Process for the compression of steam and thermal circuits for its implementation

The present invention relates to a process for the compression of steam and thermal circuits for its implementation.

It is known that the adiabatic compression of saturated steam in compressors currently used for steam, e.g. rotary or piston dry volumetric compressors, has the disadvantage that the compressed steam which is obtained is very much superheated. As a result of the difficulties in lubrication at such high temperatures, the machine can seize up. This entails the necessity of de-superheating the steam, which is also required for using it in conventional condensers, and from which there results complication of the technology of the compressor and an increase in its cost.

The aim of the invention is generally to get over this disadvantage and to define a process and circuits for the compression of pure steam without superheating it, and with the recovery, in the form of compressed saturated pure steam, of the mechanical energy for the compression.

A more particular aim of the invention is to obtain steam compression at very low pressure and even in a vacuum, to obtain saturated steam at a higher pressure.

In practice, it can be useful for obtaining saturated steam at high pressure, to create a heat supply in a drying enclosure, from very low pressure saturated steam (under partial vacuum), coming from an evaporator, the heat for vaporisation being supplied by a condenser fed with the steam and/or humid air extracted from the enclosure and coming from the dried product. Such a thermal circuit associated with a drying enclosure and using the heat pump principle is described in French patent application No. 2,341,830, in the name of the applicant.

These different aims can be reached, in accordance with the invention, in that, for the compression of the steam, particularly at very low pressure, at least one liquid ring compressor is used, using water as the compression agent.

The detailed description of such a compressor does not need to be given here. It is sufficient to recall that inside a stator of suitable shape, rotates a rotor fitted with vanes which set in motion the liquid contained in the stator and project it against the wall, in the form of a kind of ring. The gas to be compressed is admitted through ports and confined in the spaces defined by two successive vanes, the centre of the rotor and the inner surface of the liquid ring. This gas is compressed by reduction of the volume of these spaces and discharged through discharge ports.

It has been found experimentally that the use of such a machine, going moreover against the prejudices of the man skilled in this field of the art, was perfectly adapted for solving the problem posed, as this will be demonstrated hereafter. It can be noted straightaway that there will be intimate thermal exchange between the steam and the water of the liquid ring and that the steam will be saturated or will remain constantly saturated during the whole of the compression cycle, and for this reason will have a much lower temperature at the output, than that which it would have at the output of a dry compressor.

A process in accordance with the invention can again be characterized in that, before feeding it to a user circuit, the saturated and compressed steam taken from the discharge side of a liquid ring compressor is introduced into a high pressure water-steam separating balloon flask and in that the water recovered at the lower part of the balloon flask is recycled towards the liquid ring of the compressor; the lower part of the separating balloon flask also communicates with an inlet for new water and the flow of new water is controlled in accordance with the level of the water in the balloon flask.

The new water supply to the liquid ring will compensate for the amount of water of the liquid ring which is vaporized and the small amount of water which is possibly carried along with the steam.

When it is desired to obtain a fairly high determined value of the pressure of the saturated steam at the output, at least two liquid ring compressors can be used connected in series, the suction side of one being connected to the discharge side of the preceding one through the upper part of a high pressure water-steam separating balloon flask.

A process in accordance with the invention can again be characterised in that the water recovered at the lower part of the separating balloon flask of a compressor is recycled, on the one hand, towards the liquid ring of the compressor considered and, on the other hand, towards the liquid ring of the preceding compressor, through the separating balloon flask associated with the preceding compressor, when it is desired to form a closed compression circuit with recycling of the condensate of the compressed steam after use of its latent heat, following the heat pump principle.

A process in accordance with the invention, in its latter embodiment, will in any case be particularly well adapted for use in a heat pump thermal circuit associated with a drying enclosure.

It can then be characterized, in accordance with the invention, in that it is applied to the compression of the steam coming from an evaporator associated with a condenser receiving steam and/or humid air extracted from a drying enclosure, and in that the saturated steam, compressed without superheating coming from the discharge side of the last liquid ring compressor, is fed into the drying enclosure to create a heat supply in this latter, by convection or by conduction.

