EP2754979B1

EP2754979B1 - Refrigerating plant with ejector

Info

Publication number: EP2754979B1
Application number: EP13196599.8A
Authority: EP
Inventors: Maurizio Orlandi; Claudio Ferrandi; Luca Molinaroli
Original assignee: Epta SpA
Current assignee: Epta SpA
Priority date: 2013-01-15
Filing date: 2013-12-11
Publication date: 2016-04-06
Anticipated expiration: 2033-12-11
Also published as: EP2754978A1; ITPD20130004A1; EP2754979A1; ES2581063T3

Description

Field of application

The present invention relates to a refrigerating plant with ejector,
The refrigerating plant according to the invention has applications in the refrigerating and air conditioning sectors and possibly also in the more specific heat pump sector.
In particular, the plant has applications both in refrigerated cabinets with incorporated refrigerator (known in the sector as plug-in cabinets), and in large-sized plants such as refrigerating stations serving a number of refrigerated cabinets in parallel.

State of the art

As is known, a vapour compression refrigerating plant (or heat pump)of the conventional type makes it possible to transfer heat from a cold source to a hot source by means of a refrigerant fluid operating according to a thermodynamic cycle which provides in sequence for an evaporation stage, a compression stage, a cooling stage and an expansion stage. To such purpose the plant is composed of a closed circuit comprising an evaporator, a compressor, a condenser or gas cooler and an expansion device positioned in series.
The refrigerant fluid absorbs heat from the cold source (ambient to be cooled) in the evaporator passing to the vapour state. The fluid is then brought to a higher pressure level in the compressor, to transfer heat to the hot source inside the condenser or gas cooler, to return, lastly, to the evaporator flowing through the expansion device.
The section of circuit comprised between the compressor and the inlet of the expansion device is defined as the high pressure side of the circuit, while the section of circuit comprised between the outlet of the expansion device and the inlet of the compressor is defined, instead, as the low pressure side of the circuit.
As is known, a compression plant may operate according to a sub-critical cycle or alternatively according to a trans-critical cycle.
A sub-critical cycle is when the pressure at which heat is transferred to the hot source is below the critical pressure of the refrigerant fluid. In this case, during the cooling stage the refrigerant fluid comes to find itself in (two-phase)conditions of liquid-vapour equilibrium and the heat exchanger performing such stage functions as a condenser. In the high pressure branch of the plant a univocal relationship thus exists between the pressure and the temperature.
A trans-critical cycle is when the pressure is higher than the critical pressure of the refrigerant fluid. In this case, during the cooling stage the refrigerant fluid is in super critical (single-phase) conditions and may only undergo cooling without a phase change. The heat exchanger which performs such cooling stage functions as a gas cooler and not as a condenser. A univocal relationship cannot therefore exist between the pressure and the temperature in the high pressure branch of the plant, these variables being able to assume values independently of each other.
The plant solution described above comprises an additional heat exchanger as shown in Figures 1 and 2. More specifically, the refrigerant fluid is compressed (point 2a) by the compressor C, cooled at constant pressure in the condenser/gas cooler D (point 3a) and sub-cooled by a heat exchanger E (Suction Line Heat exchanger, SLHX)to increase its refrigerant capacity (point 4a); the flow of refrigerant is throttled in a throttling device B (point 5a) and sent to the evaporator A (point 6a). In output from the evaporator the refrigerant is superheated (1) to be able to sub-cool the refrigerant in output from the condenser/gas cooler in the SLHX.
The advantages of this plant solution are as follows:

simple configuration with reduced number of components,
possibility of using inexpensive components: tube in tube SLHX and capillary tube as throttling device,
possibility of introducing a two stage compressor as primary compressor group.

