EP0149413A2

EP0149413A2 - Method and apparatus for refrigeration

Info

Publication number: EP0149413A2
Application number: EP84630223A
Authority: EP
Inventors: Dori Hershgal; Mordechai Sokolov
Original assignee: GALIL ADVANCED TECHNOLOGIES Ltd
Current assignee: GALIL ADVANCED TECHNOLOGIES Ltd
Priority date: 1984-01-12
Filing date: 1984-12-31
Publication date: 1985-07-24
Also published as: EP0149413A3; AU3753685A

Abstract

A method and apparatus are described for refrigeration according to a refrigeration cycle wherein a working fluid is evaporated in a generator (11) at high pressure, the working fluid vapor is expanded and formed into a vapor stream entraining working fluid vapor from an evaporator (13), the vapor stream is condensed to a liquid, a part of the condensed liquid is returned to the generatore (11) for evaporation at high pressure, and another part of the condensed liquid is expanded and passed to the evaporator (13) for entraining in the vapor stream. According to the novel method and apparatus, the entrainment of the vapor in the vapor stream is enhanced by increasing the pressure of the vapor from the evaporator before the vapor is entrained in the vapor stream. This has been found to significantly improve the Coefficient of Performance (C.O.P.) of the refrigeration cycle.

Description

The present invention relates to a method and apparatus for refrigeration. The invention is particularly applicable to the known ejector refrigeration cycle, and is therefore described below with respect to this application.
An object of the present invention is to provide a novel method and apparatus which improves the overall Coefficient of Performance (C.O.P.) of a refrigeration cycle, particularly of an ejector refrigeration cycle. An increase in C.O.P. indicates an efficient system which requires less energy input for a given refrigeration load.
According to a broad aspect of the present invention, there is provided a method for refrigeration according to a refrigeration cycle wherein a working fluid is evaporated in a generator at high pressure, the working fluid vapor is expanded and formed into a vapor stream entraining working fluid vapor from an evaporator, the vapor stream is condensed to a liquid, a part of said condensed liquid is returned to the generator for evaporation at high pressure, and another part of said condensed liquid is expanded and passed to the evaporator for entraining in the vapor stream; the improvement wherein the entrainment of the vapor stream is enhanced by increasing the pressure of the vapor from the evaporator before said vapor is entrained in the vapor stream.
Several embodiments of the invention are described below for purposes of example. In one embodiment the pressure of the vapor from the evaporator is increased by compressing the vapor before it is entrained in the vapor stream; and in a second described embodiment, the pressure is increased by heating the vapor before it is entrained in the vapor stream. The latter is preferably done by an arrangement wherein a second working fluid is evaporated in a second evaporator, compressed, condensed in a second condenser, and expanded before being recirculated in the second evaporator, the vapor of the second working fluid in the second condenser being used to heat the vapor of the first-mentioned working fluid in the first-mentioned evaporator.
As will be shown more particularly below, both embodiments of the invention substantially improve the C.O.P. of the refrigeration cycle. Thus, in applications where an ejector cycle is suitable, the system of the present invention increases the C.O.P. of the regrigeration cycle with only a small addition of mechanical power; and in applications where the conventional compression cycle may be suitable, the present invention may be used to reduce the mechanical energy required, if a separate heat source is available.
A further advantage in the two-cycle system of the present invention is that it can operate with two different refrigerants, one of which is most suitable for the ejector part of the cycle, and the other of which is most suitable for the compression part of the cycle. The C.O.P. may be additionally increased by including both the two-cycle system of the second embodiment and the enhanced compression technique of the first embodiment wherein the vapor from the first evaporator (in the ejector cycle) is compressed before being entrained in the vapor stream.
The invention also provides new and improved apparatus for practicing the above-described two-cycle refrigeration method.
The invention will be better understood by reference to the accompanying drawings, wherein:

Figs. 1a and 1b are block diagrams illustrating the known ejector and compression refrigeration cycles, respectively; and
Figs. 2, 3 and 4 are block diagrams illustrating three novel refrigeration systems in accordance with the present invention.

