DE102014113167A1 - CO2 chiller for operation at high recooling temperatures - Google Patents

Abstract

The invention relates to a CO2 refrigerating machine with reciprocating compressors in the high pressure side under or supercritical operation. The CO2 refrigerating machine according to the invention comprises a refrigerant main circuit and a refrigerant secondary circuit, a supercharged refrigerant (6) arranged on the primary side between the refrigerant outlet of the gas cooler or condenser (3) and high-pressure control valve (8), whose cooling capacity can be generated by the refrigerant secondary circuit, and a Partial flow reciprocating compressor (2), which is operable via a switching valve (7) either as a compressor of the secondary circuit or as an additional compressor of the refrigerant main circuit. The subcooler (6) additionally cools the refrigerant flowing out of the gas cooler or condenser (3) in the refrigerant main circuit, so that an increase in efficiency is made possible by this enthalpy reduction. A control unit (15) connected to temperature sensors (14, 16, 17) and a pressure measuring sensor (15) is configured to switch the switching valve (7) in dependence on the measured pressure and temperature values and thus the additional cooling of the main flow in the subcooler (6) can switch on or off.

Description

The invention relates to a CO ₂ chiller with reciprocating compressors in the high pressure side subcritical and supercritical operation at high recooling temperatures, the principle of which can also be advantageously applied to compressed air dryer (condensate dryer) and heat pumps.

In the cold vapor chiller process with carbon dioxide (CO ₂ ) as the refrigerant, COP (Coefficient of Performance), ie the ratio of cooling capacity to applied drive power, depends crucially on the temperature of the cooling medium at the gas cooler / condenser inlet. The heat transfer from the refrigerant to the cooling medium is in this case carried out on the gas cooler / condenser in a known manner. As a cooling medium z. As cooling water from an open or closed cooling tower, a river or pond be used. The temperature of the cooling medium first determines whether the process is underpressure or supercritical on the high pressure side.

Due to the better COP, a refrigeration system with CO ₂ as the refrigerant is operated predominantly subcritical (subcritical). However, for this purpose it is necessary to liquefy the compressed carbon dioxide at the gas cooler at a temperature below 31 ° C (critical temperature), the gas cooler in this case works as a condenser. The actual liquefaction temperature for subcritical operation is determined by the recooling conditions.

In supercritical operation is u.a. decisive to which temperature the refrigerant cooled at the gas cooler outlet and with which spec. Enthalpy it is fed to the evaporator. The higher the gas cooler exit temperature, the less efficient the chiller.

It is known that the inlet temperature of the cooling medium in the gas cooler (in supercritical operation of the CO ₂ chiller) and the condenser (in subcritical operation of the CO ₂ chiller) in the summer months is much higher than in the remaining months of the year. This applies to both recooling systems with cooling tower and cooling with surface water (eg lake, cooling pond). So it may be z. B. be that the dormant water in a cooling pond in hot summer months has a temperature of 30 ° C and in cold winter months, only a temperature of about 4 ° C. This means that there is a seasonal difference of more than 25K for the inlet temperature of the cooling medium in the gas cooler and the condenser of refrigerators.

To improve the efficiency of the cold vapor chiller process by lowering the spec. Enthalpy of the refrigerant after the gas cooler / condenser outlet are known in principle three technical solutions.

In the first solution, an additional heat exchanger is used as a subcooler in the refrigerant-side flow direction after the gas cooler or the condenser. In this subcooler, which is subjected to a low-temperature cooling medium flow on the secondary side, the refrigerant emerging from the gas cooler can be further cooled. However, this solution to improve efficiency with additional heat exchanger requires a corresponding cooling medium, which is usually not available in the warm season.

