Classifications

• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
  • C09K5/00—Heat-transfer, heat-exchange or heat-storage materials, e.g. refrigerants; Materials for the production of heat or cold by chemical reactions other than by combustion
  • C09K5/02—Materials undergoing a change of physical state when used
  • C09K5/06—Materials undergoing a change of physical state when used the change of state being from liquid to solid or vice versa

Definitions

• the invention relates to a method for performing heat exchange by using a heat transfer medium wherein heating takes place by heat exchange from the target to the medium or from the medium to the target.
• the invention also relates to a medium and a heat exchange apparatus.
• a cooling medium for cooling a target is used as far as possible so that the medium does not solidify at any stage of the process nor do any of its dissolved components crystallize.
• a cooling medium should consequently remain in liquid form at its operating temperatures.
• indirect cooling the cooling medium is cooled to its lowest temperature after which it is brought into heat transfer contact with the target to be cooled which then cools by releasing heat to the cooling medium.
• Heat transfer is also influenced by the viscosity of the medium.
• the heating media belong to heat transfer media. Similar properties are required from them as the cooling media since in both cases efficient heat transfer is important between the target and the medium regardless of the direction. Particularly in spaces which are temporarily heated the heating medium should also stand low temperatures.
• Temporarily heated spaces include caravans, boats, touring vehicles, summer cottages, temporarily heated sheds and storehouses of various kinds, and small transferable houses like rest cabins which are not meant for continuous living. Heating of such inside spaces is problematic because direct heating means like e.g. electricity is not always available.
• a voluminous heating apparatus When gas, oil, wood, or petrol oil is used in direct heating a voluminous heating apparatus is required and furthermore the fire safety may be endangered. Therefore, indirect radiator heating has become more common in heating e.g. caravans wherein the heating apparatus, e.g. a gas burner may be placed outside the space to be heated and a heating fluid, normally aqueous solution of ethylene glycol, circulates in the radiators.
• the heating apparatus e.g. a gas burner may be placed outside the space to be heated and a heating fluid, normally aqueous solution of ethylene glycol, circulates in the radiators.
• the heat transfer capacity of the solution has an essential influence on the heating efficiency.
• the best heat transfer capacity can be obtained with water.
• the drawback with water is that it can cause serious damage to the apparatus in spaces which temporarily cool below zero.
• Ethylene glycol does not have this drawback but the heat transfer capacity of ethylene glycol is low due to high viscosity. Due to the surface tension of the ethylene glycol solution it tends to cause leaks e.g. as gaskets deteriorate because of large temperature variations. Leaks in turn are injurious due to the toxicity of ethylene glycol.
• the US patent 5104562 discloses a refrigerant with an optimum composition of 1 mole of potassium formate per 6 moles of sodium acetate dissolved in water. According to the US patent 5104562 it would be possible, in the case of pure potassium formate, to reach a freezing point of -35°C with the salt/water mole ratio of 1 :4,72 but even as low as -70°C with the acetate/formate mole ratio of 6:1.
• Cooling liquids are often used in conditions where they are motionless for long periods of time so that the solution according to the above mentioned US patent remains unsolidified, but it will solidify when it is set in motion due to vibration, mixing, or pumping.
• the object of the invention is to obtain a method wherein heat transfer can be realized in an efficient and non-toxic manner.
• the medium is an aqueous solution of potassium formate.
• potassium formate has excellent properties considering its use as a cooling medium.
• the method employed in a cooling process is primarily characterized in that the cooling medium is an aqueous solution of potassium formate wherein the number of moles of dissolved components of potassium formate is at least 80 % of the number of moles of all the dissolved components which depress the freezing point of water and mixing takes place in the cooling medium as it moves relative to the target to be cooled.
• Low operating temperatures can be obtained by using potassium formate in sufficiently high concentrations, preferably in a concentration of at least 30 % by weight.
• the object of the invention is also to present a heating method, heating medium and heating apparatus which are particularly suitable for nordic winter conditions where the temperature of a temporarily heated space may during the intermediate periods decrease at its lowest even 30 °C below the freezing point of water.
• the advantages of the invention are obtained by using an aqueous solution of potassium formate as the heat transfer medium.
• the eutectic point of the known ethylene glycol is at ca -50 °C and then the concentration is nearly 100 percent ethylene glycol.
• the solution according to the invention has the eutectic point at -70 °C and then the concentration is 68 % potassium formate. It is from this fact that a solution that stands even extreme cold is a solution containing about 20 -15 % potassium formate.
• the solution can be manufactured for a given region according to the expected minimum temperatures , and its concentration can be at least 20 % by weight, at least 30 % by weight, or for the North-European conditions even 50 % by weight or more potassium formate.
• Freezing of water divides the solution into two phases, the frozen part and the liquid phase, which prevents damages due to freezing and the sodium formate solution can be taken into heating use with much less pumping energy than using e.g. ethylene glycol as the solution.
• the solution of the invention is the best of the known solutions in terms of its heat transfer properties.
• Some other salt solutions e.g. calcium chloride