However, by not providing for the presence of a receiver of liquid, which acts as storage and reservoir, this plant solution has the drawback of not permitting inclusion of a removal system of the steam formed by the throttling(hereinafter simply referred to as "flash gas"), which would permit an improvement in the performance of the cycle.
In trans-critical CO2 plants, the receiver of liquid becomes a two-phase receiver and both to avoid the danger of over pressures and to improve the energy performance of the cycle, it is common practice to remove the flash gas with a dedicated removal system which controls the pressure inside the receiver.
Generally the flash gas is bled, throttled and added to the main flow in output from the evaporator. This solution is however of limited energy efficiency.
According to a possible alternative plant solution, the flash gas is returned to the high pressure side, upstream of the condenser, by means of an auxiliary compressor, as envisaged for example in the Italian patent IT1351459 in the name of Costan S.p.A.
More specifically, as shown in Figures 3 and 4, such configuration with auxiliary compressor provides for the subdivision of the throttling process into two stages and the use of a compressor for the extraction of the flash gas vapour which is generated after the first throttling (throttling which brings the refrigerant to an intermediate pressure). The refrigerant (point 3d) passes through the condenser/gas cooler D to be cooled; in output (point 4b) it undergoes a first throttling in a back pressure valve B1 (point 5b), downstream of which a receiver F is located, in which the condition of equilibrium between vapour and liquid occurs. The two phases are separated. The liquid (point 6b) proceeds towards the evaporator A (point 7b) after being further throttled in a second back pressure valve B2, and subsequently towards the primary compressor C1 (point 1b), while the vapour is compressed in an auxiliary compressor C2 (point 8b). The outlets of the two compressors ( points 2b and 9b), are mixed before input to the condenser/gas cooler D (point 3b).
This plant solution has some advantages:

possibility of replacing the traditional systems wherein the flash gas is removed with a throttling device and brought to the conditions (1) and re-compressed in the main compressor group; therefore with an auxiliary compressor system, the main group compresses less flow than the traditional systems with a consequent energy saving.
possibility of introducing a two stage compressor as primary compressor group.

This plant solution has some drawbacks however:

compared to the single compression stage configuration it requires an additional compressor, a phase separator and two back pressure valves in place of one, with an increase in costs and plant complexity;
difficulty of application to cabinet systems with incorporated refrigerator group (hereinafter simply plug-in): in the auxiliary compressor in fact volumetric flows which may be even 10-20% of those circulating in the primary compressor group may circulate; the reduced sizes of the plug-in systems would require use of auxiliary compressors of such a small size that as of today they cannot be found on the market.

