WO2008122605A1

WO2008122605A1 - Dryer

Info

Publication number: WO2008122605A1
Application number: PCT/EP2008/054089
Authority: WO
Inventors: Stefano Bernardinello; Andrea Bottazzo; Emanuele Mortin
Original assignee: M.T.A. S.P.A.
Priority date: 2007-04-05
Filing date: 2008-04-04
Publication date: 2008-10-16
Also published as: ITPD20070127A1; EP2131946A1

Abstract

A compressed-gas dryer (1) includes a cooling system (10) supplied by gas entering the dryer (1) and emitting dry gas at the output, including a cooling circuit (20) comprising a thermal exchange module (13), in which a refrigerant circulating in the said cooling circuit (20) exchanges heat with the said incoming gas. The cooling circuit (20) also includes at least a first compressor (21a) and a second compressor (21b) capable of compressing the refrigerant downstream of the heat exchange module (13), a first and a second inverter associated with the first compressor (21a) and second compressor (21b) of the cooling system (10) respectively, a control unit (27) and in which said first and second compressor (21a, 21b) deliver a smaller and larger flow rate respectively for the same pre-set operating pressure.

Description

DRYER Description

The present invention relates to a compressed-gas dryer of the type including the characteristics mentioned in the preamble of the main claim. The invention also relates to a method for managing the above-mentioned dryer.

The invention preferably, but not exclusively, applies to the specific sector of systems for drying compressed gases, such as air, used in various industrial fields that require air at a higher pressure than atmospheric pressure, containing no humidity in suspension. In this technical field, refrigerated air dryers are known in which the compressed air is cooled, by means of one or more heat exchangers, so as to separate by condensation the water in suspension in the air. Direct-expansion and thermal-mass refrigerated air dryers are known. A direct-expansion air dryer comprises at least one refrigerant heat exchanger, connected to a cooling circuit equipped with a compressor, in which the temperature of the compressed air is reduced by contact with the cold walls that delimit the volumes in which the refrigerant circulates. A thermal-mass air dryer comprises, in addition to that mentioned above, a volume of a liquid or a solid in particles interposed between the refrigerant heat exchanger and the air to be cooled.

In both types of refrigerated dryers, it is advantageous to be able to modulate the load, in other words the cooling capacity, so as to be able to suit it to the actual consumption of compressed air. Generally, this modulation is achieved by connecting the compressor of the cooling circuit to an inverter, through which the speed of rotation of the compressor motor is varied in order to vary the load.

Controlling a compressor by means of an inverter, however, has a few drawbacks, the main one being the relatively limited range of adjustment of the speed of the compressor's motor. In fact, should the number of revolutions of the compressor's motor drop below a pre-set threshold, it is no longer possible to obtain an adequate lubrication thereof since lubrication of the compressor's motor is achieved substantially by "whipping" the lubrication oil. Consequently, should it be necessary to reduce the rotational frequency of the compressor motor because the load applied drops, it is not possible to fall below a certain frequency without damaging the motor.

In the known state of the art, in order to overcome this drawback, or to drop below a certain cooling capacity, motor on/off sequence is performed, below the pre-set angular velocity threshold that can be sustained without damaging the motor, in order to adapt to the low load without excessive waste of energy and to avoid the formation of ice if temperatures that are too low are reached. In the case of thermal-mass dryers, introducing on/off operation does not create excessive drawbacks because the volume of liquid or solid used as mass is such as to be able to generally maintain the constant temperature of the cooled air even during the off phase. In direct-expansion dryers, maintaining the temperature in this way is not achieved by air which has an insufficient thermal capacity. Measures must therefore be taken to make up for these temperature variations during periods when the compressor motor is off. The main object of the present invention is to provide a compressed-gas dryer, and a method for managing it, which are structurally and operationally designed to overcome all of the drawbacks that exist with reference to the said known art.

This and other objects which will be described in further detail below are tackled and achieved by the invention by means of a dryer produced in accordance with the claims below. The features and advantages of the invention will emerge in greater detail from the following detailed description of a preferred embodiment which is shown by way of an indicative and non-limiting example with reference to the accompanying drawings, in which :

Figure 1 is a schematic side elevation of a dryer produced according to the present invention;

Figure 2 is a circuit diagram of the cooling circuit used in the dryer shown in Figure 1;

Figure 3 is a macro-block diagram of a method for managing the dryer shown in Figure 1; - Figures 4 and 4a are two detailed block diagrams of the method shown in Figure 3, in particular, Figure 4a shows, in more detail, phases 5F and

6F shown in figure 4;

Figure 5 is a first graph showing the trend of the number of revolutions per minute set by the inverter of a compressor as a function of time (top graph), a second graph showing the on or off state of the larger compressor (middle graph), and a third graph showing the on or off state of the smaller compressor (bottom graph), assuming a continuous increase in load over time;

Figure 6 is a graph of the trend of the number of rpm as a function of the pressure in the dryer shown in Figure 1. Referring initially to Figure 1, 1 indicates as a whole a refrigerated compressed- gas dryer produced according to the present invention.

The gas dryer 1 is of the refrigerated type and can be either a direct-expansion or thermal-mass dryer. The dryer 1 is capable of drying, by dehumidification, any type of compressed gas, particularly air. Other types of gas that can be used are nitrogen or methane.

The dryer 1, shown only schematically in Figure 1, comprises a box body 2 on which are made an inlet 3 and an outlet 4 for the gas, as well as an outlet pipe for the condensate 5. A connection (not shown) to an electricity generator (not shown), for example the electrical grid, is also provided.

The intended use of the dryer 1 may be very varied, for example it may be used as a dehumidifier of a compressed gas (air in particular), in industrial processes for supplying power to pneumatic equipment, equipment for stamping or blowing plastics materials, painting, for cleaning and/or drying products, as well as air for combustion.

Inside the dryer 1, the gas entering through the inlet 3 is dehumidified by a cooling system 10 (shown schematically in Figure 2) which, by means of a cooling cycle, dehumidifies the incoming gas (preferably air) so as to lower its dew point in order to minimise the tendency for liquid to form in the compressed-gas distribution lines. In fact, an increase in pressure also increases the pressure of the steam which gives rise to the tendency for liquid to form.

The cooling system 10 comprises a gas inlet manifold 11, in fluid communication with the gas inlet 3 of the dryer 1 or coinciding with it, in order to admit the gas to be dried, and a gas outlet manifold 12. Near the gas inlet 11, the system 10 also comprises one or more (in a preferred embodiment of the invention, from 5 to 10) thermal exchange modules 13, sites of the thermal exchange between the hot humid gas entering the cooling system 10 from the manifold 11 and a refrigerant, preferably of a type known in the sector, such as the refrigerant called R407c. Other refrigerants may, however, be used in the system 10 of the invention.

