DIATHERMIC OIL CIRCUIT"

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Field of the Invention

This invention concerns in general the processes or systems using diathermic oil as a heat carrier and heat exchanger with another thermovector fluid, and refers in particular to a method and system provided in order to avoid contamination of the diathermic oil by compounds that may be present in the oil while it is passing through the heat exchanger. Furthermore, the invention applies to a heat exchanger group between diathermic oil and a work fluid in the heat introduction section in ORC (Organic Rankine Cycle) power cycles, which in fact use diathermic oil or another similar fluid carrier as a heat source.

State of the Technique

Diathermic oils are widely used in industry as a heat carrier and are characterized by the fact that it has a very modest vapour tension at the use temperature, often less than the atmospheric pressure. Often it is just this characteristic that makes it preferable compared to other carriers, such as for example steam, and that enables a modest pressure to be maintained, usually between 3-6 bar, inside a heater in which the oil is heated. In particular it is the case of the ORC turbogenerators using a boiler fed by biomass or waste as a heater to heat the diathermic oil, the high temperature flows of gas coming from industrial processes and the concentration of the radiation of the sun in solar heat collectors and the like. Just as an indication, an ORC system according to the state of the technique is schematically represented in Fig. 1 of the enclosed drawings. It basically comprises

a heater 10,

an intermediate circuit 11 passed through by a carrier fluid made to circulate by at least one main pump 11 ',

a heat Exchange group with one or more heat exchangers 12, 13 between the fluid carrier and a preferably organic work fluid,

a turbo-generator 14 fed by the work fluid and usually followed by a regenerator 15, and

a condenser group 16.

The heater 10 can make use of any heat source of the type referred to above and prepare to heat the vector fluid circulated in the intermediate circuit 11. This fluid vector is preferably diathermic oil, even if other suitable fluids are not to be excluded, such as liquid water, molten salt, molten paraffin or the like, which can be used at a low pressure and however less compared with that of the work fluid being fed to the turbo-generator.

The heated diathermic oil flows along the outward line 11a and return line 11 r of the relative intermediate circuit 11 flowing through the heat exchangers 12, 13. Where required, its carrying capacity towards the heat exchanger group can be adjusted by deviating from an outward line to the return line through a three way valve 17.

On the other hand the work fluid is carried in a respective circuit 18 by means of a pump 19 to cross the heat exchangers 12, 13 against the current compared to the diathermic oil and be heated by the latter until it evaporates so as to generate steam and feed the turbo-generator group 14.

In the diagram in said Fig. 1 the heat exchangers represent respectively an evaporator 12 and a pre- heater 13.

The turbo- generator 14 comprises a turbine 20 that can be connected to an electric energy generator 21 or to a mechanical power output.

The steam that exits from the turbine 20 is usually made to pass through the regenerator 15, in which it is possible to recuperate the heat content of the steam so as to pre-heat the work fluid, and in succession in the compensator group 16 to restore the work fluid to the liquid state before it returns to the heat exchange group 12, 13 by means of the pump 19.

Nevertheless, in the actual operation of said systems an increase in the vapour tension of the diathermic oil at a set temperature can take place due to the fact that in the oil, besides the components corresponding to its initial formulization, other components can be present that accidentally enter the oil circuit, such as water or organic compounds which are more volatile than the oil itself present in the heat exchanger process between the diathermic oil and another carrier. The change of the steam tension can be very dangerous for the part of the system upstream, in that it can be, for example, the cause of cavitation problems in the circulation pumps, which besides damaging them can in fact cause the circulation of the fluid to block, with serious consequences such as for example the cracking of the oil in the heater tubes.

In fact, during the operation of the pressure in the diathermic oil circuit, set up moreover by the characteristics of the heater, is usually lower (which is typical in some bars) than the pressure of the work fluid in the heat exchangers. Consequently, a possible imperfect sealing in these exchangers, for example a sag in the connections or welding, becoming the cause of leaks with a flow of the work fluid from the ORC circuit to the diathermic oil circuit, in particular in the most critical points where the difference between the pressures of the work fluid on the one hand and the diathermic oil on the other becomes more. So, an accumulation of work fluid in the diathermic oil risks creating the problems and damage indicated above.

