The invention refers to a device able to make and keep the solvent for dry cleaning machines at a optimal temperature to better actuate the degreasing action. In the dry cleaning machines the solvent temperature has an important part. It happens, in fact, that under a certain temperature the washing capacity of the solvent is smaller, while over a limit temperature the solvent may be too aggressive onto the fabrics to dry wash. In machines that use combustible solvents, the high solvent temperature may be also a risk factor for the machine if are exceeded the combustion temperatures of the same solvent. In the art systems are known that use hot water coming, for example, to the refrigerator to heat the solvent of the dry cleaning machine. These systems, however, heating all the solvent in the machine, create perfect conditions for the generation and proliferation of bacteria. These colonies of bacteria are responsible for exhalations with bad smells in different parts of the machine, particularly concentrated in the solvent tank and in the filters. Moreover, the systems known in the art keep the solvent at a optimal temperature only for short times. A further known system is described in document US2005/0066538. In said system the bacteria are demolished with ultraviolet lamps or nitrogen generators, but the use of these technical means determine an increasing of the operating costs and of realization costs of the dry cleaning machine. The solvent temperature can, however, change in different conditions, as those when change the environment temperature in where the machine acts. Moreover, the solvent temperature also during the day changes. It happens, in fact, that in the morning the solvent has temperature lower of the medium, with consequent results of bad dry washing at the first or at the first working cycles. Other drawback of the known systems is given to the fact that, changing the solvent temperature, also change the absorption of the dampness from the same, for example in the cartridge filter, with consequent increasing of the risk of bigger concentration of water in the solvent, with washing problems and bacteria proliferation. Subject matter of the invention is a device able to avoid the stated drawbacks by the heating of the only solvent to be used during the washings. In this way the operating costs are limited, the energetic consumptions are lowered and it is created a dry cleaning machine prompt for the use till the first washing cycle, such as the solvent is heated at the optimal temperature in each using condition. The invented device keeps the temperature in its inside for a long time, i.e. till 48 hours. The invented device is particularly suitable in the case of nebulization process, as described in document EP 1722026, where the quantity of used solvent is clearly lower of which of the bath machines, so it is not necessary a heat quantity particularly high to heat the solvent in the washing. The invented device is however to be used also in dry cleaning machines with bath washing of the fabrics in the solvent. Said device comprises the use of water heated by a refrigerator of the dry washing machine to act, through heat exchanging, the heating in place and in real time of the sole solvent to be used for the fabrics washing, without heating all the solvent present in the machine, actuating in this modality an energetic saving given to the heating of the sole solvent to be used and avoiding the formation of bacteria in the tank and in the filters. The device provides, moreover, in the heat exchanging, the use of a material with high thermal inertia, such as paraffin wax, inside a tank thermally insulated from the outside, with two heat exchangers bathed in the tank with the paraffin wax and a feed back control circuit. The used paraffin wax, coming in contact inside the thermally insulated tank with a worm-pipe formed to the hot water pipes coming to the refrigerator, changes its physical state passing from solid to liquid. In this way the heating given from the hot water to the refrigerator is stored inside the tank and kept till two days. A second worm-pipe formed to the pipes with inside the solvent, placed also it inside the thermally insulated tank and bathed in the same paraffin wax, actuates the heat exchange of the sole solvent to be used. The feed back control circuit with thermostat and electrovalves permits to have the temperature inside the thermally insulated tank in a value interval clearly defined and in function of the degreasing action of the solvent. The invented device so does not employ additional heat sources and outside the device, such as electric resistances or similar, to keep in temperature the solvent to be used for the washing, this is all to advantage of the containment of the operating costs. Further aims and advantages of the present invented device are better pointed out with the following description, to be considered as example and not limiting and