Each thermal exchange module 13 is preferably known per se, for example, it may contain an evaporator 14, in which the refrigerant expands and passes from the liquid state to the vapour state, a condensate separator 15 and an air- air exchanger 16 in which the incoming air thermally exchanges with gas that has already been dehumidified. Other types of modules 13 including different components may, however, be used in the dryer according to the invention. The incoming gas, therefore, after the thermal exchange in the module 13, is emitted through the pipe 12, dry and cold, ready for the use for which it is intended.

The cooling system 10 comprises a cooling circuit 20 for cooling the above- mentioned refrigerant which is capable of thermally exchanging with the gas entering the system 10. The cooling circuit 20 is preferably a closed circuit and comprises, according to a main feature of the invention, a plurality of compressors, the function of which is to aspirate the superheated steam from the evaporator 14: that is, the refrigerant leaves the thermal exchange module 13 in the saturated (or possibly superheated) vapour state and enters the plurality of compressors where it is compressed. In the preferred embodiment shown in Figure 2, there are two compressors 21a and 21b, but the number of compressors may be varied as required, provided that there are two or more.

Preferably, each compressor 21a, 21b used is of the volumetric hermetic type, that is, the compressor and motor are enclosed in a single hermetic casing. Each compressor 21a, 21b is connected to an inverter (not shown in the figures), for its respective adjustment. The inverter of each compressor 21a, 21b is capable of varying the number of revolutions thereof to suit the different loads imposed by the dryer 1. As is known, volumetric compressors have the characteristic of having a flow rate which is directly proportional to their speed of rotation and a compression ratio which is independent thereof, therefore by varying the number of rpm the flow rate of the compressor is also varied. Furthermore, the cooling circuit 20 comprises a condenser 22 towards which the compressed refrigerant leaving the compressors 21a, 21b is driven and where the refrigerant transfers its heat (finding itself in a "hot" and compressed state) to a colder external fluid, such as air or water (in the circuit shown in Figure 2 the refrigerant is water and the water input 3A and output 4A are shown, however another fluid, such as air, may be used) so as to return to the liquid state; and one or more laminar flow devices 23 (one for each thermal exchange module 13) for reducing the pressure of the refrigerant. The thermal exchange module 13 also has a condensate receiver tank 24, the condensate being discharged as required through appropriate valves 25. The condenser 22, laminar flow device 23 and condensate tank 24, like other features of the system 10 that are not described in further detail, are known per se, are preferably standard within the sector, and will not be described in further detail. The two compressors 21a, 21b, or in the case of a plurality of compressors at least two compressors of the plurality, are distinct from each other, that is, they have a different minimum and maximum gas flow rate (or alternatively a different displaced volume), and a distinct head from each other. It is therefore possible to define a "smaller" flow rate compressor, with the same head, identified in the embodiment in Figure 2 with the compressor 21a and in the method of adjustment of the dryer 1 described below as "compressor 2", and a compressor with a "larger" flow rate, with the same head, identified in the embodiment in Figure 2 with the compressor 21b and in the method as "compressor 1". Each compressor 21a, 21b has a number of revolutions that can be varied, by means of the inverter associated with it, within a pre-set range of revolutions per minute for its optimum operation, that is, without problems of poor lubrication arising, a range that depends on the type of compressor and in particular on its characteristic curve. Consequently, for each compressor, a minimum number of revolutions per minute (equal to the minimum flow rate) and a maximum number of revolutions per minute (equal to the maximum flow rate) is established. For example, the range of rpm required to achieve the correct operation for the compressors in question ranges from 40 Hz to 65 Hz for both compressors 21a and 21b, however, distinct minimum and maximum values for each compressor may be established.

Preferably, the flow rate that can be obtained with the "smaller" compressor 21a driven at the maximum permitted number of revolutions per minute is substantially the same as or slightly less than the flow rate obtained with the "larger" compressor 21b at the minimum permitted rpm, so as not to have sharp variations in flow rate when passing from a configuration with only compressor 21a on at the maximum rpm to a configuration with only compressor 21b on at the minimum rpm.

Compressors 21a, 21b, in particular their inverters, are adjusted by means of a management method of the invention, actuated by means of a special programme residing in a control unit 27 of the dryer 1, so as to modulate their respective flow rates in a continuous manner within a wide range of overall flow rates, greater than the individual ranges of each compressor. The control unit 27 also receives a plurality of data relating to the state of the dryer 1, in particular of the cooling system 10. For example, the system 10 comprises a transducer 26 for measuring a pressure LP, equal to the pressure of the fluid upstream of the compressors 21a, 21b, and means (not shown and standard per se) for sending the pressure LP signal to the control unit 27. The inverters of the compressors 21a, 21b are subject to the control unit 27 which emits a control signal to define their operating state, as a function of the pressure LP. The control mechanism is based on the variations of the value of the pressure LP in that it is a variable which is easier to control than the temperature, to which it is connected.

In particular, according to the method for adjusting the dryer, the given two compressors 21a and 21b are capable of three different operating states: a first operating state in which only the "smaller" compressor is on (referred to hereinafter as state 1), a second operating state in which only the "larger" compressor is on (state 2) and a third operating state in which both compressors are on (state 3). Henceforth, therefore, the phrase "increase in state" means the passage from one state of the compressor to the one immediately after (namely from state 1 to state 2, or from state 2 to state 3), whereas "reduction in state" indicates the passage from one state to the one immediately before (from state 2 to state 1, or from state 3 to state 2). Within each of these operating states it is possible to vary the number of revolutions of the compressor/s that are on in order to adjust their flow rate so as to obtain a continuous adjustment ranging from the minimum flow rate value that can be obtained with the smaller compressor at the minimum number of rpm to the maximum flow rate that can be obtained with both the smaller and the larger compressors at their respective maximum rpm. There are also two separate and alternative "minimum" phases, or phases in which the load applied is less than that which can be achieved with the smallest number of revolutions of the "smaller" compressor, which will be described in detail below.

