Increased solubility of alkanes

Description

[0001] The invention relates to the neutralization of flammable gases containing refrigerant in a brine circuit, hereinafter referred to as heat transfer fluid circuit, of a heat pump system.

[0002] On the one hand, it is well known that heating circuits occasionally need to be vented because air can accumulate in the system. This usually occurs due to leaks at elevated points in the heating circuit, where a leak, combined with negative pressure, leads to air being sucked into the water circuit. In some cases, this also involves air dissolved in make-up water, which is released when it is heated. The same applies to brine-split systems, in which brine is located in the heating circuit or in an outdoor box or in a geothermal probe, and also to air conditioning systems. Depending on the purpose of such a heat transfer fluid circuit or the environmental conditions it has to cope with, additives are added to the heat transfer fluid circuit in addition to the actual heat transfer medium, usually water. These are usually antifreeze or corrosion inhibitors.

[0003] On the other hand, flammable refrigerants are now used as working fluids in heat pumps and refrigeration and freezing systems. These refrigerants have the advantage of not damaging the climate or the ozone layer if accidentally released. However, such accidental releases should be avoided wherever possible due to their flammability. In refrigeration circuits in which such working fluids are used, such unintentional releases can occur via the heat exchangers, which act as condensers and evaporators and are connected to the heat transfer fluid circuit, i.e., the heating circuit or cooling brine circuit, via their exchange surfaces. Unlike conventional gas-fired boilers, the working fluid in the refrigeration circuit is under a higher pressure than the heat transfer fluid in the heating circuit or cooling brine circuit; therefore, in the event of a leak, it could easily enter the heat transfer fluid circuit, which is under lower pressure.

[0004] To prevent this as much as possible, the conventional state of the art uses expensive double-walled heat exchangers, such as those described in patents DE 11 2019 001 344 T5, DE 11 2019 001 350 T5, and DE 11 2019 001 351 T5 for heat pumps. These heat exchangers conduct the working fluid R290 against a water-propylene glycol brine as the heat transfer fluid. In addition to the high price, this use leads to efficiency losses, as the materials, such as stainless steel, are poor heat conductors and the thin air gap between the heat exchanger surfaces acts as an insulator. In practice, this means that larger temperature differences must be set in the heat exchangers, which reduces the efficiency of heat pumps. While these double-walled heat exchangers can reduce the risk, they cannot eliminate it.

[0005] Such gaseous components that nevertheless enter the heat transfer fluid circuit through leaks could be separated using gas separators, but dissolved and very fine-bubble gas components remain. These could cause problems during temperature and pressure changes by outgassing at unfavorable points in the heat transfer fluid circuit if their solubility deteriorates due to temperature. This particularly applies to the now widely used propane in refrigerant R290, but also to isobutane in refrigerant R600a, n-butane in R600, propylene in R1270, and other alkanes.

[0006] Propylene glycol is the preferred antifreeze for aqueous heat transfer fluid circuits. This has the advantage of being completely miscible with water and non-toxic to humans and the environment in the event of an accident. One advantage of this antifreeze is that it significantly increases the solubility of R290 and other alkanes. Ethylene glycol is also frequently used. A corrosion inhibitor is typically added to prevent corrosion.

[0007] Modern heat pumps can also be designed to switch between heating and air conditioning modes, and can also produce hot water. This further increases the temperature fluctuations occurring in the system. Further problems can arise from the fact that many heat pumps are not in operation year-round, and leaks can also occur outside of operation, for example, when brine in a solar thermal system heats up too much.

[0008] Therefore, it is important to prevent the working fluid from remaining undissolved or from dissolving in the heat transfer fluid. This prevents it from outgassing at unfavorable points during temperature fluctuations and forming flammable mixtures when mixed with air.

[0009] Since alkanes are non-polar, non-polar but water-soluble substances are added to the heating brine or heating water. These significantly improve the dissolving behavior and, if they leak from the working fluid circuit into the heat transfer fluid circuit, no longer need to be separated as gas bubbles. However, it is important to ensure that the leaked working fluid can dissolve in the heat transfer fluid under all circumstances.

[0010] The addition of the solubility-enhancing substances should be as straightforward as adding conventional corrosion inhibitors, preferably only every few years as part of maintenance work. The solubility should ideally be so good that continued operation after the leak has been repaired without replacing the heat transfer fluid.

