GB1572811A - Fan blown hot air tumble driers - Google Patents

Description

(54) FAN BLOWN HOT AIR TUMBLE DRIERS (71) We, HOOVER LIMITED, a Company registered under the laws of England, of Perivale, Greenford, Middlesex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to clothes driers of fan blown hot air tumble type and is con cerned with means for automatically switch ing off at least the heater when the clothes reach a desired degree of dryness.

According to the present invention a clothes dryer of fan blown hot air tumble type includes an adjustable temperature-responsive switching device arranged to switch off the heater when the exhaust temperature reaches a set value, and means for automatically adjusting the said set value as a non linear function of elapsed time, the function corresponding to an isowetness curve of exhaust temperature plotted against time for different sizes of loads of a particular type of fabric to a given degree of wetness.

When a fan blown hot air tumble drier is operated the temperature of the exhaust air gradually increases as the load of clothes becomes progressively dryer, and it is possible to measure the relationship between exhaust air temperature and elapsed drying time for different sizes of load for a particular fabric with a given degree of initial wetness and plot curves of exhaust temperature against time. A given point on each curve represents a unique value of dryness or water retention for the size of load corresponding to that curve, and it is possible to draw a curve passing through the points representing a given degree of dryness, thereby producing what may be termed an isowetness curve.

Thus for any size and type and initial wetness of load a given degree of dryness can be defined by the exhaust temperature and the elapsed drying time, and if the heating is terminated when the exhaust temperature has risen to the value of a point on the isowetness curve corresponding to the time that has elapsed, the clothes will then be in a degree of wetness corresponding to the isowetness curve.

It has previously been proposed to control a hot air tumble dryer by a pair of contacts controlled by bimetallic strips one of which is steadily heated so that its movement is a substantially linear function of time, while the other is subject to the exhaust air temperature so that its movement is a linear function of that temperature. This is Un satisfactory in view of the fact that the isowetness curves are not straight lines and hence the characteristics of the mechanism cannot correwond to an isowetness curve.

With such an arrangement the mechanism may be arranged to switch off at the desired degree of dryness for one particular load, or even for two particular loads, where the straight line characteristic intersects the iso wetness curve, but even this is difficult ta ensure and it is impossible to make the straight line characteristic of the mechanism fit the curved isowetness curve at more than two points.

In one particular arrangement the temperature responsive switching device is of bimetallic type. The temperature responsive switching device may also include a bridge network including at least one thermistor and a variable resistor. The variable resistor may have a non-linear resistance characteristic.

In a preferred form of the present invention the means for adjusting the set value of the temperature responsive switching device includes a cam and a cam follower. Thus the shape of the cam may be chosen in accordance with any desired isowetness curve.

The means for adjusting the set value of the temperature responsive switching device may include two or more cams any one of which can be selectively brought into operation and which are shaped to match the isowetness curves of different fabrics and/or different degrees of dryness. A number of cams may be mounted axially and interconnected by a sloped surface so that they can be shifted relatively to the cam follower to change from one cam to another at any point in the cycle.

Reference has been made herein to the temperature of the exhaust but it may be desirable to substitute the temperature of the exhaust minus the ambient temperature.

Further features and details of the invention will be apparent from the following description of certain specific embodiments which is given by way of example with reference to the accompanying drawings in which: - Figure 1 shows diagrammatically an isowetness curve drawn on a graph of exhaust temperature against time for various sizes of load in a tumble drier; Figure 2 is a circuit diagram of a tumble drier in accordance with the invention; and Figure 3 is a diagrammatic representation of a cam and cam follower arrangement for adjusting the set value of temperature as a function of time.

Figure 1 shows three curves of exhaust temperature against time. As may be seen the exhaust temperature rises more slowly in the case of a large load since the presence of water tends to keep the exhaust temperature down. Isowteness curves, of which there is an infinite number, but of which only one is shown for simplicity, connect points of equal wetness. As may be seen the isowetness curve indicates that a large load takes longer to reach a certain degree of dryness than does a small load.