In the drawings, "M" denotes mass flow rate; "P" denotes pressure; "T" denotes temperature; and "Q" denotes heat flow rate.
In the known ejector-refrigeration-cycle as shown in Fig. 1a, high pressure evaporation takes place in the generator 1, and the vapor is allowed to expand through a nozzle in the ejector 2. The low pressure obtained in the ejector causes vapors from the evaporator 3 to entrain the ejector. The vapor stream leaving the ejector is then cooled in the condenser 4. The liquid at the condenser's exit is divided into two parts: One part is pumped back to the generator by the pump 6; and the other part is allowed to expand through an expansion valve 5. This results in a temperature reduction of the fluid in the evaporator 3 which then absorbs heat from the refrigeration load and evaporates.
In the above-mentioned cycle the pump is the only mechanical energy consumer. This amount of energy is normally less than the mechanical energy required in a compression cycle for the same refrigeration load.
The C.O.P. of any refrigeration cycle is defined as the ratio between the refrigeration supplied to the energy input in the cycle. Let Q , Q , Q be the amount of heat exchanged in the evaporator condenser and generator respectively. Let W_p be the energy required by the pump. Then, C.O.P. is defined as
Now let T_e, T_c, T_g be the absolute temoeratures in the e evaporator condenser and generator respectively. Then, if the cycle described is an ideal (reversible) one, the C.O.P. is then given by
The actual C.O.P. obtained in real cycle is only a fraction of the C.O.P. _ideal* This is mainly due to the small mass flow ratio of the ejector itself. The mass flow rate ratio is defined as
This ratio is very sensitive to the pressure (P ) in the evaporator.
In the known compression refrigeration-cycle shown in Fig. 1b, the working fluid (refrigerant) vapors are compressed in condenser 7 where they cool and liquify by exchanging heat through the condenser. The liquid is then allowed to expand through an expansion valve 8 which causes a drop in its temperature. In evaporator 9 the heat is added to the refrigerant which is then evaporated and compressed again in compressor 10, and so on. The C.O.P. of the ideal compression cycle is given by
In the improved ejector cycle as shown in Fig. 2, the mass flow ratio is increased by adding a compressor between the evaporator and ejector. This compressor requires a small amount of mechanical energy but improves the overall C.O._P., as will be shown in the description below.
The improved system illustrated in Fig. 2 thus comprises the same components as the known ejector refrigeration system of Fig. 1a, namely generator 11, ejector 12, evaporator 13, condenser 14, expansion valve 15, and pump 16, corresponding to components 1, 2, 3, 4, 5, and 6, respectively, in Fig. 1a. The improved system of Fig. 2, however, includes in addition, a compressor 17 between evaporator 13 and the ejector 12, which compressor compresses the vapor outputted from evaporator 13 before the vapor is entrained in the vapor stream of the ejector 12. The provision of compressor 17 requires a small amount of mechanical energy, but improves the overall C.O.P., as shown by the following illustrative example.
The table appearing below sets forth a number of examples of the invention, the mechanical energies being calculated with assumed efficiencies of unity and all heat-exchangers being assumed as ideal.
Examples 1-3 relate to three types of cycles namely: the ideal cycle, the known ejector cycle, and the improved compressor-assisted ejector cycle. In the latter, the increase in the pressure effected by the vapor outputted from the vaporator 13 to the ejector 12, is considered to be 6895 mbar above the evaporator pressure. The working fluid is considered to be R-114 (C₂Cl₂F₄, molecular weight 170.9, boiling point 3.8°C.). In addition, the following values are assumed: Qe = 3516 watts; T_g= 86°C; T = 30°C; T_e = -8^oC; and the refrigerant is R-114 Refrigerant.
It will thus be seen from Examples 1-3 of this Table that in the improved compressor-assisted ejector cycle, an increase of inputted mechanical energy by 291 watts increases the C.O.P. from 0.252 to 0.782.
The novel system illustrated in Fig. 3 includes two cycles, each having its own working fluid or refrigerant. Thus, the first cycle is comparable to the conventional ejector cycle illustrated in Fig. 1a, and includes a generator 111, ejector 112, evaporator 113, condenser 114, expansion valve 115, and pump 116, all operating similarly as the corresponding elements 1, 2, 3, 4, 5 and 6, respectively, in the ejector cycle illustrated in Fig. 1a. The second cycle in the system of Fig. 3, utilizing a separate and distinct working fluid or refrigerant, is comparable to the conventional compression cycle illustrated in Fig. 1b, and also includes a condenser 117, expansion valve 118, evaporator 119, and compressor 120, corresponding to elements 7, 8, 9 and 10, respectively, in the compression cycle illustrated in Fig. 1b.
In the two-cycle system of Fig. 3, however, the evaporator 113 of the ejector cycle is used to cool the condenser 117 of the compression cycle, as schematically shown by heat exchanger 121 between evaporator 113 and condenser 117. Thus, the C.O.P. of the compression system is increased, since its evaporator and condenser temperatures difference is decreased. The C.O.P. of the ejector cycle also increases for the same reason.
The combined system offers several advantages including the following:

