WO2008019689A2

WO2008019689A2 - A transcritical refrigeration system with a booster

Info

Publication number: WO2008019689A2
Application number: PCT/DK2007/000374
Authority: WO
Inventors: Finn Guldager Christensen
Original assignee: Knudsen Køling A/S
Priority date: 2006-08-18
Filing date: 2007-08-16
Publication date: 2008-02-21
Also published as: WO2008019689A3

Abstract

Transcritical refrigeration system (10) with a booster and a bypass valve, the system comprising a flow circuit and a high pressure compressor (20), the compressor output being connected to an input of a gas cooler (24) and having an output connected with an input of a high temperature pressure reducing device (27), and having an output connected to a receiver (26) with a liquid output connected to an input of a medium temperature expansion valve (28) with an output connected to an input of a medium temperature evaporator (30) and having an output connected with the input of the high pressure compressor (20), and a gas bypass valve (32) with an input connected to a gas output of the receiver (26) and an output connected with the output of the medium temperature evaporator (30), the gas bypass valve (32) lowering the pressure in tubing between the output of the high temperature expansion valve (27) and the input of the medium and low temperature expansion valves (28).

Description

A TRANSCRITICAL REFRIGERATION SYSTEM WITH A BOOSTER

The present invention relates to a transcritical refrigeration system with a booster and a bypass valve, for example intended for a supermarket.

Transcritical refrigeration systems with CO₂ as a refrigerant are well known in the art. The critical temperature of CO₂ is 31.0 ^GC and the critical pressure is 73.8 bar. At higher temperatures and pressures no clear distinction can be drawn between liquid and vapour, and CO₂ is said to be in the so-called super-critical fluid region.

In a conventional refrigeration system, heat release from the refrigerant is based on condensation of the refrigerant. Considering the temperature difference needed in a heat exchanger, i.e. app. 10 ⁰C, the "real life" upper limit for heat release based on condensation of CO₂ will be around 20 °C ambient temperature. Below this temperature, the CO₂ stays below the critical point and the refrigeration system operates in subcritical cycles.

For refrigeration systems used in supermarkets, the ambient temperature will exceed 20 °C during the summer in a large part of the world. At these temperatures, cooling of the CO₂ is a single-phase cooling, namely a gas cooling. CO₂ is above the critical point at the high- pressure side of the system, and the refrigeration system operates in transcritical cycles.

The efficiency and the cooling capacity of the refrigeration system are lower in transcritical operation than in subcritical operation.

In supermarkets, hypermarkets, etc., refrigeration systems for freezing and chilling, respectively, typically constitute separate systems with different refrigerants.

It is an object of the present invention to provide a CO₂ transcritical refrigeration system for both chilling and freezing is provided with very few parts and at a low manufacturing cost.

According to the invention, a transcritical CO₂ refrigeration system is provided combining a freezing system and a chilling system. According to the present invention the above-mentioned and other objects are fulfilled by provision of a transcritical refrigeration system comprising a flow circuit for recirculation of a refrigerant. The flow circuit comprises a high pressure compressor for generation of a refrigerant flow from a low-pressure side at an input of the high pressure compressor to a high-pressure side at an output of the high temperature compressor. The output of the high pressure compressor is connected to an input of a gas cooler for cooling of the refrigerant towards the ambient temperature and the gas cooler has an output that is connected with an input of a high temperature pressure reducing device for decreasing the downstream pressure and creating a mixture of gas and liquid refrigerant. An output of the high temperature pressure reducing device is connected to a receiver for accommodation of the liquid refrigerant. The receiver has a liquid output connected to an input of a medium temperature expansion valve that further has an output connected to an input of a medium temperature evaporator for evaporation of the refrigerant. The medium temperature evaporator has an output that is connected with the input of the high pressure compressor for recirculation of the refrigerant.