In each case the invention is applied to a fan blown hot air tumble clothes drier comprising a perforated druna which receives the clothes to be dried and which is mounted with its axis generally horizontal and rotated about that axis by means of a motor in order to tumble the clothes. A motor-driven fan and an electric heater produce a curler of het air which is directed through the drumin order to dry the clothes.

Figure 2 is a typical circuit diagrarn in which the main motor 2 is connected across the supply terminals in series with a fixed thermostat 4 opening below 350C and one pole 5 of a two-pole door microswitch 6.

The heater 8 is connected across the supply in series with a starting push button 10, the other pole 11 of the two pole door microswitch, and a heater thermal overload cutout 12. Also connected across the supply in series with the push button is an adjustable tem perature-responsive normally dosed switch 14 which is described in more detail below with reference to Figure 3 in series with a coil 16 of a relay 18. The relay has a normally open operating contact 20 in parallel with the fixed thermostat 4 and a normally open holding contact 22 in parallel with the starting push button 10.

The adjustable temperature responsive switching device 14 is shown diagrammatically in Figure 3 and comprises a conventional continuously adjustable thermostatic switch 24 incorporating a bimetallic strip and having a temperature adjusting shaft 26 by which the temperature setting at which it opens can be adjusted.

The thermostatic switch includes a sensor bulb 28 containing a volarile liquid in equilibrium with its vapour, or a silicone oil fluid connected through a capillary tube 30 to a bellows or diaphragm which is not shown arranged to actuate microswitch contacts through a lever which are also not shown. The rotatable adjusting shaft 26 is arranged to vary the position of the microswitch in relation to the lever, or to shift the pivot of the lever.

Mounted on the shaft 26 is a cam follower in the form of an arm 32 carrying a roller 34 which is biased by a spring 36 into contact with a cam 38 connected to a timer motor 40 through a clutch 42. In place of being driven by a timer motor the clutch 42 may be driven from the main drive motor 2 through reduction gearing, or even from the clothes drum. The dutch is connected to the srarting push button 10 so as to be released by pressure on that button and the cam 38 is provided with a spring 44 to return it to an initial position when the clutch is so released.

Accordingly each point on the cam will correspond to a certain time interval from the start of a cycle. The cam is shaped so that the temperature-responsive switch will be adjusted at any time to a temperature corresponding to the exhaust temperature which will be reached at that time when the clothes are in a predetermined degree of dryness, irrespective of the size of the load put into the drum.

Thus to start the machine the door must be closed, which closes the contacts of the door microswitch 6 and the start button musr be pushed. This initiates rotation of the cam 38 and also allows current to pass through the temperature responsive switch 14 and coil 16 thus closing the contacts of the relay 18.

The motor 2 and heater 8 begin operation and the temperature of the exhausted air which is measured by the sensor 28 increases in accordance with a temperature/time curve of the type shown at Figure 1. The temperature at which the contacts of the temperature responsive switch will open is continually changed in accordance with the particular isowetness curve determined by the shape of the cam and when these two temperatures are equal, i.e. when a certain degree of dryness is reached which is independent of the size of the load, within a certain range the contacts of the switch 14 open and the coil is deenergised thus opening the contacts of the relay 18. The heater is immediately switched off but to give a cool-down period the main motor 2 continues to be energised through the fixed thermostat 4 and the clothes steadily cool down, until, when their temperature falls below a temperature ot, say 3U(J, the hexed thermostat opens and shuts off The motor.

The machine has been described as provided with a single cam but in a modified and preferred construction it is provided with a number of cams which can be rendered operative selectively, and which correspond to different isowetness curves for different materials and/or different degrees of dryness.

The cams are mounted coaxially and provided with bridging portions so that one cam merges into another and either the cam or cam follower can be shifted axially so as to change from one cam to another. Moreover each cam surface is tapered axially so as to provide a fine adjustment, for example, for slight changes in the degree of dryness to be achieved. Thus in a particular construction there are three cams or cam surfaces, one for natural fibres fully dried, one for synthetic fibres fully dried, and one for natural fibres iron dry, which is to say sufficiently damp to be ironed.

