GB2224819A - Improved system for refrigeration - Google Patents

Abstract

An improved system for refrigeration of the vapor-composition cycle type is described. The system comprises a compressor, a condenser, expansion valve and evaporator, wherein an adsorber reactor and cooler are provided between the evaporator and compressor, and a desorber reactor is provided between the compressor and condenser. the characteristic feature being that the pressure ratio across the compressor is in the range of 1.5 to 20. Heat is transferred by means of said adsorber and desorber reactors containing a solid or liquid adsorbent having the capability of preferentially retaining one component from the stream entering into the reactor. The cooler dissipates the adiabatic compression heat and the heat of absorption through the assistance of the adsorbent bed. Using this refrigeration system, the improvement in the yield will be in the range of 3% to 40% depending on the parameters used in the cycle and the amount of external heat used. <IMAGE>

Description

2224819 Improved nys-uem ror nefrigeration is The present invention

relates to an improved system for refrigeration Nor particularly, the invention relates to an improved system for refrigtration of the vaporcompression cycle type.

BACKGROUND OF TRZ 1MVZWXON

As known, refrigeration is the process of lowering the temperature inside an insulated enclosure by extracting beat and rejecting it to an external medium of higher temperature. Since heat will not flow spontaneously from a region of low temperature to one of hig6er temperature, mechanical work or heat energy from an external source must be utilized to move the heat from the enclosure to -be refrigerated.

To-day, refrigeration is considered an essential technoloqical commodity pervading a large number of industrial a& well as consumer oriented processes. Refrigeration processes consume a substantial fraction of the total power generated and are therefore considered main candidates for energy saving initiatives.

The most widely used refrigeration cycle is the vaporcompression cycle type. A schematic representation of a such conventional refrigeration type is given in Figure 1, A high-pressure refrigerant vapour is discharged from the compressor (C I) in a superheated state and enters into the condenser (Ml). Thor the refrigerant vapor is condonsd, essentially at constant pressure by giving up its latent hoot to the cooling water flowing through coils or tubes or to the surrounding atmosphere. The saturated liquid refrigerant enters into an expansion valve (V) and reaches a lower temperature (T) corres 1 c ponding to the boiling temperature at the pr9asuro preva iling therein. The low-tomporature liquid refrigerant - containing a small fraction of its vapour - is further introduced into an evaporator (H 2), being evaporated by the heat transferred to it from the comparatively warmer space to be refrigerated.Th flow rate of the refrigerant is is so adjusted by the thermostatic expansion valve, that at the exit of the evaporator, all the liquid refrigerant - is in the vapor state. The vapor leaving the evaporator enters the auction of the compressor and is compressed to a higher pressure. A substantial amount of beat Is-roloa- sod, as a result of the compression, which in the known rfrigerotion units is not utilized. The ftsup&rheating of the hot compressed vapor is recognized as being a 1 major irreversibility In vapor-compression refrigeration cycles that prevents approach to the ultimate efficiency achievable In the theoretical reversed Carnot cycle.

The number of available refrigerants has Increased ate&dily as a result of the efforts of researchers for new fluid possessing suitable thermal, physical and chemical characteristics to meet demands of new applications and equipment design. However, up to now, It has not been found an Ideal refrigerant having simultaneously optimum 10 thermodynamic, physical and chemical properties.

In our previous M. Patent Number 4,661,133 a method for heat and mans exchange operations between two Imput streams was described. According to said method, a reactor Is provided containing a solid or fluid reagent capable is of preferentially retaining on of the components present In said streams enabling the use of thermodynamic work potential supplied by the transfer of heat between the two streams# It In an object of the present Invention to provide an Improved method for refrigeration which utilizes the beat released during the compression stop. It In another object of the present Invention to provide an Improved is method for refrigeration which reduces the power consumPtion and compressor sizes.

BRIEF DESCRIPTION OF THE INVENTION

The invention relates to an Improved system for refrigeration of the vapor-compression cycle type, comprising a compressor, condenser, expansion valve and evaporator wherein:

(a) an adsorber reactor and cooler are provided between the evaporator and compressor, (b) a desorber reactor is provided between the compressor and condenser, being characteri2ed that the pressure ratio accross the compressor is in the range of 1.5 to 20. It was found that the refrigeration system using the method according to the present invention will improve the yield for about 3 to 40P, depending on the parameters used in the cycle, and the amount of external heat used, tnus alleviating the compression requirement per refrigeration unit.Part of this improved yield is derived from utilization of the heat of compression.