[0011] When adding solubility-improving substances, the expert, taking into account the existing conditions, pays attention to the selection and dosage to prevent the solubility-improving substance reacts chemically with the other components of the heat transfer fluid, miscibility gaps arise in which demixing can occur, the flow behavior of the heat transfer fluid changes unfavourably, for example by becoming viscous in certain temperature ranges, the piping system corrodes or the solubility-improving substance reduces the anti-corrosive effect of the added corrosion inhibitor, the freezing point of an antifreeze rises, the heat transfer fluid foams.

[0012] These conditions will be examined in more detail below. First, the solubility without additives must be considered. Pure water is known to dissolve alkanes very poorly, but the working fluid R290 – propane – dissolves comparatively well in water at higher pressures and lower temperatures. If glycol is added, propylene glycol and ethylene glycol are commonly used as antifreeze agents, the solubility in the working fluid increases further. This means that brine requires fewer extensive measures than simple heating water.

[0013] The combination of propane or R290 as the working fluid for refrigeration machines and propylene glycol as the working fluid is already well known. The publication "Frank Hillerns, Thermophysical Properties and Corrosion Behavior of Refrigerants, DIE KÄLTE Klimatechnik 10 / 1999" describes the behavior of such brines and possible alternatives in detail. Refrigerants based on organic acetate or formate salts are primarily presented as alternatives. Refrigerants and the necessary corrosion inhibitors must not react chemically with each other. Available refrigerant concentrates also contain hardness stabilizers, defoamers, and possibly dyes. Here, too, chemical reactions must be avoided. A detailed description of this can also be found in EP 897 417 B1.

[0014] Further methods and devices for overcoming this problem are known from EP 3 764 001 A1 and EP 3 882 526 A1. In EP 3 764 001 A1, a bypass flow of a heat transfer fluid for a jet nozzle is used to cause a pressure drop through the resulting flow acceleration, which leads to dissolved refrigerant outgassing from the liquid in gaseous form and becoming separable. In EP 3 882 526 A1, gaseous refrigerant and air are separated together from a heat transfer fluid and then separated from each other by means of a membrane. Although both teachings can be combined successfully, it cannot be prevented that further dissolved refrigerant could be present in the heat transfer fluid of the secondary circuit and then outgassing at an unfavorable location in the heat transfer circuit under unfavorable conditions.

[0015] The object of the invention is therefore to ensure that gaseous alkane-containing working fluid components can dissolve safely in the heat transfer fluid and remain dissolved in the heat transfer fluid. The invention achieves this object by adding additives to the heat transfer fluid in conjunction with a mixing process and a mixing station.

[0016] The invention solves the problem by an additive for increasing the solubility of alkanes dissolved in heat transfer fluid, wherein the heat transfer fluid is a mixture of water, antifreeze and corrosion inhibitors, wherein the additive is at least one water-soluble hydrocarbon compound.

[0017] In particular, the medium-length, water-soluble hydrocarbon compound is selected from a group consisting of alcohols, alkanoic acids, fatty acids, fats, aldehydes, and ketones. These are preferably either ethanol, a soap, or a sugar compound, or mixtures thereof. Alternatively, it can also be an ionic liquid with medium-length chains. Coated nanoparticles, on whose surface alkanes are adsorbed, can also be added.

[0018] If working fluid leaks from one of the heat exchangers operating as a condenser or evaporator into one of the heat transfer circuits, the extent of the leak and the temporal progression of the leak are initially unknown. In many cases, the working fluid circuit can be stopped and the working fluid trapped or pumped out. However, most leaks are so small that they go unnoticed. In these cases, the working fluid dissolves without any problems in the heat transfer fluid according to the invention, and the system can continue to operate without further action until the next routine maintenance.

[0019] The situation is different if a major leak suddenly occurs, such as a large crack in one of the plate heat exchangers, perhaps due to contamination or chips from assembly residues found in the heat transfer fluid. In this case, there may not be enough time to shut off the working fluid circuit. Unfortunately, this cannot be predicted in advance, so precautions must be taken even for this rare eventuality.

[0020] The problem with a large crack and a rapid leak of working fluid is that the heat transfer fluid is displaced over long distances by the gaseous working fluid, since the working fluid is usually under significantly higher pressure than the heat transfer fluid. This displacement prevents the heat transfer fluid from exerting its dissolving effect, and also causes conventional pumping equipment such as high-efficiency circulating pumps to dry out and become damaged. Furthermore, it is no longer possible to pump heat transfer fluid, resulting in so-called gas pockets. This means that the gas pockets cannot be safely removed from the heat transfer fluid circuit if the flow collapses, and very little of the alkane-containing gas in the heat transfer fluid would be able to be released.