The temperature responsive switch also incorporates a presetting adjustment so that it may be adjusted to take into account the different spinning speeds of washing machines, which will affect the initial water content of the loads to be dried in the drier.

In certain circumstances, where similar machines are liable to be used in widely differing conditions, for example an out-house in winter compared with either summer operation or use in a heated dwelling, some compensation may be required for ambient temperature. This can be readily incorporated in the adjustable temperature responsive switch, and can for example comprise a compensating bimetal element which is subject to the ambient temperature. In this case the temperature responsive switch is exposed to ambient conditions, for example by placing it in a flow of air entering the shell of the machine. If this cannot be achieved by venting a switch placed behind the control panel, the compensating element may be remotely mounted near the rear of the base of the machine, the control being arranged by cables or other linkages.

In a further embodiment of the invention the temperature sensing is effected electrically.

In this case the temperature-responsive switching device comprises a Wheatstone bridge network having two adjacent arms formed by equal fixed resistors and the other two formed one by a thermistor of negative coefficient sensing exhaust temperature, and the other a variable resistor in series with a thermistor of negative temperature coefficient exposed to ambient temperature. The variable resitor is driven by a cam and cam follower as in the embodiment first described.

Two opposite comers of the bridge are connected to a supply, whilst the other two are connected to an amplifier or trigger circuit controlling the switching off of the heater in a manner similar to that in the first embodiment. In other words when the balance of the bridge changes direction the heater will be switched off and this will occur when the exhaust air temperature relative to ambient temperature exceeds the value given by the isowetness curve at a time equel to that during which the variable resistor has been driven.

In a still further modification, instead of employing a uniform variable resistor driven through a cam and cam follower the resistor itself is graduated so that the change of resistance is a non linear function of the move- ment of the control shaft corresponding to the appropriate isowetness curve.

WHAT WE CLAIM IS: 1. A clothes dryer of fan blown hot air tumble type including an adjustable temperature responsive switching device arranged to switch off the heater when the exhaust temperature reaches a set value and means for automatically adjusting the said set value as a non linear function of elapsed time, the funcrion corresponding to an isowetness curve of exhaust temperature plotted against time for different sizes of loads of a particular type of fabric to a given degree of wetness.

2. A dryer as claimed in Claim 1 in which the temperature responsive switching device is of bimetallic type.

3. A dryer as claimed in Claim 1 in which the temperature responsive switching device includes a bridge network including a variable resistor and at least one thermistor.

4. A dryer as claimed in Claim 1 or Claim 3 in which the temperature responsive switching device includes a variable resistor having a non-linear resisrance characteristic.

5. A dryer as claimed in any one of Claims 1 to 3 in which the means for adjusting the set value of the temperature responsive switching device includes a cam and a cam follower.

6. A dryer as claimed in Claim 5 in which the means for adjusting the set value of the temperature responsive switching device includes two or more cams any one of which can be selectively brought into operation and which are shaped in accordance with the isowetness curves of different fabrics and/or different degrees of dryness.

7. A dryer as claimed in Claim 6 in which the cams are mounted coaxially and interconnected by a sloped surface so that they can be shifted relative to the cam follower to change from one cam to another at any point in the cycle.

8. A dryer as claimed in any one of Claims 5 to 7 in which the surface of the or each cam is tapered axially to provide a fine adjustment.

