WO2021051194A1

WO2021051194A1 - Low compression ratio refrigeration system with low-pressure booster

Info

Publication number: WO2021051194A1
Application number: PCT/CA2020/051243
Authority: WO
Inventors: Benjamin Joseph KUNGL
Original assignee: Oxford Gas Compression Systems Inc.
Priority date: 2019-09-16
Filing date: 2020-09-16
Publication date: 2021-03-25
Also published as: CA3151295A1; US20220341632A1; EP4031816A4; EP4031816A1

Abstract

A refrigeration system, according to the present invention, has a standard refrigeration circuit, with a condenser connected to a low temperature evaporator and a medium temperature evaporator by a high-pressure liquid line, which are in turn connected to a compressor by a low-pressure vapour return line and a medium-pressure vapour return line, and the compressor is connected to the condenser by a high-pressure vapour line. A low-pressure booster is connected on the low-pressure vapour return line operating at a low compression ratio to boost the pressure of the low-pressure vapour return line to substantially the same pressure as the medium-pressure vapour return line.

Description

LOW COMPRESSION RATIO REFRIGERATION SYSTEM WITH LOW-

PRESSURE BOOSTER

Field of the Invention [0001] The present invention relates to refrigeration systems, in particular, to a refrigeration system that operates at low compression ratios using a low-pressure booster.

Background

[0002] Refrigeration systems, based on a liquid/vapour cycle, are widely used in a variety of residential, commercial, and industrial applications. Larger, more complex refrigeration systems, such as those in grocery stores, require significant amounts of energy to operate, resulting in a significant cost to the operation. Many attempts to increase the efficiency of such systems have included adding hydraulic pumps, sometimes called boosters, on the high-pressure, liquid side of the system. These high-pressure boosters increase the pressure of the liquid refrigerant, thereby increasing its heat absorption capacity, before it arrives at the expansion valve and into the evaporator. As a result, the overall amount of cooling in the evaporator phase is increased.

[0003] However, boosting the pressure on the liquid side of a refrigeration system is problematic. High refrigerant leak rates are a common problem, which results in increased cost of operation and damage to the environment. Reliability can also be impacted, due to the generation of refrigerant vapour in the liquid line, known as flash gas. This occurs because the liquid refrigerant on the high-pressure, liquid side of the system is at or close to saturation and small drops in pressure can cause the liquid refrigerant to evaporate. When flash gas enters the impeller of the hydraulic pump it results in cavitation and cause severe damage to the pump. If a booster fails, it causes a massive drop in pressure in the liquid line. [0004] Since the liquid refrigerant is at or near saturation, cavitation can be caused by the hydraulic pump. This occurs because, as the pump increases the downstream pressure in the liquid line, the pulling effect of the pump can decrease the pressure at the inlet, causing evaporation of flash gas and cavitation.

[0005] Other attempts to introduce boosters into a refrigeration system have suffered from the limitations of specific types of booster technology, such as piston compressors. Every type of compressor has upper and lower pressure differential limits between the compressor’s high and low-pressure sides. If the pressure differential is below the lower limit of the compressor, it is unable to create a compression seal and thus provide compression. Conversely, if the pressure differential is above the upper limit, the amount of blow by, or re-expansion of gases, reduces the effectiveness and efficiency of the compressor. [0006] Previous attempts to provide a booster on the low-pressure, vapour side of the system, using a piston compressor have proven inefficient or ineffective, due to the limitations of piston compressors. Piston compressors have a valve plate on the piston, which seals with pressure differential. Accordingly, piston compressors are required to operate at a high pressure differential, and therefore higher compression ratio, relative to other types of compressors. This results in an over-pressurization of the vapour line, reducing or preventing flow of other vapour lines. In order to address this over-pressurization in existing refrigeration systems, pressure limiting valves are used to reduce the pressure created by the compressor. However, this causes overheating of the booster, which can lead to failure.

