US3583688A

US3583688A - Dryer control

Info

Publication number: US3583688A
Application number: US54972A
Authority: US
Inventors: Norman L Fuqua; William H Gartley
Original assignee: Whirlpool Corp
Current assignee: Whirlpool Corp
Priority date: 1960-09-09
Filing date: 1960-09-09
Publication date: 1971-06-08
Anticipated expiration: 1988-06-08

Abstract

A temperature control system for a fabric dryer that includes a heated fluid inlet means to the dryer, means for supporting said fabric for contact by a heated fluid and a moisture and fluid exhaust means from the dryer, comprising: a variable heat supply means for supplying heat to said inlet means; an exhaust thermostatic control associated with said exhaust means exposed to exhaust heat therein; means operated by said exhaust control for progressively reducing the heat supplied by said heat supply means as said exhaust heat increases; a second thermostatic control associated with said inlet means exposed to heat from said supply means; means for subjecting said second thermostatic control to ambient air in addition to said supply means heat; and means for stopping said heat supply means when said second control reaches a predetermined temperature.

Description

United States Patent [72] Inventors Norman L. Fuqua Mansfield, Ohio; William H. Gartley, St. Joseph, Mich. [21] AppLNo. 54,972 [22] Filed Sept.9, 1960 [45] Patented June8,l97l [73] Assignee Whirlpool Corporation [54] DRYER CONTROL 17 Claims, 7 Drawing Figs.

[52] U.S.Cl 263/33, 34/30, 34/45, 34/48 [51] InLCl F27b7/00 [50] FieldofSearch 34/45,48, 30,3];263/333 [56] References Cited UNITED STATES PATENTS 2,505,041 4/1950 Gorsuch 34/45 3,022,987 2/1962 Thorsheim... 34/45 3,028,680 4/1962 Conlee 34/45 3,037,296 6/1962 Cooley 34/45 Primary ExaminerCarro11 B. Dority, Jr.

Attorneys-Herbert K. Anspach, William l-louseal, James S.

Nettleton and l-lofgren, Wegner, Allen, Stellman & McCord ABSTRACT: A temperature control system for a fabric dryer that includes a heated fluid inlet means to the dryer, means for supporting said fabric for contact by a heated fluid and a moisture and fluid exhaust means from the dryer, comprising: a variable heat supply means for supplying heat to said inlet means; an exhaust thermostatic control associated with said exhaust means exposed to exhaust heat therein; means operated by said exhaust control for progressively reducing the heat supplied by said heat supply means as said exhaust heat increases; a second thermostatic control associated with said inlet means exposed to heat from said supply means; means for subjecting said second thermostatic control to ambient air in addition to said supply means heat; and means for stopping said heat supply means when said second control reaches a predetermined temperature.

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IN ET THEHMO CLOSING TEMP TEHPERnTuR TEMPs H H [Infil -NT TEMP Low firm/5N1 TEMP I N T! R VRLS Y 6 TIME INTERVRLS DRYER CONTROL This invention relates tov a control system for controlling automatically the operation of a dryer such as a home laundry dryer.

The dryer control system of this invention is ideally suited for use on domestic laundry dryers and combination washerdryers. The system is completely automatic with overdrying being prevented and with the drying being terminated automatically at the end of a drying cycle. The control system is particularly adapted to those dryers in which the load such as a load of laundry is placed in a drum which is then rotated while heated fluid such as heated air or air mixed with the products of combustion of domestic heating gas is forced through the drum. Such dryers are illustrated in U.S. Pat. Nos. 2,635,354 and 2,799,948.

One of the features of this invention therefore is to provide an improved dryer control system with automatic controls for regulating the temperature and for terminating the drying cycle.

Another feature of the invention is to provide such a system in which the temperature is controlled by a temperature responsive device associated with the exhaust duct of the dryer for controlling the heat input to the dryer and in which means are provided for discontinuing the drying when the material being dried has reached a desirable low moisture content.

Other features and advantages of the invention will be apparent from the following description of certain embodiments thereof taken in conjunction with the accompanying drawings. Of the drawings:

FIG. 1 is a schematic wiring diagram of the controls for a gas fired dryer first embodiment of the invention.

FIG. 2 is a chart showing the sequence of operation of the control switches of the wiring diagram of FIG. 1.

FIG. 3 is a schematic view showing the heated fluid and ambient air supply to the dryer of this first embodiment.

FIG. 4 is a graph illustrating the heating conditions within the inlet and exhaust of the dryer under typical high ambient and low ambient environments.

FIG. 5 is a view similar to FIG. 1 but showing a wiring diagram for an electric dryer second embodiment of the inventron.

FIG. 6 is a view similar to FIG. 3 but showing the heated fluid and ambient air intake to this second embodiment.

FIG. 7 is a view similar to FIG. 4 but relating to this second embodiment of the invention.

Present temperature controlled domestic dryers or combination washer-dryers use either time selection systems or automatic dryer control systems. In the time selection system the operator first places the wet load in the dryer drum and then sets a time which she judges will be necessary to dry the load. This prejudgment of time is of course extremely difficult and ordinarily impossible because of the many variables that are present and cannot be easily evaluated. In such an arrangement it is very easy to overdry the fabric as the operator naturally tends to use enough time to be sure that the load is dry. If she underestimates and the load is still damp, she then resets the time and here it almost invariably follows that she allows too much time with the result that some of the fabrics become overdried, since drying will continue at the maximum input of the heat source.

