EP1861662A1

EP1861662A1 - Cold circuit

Info

Publication number: EP1861662A1
Application number: EP06707479A
Authority: EP
Inventors: Andreas Kemle Kemle; Bernd Schäfer; Thomas Tscheppe
Original assignee: Behr GmbH and Co KG
Current assignee: Mahle Behr GmbH and Co KG
Priority date: 2005-03-15
Filing date: 2006-03-08
Publication date: 2007-12-05
Also published as: WO2006097229A1

Abstract

The invention relates to a cold circuit (1) comprising a compressor (2), a gas cooler (3), an internal heat exchanger (4), an expansion element (5), an evaporator (6) and, optionally, an accumulator (7). The accumulator (7) is arranged in the low pressure part of the cold circuit (1). The average density of the coolant in the cold circuit (1) has a maximum of 260 kg/m3 +/-10%. The volume of the accumulator (7) is obtained, preferably, approximately by the formula VAkku = [mLeckage- V KKL (Pmax_

Description

Refrigeration cycle

The invention relates to a refrigeration cycle according to the preamble of claim 1.

EP 0 424 474 B2 discloses a method for operating a cold vapor process under transcritical or supercritical conditions. Here, in the vapor compression cycle, a compressor, a radiator, a throttle device and an evaporator are provided, which are serially connected to each other to form an integral closed circuit. The circuit operates with supercritical pressure on the high pressure side of the circuit, wherein the pressure on the high pressure side is regulated by changing the respective refrigerant charge on the high pressure side of the circuit by changing the content of a refrigerant buffer tank disposed in the circuit, the pressure decreasing the content is increased, and vice versa. This affects the specific capacity of the circuit. The regulation of the supercritical pressure is carried out by changing the liquid content of a low-pressure refrigerant tank, which lies between the evaporator and the compressor, using only the Throttle device as a control device. The condition of the evaporator outlet is maintained as a two-phase mixture of vapor and liquid, creating an excess of liquid at the low pressure inlet of an additional heat exchanger where the low pressure refrigerant is heated by the high pressure refrigerant before entering the compressor evaporator and overheating. The refrigerant is carbon dioxide.

However, such a cycle still leaves something to be desired.

It is an object of the invention to provide an improved circuit.

This object is achieved by a circuit having the features of claim 1. Advantageous embodiments are the subject of the subclaims.

According to the invention, a refrigeration circuit which can be operated under supercritical conditions is provided with a compressor, a gas cooler, an internal heat exchanger, an expansion element, an evaporator and optionally an accumulator, the accumulator, if present, being arranged in the low-pressure part of the refrigeration circuit. The accumulator is preferably arranged between the evaporator and the inner heat exchanger. In this case, the refrigeration cycle at the time of filling - which may be, for example, the first filling or refilling in the context of repair or maintenance - filled in addition to the minimum amount of refrigerant required with an additional volume of refrigerant, which compensates for the leakage losses occurring over an operating period , In this context, the term "operating time duration" is to be understood as meaning, in particular, a time interval up to scheduled maintenance or the planned service life of the plant. By contrast The minimum required amount of refrigerant is usually understood as the amount of refrigerant that is necessary to realize the maximum cooling capacity of the system. This is usually determined in practice by the fact that in every operating state of the refrigeration system (starting operations or short-term operating fluctuations excepted) the refrigerant at the evaporator outlet is just two-phase, ie vaporous and liquid.

Particularly when R744 is used as the refrigerant, the refrigerant can be in a supercritical state during a plant standstill at temperatures which occur in practice (T _{crit R744} = 31 ^° C.). In such a case, the standstill pressure is both a function of the temperature, as well as the average density of the refrigerant in the refrigeration cycle, the standstill pressure increases with increasing average density and with increasing temperature. Otherwise, the use of R744 (carbon dioxide) as a refrigerant compared to the use of R134a generally causes a significantly increased pressure level. A mean density in the refrigeration cycle of a maximum of 280 kg / m ³ , preferably a maximum of 260 kg / m ³ , in particular a maximum of 250 kg / m ³ (tolerances of +/- 10% are possible) can ensure that in particular even with a standstill of the refrigeration system or when operating at high ambient temperatures in conjunction with high refrigerant charge in the refrigeration cycle, the refrigeration system takes no harm as a result of the significantly increased pressure. It is, if an accumulator is present, make sure that the accumulator volume on the one hand is not too small to ensure appropriate leakage retention, on the other hand is not chosen too large to ensure cost-effective production and the space in the engine compartment as small as possible to keep. Leaks can usually occur at the connection points in the refrigeration circuit and at the radial shaft sealing ring of the compressor. If the accumulator is dimensioned relatively small, or in extreme cases no accumulator is present, the lines and other components take over. components whose function they are to be interpreted accordingly. By providing a sufficient provision of leakage among other things, the maintenance intervals can be extended without sacrificing performance and reliability can be increased.