[0007] Recently, the move away from traditional hydrofluorocarbon (HFC) refrigerants to more environmentally friendly alternative refrigerants, such as low-pressure HFO blends, has created an additional challenge to overcome in providing a booster on the low-pressure, vapour side of the system. This is a result of the low density of the newer refrigerants, compared to traditional refrigerants. A typical low-pressure HFO blend refrigerant has about half of the density of standard refrigerants. [0008] Accordingly, there is a need for an improved refrigeration system that improves the efficiency of the system, without the drawbacks associated with existing boosters.

Summary of the Invention [0009] A refrigeration system, according to the present invention, has a standard refrigeration circuit, with a condenser connected to a low temperature evaporator and a medium temperature evaporator by a high-pressure liquid line, which are in turn connected to a compressor by a low- pressure vapour return line and a medium-pressure vapour return line, and the compressor is connected to the condenser by a high-pressure vapour line. A low-pressure booster is connected on the low-pressure vapour return line operating at a low compression ratio to boost the pressure of the low-pressure vapour return line to substantially the same pressure as the medium-pressure vapour return line.

[0010] In another embodiment, the low compression ratio of the low-pressure booster is between 1.8:1 and 3:1. In another embodiment, the low compression ration is between 1.8:1 and 2.8:1, and preferably about 2:1.

[0011] In another embodiment, the low-pressure booster is a scroll compressor, such as a double-sided scroll compressor or a magnetic drive oil-less scroll compressor. Alternatively, the low-pressure booster may be a turbine.

[0012] In another embodiment, a medium-pressure manifold is connected to the low-pressure vapour return line, after the low-pressure booster, and connected to the medium-pressure vapour return line to receive and combine refrigerant therefrom. The medium-pressure manifold is also connected to the compressor to supply the combined refrigerant from the low and medium- pressure vapour return lines thereto. [0013] A double-sided scroll compressor, according to the present invention, has a motor that drives a crank shaft having first and second ends which extend outwardly from opposing sides of the motor. A first orbiting scroll is attached at a first end of the crank shaft and is engaged with a first stationary scroll. A second orbiting scroll is attached at a second end of the crank shaft and is engaged with a second stationary scroll.

Brief Description of the Drawings

[0014] In order that the invention may be more clearly understood, a preferred embodiment thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:

[0015] Figure 1 is a schematic view of a refrigeration system, according to the present invention.

[0016] Figure 2A is a schematic view of the booster/compressor portion of the refrigeration system, shown in Figure 1. [0017] Figure 2B is a schematic view of an alternative configuration for the booster/compressor portion of the refrigeration system, shown in Figure 1

[0018] Figure 3 is a cross-sectional view of a double-sided scroll compressor, according to the present invention. Description of the Invention

[0019] The refrigeration system, according to the present invention, has a low-pressure booster, which operates at a low compression ratio, to boost the pressure of the low-pressure vapour return line, so that all of the refrigerant vapour entering the medium-pressure compressor is supplied at the same medium pressure. The overall efficiency of the system is thereby improved, since there is a lower pressure differential across the compressor, which therefore can operate at a low compression ratio. The low compression ratio allows the compressor to run with less re-expansion and less blow by in the compressor, itself. Additionally, less energy is required and less heat is generated to create the compression. The traditional sources of secondary heat, being the heat of the compressor and motor heat, which reduce the efficiency of refrigeration systems, are reduced by the use of the low-pressure booster of the present invention.

[0020] The system may be run with an overall lower amount of refrigerant, while providing the same amount of cooling. The efficiency gains reduce the secondary heat entering the system, which in turn lowers the total amount of heat the system is required to release at the condenser phase. Additionally, when operating at the same refrigerant flow rate (m³/hr), a significantly lower amount of overall energy is required to produce the same amount of cooling as other liquid/vapour refrigeration systems. Some embodiments of the present invention have been shown to reduce the electricity consumption of a commercial refrigeration system by up to 80%. The present invention also permits a simpler design of the refrigeration system by eliminating the need for many of the special components otherwise required, including control valves, bypass valves, pressure-limiting valves, and oil mitigation or oil balancing systems.