Automatic drying control systems have been proposed but here again there is danger as ordinarily the control thermostats are set very high to insure complete drying regardless of drying variables. Here again, this type of control can result in very serious damage to the fabric as the result of overdrymg.

The dryer control system of this invention avoids these difficulties as it provides automatic control which dries at a much lower temperature, and yet maintains high inputs, than previous dryers and which automatically compensates for all variables whether or not the operator even realizes that they are present.

In the embodiment of FIGS. 1 -4 inclusive the control is used on an ordinary gas dryer having a drying chamber (not shown) and which is indicated schematically at 10. The dryer is heated by an ordinary gas burner (not shown) supplied by a gas inlet line 11 in which is located a valve 12 controlled by an electric solenoid 13 for opening and shutting the line. This gas line has a branch 14 going to the main burner of the dryer and flow into this branch line is controlled by a throttle valve 15 that is movable toward and away from a valve seat 16. This throttle valve 15 is urged toward open position by means of a spring 17. The throttle valve is controlled by an ordinary thermostat 18 of the tube pneumatic type having a fluid line 19 leading to a chamber 20 where fluid pressure can operate on a piston 21 attached to the valve 15. As can be seen in FIG. 1, the spring 17 and fluid pressure within the chamber operate against each other so that increasing pressure in the chamber 20 forces the valve 15 toward the minimum input position while decreasing pressure in this chamber pulls the valve 17 toward open or maximum position. The exhaust thermostat 18 is located in an exhaust passage 22 from the dryer 10. Naturally, means other than that shown in FIG. 1 could be used to throttle the valve 15 as the result of the temperature sensed by exhaust thermostat 18.

The control system is electrically powered and receives its power from electric lines indicated at L1 and L2. In this control system there is provided a first electrical circuit for controlling the supply of gas by controlling the solenoid 13 with this first electrical circuit including a first switch indicated at A-C in FIG. 1, an electric line 23 to one side of the solenoid 13, a safety thermostat switch 24 in this line and a normally closed switch 25 also in this line.

The circuit also includes an electrically operated means for providing airflow through the dryer 10 including a motor 26 having starting windings 27, running windings 28 and a centrifugal switch 29. The motor 26 operates an ordinary blower 30 for causing this flow of air through the dryer. Providing power for the motor 26 is a second electrical circuit including the

electrical leads

31, 32, 32a and 33, a start switch 34 in line 33 that is normally open and an on-ofi switch 35 in line 32. The second electric circuit for this air moving means includes a second switch designated by the contacts A-B in FIG. 1. As can be seen in FIG. 1 the other side of the solenoid from the lead 23 is connected to the centrifugal switch 29 of the motor 26. The control system is also provided with a third electrical circuit including a third switch designated by the contacts A-D with this circuit including the electric lead 36 and an ordinary timer motor 37 for operating the sets of contacts A-B, A-C and A-D. Extending between the contact B and the electric line 36 is a line 38 which includes an inlet thermostat 39.

As is shown in FIG. 3 the inlet thermostat 39 in this embodiment extends into the heated fluid inlet passage 40 which is divided over a portion of its length by means of a partition 41 into a main passage 42 and an auxiliary passage 43. The heated fluid which is a mixture of ambient air and products of combustion of the gas burner (not shown) enters at the bottom of the main passage 42 as indicated by the opening 44. This heated fluid travels up the inlet passage 40 to enter the drying chamber as indicated by the opening 45. In the meantime ambient air is drawn into the auxiliary passage 43 through the series of openings indicated at 46 with some of this air passing through openings 47 in the partition 41 to mingle with the heated fluid in the main passage 42. Other portions of the ambient air flow upwardly in the passage 43 to contact the thermostat 39 and mix with the heated fluid at the top of the main passage 42 just prior to entry to the drying chamber through the opening 45. The inlet thermostat 39 is of the type that is effective over its entire length so that both the heated fluid in the passage 42 and the ambient air in the passage 43 serve to control the operation of the thermostat 39.

The closing sequence of the switches A-B, A-C and A-D as controlled by the timer 37 is indicated in FIG. 2. The letter "C" is used to indicate the succeeding intervals of time 1 to 10 when the switches are closed. In the particular embodiment of the invention illustrated in FIGS. l-4 these intervals of time are 1 minute each so that the intervals total 10 minutes of timer 37 operation.

In operating the dryer control of this invention the dryer is started in its dry cycle by indexing the timer back to the beginning of interval No. 1. In some ordinary timers this is done manually while in others it is accomplished by pushbuttons. The timer used in this invention is purely conventional and is therefore not illustrated except schematically. When this is done, as is shown in FIG. 2, all three switches A-B, A-C

and A-D are closed. To start the dryer the switch 34 is depressed or closed and momentarily held closed to put power through the starting windings 27 and running windings 28 of the motor 26. As soon as the motor picks up speed the centrifugal switch 29 leaves the contact 47 to engage contact 48 deenergizing the starting windings 27 and to complete the circuit through the running windings 28 with the start switch 34 being bypassed by a parallel circuit through lead 32a. The switch 34 can then be released and it moves toward its normally open position. The motor 26 then continues to run by power being supplied to the running windings 28 to operate the blower 30 which causes the flow of heated fluid through the passage 42, ambient air through the passage 43 and mixed fluid and air through the opening 45 into the dryer to contact the clothes load being tumbled in the dryer. From here the mixed heated fluid with moisture from the drying load then flows out the exhaust 22 in 39 contact with the exhaust thermostat 18.