The volume of the accumulator preferably results according to the equation given in the claim, with deviations of +/- 20% being possible. However, the deviations are preferably only a maximum of +/- 10%, in particular a maximum of +/- 5%, in order to obtain an optimum accumulator volume. It should be ensured that for use of the accumulator as a buffer volume to compensate for operational fluctuations, the free accumulator volume should be greater than 450 cm ³ . The free accumulator volume for mobile R744 refrigeration systems is preferably between 700 and 4000 cm ³ , in particular between 900 and 3000 cm ³ , and particularly preferably between 1100 and 2100 cm ³ .

The accumulator volume may also be distributed to a plurality of containers, which are arranged between the evaporator and the compressor. In this case, correspondingly, in particular with an enlarged inner diameter, formed lines form a part of the accumulator volume.

In the following the invention with reference to several embodiments, partially with reference to the drawings, explained in detail. It shows:

1 is a schematic schematic diagram of a refrigeration cycle according to the first embodiment,

2 is a schematic diagram of a refrigeration cycle GE measure the second embodiment, Fig. 3 is a p, h diagram of a typical R744 refrigerant circuit at the design point, and

Fig. 4 is a diagram illustrating a typical course of the R744 pressure over time in the accumulator at the design point.

1 shows as a first exemplary embodiment a schematic diagram of a refrigeration cycle 1 of a motor vehicle air conditioning system which includes a refrigerant compressor 2, a gas cooler 3, an internal heat exchanger 4, an expansion element 5, an evaporator 6 and a collector or accumulator 7 having.

As refrigerant 1 R744 (carbon dioxide) is circulated in the circuit. The gaseous refrigerant passes from the compressor 2, in which it is compressed, via the gas cooler 3, in which the refrigerant heated as a result of the compression is cooled, to the internal heat exchanger 4 and to the expansion element 5, in which the compressed refrigerant is expanded and thereby is cooled further. Because of the high pressure prevailing in this part of the refrigeration cycle 1, it is referred to as the high pressure side. Subsequently, the cold, relaxed refrigerant from the expansion device 5 to the evaporator 6, in which it absorbs heat from the environment and which it leaves partly vaporous and partly liquid, then to the accumulator 7, in which liquid refrigerant collects and, if necessary, the Circuit can be supplied, and via the inner heat exchanger 4 back to the compressor 2. Because of the prevailing in this part of the refrigeration cycle 1, lower pressure, it is referred to as the low pressure side.

The accumulator 7, which is arranged on the low pressure side between the evaporator 6 and the inner heat exchanger 4, has two tasks. To egg NEN the accumulator 7 is to serve as a buffer volume to compensate for fluctuations in operation, since different levels of refrigerant is required in different operating points. On the other hand, in addition to the amount of refrigerant required for the operation of the refrigeration system additional refrigerant to be filled in the circuit to buffer over a defined period, the inevitable refrigerant leakage, for example, at the compressor radial shaft seal. Also, this additional refrigerant buffer or supply should be kept in particular in the operation of the system in the accumulator 7.

Under real operating conditions and with meaningful provision of leaks, the refrigerant leakage is decisive for the design of the free accumulator volume. The free accumulator volume is defined as the internal volume of the accumulator minus the volume of any internal installations of the accumulator. Depending on the cycle volume without accumulator 7 and the refrigerant pressure in the accumulator 7 at the design point, the free accumulator volume V _accumulator can be calculated according to the following equation:

V _Accumulator = [m _Leakage - V _KKL (p _max - PκκL _/ 1 '(Pmax "~ Pτaulinie [PAkku.)

Here, m _{leakage is} the refrigerant for provision for the leakage, p _accumulator the pressure of the refrigerant in the accumulator (in the design point), p _max the maximum permissible density in the refrigeration circuit and accumulator, p _KKL the average density of the refrigeration cycle without accumulator, p _TaU | _inle the density of the refrigerant on the dew line, V _battery the volume of the accumulator and V _KKL the volume of the refrigeration circuit without accumulator. The formula applies primarily to the filling time of the system.