[0021] The basic operating principles of a liquid/vapour refrigeration circuit are well understood by those skilled in the art and are not set out in detail herein. Various refrigerants may be used with the refrigeration system, according to the present invention, for example, all HFC refrigerants, such as R-404a, 507 407(a,b,c), and all HFO refrigerants, such as R-513a, are suitable refrigerants. Other refrigerants, suitable for a liquid/vapour refrigeration cycle, may also be used.

[0022] As shown in Figure 1, the refrigeration system has a condenser 1, a low-temperature evaporator 2, a medium-temperature evaporator 3, a low-pressure booster 4, and a compressor 5. The standard refrigeration circuit is made up of the condenser 1, which is connected to the low- temperature evaporator 2 and the medium-temperature evaporator 3 by a high-pressure liquid line 6. The low and medium-temperature evaporators 2 and 3 are connected to the compressor 5 by a low-pressure vapour return line 7 and a medium-pressure vapour return line 8, respectively. The compressor 5 is connected to the condenser 1 by a high-pressure vapour line 9, completing the standard refrigeration circuit.

[0023] The standard refrigeration circuit may have a number of optional or additional components, including a plurality of either or both of the low or medium-temperature evaporators 2 and 3, and compressors 5, depending on the requirements of the particular application. The present description will refer to a single low-temperature evaporator 2, medium-temperature evaporator 3, and compressor 5, but it should be understood by a person skilled in the art that the number of any of these components may vary, depending on the system requirements. In most cases, the low and medium-pressure return vapour lines 7 and 8 feed into a medium-pressure manifold 10, which in turn feeds the combined refrigerant from both lines into the compressor 5. The medium-pressure manifold 10, may also feed into an accumulator 11 before the medium- pressure refrigerant is drawn from the accumulator 11 into the compressor 5.

[0024] The liquid refrigerant from the condenser 1 is fed into a receiver 12 along the high- pressure liquid line 6, before continuing on to the low and medium-temperature evaporators 2 and 3. Optionally, the receiver 12 may feed the liquid refrigerant in the high-pressure liquid line 6 through one or more subcooling stages. This lowers the temperature of the liquid refrigerant before it passes on to the expansion valves 13 and the low and medium-temperature evaporators 2 and 3. One potential subcooling stage may take place in the condenser 1, by way of a subcooling loop 14 within the condenser 1. Alternatively, or additionally, the high-pressure liquid line may pass through a subcooling coil 15 in the accumulator 11. [0025] The low-pressure booster 4 is located on the low-pressure vapour return line 7, between the low-temperature evaporator 2 and the medium-pressure manifold 10, where one is used. It is fed with low-pressure refrigerant vapour from the low-temperature evaporator 2 and boosts the pressure of the refrigerant vapour in the low-pressure vapour return line 7 to about the same pressure as the refrigerant vapour in the medium-pressure vapour return line 8. It then enters the medium-pressure manifold 10 at about the same pressure as the refrigerant vapour from the medium pressure vapour return line 8. The low-pressure booster 4 may increase the pressure of the refrigerant vapour in the low-pressure vapour return line 7 slightly higher than the pressure in the medium-pressure vapour return line 8, but it is preferable that the pressure not be lower. The compressor 5 is therefore only required to increase the pressure of the refrigerant vapour from the pressure in the medium-pressure vapour return line 8 to the pressure in the high-pressure vapour line 9. As a result, the low-pressure booster 4 and, in certain conditions the compressor 5, can operate at a low compression ratio, since the refrigeration system incrementally increases the pressure of the refrigerant in a two-step process from low-pressure to medium-pressure and then from medium-pressure to high-pressure.

[0026] In traditional refrigerant systems, separate compressors are used to compress the refrigerant vapour from the low and medium-pressure lines, independently. As a result, the compressors boosting the low-pressure line(s) must operate with a very high compression ratio, due to the pressure differential between their low and high-pressure sides, reducing their efficiency and increasing the amount of secondary heat added to the system. In a conventional low-pressure compressor, the compressor is required to operate at a compression ratio of about 8.5:1. Because of this, the amount of heat leaving the compressor is about 1.6 times the amount of heat entering. This means that if the low-temperature evaporator 2 is absorbing 10,000 btu/hr, when a conventional low-pressure compressor is finished compressing the low-pressure refrigerant, it will pass it on to the condenser with 16,000 btu/hr of heat to remove from the refrigeration system. This secondary heat, added by a low-pressure compressor operating at a high compression ratio, significantly reduces the efficiency of the system.