The medium temperature evaporator is situated in cooling furniture, e.g. keeping the temperature in the cooling furniture at app. 4 ⁰C. In a typical system, e.g. in a supermarket, the system comprises several medium temperature evaporators situated in different cooling furniture and connected in parallel. Further, the system may comprise a gas bypass valve with an input connected to a gas output of the receiver and an output connected with the output of the medium temperature evaporator, the gas bypass valve lowering the pressure in tubing between the output of the high temperature expansion valve and the input of the medium temperature expansion valve and the input of the low temperature expansion valve. The gas bypass valve may for example lower the pressure from app. 70 bar to app. 30 bar. This leads to considerable savings in installation cost since the steel tubing (typically more than 20 m in a small installation) required for withstanding 70 bar may now be substituted by cupper tubing.

The transcritical refrigeration system according to the present invention may further comprise a freezing system coupled in parallel between the receiver output and the medium temperature evaporator output.

The freezing system has a low pressure compressor for generation of a refrigerant flow from a low-pressure side at an input of the low pressure compressor to a high-pressure side at an output of the low pressure compressor. The output of the low pressure compressor is connected with the output of the medium temperature evaporator and the output of the gas bypass valve and the input of the high pressure compressor.

The freezing system further has a low temperature expansion valve with an input connected with the liquid output of the receiver and an output connected to an input of a low temperature evaporator for evaporation of the refrigerant generating freezing temperatures and having an output connected with the input of the low pressure compressor.

The low temperature evaporator is situated in freezers, e.g. keeping the temperature in the freezers at app. - 18 ⁰C. In a typical system, e.g. in a supermarket, the system comprises several low temperature evaporators situated in different freezers and connected in parallel. Preferably, the refrigerant is CO₂ due to its low global warming potential (GPW = 1), availability, and reasonable cost.

The pressure reducing devices are preferably expansion valves.

The transcritical refrigeration system according to the present invention may further comprise a high temperature heat exchanger for improvement of the efficiency of the system. The high temperature heat exchanger has a first flow circuit with an input connected to the output of the gas cooler and an output connected to the input of the high temperature expansion valve. The high temperature heat exchanger further has a second flow circuit in thermal communication with the first flow circuit and having an input connected with the output of the medium temperature evaporator and an output connected with the input of the high pressure compressor.

The transcritical refrigeration system may further comprise a low temperature heat exchanger for improvement of the efficiency of the system. The low temperature heat exchanger has a first flow circuit with an input connected to the liquid output of the receiver and an output connected to the input of the low temperature expansion valve. The low temperature heat exchanger further has a second flow circuit in thermal communication with the first flow circuit and having an input connected with the output of the low temperature evaporator and. an output connected with the input of the low pressure compressor.

It is an important advantage of the present invention that a CO₂ transcritical refrigeration system for both chilling and freezing is provided with very few parts and at a low manufacturing cost.

For improved performance of the system, the high temperature pressure reducing device may be controlled for adjustment of a desired pressure in the gas cooler. As will be further explained below with reference to the drawing, an optimum gas cooler pressure exists during transcritical operation of the system. The high temperature pressure reducing device, such as an expansion valve, may be controlled so that the gas cooler pressure attains the optimum value or approximately the optimum value.

Below the invention will be described in more detail with reference to the exemplary embodiments illustrated in the drawing, wherein Fig. 1 is a blocked schematic of a preferred embodiment of a transcritical refrigeration system according to the present invention,

Fig. 2 is a plot of a subcritical cooling cycle,

Fig. 3 is a plot of a transcritical cooling cycle, and

Fig. 4 is a plot illustrating control of gas cooler pressure. Fig. 1 shows a blocked schematic of an embodiment 10 of a CO₂ refrigeration system according to the present invention. The system 10 comprises a chilling system 12 and a freezing system 14.

The chilling system 12 has a CO₂ refrigerant flow circuit for recirculation of a CO₂ refrigerant 18, the flow circuit comprising a high pressure compressor 20 for generation of a CO₂ refrigerant flow in the direction of the arrow 22 from a low-pressure side to a high-pressure side of the compressor 20. The high pressure compressor receives low temperature CO₂ gas in a saturated state. The gas is then compressed, exiting at a superheated state. In the illustrated embodiment, the high pressure compressor is realized with one or several compressors. CO₂ gas enters the compressor 20 at an approximate pressure and temperature of 26 bar, 10 ⁰C and is compressed to approximately 90 bar, 130 °C.

