EP2061999A2

EP2061999A2 - Method and apparatus for generating ice slurry

Info

Publication number: EP2061999A2
Application number: EP07866694A
Authority: EP
Inventors: Dilip Waman Bapat; Samir Vasudeo Kulkarni
Original assignee: Thermax Ltd
Current assignee: Thermax Ltd
Priority date: 2006-08-31
Filing date: 2007-08-31
Publication date: 2009-05-27
Also published as: WO2008062438A3; WO2008062438A2; EP2061999A4

Abstract

A method and apparatus (100) for generating ice slurry is disclosed. The method involves creating vacuum in an ice slurry generator using a lithium bromide vacuum absorption machine and the apparatus (100) involves connecting the absorber (5) of a lithium bromide vacuum absorption machine to the chamber (1) of an ice slurry generator (100).

Description

METHOD AND APPARATUS FOR GENERATING ICE SLURRY

FIELD OF INVENTION:

This invention relates to a method of generating of ice slurry. In particular, this invention relates to method of generating of ice slurry under vacuum conditions.

BACKGROUND AND INTRODUCTION:

Ice slurry is a suspension of water based ice crystals in a solution of either pure water or water containing additives like freezing point depressants.

The ice contained in the ice slurry makes it very effective for preserving the cold. Ice slurry can be pumped to consumption points easily. It has potential use as a secondary cooling medium, either by direct application on the product or alternatively as a thermal energy storage media, whilst remaining fluid enough to pump. Ice slurry offers superior performance over the conventional flake and block ice systems as a chilling medium. Ice slurry offers higher efficiency and cost effective ice production and also unique pumping and easy handling characteristics.

Ice slurry is called by brand names of "Liquid-ICE" or "Pumpable-ICE" or "Binary Ice" or "Flo Ice" or "Vacuum Ice".

Ice slurry systems provide totally sealed "Hygienic Systems" for applications in fisheries, supermarkets etc., enhancing productivity, flexibility of operating temperature and consistency of application. To remain pump-able only a portion of the water (5 to 30%) should be transformed into small ice crystals of size around 1 mm., but not more. This ensures a uniform solution while enhancing cold transfer.

Various methods of producing ice slurries, known in the prior art include scraped-surface generators where ice crystals are formed by mechanically scraping on a refrigerated surface over which water along with freezing point depressants is allowed to flow.

Another known method producing ice slurry is under vacuum utilizing triple point conditions of water.

The triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium. By creating vacuum and cooling either pure water or water containing additives like freezing point depressants to about 0° C or less, ice slurry is produced. The vacuum is created by using water vapor compressors. The system utilizes electricity as energy source.

There are even systems where thermal compressors have been used to create vacuum. The thermal compressors use thermal energy in the form of steam as energy source. The thermo compressors are generally low efficiency devices which require much larger quantity of steam to produce unit refrigeration effect. For example US patent 4,734,116 discloses a method and apparatus for generating ice slurry with ethanol as an additive, producing vacuum by sucking water vapors using a compressor #nd compressing a flow of water vapor to a condensation chamber in which the compressed water vapor is condensed.

Other types of ice slurry generators employ mechanical refrigeration for slurry production. Ice crystals are generated on refrigerated surface over which water along with freezing point depressants is flowing. Ice crystals are then scraped off mechanically from the surface to form slurry with water.

The main limitation of the prior art methods of ice slurry production under vacuum is the large size of water vapor compressors due to very high specific volume of water vapor under triple point condition.

Another limitation is energy consumption in the form of electrical power and associated costs which are comparable to mechanical refrigerated systems.

The limitation of thermally driven ice slurry generators under vacuum is that low efficiency of thermo compressors resulting in very high steam consumption figures.

Again, in the case of mechanical systems, the surface temperatures are of the order of -10 to -15°C leading to higher electrical power consumption. Yet another limitation of mechanical systems is that there is a much higher requirement of freezing point depressants since ice is produced at much lower temperatures.

OBJECTS OF THE INVENTION:

An object of this invention is to provide a method and apparatus for generation of ice slurry which is efficient and consumes considerably less energy to produce unit refrigeration effect.