With reference now to Figure 3, the method of managing the dryer 1, in particular the two compressors 21a, 21b, is shown in distinct macro-phases. A first macro-phase relates to turning on the dryer (macro-phase IF) in which some variables of compressors 21a, 21b are initialised (described below) and one of the two compressors is chosen to be the first to come on, for example preferably the smaller compressor 21a. Subsequently, depending on the load detected, the operating state of the compressors is either increased or reduced or maintained (macro-phase 2F, macro-phase 4F and macro-phase 3F respectively). Increasing the operating state involves either increasing the number of compressors used (from state 2 to state 3 : from just one compressor on to two compressors on) or passing from using only the "smaller" compressor to using only the "larger" compressor (from state 1 to state 2). Maintaining the same operating state means modulating the number of rpm of the compressor/s that are on always, however, remaining within that particular operating phase (macro-phase 3F). Lastly, reducing the operating state (macro-phase 4F) means passing from two compressors on to just one (from state 3 to state 2) or from using the "larger" compressor 21b to the "smaller" compressor 21a (from state 2 to state 1) depending on the value of the input pressure LP at the compressors measured by means of the transducer 26.

Typical values of the input pressure LP are in the order of 4-6 bar. According to the method of the invention, it is possible to pass from one phase to the next 2F-3F-4F depending on whether variations in the load are detected, in the manner exemplified below.

In order to pass to macro-phase 2F or 4F, starting from modulation macro- phase 3F, the following check is performed : at a set phase of the method of the invention, the value of the pressure LP is measured and a check is performed to determine whether the value of the pressure LP measured has reached a set pressure threshold value (either upper or lower), in which case we pass to a different operating phase, thus entering into macro-phases 2F or 4F. Having performed the change of operating state through macro-phases 2F or 4F, we automatically return to the modulation macro-phase 3F in which the number of rpm of the compressor/s that is/are on is modulated (even by keeping it unchanged).

For example, on reaching the maximum threshold value for the pressure LP and at the same time being in a state in which the maximum rpm for the "smaller" compressor 21a has been reached, said compressor 21a is switched off and the second "larger" compressor 21b is turned on, whereas on reaching a further threshold value for the pressure LP if also the larger compressor is working at its maximum rpm, the smaller compressor is also switched on in addition to the larger one (the two compressors work together).

Preferably, since at the minimum rpm the larger compressor generates a flow rate which is substantially equal to or slightly higher than the flow rate generated by the smaller compressor at the maximum rpm, the variation between the first and the second operating state is a continuous variation. Similarly, on reaching the minimum threshold value for the pressure LP and the minimum rpm for the "larger" compressor, said "larger" compressor is switched off and the "smaller" compressor is switched on. Further details are given in the precise analysis of the method of the invention described below. On reaching the minimum frequency level for the smaller compressor, and at the request to further reduce the load, either one of the two "minimum" macro- phases can be actuated as provided for by the method of the invention, the first being called "hot gas" (macro-phase 5F) in which fluid is introduced upstream of the compressors 21a, 21b, and the second "on/off" (macro-phase 6F) in which the said compressors are switched on and off according to a set pattern, particularly the "smaller" compressor 21a. In more detail, referring now to the accompanying Figures 4 and 4a, in phase IF of switching on the dryer 1, having checked that it is actually on (phase lfa), a variable called the "inverter", which represents the value of rpm of the compressor set by the inverter associated with the compressor chosen, is initialised with the minimum possible frequency value, called inv_min_2 (phase 2fa), and then the "smaller" compressor 21a is started (phase 3fa). This state with only the smaller compressor on is called operating state 1 of the compressors 21a, 21b.

In the preferred embodiment shown in Figure 4, the minimum rpm value of the "smaller" compressor is equal to the minimum number of the "larger" compressor, therefore the constant inv_min_2 is the same associated with both compressors (inv_min_2=minimum number of rpm both for compressor 21a and for compressor 21b). The invention states, however, that this constant may differ for the two distinct compressors 21a, 21b (and therefore that there are two constants). Similarly, the constant "inv_max", used below, is defined as equal to the maximum permitted number of rpm for the correct operation both of the smaller and the larger compressor. In this case too, however, the invention gives two separate constants, one per compressor, indicating the maximum permitted number of rpm. In order to perform the appropriate modulation during operation of the dryer 1, the intake pressure LP of the compressors is measured, by means of the transducer 26, and compared with a set point value for the inverter (constant called SET_INV, generally between 4.5 and 5.5 bar, for example) which is predefined, according to the method of the invention, in phase 4fa and may be varied before turning on the dryer 1. The value of the set point constant SET_INV is substantially a value in the middle of the compressor's working range.

If the pressure LP measured is the same as the set point pressure SET_INV, the compressor 21a shall be deemed to be operating at the correct working point and therefore its number of revolutions must not be changed : in terms of operations performed by the management programme, the "inverter" variable, which as stated represents the number of rpm of the compressor in question, remains the same (phase lOfa) and a further check of the value of the pressure LP is therefore made, comparing it to a different constant SET_A in a checking phase 5fa explained in detail below. If, however, the measured pressure LP differs from the set point pressure, in particular preferably differs compared to the set point value by plus or minus a certain pre-set value range (for example set point value ±0.1 bar) so as not to vary the number of revolutions per minute at minimum pressure variations, we enter one of the three possible macro-phases 2F, 3F or 4F. In fact, if the pressure LP is distinct from the set point value it means that the number of revolutions of the compressor should be varied.

First of all, in phase 7fa a check is performed to determine whether it is necessary to increase the operating state of the compressors (or increase the number of compressors on or the type of compressor used, from smaller to larger) and, if not, a check is also performed to determine whether by contrast it is necessary to reduce the operating state (phase 8fa) of the compressors (that is, reduce the number of compressors on or the type of compressor used, from larger to smaller) in other words, whether it is necessary to pass from one operating state to the other. The checks performed in phases 7fa and 8fa select entry into one of the three separate macro-phases listed above: we enter into macro-phase 2F in the event of an affirmative response to the check performed in phase 7fa to increase the operating state, into modulation phase 3F to maintain the existing operating state in the event of a negative response to both of the checks performed in phases 7fa and 8fa, and into macro-phase 4F to reduce the state in the event of an affirmative response to the check performed in phase 8fa. Since the smaller compressor 21a has just come on, the only possible response to the checks performed in phases 7fa and 8fa is a negative one, and so we enter into macro-phase 3F to modulate the number of rpm of the compressor, maintaining the initial state (state 1).

