Title: SYSTEM FOR CREATING A VACUUM IN A TANNING DRYER.

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The present invention relates to a system for creating a vacuum in a multi-layer tanning dryer, according to the general definition in claim 1.

So-called "vacuum" machines are already well known and widely used in the tanning sector, and are used to dry wet skins after they have been subjected to tanning and dyeing processes .

Basically, a "vacuum" dryer comprises a range of heated levels on which the wet skins are spread. These are then closed with a "lid" cover in order to form a drying chamber, inside which a vacuum is then created to accelerate the evaporation of the liquid substances contained in said skins, substances which are transformed into gases and vapours .

In particular, these dryers comprise a system for heating the levels and a system to create the vacuum in the drying chambers on the various levels.

In order to create the vacuum, the system is basically composed of one or more suction units, defined by the term "vacuum pump", which suck in the gases and vapours from the drying chamber and the condensers, located upstream of the aforesaid pumps, and which are used to cool part of the vapours until they are condensed.

Current technical state of the art uses "liquid ring" or "vane" type vacuum pumps (such as those manufactured by the companies TRAVAINI, ROBUSCHI and P. V. R.) and it is also possible to associate a secondary suction pump with the vacuum pumps, positioned in series and upstream of the main vacuum pump (refer to patent document N° 01270767).

A drawback that arises with this type of system is represented by the fact that when the damp gaseous substances (gases and vapours released by the drying skins) are sucked in by the liquid ring or vane pumps, they mix with the working fluids (the water or oil that must ensure the hermetic sealing and lubrication of moving parts) , to form a strongly corrosive polluting mixture that provokes rapid deterioration of the vanes and other components of the aforesaid pumps, thus requiring constant maintenance and creating considerable running costs.

Furthermore, said polluting mixture (the lubrication fluids necessary for liquid ring or vane vacuum pump operation, plus the gases and vapours released in the hermetically sealed vacuum chamber during skin drying action, and that are sucked out because of vacuum action) cannot be discharged directly into the sewer system because it would provoke strong pollution, and therefore the system must also be equipped with a separator.

Moreover, it must be remembered that while running, the pump must be constantly supplied with enough working fluid to dispose of the heat generated by the gas compression, and to replace the working liquids that exit from the pump together with the gas. This means that the size of said separator must be considerable, and this too has an influence on purchase costs and also involves the running of the separator equipment.

The object of the present invention is to create a vacuum system that eliminates the aforesaid problems, in other words, that does not result in the emission of polluting substances such as emulsified oil and polluted water that require complex and expensive processing; in particular, during skin drying operations, the system must release vapours and gases that, when condensed, can be easily collected and taken to disposal plants.

A further aim of the invention is to create a vacuum system that is able to provide very strong suction and reduced pressure performance levels, with power consumption, which is considerably lower than that of prior art.

These aims are achieved with the creation of a vacuum system that does not require the use of any traditional type of vacuum pump, in other words, liquid ring or vane pumps, while, on the contrary, the vacuum is created advantageously by using so-called "dry" low head blowers, arranged and operating in an appropriately defined circuit.

These blowers, familiarly known by the term "Roots blower", are very simple to build, and have the added advantage of not requiring lubricants that come into contact with the gaseous substances to be compressed, as occurs with liquid ring or vane pumps, and therefore they do not generate polluting mixtures and thus prevent the problems described above.

As far as construction is concerned, the system is characterised in that it involves a circuit positioned downstream of the vacuum condenser, inside which at least two 3-lobed Roots blowers are inserted. These are better known as vacuum pumps with pre-inlet and a heat exchanger with condensation recovery.

The system according to the invention can be possibly completed with a third 2-lobed Roots blower, better known as a vacuum blower, set between the vacuum condenser and the aforesaid circuit. Said Roots blower operation is controlled by a by-pass valve.

The system is completed with suitable auxiliary equipment such as at least one gaseous substance collector tank, a silencer with a suction filter on environment inlet, and a silencer on the fume discharge.

Structurally, the two Roots blower vacuum pumps are positioned in series and their suction ratio is approximately 5:1; the first Roots blower vacuum pump is positioned with the suction opening for cooling at pre-inlet in communication with the environment, and therefore the absolute pressure at the suction opening is approximately 200 mbar.

The second Roots blower vacuum pump is positioned upstream and in series with the first Roots blower, and therefore its suction opening pre-inlet communicates with the reduced pressure pipe at 200 mbar, and because of the suction ratio of 5:1, the absolute pressure at its suction opening is approximately 40 mbar.