In the second solution, refrigerant vapor is extracted from a medium-pressure collector, which is arranged in the refrigerant flow of the refrigerant circuit between the gas cooler or condenser and the expansion device. In the case of a supercritical process, the high pressure must be regulated; The control valve used for this purpose is arranged between the gas cooler or condenser and the refrigerant collector. In the refrigerant collector, a separation of the liquid and vapor refrigerant phase takes place. By sucking off the flash gas produced during the expansion into the two-phase region, it is achieved that only liquid in the saturation state at medium pressure is supplied to the expansion valve. The refrigerant occurs after relaxation with practically unchanged spec. Enthalpy into the evaporator. The compression of the extracted flash gas from the medium pressure to the high pressure takes place in screw compressors via an economizer connection opening connected to the compression space. The pressure at this point is, depending on the geometric arrangement, between suction and high pressure.

This possibility for charging at an economizer connection can also be realized in scroll compressors, but not in reciprocating compressors. The increase in the coefficient of performance through this circuit has been proven many times on subcritically operated refrigeration machines. In DE 103 26 467 A1 the embodiment is described by way of example.

A refrigerant-loaded subcooler represents the third known solution for improving the efficiency. Here, the gas cooler or the condenser is followed by a refrigerated subcooler. This subcooler is operated with refrigerant from the refrigerant circuit of the refrigerator. On the primary side of the subcooler, the refrigerant to be cooled flows, on the secondary side, refrigerant flows from a secondary circuit, which has been expanded to medium pressure. The partial flow of the secondary circuit evaporates in the designed as an evaporator subcooler, it deprives the mass flow of the main circuit, the heat of evaporation and thus cools the refrigerant of the main circuit to a lower temperature. The injection control in the secondary circuit is carried out with a thermostatic or electronic expansion valve. The control variable is the overheating of the refrigerant in the secondary circuit at the subcooler outlet. The compression of the emerging from the subcooler refrigerant partial flow takes place - analogous to Flashgasverdichtung from the medium pressure collector - via an economizer port opening in screw or scroll compressors.

Due to an economizer connection which is not technically feasible in reciprocating compressors, this third solution for charging the compression chamber, as well as the second one, has not previously been applicable to single-stage refrigerators with reciprocating compressors.

Since scroll compressors are less suitable for single-stage refrigerators with the refrigerant CO ₂ , a solution for improving the efficiency under unfavorable recooling conditions, for example in the summer, would be desirable for CO ₂ refrigerators with reciprocating compressors.

The invention has for its object to develop a technical solution to improve the efficiency of working with reciprocating compressors, single-stage CO ₂ chiller for, for example, seasonal, fluctuating recooling conditions. The improvement in efficiency aims at improving the high-pressure side cooling of the CO ₂ , wherein the CO ₂ chiller at a high temperature of the cooling medium applied to the gas cooler to work supercritically high pressure side, while at a low temperature, a subcritical operation is to be realized.

The solution of this task is carried out with a CO ₂ chiller with reciprocating compressor according to the features of claim 1; expedient embodiments of the invention are listed in the subclaims 2 to 9.

The refrigerant circuit of the CO ₂ refrigerating machine according to the invention may have one or more reciprocating compressors connected in parallel. The singular "reciprocating compressor" used in the following stands synonymously for one or more parallel reciprocating compressors.

According to the invention, the CO ₂ chiller on the gas cooler or the condenser in the refrigerant circuit downstream subcooler for additional cooling of the refrigerant, which is also acted upon by the secondary side with refrigerant. The subcooler is therefore a refrigerant-refrigerant heat exchanger. This subcooler is integrated in a working on medium pressure refrigerant secondary circuit whose refrigerant from the actual refrigerant circuit, hereinafter referred to as "main circuit", branched off at a branch point and at a return point, which is arranged in the refrigerant flow in front of the gas cooler, back again. In addition, the CO ₂ chiller another, at least one reciprocating compressor of the main circuit in parallel switchable reciprocating compressor, hereinafter referred to as partial flow reciprocating compressor whose suction pressure with a switching valve to a medium pressure level (resulting from an interaction between compressor and heat exchanger) switchable is on.

The term "main compressor" used hereinafter refers to the one or more d. H. several parallel, reciprocating compressors of the main circuit.