Another object of this invention is to provide a method and apparatus for generation of ice slurry which is compact in size and at the same time consumes much less electrical energy.

Still another object of the invention is to provide a method and apparatus for generation of ice slurry which requires comparatively less amount or even no freezing point depressants.

One more object of the invention is to provide a method and apparatus for generation of ice slurry which is economical and in which the method can be precisely controlled.

Yet another object of this invention is to produce ice slurry in a slurry generator under stable and homogeneous conditions. SUMMARY OF THE INVENTION:

In accordance with this invention there is provided a method of generating ice slurry in a ice slurry generating chamber maintained under vacuum; said vacuum being created by lithium bromide vapour absorption machine.

Typically, vacuum is created in the ice slurry generating chamber by permitting communication between the absorber unit of a lithium bromide vapour absorption machine and the said chamber.

In accordance with a preferred embodiment of the invention, ice slurry is re- circulated in said chamber until desired concentration of ice in the ice slurry is achieved.

In accordance with another aspect of the invention there is disclosed apparatus for making ice slurry comprising

[i] an ice slurry generating evaporator chamber adapted to contain water with or without a freezing point depressant;

[ii] an agitator provided within said chamber;

[iii] a first inlet for introducing water or dilute ice slurry solution into said chamber;

[iv] a first outlet having pumping means for pumping out ice slurry solution from said chamber;

[v] a Lithium bromide vapour absorption machine comprising an absorber unit having : cooling tubes; means for spraying concentrated lithium bromide solution on said tubes; a shell in communication with said chamber to permit contact with vapour in said chamber with the lithium bromide solution and absorbing said vapour in said unit for creating vacuum in said chamber; means for collecting dilute lithium bromide solution with water vapour absorbed from said unit and pumping said dilute lithium bromide solution to a high temperature generator unit to regenerate concentrated lithium bromide solution and water.

Typically, the said chamber is located operatively directly below said absorber unit.

Alternatively, said chamber is located operatively below and towards one side of the absorber unit.

Typically, the absorber unit has a single set of cooling tubes and a trough is provided below said column for collecting and removing dilute lithium bromide solution with absorbed vapour.

Alternatively, the absorber has at least two sets of cooling tubes and troughs are provided operatively below each set for collecting and removing dilute lithium bromide solution with absorbed vapour.

In accordance with a preferred embodiment of the invention, the chamber is provided with a defrosting mechanism. BRIEF DESCRIPTION OF THE DRAWINGS:

The invention will now be described with reference to the accompanying drawings, in which all the aspects and advantages of the invention will become apparent with the description of the preferred, non-limiting embodiment, when read together with the accompanying drawings in which:

Figure 1 shows a schematic sectional view of the Ice slurry generator configured to produce ice slurry as per this invention ;

Figure 2 is the schematic diagram of an alternative embodiment of the Ice slurry generator of Figure 1; and

Figure 3 is one more alternative embodiment of the generator of figure 1.

DETAILED DESCRIPTION OF THE DRAWING:

In a conventional LiBr Vapour Absorption Machine with double effect cycle, the weak solution from an Absorber is pumped by a solution pump to a high temperature generator (HTG) via a low temperature heat exchanger and a high temperature heat exchanger. Heat is added from a heat source like steam to the weak LiBr solution in this generator and the refrigerant which is typically water is evaporated. The high temperature heat source is generally at a temperature higher than 14O⁰C. This step concentrates the LiBr solution to make it stronger. The stronger LiBr solution flows to a low temperature generator(LTG) via the shell side of the high temperature heat exchanger. The refrigerant generated in the high temperature generator is then condensed on the tube side of LTG, thereby providing more heat to the solution arriving from the HTG. More amount of refrigerant is generated in LTG . The stronger solution from the LTG is then passed back to the absorber via the shell side ofthe LTHE to complete the cycle.

However, one limitation of the aforesaid vapour absorption machines is that they produce refrigeration effect only at temperatures above the freezing point of water. Further, in the conventional LiBr VAM at lower pressures, the liquid refrigerant (water) freezes in an uncontrolled manner and forms lumps of ice over the evaporator tubes or refrigerant storage below it. In fact it becomes hindrance to the operation and there is possibility that chilled water inside the tubes may also freeze. The machine fails to perform further under such conditions.