The high pressure compressor 20 is connected in series with a gas cooler 24 for cooling of the CO₂ refrigerant 18 towards the ambient temperature. The gas cooler is cooled by forced air and is preferably placed outside. CO₂ gas enters the gas cooler 24 as vapour at approximately 90 bar, 130 ⁰C and is cooled to 35 ⁰C with 32 ⁰C ambient air.

The gas cooler 24 output is connected with a first flow circuit of a high temperature heat exchanger 34 that uses excess heat from the gas exiting the gas cooler 24 to preheat the gas flowing through a second flow circuit of the high temperature heat exchanger 34 for entrance into the high pressure compressor 20. CO₂ from the gas cooler 24 enters the heat exchanger 34 at an approximate pressure and temperature of 90 bar, 35 ⁰C and is cooled to 32 ⁰C remaining at approximately the same pressure. CO₂ gas flowing towards the high pressure compressor 20 enters at approximately 26 bar, -1.5 ^CC and is heated to 26 bar, 9 ⁰C. The heat exchanger 34 improves the overall efficiency of the system and further operates as a safety device that prevents liquid from entering the compressor 20. The output of the first flow circuit of the high temperature heat exchanger 34 is connected with a high temperature expansion valve 27 for pressure reduction and formation of a mixture of gaseous and liquid CO₂. In the illustrated embodiment, the CO₂ enters the valve 27 at approximately 90 bar, 32 ⁰C and exits the valve 27 as a gas-liquid mixture at 30 bar, -5.5 ⁰C.

The CO₂ liquid is collected in a receiver 26 for accommodation of the refrigerant 18. The gas- liquid mixture is separated in the receiver 26 that has separate outputs for the gas and for the liquid. In the illustrated embodiment, the gas content can be 55 % by mass.

The liquid output of the receiver 26 is connected with a medium temperature expansion valve 28 delimiting the low pressure side of the high pressure compressor 20. The expansion valve 28 regulates the amount of CO₂ liquid entering the medium temperature evaporator 30. In the illustrated embodiment, the valve 28 is controlled by feedback from the output of the evaporator 30 in such a way that super heated gas is output from the evaporator 30. In the illustrate embodiment, the CO₂ liquid enters the expansion valve at approximately 30 bar, - 5.5 ⁰C and exits at approximately 26 bar, -10 ⁰C.

The CO₂ refrigerant 18 evaporates in the medium temperature evaporator 30 connected to the output of the medium temperature expansion valve 28. The medium temperature evaporator 30 is situated in cooling furniture, e.g. keeping the temperature in the cooling furniture at app. 4 ⁰C. The evaporator receives CO₂ in liquid form and literally evaporates it to vaporous form. Warm air is blown across coils containing liquid CO₂, creating a superheated vapour to enter the high pressure compressor 20. The air blown across the coils is cooled and in turn distributed to the medium temperature cooling furniture, e.g. for meat, dairy products, etc. CO₂ liquid enters the evaporator at approximately 26 bar, -10 ⁰C and is evaporated to 26 bar, -5 ⁰C.

The output of the medium temperature evaporator 30 is connected to the second flow circuit of the high temperature heat exchanger 34 for preheating of the evaporated cold refrigerant 18 before re-entrance into the high pressure compressor 20.

Further, a gas bypass valve 32 is connected between the receiver 26 and the output side of the medium temperature evaporator 30, the gas bypass valve 32 lowering the pressure in the tubing between the output of the high temperature expansion valve 27 and the input of the medium temperature expansion valve 28 and the input of the low temperature expansion valve 42 from app. 70 bar in conventional CO₂ transcritical refrigeration systems to app. 30 bar. This leads to considerable savings in installation cost since the steel tubing (typically more than 20 m in a small installation) required in a conventional CO₂ transcritical refrigeration system can now be substituted by cupper tubing.

Still further, a freezing system 14 is booster coupled in parallel between the receiver 26 output and the medium temperature evaporator 30 output. The freezing system 14 has a low pressure compressor 38 for generation of a CO₂ refrigerant flow in the direction of the arrow 40 from a low-pressure side to a high-pressure side of the low pressure compressor 38. In the illustrated embodiment, CO₂ gas enters the compressor 38 at approximately 14 bar, -19 ⁰C and is compressed to 26 bar, 35 ⁰C. The low pressure compressor 38 may be fitted with a frequency converter in order to be capable of adapting to fluctuating capacities.

