GB2210485A - Tumble dryer - Google Patents

Abstract

A tumble dryer comprising a cabinet 11 housing a drum 14 supported for rotation, and incorporating a control system having: means 25, 26 for monitoring the inlet and outlet air temperatures; and means responsive thereto to generate an output signal in the event that the period, during the early part of a drying operation, for which the outlet temperature is lower than the inlet temperature is greater than a predetermined period, e.g. 15 minutes. The output signal can be used to terminate the drying operation or to provide a warning signal. <IMAGE>

Description

Tumble Dryer The present invention relates to control systems for tumble dryers. Tumble dryers are well known and usually comprise a cabinet housing a drum supported for rotation about a horizontal axis. In such dryers wet clothes, and other articles of fabric, placed in the drum are lifted by means of paddles within the drum and at the same time heated air is moved through the drum to carry away moisture from the clothes or said other articles, hereinafter referred to collectively as 1clothes" for simplicity. Control systems are known in which the temperature of the air entering the dryer and leaving the drum, hereinafter called the inlet and outlet temperature respectively, is monitored, and the flow of heated air controlled so as to ensure a satisfactory drying performance for the clothes.Such systems make use of a known characteristic of moisture removal as related to exhaust temperature which is such that after reaching a steady rate of moisture removal during which the outlet temperature remains relatively constant the drying operation proceeds until dryness of the clothes has reached the stage where further heat cannot be absorbed thus causing the outlet temperature to rise. Such increase, as related to the still steady level of inlet temperature, can be used to limit the heat input and/or to determine the end of the drying period (positive differentiation).

It is also usual in tumble dryers to have a removable lint filter built into the air circulation system, the purpose of the filter being to trap any entrained lint in the air passing through the dryer so as to leave the clothes free of any undesirable and unsightly lint deposits and to ensure that any air passing from the dryer into the atmosphere is lint-free. However the presence of the lint filter, if not cleaned regularly, can have the undesirable effect of preventing proper circulation of air through the dryer.

Likewise blockage or partial blockage of the inlet or outlet of the dryer or kinking of the outlet hose can similarly prevent proper air circulation.

In the event of such an occurrence the Internal temperature of the dryer may rise to an unacceptable level and whilst thermal cut-out devices may be fitted to the dryer to ensure the de-energisation of the heater if a pre-determined temperature is exceeded it is clearly preferable to provide early warning of such a situation so that remedial action can be taken by the user and if necessary for the heater to be de-energised and for operation of the dryer to be stopped.

It has been found that in these circumstances the relation between inlet and outlet temperatures is such that a negative differential i.e. T inlet outlet persists beyond a normal initial negative differential and the object of this invention is to harness that characteristic so as to provide the user with an indication of a fault so as to enable the adverse condition to be corrected.

According to the invention a tumble dryer comprising a cabinet housing a drum supported for rotation, incorporates a control system having: means for monitoring the inlet and outlet air temperatures; and means responsive thereto to generate an output signal in the event that the period, during the early part of a drying operation, for which the outlet temperature is lower than the inlet temperature is greater than a predetermined period.

The control system preferably comprises means responsive to the output signal either to terminate the drying operation, or to provide a warning signal, or both.

Such a warning signal may be in the form of a visual or an audible indication.

Where the dryer incorporates a microprocessor for controlling its operation the microprocessor can readily be arranged to provide the output signal in the event that information on the inlet and outlet temperatures fed into the microprocessor persists for more than said predetermined time. The maximum allowable time before the output signal is generated will depend upon the particular construction of dryer embodying the invention, but may readily be ascertained by calculation or trial. Typically, where no blockage exists in the air flow path the negative temperature differential will last for about five to six minutes, and accordingly the microprocessor may be programmed to generate the output signal in the event that it persists for a period of, say, between ten and fifteen minutes.

An embodiment of the invention, by way of example, will now be described with reference to the accompanying drawings in which Figure 1 shows a cross-sectional side view of one form of tumble dryer to which the invention is applied; Figure 2 shows a partly block and partly diagrammatic representation of a control system for use in the dryer; Figures 3 to 7 are graphs showing typical temperature curves associated with the performance, both normal and adverse, of a dryer according to the invention.