Let us now assume that an increase in the pressure LP compared to the set point SET_INV has been detected (by measuring the pressure LP in phase 4fa) and therefore an increase in the number of rpm of the smaller compressor 21a is required. In order to vary the number of revolutions of the compressor, a cyclic time check (phase 9fa) is preferably performed by the method of the invention. Since the pressure LP differs from that of the set point, it is necessary to vary the number of revolutions per minute of the compressor, however this variation is made only after a set interval of time has passed during which this increase in pressure LP continues to be present. It is in fact preferable, according to the invention, that variations to the number of rpm of the (larger and smaller) compressor are made after an increase or reduction in the value of the pressure LP has occurred more than once, based on the measurement and comparison of the pressure LP made in phase 4fa or 5fa, in order to prevent changes in the rpm value of the compressor as a result of an extemporaneous and short-lived variation in load. A check is therefore performed to determine whether this pre-set time has passed and, if it has, the number of rpm of the compressor is changed by the inverter by checking the value assumed by a variable called "time", which in the method of the invention corresponds for example to the number of cycles, or loops, of the adjustment programme in question. Other measurements of the time variable may, however, be considered. In detail, a constant is defined, called time_set_inv, whose set value determines after how many cycles of the operating programme of the invention the number of rpm of the compressor which is on is modified by the inverter. Consequently, if the "time" variable is lower than the pre-set value, the number of revolutions of the compressor remains unchanged (the "inverter" variable remains the same as itself, see phase lOfa) and we pass to the checking of the pressure LP (phase 5fa described below), otherwise if the "time" variable is greater than the pre-set value time_set_inv, the number of rpm of the compressor is changed by changing the "inverter" variable by a quantity called "func A" which represents a positive or negative value depending on the value of the pressure LP measured by the transducer 26 (phase Ufa) and the "time" variable is then reset to zero (phase 12fa). Alternatively, the value time_set_inv is pre-set and the time variable, set to zero at each 12fa phase, changes after a "delay" equal to time_set_inv.

The value of "func A" is a discrete value obtained by linear interpolation between the set point value and the new measured value of the pressure LP. As shown in the graph in Figure 6, the number of rpm set in the compressor depends on the measured value of the pressure LP. A certain set point is fixed (indicated in the graph by SP) which substantially represents the "zero" of the interpolation function (middle working point) and a certain frame of amplitude SET_INV. The possible minimum and maximum number of rpm for the compressor in question is also fixed, beyond which the number of rpm is maintained constant regardless of any pressure variations detected. A linear interpolation is then made between these points and having measured the pressure LP from an interpolation the number of rpm at which to set the compressor is found. Consequently, the value "func A" represents the instantaneous value of interpolation which is added to or subtracted from the number of "inverter" revolutions depending on the measured pressure LP both below and above the set point value respectively.

Having made this increase (it is assumed that the smaller compressor is at minimum rpm since the dryer has just started and therefore an increase in the number of rpm is required) we pass to phase 5fa in which the pressure LP is checked, a phase to which we would have been re-directed even if the value of the "time" variable had not been greater than the value time_set_inv. In this latter case, if a pressure value greater that the set point had persisted, the cycle described would have been repeated until the value of the "time" variable was not greater than time_set_inv, a value above which the number of rpm of the compressor can be changed. In phase 5fa, the pressure LP is measured by means of the appropriate transducer 26 and compared with a constant (which can be reprogrammed as desired) called SET_A. If the measured pressure LP is greater than SET_A, which is substantially a pre-set pressure "threshold" value, the operating state of the compressors changes - if other conditions detailed below also occur - in particular we change state by passing from one operating state to the one above (in other words, we enter into macro-phase 2F). In the case in question, assuming that the pressure LP did not undergo this sharp increase, this SET_A threshold value is not exceeded, a negative response to the check performed in phase 5fa is given and we pass to phase 20fa. This phase 20fa represents the phase of possible entry into macro-phase 4F in which a check is performed to determine whether it is necessary to reduce the operating state of the compressors.

Initially a check is therefore performed to determine whether we are in operating state 1, in which case we obviously immediately come out of macro- phase 4F since it is not possible to "go down a state", that is to reduce the number of compressors or change the type of compressor used (always going towards a smaller one) if we are in state 1 in which only the smaller compressor is on. Finding ourselves in this state, or having turned on only the smaller compressor, we move on to analyse whether the conditions have occurred that dictate that one of the minimum macro-states 5F or 6F covered by the dryer management method must be activated, that is whether the set load is less than that which can be obtained by the minimum number of rpm at which the smaller compressor can work, or whether considerable reductions in pressure LP have occurred which require the intervention of the "hot gas" macro-phase 5F (if present), detailed below.

Obviously it may also be that it is not necessary to check whether it is necessary to enter into these macro-phases 5F, 6F each time phase 20fa is reached, but that the check is performed only when a particular condition occurs.

According to the preferred embodiment shown in Figures 4 and 4a, a check is performed sequentially to determine whether the conditions for activating phase 5F or alternatively phase 6F have been met (the two minimum states are exclusive of one another) and, in the negative case, we return to phase 4fa of checking the value of the pressure LP and comparing the measured value of LP with the set point value SET_INV.

If the minimum macro-phase called "hot gas" 5F is included in the management method of the invention, changes are made to the operation of the compressors or the cooling circuit 10 even if the smaller compressor is not at the minimum number of revolutions possible for its correct operation, however, full details of the hot gas macro-phase 5F will be given below for the sake of practicality. Following the method for managing the operation of the dryer 1, it is now assumed that the value of the "inverter" variable will be further increased (according to the above-described phases), until the smaller compressor 21a is substantially brought up to the maximum permitted number of revolutions (equal to the constant inv_max, or to 100 %) for its correct operation. This increase is achieved by means of phases 9fa and Ufa. An illustration is now given of macro-phase 4F, that is the phases of the management method according to the invention which relate to a passage from one operating state to the other which involve an increase in the number of compressors or switching from using the smaller compressor to the larger compressor.

In this situation, in the event of an increase in the pressure LP in such a way as to exceed the value SET_A, in phase 5fa of the management method, we go on, responding in the affirmative, by means of phase 13fa, in which a check is performed to determine whether the operating state of the compressors is other than three (that is, we are not in the state in which both compressors are on: if this were the case it would not be possible to further increase the state), something which in this case is true since only the smaller compressor is operating (therefore the state of the compressors is one) and therefore the response to phase 13fa is affirmative.

A check is then made in phase 14fa to determine whether the inverter is at the maximum number of revolutions possible by comparing the value of the "inverter" variable with the constant (inv_max) which represents the maximum permitted number of revolutions for its correct operation.