As shown in FIG. 2, all three contacts A-B, A-C and A-D are. closed for the first three time intervals or, in this embodiment, for the first 3 minutes. At the end of this time the contacts A-D open. While contacts A-D are closed during these first three intervals the inlet thermostat 3 is bypassed by way of the electric line 36. The inlet thermostat is closed when the dryer is not operating but during this three-interval period the temperature in the inlet 40 reaches a temperature such that the inlet thermostat 39 is opened. This initial three-interval period is indicated by the drying time line 48 on the graph of FIG. 4. As is indicated on this graph, the abscissa is the drying time in any units desired while the ordinate is the temperature. The second vertical line 49 on the graph of FIG. 4 indicates the approximate time when inlet thermostat 39 closes to energize timer motor 37 to the end of the drying cycle after a predetermined timed period. On the graph of FIG. 4, showing a plot of inlet and exhaust temperatures during the drying cycle, the conditions when the ambient air is at a high temperature, as in summer, are indicated by the solid line. When the ambient temperature is low, as in winter, the inlet temperature is indicated by the broken line, both as indicated by the legends on FIG. 4. As is seen in this Figure, during the first three time intervals the inlet temperature rises above the closing temperature of the inlet thermostat as indicated by the horizontal line 50. By reason of the higher temperature of high ambient air in combination with the heated air, the inlet temperature will rise to a higher maximum for this condition than when the ambient air temperature is low.

Assuming high ambient temperature such as approximately 90 F., the timer will operate during the first three intervals with all switches closed so that the inlet temperature will rise as indicated by the line 51 on the graph of FIG. 4 while the exhaust temperature will rise as indicated by the line 52 on FIG. 4. Both of these temperatures are rising as they cross the end of the three-interval period as indicted by the line 48. After these first three time intervals the switch A-D opens. The inlet thermostat 39 is open as it has been opened during the first three timer intervals by reason of the rising temperature moving above the line 50 on the graph of FIG. 4. This line 50 indicates the setting for the inlet thermostat. Actually, inlet thermostat 39 would open up somewhat above the inlet thermostat 39 closing temperature represented by the line 50, but for purposes of this explanation it can be assumed that the thermostat 39 both opens and closes at the temperature represented by line 50. As the inlet thermostat 39 is open, switch A-D deenergizes the timer 37. The operation of the timer between the time lines 48 and 49 is then controlled solely by the inlet thermostat 39 and since in FIG. 4 the inlet thermostat 39 is open between the time period represented by the lines 48 and 49, this time will be of variable time with timer motor 37 deenergized. i

During operation of the dryer the exhaust temperature in the exhaust 22 continues to rise slowly until the high ambient exhaust temperature curve as indicated by the line 52 reaches the exhaust thermostat modulation initiation point as indicated by the horizontal line 53 on the graph. At this point, which is indicated at 54, the exhaust thermostat 18 begins to apply pressure within the chamber 20 and againstthe piston 21 to move the gas valve 15 to the left as viewed in FIG. 1 to throttle the gas supply. This immediately limits the maximum value of the inlet temperature as indicate by the point 55 on FIG. 4 as after this the inlet temperature begins to fall. As moisture is removed from the load within the dryer more and more of the heat input becomes available as sensible heat at the exhaust thermostat 18. Although the exhaust temperature 52 continues to rise on a small slope as indicated in FIG. 4, the inlet temperature is throttled down more and more with this increasing sensible heat in the exhaust. The inlet temperature continues to fall commensurate with the dryness of the load from the point 55 until it reaches the closing temperature of the inlet thermostat as indicated by the line 50 in FIG. 4. This closing temperature of the inlet thermostat is selected arbitrarily as being the temperature at which experience has indicated that the load is sufficiently dry. If desired, this inlet thermostat 39 can be of the adjustable type whereby adjustments can be made to change the closing value 50 of this thermostat and thus control the dryness of the clothes load.

As soon as the inlet heat has thus closed the inlet thermostat 39, the circuit to the timer37 is closed to start the operation of the timer. The timer then continues its movement through intervals 4l0 inclusive. During the

intervals

4 and 5 contacts A-C remain closed. This continues to supply heat to the dryer at a relatively low temperature as the valve 15 is then throttled down to a substantial extent.

At the end of interval 5, as indicated in FIG. 2, switch A-C is then opened to break the circuit to the solenoid 13 and permit the main gas control valve 12 to close. This shuts off the gas supply and stops the heating. The timer which is energized through the inlet thermostat 39 continues to operate through the cooldown intervals 6-10 inclusive with contacts A and B being closed to continue the air supply which serves to cool the dried load. At the end of interval 10 the machine is automatically shutoff.