[0027] By using the low-pressure booster 4 to boost the pressure of the refrigerant vapour in the low-pressure vapour return line 7, both the compressor 5 and the low-pressure booster 4 are able to operate at lower compression ratios, because of the lower pressure differentials between their low and high-pressure sides. As a result, both operate more efficiently, lowering the energy cost associated with operating the refrigeration system.

[0028] In preferred embodiments of the present invention, the low-pressure booster 4 operates at a compression ratio of between 1.8:1 and 3:1, more preferably between 1.8:1 and 2.8:1, and most preferably about 2:1. The low-pressure booster 4 is thereby able to operate with very low motor wattage, which results in an input-to-output heat factor multiplier of 1.1, due to the very low pressure differential between its low and high-pressure sides. The compressor 5 preferably operates at a compression ratio of between 1.8:1 and 5:1, more preferably 2.8:1. The compressor 5 is controlled, depending on the circumstances such as ambient temperature, to operate at a lower compression ratio when less compression is required to produce the necessary amount of cooling conventional medium-pressure compressors will normally operate at a compression ratio around 5:1, but do not vary in response to cooling demand. This is because conventional systems are designed to operate at fixed high pressure differentials and do not have the flexibility to reduce their compression ratios and pressure differentials in response to lower cooling demand. [0029] Because the present system operates at low compression ratios, small variations in refrigerant pressure have a greater impact on the operation of the system. The optimum operating pressure of any system depends on the application and the refrigerant used. Nonetheless, the present system requires a relatively consistent refrigerant pressure compared to conventional systems. For example, in a conventional system, with a medium-pressure refrigerant having an optimum low-side operating pressure of 50psi, the conventional system will experience pressure variations of up to +/- 18psi. The present system requires refrigerant pressure to be maintained within +/- 20% of the optimum operating pressure (lOpsi in the above example), more preferably within 15%, and most preferably within 10%. Pressure variations beyond 20% from the optimum operating pressure can trigger the system’s pressure safety.

[0030] Not all types of compressors are able to operate at such low compression ratios and require a higher pressure differential between their low-pressure and high-pressure sides to form a compression seal. A scroll-type compressor is particularly well-suited for use as the low- pressure booster 4 in the present invention, because it is able to operate at low compression ratios, with a very low pressure differential between its low and high-pressure sides. It is thereby able to provide the small boost in pressure required to bring the pressure of the refrigerant vapour in the low-pressure vapour return line 7 up to the same pressure as the refrigerant vapour in the medium-pressure vapour return line 8. Additionally, the compression section of a scroll-type compressor is able to be switched off in an unsealed, or unseated, state and provide very little resistance to the flow of refrigerant vapour. For example, this allows the low-temperature circuit to operate in a medium-temperature setting, if necessary, by adjusting the temperature set point and leaving the low-pressure booster 4 switched off.

[0031] Optionally, the low-pressure booster 4 may be a double-sided scroll compressor 18, due to its ability to provide higher volume displacement to account for the low density of many modem refrigerants. The double-sided scroll compressor 18 provides double the volume displacement of a traditional scroll-type compressor, while using the same single motor 19. An existing scroll-type compressor model, designed for use in a refrigeration system with standard refrigerants, may also be modified with a second scroll to operate efficiently in a modem refrigeration system that uses newer, lower density refrigerants, such as low-pressure HFO blends. [0032] As shown in Figure 3, the double-sided scroll compressor 18 has a motor 19 that drives a crank shaft 20 to operate the scroll in the same manner as traditional scroll-type compressors. The crank shaft 20 has first and second ends 20a and 20b which extend outwardly from opposing sides of the motor 19. A first orbiting scroll 21a is attached at a first end 20a of the crank shaft 20 and is engaged with a first stationary scroll 22a. A second orbiting scroll 21b is attached at a second end 20b of the crank shaft 20 and is engaged with a second stationary scroll 22b. The first orbiting scroll 21a and the first stationary scroll 22a operate together as a first scroll 23, while the second orbiting scroll 21b and the second stationary scroll 22b operate together as a second scroll 24. Each of the first and second scrolls 23 and 24 have a suction port and a discharge port. Refrigerant enters the scroll 23 or 24 through its respective suction port, is compressed within the scroll, and leaves the scroll via its respective discharge port.