The adsorber and desorber reactors are quite similar to those described in our previous U.S. Patent No.4,661,133 The roagents used in these reactors are selected from a I large groups of known compounds used in the art, having the capability of preferentially retaining ono, component from the stream entering into the reactor. Typical useful reagents include alumina, silica gel, zolites, various forms of carbon icharcoal, carbon black etc,), metal powders, variouE- polymers, organic solvents, etc.

BRIEF DESCRIPTION OF TOR DRAWINGS.

is Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 1. is a scheme of a conventional refriqeration cycle. is a scheme of the boat mass exchange enhanced refrigeration accordinq to the Invention. Is a scheme of operation of a Liquid Heat and mass Exchange (LNME) unit (cascade with three stages) Is a scheme of valving arrangement for LHME unit. is a schematic diagram of a continuous LHME unit (with external low-grade heat source). is a scheme for continuous LfiME with periodic absorbent recycle.

is in its simplest form, the Improved system comprises the Incorporation of a pair of adsorbent& beds -in the refri9ration system - out at suction and one at the discharge of the compressor. The two beds are periodically interchanged. The bad on the auction side of the compressor adsorbs part of the refrigerant which is subsequently dsorbod from the bed, when this is relocated at the hot compressor discharge. Thus, a fraction of the refrigerant that is circulated around the loop is conveyed from the suction (low pressure) side to the discharge ( i,e. high pressur) side without requiring any mechanical compression. The thermodynamical work required to achieve this goal is derived from the temperature difference between the cold auction and the hot discharge which is utilized to drive the adsorption - desorption process. As encountered in all beat exchange operations, heat is transferred by means of the beds from the compressor discharge to the compressor auction. In order to avoid exc_ solve power consumption, and to maintain capacity of the refrigeration cycle the refrigerant going out from the adsorbent bed should be cooled to remove this beat,, plus the heat of absorption. The beat of absorption removed is have adds to the potential refrigeration effect generated In the cycle.

A schematic representation of the refrigeration system according to the present Invention Is given In Pigure 2. An can he noticed the system In quit similar to that of the conventional system,the difference being: the presence of (a) the adsorber bed at the exit of the evaporator (E 2)o (h) the cooler (H lb) located between the adsorber bed and compressor (P 2) and (c) the denorber bed located between the compressor (P 2) and condenser (H l&). The cooler has the object to dissipate the adiabatic compression beat plus the heat absorption through the assistance of the adsorbent bed.

The expected reduction in power rwluired per ton refrigeration - as compared to conventional vapor- compression cycle refrigeration - is proportional to the amount of refrigerant that can be adworbedldsorbed per unit time. This In determined by the following two criteriat (a) the adsorption equilibrium. and (b) the adsorbent circulation rate.

The u adsorption equilibrium is determined by the conditions prevailing at the compressor auction on the one is hand, and at the compressor discharge on the other. The difference between the adsorbent load at those two points is the "differential adsorption load". The higher this differential, the more refrigerant is Opumped" by adsorption. This differential is clearly specific to each particular adsorbent/refrigerant pair. It also depends on the conditions of pressure and temperature prevailing at both ends of the cycle. Thus, to obtain a high yield, one must "fit" an adsorbent to a set of parameters such as type of refrigerant, pressures and temperatures prevailing in the system.

The "adsorbent circulation rate" is defined as the mass of the adsorbent in each bed divided by the cycle period. The higher the adsorbent circulation rate, the more refrigerant is "pumped" by adsorption. This circulation rate is limited by the capacity of the regenerating gas (compressor discharge effluent) to provide the heat necessary for heating up the adsorbent mass and desorb the amount adsorbed. A same adsorbent circulation rate can be obtained by frequent switching of a small amount of adsorbent, or by infrequent switching of a larger bed size. The bed size and geometry and the corresponding is permissible switching rate depend on diffusional limitations. Thus, the physical properties (density, thermal capacity, physical structure) of the refrigerant and adsorbent phases, the equilibrium adsorption load between the two phases, as well as the temperatures and pressures prevailing, will influence the resulting yield.