The need therefore exists in the refrigeration sector to perform a removal of flash gas in a more efficient manner from an operating point of view and in a less expensive and complex manner as regards plant design.
In general to improve the efficiency of refrigerating plants, plants provided with an ejector have been proposed.
The ejector is a machine without moving parts which can be used both as a compressor and as a pump to obtain a raising of the pressure of a fluid by supplying a fluid (of the same type or different) at different pressure and temperature conditions. The ejector works according to a basic principle, according to which when a fluid with a high momentum encounters one with a low momentum, it raises the pressure thereof. The fluid with greater momentum (high pressure) is called the primary flow or driving flow, while the fluid with lesser momentum (low pressure) is called the secondary flow or driven flow. The ejector has a structure with a first converging element, followed by a throat and then by a divergent element (diffuser). The internal energy possessed by the primary flow is transformed into kinetic energy. The effect is to lower the pressure to aspirate the secondary flow. Mixing takes place in the convergent section of the ejector and the speed of the two flows becomes uniform. Downstream, in the throat section, a normal shock wave is generated which causes a violent transformation from kinetic energy to pressure energy. The outgoing flow obtained is generally a uniform two-phase mixture. The normal shock wave modifies stagnation pressure, lowering it. This reduces the efficiency of the ejector. An alternative to the normal wave is the oblique wave which consists of a less violent transformation which generates a loss of stagnation pressure on the normal component only of the flow crossing it.
A known plant solution provides for the use of an ejector on the low pressure side (low side) to increase the pressure of the vapour in output from the evaporator thereby reducing the work of the compressor. The plant diagram of this configuration is described in figures 5 and 6. The primary flow (driving flow) in input to the ejector G is the refrigerant in output from the condenser D (gas cooler), while the secondary flow (driven flow) in input to the ejector is the refrigerant in output from the evaporator A. In this configuration, due to the presence of a two-phase liquid-vapour flow at the output of the ejector, a phase separator F needs to be positioned, which separates the saturated liquid to be sent to the back pressure valve B which feeds the evaporator A, from the saturated vapour, to be sent to the compressor C. A plant of this type is described in the British patent GB1132477 . Another plant solution provides for the use of an ejector on the high pressure side (high side) to increase the pressure of the vapour in output from the compressor thereby reducing the work of said compressor. The plant diagram of this configuration is described in Figures 7 and 8. The primary flow (driving flow) in input to the ejector G is the refrigerant in output from a pump P fed by a fraction of refrigerant (in liquid phase in the case of a sub-critical work cycle, otherwise gaseous for a trans-critical work cycle) in output from the condenser D (gas cooler in the case of a trans-critical work cycle), while the secondary flow (driven flow) in input to the ejector G is the vapour in output from the compressor C. In this configuration, an active component such as the pump P must be provided to enable the primary flow to effectively drive the secondary flow. A plant of this type is described in the US patent US20070101760 .
Plant solutions may also be hypothesised wherein in a simple single stage refrigerating cycle with or without heat exchanger SLHX (Suction Line Heat exchanger)an ejector has been introduced as a pressure recoverer, to reduce the compression ratios developed by the compressor to reduce the consumption of the cycle. Currently none of the solutions proposed have found a practical application in marketed products. Among the main causes is the fact that the ejector is a static device, in other words it has an optimal project design to which predefined input flow (primary and secondary) conditions correspond. Deviations from these optimal conditions lead to a reduction in the efficiency of the ejector and thus of the benefit to the refrigerating cycle. A typical example is the modification of the output temperature from the condenser/gas cooler following variations of the environmental conditions in which the refrigerating plant works.

Presentation of the invention

Consequently, the purpose of the present invention is to eliminate or at least attenuate at least some drawbacks of the prior art mentioned above, by making available a refrigerating plant with ejector, which upon a variation of the operating conditions of use of the plant permits an efficient use of the ejector to increase the pressure of the refrigerant so as to reduce the compression ratios developed by the compressor and by so doing, reduce the consumption of the cycle.
A further purpose of the present invention is to make available a refrigerating plant with ejector which is simple to make as regards construction and operatively simple to run.

Brief description of the drawings

The technical characteristics of the invention, according to the aforementioned purposes, can be seen clearly from the contents of the following claims and the advantages thereof will be more clearly comprehensible from the detailed description below, made with reference to the attached drawings, showing one or more embodiments by way of non-limiting examples, wherein:

Figures 1 and 2 respectively show a simplified diagram of a plant and the relative thermodynamic cycle in a pressure-enthalpy P-h diagram of a vapour compression refrigerating plant of the traditional type, in currently used plug-in cabinets;
Figures 3 and 4 respectively show a simplified diagram of a plant and the relative thermodynamic cycle in a pressure-enthalpy P-h diagram of a known vapour compression refrigerating plant with removal of the flash gas by means of the auxiliary compressor;
Figures 5 and 6 respectively show a simplified diagram of a plant and the relative thermodynamic cycle in a pressure-enthalpy P-h diagram of a known vapour compression refrigerating plant with ejector on the low pressure side;
Figures 7 and 8 respectively show a simplified diagram of a plant and the relative thermodynamic cycle in a pressure-enthalpy P-h diagram of a known vapour compression refrigerating plant with ejector on the high pressure side;
Figures 9 and 10 respectively show a simplified diagram of a plant and the relative thermodynamic cycle in a pressure-enthalpy P-h diagram of a vapour compression refrigerating plant with ejector as pressure recoverer according to the invention.

The elements or parts of elements common to the embodiments described below will be indicated using the same reference numerals.