As stated above, the smaller compressor is at the maximum number of revolutions and therefore also the condition of phase 14fa is checked. If both these conditions are true (that is, the pressure LP is greater than SET_A and the smaller inverter is at the maximum possible rpm) at least for a certain programmable time (called the "START_DEL" constant), a possibility checked in phase 22fa of the method, we pass to phase 7fa in which the actual go-ahead is given to pass to another state and therefore the state of the compressors is changed according to Table 1 below. In this case too, the check to determine whether a time constant START_DEL has been exceeded is set in such as way as to change the state of the compressors only if the pressure LP exceeds the SET_A threshold for a prolonged period, and not extemporaneously.

If the time threshold is not exceeded ("time" < START_DEL), we return to checking the conditions for entering into the minimum macro-phase 5F or alternatively 6F, the hot gas macro-phase 5F may be performed, and then we pass on to measuring the pressure LP (phase 4fa) and comparing it with the set point value SET_INV.

Preferably, as stated above, Table 1 is sequential, that is we can pass only from one operating state to the other state immediately adjacent to it in the Table. State Compressor Compressor

1, larger 2, smaller

OFF

1 X

2 X

3 X X

TABLE 1

An X in the table indicates the compressor that is on. From phase 22fa, we therefore pass to increasing the "operating state", that is the number of compressors or type thereof, which in this case corresponds to passing from state 1 to state 2.

The "inverter" variable is set equal to the minimum number of revolutions of the compressor (phase 23fa) and the state is changed : from the first state with only the smaller compressor on we pass to the increase in state (phase 24fa), that is, to state 2 in which only the larger compressor is on and running at the minimum number of rpm (having set the inverter variable = inv_min_2) and the smaller one is off. As stated previously, the passage between one state and another can occur in a preferred embodiment of the method of the invention, solely in a sequential manner, that is from the first to the second to the third state, and respectively from the third to the second to the first, without jumping intermediate states.

The method of the invention also includes a particular phase if the pressure LP exceeds the threshold value SET_A but the compressor is not at the maximum permitted rpm. In this case, an affirmative response is given to the check performed in phase 13fa, but a negative one to the check performed in phase 14fa: we therefore pass to a new phase (called phase 30fa) provided optionally by the method of the invention in which a check is performed to determine whether the pressure LP exceeds an additional threshold value called SET_C (where SET_C > SET_A), which represents a set point of an instant increase in the operating state of the compressors. If the pressure LP also exceeds this SET_C threshold value, we then pass to the next operating state of the compressors (that is, we pass to the increase state phase 24fa), even if the compressor is not at its maximum rpm, since it means that the load has suddenly increased greatly and an immediate greater flow rate of compressed air is required. Consequently, there is no need to wait for a time longer than START_DEL before increasing the operating state. Alternatively, if the threshold value for an instant increase in state SET_C is not exceeded by the pressure LP, we return to checking the parameters for entering into the minimum macro-phases 5F or 6F and in the negative case (or at the end of the "hot gas" phase 5F) we return to checking the pressure LP (phase 4fa). From operating state 2, assuming now that the pressure LP continues to increase and we are already at the maximum number of rpm possible for the correct operation of the larger compressor, we return to the go-ahead phase to increase the present state (7fa), and in the event of an affirmative response we pass to the third operating state in which both compressors are on, all this in a manner similar to that described above. If both compressors are at the maximum of their operation, that is both with the inverter at 100 %, it is no longer possible to make any variations and so no change is made even in the event of a further variation in load (an event which is, however, difficult to achieve since the dryer is made to the appropriate size to handle particular loads). With reference to Figure 5, the example shown is of the passage from one operating state to the next one up. The top graph represents the trend of the compressor inverters and the first "saw tooth" of the graph represents operating state 1, in which the inverter of the smaller compressor passes from 0 % to 100 % operation, assuming a continuous increase in load. In this state 1, the larger compressor is off (the second middle graph represents a step function in which the OFF value indicates that the compressor is off and the ON value indicates that the compressor is on), while the smaller compressor (bottom graph, in this case too a step function similar to the middle graph is shown) is on. On reaching the maximum rpm (inverter of the smaller compressor at 100 %) for a set time (equal to START_DEL which is the checking time and the duration of the "flat" area between one saw tooth and another) in which the compressor works at its maximum, and assuming always that LP is greater than SET_A (but not SET_C), we pass to the second operating state, in which the smaller compressor is off (in the third graph at the bottom the step function moves to OFF) and the larger compressor begins to work from the minimum permitted rpm (in the second graph the step function moves to ON). Continuing to observe the second saw tooth of the first graph, it shows the operation of the inverter of the larger compressor, which passes from 0 % to 100 % (always assuming an increase in pressure LP). Having reached 100 % of the operation of the inverter of the larger compressor, we pass to state 3, with both compressors on, and in this case too the inverters are modulated until they both reach 100 %. The modulation of the compressors occurs in the same way, that is the same increase in rpm is set on both compressors moment by moment, however the two compressors may also be set with rpm that are different from each other when they are working contemporaneously.

Assuming now that the pressure LP begins to drop and, not immediately having the go-ahead to drop an operating state (negative response in phase 8fa), we remain in macro-phase 3F to modulate, this time downwards, the number of rpm of the compressor contemporaneously decreasing by the same value the number of rpm of both compressors 21a, 21b in phase Ufa (assuming that a time longer than time_set_inv has passed resulting in an affirmative response being given in phase 9fa): the reduction is made by calculating the value of func A, which is negative, and the value of the "inverter" variable is varied by func A reducing the number of revolutions.

We then pass to the next phase (phase 5fa) in which the pressure LP is measured again and is now assumed not to be greater than SET_A (in that LP is dropping) and we are then moved on to phase 20fa. In this phase, the compressors are not in state 1 (assumed to be in state 3) and we pass to an additional new measurement of the value of the pressure LP (phase 27fa) in which a check is performed to determine whether the pressure is less than an additional threshold value, a set point for reducing the operating state, called SET_B. Assuming that we are not in this condition, that is if the drop in pressure is not very great, we return to performing the check to determine whether it is necessary to enter into the minimum macro-phases 5F or 6F (phase 40fa) and then we return to measuring the value of the pressure LP and comparing it with the set point value SET_INV, phase 4fa. Assuming now, by contrast, that the pressure LP is further lowered, we enter into macro-phase 4F for reducing the operating state of the compressors.