According to another embodiment a liquid adsorbent is utilized for enhancing the performance of vapor compression refrigeration cycles. A liquid heat-mass exchanger (LHME) unit can modify the refrigeration cycle in one of two wayst a. An analog to the adsorption unit, where the liquid absorbent is stationary and the refrigerant streams are periodically switched.

b. Continuous operation in which the liquid absorbent is circulated continuously between the high and low pressure refrigerant streams.

The system for the former is similar to the operation described above, except that a cascade of small cells, each partially filled with liquid absorbent, replaces each adsorbent bed. The cells are connected in series such that the refrigerant vapor (absorbate) passes from is one cell to the other. Successful operation of the LHHE unit requires the use of a cascade of cells (rather than a single large tank) to prevent backmixing of hot and cold gas. A schematic presentation is given in Figure 3.

S The refrigerant flow in periodically switched between the absorption mode (Figure 3a) and the desorption mode (Figure 3b) one valving arrangment using two 3-way switch valves for each stage is shown in Figure 4. It should be noted that a simpler arrangement avoiding interstage valves altoge thor by suitable structuring of each cell in also possi ble.and that in actual implementation, probably a single, multi-port switching valve can be used instead of the individual switching valves. This will reduce the size of the system and also minimize the added pressure drop.

Also, to minimize pressure drop, while also utilizing external beating/ cooling media to boost the absorption/ desorption capacity, a wet wall gas-liquid contacting arrangement can be used as shown in Figure 5, instead of tho contacting arrangement shown in Figure 3.

Liquid adsorbents are well known and are preferred in specific cases. A review on liquid adsorbent3 was presented in a paper by L.F. Albright et al. at the ASHRHI, 67th annual meeting. Vancover, British Columbia, June 1960. Typical examples of liquid adsorbent& are:

glycerol triacotate (known as triacetin), dimethyl other of tetraothylano glycol(hreafter referred to as (DMETED) etc, A continuous LHME in shown schematically in Figure 6. Here, the liquid absorbent is periodically recirculated by several small pumps, between corresponding stages in two continuous columns serving one as absorber and the other as a dsorber. in each column the gaseous refrigerants flow from one stage to the next. Again, in implementation one aingl drive will be used to pump in parallel the liquid absorbent as necessary. This configuration is is an exact emulation of the time-poriodic HME. It has the added advantage of allowing heating/cooling of the absorbent along its path from no column to the next and thus precondition it to its next role. This arrangement can also be further simplified, while sacrificing some efficiency by colAapsing the desorber on the absorber or both into a simple stag thereby using a simple pump.

According to another embodiment, a low grade heat source in utilized for the heat-mass-exchange processes.

Processes Involving hoat-mass exchange can take place only if the following two conditions are fulfilled:

a. The sorption (adsorption or absorption) equilibrium value of the refrigerant at the temperature and pressure prevailing at the compressor suction side, in sufficiently higher than that corresponding to the temperature and pressure on the discharge side of the compressor, and b. there is sufficient thormodynamic driving force to extract work from the fluid at these conditions.

In some operations the temperature of the refrigerant leaving the compressor is not high enough to drive the is RME unit. In fact, in many instances it is desirable to keep this temperature low to prevent loss in compressor efficiency. However, the operation of the HME unit can be significantly improved by using an external heat source which would raise the desorption tmperature. Interestingly, energy in strecma at moderate temperatures of about 70- 120 a C which is not commonly considered recoverable, can be used for this purpose. Thus,through the service of is an appropriate solvtnt, the NNE process provides economical means to exploit low-.grad heat sources which would not otherwise be considered useful. The situation where such a low-grad beat sources are available in the vicinity of a refrigeration system is quite common, notably in industrial plants, solar systems and in transportation vehicles.

A soatias Interesting aspect of waste heat enhanced refrigeration, concerns the capability to sustain a marginal amount of refrigeration capacity in the absence of power as long as the source of heat is still there. Such cases or particularly ccounterod in cooling of vehicls and other industrial uses. When condensing, pressure is saintained at a low level by effective cooling or other. rwise, the effectivity of the waste heat enhanced scheme to actually improved.

