through the connecting line 21 with the branches 22a, 22b and at least one open defrost valve 25 to at least one heat exchanger 16 to be defrosted. The hot gas is expanded as it flows through the defrost valve 25 from state B to state R to the pressure level of the lower evaporation pressure stage and is passed through the heat exchanger 16 to be defrosted.

The operation in cooling mode is interrupted for the heat exchanger (s) 16 to be defrosted for the time of the defrosting and thus the pressure gas passage. The respective expansion valve 15 is closed.

The hot gas is heated while releasing heat. The heat transfer surface of the heat exchanger 16 now operated as a desuperheater is de-iced. The heat dissipated by the refrigerant is used to melt the ice and evaporate the water. The refrigerant preferably exits the heat exchanger 16 with the state A.

The refrigerant mass flows emerging from the heat exchangers 16 of the lower evaporation pressure stage are then mixed at the outlet point 18 and sucked in by the compressor unit 2. The further method steps correspond to the steps of operating the refrigerant circuit 1a, 1b in the cooling mode of the heat exchangers 9, 16.

It must again be ensured that only superheated and therefore gaseous refrigerant is drawn in by the compressor unit 2 in order to avoid liquid hammer which can damage or destroy the compressor 2. The refrigerant circuit 1a, 1b, in particular with regard to the refrigerant mass flows through the heat exchangers 16, is to be controlled accordingly.

Since the provision of the arrangement for defrosting as components of the refrigerant circuit 1a, 1b additional devices for defrosting the / 33rd

Heat transfer surfaces, such as electrical defrosting devices, can be saved, material costs can be saved and the installation effort can be minimized. In addition, the operation of the system causes minimal costs.

/ 33

LIST OF REFERENCE NUMBERS

1a, 1b refrigerant circuit

Compressor, compressor unit lower evaporation pressure level

Heat exchangers, desuperheaters

Compressor, compressor unit upper evaporation pressure stage

Heat exchanger, condenser / gas cooler

Expansion device, expansion valve high pressure

Separators, medium pressure separators

Expansion device, expansion valve upper evaporation pressure stage

Heat exchanger, evaporator upper evaporation pressure level

10, 17 branch

11, 18 muzzle

Expansion device, expansion valve medium pressure

13, 14 muzzle

Expansion device, expansion valve lower evaporation pressure stage

Heat exchanger, evaporator lower evaporation pressure level

branching point

Three-way valve

connecting line

22a, 22b branching connecting line 21 23 defrost valve, shut-off valve

24a, 24b mouth

defrost valve

Valve, shut-off valve

Valve, differential pressure valve

A - R condition, condition points refrigerant

Claims (14)