By supplying the secondary circuit with refrigerant from the main circuit of the CO ₂ refrigerator, wherein the subcooler is formed as an evaporator of the refrigerant secondary circuit and the gas cooler of both circuits (ie, main and secondary circuit) is shared, is between the gas cooler and the Subcooler a separation point, ie branch, with a downstream expansion valve inserted into the refrigerant line. From this separation point, part of the carbon dioxide flowing out of the gas cooler flows into the expansion valve of the secondary circuit, where it is expanded to a medium pressure. The expanded carbon dioxide is supplied from the expansion valve of the secondary circuit to the subcooler on the secondary side, that is introduced into the evaporator side of the subcooler, where it by evaporation of the refrigerant of the main circuit, ie the gas flowing from the gas cooler and remaining at the separation point in the main stream of carbon dioxide, which on the primary side of Subcooler (ie evaporator) is introduced, heat extracts.

The secondary circuit has its own compressor, which is a reciprocating compressor according to the object of the invention, namely the partial flow reciprocating compressor. Its suction pressure is at a medium pressure level, which adjusts according to the subcooler surface and the flow rate of the partial flow reciprocating compressor. In refrigerating machines with only one compressor, this reciprocating compressor of the secondary circuit is additionally provided, in systems with a main compressor in the form of several parallel-operated compressors, one of these existing compressors can be used as part-flow reciprocating compressors.

According to the invention, the suction side of this partial flow reciprocating compressor can be connected via the changeover valve either to the secondary side refrigerant outlet of the subcooler or to the suction side of the main compressor. In the latter case, the partial flow reciprocating compressor is connected in parallel with the compressors of the main circuit, that is, the partial flow reciprocating compressor becomes a part of the main flow reciprocating compressor.

In its function as a compressor of the secondary circuit, the partial flow reciprocating compressor compresses the vaporized partial flow back to the high pressure level. The switching valve is in this case set such that the secondary-side subcooler outlet is connected to the suction side of the partial flow reciprocating compressor. From the high pressure side of the partial flow reciprocating compressor, the carbon dioxide is returned to the main circuit in the refrigerant line between the main compressor and gas cooler.

The overheating of the partial flow at the secondary-side refrigerant outlet of the subcooler is regulated by the expansion valve of the secondary circuit.

The main stream, which is further cooled in the subcooler on the primary side, flows into a coolant collector, preferably through a high-pressure regulating valve. Subsequently, the expansion of the CO ₂ in the main stream to evaporation pressure, preferably with an expansion valve, and the evaporation takes place in an evaporator. The refrigerant vapor leaving the evaporator is compressed by the main compressor and fed to the gas cooler or condenser.

In the main circuit, an expansion valve may be provided in combination with a dry evaporator, but it may also be a flooded evaporator used, in which case the refrigerant collector and expansion valve omitted.

The CO ₂ refrigerating machine according to the invention also has a control unit. The partial flow reciprocating compressor may be equipped with a frequency converter connected to the control unit, the frequency being controlled by the control unit so that the main refrigerant flow at the subcooler outlet reaches a predetermined value. In the control unit, a corresponding algorithm is deposited, which can set this value for the main refrigerant flow in such a way that either the cooling capacity or the COP reaches a predetermined value.

The control unit is for example a suitably programmed PLC (memory programmable controller) or a PC (personal computer) with a software running on it.

In the case of heat extraction from the chiller, the gas cooler can be designed in the form of two separate heat exchangers.

Due to the subcooler is in subcritical process control even with unfavorable recooling conditions, eg. B. a cooling medium temperature of more than 27 ° C, a further supercooling of the liquefied CO ₂ allows.