Examples 2 and 3 illustrate two experiments where a lowgrade beat source at a temperature of 100 0 C. or even at 700 C, can be used to aid NNE enhancement and provide significant power savings. Both Zxanples consider a vapor compression cycle using R-22 refrigerant which is consi- is dered to be the refrigerant that most likely will survive for some time the now environmental restrictions.

Claims (7)

1. The invention will be further illustrated by the following Examples

   without being limited thereto. A person skilled in the art after reading the present specification will appreciate the merits of the invention being in a position to insert any modification without being outside the scope of the invention as defined by the Claims. ZZAMPLE 1.

   The refrigeration system used had utilized ammonia as a refrigerant with the following operation conditions (refer to Figure 2 for the symbols):

   = -14 0 C; T = 20 0 C. P w 2.5 atmospheres.

   Te 1 0 1 T2 " 135C-1 T4 ' 30C; P 2 - 10.4 atmospheres.

   The amount of refrigerant recirculation was q-186 g/min per ton refrigeration.

   The adsorbent and desorbent beds contained each 370 9 of activo carbon per ton refrigeration capacity so that the beat-mass transfer unit revolved at a period of about 4 minutes. To isolate the NNE contribution to myxtm ffIciency, the cooling water to condenser Hla was restric- 2.6 is ted such as to maintain T 4 at Its original nominal value of 3011C.

   The beat exchange efficiency of the heat-wass exchange was 75% and the net adsorption load was 0.053 kg ammonia per kilogram of adsorbent.The data of T1, T 21 T4, T c were the same as mentioned above, while T 3 was 49 0 C# T 5 was 0 oc C and T 6 was 105. The gas comes out of the cooler Elb at 300C and is further cooled to the original compressor suction temperature of 200C by injecting into It g/min liquid.

   The compressor suction capacity and power consumption thus remained unchanged, while a 4% Increase An refrigeration load at the same conditions was recorded.This Is equivalent to a 4% saying in the compressor power required or 4% Improvement In the compressor capacity.

   The 4% reduction in the power consumption is equivalent of savings of about 260 KwhIton refrigeration per year which Is a substantial reduction In the operation coats of an Industrial size refrigeration system.

   ZXAKPLZ 2. LHME-Enhanced le Boosted kú a Waste-Neat wourcoUsing Triacetin as the Absorbent.

   - 16 To illustrate the operation of the LEME-enhanced refrigeration cycle boosted by an external low-grade heat source, consider the conventional cycle of Figure 1 using R-22 an a refrigerant. The Example In structured to demonstrate the potential of replacing R-12 refrigerant by R-22 In mobile air conditioning systems.lt assumes a countercurrent approach temperature difference of 100c and a LEME beat exchange efficiency of 75%.

   A conventional R-22 vapor compression cycle (Figure 1), Is operated at the following conditions (basis: one ton refrigeration):

   Tl m 150CP P1 m 5.19 bar, T2 - 500CP P2 14.7 bar.

   T4 m 38 0 c# TC - soc, Refrigerant circulating rate 1.3 kg/min.

   Consider now the same system, modified an shown In Figure 6 by the addition of a LNU9 unit. tach cascade consists of 3 stages each containing about 160 g of TriacatIn. Two additional, relatively mall, heat exchangers are Insta lled, one (0.63 Nw) for the low-grade beet supply located on the compressor discharge line, and the second (0.39Kw) 1 Is a water cooler located on the compressor auction line. For comparison. 3.53 Kw are transferred at the evaporator and 4.06 Kw at the condenser. The compressor auction Is now at 38 0 C causing a reduction in the mass of R-22 compressed by 100xl-L273+153- 7.4%, using the same power. [273+381 Regenerating the absorbent at T8 m 88 0 C allows for the diversion of 264g/min of R-22 to be "pumped" from the low pressure side to the high pressure side by absorption. As a result, the rate the refrigerant In pumped by mechanical compression is 1.3 x U-0.074) - 1.204 kg/sin.