The line 23 from the contacts A-D contains a manual switch 25. This switch may be opened when it is desired to blow air only through the dryer without the application of heat. This open switch prevents energization of the gas supply so that the timer will run through the entire 10 intervals with only the motor operating. This will occur since inlet thermostat 39 will remain closed in the absence of heat, therefore timer motor 37 will be energized continuously.

The positioning of the inlet thermostat 39 as shown in FIG. 3 serves a very important function with respect to the ability of the control system of FIG. 1 to operate properly in any installation and yet use exhaust and inlet thermostats having the same operating values for all installations. It is well known that the drying variables of ambient air temperature, airflow rate through the machine, and heat input have a marked influence on any dry control system and that common extremes of these variables individually or in combination can actually make a dry control system ineffective. The positioning of the inlet thermostat 39 in this invention allows the control system of FIG. I to be compensated for the above-mentioned drying variables with the result that like clothes loads will be dried to thedesired degree of dryness and in approximately the same time regardless of the variable installation conditions prevailing.

An explanation of the compensation effect of the inlet thermostat 39 will be explained by referring to FIG. 4 in which it will be assumed that two identical clothes loads are being dried under identical installation conditions with the sole exception being that one load is being dried in the presence of high room ambient temperature (90 F. or above) and the other load in a low room ambient temperature (60 F. or below). As earlier explained with regard to FIG. 4, the solid lines represent the high ambient inlet and exhaust temperatures and the dotted lines represent the low ambient inlet and exhaust temperatures as sensed by the respective inlet and exhaust thermostats. The valves 53 and 50 for both loads will naturally be the same.

As seen in FIG. 4 and due to the differences in the ambient conditions, the corresponding inlet and exhaust temperature curves will be displaced from each other by an amount approximately equal to the differences in ambient temperature. Even though the ambients are different, the actual drying rate of moisture from both loads is practically identical. Thus, it can be seen from FIG. 4 that when the high ambient exhaust temperature curve reaches the modulation initiation temperature 53 of the exhaust thermostat 18 at the point 54 there will be considerably more moisture remaining in the load being dried under high ambient conditions at this point than there will be when at the point 57 when the low ambient exhaust temperature curve reaches its modulation initiation point. Since it is very desirable that the dry control system of FIG. 1 dry both loads in approximately the same amount of time, it can be seen that considerably more moisture will have to be dried from the high ambient load during the modulation time period between point 54 and the line 49 when the inlet thermostat 39 closes than will have to be removed during the modulation period of the low ambient load between the point 57 and the line 49. The ambient air as shown in FIG. 3 that passes over the thermostat 39 assures this condition.

Due to low ambient drying characteristics, the load being dried under low ambient conditions would be almost completely dry at the point 57 when input modulation begins. This means that it would be very desirable to have the inlet temperature cool as rapidly as possible to the inlet thermostat closing temperature as indicated at 50 to expedite the completion of the overall drying time. As the input is reduced after point 57, the cool ambient air passing over the inlet thermostat 39 in combination with the reduced input results in the desired rapid cooling of this thermostat to the line 50. In the case of the high ambient load the higher temperature ambient air, after modulation beings to reduce the input at point 54, does not have such a pronounced cooling effect on the inlet thermostat as in the low ambient load and thus for comparable inputs the inlet temperature curve will cool with a smaller slope to the inlet thermostat closing temperature which allows enough time for the moisture remaining after the point 54 to be removed and yet not unnecessarily prolong the cycle. Thus the compensation effect of the ambient air passing over the inlet thermostat serves to allow equal loads to be dried in approximately the same minimum amount of time using approximately the same total heat input regardless of the ambient air temperature at the installations. Loads that are dried under identical conditions with only the airflow rate through the dryer or the maximum available heat input being different are compensated for in the same way as explained for ambient variations.

The dry control system of FIG. 1 is also able to dry loads efficiently and safely of any size within the capacity of the dryer and of any fabric makeup regardless of whether or not the fabrics are mixed.

The dryer control system of this invention prevents fabric damage due to overheating as the inlet temperatures are controlled to low enough values when the fabrics are dry that damage does not. occur. Furthermore, as more and more moisture is evaporated from the load so that portions of the fabric become dry and thus susceptible to heat damage the inlet heat is automatically reduced to prevent this damage. In addition, the dryer control system of this invention permits a mixture of such fabrics as synthetics, heavy cottons such as towels and delicate fabrics to be dried in the same load without damage to any of the fabrics. This is true because as drying progresses with this mixed fabric load certain of the articles such as synthetic fabrics and delicate fabrics dry first before the heavier cottons and the like. When these earlier dried fabrics become completely dry more of the inlet heat to the dryer becomes available as sensible heat to raise the temperature of the exhaust airstream. When the exhaust airstream reaches a safe temperature, for example 155 F., the reduction of the heat input to the dryer begins by throttling down the gas supply as has been described. This 155 F. is a very low control temperature for a high heat input dryer. Thus with the time control systems, thermostats are set at much higher values such as l75-- 1 F. resulting in the dry portions of the load being exposed to extremely high inlet temperatures for a long time period. As soon as the exhaust gases have reached the temperature of the exhaust thermostat setting, throttling of the inlet heat and subsequent reduction of inlet temperature begins. This is very important as it is the very high inlet air temperatures to the load, and particularly to the partially dry load, that primarily cause fabric damage.