This invention proposes a method to produce ice slurry using Vacuum absorption cycle and lithium bromide as an absorbent with water as the refrigerant.

This invention also proposes a method to produce ice slurry in a slurry generator using a modified LiBr VAM configured to operate at temperatures close to freezing point of water.

Referring to the accompanying drawings the ice slurry generator configured to produce ice slurry as per this invention is generally indicated by reference numeral 100. Referring to Figure 1 the ice slurry generator 100, comprises an evaporator chamber (1) which is provided with an agitator (2). and is subject to vacuum conditions of a vapour absorption machine. The chamber 1 is preferably provided with a defrosting mechanism (3) which has provisions for controlled circulation of heating water. An ice slurry pump (4) is provided to pump out the ice slurry from the chamber 1 to the desired application point. A return pipe (14) is provided through which dilute ice slurry or make up refrigerant is returned to the chamber 1. The pump 4 is adapted to return the ice slurry solution to the chamber until the ice slurry is of suitable concentration for the particular application.

The chamber 1 is in communication with the absorber unit 5 of a Lithium Bromide vapour absorption machine. The machine has an Absorber(5) a Condenser (6) Low Temperature Generator (LTG) 7, High Temperature Generator (HTG) 8, Low Temperature Solution Heat Exchanger (LTHE) 9, High Temperature Solution Heat Exchanger( HTHE ) 10, Heat Reclaimer (HR) 11, Interconnecting Piping, 12 Solution Pump 13 all common as in any conventional LiBr VAM.

Operation of ice slurry generator coupled with LiBr VAM is explained with reference to figures 1, 2 and 3.

In this cycle, the weak solution from Absorber (5) is pumped by solution pump (13 ) to (8) [HTG] via LTEE (9) and HTHE (10). Heat is added from a high temperature heat source like steam to the LiBr solution in this generator and refrigerant is evaporated. The high temperature heat source is generally at a temperature higher than 140⁰C can be any thing like high temperature pressurized hot water, steam, direct firing of oil / gas or waste heat from any exhaust gases such as DG sets.

The stronger LiBr solution flows to (7) [LTG] via shell side of high temperature heat exchanger (10). The refrigerant generated in HTG is then condensed on the tube side of LTG, thereby providing more heat to the solution arrived from HTG. More amount of refrigerant is generated in LTG (7). The strong LiBr solution from LTG (7) is then passed back to absorber (5) via shell side of the LTHE (9) to complete the cycle. The typical temperature of the strong LiBr solution is around 45 degrees Celsius.

The refrigerant vapor generated in LTG (7) is condensed in the condenser (6). This liquid refrigerant [water] is then passed to the ice slurry generator (1) to complete the refrigerant cycle. In another embodiment of ice slurry generator the liquid refrigerant can also be taken out of the system and replenished with the new refrigerant.

By use of the defrosting mechanism 3 the formation of ice in the ice slurry generator chamber (1) is controlled. The defrosting mechanism allows relatively hot water around 30 degrees Celsius to jacket the outer wall of the chamber 1. This hot water jacket prevents icing of the walls of the chamber 1 and is used intermittently as a defrosting unit. The chamber 1 is in communication with the absorber unit 5 . The absorber unit 5 has cooling tube sets where cold water again at temperatures around 30 degrees Celsius is circulated through sets of tubes. There may be one central set 15 as seen in figure 1 or two sets 16 and 17 as seen in figure 2. More than two sets of tubes may also be fitted in the absorber 5. Concentrated strong Lithium bromide solution from the LTG 7 unit of the machine is sprayed by sprayers 18 on the tube sets 15,16 and 17. The tube sets are provided with cooling water inlet 19 and cooling water outlet 20. During this spraying process the concentrated lithium bromide solution absorbs vapour in the absorber from the chamber 1. The weak lithium bromide solution is collected in the trough/troughs 21 and led off through conduit/conduits 22 to the pump 13 This absorption performs two functions: it generates heat, because the absoption of vapour in the LiBr is an exothermic reaction which is taken up by the cooling tube sets and 2 it reduces the pressure in the absorber chamber .space causing more water present in the chamber to evaporate. The heat for this evaporation is taken from the resident water/dilute ice slurry solution in the chamber 1 thereby cooling the water. Due to this operating condition, ice crystals are formed in the ice slurry generator. With the help of agitator (2), rotating at a range of 400 to 1500 revolutions per minute and typically 600 rpm , ice crystals are mixed thoroughly with the solution present in the ice slurry generator and thus ice slurry is formed. Slurry pump(4) is used to take away slurry, once it has reached the desired concentration. The diluted ice slurry or make up refrigerant is returned to the ice slurry generator via pipe (14). Part of the ice slurry is re circulated back to the slurry generator for maintaining stable operation and consistency of ice slurry. In the above operation, it is possible to drop the vacuum in evaporator/absorber to level of triple point of water i.e. 4.5 mm of Hg absolute and 0.015⁰C. Typically, in the tube sets 15,16 and 17 the inlet water is at around 30 degrees Celsius and the outlet water is at 33 degrees.