Aims and Summary of the Invention

One aim of this invention is to create in general the conditions to prevent or at least to make very improbable the input of foreign fluids in the diathermic oil during the heat exchange process with other fluid.

A further aim of the invention is to propose a system for preventing or at least making, in particular, an input into the diathermic oil circuit of the circulating work fluid in the heat exchangers of the ORC power cycles very improbable, even in the presence of an imperfect fluid seal in said exchangers.

These aims are reached, according to the invention, by means of a localized pressurization of the diathermic oil circuit in line with only a part of its course in which the heat exchange is carried out with a second fluid flowing in a second circuit. Especially in an ORC system the pressurization of the diathermic oil circuit is carried out within the ambient of the heat exchange group between the diathermic oil and the work fluid feeding the ORC cycle, until the diathermic oil is equal in pressure to the work fluid in any part of the heat exchange group so as to avoid a flow of fluid from one circuit to another should there be faults or defects in the hydraulic seals.

The pressurization localized in this way in the diathermic oil circuit becomes irrelevant on the heater side and can be achieved, according to the invention, by an increase of the pressure of the diathermic oil in an area upstream of the heat exchange group and then by a reduction of the diathermic oil pressure downstream of the heat exchange group until it reaches the operating pressure level of the heater.

Brief Description of the Drawings

The invention will however be explained better in the course of the description provided in relation to an ORC system and in reference to the enclosed illustrative drawings.

In said drawings:

Fig. 1 shows a diagram of an ORC system according to the known technique previously described;

Fig. 2 shows a similar diagram to the one in Fig. 1 and incorporating a first realization method of the invention;

Figs. 3, 4, 5, 6 and 7 show as many different ways of carrying out the invention always in the ambient of a same ORC system; and Fig. 8 shows schematically the system of the invention applied to the ORC system of the type with at least one additional heat exchanger. Detailed Description of the Invention

Starting from the diagram of the ORC system shown in Fig. 1 and described above, a first realization method of the invention, as shown in Fig. 2, consists in inserting on the outward line 11a of the diathermic oil circuit 11 , upstream of the heat exchange group 12, 13, a pressurization pump 31 and on the return line 11 r, downstream of said heat exchanger group 12, 13, a throttle valve 32, that is to say a pressure reduction means. The pressurization pump 31 can be either positive-displacement or centrifugal, operated by a respective motor 33. The throttle valve 32 can be the controlled type, discharge or the like.

The pressurization pump 31 and the throttle valve 32 are managed so as to cause, the first, an increase of the pressure of the diathermic oil in the outward line 11a from the heater 10 towards the heat exchanger group 12, 13 and then the second, a drop in the diathermic oil pressure in the return line 11 r towards the heater. In this way, the diathermic oil pressure can be increased up to the levels of the work fluid pressure in the respective circuit 18. The pressurization of the fluid in the diathermic oil circuit 11 becomes basically localized in the part of the path between the input and output heat exchange group 12,

13 between the diathermic oil and the work fluid that flows in the circuit

18, without influencing the diathermic oil pressure on the heater 10 side.

Therefore the oil which by means of the heat exchange group 12, 13 can be carried and kept at a pressure almost equal to that of the work fluid in the feed circuit 18 of the turbo-generator 14, so as to avoid the oil pressure dropping below the work fluid pressure at any point of the ambient of the heat exchange group and that, should there be leakages, the work fluid can migrate towards the diathermic oil.

This condition becomes checked and reached by means of a direct comparison of the pressures of the two different fluids, diathermic oil and work oil, in the most critical part of the system, corresponding reliably to the A, B zones with less oil pressure and higher work fluid pressure respectively. Zones which, in the absence of important variations in the pressure height in the various points of the system and moderate speed of the fluids in the respective conduits, corresponding basically to the exit of the preheater 13 on the side of the return line 11 r of the diathermic oil circuit 11 and the input of the work fluid in the same preheater 13.