In a typical embodiment of a gas heated dryer controlled by the system of this invention and assuming average gas quality the minimum heat input when the valve 15 is fully throttled down is approximately 8000 B.t.u. per hour. This has been found to be a safe minimum that can continue until the main gas valve 12 is closed at the end of the fifth time interval on the timer 37. In a typical embodiment the maximum heat input is 37,000 Btu per hour. During the operation there is no cycling on and off of the heat input but rather a continual input at the correct heat level with the result that the load becomes dry with no fabric damage or overdrying in a minimum overall time period.

In the embodiment of FIGS. 57 inclusive

electric heaters

60 and 61 are arranged in parallel with an exhaust thermostat 62 being located between the heaters. In this typical embodiment the first heater 60 may be of 2000 watts while the second heater may be of 3600 watts for a total of 5600 watts. If desired, the control may include an additional heater or heaters such as the electric heater 63, of 2800 watts, in parallel with the heater 61. This gives the entire heat input assembly a capacity of 8400 watts. Although 8400 watts could also be used, the following explanation of the second embodiment will be based on 5600-watts operation (elements 60 and 61). When heating element 63 is used, it is controlled in the same manner as heating element 61 is controlled.

In the embodiment of FIG. 5 the same switch contacts A, B, C and D are provided and the timing arrangement is the same as that shown in FIG. 2 for the first embodiment. The motor 26 is the same, and the starting switch 34, blower 30 and timer 37 are the same. In this embodiment the switch A-C is in a line 64 to one side of the

parallel heaters

60 and 61 and 63, when used, and with the exhaust thermostat 62. Similarly, contact D of the switch A-D is in an electric line 65 that bypasses the inlet thermostat 39. The door switch 66 which is the same as the door switch 66 of the first embodiment is connected to a line L1. The on-off switch 35 which is the same as the corresponding switch in the first embodiment is connected to the second line L2. The third line L3 to provide the necessary power for the electric heaters is connected through a centrifugal switch 67 and a safety thermostat 68 by means of a line 69. This line is connected to the side of the parallel electric heaters opposite the line 64.

The centrifugal switch 67 is controlled by rotation of the motor 26 and is ganged as at 67a to centrifugal switch 29. In order to start operation of this embodiment, the switch 35 is closed and the start switch 34, which is normally open, is closed. This starts operation of the motor by energizing the starting windings 27 and the running windings 28 as in the first embodiment and as soon as the centrifugal switch 29 moves to the contact 48, which deenergizes the starting windings 27, the start switch 34 can be released. This operation is exactly the same as described in the first embodiment and occurs in a very short period of time. As soon as the motor 26 reaches a speed to move switch arm 29 from contact 47 to contact 48, the centrifugal switch 67 which is also controlled by switch arm 29 as shown at 67a also closes to provide power to the electric heaters.

As soon as the timer 37 is indexed to the starting position in the manner described previously, contacts A B, A-C and A-D are closed. As pointed out in the first embodiment, this provides a parallel energization circuit for inlet thermostat 39 and also causes immediate energization, after centrifugal switch 67 closes, of the

electric heaters

60 and 61. This heating continues through the first three timed intervals as previously described during which the inlet thermostat 39 opens. In conditions of high ambient temperature, the exhaust temperature, as indicated by the line 72, continues to increase until it reaches the upper setting or opening point of the exhaust thermostat 62 indicated by the line 73. As is customary in thermostats of this type, the exhaust thermostat 62 has an upper setting or opening point and lower setting or reset point with the lower setting indicated by the line 74 in order to switch the heater 6] into and out of the circuit. When the exhaust temperature line 72 reaches the opening temperature line 73 of the exhaust thermostat, the circuit is broken in the exhaust thermostat 62 to deenergize heating element 61. The exhaust thermostat then continues to open and close in the customary manner between the temperature setting lines 73 and 74 during the drying cycle.

As soon as the exhaust temperature has reached this maximum or opening setting of the exhaust thermostat as indicated by the point 75 and the heater 61 is deenergized, the inlet temperature as indicated by the line 76 has reached its maximum value as indicated by the point 77. The inlet temperature then falls because the circuit to the electric heater 61 has been broken and operation continues with element 60 (2000 watts) only operating. As the circuit to the electric heater 61 continues to be made and broken by the operation of the exhaust thermostat, the inlet temperature falls in a series of steps until it reaches the inlet thermostat closing temperature as indicated by the line 70 whereupon the inlet thermostat closes to energize the timer 37. The timer thereupon resumes its movement through intervals 4 inclusive with results as described above in connection with the first embodiment. The beginning of interval 4 is indicated by the vertical line 78 on FIG. 7.

In this second embodiment the inlet thermostat 39 is located in the inlet passage 79 to the dryer in which the electric heaters (such as 60, 61 and 63) are located as indicated at 80. In this embodiment ambient air is admitted through openings such as the openings 81 and through other openings as indicated by the arrows 82 to contact the thermostat 39 with the results described in connection with the first embodiment.

As can be seen from the circuit of FIG. 5, heater 60 is always in the circuit, whenever timer switch A-D is closed, during the drying cycle with the exhaust thermostat controlling the heater 61.