claims 1. refrigerant circuit (1a, 1b) of a refrigeration system with an arrangement for Defrosting at least one heat exchanger with refrigerant, the refrigerant circuit (1a, 1b) having at least two evaporation pressure levels, wherein - A first, lower evaporation pressure stage with at least one compressor (2) and at least one heat exchanger (16) that can be operated as an evaporator with an upstream expansion element (15) and - A second, upper evaporation pressure stage with at least one compressor (4) and at least one heat exchanger (9) that can be operated as an evaporator with an upstream expansion element (8) are formed, the heat exchangers (9, 16) of the different evaporation pressure stages being arranged to be acted upon by refrigerant at different pressure levels characterized in that the arrangement for defrosting has a connecting line (21) for conducting refrigerant at a constant pressure level, which extends from a branch point (19) formed at an outlet of the at least one compressor (2) of the first, lower evaporation pressure stage an outlet point (24a) formed at an inlet of the at least one heat exchanger (9) of the second, upper evaporation pressure stage that can be operated as an evaporator, and that the heat exchangers (9, 16) are arranged to flow through unidirectionally. 2. Refrigerant circuit (1a, 1b) according to claim 1, characterized in that between the branch point (19) of the arrangement for defrosting and an inlet of the at least one compressor (4) of the second, upper evaporation pressure stage as a desuperheater for 25/33 from the at least one compressor (2) of the first, lower evaporation pressure stage gaseous refrigerant operated heat exchanger (3) is arranged such that the refrigerant emerging from the heat exchanger (3) from the at least one compressor (4) of the second, upper Evaporation pressure stage is sucked. 3. Refrigerant circuit (1a, 1b) according to claim 1 or 2, characterized in that a shut-off valve (23) is arranged in the flow direction of the refrigerant through the connecting line (21) in front of the outlet point (24a). 4. Refrigerant circuit (1a, 1b) according to any one of claims 1 to 3, characterized in that the mouth (24a) of the connecting line (21) is formed in a refrigerant line arranged between the expansion element (8) and the heat exchanger (9). 5. Refrigerant circuit (1a, 1b) according to one of claims 1 to 4, characterized in that the connecting line (21) is formed with a branch (22b) for integrating a refrigerant line, which extends from the branch (22b) to an outlet point (24b) extends at an inlet of the at least one heat exchanger (16), which can be operated as an evaporator, of the first, lower evaporation pressure stage. 6. Refrigerant circuit (1a, 1b) according to claim 5, characterized in that a defrost valve (25) is arranged in the flow direction of the refrigerant through the refrigerant line in front of the outlet point (24b). 26/33 7. The refrigerant circuit (1a, 1b) according to one of claims 1 to 6, characterized in that the first, lower evaporation pressure stage is formed from at least two heat exchangers (16) operated in parallel and acted upon in parallel with refrigerant, each heat exchanger (16) having an expansion element (15) is arranged upstream and / or that the second, upper evaporation pressure stage is formed from at least two heat exchangers (9) which are operated in parallel and can be acted upon in parallel with refrigerant, each expansion unit (8) being arranged upstream of each heat exchanger (9). 8. refrigerant circuit (1 a) according to one of claims 1 to 7, characterized in that the branch point (19) is designed as a three-way valve (20). 9. Refrigerant circuit (1b) according to one of claims 1 to 7, characterized in that a shut-off valve (26) and in a suction area of the at least one compressor (4) of the upper evaporation pressure stage, a differential pressure valve (27) are arranged within the connecting line (21) , 10. The method for operating the refrigerant circuit (1a, 1b) according to one of claims 1 to 9 in a defrost mode for defrosting at least one heat exchanger (9) operated in a cooling mode for heat absorption as an evaporator of an upper evaporation pressure stage, comprising the following steps: - Detection of icing of a heat exchanger surface of the heat exchanger (9) operated in a cooling mode for absorbing heat, - Closing an expansion member (8) associated with the at least one heat exchanger (9) to be defrosted 27/33 - Opening a shut-off valve (23) associated with the at least one heat exchanger (9) to be defrosted and at least partially opening a connecting line (21) at a branch point (19) and directing at least a partial mass flow of hot compressed gas from at least one compressor (2) of a lower evaporation pressure stage to the heat exchanger (9) of the upper evaporation pressure stage to be defrosted, - Passing the hot compressed gas at the pressure level of the upper evaporation stage through the at least one heat exchanger (9) and - suction of the refrigerant emerging from the at least one heat exchanger (9) through at least one compressor (4) of the upper evaporation pressure stage, - Closing the connecting line (21) at the branch point (19) and the defrost valve (23) when an end defrost temperature is reached on the heat exchanger (9). 11. The method according to claim 10, characterized in that a Overheating of the refrigerant at the inlet into the at least one compressor (4) of the upper evaporation pressure stage is monitored and the connecting line (21) at the branch point (19) and / or the shut-off valve (23) associated with the at least one heat exchanger (9) to be defrosted is closed, when the overheating falls below a specified setpoint. 12. The method according to claim 10 or 11 in a defrost mode Defrosting at least one heat exchanger (16) of the lower evaporation pressure stage operated as evaporator in a cooling mode for absorbing heat, comprising the following steps: - Detection of icing of a heat exchanger surface of the evaporator operated in a cooling mode for heat absorption 28/33 Heat exchanger (16), - Closing an expansion member (15) assigned to the at least one heat exchanger (16) to be defrosted - Opening a defrost valve (25) assigned to the at least one heat exchanger (16) to be defrosted and at least partially opening the connecting line (21) at the branch point (19) and directing at least a partial mass flow of hot compressed gas from the at least one compressor (2) of the lower evaporation pressure stage to the heat exchanger (16) to be defrosted in the lower evaporation pressure stage, - depressurization of the compressed gas as it flows through the defrost valve (25) to the pressure level of the lower evaporation stage, - Passing the hot compressed gas through the at least one heat exchanger (16) and - suction of the refrigerant emerging from the at least one heat exchanger (16) through at least one compressor (2) of the lower evaporation pressure stage, - Closing the connecting line (21) at the branch point (19) and the defrosting valve (25) when a defrosting end temperature is reached on the heat exchanger (16). 13. The method according to any one of claims 10 to 12, characterized in that the mass flow of the hot compressed gas when opening the connecting line (21) at the branch point (19) is divided in a ratio between 0 and 100% into the partial mass flows. 14. The method according to any one of claims 10 to 13, characterized in that in the formation of the lower Evaporation pressure stage comprising at least two heat exchangers (16) which can be acted upon in parallel with at least one of the 29/33 Heat exchanger (16) is operated in defrost mode, while at least one second heat exchanger (16) is operated in cooling mode and / or that when the upper evaporation pressure stage is formed from at least two heat exchangers (9) which can be acted upon in parallel with refrigerant, at least one of the heat exchangers (9) in Defrost mode is operated while at least one second heat exchanger (9) is operated in cooling mode.