[0021] To prevent this, a mixing station according to claim 1 and a method according to claim 4 are provided. The mixing station causes gaseous working fluid in the heat transfer fluid to be brought into contact with a quantity of heat transfer fluid sufficient for dissolution before it can enter the brine circuit.

[0022] For this purpose, the circulation pumps for the heat transfer fluid are arranged on the inflow side of the respective heat exchanger. To ensure that in the event of a leak, the working fluid cannot force the heat transfer fluid through the circulation pump in the opposite direction to the intended flow direction and thus enter the circulation pump, the circulation pump is protected by a non-return valve.

[0023] A closed storage tank for heat transfer fluid is arranged on the downstream side of the respective heat exchanger. Such storage tanks are often used for the temporary storage of heat transfer fluid; if this is the case, such a tank can be used. It is continuously flowed through with heat transfer fluid during operation. A float is provided in the upper area of ​​the storage tank; in the event of gas ingress, its sinking sends a signal to a circulation pump. This circulation pump has a forced flow system (Roots pumps or peristaltic pumps are suitable, for example) and pumps the fluid through the storage tank. Optionally, a static mixer is used. The fluid can be added to the storage tank, for example, via a spray device in the top area of ​​the tank.As soon as the float signals that there is no more volume of undissolved gas in the storage tank, the heat transfer fluid circuit can continue to operate until the defective heat exchanger is replaced or repaired.

[0024] Smaller dimensions can be selected for this forced-feed circulation pump, as well as for the downstream static mixer. Both can be integrated into the storage tank as a set together with the float. Alternatively, the float can simply shut off the heat transfer fluid circulation until a pumping device is connected to the designated connections in the storage tank during repair work.

[0025] Fig.1shows a process flow diagram with a mixing device for each heat exchanger. The heat pump 1 has a counterclockwise cycle 2 with a compressor 3, a condenser 4, an expansion valve 5, and an evaporator 6. In heating mode, the heat transfer fluid 7 is drawn from the condenser 4 and fed via the connecting line 11 into the mixing station 100. There, it passes through the mixing tank 101, which is normally completely filled with heat transfer fluid. In this normal case, the float switch 102 indicates that no gas is present or accumulating in the upper area of ​​the mixing tank. If traces of working fluid were present in the heat transfer fluid, they would dissolve in the mixing tank. The heat transfer fluid can therefore be fed to the heating circuit as heated heat transfer fluid via the valve 104 and the connecting line 12.After the heat has been released, it is fed back into the return line 8 of the condenser 4 by the circulation pump 10 and the non-return valve 9.

[0026] However, in the event that gaseous working fluid should collect in the upper area of ​​the mixing tank, which can only occur in the event of a major leak, the float switch 102 triggers a signal that starts the forced-feed circulation pump 105. Furthermore, the gas-liquid mixture is directed from the mixing tank into the static mixer 103, and the connection to the circulation pump 105 is opened in valve 104, while the distribution valve 104 blocks the flow into the connecting line 12. As soon as the float switch 102 indicates that the gaseous working fluid has dissolved, the path to the heating circuit can be opened again via the valve 104.

[0027] Shut-off valve 13 serves only as a safety measure and can remain closed throughout the entire mixing process. Only in special cases, when gas is already present in the heating circuit, can shut-off valve 13 be opened, allowing backflow into connecting line 11 and subsequent mixing in mixing station 100. This is particularly useful when mixing station 100 is used as a mobile station, which is only connected when other indications indicate that undissolved working fluid is present in the heating circuit.

[0028] On the cold side with evaporator 6, the same scheme is used in an analogous manner. Here, mixing station 200 corresponds to mixing station 100. Circulation pump 16 conveys heat transfer fluid via non-return valve 15 into inlet 14 of evaporator 6, where the heat transfer fluid exits via outlet 17 and is fed via connecting line 18 into mixing station 200. There, it passes through mixing tank 201, which is normally completely filled with heat transfer fluid. Float switch 202 normally indicates that no gas is present or accumulating in the upper area of ​​the mixing tank. The static mixer 203 corresponds to the static mixer 103, the distribution valve 204 to the distribution valve 104, the circulation pump 205 to the circulation pump 105, each with forced conveyance, the connecting line 19 to the connecting line 12, the shut-off valve 20 to the shut-off valve 13. List of reference symbols