Under certain operating conditions (eg, low temperature of the cooling medium, for example, less than 24 ° C at the gas cooler inlet and high evaporation temperature), it may be energetically more effective to operate the partial flow reciprocating compressor in parallel to the main compressor. Thus, the refrigerant circuit is operated with deactivated subcooler. The change-over valve is for this purpose placed in a position which connects the evaporator outlet (the main circuit) directly or indirectly with the inlet of the partial flow reciprocating compressor. An optionally arranged in the main flow behind the gas cooler high pressure control valve is fully open in this case. The partial flow reciprocating compressor thus operates parallel to the main compressor. The main stream passes in this case, the subcooler without further cooling; the secondary circuit is prevented due to the closed state of the switching valve for the partial flow. The course of the process corresponds to that of a simple one-stage chiller process.

The control unit may include an additional algorithm that achieves a maximum COP value by turning on or off the subcooler by switching the changeover valve. Depending on the present operating conditions, the control unit can thus switch the changeover valve into the position suitable for the maximum COP.

Since due to the highly nonlinear course of the specific heat of CO ₂ as a function of the temperature (and the pressure) - especially at the pressures commonly used in CO ₂ refrigeration machines up to 115 bar - comparatively large amounts of heat are necessary to reduce the refrigerant of z , B. 23 ° C (at 115 bar) to cool a few Kelvin on. In this case, the operation of the subcooler would be inefficient, or it is more effective to operate the partial flow reciprocating compressor in parallel with the main compressor.

So it may be z. B. be provided, the temperature of the cooling medium at the gas cooler or condenser inlet, the temperatures of the refrigerant on the primary side inlet and outlet of the subcooler and measure the suction pressure of the CO ₂ at the main compressor with appropriate sensors. From the refrigerant pressure measured on the suction side of the main compressor, the associated evaporation temperature is determined by means of known equations.

Climb z. B. the temperature of the cooling medium at the gas cooler inlet or the temperature of the CO ₂ at the primary side subcooler inlet above a predetermined temperature switching value or decreases the difference between the measured at the primary side subcooler inlet and outlet temperatures of CO ₂ below a predetermined temperature difference switching value and the evaporation temperature in Main circuit is above a predetermined evaporation temperature switching value, the subcooler is switched off by the control unit, ie, the partial flow reciprocating compressor is operated by switching the switching valve in parallel to the main compressor. The switching valve is thus in the position which connects the evaporator outlet directly or indirectly with the inlet of the partial flow reciprocating compressor.

In an analogous manner, the control unit switches the switching valve to that position which separates the inlet of the partial flow reciprocating compressor from the evaporator outlet and connects it to the secondary refrigerant outlet of the subcooler (i.e., by activating the partial flow, the subcooler is put into operation). B. the temperature of the cooling medium at the gas cooler inlet is below a predetermined cooling medium temperature switching value and the evaporation temperature (in the main circuit) drops below a predetermined evaporation temperature switching value.

By means of a hysteresis of the respective switching values too frequent a reversal of the chiller can be prevented, d. h., Two switching values for the cooling medium temperature and the evaporation temperature are respectively defined, wherein the first pair of switching values for the activation of the subcooler and the second pair of switching values for switching off the subcooler is used.

An advantage of the invention is the solution of the problem by a low technical effort.

The invention may also be advantageously designed such that the CO ₂ chiller has an additional internal heat exchanger (IHE - Inner Heat Exchanger). The IHE is arranged such that the refrigerant exiting the gas cooler before it enters the subcooler is cooled by the refrigerant exiting the evaporator and drawn by the main compressor. The IHE is thus arranged in the refrigerant flow between the gas cooler and subcooler in the refrigerant line of the main circuit.

In operating conditions such. As a constantly high difference in the temperatures of the cooling medium at the gas cooler inlet and outlet and a low evaporation temperature in the main circuit, it may be energetically effective to use such an additional internal heat exchanger.

The structure of the CO ₂ refrigerating machine described herein is not limited to a refrigerating machine, but can be equally advantageously applied to compressed air dryers, condensate dryers and heat pumps with reciprocating compressors.

The invention will be explained in more detail with reference to two embodiments. These show in a schematic representation of the

1 : a refrigerant circuit of a CO ₂ refrigeration machine with inventive partial flow reciprocating compressor and changeover valve,

2 : A variant of the CO ₂ chiller with internal heat exchanger.