The invention concerns moreover thermal circuits for the implementation of the process, circuits which will be defined more specifically below by way of examples which are in no wise limiting.

In the accompanying drawing:

FIG. 1 shows a basic thermal circuit in accordance with the invention comprising a single liquid ring compressor, in an open steam circuit;

FIG. 2 shows the Mollier diagram for the water;

FIG. 3 is a graph giving the suction flow of steam in terms of the increase Δp of the pressure, for different values of the pressure at the discharge side of a small commercial liquid ring compressor; and

FIG. 4 shows a thermal circuit according to the invention comprising several liquid ring compressors connected in series, and associated with a drying enclosure, following the heat pump principle.

Referring to FIG. 1, the suction side 1 of a water liquid ring compressor 2 is connected to a low pressure steam inlet pipe 3, through a non-return valve 4. The rotor of the compressor is driven by a motor 5. The discharge side 6 of compressor 2 supplies saturated and compressed steam to a user circuit 7, e.g. a condenser, through the upper part of a high pressure separating balloon flask 8 and a valve 9. The condensates are discharged at 10. The water from the lower part of the balloon flask 8 is recycled towards the liquid ring of compressor 2 by a pipe 11 and pump 12 driven by a motor 13. The temperature of the water supplying the liquid ring is then equal to that of the saturated and compressed steam supplied at 6 by compressor 2.

The mechanical energy supplied by motor 5 and circulating pump 12 is transmitted in its entirety to the water of the liquid ring, apart from radiation or convection losses. The part of the liquid ring which is vaporised is replaced by the new water fed to the circuit by a pipe 14 connected to pipe 11 and provided with a valve 16. Valve 16 is governed by a regulator 17 receiving the signal from a water level sensor 18.

The circuit may also comprise temperature sensors, such as 19 and 20, manometers, flowmeters, drain, adjusting, insulating valves etc. and any conventional safety devices.

FIG. 2 shows the enthalpy-entropy Mollier diagram for the steam. Point A represents the state of input saturated steam at pressure P_o and temperature T_o, such as supplied by pipe 3. If, to compress this steam at pressure P₁, a perfect compressor were used, with isentropic efficiency ζ_s =1, the point representative of the condition of the steam would arrive at C, following the isentropic straight segment AC, and correspond to a temperature T₂ much greater than T_o. With a piston or rotary dry conventional compressor not perfectly isentropic (ζ_s <1), the condition of the superheated steam compressed at pressure P₁ would be represented by point D, where temperature T₃ is again greater than T₂. In practice, there could thus be a considerable difference between the temperatures T₃ of the steam at the discharge side of the compressor and T₀ at the suction side, which would necessitate de-superheating the steam, not only during compression but also afterwards, to avoid disadvantages of a technological kind in the compressor or in the use by exchangers.

On the contrary, with the use of a water liquid ring compressor, the point representative of the compressed saturated steam would follow the saturation curve Γ to reach, at the same pressure P₁, point B where temperature T₁ is only a little higher than the initial temperature T₀. We can then see the advantage in using, in accordance with the invention, a water liquid ring compressor.

It has been found experimentally:

(1) That the temperature of the steam measured at the discharge side of the compressor is indeed the saturated steam temperature at the measured discharge pressure;

(2) That there exists a maximum constant value of the difference ΔT=T₁ -T₀ for a water liquid ring compressor rotating at constant speed, for zero suction flow, whatever the suction pressure. Maximum ΔT depends only on the speed and the size of the compressor; the higher these parameters the greater ΔT.

This last result can be deduced by analysis of the curves of FIG. 3 which show test results obtained with a small commercial liquid ring compressor, for different constant pressure values P_r at the discharge side: 2.4 bars, 3 bars, 4 bars, 5 bars etc. The characteristics under partial vacuum have not been given for the sake of clarity, but they would also show the second assertion made above.

For P_r =P₁ =4 bars, for example, we can have an operating point at B, at which ΔP=P₁ -P₀ =0.8 bars, with a flow of steam at the input of 50 kg/h, and a suction pressure P₀ =P₁ -Δp=3.2 bars.

