GB2052709A - Controls for Drying Machines - Google Patents

Abstract

To control the switch-off of a tumble dryer, a sample flow of the moist air leaving the drying chamber (10) is drawn off through a tube (26), and cooled (at 28) to below its dew- point. The temperature of the cooled air, which is representative of the specific enthalpy of the air leaving the drying chamber (10), is sensed (at 24), and, when the sensed temperature begins to fall, the heating element (20) of the dryer is switched off. After a further fall in the sensed temperature, the motor (14) of the dryer is also switched off. <IMAGE>

Description

SPECIFICATION Controls for Drying Machines This invention relates to controls for drying machines, and particularly to controls for adiabatic dryers, that is to say, dryers in which the gas (usually air) used as a drying medium becomes cooled adiabatically by evaporation of water from the material being dried. One example of a dryer to which the invention is applicable is the domestic laundry drying machine known as a tumble dryer. A tumble dryer usually includes a fan and an electrical heating element by means of which air is drawn directly from the atmosphere, heated, and directed into a laundry-drying chamber. The air, whose moisture content is thereby increased, is then discharged directly to atmosphere, or in certain cases the air from the drying drum is passed through a condenser and then partly or wholly recirculated back to the drying drum.

The present invention is concerned with controls which control the operation of a laundry drying machine in response to variations in the condition of drying air leaving a laundry-drying chamber.

Thus, according to one aspect of the invention, a drying machine comprises a drying chamber, means arranged to draw air from the atmosphere, to heat such air, and to direct such heated air into the drying chamber, and a discharge leading air from the drying chamber to the atmosphere, and means arranged to sense a parameter related to the specific enthalpy (or total heat) of the air flowing in the discharge from the drying chamber, and to terminate a phase of a drying operation when the said parameter changes in such a way as to indicate a fall of more than a certain magnitude in the said specific enthalpy (or total heat).

In operation of the drying machine, the air heating means provides a constant heat input to the air, so that the specific enthalpy of the air entering the drying chamber does not vary. If the conditions in the drying machine are steady, so that no heat is gained or lost from the thermal capacities of the parts of the dryer or the stock being dried, then the specific enthalpy of the drying air will be unchanged in its passage through the drying chamber. If, the other hand, the temperature of the drying chamber and its contents is rising, some heat is lost from the drying air to the thermal capacity of the drying chamber and its contents, so that the specific enthalpy of the air leaving the drying chamber is less than in the steady state.Usually, in a drying operation, the temperature of the drying chamber first rises to a steady value, at which it remains until most of the water has been evaporated from the stock; the temperature then begins to rise again. This means that the specific enthalpy of the air leaving the drying chamber will rise somewhat as steady conditions are approached and will fall again when the drying chamber temperature begins to rise again at the end of the steady state phase. Thus, this fall indicates that a certain degree of dryness has been reached; and this is why it is used to control termination of a phase of the drying operation.

One way in which a parameter related to total heat or enthalpy can be obtained is to cool a sample of the discharge air to saturation point or below. This means that during the initial stage of the drying process, most of the heat exchange surfaces by which the sample is cooled will become moist; the only exception is that those surfaces which the air contacts initially, before being cooled down to its saturation point, will remain dry. During subsequent stages of the drying process, the moist surfaces will still be moist. If the state of the air leaving the drying chamber is such that it is saturated where it passes over the moist surfaces, then none of this moisture will evaporate, and the only cooling applied to the air will be simple heat exchange cooling.However, if the state of the air should now change in such a way that it is unsaturated over part of the moist surfaces, then it will also be cooled by evaporative cooling, in the process of which the total heat or enthalpy of the moist air remains constant. This cooling is additional to the simple heat exchange cooling, whose effect on the total heat or enthalpy remains more or less unchanged, and therefore the total heat or -enthalpy of the air after cooling remains substantially constant, for a given total heat or enthalpy at the exit from the drying chamber.

However, the air is always saturated at the end of the cooling process, and therefore its temperature is a direct indication of its total heat or enthalpy, which is in turn indicative of the total heat or enthalpy of the air leaving the drying chamber.

As an alternative, a sample of the discharge air can be brought into heat exchange relationship with a separate air flow, which will cool the sample to saturation point and bring about a rise in temperature in the separate cooling air supply which is indicative of the total heat or specific enthalpy of the discharge air. In one arrangement, for example, the separate air flow could be a portion of the air being drawn from the atmosphere for supply to the drying chamber.