In order to permit the aforesaid Roots blower to operate with an absolute pressure of 40 mbar at the suction opening inlet, in relation to a pressure of 200 mbar, the delivery opening of the aforesaid is connected by means of a second pipe to said first pipe, which is the inlet pipe for the suction opening of the first Roots blower.

Since the maximum admitted differential pressure on this type of machine depends on the thermal load, a heat exchanger is inserted into the second pipe, to provide for the condensation of the vaporised liquids present in the gases sucked in from the vacuum chamber, as well as for cooling the fluid.

In order to lower the vacuum level even further, a third two-lobed Roots blower is activated in the connecting pipe between the second Roots blower and the vacuum condenser, which when activated by a by-pass valve, and working together with the other two Roots blower, will provide a final vacuum level lower than 2mbar.

The operating mode described above is "running mode" which means that this occurs when the vacuum chambers are

hermetically sealed and the system operates with the highest possible vacuum level.

In cases where there is a variation in the maximum vacuum level, in other words, a sudden rise in reduced pressure level due to the opening and successive closing of one of the drying plates, and therefore the need for suction action to generate a vacuum again in the chamber on said shelf, the aforesaid circuit will operate in "self-balancing mode".

In practice, the third blower is excluded and the flow rate of the fluid sucked in from the machine and discharged by the second Roots blower is subdivided by the two aforesaid pipes into a first part which is sucked in by said second Roots blower, and necessary for said Roots blower to operate, and the remaining part in excess, which is sucked in by the first Roots blower and expelled into the atmosphere.

The self-balancing differential in the suction by the two Roots blowers is gradually reduced with the progressive increase of the vacuum in the circuit, and it will finally disappear when the second Roots blower begins operating at its maximum differential pressure, in other words, when the absolute suction pressure returns to approximately 40 mbar, as opposed to an inlet/discharge pressure of approximately 200 mbar.

The invention will become more apparent with reference to the appended drawing where:

Figure 1 shows a layout of the system applied to a multi-level vacuum dryer.

As can be seen in figure 1, the system according to the invention is connected to a condenser "K", into which are directed the vacuum intakes of the various levels of the vacuum "S" by means of on-off valves "V", and which is

basically composed of three Roots blowers, identified by numerals 1, 2 and 3, of a by-pass valve 4, a heat exchanger 5, equipped with a silencer with a suction filter 7, and a silencer mounted on the gaseous substance discharge 8, said devices being connected by means of piping as specified further on.

In particular, the Roots blowers 1 and 2 are three-lobe types, better known as pre-inlet vacuum pumps, while Roots blower 3, is a two-lobe vacuum blower.

Figure 1 shows the two Roots vacuum pumps 1 and 2 positioned reciprocally in series, since their suction ratio is 5:1, and since the first Roots vacuum pump 1 is positioned with the suction opening for cooling at the pre- inlet, in communication with the environment, it has an absolute pressure of approximately 200 mbar in its suction opening "A" .

The second Roots blower is positioned upstream and in series with the first Roots blower 1, and therefore results as having the pre-inlet suction opening in communication through a first connection pipe 10, with a zone in reduced pressure at 200 mbar. Therefore because of the ratio 5:1, it has an absolute pressure of approximately 40 mbar in its suction opening "B".

In order to permit the Roots blower 2 to operate with an absolute pressure of 40 mbar at the suction opening inlet, in relation to a pressure of 200 mbar, the delivery opening must be connected by a second pipe 20 to the aforesaid first connection pipe 10, inside which a heat exchanger 5 has been inserted, and equipped with a condensate discharge tank 6.

Lastly, in pipe 30 that connects Roots blower 2 to the vacuum condenser "K", a possible third Roots blower 3 can operate, activated by a by-pass valve 4 and working together with the other two Roots blowers 1 and 2. This will produce

a final vacuum level in the circuit that is lower than 2 mbar .

The description above refers to "running condition", but when the reduced pressure level rises in "self- balancing" condition, or rather when the third blower is excluded, through valve 4 the flow rate of the fluid sucked in by the machine "S" and discharged by the second Roots blower 2 is subdivided, by means of thethe two aforesaid pipes 10 and 20, into two parts: the first part which is sucked by said second Roots blower 2, and which is necessary for it to operate; and the remaining part in excess, sucked in by the first Roots blower 1, and expelled into the atmosphere (see direction of arrows) . Said self-balancing action of the two Roots blowers 1 and 2 is maintained until the second Roots blower 2 returns to working at its maximum differential pressure- in other words, when the absolute suction pressure in opening "B" returns to approximately 40 mbar, in relation to the pre-inlet pressure of approximately 200 mbar.