Detailed description

With reference to the figures 9 and 10, reference numeral 200 globally denotes the refrigerating plant with ejector according to the invention.
In particular the plant 200 - upon a variation of the operating conditions of use of the refrigerating plant (i.e. temperature at the condenser and temperature at the evaporator) - permits an efficient use of the ejector as a pressure recoverer, to reduce the compression ratios developed by the compressor and thus reduce the consumption of the cycle.
The refrigerating plant 200 operates with a refrigerant according to a vapour compression cycle. The cycle may be either sub-critical or trans-critical. In particular CO 2 may be used as the refrigerant.
According to a general embodiment of the invention, shown in the appended Figures 9 and 20, the plant 200 comprises a main circuit 200A and such main circuit 200A comprises:

a condenser 210;
an expansion device 211 positioned downstream of the condenser 210;
an evaporator 214 positioned downstream of the second expansion device 113;
compression means 215 which are positioned downstream of the evaporator 214 and comprising a first low pressure compression stage 215b fluidically connected to the evaporator 214 and a second high pressure compression stage 215a fluidically connected to the condenser 210.

Preferably, the expansion device 211 is composed of a back pressure valve.
The plant 200 comprises an ejector 216 positioned between the two compression stages 215a, 215b.
The ejector 216 is of the converging-diverging type. The structure and functioning of the ejector are known to a person skilled in the sector and will not therefore be described in detail.
The ejector 216 comprises a first inlet 216a for a driving flow, a second inlet 216b for a driven flow and an outlet 216c for ejection of the mixture of the two flows.
As shown in Figure 9, the ejector 216 is fluidically connected to the first low pressure stage 215b at the second inlet 216b and to the second high pressure stage 215a at the outlet 216c.
The plant 200 further comprises:

a receiver of liquid 212 positioned in the main circuit between the outlet 216c of the ejector 216 and the second high pressure stage 215a; in the receiver 212 the refrigerant ejected by the ejector separates into the liquid phase and the vapour phase; and
a secondary branch 200B which connects the receiver 212 in parallel to the first inlet 216a of the ejector 216 and comprises at least one pump 221 which recirculates to the liquid phase to the first inlet of the ejector 216; the vapour phase of the refrigerant is aspirated by the second high pressure stage 215a of the compression means.