It may be noted here that, because the wetbulb temperature of air is constant, or very nearly so, for a given specific enthalpy, the said parameter indicative of specific enthalpy could in principle be wet-bulb temperature, and be measured by a wet-bulb thermometer. However, it will usually be more convenient to use one of the other methods of measurement described above.

In any case in which the said parameter is itself a temperature, variations in the latter temperature may be sensed by a temperature-sensitive mechanical actuator, such a bimetallic strip or a capsule or bellows filled with a fluid which expands on heating. Various methods may be used to detect a movement of such an actuator corresponding to a fall in the specific enthalpy (or total heat).In one arrangement, the movement of an output member of the mechanical actuator is applied to a switch unit which is mechanically coupled in series with a friction arrangement, and a stop is associated with the switch, the stop being so arranged that movements of the said output member resulting from rises in the specific enthalpy (or total heat) result in slipping of the friction arrangement, without affecting the switch, while movements of the said output member in the opposite direction cause the switch to change state, without slipping of the friction arrangement. The switch may be connected to control the heating element of the dryer; its change of state would de-energise the heating element.

With such an arrangement, the stop may be adjustable, in such a manner that the amount of movement of the output member in the said opposite direction required to cause the switch to change state can be adjusted. This should allow laundry to be dried to a selected degree of dryness.

As an alternative to a friction arrangement, a one-way freewheel arrangement may be used.

The drying machine may also include a second switch which controls the operation of a fan arranged to direct the heated air into the drying chamber and which, like the first, is mechanically coupled in series with a friction or freewheel arrangement, to be actuated by the output member of the actuator. A stop would be associated with this switch also, and would normally be so positioned that the first switch changes state, to de-energise the heater, before does the second switch, to de-energise the fan. In this way, a cooling-off period can be provided, without having to provide a separate timer for this purpose.

The two stops may be coupled together, as for example by being formed as two cams on the same shaft, controlled by a manual control knob.

The provision of a cooling-off period, without the use of a separate timer, forms a second aspect of the invention. Thus, according to this second aspect, a drying machine comprises a drying chamber, means arranged to heat a gas flow, and to direct such heated gas through the drying chamber, and a control unit arranged to sense a parameter related to the specific enthalpy, (or total heat), of gas leaving the drying chamber, and to generate a mechanical movement representative of the said specific enthalpy, (or total heat), the control unit also including first and second switches connected to control the gas heating means and the gas flow directing means respectively, the switches being so arranged, that, when a mechanical movement occurs indicative of a fall in the said specific enthalpy (or total heat), first the first and then the second switches change state. at different points of the mechanical movement, thereby disabling first the gas heating means and second the gas flow directing means.

The invention may be carried into practice in various ways, but one specific example and a modification thereof will now be described by way of example, with reference to the accompanying drawings, of which Figure 1 is a diagrammatic illustration of a tumble dryer for clothes, showing the paths taken by the drying air; Figure 2 is a graph, showing the changes in specific enthalpy and temperature of the discharged drying air which occur during a typical drying operation using the dryer of Figure 1; Figure 3 is a plan view, partly in section, of a temperature sensitive control switch unit forming part of the dryer of Figure 1; Figure 4 is a rear view of the switch unit of Figure 3; Figure 5 is a diagrammatic illustration, similar to Figure 1, of a modified form of tumble dryer, in which the switch unit of Figures 3 and 4 can be used;; Figure 6 is a rear view, similar to Figure 4, of a modified form of control switch unit; and Figure 7 is a view in section on the line VIl-VIl of Figure 6.

Referring first to Figure 1 , the tumble dryer has a rotatable drum 10, which is mounted within an air-tight casing 12. During operation of the dryer, the drum 10 is rotated by an electric motor 14.

The motor 14 also drives a fan 16, by which air is drawn from the atmosphere, through an inlet duct 18, and blown into the casing 12. An electric heating element 20 is mounted in the inlet duct 18, and is energised during most of the period during which the motor 14 is energised, so that hot air is supplied to the casing 12, to dry clothes contained in the drum 10. The walls of the drum 10 are perforated to allow this hot air to reach the clothes.

After passing through the drum 10, the hot air, which now contains moisture evaporated from the clothes, is discharged directly to the atmosphere through an exhaust duct 22.