[0033] Preferably, the double-sided scroll compressor 18 is a horizontal type compressor, due to the nature of oil return in a refrigeration system. The double-sided scroll compressor 18 has an oil-free sumpless design, which prevents oil from accumulating in the scroll or motor housing. The bearings in the double-sided scroll compressor 18 do not require oil lubrication. The oil that is contained in the mass flow of refrigerant through the refrigerant system flows through the double-sided scroll compressor 18 and out the discharge ports, without collecting in the motor area. This provides equal flow and prevents any low pockets of oil that could otherwise accumulate in a vertical design.

[0034] The double-sided scroll compressor 18 provides the flexibility to operate in two different refrigerant system configurations. As shown in Figure 2a, the first scroll 23 of the double-sided scroll compressor 18 can operate as the low-pressure booster 4, while the second scroll 24 operates as the compressor 5. The low-pressure vapour return line 7 is fed into the suction port of the first scroll 23, which boost the pressure of the refrigerant to the pressure of the refrigerant in the medium-pressure vapour return line 8 and feeds it into the medium-pressure manifold 10 or directly into the accumulator 11. The medium-pressure refrigerant from the accumulator 11 is fed into the suction port of the second scroll 24, which boosts the pressure of the refrigerant to the pressure of the high-pressure vapour line 9 and feeds it into the condenser 1. This provides additional efficiency and less secondary heat in applications where higher volume displacement is not essential, as this configuration uses a single motor where a traditional configuration uses two motors to accomplish the same compression. The heat rejected in a refrigeration system is comprised of the heat absorbed in the evaporator, plus the heat of compression and motor heat. This configuration does the work of two stages of compression with one motor, reducing losses to motor heat and increasing efficiency.

[0035] Alternatively, as shown in Figure 2b, the low-pressure vapour return line 7 may split into two lines, which are fed into the suction ports of the first and second scrolls 23 and 24 of a double-sided scroll compressor 18, which operates as the low-pressure booster 4. The pressure of the refrigerant is boosted to the pressure of the refrigerant in the medium-pressure vapour return line 8 and is fed into the medium-pressure manifold 10 or the accumulator 11. Another double-sided scroll compressor 18 operates as the compressor 5. This second double-sided scroll compressor 18 works in the same way to increase the pressure of the refrigerant vapour from the pressure in the medium-pressure vapour return line 8 to the pressure in the high-pressure vapour line 9.

[0036] Whether a double-sided scroll compressor 18 is used, or a traditional scroll-type compressor is used, the motor 19 may be any suitable type of motor, including a traditional alternating current motor or direct current motor. Preferably, the motor 19 is an electronically commutated motor (ECM). The use of an ECM improves mass flow of refrigerant through the system, because it provides smoother and non-surging drive to the compressor. An ECM also allows the compressor to be ramped up or down to match the load profiles within the system. Full speed control is available with an ECM, without the need for a secondary electrical control device, such as a variable frequency drive. An ECM also reduces or eliminates lock rotor start ups, high cycling events, and peak energy penalties from start-up events charged by electricity providers. Alternatively, where power is being generate by solar panels or other direct current sources, the system can use direct current motors to increase the efficiency by eliminating the need to convert DC to AC power or vice versa.