9. A dryer as claimed in any one of the preceding Claims in which the motor is asso

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. below a temperature ot, say 3U(J, the hexed thermostat opens and shuts off The motor. The machine has been described as provided with a single cam but in a modified and preferred construction it is provided with a number of cams which can be rendered operative selectively, and which correspond to different isowetness curves for different materials and/or different degrees of dryness. The cams are mounted coaxially and provided with bridging portions so that one cam merges into another and either the cam or cam follower can be shifted axially so as to change from one cam to another. Moreover each cam surface is tapered axially so as to provide a fine adjustment, for example, for slight changes in the degree of dryness to be achieved. Thus in a particular construction there are three cams or cam surfaces, one for natural fibres fully dried, one for synthetic fibres fully dried, and one for natural fibres iron dry, which is to say sufficiently damp to be ironed. The temperature responsive switch also incorporates a presetting adjustment so that it may be adjusted to take into account the different spinning speeds of washing machines, which will affect the initial water content of the loads to be dried in the drier. In certain circumstances, where similar machines are liable to be used in widely differing conditions, for example an out-house in winter compared with either summer operation or use in a heated dwelling, some compensation may be required for ambient temperature. This can be readily incorporated in the adjustable temperature responsive switch, and can for example comprise a compensating bimetal element which is subject to the ambient temperature. In this case the temperature responsive switch is exposed to ambient conditions, for example by placing it in a flow of air entering the shell of the machine. If this cannot be achieved by venting a switch placed behind the control panel, the compensating element may be remotely mounted near the rear of the base of the machine, the control being arranged by cables or other linkages. In a further embodiment of the invention the temperature sensing is effected electrically. In this case the temperature-responsive switching device comprises a Wheatstone bridge network having two adjacent arms formed by equal fixed resistors and the other two formed one by a thermistor of negative coefficient sensing exhaust temperature, and the other a variable resistor in series with a thermistor of negative temperature coefficient exposed to ambient temperature. The variable resitor is driven by a cam and cam follower as in the embodiment first described. Two opposite comers of the bridge are connected to a supply, whilst the other two are connected to an amplifier or trigger circuit controlling the switching off of the heater in a manner similar to that in the first embodiment. In other words when the balance of the bridge changes direction the heater will be switched off and this will occur when the exhaust air temperature relative to ambient temperature exceeds the value given by the isowetness curve at a time equel to that during which the variable resistor has been driven. In a still further modification, instead of employing a uniform variable resistor driven through a cam and cam follower the resistor itself is graduated so that the change of resistance is a non linear function of the move- ment of the control shaft corresponding to the appropriate isowetness curve. WHAT WE CLAIM IS:

1. A clothes dryer of fan blown hot air tumble type including an adjustable temperature responsive switching device arranged to switch off the heater when the exhaust temperature reaches a set value and means for automatically adjusting the said set value as a non linear function of elapsed time, the funcrion corresponding to an isowetness curve of exhaust temperature plotted against time for different sizes of loads of a particular type of fabric to a given degree of wetness.

2. A dryer as claimed in Claim 1 in which the temperature responsive switching device is of bimetallic type.

3. A dryer as claimed in Claim 1 in which the temperature responsive switching device includes a bridge network including a variable resistor and at least one thermistor.

4. A dryer as claimed in Claim 1 or Claim 3 in which the temperature responsive switching device includes a variable resistor having a non-linear resisrance characteristic.

5. A dryer as claimed in any one of Claims 1 to 3 in which the means for adjusting the set value of the temperature responsive switching device includes a cam and a cam follower.

6. A dryer as claimed in Claim 5 in which the means for adjusting the set value of the temperature responsive switching device includes two or more cams any one of which can be selectively brought into operation and which are shaped in accordance with the isowetness curves of different fabrics and/or different degrees of dryness.

7. A dryer as claimed in Claim 6 in which the cams are mounted coaxially and interconnected by a sloped surface so that they can be shifted relative to the cam follower to change from one cam to another at any point in the cycle.

8. A dryer as claimed in any one of Claims 5 to 7 in which the surface of the or each cam is tapered axially to provide a fine adjustment.

9. A dryer as claimed in any one of the preceding Claims in which the motor is asso

ciated with a second temperature responsive switch so arranged that after the first temperature responsive switch has switched off the heater the motor continues to operate until the temperature of the load sinks to a predetermined value, whereafter the motor is switched off by the second temperature responsive switch.

10. A dryer as claimed in any one of the preceding Claims which also includes compensation for ambient temperature so that the heater is switched off when the difference between the exhaust temperature and ambient temperature reaches a set value.

11. A dryer substantially as specifically described herein with reference to Figures 2 and 3 of the accompanying drawings.