The following equation can be used as a further basis for interpretation: ^ Accumulator ⁼ [ ^m leakage ~ V _KKL (p _max - P _KK L)] / (Pmax ~~ PGas)

Here are m _Leokage the refrigerant for provision for the leakage, p _accumulator the pressure of the refrigerant in the accumulator (in the design point), p _max the maximum permissible density in the refrigeration circuit and accumulator, p _KKL the average density of the refrigeration cycle without accumulator, p _Gas the density of the gaseous refrigerant in the accumulator, V _accumulator the volume of the accumulator and V _KKL the volume of the refrigeration cycle without accumulator. This formula also applies primarily to the filling time of the system.

Design point for the battery volume is an operating point at very high ambient temperatures (eg 40 to 45 ⁰ C and / or 45 to 48 ⁰ C) and a high solar load (eg 1000 W / m ² ). The vehicle is operated in idle, possibly in outdoor air mode. In order to operate the refrigeration system optimally, the refrigeration system must be filled with refrigerant in such a way that the overheating of the refrigerant disappears after the evaporator, or within the scope of the measuring accuracy is less than 6 K, preferably 4 K, particularly preferably 2 K. In this case, set a certain refrigerant pressure in the accumulator 7 and a certain refrigerant density (density on the dew line, or density at 2, 4 or 6 K overheating and p = p _battery ) (stationary state or value after 10 min). A typical course of the pressure in the accumulator 7 is shown in Fig. 4. As can be seen from the diagram, the pressure drops sharply after starting within a short time.

The free accumulator volume should be determined approximately according to the above equation, the equation being optimum. Deviations in the design up or down are of course possible, but should be kept low, in particular amount to a maximum of 20%. When designing it is important to ensure that the use of the accumulator 7 as a buffer volume to compensate for operating fluctuations expediently, the accumulator volume should be greater than 450 cm ³ . In the present case, the free accumulator volume is about 1600 cm ³ .

The average density in the refrigeration cycle 1 without accumulator 7 should be less than or equal to the maximum permissible density of the refrigeration cycle 1 including accumulator volume in the design of the accumulator 7, which is advantageously 260 kg / m ³ . The refrigeration cycle 1 is operated so that in any operating state (except for the starting or short-term fluctuations in operation) the refrigerant at the evaporator outlet two-phase, ie vapor and liquid, is present, whereby at least a portion of the refrigerant in the liquid state reaches the accumulator 7 and there collected and saved.

The illustrated in Fig. 2, second embodiment corresponds essentially borrowed the first embodiment, but in this case the expansion element 5 is integrated into the inner heat exchanger 4. Apart from the fact that the refrigerant passes directly from the inner heat exchanger 4 to the expansion element 5, which in the present case is directly integrated into the inner heat exchanger 4 at the rear end, and does not have to flow through a line, nothing changes at the function of the refrigeration cycle.

By an appropriate design of the line between the inner heat exchanger 4 and the expansion element 5 is made possible that the line in which there is refrigerant with the highest density, if possible lent can be kept short or, as in the second embodiment, completely eliminated.

According to a third, not shown in the drawing embodiment, but which corresponds in its basic structure substantially to the first embodiment, the refrigerant pipes between the evaporator and the accumulator, the accumulator and the inner Heat exchanger and the inner heat exchanger to the compressor with an enlarged inner diameter (for example, more than and / or = 10 mm inner diameter) and the refrigerant lines between the compressor and gas cooler, the gas cooler and the expansion element and the expansion device to the evaporator with a reduced inner diameter ( of, for example, less than 8 mm, 6 mm or 4 mm inner diameter). In addition, the line lengths are formed enlarged in the areas with increased inner diameter and the line lengths in the areas with reduced inner diameter formed shortened, so that in the corresponding areas more or less refrigerant is absorbed. As a result, among other things, the accumulator can be made smaller or even completely eliminated in the extreme case. In particular, in the latter case, other components can take over a protection of the compressor before a "wet driving".

The refrigeration cycle is operated according to the third embodiment in each case such that the average density of the refrigerant in the refrigeration cycle is at most 260 kg / m ³ + 10%, ie a maximum of 286 kg / m ³ . Also in this case, the circuit is operated so that in any operating condition (except for the starting and short-term fluctuations in operation), the refrigerant at the evaporator outlet two-phase, ie vapor and liquid, is present.