The dryer control system of the embodiment of FIGS. 5, 6 and 7 functions similarly to the dryer control system of the first embodiment and has the same advantages. The principal difference here is that the exhaust thermostat cycles on and off as indicated by the upper and lower setting lines 73 and 74 of FIG. 7 and that the inlet and exhaust temperature curves instead of being smooth curves are in the form of jagged lines corresponding to the cycling of the exhaust thermostat, also as shown in FIG. 7. This is caused by the inlet air heater 61 cycling on and off by reason of the closing and opening of the exhaust thermostat 62. In this embodiment of the invention the upper exhaust temperature line 73 is 145 F. for the 5600- watt input of

heaters

60 and 61. Under these conditions the lower line 74 of FIG. 7 of the 5600-watt input is equivalent to 137 F. This differential between 145 F and 137 F. is a very small differential for a thermostat of this type and permits the inlet temperatures to rise and fall over smaller temperature ranges to lessen the chance of false closing of the inlet thermostat. A correspondingly small differential is provided with the 8400-watt input using an opening value of 165 F. and a closing value of 157 F.

In operation when the exhaust temperature as indicated by the line 72 for high ambient conditions and by the line 84 for low ambient conditions reaches the thermostat setting line 73, the exhaust thermostat 62 opens with the result that heating element 61 of the electrical input to the dryer is cycled off. This causes a drop in temperature in both the exhaust fluid stream from

points

75 and 86 and the inlet temperatures from the peaks 77 and 87 to drop. Drying still continues, however, as the heater 60 is still in the electrical circuit. As the exhaust temperature decreases it reaches the value indicated by the line 74 on FIG. 7 where the exhaust thermostat resets and element 61 is reenergized. This cycling of a part of the input heat as indicated by the

lines

76 and 85 for high and lowambient conditions continues until the inlet thermostat closes at its setting indicated by the line 70 on FIG. 7. This is low enough to insure that the load will be completely dry.

Compensation for the important drying variables in the dry control system of FIG. 5 is very similar to that as explained for the first embodiment of FIG. 1. Of course in the electric heat input system of FIG. 5 the heat input does not vary infinitely over its operating range as it does for the gas input embodiment of FIG. 1. In FIG. 7 the same conditions of dryness for the low and high ambient drying conditions exist at the

points

86 and 75 as at the

points

57 and 54 of FIG. 2. The compensating effect of the ambient air passing over the inlet thermostat 39 as shown in FIG. 6 is therefore very similar as for FIG. 3. When the low ambient load is dry or is almost dry as at the point 86 in FIG. 7, the exhaust airstream cools slowly after the first cycle point resulting in a longer period of time to allow cool ambient air to pass over the inlet thermostat 39 to cool it so therefore the inlet temperature drops a greater amount under one element operation than with a high room ambient temperature under the operation of one heating element only. Under full input after the exhaust thermostat resets, the low ambient exhaust temperature-will reheat to the trip temperature of line 73 faster because of the lack of moisture than with the high ambient, thus the maximum value that the low ambient inlet temperature attains will be considerably less than at the point 87 when the input was first reduced. Therefore, the low ambient inlet temperature will drop to the inlet thermostat closing temperature 70 much sooner after the first cycle point than with the high ambient condition which is the desired and necessary condition to assure that like loads will dry as explained for the first embodiment.

Having described the invention as related to the embodiments shown in the accompanying drawings, it is our intention that the invention be not limited by any of the details of description, unless otherwise specified, but rather be construed broadly within its spirit and scope as set out in the accompanying claims. The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

Iclaim:

1. A temperature control system for a fabric dryer that includes a heated fluid inlet means to the dryer, means for supporting said fabric for contact by a heated fluid and a moisture and fluid exhaust means from the dryer, comprising: a variable heat supply means for supplying heat to sad inlet means; an exhaust thermostatic control associated with said exhaust means exposed to exhaust heat therein; means operated by said exhaust control for progressively reducing the heat supplied by said heat supply means as said exhaust heat increases; a second thermostatic control associated with said inlet means exposed to heat from said supply means; means for subjecting said second thermostatic control to ambient air in addition to said supply means heat; and means for stopping said heat supply means when said second control reaches a predetermined temperature.

2. A temperature control system for a fabric dryer that includes a heated fluid inlet means to the dryer, means for supporting said fabric for contact by a heated fluid and a moisture and fluid exhaust means from the dryer, comprising: means for supplying heated fluid to said inlet means during a drying cycle including gas heating means having a gas supply line; airflow means for flowing ambient air into said inlet means; a normally open throttle valve in said supply line; means for opening said gas supply line at the beginning of said cycle to produce a rising temperature in said inlet means; a thermostatic control in said exhaust means operable at a first predetermined temperature to throttle down said valve to maintain an at least approximately constant temperature in said exhaust means whereby the temperature of said inlet means is reduced as moisture is removed; a thermostatic control in said inlet means operable after reaching a predetermined temperature to close said supply line, said inlet thermostatic control including a thermostat; and means directing a portion of said ambient air in operating contact with said thermostat.