[0029] 1 Heat pump 2 Cycle 3 Compressor 4 Condenser 5 Expansion valve 6 Evaporator 7 Heat transfer fluid 8 Return 9 Check valve 10 Circulation pump 11 Connecting line 12 Connecting line 13 Shut-off valve 14 Inlet 15 Check valve 16 Circulation pump 17 Outlet 18 Connecting line 19 Connecting line 20 Shut-off valve 100 Mixing station 101 Mixing tank 102 Float switch 103 Static mixer 104 Valve 105 Circulation pump 200 Mixing station 201 Mixing tank 202 Float switch 203 Static mixer 204 Valve 205 Circulation pump

Claims

1. A mixing station (100, 200) for mixing alkane-containing gas from a leakage occurring in a heat exchanger of a heat pump (1), wherein - the heat pump (1) has a counterclockwise thermodynamic cycle (2) by means of an alkane-containing working fluid which is guided in a closed, hermetically sealed working fluid circuit, - furthermore at least one compressor (3) for working fluid, at least one expansion device (5) for working fluid, at least two heat exchangers (4, 6) for working fluid, each with at least two connections (7, 8, 14, 17) for heat transfer fluids, of which at least one is connected to a heat transfer fluid circuit with a heat transfer fluid and a circulation pump (10, 16), - wherein the mixing station (100, 200) comprises at least one container (101, 201), another circulation pump (105, 205) and a static mixer (103, 203), - the tank (101, 201) has a connection to the static mixer (103, 203) and the other circulation pump (105, 205), - the circulation pump (10, 16) is arranged on the inflow side of the heat exchanger (4, 6) and a backflow preventer (9, 15) is provided between the circulation pump (10, 16) and the heat exchanger connection (8, 14), which prevents backflow from the direction of the heat exchanger (4, 6), - the container (101, 201) with heat transfer fluid is connected to the outflow side (7, 17) of the heat exchanger (4, 6), characterised in that - the container (101, 201) is equipped with a float switch (102, 202) in the upper area of the container (101, 201), - the other circulation pump (105, 205) is equipped with forced delivery and has a return line to the tank (101, 201), - the tank (101, 201) or the static mixer (103, 203) or the other circulation pump (105, 205) with forced conveying is connected to the heat transfer medium circulation.

2. Mixing station according to claim 1, characterised in that the tank (101, 201) forms a structural unit with the float switch (102, 202), the other circulation pump (105, 205) with forced delivery and the static mixer (103, 203), which unit can be dismantled or connected as a whole and can be operated in the bypass flow.

3. Mixing station according to one of claims 1 or 2, characterised in that a gas-liquid separator is arranged between the heat exchanger (4, 6) and the container (101, 201), and the gaseous portion is conducted either through a line into the open air or into the connection to the container (101, 201).

4. Method for dissolving refrigerant-containing gas in the heat transfer fluid from a leak in a heat exchanger (4, 6) of a heat pump (1), characterised in that an additive for increasing the solubility of refrigerant dissolved in heat transfer fluid is added to the heat transfer fluid and the heat transfer fluid is passed from the heat exchanger (4, 6) into a mixing station (100, 200) according to one of claims 1 to 3, and the gaseous refrigerant is circulated in this mixing station (100, 200) until the refrigerant has completely dissolved in the heat transfer fluid and the heat transfer fluid with the dissolved refrigerant is then fed into the heat transfer fluid circulation.

5. The method according to claim 4, wherein the mixing station (100, 200) corresponds to claim 3, characterised in that the presence of undissolved gaseous working fluid is measured by means of the float switch (102, 202) or the gas-liquid separator.

6. Process according to one of claims 4 or 5, wherein the heat transfer fluid is aqueous brine with additives of antifreeze and corrosion inhibitors and the refrigerant is a paraffin or an olefin or a mixture thereof, characterised in that, in order to increase the solubility of refrigerant dissolved in heat transfer fluid, so much water-soluble carbon-hydrogen-oxygen compounds are added that all refrigerant entering the brine dissolves in the brine.

7. Process according to claim 6, characterised in that the water-soluble carbon-hydrogen-oxygen compounds to be admixed are selected from a group formed from alcohols, alkanoic acids, fatty acids, fats, aldehydes and ketones, each with chain lengths of 2 to 6 carbon atoms.

8. Process according to claim 7, characterised in that the water-soluble carbon-hydrogen-oxygen compound to be admixed is ethanol.

9. Process according to claim 7, characterised in that the water-soluble carbon-hydrogen-oxygen compound to be admixed is a soap.

10. Process according to claim 7, characterised in that the water-soluble carbon-hydrogen-oxygen compound to be admixed is a sugar compound.