Claims

Patent claims

1. refrigeration cycle with a compressor (2), a gas cooler (3), an inner heat exchanger (4), an expansion element (5), an evaporator (6) and a refrigerant, characterized by an additional volume of refrigerant, the over compensates for an operating period occurring leakage losses.

2. Refrigeration circuit according to claim 1, characterized in that the average density of the refrigerant in the refrigeration cycle (1) maximum 280 kg / m ³ , preferably at most 260 kg / m ³ , in particular at most 250 kg / m ³ , wearing.

3. Refrigeration circuit according to claim 1 or 2, characterized in that an accumulator (7) is arranged on the low pressure side in the refrigeration circuit (1) between the evaporator (6) and the inner heat exchanger (4).

4. Refrigeration circuit according to claim 3, characterized in that the volume of the accumulator (7) results from:

"Accumulator ⁼ [^ Leakage ~ ^ _KKL (P _ma χ ~ PKKL / J '(Pmax" ~ Pτaulinie-PAkku.)> Where deviations of +/- 20% are possible, and where m _{leakage is} the refrigerant for maintaining the leakage , p _battery the pressure of the refrigerant in the accumulator (7), p _max the maximum permissible density in the refrigeration cycle (1) and accumulator (7), p _KKL the average density of the refrigeration cycle (1) without accumulator (7), p _TaU | _inie the density of the refrigerant on the dew line, V _battery the volume of the accumulator (7) and

V _{KKL is} the volume of the refrigeration cycle (1) without accumulator (7).

5. Refrigeration circuit according to claim 3 or 4, characterized in that the deviations of the volume of the accumulator (7) a maximum of +/- 10%, in particular a maximum of +/- 5%.

6. Refrigeration circuit according to one of claims 3 to 5, characterized in that the free accumulator volume is greater than 450 cm ³ .

7. Refrigeration circuit according to one of claims 3 to 6, characterized in that the free accumulator volume between 700 cm ³ and

4000 cm ³ , preferably between 900 cm ³ and 3000 cm ³ , particularly preferably between 1100 cm ³ and 2100 cm ³ .

8. Refrigeration circuit according to one of claims 3 to 7, characterized in that at least two containers for receiving the refrigerant low-pressure side between the evaporator (6) and the compressor (2) are provided.

9. Refrigeration circuit according to one of the preceding claims, characterized in that the lines in the region between the evaporator (6) and the compressor (2) with an enlarged inner diameter of more than and / or = 10 mm inner diameter and / or increased length are formed.

10. Refrigeration circuit according to one of the preceding claims, characterized in that the lines in the region between the compressor (2) and the evaporator (6) with reduced inner diameter of in particular less than 8 mm, preferably less than 6 mm inner diameter and / or reduced length are formed.

11. Refrigeration circuit according to one of the preceding claims, characterized in that the expansion element (5) is arranged in the vicinity of the high-pressure-side outlet of the inner heat exchanger (4) or integrated in the same.

12. Refrigeration circuit according to one of the preceding claims, characterized by carbon dioxide as the refrigerant.

13. A method for filling a refrigeration cycle (1) with a compressor (2), a gas cooler (3), an inner heat exchanger (4), an expansion element (5) and an evaporator (6) with a refrigerant, characterized an additional volume of refrigerant is provided which compensates for the leakage losses occurring over an operating period.

14. The method according to claim 13, characterized in that the average density of the refrigerant in the refrigeration cycle (1) is adjusted so that it is at most 280 kg / m ³ , preferably at most 260 kg / m ³ , in particular at most 250 kg / m ³ , is.

15. The method according to claim 13 or 14, characterized in that an accumulator (7) is arranged on the low pressure side in the refrigeration circuit (1), in which liquid refrigerant is collected.

16. The method according to claim 15, characterized in that the volume of the accumulator (7) results from:

"Accumulator ⁼ [f ^ Leakage KKL (Pmax ~ PKKL) J '((Pmax ~ ^" Pta_line-PAkkuJ)> where deviations of +/- 20% are possible, and where m _{leakage is} the refrigerant for maintaining the leakage , p _Battery the pressure of the refrigerant in the accumulator (7), p _max the maximum permissible

Density in the refrigeration cycle (1) and accumulator (7), p _KKL the average density of the refrigeration cycle (1) without accumulator (7), p _TaU | _inie the density of the refrigerant at the dew line V _battery, the volume of the accumulator (7) and V _KKL of the refrigerant circuit (1) is the volume without the accumulator (7).

17. The method according to any one of claims 13 to 16, characterized in that carbon dioxide is used as the refrigerant.