The energisation of the motor 14 and the heating element 20 is controlled by a switch unit 24, which is arranged to be sensitive to changes in the thermodynamic state of the air leaving the drum 10. More specifically, a small proportion of the air flowing in the exhaust 22 is bled off through a bleed duct 26. This bleed duct is formed with a coiled cooling portion 28, over which air is blown by a further fan 30 driven by the motor 14; as an alternative, the cooling portion 28 could be cooled by all or part of the flow induced by the fan 1 6, before heating by the element 20. After passing through the cooling portion 28, the bleed airflow is directed to the switch unit 24.

The effect of the cooling portion 28 is to cool the bleed airflow to a temperature below its saturation temperature. As previously explained, this will mean that the temperature of the airflow supplied through the bleed duct 26 to the switch unit 24 will normally give a good indication of the specific enthalpy of the air leaving the drum 10.

The variation of the specific enthalpy of the air leaving the drum 10 in the course of drying a load of clothes is illustrated by the curve A in Figure 2.

Figure 2 also illustrates, as curve B, the variation of the temperature of the air leaving the drum 10.

It will be seen that, during the first stage of the drying process, both curves are rising; this is because the clothes, and various parts of the dryer, are being heated up during this stage.

When stable temperatures have been reached, moisture is being evaporated steadily from the clothes, and, as will be clear from Figure 2, conditions will remain more or less stable until the moisture content of the clothes has fallen to about 20%. After this point, the temperature (curve B) will begin to rise; this is indicative of the fact that the rate of evaporation of water from the clothes is decreasing, so that less heat is absorbed as latent heat. The specific enthalpy, on the other hand, will begin to fall, and this can be accounted for by the fact that the drum and clothes are now rising in temperature, and absorbing heat as a result, whereas previously their temperature was constant.

The switch unit 24 is so arranged that, when the temperature of the cooled bleed airflow has fallen by more than a certain amount below the maximum temperature attained by this airflow, the heating element 20 will be de-energised. The motor 14 continues to run, so that the clothes will be cooled off somewhat, until the temperature of the cooled bleed airflow has fallen by a further margin. The motor 14 is then switched off by the switch unit 24, and the drying operation is complete.

Referring now to Figures 3 and 4, most of the parts of the switch unit 24 are housed within a channel-shaped pressing 32, whose front and back walls are shown at 34 and 36, and whose bottom wall is shown at 38. The bleed air duct 26 passes beneath the bottom wall 38, and this part of the bleed duct contains a temperaturesensitive capsule 40, which has an operating rod 42 extending upwards through the bottom wall 38 of the pressing 32. A rise in temperature in the bleed duct 26 will result in expansion of the capsule 40, and a corresponding upward movement of the rod 42, and vice versa. The upper end of the rod 42 bears against a first onearmed lever 44 which is pivoted on a lever shaft 46 extending between the walls 34 and 36, and a tension spring 48 biases the lever 44 downwards so that it is always in contact with the operating rod 42.Two further one-armed levers 50 and 52 are also pivoted on the lever shaft 46, and are coupled to the first lever 44 by a friction clutch arrangement, so that, whenever possible, the levers 44, 50 and 52 pivot as one body. In the position illustrated, both the levers 50 and 52 are resting on respective control cams 54 and 56, carried on a cam shaft 58 extending between the front and back walls 34 and 36, and therefore anticlockwise movement (as seen in Figure 4) of the levers 50 and 52 cannot occur. Thus, any rise in the temperature of the cooled bleed air will result in slipping of the friction clutch arrangement, since the upward movement of the operating rod 42 will force the lever 44 to move anti-clockwise.However, the cams 54 and 56 do not block movements of the levers 50 and 52 in the opposite direction, so that, as soon as the temperature of the cooled bleed airflow begins to fall, the free ends (the left-hand ends, as seen in Figure 4) of the levers 50 and 52 begin to rise, as a result of the contraction of the capsule 40.

Two microswitches 60 and 62, which control, respectively, the motor 14 and the heating element 20, are mounted on the rear wall 36; each microswitch has an operating lever 64 or 66, which, in the position illustrated in Figures 3 and 4, is held in the switch-operated position by a respective one of the levers 50 and 52. However, a small rise of the left-hand tip of the lever 50 or 52 will free the operating lever 64 or 66, so that the associated microswitch opens.

The amount of upward movement which is necessary for the lever 50 or 52 to release the microswitch 60 or 62 can be adjusted by means of the cam shaft 58, which carries a control knob 68 at its front end. If, for example, the cam 56 is so adjusted that the lever 52, when resting on the cam 56, is only slightly below the position at which it releases the lever 66, then the heating element 20 will be de-energised almost as soon as the cooled bleed airflow temperature has passed its maximum. This will leave the clothes with about 20% moisture content (see Figure 2), which is suitable for ironing.