As in the state-increase macro-phase 2F, also for entering into the state- reduction macro-phase, a check is performed to determine whether the pressure LP is lower than the threshold value of the pressure LP SET_B (check performed in phase 27fa), substantially a "minimum" value (similar to SET_A "maximum" value), that is a pressure value at which we must pass to a lower distinct state (for example from operating state 3 to operating state 2 or from state 2 to state 1). If the response is affirmative, a check is performed to determine whether the number of rpm of the compressor is at the minimum in phase 28fa (in the case of state 3, since the compressors are modulated by setting them at the same number of rpm, if the smaller compressor is at minimum so too is the larger compressor and so only the state of one compressor need be checked; in the case of state 2, the state of the larger compressor, the only one operating, is checked), in other words whether the "inverter" variable equals inv_min_2. If both these conditions are true (that is the pressure LP is less than SET B and the compressor - larger one if the dryer is in operating state 2, both if in operating state 3, as indicated above - is at the minimum possible rpm) at least for a set programmable time (called the START_DEL constant, a constant similar to that used in phase 22fa of macro-phase 2F), a possibility checked in phase 29fa of the method, we pass to phase 8fa in which the actual go-ahead is given to reduce the state and therefore the state of the compressors is changed according to Table 1.

In particular, from phase 29fa we pass to the go-ahead phase 8fa to decrease the present state, the inverter variable is brought to the minimum (phase 25fa) and we move to the lower state (26fa), for example from operating state 3 to operating state 2 in which only the larger compressor is on while the smaller compressor is off. The larger compressor is left on at the minimum number of rpm. Alternatively, if the time threshold START_DEL is not exceeded, we return to checking the conditions for entry into the minimum macro-phases 5F or alternatively 6F (and possibly to the hot gas macro-phase, if applicable) and then to checking the pressure LP (phase 4fa) and its comparison with the set point value SET_INV. In this case too, the method of the invention also includes a special phase if the pressure LP is lower than the threshold value SET_B, but the compressor is not at the minimum permitted rpm. In this case, an affirmative response to the check performed in phase 27fa is given, but a negative response is given to the check performed in phase 28fa: we therefore pass to a new phase (phase 31fa) provided optionally by the method of the invention, for which a reduction in state is still made if the pressure LP exceeds another threshold value called SET_D (with SET_D < SET_A), a set point for an instantaneous reduction in operating state.

If the pressure is less than this threshold value we then pass to the previous operating state of the compressors (that is we pass to state-reduction phase 26fa), without even waiting for a START_DEL time because it means that the load has dropped to an extremely low level.

Alternatively, or in the case in which LP is not also less than the threshold value SET_D, we return to checking the parameters for entering into macro-phases 5F or 6F and in the negative case (or on their completion) we return to checking the pressure LP (phase 4fa).

It should be noted that according to the method of the invention at any passage of state, whether up or down, the smaller/larger inverter or both is forced to the minimum achievable rpm. The method according to the invention includes two alternative macro-phases for the management of compressors 21a and 21b if the pressure LP falls and the smaller compressor is already at the minimum number of rpm for its correct operation.

A first macro-phase 5F called "hot gas" is provided as an option. Returning to Figure 2, downstream of the condenser 22 is a valve called a "hot gas" valve, on the opening of which part of the fluid in the cooling system is "bled off" and brought upstream of the two compressors 21a and 21b jumping the thermal exchange modules 13 (substantially creating a by-pass). In this way the quantity of refrigerant that reaches the modules 13 is less than it would be if the valve 28 were shut and the cooling capacity of the cooling system 10 decreases.

In the method of the invention, a check is performed to determine whether the "hot gas" macro-phase 5F is present (see phase 40fa), and if the response is affirmative first of all a check is performed to determine whether the value of the "inverter" variable is equal to inv_min_2, or if the rpm of the smaller (larger) compressor are equal to the permitted minimum (phase 41fa). If this condition is not checked, or the smaller (larger) compressor is not at the lowest number of rpm possible (and therefore it is possible to further reduce the number of rpm) we return to the normal cycle of checking the pressure LP (phase 4fa). A new variable is introduced, however, called "hot gas", which is set at zero (phase 42fa). This variable is substantially a safety variable: the opening of the valve 28 is preferably controlled by means of a stepper motor and by setting this variable to zero the valve is made to close through a number of steps of the motor greater than the number of steps of maximum opening, so as to be certain that the valve is properly closed.

If by contrast the smaller (larger) compressor is at the minimum permitted revolutions, and so an affirmative response is given to the question posed in phase 41fa, we pass from said phase 41fa to phase 42fa in which a check is performed to determine whether we actually are in operating state 1 (only smaller compressor on). In the event of a negative response, the time variable is set to 0 in phase 44fa and a first set point for the operation of a PID controller 27a is introduced.

The PID controller 27a is capable of actuating the "hot gas" valve 28 which admits a set quantity of refrigerant in the liquid state from the cooling system 10, upstream of the two compressors 21a and 21b in order to reduce the load. The function of the PID controller 27a is therefore to open the valve 28 for bleeding off the refrigerant (which in passing between the two indicated points of the cooling system 10 evaporates) for a valve-opening extent and time that are pre-set and dependent upon the parameters input into the PID. The first set point introduced in phase 46fa is called Set_PIDl. So the PID 27a, depending on the value of the pressure LP and the Set_PIDl, actuates the hot gas valve for a pre-set time/opening depending both on the pressure LP and on Set_PIDl.

This operation of the "hot gas" macro-phase even when the larger compressor, not the smaller, is running at the lowest permitted number of revolutions, prevents sharp variations in the operation of the machine, slowing down any rapid drops in state due to rapid variations in load, thus preventing the formation of ice. If, however, the operating state is 1, or the smaller compressor is at the minimum rpm and a further reduction in load is required, we pass from phase 43fa to phase 45fa which is an additional time check. A check is in fact performed to determine whether the compressors have substantially remained in operating state 1 at least for a set interval of time called Time_PID. In the affirmative case, a set point that is different from the one mentioned above, called Set_PID2 (phase 49fa) and of a greater value than Set_PIDl is set for PID 27a. We then pass to the hot gas adjustment phase 48fa described above with the consequent opening of the valve 28 depending on the parameters set and the value of the pressure LP thus bleeding off gas and consequently reducing the load. In the negative case, we pass from phase 45fa to phase 46fa in which Set_PIDl is set as a set point for the PID 27a and we again pass to phase 48fa of actuating the hot gas valve 28.