The output of the low pressure compressor 38 is interconnected with the output of medium temperature evaporator 30 and the output of the gas bypass valve 32. The gas from the low pressure compressor 38 is mixed with gas from the gas bypass valve 32 as well as the medium temperature evaporator 30. Further, the by-pass gas is flashed at the output of by- pass valve 32, and a small amount of liquid is formed, and this liquid is used to further cool the gas exiting the low pressure compressor 40 so that the gas exiting the low temperature compressor 38 can safely enter the high temperature compressor 20.

The freezing system 14 further has a low temperature heat exchanger 36 with a first flow circuit that is connected to the liquid output of the receiver 26. The output of the first flow circuit is connected to a low temperature expansion valve 42 delimiting the low pressure side of the low pressure compressor 40. Similar to the valve 28 found earlier in the system, this valve 42 regulates the amount of CO₂ liquid that can enter the low temperature evaporator 44 based on the exit conditions. Liquid CO₂ enters the expansion valve 42 at approximately 30 bar, -8 ⁰C and exits at 14 bar, -30 ⁰C. The CO₂ refrigerant 18 evaporates in the low temperature evaporator 44 connected to the output of the low temperature expansion valve 42. The low temperature evaporator 44 is situated in freezers, e.g. keeping the temperature in the freezers at app. - 18 ⁰C. Liquid CO₂ from the expansion valve 42 enters the evaporator 44 at approximately 14 bar, -30 ⁰C and leaves as a gas at 15 bar, -25 ⁰C. The output of the low temperature evaporator 30 is connected to the second flow circuit of the low temperature heat exchanger 36 for preheating of the evaporated cold refrigerant 18 before entrance into the low pressure compressor 40. Liquid enters the heat exchanger 36 at approximately 30 bar, -5 ⁰C and is cooled to 30 bar, -8 ⁰C. Gas enters at 14 bar, -25 ⁰C and is heated to 14 bar, -19 ⁰C. The heat exchanger 36 increases the COP of the system 10 and reduces the risk of liquid entering the low pressure compressor 38.

According to the invention, a CO₂ transcritical refrigeration system for both chilling and freezing is provided with very few parts and at a low manufacturing cost.

Fig. 2 illustrates subcritical operation of the system 10 in a conventional Log (p), h (enthalpy) diagram. The enthalpy H is defined by the equation: H = U + pV, where U is the internal energy, p is the pressure, and V is the volume of the system. Between point 1 and 2, the compressor 20 compresses the CO₂ refrigerant, and subsequently heat is released from the refrigerant from point 2 to 3 below the critical point 32 by condensation of the refrigerant in the gas cooler 24 at a constant pressure. The expansion from point 3 to 4 takes place at constant specific enthalpy at passage of the expansion valve 28. The heat absorption takes place in the evaporator 30 in the cooling furniture of the system 10 from point 4 to 1 at constant pressure.

Fig. 3 illustrates transcritical operation of the system 10. The most important difference between the plot of Fig. 3 and the plot of Fig. 2 is that the CO₂ refrigerant is above the critical point 32 at the high-pressure side of the compressor 14 and thus, heat is released from the refrigerant by CO₂ gas cooling in the gas cooler 18. The coefficient of performance (COP) of the system 10 is less for transcritical cycles than for subcritical cycles due to the lacking phase transition, i.e. no condensation, during heat release.

The expansion from point 3 to 4 takes place in two steps, namely from point 3 to 5, and subsequently from point 5 to 4. The valve 27 reduces the pressure from point 3 to point 5 for pressure reduction and formation of a mixture of gaseous and liquid CO₂. The CO₂ liquid is collected in the receiver 26.

The valve 27 may be controlled in such a way that the pressure in the gas cooler 18 attains a value that gives the best possible COP. This is further illustrated in Fig. 4 that illustrates transcritical operation of the system 10 with the bypass valve 32. Numbers in circles in Fig. 4 indicate the pressure, temperature and enthalpy at positions in the system 10 indicated by the same numbers in circles in the blocked schematic of Fig. 1.