3. A temperature control system for a fabric dryer that includes a heated fluid inlet means to the dryer, means for supporting said fabric for contact by a heated fluid and a moisture and fluid exhaust means from the dryer, comprising: electrically controlled means for supplying heated fluid to said inlet means during a drying cycle; a first electrical circuit for said heated fluid means including a first switch; electrically operated means for flowing ambient air into said inlet means; a second electrical circuit for said air means including a second switch; a first thermostatic control including a thermostat associated with said exhaust means operable at a first predetermined temperature for controlling said heat supply to said inlet means to reduce said heat supply as the temperature in said exhaust means tends to exceed said predetermined temperature; a second thermostatic control including a thermostat associated with said inlet means and having an open position and a closed position, said thermostat opening at a second predetermined temperature in said inlet means and closing at a third predetermined temperature in said inlet means to inactivate said supplying means; a third electrical circuit including a third switch bypassing said second control thermostat; and an electrically operated timer in said third circuit having means for closing said second switch for a first time interval for flowing of ambient air, for closing said first switch for a second time interval less than said first time interval to heat said flowing ambient air, and for closing said third switch for a third time interval less than said second time interval to bypass said second thermostatic control to allow said thermostatic control to open only as a result of the heating of said ambient air and to inactivate said timer at the end of said third time.

4. A temperature control system for a fabric dryer that includes a heated fluid inlet means to the dryer, means for supporting said fabric for contact by a heated fluid and a moisture and fluid exhaust means from the dryer, comprising: electrically controlled means for supplying heated fluid to said inlet means during a drying cycle; a first electrical circuit for said heated fluid means including a first switch; electrically operated means for flowing ambient air into said inlet means; a second electrical circuit for said air means including a second switch; a first thermostatic control including a thermostat associated with said exhaust means operable at a first predetermined temperature for controlling said heat supply to said inlet means to reduce said heat supply as the temperature in said exhaust means tends to exceed said predetermined temperature; a second thermostatic control including a thermostat associated with said inlet means and having an open position and a closed position, said thermostat opening at a second predetermined temperature in said inlet means and closing at a third predetermined temperature in said inlet means to inactivate said supplying means; a third electrical circuit including a third switch bypassing said second control thermostat; and an electrically operated timer in said third circuit having means for closing said second switch for a first time interval for flowing of ambient air, for closing said first switch for a second time interval less than said first time interval to heat said flowing ambient air, and for closing said third switch for a third time interval less than said second time interval to bypass said second thermostatic control to allow said thermostatic control to open only as a result of the heating of said ambient air and to inactivate said timer at the end of said third time.

5. In a dryer control system for a dryer having a heat source and means for moving hot air through the dryer, the combination comprising first means for varying the heat output of the heat source between a high output and a low output, temperature responsive means responsive to the temperature of exhaust air, means connecting said temperature responsive means to said first means whereby as the moisture is removed and less heat is needed said first means is adapted to reduce the heat output of the heat source, second means for terminating the operation of the heat source, a timer for controlling said second means, second temperature responsive means responsive to a predetermined level of output of the heat source and having contacts that are closed below said level and open above said level, and means connecting said second temperature responsive means to said timer so that said second temperature responsive means only determines the termination of operation of the dryer by providing a fixed time of operation of the dryer after the output of the heat source has reduced to a predetermined level to close said contacts.

6. In a dryer control system for a dryer having a heat source and means for moving hot air through the dryer, the combination comprising first means for varying the heat output of the heat source between a high output and a low output, temperature responsive means responsive to the temperature of exhaust air, means connecting said temperature responsive means to said first means whereby as moisture is removed and less heat is needed the output of said first means controlled by said temperature responsive means only is adapted to vary without cutting off the heat output of the heat source, second means for terminating the operation of the heat source, a timer for controlling said second means, second temperature responsive means responsive to a predetermined level of output of the heat source and having contacts that are closed below said level, and means connecting said second temperature responsive means to said timer to cause energization of said timer by said second temperature responsive means only when the output drops to said level to cause termination of operation of the dryer at a fixed time after the said level is reached.

7. The combination of claim 6 wherein said timer is manually operable to its on position and has a holding circuit switch connected to said timer in parallel with said second temperature responsive means for keeping the timer operating for a predetermined short time regardless of the condition of said second temperature responsive means to assure the dryer coming up to operating temperature above said predetermined level and then causing said timer to stop operating until said predetermined level is reached thereafter whereupon the timer will start and run the remainder of the drying time.

8. The combination of claim 7 wherein said timer also has switch means connected to said means for moving the air and to said first means for controlling operation of the means for moving the air and for operating said first means.

9. In a dryer, the combination comprising a fuel burner furnishing heat thereto, a modulating valve arranged to control the supply of fuel to the burner, means for circulating air through said dryer, a first temperature responsive means responsive to exhaust air temperature of the dryer, means connecting said first temperature responsive means to said modulating valve so that the burner heat output is gradually decreased as the temperature of the exhaust air increases, second temperature responsive means responsive to a temperature indicative of the output of the burner, control means for said valve for shutting off the flow of gas to the burner, and

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a timer for controlling said control means and controlled only by said second temperature of responsive means to initiate operation of the timer near the end of the drying cycle.

10. The combination of claim 9 wherein said timer has switch means for controlling operation of said control means and switch means for controlling said timer, said switch means for said control means being arranged to keep said control means energized after said switch means for the timer has opened.