referring to the enclosed drawing of sheet 1 where the figure 1 is diagram of the used components in a preferred but not limiting embodiment. The device to optimize the washing in dry cleaning machine comprises an inlet pipe 1 for cooling water to a refrigerator 2 and a outlet pipe 3 for hot water from the refrigerator. The outlet pipe 3 provides a T-shaped connection 4, with pipe 5 of connection to an electrovalve 6 normally opened. In exit from the electrovalve 6 a pipe 7 brings hot water in exit to a heat exchanger 11 consisting, for example, in a copper warm-pipe with fins. The heat exchanger 11 is placed inside a thermal insulated tank 8 with, at the walls and at the upper cover, insulated material 9, such as thermal insulator armaflex ^® . Inside the thermally insulated tank 8 is present also a second heat exchanger 14, consisting for example of a copper warm-pipe with fins. Said heat exchanger 14 is crossed in its inside to solvent to be heated, coming to pipe 15 flowed into the tank 8. The pipe 15 is connected with a filter 16. The filter 16 receives the solvent in entrance by the pipe 17, where the solvent is pushed from the tank to the filter 16 and to the thermal insulated tank 8 by a solvent pump 18. The heat exchanger 11 is connected in exit to a pipe 10 and, by T-shaped connection 12, to the pipe 3 of hot water in exit from the refrigerator and the device. To the T-shaped connection 4 is connected a pipe 19 in connection with an electrovalve 20 normally closed. At the exit of the electrovalve 20, the pipe 21 puts in connection said electrovalve with the T- shaped connection 12. At the end the hydraulic circuit is completed to pipe 22 connected to the heat exchanger 14 in where flows in exit from the thermal insulated tank 8 the solvent to reach to the electrovalve 23. By the T-shape connection 24, placed on pipe 25 and connected at the exit of the electovalve 23, can be connected, in other embodiment and with suitable T-shaped connections 24A, electrovalves 23A and 23B to other uses of the heating solvent. From the T- shaped connection 24 the pipe 26 brings the heating solvent to the connection 27, to which are connected by pipes 28 and 29 nebulizing nozzles 30 from which the heating solvent is sprayed in the basket of fabric washing. In other embodiment for dry washing machines with bath of the fabrics the heating solvent reaches the basket with direct pipe to the basket, i.e. without nebulizing nozzles. The device is completed to the feed back control circuit comprising for example a thermostat 31 , with probe placed through hole onto the cover of the thermal insulate tank 8 in the paraffin wax 13. The thermostat 31 has working values that can be regulated on the base of the solvent characteristics. Through cable 32 the thermostat 31 is connected with the electrovalves 6 and 20. In operating phase the cooling water reaches the refrigerator 2 by pipe 1. In exit of the pipe 3 the hot water flows into the T-shaped connection 4, into the pipe 5 and crosses the electrovalve 6 normally opened to reach, passing into the pipe 7, the heat exchanger 11. The heat of the hot water, in the warm-pipe of the heat exchanger 11, melts the paraffin wax 13 inside the thermal insulated tank 8. The heating of the water inside the warm-pipe of the heat exchanger makes fluid the paraffin wax 13 and the temperature increases till a temperature that depends to the solvent characteristics. This temperature threshold is stated in function of the degreasing capacity of the solvent and to the machine security. Inside the thermal insulated tank 8 happens the heat exchanger between the hot water and the solvent, placed inside their heat exchangers 11 and 14. The thermal insulation offers to the insulating material 9 of the tank 8 keeps the paraffin wax liquid till two days, with the immediate availability of the solvent heated to the stated values. At the overcoming of the value of stated temperature, the thermostat sets at this temperature gives electric impulse to the cable 32 and so to the solenoids of the electrovalves 6 and 20, bringing the electrovalve 6 from normally opened to closed and the electrovalve 20 from normally closed to opened. In this way the hot water in exit to the refrigerator 2 trough the pipe 3 does not reach inside the thermal insulated tank 8 but it is dispelled from the pipe 3 in other parts of the dry washing machine. The heating solvent, through the pipes 22, 25, 26, 28 and 29, reaches the washing basket to be poured in it or to be nebulized by the nozzles 30. The paraffin wax 13 to be used for the thermal exchange between the heat exchangers 11 and 14 is cheap, without smells and without risks per the health and the environment. The solvent present in the tank and in the filter cartridge is kept cold, avoiding the generation and the growth of bacteria. The invented device is possible of changes and variations all part of the present subject matter and in the protection field stated to the claims.