In addition to the transcritical cooling cycle, Fig. 4 shows two isotherms 34, 36. It should be noted that a decrease of the gas cooler pressure at the point 3 moves the point 5 to the right by a large amount because of the low and almost horizontal slope of the isotherm 34 so that the available specific enthalpy for release in the evaporator decrease by a large amount. Since the specific enthalpy added by the compressor 14 decreases by a small amount, the resulting COP decreases by a large amount. Conversely, an increase of the gas cooler pressure at the point 3 moves the point 3 to the left by a small amount because of the steep slope of the isotherm 34 so that the available specific enthalpy for release in the evaporator increases by a small amount. Since the specific enthalpy added by the compressor 14 also increases by a small amount, the resulting COP hardly changes.

It should be noted that if the slope of the isotherm 34 is larger than the slope of the line between points 1 and 2, the COP decreases for increased gas cooler pressure. This illustrates that there is an optimum value for the gas cooler pressure that maximizes the COP, and preferably the valve 20 is adjusted in such a way that the gas cooler pressure attains, at least approximately, this optimum pressure value. Typically, the gas cooler pressure is app. 90 bar while the pressure at the low-pressure side of the compressor 14 is app. 30 bar.

A processor (not shown) may be adapted to control the valve 27 based on the temperature and pressure on the low pressure side of the compressor 20 downstream the evaporator 30 and on the pressure at the output side of the gas cooler 24 in such a way that the gas cooler pressure attains, at least approximately, its optimum pressure value.

Claims

1. A transcritical refrigeration system comprising a flow circuit for recirculation of a refrigerant, the flow circuit comprising a high pressure compressor for generation of a refrigerant flow from a low-pressure side at an input of the high pressure compressor to a high-pressure side at an output of the high temperature compressor, the output being connected to an input of a gas cooler for cooling of the refrigerant towards the ambient temperature and having an output connected with an input of a high temperature pressure reducing device for decreasing the downstream pressure and creating a mixture of gas and liquid refrigerant and having an output connected to a receiver for accommodation of the liquid refrigerant with a liquid output connected to an input of a medium temperature expansion valve with an output connected to an input of a medium temperature evaporator for evaporation of the refrigerant and having an output that is connected with the input of the high pressure compressor for recirculation of the refrigerant, and a gas bypass valve with an input connected to a gas output of the receiver and an output connected with the output of the medium temperature evaporator, the gas bypass valve lowering the pressure in tubing between the output of the high temperature expansion valve and the input of the medium temperature expansion valve and the input of the low temperature expansion valve.

2. A transcritical refrigeration system according to claim 1 , further comprising a freezing system coupled in parallel between the receiver output and the medium temperature evaporator output, the freezing system having a low pressure compressor for generation of a refrigerant flow from a low-pressure side at an input of the low pressure compressor to a high-pressure side at an output of the low pressure compressor, the output of the low pressure compressor being connected with the output of the medium temperature evaporator and the output of the gas bypass valve and the input of the high pressure compressor, a low temperature expansion valve with an input connected with the liquid output of the receiver and an output connected to an input of a low temperature evaporator for evaporation of the refrigerant generating freezing temperatures and having an output connected with the input of the low pressure compressor.

3. A transcritical refrigeration system according to claim 1 or 2, wherein the refrigerant is CO₂.

4. A transcritical refrigeration system according to claim 3, wherein

5. A transcritical refrigeration system according to any of the previous claims, further comprising a high temperature heat exchanger for improvement of the efficiency of the system and having a first flow circuit with an input connected to the output of the gas cooler and an output connected to the input of the high temperature expansion valve, and a second flow circuit in thermal communication with the first flow circuit and having an input connected with the output of the medium temperature evaporator and an output connected with the input of the high pressure compressor.

6. A transcritical refrigeration system according to any of the previous claims, further comprising a low temperature heat exchanger for improvement of the efficiency of the system and having a first flow circuit with an input connected to the liquid output of the receiver and an output connected to the input of the low temperature expansion valve, and a second flow circuit in thermal communication with the first flow circuit and having an input connected with the output of the low temperature evaporator and an output connected with the input of the low pressure compressor.