In figures 1 and 2 the evaporator/ice slurry generator chamber 1 is located operatively directly below the absorber unit 5 of the vapour absorption machine and the shell of the absorber and the that of the chamber 1 are continuous whereas in figure 3 the absorber 5 opens on one side and the chamber is positioned on one side of the absorber affording a more compact design.

An ice slurry generator with LiBr Vapor Absorption Machine was built to deliver 35 Tonnes of refrigeration capacity (Approx. 25000 kg/ per day) of fresh ice generation. The machine was operated with steam at 8 kg/cm2 pressure as heat source. Cooling water at 32°C was used as heat sink in the absorber tubes. It was observed that as the pressure in the absorber reached below triple point of water small ice crystals formed in the solution which was in the ice slurry generator. Different types of inorganic salts were used in small proportions as freezing point depressants. The machine delivered ice slurry with ice percentage between 2% to 15%. The slurry was taken out of slurry generator with the help of slurry pump, melted in heat exchangers and solution was fed back to the slurry generator, as shown in the figure to operate the machine in continuous mode. The flow rate of slurry pump was adjusted for different slurry concentrations. The machine delivered refrigeration capacity approx. equivalent to 35 TR when the ice concentration in slurry was 6% to 15%. While considerable emphasis has been placed herein on the particular features of "an ice slurry generator and LiBr VAM configured to produce ice slurry" and the improvisation with regards to it, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiment without departing from the principles of the invention. These and other modifications in the nature of the invention or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

Claims

[1] A method of generating ice slurry in a ice slurry generating chamber maintained under vacuum; said vacuum being created by lithium bromide vapour absorption machine.

[2] A method of generating ice slurry as claimed in claim 1, wherein vacuum is created in the ice slurry generating chamber by permitting communication between the absorber unit of a lithium bromide vapour absorption machine and the said chamber.

[3] A method of generating ice slurry as claimed in claim 1, wherein ice slurry is re-circulated in said chamber until desired concentration of ice in the ice slurry is achieved.

[4] Apparatus for carrying out the method of making ice slurry as claimed in any one of claims 1 to 3 , comprising

[ii] an agitator provided within said chamber;

[5] Apparatus for carrying out the method of making ice slurry as claimed in 4, wherein the said chamber is located operatively directly below said absorber unit.

[6] Apparatus for carrying out the method of making ice slurry as claimed in 4, wherein said chamber is located operatively below and towards one side of the absorber unit.

[7] Apparatus for carrying out the method of making ice slurry as claimed in 4, wherein, the absorber unit has a single sets of cooling tubes and a trough is provided below said sets of tubes for collecting and removing dilute lithium bromide solution with absorbed vapour.

[8] Apparatus for carrying out the method of making ice slurry as claimed in 4, wherein, the absorber has at least two sets of cooling tubes and troughs are provided operatively below each set for collecting and removing dilute lithium bromide solution with absorbed vapour. [9] Apparatus for carrying out the method of making ice slurry as claimed in 4, wherein the chamber is provided with a defrosting mechanism.