1. When the ejector cycle is suitable, the novel system can be used to increase the C.O.P. of the refrigeration cycle with only a small addition of mechanical power.
2. When the compression cycle is suitable, the novel system can be used to reduce mechanical energy required if a separate source of heat is available.
3. The novel system can operate with two types of refrigerants, one of which is most suitable for the ejector part of the cycle, and the other of which is most suitable for the compression part.
4. The novel system can provide a second cooling temperature T if M₀ is increased.

Examples 4-6 in the Table appearing below relate to typical cycles in the system illustrated in Fig. 3. In all these cases the ejector part is assumed to operate with R - 114; also, all heat exchangers, pumps, and compressors are assumed to be ideal. The temperature T of the unit including the condenser 117 (of the compression cycle) and evaporator 113 (of the ejector cycle) is maintained constant at 10.1°C. It will be seen that the C.O.P. is increased from 0.252 to e.g., 0.782 (Example 4) with an increase of mechanical energy from 49 to 340 watts.
It has been found that the C.O.P. may be even further increased by including the enhanced compression technique of Fig. 2, wherein the vapor from the evaporator in the ejector cycle is compressed before being entrained in the ejector vapor stream. Thus, the modified system illustrated in Fig. 4 is identical to that illustrated in Fig. 3 (and therefore carries corresponding reference numerals) except that a compressor 130 has been added between evaporator 113 and ejector 112 to compress the vapor from the evaporator before the vapor is entrained in the vapor stream of the ejector. Examples 7-9 of the Table appearing below summarize the performance of such a modified cycle for various types of refrigerants and for the same conditions as described with respect to the system of Fig. 3. In the system illustrated in Fig. 4, the temperature T₀ of the unit including the evaporator (13) of the ejector cycle and the condenser (17) of the compression cycle, is maintained at 0°C. Thus the C.O.P. is increased from 0.252 (Example 2) to e.g., .801 (Example 8) with an increase of mechanical power required from 49 to 261 watts in these examples.
The refrigerants referred to in this Table are well-known refrigerants. Thus, R114 is C₂Cl₂F₄, molecular weight 170.9, boiling point 3.80°C; R12 is CC1₂F₂, molecular weight 120.92, boiling point -29.8°C; and R22 is CH Cl F₂ molecular weight 86.47, boiling point -40.75°C;
It will be appreciated that the ejector in Figs. 3 and 4 could be a turbo-compressor, which may replace the ejector. Thus, the vapor from the generator would be expanded in the turbine to provide the power for the compressor of the turbo-compressor, which compressor forms the vapor stream entraining vapor from the first evaporator. While the absolute members set forth in the table will change, the trend will be the same.