   Total R-22 condensed is: 1.204 + 0.26 m 1.47 kg/min, representing a 13% increase In refrigeration capacity using the same amount of power, The following operating conditions were determined for the modified cycle:

   T1 m 380C; P1 m 5.8 bar (unchanged), T2 m 74 0 C; P2 m 14.7 bar (unchanged), T3 m 38 0 C (unchanged), T4 - SOC (unchanged). TS m 15 OC (unchanged), T6 m 380C3 T7 m 41 0 C# - is is TS m 88 0 c# Refrigeration capacity: Power used:

   Waste beat used: ZZAMPLZ 3.

   Continuous LHMB-Enhanced Cycle Boosted by a Waste-Heat source using DMETEG.

   The following example is intended to demonstrate how intermediate supply and removal of the heat absorption at, respectively, the absorption and desorption steps. permits an exploitation of a low-grade heat source to obtain a substantial yield improvement. Reference conditions are Identical to those in Example 2. An LHNE unit consisting of two 3-stage cascades is depicted In Figure 6, Each stage contains 100 9 DNETEG. Two small heat exchangers through which heat is supplied prior to the regeneration step in the form of moderately hot water at 930C, and removed prior to the absorption step Into cooling water supplied at 28 0 C (in practice, the entire LHNE unit would be compactly fabricated at an integral unit). The DNETEG in cycled to the columns at &.rate of 100 9/min per stage. As a rosult, the amount f R-22 T9 m 65 0 C. m 4Kwal.13 Ton refrig a 0,5 Kw (unchanged) m 0.63 Kv at 930C.

   11 1 m - 19 absorbed In 356 glin. The R-22 gas exiting the absorber at 50 0 C In cooled In cooler E1B to 380C. The higher com prsolon auction causes, an In Xxple 2, a reduction of 7.4% In the mass compressed using tho same power.

   Thus, 1.3 x U-0.074) m 1.204 kg/min R-22 mechanically pumped and the total R-22 condon is thereforer 1.204 + 0.356 mJ,56 kg/min or a not increase in refrige ration capacity of 20%, using the same amount of power.

   Summarizing the calculated now operating conditions:

   T1 m 150C (Unchanged); P1 - 5.8 bar (unchanged).

   T2 m 740 C P2 m 14.7 bar (unchanged).

   T3 m 78 0 C T4 m 380 C (unchanged); T6 m 500C Refrigeration capacItyt Waste Heat supplied Power Used T5 m 5 0 C (unchanged), 4.23 Kw - 1.2 Ton refrig. 1.19 KW at 1980C. 05 XW (unchanged), -- 20 C L A 1 At As- 1. An Improved system for refrigeration of the vaporcompression cycle type, comprising a compressor. condon&or, expansion valve and evaporator whercint (a) an adsorker reactor and cooler are provided between the Yaporator and compressor.

   (b) a dcaorber reactor Is provided between the compressor and condenser, being characterized that the pressure ratio accroas the compressor Is In the range of 1.5 to 20.
2. 2. An Improved system according to Claim 1, wherein beat In transferred by moan& of a reactor containing a reagent. from the compressor discharge to the compressor auction,
3. 3. An Improved system for refrigeration according to Claim 2. wherein the refrigerant going out from the reactor containing the adsorbent Is cooled, dissipating the adiabatic compression heat.
4. 4. An Improvad system for refrigeration according to Claims 1 to 3, wherein the adsorber and dosorber reactors contain a reagent selected from &lumina silica gel.

   1 - 21 lit&, charcoal, active carbon, metal powder and mixture& thereof.
5. 5. An Improved system for refrigeration according to Claims 1 to 3. wherein a liquid adsorbent in utilized for enhancing the performance of vapor compression refrigeration cycle.
6. 6. An Improved system for refrigeration according to Claim 5, wherein maid liquid adsorbent Is selected from glycerol trIacetate, dlathyl ether of totroothylene glycol.
7. 7. An Improved system for refrigeration according to claim& 1 to 6, wherein a low grade heat source I& utilized for the beat-mass-exchange processes.

   An Improved system for refrigaretlon substantially as described In the specification and claimed In any one of Claims 1 to 7.

   Published 1990 at The Patent Office, State House. 66'71 High HolborrLondon WCIR4TF. Further copies maybe obtained from The Patent OfficeSales Branch. St Mary Cray. Orpington. Kent BRS 3RD Printed by Multiplex techniques ltd- St Ma,3r Cray- Kent. Con 1 87