Thus, this position of the control knob 68 is marked 'iron dry'.

If alternatively, the cam 56 is so adjusted that a considerable movement is necessary to move the lever 52 up away from the cam to the position at which it releases the microswitch 62, energisation of the heating element 20 will continue for an appreciable period after the curve A (Figure 2) has passed its peak This will result in more or less complete drying of the clothes, and therefore this position of the control knob 68 is marked 'Dry'.

At both these settings, the control cam 54 sets a position for the lever 50 which is somewhat lower than the position set by the cam 56 for the lever 52. Thus, a further drop in the temperature of the cooled bleed airflow, after the heating element 20 has been de-energised, will be required before the lever 50 releases the microswitch 60 to de-energise the motor 14. This provides the cooling-off period mentioned above; at the 'Iron dry' setting, only a very short coolingoff period-is provided, while a rather more prolonged cooling-off period is provided at the 'Dry' setting.

Of course, settings intermediate between 'Iron dry' and 'Dry' can be used.

The control knob 68 also has an 'Off' setting, in which the cams 54 and 56 lift the levers 50 and 52 so high that neither of the microswitches 60 and 62 can be kept operated.

To ensure that the motor 14 cannot be deenergised at any time when the heating element 20 is energised, the operating lever 66, which controls the energisation of the heating element, has a lateral extension 70 which acts on the lever 64 to hold the microswitch 60 operated at any time when the microswitch 62 is operated. This ensures that overheating of the element 20 cannot occur as a result of being energised at a time when there is no airflow in the inlet duct 18.

When a load of clothes has been dried, and a further load of clothes is to be dried, the switch unit 24 has to be restored to the position of Figures 3 and 4, in order for the drying operation to begin. This is achieved by operating a 'Start' button 72, which both releases the friction clutch arrangement, and moves the switch operating levers 64 and 66 to their switch-operated positions, so that the levers 50 and 52 can drop down on to the cams 54 and 56. To help in an understanding of the operation of the 'Start' button, the friction clutch arrangement will now be described in more detail.

The friction clutch arrangement comprises a number of components which are carried on the lever shaft 46, being sandwiched together and normally held under compression by a clutch spring 74. Starting from the end closest to the rear wall 36, these components are, in order: a pressure disc 76 which is pinned to the lever shaft 46, and against which the clutch spring 74 acts; the lever 44, a large washer 78 of friction material; the lever 52; a further large washer 80 of friction material; the lever 50; and a small washer 82 of polytetrafluoroethylene. The washer 82 bears against a fixed stop plate 84, which projects upwards form the bottom wall 38 of the pressing 32.

As long as these components are squeezed together by the spring 74, the levers 44, 50 and 52 will tend to move as one body, because of the friction between them and the friction washers 78 and 80. The small size and slippery nature of the washer 82 mean that it does not appreciably impede movements of the lever 50.

The 'Start' button 72 is mounted on the front end of the lever shaft 46, so that, by pressing this button, the pressure disc 76 can be moved to the rear, compressing the clutch spring 74 and freeing the other clutch components from compression.

The lever shaft 46 also carries a downwardlydirected projection 86, which moves a bell crank 88 to press on the extension 70 of the switch operating lever 66. Thus, operation of the 'Start' button will cause both the switch-operating levers 64 and 66 to move to the left (as seen in Figure 4) so that the levers 50 and 52, which are simultaneously released from the action of the clutch arrangement, can drop down onto the cams 54 and 56. A light tension spring 90 is provided for each of the levers 50 and 52, to ensure that the levers do in fact drop down in this way.

The tumble dryer shown in Figure 5 has many parts, including the switch unit 24, which are identical to the corresponding parts of the tumble dryer of Figure 1. The same reference numbers will be used for corresponding parts, and the following description will be confined to those parts where differences occur.

As with the dryer of Figure 1, a proportion of the exhaust air form the drum 10 is bled off through a bleed duct 26, but in this case the bleed duct leads to a counter-flow heat exchanger 100, consisting of two coaxial tubes. The bleed airflow passes through the annular space between the inner and outer tubes, and is then discharged directly to atmosphere; this air does not act directly on the switch unit 24. The inlet duct 18 still draws most of its air directly from the atmosphere, but a small proportion of this flow is drawn from atmosphere through the central tube of the heat exchanger 100; this air then flows over the sensing capsule 40 of the switch unit 24, before passing through a conduit 102 to the fan 16.