Claims

1. A method for refrigeration according to a refrigeration cycle wherein a working fluid is evaporated in a generator at high pressure, the working fluid vapor is expanded and formed into a vapor stream entraining working fluid vapor from an evaporator, the vapor stream is condensed to a liquid, a part of said condensed liquid is returned to the generator for evaporation at high pressure, and another part of said condensed liquid is expanded and passed to the evaporator for entraining in the vapor stream; the improvement wherein the entrainment of the vapor in the vapor stream is enhanced by increasing the pressure of the vapor from the evaporator before said vapor is entrained in the vapor stream.

2. The method according to Claim 1, wherein the pressure of the vapor from the evaporator is increased by compressing same before entrained in the vapor stream.

3. The method according to Claim 2, wherein the working fluid vapor from the generator is expanded and formed into a vapor stream by passing same through a nozzle in an ejector which causes vapor from the evaporator to be drawn into the ejector for entrainment in said vapor stream.

4. The method according to Claim 1, wherein the pressure of the vapor from the evaporator is increased by heating said vapor before it is entrained in the vapor stream.

5. The method according to Claim 4, wherein a second working fluid is evaporated in a second evaporator, compressed, condensed in a second condenser, aand expanded before being recirculated to said second evaporator; the vapor of said second working fluid in said second condenser being used to heat the vapor of said first working fluid in said first evaporator to thereby increase the pressure of the vapor from the evaporator before said vapor is entrained in the vapor stream.

6. The method according to Claim 5, wherein said first working fluid vapor from the generator is expanded and formed into a vapor stream by passing same through a nozzle in an ejector which causes vapor from the evaporator to be drawn into the ejector for entrainment in said vapor stream.

7. The method according to Claim 5, wherein said first working fluid vapor from the generator is expanded and formed into a vapor stream by passing same through a turbo-compressor in the turbine of which the vapor from the generator is expanded to provide the power for the compressor thereof, which compressor forms the vapor stream entraining vapor from said first evaporator.

8. Apparatus for refrigeration including a generator for evaporating a working fluid at high pressure, means for expanding the working fluid vapor and forming same into a vapor stream, a condenser for condensing the vapor stream to a liquid, a pump for pumping a part of said condensed liquid to the generator, an evaporator, and means for expanding another part of said condensed liquid and inletting same into said evaporator, which evaporates same and outlets the vapor for entraining in said vapor stream; the improvement comprising means for enhancing the entrainment of the vapor in the vapor stream by increasing the pressure of the vapor from the evaporator before it is entrained in said vapor stream.

9. Apparatus according to Claim 8, wherein said last-mentioned means comprises a compressor at the outlet of said evaporator for compressing the vapor before entrained in said vapor stream.

10. Apparatus according to Claim 8, wherein said means for expanding the working fluid vapor and forming same into a vapor stream comprises an ejector including a nozzle through which the vapor issues in the form of a vapor stream entraining vapor from said evaporator.

11. Apparatus according to Claim 8, wherein said last-mentioned means comprises heating means for heating said vapor before it is entrained in the vapor stream.

12. Apparatus according to Claim 11, wherein said last-mentioned means comprises a second evaporator for evaporating a second working fluid; a compressor for compressing the vapor of said second working fluid; a second condenser for condensing the vapor of said second working fluid; means for expanding the condensed working fluid from said second condenser and for circulating same back to said second evaporator; and a heat exchanger utilizing the vapor of said second working fluid in said second condenser for heating the vapor of said first working fluid in '. said first evaporator.

13. Apparatus according to Claim 8, wherein said means for expanding the vapor of said first working fluid and forming same into a vapor stream comprises an ejector including a nozzle through which the vapor issues in the form of a vapor stream entraining vapor from said first evaporator.

14. Apparatus according to Claim 8, wherein said means for expanding the vapor of said first working fluid and forming same into a vapor stream comprises a turbo-compressor in the turbine of which the vapor from the generator is expanded to provide the power for the compressor thereof, which compressor forms the vapor stream entraining vapor from said first evaporator.