1 shows the refrigerant circuit of the CO ₂ chiller. The refrigerant main circuit shows the compressor 1 , the evaporator 11 in which a medium to be cooled is cooled by means of the evaporating refrigerant, the gas cooler 3 in which the compressed refrigerant is cooled by means of water, and the refrigerant collector 9 on. The CO ₂ chiller can be operated both under- and supercritical. In this embodiment, it is operated under critical, so that the gas cooler 3 works as a liquefier. Before entering the refrigerant collector 9 is the high pressure control valve 8th , which is fully open in subcritical operation, and behind the outlet is the expansion valve 10 Installed.

In supercritical operation of the CO ₂ chiller would with the high pressure control valve 8th the refrigerant charge on the high pressure side of the circuit is controlled so that the optimum high pressure in the chiller is established. However, in this example, since the chiller is operated subcritically, the high pressure control valve is 8th without meaning or always fully open.

At evaporation pressure evaporated refrigerant is by means of the compressor 1 from the evaporator 11 into the liquefier 3 promoted, wherein refrigerant pressure and temperature are increased. By heat to the secondary side through the condenser 3 Guided water becomes the refrigerant in the condenser 3 liquefied.

In addition, the refrigerant circuit of the refrigerator includes the refrigerant subcooler 6 which provides by a refrigerant secondary circuit, the cooling capacity for further cooling of the refrigerant. That from the condenser 3 escaping refrigerant is at the point of separation 4 split into a main and a partial flow.

The partial flow is via the expansion valve 5 relaxed to a medium pressure and the subcooler 6 supplied on the secondary side. The overheating of the partial flow at the outlet of the subcooler is from the expansion valve 5 regulated. The partial flow reciprocating compressor 2 compacts in the subcooler 6 vaporized partial flow back to the high pressure level. The changeover valve 7 is set so that the secondary-side refrigerant outlet of the subcooler 6 with the partial flow reciprocating compressor 2 connected is. The condenser 3 the main circuit is also used as a gas cooler or condenser of the refrigerant secondary circuit.

The secondary circuit is thus an almost independent refrigerant circuit, which is in the main circuit for cooling the out of the condenser 3 flowing refrigerant serves.

The suction-side refrigerant pressure is measured with a pressure measuring sensor 13 at the refrigerant inlet of the compressor 1 measured. The measurement of a cooling water temperature takes place at the cooling water inlet of the condenser 3 with a temperature sensor 14 , Furthermore, the CO ₂ temperatures on the primary side refrigerant inlet and outlet on the subcooler 6 with the temperature sensors 16 and 17 measured The control unit 15 is with the pressure measuring sensor 13 and the temperature sensors 14 . 16 and 17 connected so that they can read the respective measured values. In the control unit 15 is a control algorithm deposited, depending on the measured values for pressure and temperature, the switching valve 7 switched in such a way that the COP reaches a maximum.

2 shows an embodiment variant of the CO ₂ chiller with the additional internal heat exchanger 12 , This will remove the liquefier 3 Exiting refrigerant flow is cooled by the suction gas before it enters the subcooler 6 entry.

LIST OF REFERENCE NUMBERS

1

compressor

2

Partial flow reciprocating compressors

3

Gas cooler / condenser

4

split point

5

expansion valve

6

subcooler

7

switching valve

8th

High-pressure control valve

9

Refrigerant collector

10

expansion valve

11

Evaporator

12

internal heat exchanger

13

Pressure measuring sensor

14

temperature sensor

15

control unit

16

temperature sensor

17

temperature sensor

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10326467 A1 [0009]

Claims (9)