11. The combination comprising drying apparatus having a source of heat for heating air flowing into said apparatus and means for exhausting the air from said apparatus, a timer having first switch means for controlling said heat source and second switch means for controlling said timer, and control means responsive to the heat input to the apparatus to control energization of said timer independently of said second switch means, said control means being arranged to cause energization of said timer only after the heat input to the apparatus has been relatively high and has dropped below a predetermined level, and said second switch means being arranged to break the circuit to said timer a short time after the timer is initially placed into operation, said time being sufficient to have the heat input rise above said predetermined level.

12. The combination defined in claim 1 1 wherein said timer is manually operable to start its operation.

13. Drying apparatus comprising a chamber, a source of heat for heating air flowing to said chamber, means for exhausting the air from said chamber after moisture is collected by the air, first means responsive to the temperature of exhaust air, said means being adapted to control the source of heat whereby the heat is reduced as the temperature of the exhaust air reaches a predetermined high limit, second means providing on-o ff control of the source of heat in addition to said first means, said second means being responsive to a parameter affected by the reduction of the heat from said source, and timer means controlled by said second means for providing a fixed terminal period of heat supply following said parameter lowering to a predetermined level.

14. The apparatus defined in claim 13 in combination with means responsive to the operation of said air exhausting means to also control said source of heat.

15. The method of drying laundry in a laundry dryer which comprises drying said laundry at a maximum rate of heat input, with air introduced into the dryer inlet at a high temperature for a period of variable length, terminating said period of variable length upon detection of a temperature rise in air outlet temperature from said dryer indicating a reduction in the rate of heat absorption due to evaporation below the rate of heat input, drying said laundry for a subsequent variable period of time with gradually reducing heat input, and drying with reducing heat input for afixed portion of a predeter mined terminal period of fixed duration, commencing upon the air inlet temperature dropping to a predetermined temperature.

16. A laundry dryer comprising a motor driven tumbling drum, means for moving air through said drum including an inlet and an outlet, an inlet thermostatic switch associated with said inlet, and adapted to open at a predetermined elevated temperature in response to heated air flowing through said inlet, and close upon a drop in inlet air temperature, means for supplying heated air to said inlet at a temperature above said predetermined temperature, an outlet thermostatic switch associated with said outlet adapted to open at a temperature and close upon a drop in outlet temperature, and means for cycling said air heating means off and on in response to said outlet thermostat temperature after detecting a temperature rise in air outlet temperature from said dryer indicating a reduction in the rate of heat absorption due to evaporation below the rate of heat input, a timer having cam actuated contacts for completing a circuit to said air heating means except during a fixed run out period, a motor for driving said timer, timer cam controlled contacts connected in parallel with the contacts of said inlet thermostatic switch and adapted to energize said timer motor, said timer controlled contacts being adapted to open to suspend timer operation and preset the timer for a terminal period of fixed length greater than said run out period, and the contacts of said inlet thermostatic switch being adapted to close to energize said timer motor to initiate said terminal period upon a drop in inlet air temperature in response to the cycling of said outlet thermostat switch.

17. The method of drying laundry as recited in claim 15 wherein said laundry is cooled off during the remainder portion of said terminal period without heat input.

Claims

7. The combination of claim 6 wherein said timer is manually operable to its ''''on'''' position and has a holding circuit switch connected to said timer in parallel with said second temperature responsive means for keeping the timer operating for a predetermined short time regardless of the condition of said second temperature responsive means to assure the dryer coming up to operating temperature above said predetermined level and then causing said timer to stop operating until said predetermined level is reached thereafter whereupon the timer will start and run the remaiNder of the drying time.

9. In a dryer, the combination comprising a fuel burner furnishing heat thereto, a modulating valve arranged to control the supply of fuel to the burner, means for circulating air through said dryer, a first temperature responsive means responsive to exhaust air temperature of the dryer, means connecting said first temperature responsive means to said modulating valve so that the burner heat output is gradually decreased as the temperature of the exhaust air increases, second temperature responsive means responsive to a temperature indicative of the output of the burner, control means for said valve for shutting off the flow of gas to the burner, and a timer for controlling said control means and controlled only by said second temperature of responsive means to initiate operation of the timer near the end of the drying cycle.

12. The combination defined in claim 11 wherein said timer is manually operable to start its operation.

13. Drying apparatus comprising a chamber, a source of heat for heating air flowing to said chamber, means for exhausting the air from said chamber after moisture is collected by the air, first means responsive to the temperature of exhaust air, said means being adapted to control the source of heat whereby the heat is reduced as the temperature of the exhaust air reaches a predetermined high limit, second means providing ''''on''''-''''off'''' control of the source of heat in addition to said first means, said second means being responsive to a parameter affected by the reduction of the heat from said source, and timer means controlled by said second means for providing a fixed terminal period of heat supply following said parameter lowering to a predetermined level.

15. The method of drying laundry in a laundry dryer which comprises drying said laundry at a maximum rate of heat input, with air introduced into the dryer inlet at a high temperature for a period of variable length, terminating said period of variable length upon detection of a temperature rise in air outlet temperature from said dryer indicating a reduction in the rate of heat absorption due to evaporation below the rate of heat input, drying said laundry for a subsequent variable period of time with gradually reducing heat input, and drying with reducing heat input for a fixed portion of a predetermined terminal period of fixed duration, commencing upon the air inlet temperature dropping to a predetermined temperature.