Referring to the drawings, and first, particularly to Figure 1, there is shown a tumble clothes dryer 10 comprising a substantially air-tight cabinet 11 with an access door 12 in the front through which clothes to be dried are loaded, an air inlet 13 to the left-hand side of the lower part of the rear of the cabinet and a cylindrical drum 14 mounted within the cabinet for rotation about a horizontal axis. Paddles 14a in the drum serve to lift and turn the clothes during a drying operation. Beneath and to one side of the drum is mounted an electric motor 15 driving at one end a centrifugal fan 19 for drawing in air through the inlet 13, and at the other end a belt 16 which extends around the body of the drum 14 to rotate the drum.The drum is open at its front end where clothes are admitted and closed at its rear end apart from a centrally positioned circular opening 17 which coincides with a similar opening 18 formed in the rear panel lla of the cabinet. A removable filter 20 is provided at the rear of the inside of the drum so as to collect any extraneous air-borne material such as fluff or lint produced from clothes tumbling in the drum. An annular enclosing plate 21 at the front of the drum is provided with a series of apertures 21a through which heated air may be forced into the drum, the heater 22 being mounted around the front opening of the drum adjacent the apertures. The annular plate 23 supporting the heater also carries an over-temperature device 24. The access door 12 covers the opening in the front end of the drum in air tight manner when closed.Thermal sensing devices, e.g. thermistors are positioned one, 25 adjacent the ambient air inlet 13, being mounted on the floor of the cabinet, the other, 26, being attached to the wall of the opening 18 where it is subjected to flow of air leaving the drum. A cowl 27 for directing the expelled air upwards from the opening 17 so as to prevent as far as possible any moisture-containing air from being re-drawn into the dryer and recirculated Is attached to the rear panel over the opening, as shown. A flexible hose is connected between the cowl 27 and a convenient outlet for the damp air, for example in a window adjacent the tumble dryer.

The cowl 27 could direct the air in a different direction, e.g. horizontally to the right-hand side or downwardly and to the right, away from the air inlet 13. Alternatively, it could direct the air#rearwardly towards a tube extending through a wall aperture.

Operation of the dryer is such that air drawn in by the fan through the inlet is pressurised within the cabinet, the air circulation path being shown by broad arrows. Since the air outlet is at atmospheric pressure air passes into and through the drum from the pressurised cabinet and is expelled from the dryer through the openings 17 and 18. During its passage from the cabinet to the drum it passes over the heater 22 and is fed into the drum via the apertures 20a where, having passed over and through the clothes in the drum any extraneous material picked up is carried in the air stream and lodged in the filter from where it can subsequently be removed.

The control system for the dryer is shown in Figure 2 and comprises a control module 30 and power module 31 from which the heater bank and motor are controlled, a mains supply 32 feeding the modules via an on-off switch 33 and an inter-locking door switch 34.

Thermistors 24, 25, 26 for the heater plate, air inlet and air outlet locations respectively are provided, these being interfaced with a microprocessor (not shown) associated with the control module via a frequency sensitive oscillator or some other form of temperature-measuring device. Further dryer console controls (not shown) interface with the microprocessor, these enabling the drying programmes to be set up. An alarm buzzer and vacuum fluorescent display units (not shown) are activated via the microprocessor on sensing of fault conditions, in particular a 'BLOCKED' display is featured and the alarm buzzer is sounded in the event of a blockage of the air flow system.

In operation, when the loaded dryer is switched on from cold, and assuming there are no blockages or restrictions in the air circuit, the inlet and exhaust temperatures will rise in accordance with the typical graphs shown in Figures 3 or 4 from which it can be seen that after an initial heat-up period P when the inlet temperature is higher than the exhaust temperature the exhaust temperature rises above the inlet temperature and remains higher for the rest of the drying cycle. Although the period shown is six minutes this is likely to vary slightly according to the conditions of ambient and air flow. Under these conditions the microprocessor which is continuously monitoring the temperature differential allows the drying cycle to proceed. When, however, a fault occurs such as a half-blocked inlet and/or a partly blocked filter - see Figure 5 the period of negative differential is extended to some fourteen minutes and this time the microprocessor intervenes sending a signal to the alarm/display which indicates to the user that a fault condition exists. Similarly referring to Figures 6 and 7 the blockage of the inlet and the outlet exhaust hose respectively give rise to extended periods of negative differential. Thus for a given type of tumble dryer the parameters necessary for adequate control can be determined and the microprocessor programmed accordingly.

Whilst the foregoing information has been related to a pressurised cabinet tumble dryer it will be appreciated that such invention can also relate to a non-pressurised tumble dryer and to a combination washing and drying machine having a similar drying facility.

Claims (8)

1. A tumble dryer comprising a cabinet housing a drum supported for rotation, and incorporating a control system having: means for monitoring the inlet and outlet air temperatures; and means responsive thereto to generate an output signal in the event that the period, during the early part of a drying operation, for which the outlet temperature is lower than the inlet temperature is greater than a predetermined period.

2. A tumble dryer according to Claim 1, in which the control system comprises means responsive to the output signal to terminate the drying operation.

3. A tumble dryer according to Claim 1 or 2, in which the control system comprises means responsive to the output signal to provide a warning signal.

4. A tumble dryer according to Claim 3, in which the warning signal comprises a visual indication.

5. A tumble dryer according to Claim 3 or 4, in which the warning signal comprises an audible indication.

6. A tumble dryer according to any preceding Claim, in which the monitoring means of the control system comprises programmable data processing means including a memory for storing a representation of the predetermined period.

7. A tumble dryer according to any preceding Claim, in which the cabinet is of the pressurised type.

8. A tumble dryer substantially as described herein with reference to the accompanying drawings.