The increased pressure operation corresponds to absorption, consequently a consumption of energy on the part of the pressurization pump 31 which is then dissipated in the throttle valve 32. To minimize this consumption it is preferable for the rotation speed of the pump to be variable. Therefore and preferably, the motor 33 of the pressurization pump 31 and the throttle valve 32 may be managed by a control device 34 fitted out and bearing in mind some pressure and temperature variations in the diathermic oil and work fluid and in particular the difference in ΔP pressure in zone A, and B, individualized above or specifically the pressures of the P1oil and the P2 work fluid found in said same zones. In the example in Fig. 2 a balancing conduit 35 of the flow rate that extends between the outward lines 11a and return lines 11 r of the diathermic oil circuit 11 connecting to them in the junctions 36, 37 is also shown. The control device 34 can therefore be equipped so as to individualize from time to time the flow direction of the oil and its value compared to the one that crosses the heat exchange group 12, 13, by measuring the temperatures T1 , T2 carried out respectively upstream and downstream of junction 36 on the outward line 11a and at least T3 downstream of junction 37 on the return line 11 r, so that said control device will be able to restore the flow in the conduit 35 to very small amounts so as to balance the oil flows outwards and inwards and reduce the energetic load of the pressurization pump 31.

Furthermore, between the input and output of the pressurization pump 31 a by-pass circuit can be provided with a non-return valve 38 to make up for possible pump breakdowns without having to stop the system.

The system may also envisage and provide temperature sensors T4 on the outward line 11a of the circuit 11 upstream of the three-way valve, T5 on the return line upstream of the throttle valve 32 and T6 along the balancing line 35, plus P3 pressure calculators on the outward line 11a and P4 on the output side of the work fluid from the heat exchange group. Other valves and tubes, although useful for the operation of the unit, are not shown in the drawings in that they are normal and unimportant as regards to the description of this invention.

The operating method of the invention shown in Fig.3 is completely analogous to the one described above in Fig. 2 with the only difference that on the return line 11r of the diathermic oil line 11 , downstream (or upstream) of the throttle valve 32 is inserted an hydraulic motor 40, such as a turbine or an auxiliary centrifugal pump used as a turbine, driven by the oil flow in return pressure from the heat exchange group 12, 13 and connected, for example, to an electric generator 41. In this case, the turbine or auxiliary pump 40 carries out the dual function of contributing to lowering the oil pressure going towards the heater 10 and at the same time producing power, enabling a useful recovery of at least a part of the energy supplied by the pressurization pump 31 on the outward line 11a of the fluid circuit 11.

In addition, to the return line between the input of the throttle valve 32 and the output of the hydraulic motor 40, a by-pass circuit 42 with a controlled valve 43 to by-pass said two components 32, 40 can be connected each time it becomes necessary to optimize the adjustment and operation of the system.

According to the operating method of the invention shown in Fig. 4, also fundamentally analogous to the one in Fig. 2, on the return line 11 r of the diathermic oil circuit 11 , downstream of the throttle valve 32 (or upstream of it ) is inserted an hydraulic motor 50 comprising a turbine or a centrifugal pump used as a turbine. The hydraulic motor 50 and the pressurization pump 31 are connected to the same variable speed electric motor 51 depending on the same shaft 52. So the hydraulic motor 50 is operated by the oil flow under return pressure from the heat exchange group 12, 13 and, besides lowering the pressure of the fluid itself, supplies power to operate the pressurization pump 31 , consequently reducing the feed, and therefore the input of external electric energy.

Also in this realization, a by-pass circuit 53 with a respective valve 54 on the return line 11 r between the input of the throttle valve 32 and the output of the hydraulic motor 50 can be connected so as to bypass those two components 32, 50, in the same way as what has been said regarding Fig. 3 and once more so as to optimize the adjustment and function of the system. On the side of the outward line 11a of the circuit 11 an additional pump 55 can be connected in parallel with the pressurization pump 31 with the aim of amplifying the function field in terms of pressure and capacity, in particular by means of a variation in the rotation speed of the same pump 55.