These saturated steam pressures correspond to input, T₀ =about 136° C., and output, T₁ =about 145° C., temperatures. The temperature increase ΔT is then very small: about 9° C., whereas with an excellent conventional dry compressor it could reach 27° C.

In the case where a single liquid ring compressor is not sufficient to obtain a definite pressure, several water liquid ring compressors can be used connected in series as indicated hereabove. An example of a thermal circuit of this kind is shown diagrammatically in FIG. 4, for the case where it forms a heat pump applied to a dryer.

This circuit comprises a number of liquid ring compressors 2₁, 2₂ . . . 2_n. The discharge side 6₁, 6₂ . . . 6_n of each compressor is connected respectively to the input 1₂ . . . 1_n, 1_n+1 of the following circuit through a high pressure separating balloon flask, respectively 8₁, 8₂ . . . 8_n whose lower part communicates with the liquid ring of the compressor considered through recycling pipes and pumps, respectively 12₁, 12₂ . . . 12_n. The lower part of each balloon flask communicates in addition with a pipe for supplying new water, respectively 14₁, 14₂ . . . 14_n each provided with a valve, respectively 16₁, 16₂ . . . 16_n. The lower parts of two successive balloon flasks are moreover connected by return pipes, respectively 21₁, 21₂ . . . 21_n, each provided with a valve, respectively 22₁, 22₂ . . . 22_n. These valves, as well as valves 16₁ to 16_n, are controlled by level regulators, respectively 17₁, 17₂ . . . 17_n, similarly to the case of FIG. 1, so as to permit either the draining of the corresponding balloon flask by acting on the valve 22₁, 22₂ . . . or its filling by acting on the new water input valve 16₁, 16₂ . . . .

A common motor 5 may be provided for driving all the compressors, a constant or variable speed electric motor or similar, or else a motor for each compressor.

For the first comparison stage, the very low pressure standard pure steam is supplied by an evaporator 23 fed with water by a low pressure separating balloon flask 24 whose upper part communicates with the outlet of evaporator 23 and with inlet 1₁ of compressor 2₁. The heat is supplied by a condenser 25 receiving, through a pipe 26, the steam and/or the humid air extracted from a drying enclosure (not shown) which may be similar to the drying enclosure described in French patent application No. 76 04597.

The level of water in the balloon flask 24 is kept constant by a level regulator 27 acting on a new water inlet valve 28.

The high pressure saturated steam produced by the last compressor 2_n is directed, through pipe 1_n+1, towards a heat exchanger-condenser 7 of the drying enclosure, producing the heat supply in this latter.

The condensates, occurring at 10, of the exchanger of the dryer, are recycled in a high pressure separating balloon flask 29 whose lower part is connected to the lower part of the separating balloon flask 8_n of the last compression stage by a pipe 30 provided with a circulating pump 31 and a valve 32. The level of the balloon flask 29 is regulated by a level regulator 33 acting on valve 32. These condensates from the exchanger of the dryer are then recycled to the separating balloon flask 24, passing successively through the separating balloons 8_n . . . 8₂, 8₁ of the compression stages.

In such a closed thermal circuit, the new water inlet valves only serve at start-up for filling the different balloon flasks 24 and 8₁ to 8_n.

A heating system for start-up can consist simply of a direct steam supply to the low pressure balloon flask 24.

So as not to complicate the diagram, the associated adjusting, controlling and regulating devices of the circuit such as flowmeters, thermometers, non-return valves, adjusting and isolating valves etc. have not been shown.

It goes without saying, finally, that such a circuit will provide the same basic advantages as those of the simple circuit of FIG. 1, i.e. essentially to allow compression of saturated steam taken at very low pressure, without superheating of the compressed steam.

Such a circuit as that shown in FIG. 4 will be very advantageously used with an installation for the continuous heat treatment of a product moving through an enclosure, such as the one described in French patent application No. 76 04597 in the name of the applicant; this circuit will contribute to making even greater the advantage of this installation from the economical point of view, because of the absence of superheating of the steam produced.

As is evident, and as follows moreover from what has gone before, the invention is in no wise limited to those of its embodiments and modes of application which have been more especially considered; it embraces, on the contrary, all variations thereof.