Once the operation of the hot gas valve 28 has ended, we return to the normal check of the pressure LP performed in phase 4fa. As described, therefore, the "minimum" macro-phase 5F is activated whenever the smaller inverter is at the minimum number of rpm. When the hot gas macro-phase 5F is included in the method of the invention, no operating state of the compressors exists with dryer 1 on and both compressors off - at least one is always working. As an alternative to the hot gas macro-phase 5F, according to the method of the invention, another minimum macro-phase 6F is provided in which a sequence of switching the compressors on and off is performed when the load set is excessively low (less than that which can be obtained with the minimum of rpm attainable by the compressors). Since the two macro-phases are alternative, if this second macro-phase 6F is present, in phase 40fa of checking to determine whether the hot gas macro- phase 5F is present, a negative responsive is given and we pass to phase 50fa. It should be noted that this different minimum macro-phase 6F is actuated, unlike the hot gas macro-phase 5F, only when the state of the compressors is equal to 1, without checking the number of rpm of the smaller compressor. Although the number of rpm is not checked, at the state-change set point the inverter decelerates very rapidly so that the times for checking the pressure set to encourage the stability of the system are generally sufficient to ensure that the compressor is at the minimum speed possible, thus guaranteeing an indirect check.

In phase 50fa a check is therefore performed to determine whether the state of the compressors is in fact equal to 1 : on receiving a negative response, macro- phase 6F is not used and we return to phase 4fa of checking the pressure LP. Supposing instead that the state of the compressors equals 1, an initial check of the value of the pressure LP is made; that is, whether it is lower than a threshold set point value SET_E, a constant, in phase 51fa. An affirmative response, that is if the pressure is below this threshold value, means that the pressure LP is extremely low and the compressors are no longer capable of effectively regulating the load since the minimum settable number of rpm of the smaller compressor is too high. We then pass to a time check, that is a check is performed to determine whether we have remained below the SET_E pressure for a time less than a constant called time_of.

In detail, the "time2" variable is introduced and compared with the time_of constant (phase 52fa) : if the "time2" variable is not greater than the constant indicated, it means that the pressure dropped a fairly short time ago and it would be advisable to re-check that this condition of pressure LP < SET_E is still present by returning to phase 4fa of checking the pressure LP so as to avoid having to change the setting of the compressors in the presence of an extemporaneous variation of the pressure LP. If, by contrast, the pressure LP is below the SET_E threshold for a prolonged period of time, that is for a time longer than time_of, the check performed in phase 52fa receives an affirmative response, the smaller inverter is commanded to run at a number of revolutions that is lower than its minimum for correct operation inv_min_2, that is it is set at a number of rpm equal to inv_min_l (< min_inv_2) in phase 53fa. The smaller compressor is then switched off (phase 54fa).

The method of the invention also introduces a variable called VAR which is a supporting variable capable of easily identifying whether the minimum on/off macro-phase 6F is being used. In particular, when the macro-phase 6F is used, the variable VAR is set equal to 1 (phase 57fa). Alternatively, if a negative response was given at phase 51fa, but the pressure LP, however, is within a range of values between SET_E and SET_I (SET_I < pressure LP < SET_E, phase 55fa) at least for a time of time_of2 (that is a "time 5" variable is greater than a time constant called time_of2; phase 56fa), also in this case we pass to phase 53fa and then to the switching off of the smaller compressor 54fa.

If, by contrast, the pressure LP is not even within this range between the two above-mentioned threshold values, and if a check has also been performed to determine whether the variable VAR is other than 1 (phase 58fa), we return to the phase of checking pressure LP (phase 4fa).

From the phase of switching off the compressor 54fa, we pass, as in the case in which the variable VAR had by contrast been equal to 1 in response to the question posed in phase 58fa, to the phase of monitoring the pressure LP in order to check that such pressure does not exceed an additional threshold value SET_F, which substantially represents the set point for actuating the compressors in on/off operation, on reaching which the smaller compressor is re-started.

In phase 59fa, therefore, a check is performed to determine whether LP > SET_F and if it is, provided that the condition that an additional time variable called "time 3" is greater than time_of/phase 60fa is met, that is the pressure LP is greater than the threshold value SET_F for a certain period of time which is sufficiently long as not to be a momentary variation, a check is performed to determine whether the smaller compressor is available (phase 61fa) and, if it is, it is re-started (phase 62fa). It should be noted that on re-starting the smaller compressor in this minimum macro-phase 6F, it works at a number of rpm equal to inv_min_l, lower than the minimum set during macro-phases 3F, 4F and 2F (which is inv_min_2). A compressor availability check is performed because it is damaging to turn a compressor on and off at arbitrary intervals of time: if a compressor has just been switched off, it is necessary to wait at least a certain interval of time before it can be switched on again.

If both compressors are available, priority is always given to the smaller one. If, however, insufficient time_of time has passed, we pass from phase 60fa back to the phase of checking the pressure LP and comparing it with the threshold value SET_F of phase 59fa and the loop continues until the "time 3" variable does not exceed this time_of constant.

If the smaller compressor is not available, a check is performed to determine whether the larger compressor is available (phase 63fa) and, if it is, it is switched on (phase 64fa). If this compressor is not available either the availability check of at least one of the two continues, passing back to phase 61fa.

From the phase of switching on the smaller compressor 62fa or from the phase of switching on the larger compressor 64fa, we pass to an additional check of the pressure LP: in fact, a check is performed to determine whether the pressure LP is greater than a set point SET_H (phase 65fa), which represents the de-activation set point of the on/off operation of the compressors. In the affirmative case, the smaller compressor is set to the minimum number of rpm of normal operation inv_min_2 (phase 66fa; until this moment it was operating at a lower number of rpm equal to inv_min_l), and the variable VAR is set to zero (phase 67fa) before leaving macro-phase 6F. The state of the compressors is checked since either the smaller or the larger compressor could have access depending on their availability. From macro-phase 6F we then pass to the normal checking of the pressure LP in phase 4fa. Alternatively, if the pressure LP does not exceed the threshold value SET_H, but is still within a range SET_G < pressure LP < SET_H (where SET_G is substantially a set point of forced exit from the on/off operation of the compressors (this check is performed in phase 68fa)) for at least a time of time_of2 (check performed in phase 69fa by means of another time variable referred to as "time 4"), we then exit the macro-phase 6F, going to phase 4fa, passing first to setting the number of rpm of the smaller compressor to inv_min_2 (phase 66fa) and setting VAR=O (phase 67fa). If the pressure is not within this SET_G < pressure LP < SET_H range either, we return to phase 51fa to check whether the pressure is still below the SET_E threshold. The invention therefore meets the aims proposed and offers numerous advantages compared to the known state of the art.