In the way of operating, the invention represented in Fig. 5, and which is basically analogous to the one shown in Fig. 4, a pressurization pump 31 on the outward line 11a and an auxiliary pump or an hydraulic motor 60 on the return line 11 r are provided which are connected to the same electric motor 61 by means of a shaft 62. The pressurization pump 31 and the auxiliary pump or the hydraulic motor 60 are both the volumetric type, the pressurization pump 31 having however a displacement greater than that of the auxiliary pump or hydraulic motor 60.

For the rest, between the input of the throttle valve 32 and the output of the hydraulic motor 60 there can again be provided a by-pass circuit 63 with a respective valve 64 to eventually by-pass said two components 32, 60, and on the outward line of the circuit 11 an auxiliary pump 55 again, in parallel with the additional pump 31.

As regards to the way of operating, the provisions in the invention shown in Fig. 6, are once more provided, and are similar to what has been described in Fig. 5, a pressurization pump 31 and a second pump in parallel 55 on the outward line 11a, whereas on the return line 11 r an auxiliary pump or a hydraulic motor 70 is inserted. The pressurization pump 31 and the auxiliary pump or hydraulic motor 70 are connected to the same electric motor 71 using the same shaft 72. In this case with an opportune scaling of the pump or hydraulic motor 70 it is possible to avoid the presence of the throttle valve.

According to a further way of operating the invention, as shown in Fig. 7, on the outward line 11 a of the diathermic oil circuit 11 is inserted a pressurization pump 31 connected directly to an auxiliary pump 80 by means of a shaft 81 inserted on the return line 11 r of the circuit itself, so that the auxiliary pump 80, operated by the flow of fluid returning from the heat exchanger group supplying power of the pressurization pump 31 by means of the shaft 80. In this case the pressurization pump 31 becomes operated by the pressure coming from the main pump 11 ' provided for the circulation of diathermic fluid in the relative circuit with the addition of the power supplied by the auxiliary motor 80, without the contribution of locai power, such as electric power from external systems.

Ultimately It should be pointed out that a pressurization system such as the one described above is applicable also in systems of the aforementioned type where at least an additional exchanger is present provided to work with a heat exchanger between a fraction of the work fluid from the circuit of the ORC power cycle and a part of the diathermic oil from the vector fluid circuit. This is always aimed at preventing an input of the work fluid, at a higher pressure, into the diathermic oil at a lower pressure, should there be an imperfect seal of the fluid in said additional exchanger.

It is the case, for example, of at least one exchanger 90 provided to pre-heat a part of the work fluid that makes it preferable not to have it pass through the regenerator 15 of the ORC power cycle.

So, from the work fluid side, and as schematically shown in Fig. 8, the additional heat exchanger 90 receives, on entering, a flow of fluid tapped from the circuit 18 in a point situated downstream of the relative circulation pump 19 by means of an off take conduit 91 , and upstream of the regenerator 15, whereas in exit it is connected at a point 92 in said circuit 18 upstream of the heat exchanger group 12, 13, between the latter and the regenerator 15.

On the side of the vector fluid, the additional heat exchanger 90 is connected, at the entrance, by means of a derivation conduit 93, to the return line 11r of the diathermic oil circuit 11 in a point upstream of the throttle unit 32 (where the oil pressure is basically equal to that Of ₁ the work fluid) and in exiting again to said return line 11 r, but in any point 94 downstream of said throttle unit 32 (where the oil pressure is lower).

Also in this case, along the oil flow conduit 95 from the additional exchanger 90 to point 94 connecting to the return line 11 r, a throttle means 96 is inserted to reduce the diathermic oil pressure to be compatible with the pressure in the fluid vector circuit 11 on the heater side. The throttle means 96 can be of any type defined above and also managed by the control device 34 and in relation to the pressurization pump 31 on the outward line 11 a of the fluid vector circuit 11.