With this arrangement, the air bled off from the exhaust duct 22 is cooled in the heat exchanger 100 to a temperature below its saturation point.

The amount of heat transferred to the ingoing air produces a temperature rise which is an indication of the specific enthalpy of the moist air in the exhaust duct 22. As long as the temperature of the clothes being dried and of the various parts of the dryer remains constant, the specific enthalpy of the moist air in the exhaust duct will also remain constant, because the only heat lost by the air in its passage through the drum 10 is that taken up by the evaporating moisture as latent heat, and this moisture is present with the air in the exhaust duct.However, when the temperature of the clothes and the various parts of the dryer begins to rise, as happens when the moisture content of the clothes has fallen to about 20%, some of the heat supplied by the air entering the drum 10 will be absorbed by the clothes, and by those parts of the dryer whose temperature is also rising, and the specific enthalpy of the discharged air will therefore decrease from its previous value. This decrease will be reflected as a decrease in the temperature of the air reaching the switch unit 24 after being heated in the heat exchanger 100.

Thus, the switch unit 24 can control the operation of the dryer in much the same way as in Figure 1.

Figures 6 and 7 illustrate an alternative form of control switch unit. In this switch unit, the friction clutch arrangement of Figures 3 and 4 is replaced by a one-way freewheel arrangement, but, apart from changes necessitated by this difference, the parts of the switch unit are generally similar to those shown in Figures 3 and 4, and the same reference numerals will be used for corresponding parts. The following description will be confined to those parts which differ from Figures 3 and 4.

The levers 44, 50 and 52 are still pivoted on the lever shaft 46, but in Figures 6 and 7, there is no clutch spring tending to squeeze the levers together; the levers are simply kept in place between a pair of circlips on the shaft 46. The lever 44 lies between the two levers 50 and 52, and has a downwardly extending portion 120 beneath the shaft 46, which portion has, on each side, a protrusion 122 which extends beneath an arcuate portion of the periphery of one of the levers 50 and 52. The upper surfaces of the protrusions 122 are so shaped that, with the aforementioned arcuate portions of the levers 50 and 52, they define spaces which decrease in vertical depth from left of right (as seen in Figure 6). A small roller 124 is received in each of these spaces, and these rollers are biased towards the right by a spring 126.In more detail, each roller is hollow, and receives one of the free ends of the spring 126 in its interior. From one free end, the spring 126 makes several turns about a peg 128 on the portion 120 of the lever 44, and then passes beneath the lever, to follow a similar path on the other side of the lever. Thus, the rollers 124 act as wedging clutch elements. An anticlockwise movement of the lever 44 will tend to move the rollers 124 towards a wider part of the space between the respective lever 50 or 52 and the protrusion 122, so that no wedging of the rollers 124 occurs, and the lever 44 can move freely relative to the levers 50 and 52, which at this stage rest on the control cams 54 and 56.

The spring 126 keeps the rollers 124 in contact both with the levers 50 and 52 and with the protrusions 122, so that, as soon as the lever 44 starts to move clockwise, indicating that the specific enthalpy of the air leaving the drum has begun to fall, the rollers 124 becomes wedged, because they tend to move to narrower parts of the spaces within which they are received. The levers 44, 50 and 52 therefore move as a single body, as with the embodiment of Figures 3 and 4, and the microswitches 60 and 62 are therefore released in the same manner.

To restore the switch unit to the position of Figures 6 and 7 after the microswitches have been released, the 'Start' button 72 must again be pressed; in this case, however, the button 72 is mounted directly on the bell crank 88, and does not affect the lever shaft 46. To ensure that the one-way wedging roller freewheel arrangement is released when the 'Start' button is pressed, a small, generally U-shaped yoke 130 (best seen in Figure 7) is connected by a link 132 to the part of the bell crank 88 which acts on the microswitch lever extension 70. The yoke 130 embraces the downwardly-extending portion 120 of the lever 44, and has two hooked portions 134 which can engage against the right-hand sides of the rollers 124. Thus, when the 'Start' button 72 is pressed, the yoke 1 30 will move to the left, and in so doing will move the rollers 124 to a position in which they are not jammed between the levers 50 and 52 and the protrusions 122. The levers 50 and 52 can then drop down to hold closed the microswitches 60 and 62, which will already have been closed by the action of the bell crank 88.

It will be appreciated that the invention may employ forms of one-way freewheel device other than those which rely on friction on wedging elements for their operation.