A first advantage is that of being able to modulate the load of the dryer within a much wider range by using two compressors both controlled by two different inverters. Furthermore, optional measures are provided in order to avoid modifying the operation of the compressors in the event of momentary and short-lived sudden changes in pressure.

Claims

1. Compressed-gas dryer (1), including : a cooling system (10) supplied by gas entering the said dryer (1) and emitting dry gas at the output and including a cooling circuit (20) comprising a thermal exchange module (13), in which a refrigerant circulating in the said cooling circuit (20) exchanges heat with the said incoming gas, the said cooling circuit (20) also including at least one first compressor (21a) and one second compressor (21b) capable of compressing the said refrigerant downstream of the said heat exchange module (13), characterised in that the said cooling circuit (20) also comprises a first and a second inverter associated with the said first compressor (21a) and said second compressor (21b) of the said cooling system (10), respectively, a control unit (27) capable of commanding the said first and second inverter to set the flow rate of air emitted by the said first and/or said second compressor (21a, 21b) and in which the said first and second compressor (21a, 21b) deliver a smaller and larger flow rate respectively for the same pre-set operating pressure.

2. Dryer (1) according to claim 1, in which the said cooling circuit (20) comprises a condenser (22) towards which the said compressed refrigerant leaving the said first and second compressor (21a, 21b) is driven and where the said refrigerant transfers its heat to a colder external fluid, the said cooling circuit (20) also comprising an inlet and an outlet for the said external fluid.

3. Dryer (1) according to claim 2, in which the said external fluid is water.

4. Dryer (1) according to claim 2, in which the said external fluid is air.

5. Dryer (1) according to any one of the preceding claims, including a pressure transducer (26) for measuring the pressure (LP) upstream of the said first compressor (21a) and the said second compressor (21b) of the cooling system (10) and means for sending the said measure to the said control unit (27).

6. Dryer (1) according to any one of the preceding claims, including a condenser (22) capable of condensing the said refrigerant compressed by the said first and second compressor (21a, 21b) of the cooling system (10), and a valve (28) arranged downstream of the said condenser (22) capable of bleeding off a pre-set quantity of refrigerant.

7. Dryer (1) according to claim 6, in which the opening of the said valve (28) is subject to the said control unit (27).

8. Dryer (1) according to one or more of the preceding claims, in which the said compressed gas is air.

9. Dryer (1) according to one or more of the preceding claims, in which the said first and said second compressor (21a, 21b) are commanded by the said control unit (27) in one of the following alternative operating states: a first operating state in which only the first compressor is on, - a second operating state in which only the said second compressor is on, a third operating state in which both the said first and the said second compressors are on.

10. Method of managing a dryer (1) supplied by incoming gas and emitting dry gas at the output and including a cooling circuit (20) comprising a thermal exchange module (13), in which a refrigerant circulating in the said cooling circuit (20) exchanges heat with the said incoming gas, the said cooling circuit (20) also including at least one first compressor (21a) and one second compressor (21b) capable of compressing the said refrigerant downstream of the said heat exchange module (13), comprising the phases of: - setting a first set point value (SET_INV), an upper threshold value

(SET_A) and a lower threshold value (SET_B), measuring an input pressure (LP) at the said first and said second compressor (21a, 21b), comparing the said pressure (LP) with one or more of the said set point, upper threshold and lower threshold values (SET_INV, SET_A, SET_B), depending on the result of the said comparison, performing alternatively one of the following phases:

• modulating the flow rate of the said first compressor (21a) and/or of the said second compressor (21b), • passing from an operating state in which only the first compressor (21a) is on to an operating state in which only the second compressor (21b) is on, or vice versa,

• passing from an operating state in which only one of the said two compressors, first or second, is on, to an operating state in which both the said first and second compressors are on, or vice versa.

11. Management method according to claim 10, comprising the phase, upon turning on the said dryer (1), of setting the said first compressor (21a) to the minimum number of revolutions per minute for its correct operation.

12. Management method according to either claim 10 or claim 11, in which the said phase of modulating the flow rate of the said first and/or of the said second compressor is performed when the said measured pressure differs from the said first set point value (SET_INV) and is between the said lower threshold value (SET_B) and the said upper threshold value (SET_A), SET_B<LP<SET_A.

13. Management method according to claim 10 and/or claim 11, in which the said phase of passing from an operating state in which only the said first compressor (21a) is on to an operating state in which only the said second compressor (21b) is on is performed when the said measured pressure (LP) is greater than the said upper threshold value (SET_A), LP>SET_A.

14. Management method according to any one of claims 10 to 13, in which the said phase of passing from an operating state in which only one of the said two compressors, first or second, is on, to an operating state in which both the said first and second compressor are on, is performed when the said measured pressure (LP) is greater than the said upper threshold value (SET_A), LP>SET_A.

15. Management method according to one or more of claims 10 to 14, in which the said phase of passing from an operating state in which only the second compressor (21b) is on to an operating state in which only the first compressor (21a) is on is performed when the said measured pressure (LP) is less than the said lower threshold value (SET_B), LP<SET_B.

16. Management method according to one or more of claims 10 to 13, in which the said phase of passing from an operating state in which both the said first and second compressor are on to an operating state in which only one of the said two compressors, first or second, is on, is performed when the said measured pressure (LP) is less than the said lower threshold value (SET_B), LP<SET B.

17. Management method according to one or more of claims 10 to 16, comprising the phases of: setting a waiting time (START_DEL), modifying the said operating state of the said dryer only if the said exceeding by the said measured pressure (LP) of the said upper threshold (SET_A) or said lowering under the said lower threshold (SET_B) persists for a time which is at least equal to or greater than the said waiting time.

18. Management method according to claim 17, comprising the phases of: setting a second upper threshold value (SET_C) greater than the said upper threshold value (SET_A), comparing the said measured pressure value with the second said upper threshold value (SET_C), if the said measured pressure value (LP) is greater than the said threshold value (SET_A), but the said waiting time has not yet elapsed, - if the said measured pressure value is greater than the said second threshold value, modifying the said operating state of the said dryer as if the said waiting time had elapsed.

19. Management method according to either claim 17 or 18, comprising the phases of: - setting a second lower threshold value (SET_D) below the said lower threshold value (SET_B), comparing the said measured pressure value with the said second lower threshold value (SET_D), if the said measured pressure value (LP) is below the said lower threshold value (SET_B), but the said waiting time has not yet elapsed, if the said measured pressure is less than the said second lower threshold value, modifying the said operating state of the said dryer as if the said waiting time had elapsed.