Operatively, the ejector 216 defines a third compression stage, intermediate between the two low pressure 215b and high pressure 215a compression stages.
Thanks to the invention, the ejector 216 works between two pressures, that is that of the driving flow and that of the driven flow, which are intermediate to the pressure of the evaporator 214 and to that of the condenser 210. Such two pressures correspond to the pressures impressed on the flows by the pump 221 and by the first low pressure compression stage 215b. Such two pressures are thus adjustable, acting respectively on the pump and on the first compression stage 215b.
This way, it is always possible to make the ejector work at fixed and not variable working conditions. In particular, it is thus possible to make the ejector (in itself a static device) work at optimal design conditions to which predefined conditions of the driving flow (primary) and driven (secondary) flow in input correspond. Thanks to the invention, for example modifications of the temperature in output from the condenser following variations of the environmental conditions in which the refrigerating plant works do not make the ejector deviate from the optimal conditions, thus avoiding reductions of the efficiency of the ejector and thus of the benefits to the refrigerating cycle.
Preferably, the plant 200 comprises a heat exchanger 217 which thermally connects the section of secondary branch between the receiver 212 and the pump 221 with the section of main circuit comprised between the evaporator 214 and the first low pressure compression stage 215b. This gives the certainty of pumping liquid, and not liquid and vapour, into the pump.
Advantageously, the compression means 215 are composed of a single two-stage compressor 215, the two stages of which define said first low pressure stage 215b and said second high pressure stage 215a.
Alternatively, the compression means may be composed of two separate primary compressors 215a, 215b, of which a first compressor 215b defines the aforesaid first low pressure stage and a second compressor 215a defines the aforesaid second high pressure stage.
The functioning of the plant 200 with reference to figures 9 and 10 will be now described in detail. The alphanumerical references from 11 to 111 identify the various sections of the plant in the pressure-enthalpy diagram P-h of Figure 20.
The two-phase flow in output from the ejector (point 31) enters the receiver of liquid 212 in which the flow separates into the liquid phase and the gas phase; the gas (point 41) is compressed at the second compressor stage 215a (point 51) and enters the condenser 210, which it comes out of (point 61) to be throttled in the back pressure valve 211 (point 71). The liquid (point 91), after going through the heat exchanger 217 (point 101), goes through the pump 221 (point 111) which increases the pressure and is used as a driving flow in the ejector 216 for the gas in output from the first compression stage 215b (point 21). The main flow enters the evaporator 214 (point 71), to then enter the heat exchanger 217 (point 81) and, subsequently, the first compression stage 215b (point 11).
The plant 200 according to the invention makes the ejector work at constant pressure conditions, unlike the solutions of the prior art mentioned in the introduction.
Compared to the solutions of the prior art without ejector, the compressor group works with lower pressure differences, with a consequent saving of energy.
The plant 200 may be applied both in plug-in cabinets (small sized plants) and in large sized systems (refrigerating stations).
The plant 200 according to the invention, by dividing the pressure difference into 3 differences, is particularly useful for those refrigerating systems presenting a wide pressure difference.
The invention makes it possible to achieve several advantages which have been expounded in the description.
The refrigerating plant 200 with ejector according to the invention upon a variation of the operating conditions of use of the plant permits an efficient use of the ejector as a pressure recoverer, to reduce the compression ratios developed by the compressor and thus reduce the consumption of the cycle.
The refrigerating plants 200 is constructionally simple to make and operatively simple to run.
The invention thus conceived thereby achieves the intended objectives.
Obviously, its practical embodiments may assume forms and configurations different from those described while remaining within the scope of protection of the invention. Furthermore, all the parts may be replaced with technically equivalent parts and the dimensions, shapes and materials used may be varied as required.

Claims

Refrigerating plant with ejector, operating with a refrigerator according to a vapour compression cycle and comprising in a main circuit (200A): - a condenser (210); - an expansion device (211) positioned downstream of the condenser (210); -an evaporator (214) positioned downstream of the expansion device (211); -compression means (215) which are positioned downstream of the evaporator (214) and comprise a first low pressure compression stage (215b) fluidically connected to the evaporator (214) and a second high pressure compression stage (215a) fluidically connected to the condenser (210);
comprising:
- an ejector (216) positioned between the two compression stages (215a, 215b), which comprises a first inlet (216a) for a driving flow, a second inlet (216b) for a driven flow and an outlet (216c) for ejection of the mixture of the two flows, said ejector (216) being fluidically connected to the first low pressure stage (215b) at the second inlet (216b) and to the second high pressure stage (215a) at the outlet (216c) ;

- a receiver of liquid (212) positioned in the main circuit between the outlet (216c) of the ejector (216) and the second high pressure stage (215a), in said receiver (212) the refrigerant ejected by the ejector separating into the liquid phase and the vapour phase;

- a secondary branch (200B) which connects the receiver (212) in parallel to the first inlet (216a) of the ejector (216) , the vapour phase of the refrigerant being aspirated by the second high pressure stage (221) of the compression means; characterized in that the secondary branch (200B) comprises at least one pump (221) which recirculates the liquid phase at the first inlet of the ejector (216).
Plant according to claim 1, wherein the compression means are composed of a single two-stage compressor (215), the two stages defining said first low pressure stage (215b) and said second high pressure stage (215a).
Plant according to claim 1, wherein the compression means are composed of two separate primary compressors (215a, 215b), of which a first compressor (215b) defines said first low pressure stage and a second compressor (215a) defines said second high pressure stage.
Plant according to one or more of the claims from 1 to 3, comprising a heat exchanger (217) which thermally connects the section of the secondary branch (200B) between the receiver (212) and the pump (221) with the section of the main circuit (200A) comprised between the evaporator (214) and the first low pressure compression stage(215b).