Claims (15)

Claims

1. A drying machine comprising a drying chamber, means arranged to draw air from the atmosphere, to heat such air, and to direct such heated air into the drying chamber, and a discharge leading air from the drying chamber to the atmosphere, and means arranged to sense a parameter related to the specific enthalpy (or total heat) of the air flowing in the discharge from the drying chamber, and to terminate a phase of a drying operation when the said parameter changes in such a way as to indicate a fall of more than a certain magnitude in the said specific enthalpy (or total heat).

2. A-drying machine as claimed in Claim 1, in which the said parameter sensing means comprises means arranged to cool a sample of the air flowing in the discharge from the drying chamber, and means arranged to sense the temperature of the cooled sample, the sensed temperature constituting the said parameter related to specific enthalpy (or total heat).

3. A drying machine as claimed in Claim 2, in which the cooling means comprises a tube through which a sample flow of air continuously passes during operation of the machine.

4. A drying machine as claimed in Claim 3, which includes a motor-fan unit arranged both to circulate air from the atmosphere through the drying chamber, and to pass a flow of cooling air over the tube carrying the sample flow of air.

5. A drying machine as claimed in Claim 1, in which the said parameter sensing means comprises a heat exchanger arranged to transfer heat from a sample of the air flowing in the discharge from the drying chamber to a flow of cooling air, and means arranged to sense the temperature of the cooling air after passing through the heat exchanger, the sensed temperature constituting the said parameter related to specific enthalpy (or total heat).

6. A drying machine as claimed in Claim 5, which includes a motor-fan unit arranged both to circulate air from the atmosphere through the drying chamber, and to pass the cooling air flow through the heat exchanger.

7. A drying machine as claimed in Claim 1, in which the said parameter sensing means comprises a wet-bulb thermometer device.

8. A drying machine as claimed in any of Claims 2 to 6, in which the temperature sensing means comprises a temperature-sensitive actuator providing a mechanical movement of an output member in response to changes in sensed temperature.

9. A drying machine as claimed in Claim 8, in which the movement of the actuator output member is applied to a switch unit which is mechanically coupled in series with a friction arrangement, and a stop is associated with the switch, the stop being so arranged that movements of the said output member resulting from rises in the specific enthalpy (or total heat) result in slipping of the friction arrangement, without affecting the switch, while movements of the said output member in the opposite direction cause the switch to change state, without slipping of the friction arrangement.

10. A drying machine as claimed in Claim 8, in which the movement of the actuator output member is applied to a switch unit which is mechanically coupled in series with a one-way freewheel arrangement, and a stop is associated with the switch unit, the freewheel and the stop being so arranged that movements of the said output member resulting from rises in the specific enthalpy (or total heat) result in freewheeling of the freewheel, without affecting the switch, while movements of the said output member in the opposite direction cause the switch to change state, without relative movement occurring in the freewheel arrangement.

11. A drying machine as claimed in Claim 9 or Claim 10, in which the stop associated with the switch unit is adjustable, in such a manner that the amount of movement of the output member in the said opposite direction required to cause the switch to change state can be adjusted.

12. A drying machine as claimed in Claim 9 or Claim 10 or Claim 11, in which the switch unit includes a switch which controls a heating element of the machine, and a second switch which controls the operation of a fan of the machine, the fan being arranged to direct the heated air into the drying chamber, and the second switch being coupled in series with a friction or freewheel arrangement, to be actuated by the output member of the actuator, and a second stop being associated with the second switch, the second stop being so arranged that the first switch changes state, on a fall in specific enthalpy (or total heat) before does the second switch.

13. A drying machine as claimed in Claim 12, in which the first and second stops are coupled together for simultaneous adjustment.

14. A drying machine comprising a drying chamber, means arranged to heat a gas flow, and to direct such heated gas through the drying chamber, and a control unit arranged to sense a parameter related to the specific enthalpy (or total heat), of gas leaving the drying chamber, and to generate a mechanical movement representative of the said specific enthalpy (or total heat), the control unit also including first and second switches connected to control the gas heating means and the gas flow directing means respectively, the switches being so arranged, that, when a mechanical movement occurs indicative of a fall in the said specific enthalpy (or total heat), first the first and then the second switches change state, at different points of the mechanical movement, thereby disabling first the gas heating means and second the gas flow directing means.

15. A drying machine substantially as herein described, with reference to either of Figures 1 and 5 and either Figures 3 and 4 or Figures 6 and 7 of the accompanying drawings.