CO ₂ chiller with CO ₂ as refrigerant and a control unit ( 15 ), characterized in that: a) the CO ₂ refrigeration machine has a refrigeration main circuit, comprising an evaporator ( 11 ), a reciprocating compressor ( 1 ) and a gas cooler or condenser ( 3 ), flowing main flow of the refrigerant comprises; b) the CO ₂ refrigerating machine a refrigerant secondary circuit operable with a partial flow of the refrigerant of the CO ₂ refrigerating machine, comprising an expansion valve ( 5 ), acting as an evaporator and with its primary side in the refrigerant main circuit in the refrigerant flow direction behind the gas cooler or condenser ( 3 ) integrated subcooler ( 6 ) and a partial flow reciprocating compressor ( 2 ); c) the CO ₂ chiller in the flow direction of the refrigerant upstream of the partial flow reciprocating compressor ( 2 ) a switching valve ( 7 ), wherein the switching valve ( 7 ) the suction side of the partial flow reciprocating compressor ( 2 ) in a first switching state with the secondary-side refrigerant outlet of the subcooler ( 6 ) and in a second switching state with the suction side of the reciprocating compressor ( 1 ) of the refrigerant main circuit connects; and d) the control unit ( 15 ) is designed such that by switching the switching valve ( 7 ) in the first switching state, a refrigerant flow from the refrigerant main circuit in the refrigerant secondary circuit and in the secondary side of the subcooler ( 6 ) can be introduced and by switching to the second switching state of the partial flow reciprocating compressor ( 2 ) parallel to the reciprocating compressor ( 1 ) of the refrigerant main circuit is operable. CO ₂ refrigerating machine according to claim 1, characterized in that the control unit ( 15 ) is a programmable logic controller (PLC). CO ₂ chiller according to one of the claims 1 or 2, characterized in that the partial flow reciprocating compressor ( 2 ) is equipped with a frequency converter, the control unit ( 15 ) is arranged to regulate the frequency in such a way that the refrigerant flow of the refrigerant main circuit at the primary-side outlet of the subcooler ( 6 ) has a predetermined value. CO ₂ chiller according to one of the preceding claims, characterized in that it comprises an internal heat exchanger ( 12 ) whose primary side into the refrigerant flow of the refrigerant main circuit between the gas cooler or condenser ( 3 ) and subcooler ( 6 ) and its secondary side into the refrigerant flow of the refrigerant main circuit between evaporator ( 11 ) and reciprocating compressors ( 1 ) is involved. CO ₂ refrigerating machine according to one of the preceding claims, characterized in that it is equipped with a control unit ( 15 ) connected pressure sensor ( 13 ) for detecting a refrigerant pressure at the suction side of the reciprocating compressor ( 1 ), one each with the control unit ( 15 ) connected temperature sensor ( 16 . 17 ) for detecting a refrigerant temperature at the primary-side refrigerant inlet and refrigerant outlet of the sub-cooler ( 6 ) and one with the control unit ( 15 ) connected temperature sensor ( 14 ) for detecting a coolant temperature at the cooling medium inlet of the gas cooler or condenser ( 3 ), wherein the control unit ( 15 ) is designed such that with selbiger from the refrigerant pressure an evaporating temperature determined and the switching valve ( 7 ) on reaching, under or exceeding in each case for the cooling medium temperature, one of the refrigerant temperatures on the primary-side refrigerant inlet and refrigerant outlet of the sub-cooler ( 6 ) certain refrigerant temperature difference and the evaporation temperature of predetermined setpoints can be switched. CO ₂ chiller according to claim 5, characterized in that the setpoint values of cooling medium temperature, refrigerant temperature difference and evaporation temperature for switching the switching valve ( 7 ) are different in the first switching state to the setpoint values of cooling medium temperature, refrigerant temperature difference and evaporation temperature for switching the switching valve ( 7 ) in the second switching state. CO ₂ refrigerating machine according to one of the preceding claims, characterized in that the evaporator ( 11 ) is a flooded evaporator. CO ₂ chiller according to one of the claims 1 to 6, characterized in that the evaporator ( 11 ) is a dry evaporator. CO ₂ refrigerator according to one of the preceding claims, characterized in that the gas cooler ( 3 ) is designed in the form of two separate heat exchangers.

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