• 90 % of the heat transfer capacity can be explained by the viscosity of the corresponding aqueous solution so that also from this point of view the solution of the invention is the best as will be shown later in the presented viscosity comparison in which viscosities of different solutions are compared as a function of the operating temperature.
• the operating temperature denotes the temperature 10 °C above the freezing point of the corresponding solution.
• Fig. 1 shows schematically use of a heat transfer medium
• Fig. 2 shows the phase diagram of aqueous solution of potassium formate
• Fig. 3 shows the viscosity of aqueous solution of potassium formate at different concentrations as a function of temperature
• Fig. 4 shows the viscosity of potassium formate as a function of operating temperature compared with other media.
• Fig. 1 shows the principles in the use of a heat transfer medium.
• the question is about a two-stage method in simplified terms.
• the medium is pumped through a conduit 2 to the target B in which heat transfers between the target B and the medium through a heat transfer surface 3 in a direction reverse of that at the starting point A.
• the target B is in general a space of a specific size where the medium flows as a flow separated from the space by a heat transfer surface 3 such as cooling pipe system, heating radiator, or the like.
• From the target B the medium returns to point A through a conduit 4. It is clear that in case of a cooling medium the apparatus 1 cools the medium and in the case of a heating medium it is heated.
• Freezing point of the solution was measured in the following way.
• the tested solution was dosed in a ( ca. 15 ml) test tube provided with a top. Before closing a glass sphere with a diameter of 8 mm was also placed into the test tube.
• the test tube ( ca. 30 pieces) was placed on a rack which was immersed in the cooling bath of a cryogenic apparatus. The temperature of the cooling liquid was lowered 5 °C / day from the temperature of 0 °C down to -60 °C.
• the test tube was checked daily by lifting each tube from the cold bath, inclining the tube so that the glass sphere moved to and fro in the tube producing a mixing motion. Formation of ice crystals was observed visually.
• Fig. 2 shows the phase diagram of potassium formate in which the freezing point curve is a polynomial fit to the freezing points of potassium formate solutions of various concentrations measured by the applicant and the solubility curve is a polynomial fit to solubility data obtained from literature.
• the concentration of potassium formate is 68 % by weight and the freezing point is ca -70°C.
• Fig. 3 shows the viscosity of potassium formate of different concentrations as a function of temperature measured with an Ostwald capillary viscosimeter tube held in the cold bath
• Fig. 4 shows the dependence of viscosity of potassium formate and known coolants on operating temperature. Operating temperature, in this case, is a temperature 10 °C above the freezing point of the liquid being used i.e.
• potassium formate as such is the best alternative for a cooling medium.
• the freezing point of potassium formate could not be depressed by any additive the applicant tested including potassium acetate, sodium formate, sodium acetate, ethylene glycol, ethanol, urea, and calcium chloride.
• the number of moles of dissolved components of potassium formate must constitute at least 80 % of the total number of moles of the dissolved components which depress the freezing point. These dissolved components may be ions or undissociated molecules.
• a gas operated Primus-type heater was installed, which was kept in a box outside the caravan, and inside the caravan 8 m long series of radiators, pump and thermostat were installed.
• the heating solution an ethylene glycol solution with a concentration of 30 % of ethylene glycol was used according to the directions provided by the supplier of the apparatus. A solution which had the same concentration as the ethylene glycol solution and which thereby had lower resistance to freezing than the ethylene glycol was used as the test solution. Another solution was a 20 % potassium formate solution which was expected to stand -15 °C operating temperature and temporarily even lower temperatures.
• the heat transfer medium may also contain other components which are appropriate for the use of the medium but potassium formate is always the main component constituting over 50 % by weight, preferably over 70 % by weight of all the dissolved components.
• the other components may be e.g. corrosion inhibitors.
• Potassium formate used in the invention may be prepared by direct synthesis from carbon dioxide and potassium hydroxide in the presence of a suitable catalyst. Potassium acetate cannot be prepared by direct synthesis and thus it is much more expensive than potassium formate manufactured as mentioned above. So it seems obvious that potassium formate made by direct synthesis from carbon monoxide and potassium hydroxide is a useful cooling medium which suits to heat transfer in most applications in which the solution must remain unfrozen down to -40 °C. It is also to be noted that potassium formate has considerably better heat transfer properties than the currently used ethylene glycol. Furthermore, contrary to ethylene glycol, potassium formate is less toxic and the decomposition products are not harmful to nature. Consequently, potassium formate is also well suited to a heating medium and due to its resistance to freezing particularly for use in the heating systems of temporarily heated spaces.
• the invention may be used in many existing cooling systems and generally in applications requiring low temperatures and respectively in many existing heating systems.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Combustion & Propulsion (AREA)
• Thermal Sciences (AREA)
• Materials Engineering (AREA)
• Organic Chemistry (AREA)
• Materials Applied To Surfaces To Minimize Adherence Of Mist Or Water (AREA)

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• 1995
  • 1995-11-29 FI FI955754A patent/FI955754L/fi unknown
• 1996
  • 1996-05-31 EP EP96919847A patent/EP0830437A1/de not_active Withdrawn
  • 1996-05-31 WO PCT/FI1996/000327 patent/WO1996039472A1/en not_active Ceased

Non-Patent Citations (1)

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