[0037] An oil separator 16 will typically be connected on the high-pressure vapour line 9, between the compressor 5 and the condenser 1. Lubricating oil is supplied to the compressor 5 and, optionally, to the low-pressure booster 4, by oil line 17 from the oil separator 16. If a horizontal, or oil-free sumpless, design of double-sided scroll compressor 18, as described herein, is used as the low-pressure booster 4, then no lubricating oil is required by the low-pressure booster 4. During operation, lubricating oil is introduced into the refrigerant vapour in the compressor 5 and, depending on the type used, the low-pressure booster 4. The oil separator 16 removes this lubricating oil as it passes through the high-pressure vapour line 9, before reaching the condenser 1. [0038] Alternatively, the need for the oil separator 16 and oil line 17 can be avoided by using a magnetic drive oil-less scroll compressor as the low-pressure booster 4 and the compressor 5. With a magnetic drive scroll compressor, there is no motor in the refrigerant flow, and so oil is not introduced into the refrigerant flow. With no oil in the refrigerant flow, there is no need for the oil separator 16 and oil line 17. [0039] A further alternative to a traditional scroll compressor is to use a turbine as the low- pressure booster 4 or the compressor 5 or both. Because the system operates at very low pressure differentials between the low-pressure side and high-pressure side of both the low-pressure booster 4 and the compressor 5, a turbine is able to generate the required low compression ratios very efficiently. With a system operating at higher, conventional compression ratios, turbines would suffer from too much blow by, due to the high pressure differential.

[0040] The present invention has been described and illustrated with reference to an exemplary embodiment, however, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as set out in the following claims. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed herein.

Claims

What is Claimed is:

1. A refrigeration system, with a standard refrigeration circuit having a condenser connected to a low temperature evaporator and a medium temperature evaporator by a high- pressure liquid line, which are in turn connected to a compressor by a low-pressure vapour return line and a medium-pressure vapour return line, and the compressor is connected to the condenser by a high-pressure vapour line, comprising: a low-pressure booster on the low-pressure vapour return line operating at a low compression ratio to boost the pressure of the low-pressure vapour return line to substantially the same pressure as the medium-pressure vapour return line.

2. The refrigeration system of claim 1, wherein the low compression ratio of the low- pressure booster is between 1.8:1 and 3:1.

3. The refrigeration system of claim 1, wherein the low compression ratio of the low- pressure booster is between 1.8:1 and 2.8:1.

4. The refrigeration system of claim 1, wherein the low compression ratio of the low- pressure booster is about 2:1.

5. The refrigeration system of claim 2, wherein the low-pressure booster is a scroll compressor.

6. The refrigeration system of claim 5, wherein the scroll compressor is a double-sided scroll compressor having a first scroll and a second scroll.

7. The refrigeration system of claim 6, wherein the first scroll is connected to the low- pressure vapour return line and the medium-pressure vapour return line to operate as the low- pressure booster, and wherein the second scroll is connected to the medium-pressure vapour return line and the high-pressure vapour line to operate as the compressor.

8. The refrigeration system of claim 6, wherein the low-pressure vapour return line is split into two lines one of which is connected to the first scroll and the other of which is connected to the second scroll.

9. The refrigeration system of claim 5, wherein the scroll compressor is a magnetic drive oil-less scroll compressor.

10. The refrigeration system of claim 2, wherein the low-pressure booster is a turbine.

11. The refrigeration system of claim 2, comprising a medium-pressure manifold connected to the low-pressure vapour return line, after the low-pressure booster, and the medium- pressure vapour return line to receive and combine refrigerant therefrom, and connected to the compressor to supply the combined refrigerant thereto.

12. The refrigeration system of claim 11, comprising an accumulator connected between the medium-pressure manifold and the compressor.

13. The refrigeration system of claim 2, comprising one or more subcooling stages on the high-pressure liquid line.

14. The refrigeration system of claim 13, wherein the one or more subcooling stages comprise a subcooling loop in the condenser.

15. The refrigeration system of claim 13, wherein the one or more subcooling stages comprise a subcooling coil in the accumulator.

16. A double-sided scroll compressor, comprising a motor that drives a crank shaft having first and second ends which extend outwardly from opposing sides of the motor, a first orbiting scroll attached at the first end of the crank shaft and engaged with a first stationary scroll, and a second orbiting scroll attached at the second end of the crank shaft and engaged with a second stationary scroll.