US2643464A

US2643464A - Automatic drying control system

Info

Publication number: US2643464A
Application number: US79949A
Authority: US
Inventors: Robert E Hadady
Original assignee: AMERICAN INSTR CO Inc
Current assignee: AMERICAN INSTR CO Inc; AMERICAN INSTRUMENT COMPANY Inc
Priority date: 1949-03-07
Filing date: 1949-03-07
Publication date: 1953-06-30
Anticipated expiration: 1970-06-30

Description

.lune 30, 1953 R. E. H'ADADY AUTOMATICDRYING CONTROL SYSTEM 3 Sheets-Sheet 1 Filed March '7, 1949 Tw y D@ u :WIKRST YuUUUnn-HH III NGQSG June 30, 1953 R. E. HADADY AUTOMATIC DRYING CONTROL SYSTEM Filed March 7, 1949 3 Sheets-Sheet 2 June 30, 1953 R. E. HADADY AUTOMATIC DRYTNG CONTROL SYSTEM 3 Sheets-Sheet 5 Filed March '7, 1949 Patented June 30, 1953 UNITED STATES PATENT OFFICE AUTOMATIC DRYTNG CONTROL SYSTEM Application March 7, 1949, Serial No. 79,949

(Cl. 3ft- 48) Claims. l

VThis invention relates to drying apparatus, and more particularly to a drying apparatus of the hot air automatic type.

A main object of the invention is to provide a novel and improved drying apparatus which is simple in construction, automatic in operation,

and which is very reliable in performance.

A further object of the invention is to provide an improved drying system of the hot air automatic type wherein the moisture content of the kair leaving the drying chamber is compared with that of the atmosphere and wherein the source of drying heat is `autonlatically controlled in accordance with the results of such comparison, whereby no human supervision of the heat source is required and whereby uniformity of drying of the material to be dehydrated is obtained.

A Vstill further object of the invention is to Aprovide an improved automatic drying system of the hot air type employing the principle of Ycomparing the moisture content of a sample of the air leaving the drying chamber with that of atmospheric air at a common temperature, and wherein means are provided for preventing condensation of the moisture in the sample of air leaving the drying chamber, whereby inaccurate operation of the system due to such condensation is prevented, and whereby the reli-l ability of operation of the system is greatly increased.

A still further object of the invention is to provide an improved automatic drying system of the hot 'air type wherein a sample of the air leaving the drying chamber is compared as to vmoisture content with atmospheric air at a common temperature and wherein automatic means are provided for mixing the hot air sample with atmospheric air and maintaining the mixture at all times in a condition below saturation as to moisture content, whereby condensa- Figure 3 is a schematic wiring diagram illustrating the electrical connections of a second form of the invention wherein control of the system in terms of the absolute moisture content of the material being dried is obtained.

Referring to the drawings, Ii designates a suitable chamber or oven in which the articles to be dried are placed. Chamber II is provided with an air intake section I2 in which iS mounted an electric heater I3 or other electrically controlled heater. Said heater may be equipped with a conventional blower unit I4 for moving air past the heating coils of heater I3 into the drying chamber II. Designated at I5 is the air intake duct which communicates with the atmosphere. The exhaust duct leading from drying chamber II is shown at I6. A conventional exhaust blower I'I may be mounted within exhaust duct I5 if so desired.

At its connection to drying chamber II the exhaust duct I6 is formed with an enlarged portion I8, and projecting into said enlarged portion through its wall is an air sampling conduit I9. Designated at 20 is an air intake conduit which also extends through the wall of said enlarged portion I8 adjacent conduit I9 and connects with conduit I9 inside said enlarged portion. Fivoted within conduit I9 at its junction Awith conduit 2t is a flap valve 2i.

Valve 2I may be rotated from a normal position, wherein the upper portion of conduit I9 is closed off and conduit 2l! connects the remainder of conduit I9 to atmosphere, toward a position wherein conduit Eil is closed off with respect to conduit E9 and the upper portion of conduit I9 is in unrestricted communication with the lower portion of conduit I9. Flap valve 2l is connected ina suitable manner to the shaft of an external motor 22 so that when motor 22 is energized the valve 2li is rotated from its normal position, shown in Figure 1, toward the position wherein the conduit 20 is closed off with respect to conduit I9 and conduit I9 is unrestricted. During such periods wherein flap valve 2I is in an intermediate position, the lower portion of conduit I9 vmay receive air from both the drying chamber l l and from the atmosphere.

As shown in Figures 2 and 3, the shaft of motor 22, shown at 23, is connected to the shaft of flap valve 2|, shown at 2d, through a speed reducing unit and a friction coupling 26. Friction coupling 2t transmits driving torque to shaft 24 in one direction but allows shaft 24 to be rotated in the opposite direction without requiring motor 22 to be reversed. This reversal of shaft 24 may be accomplished by means of a reset solenoid, shown at 21, whose plunger is connected by a link 28 to an arm 29 secured to shaft 24. The winding of solenoid 21 is connected by conductors 38 and 3| to the power lines, shown respectively at 32 and 33. A normally open push button reset switch 34 is interposed in conductor 30, whereby solenoid 21 is normally deenergized.

Designated at 35 is a suitable tank or container having liquid 36 therein maintained by suitable thermostatic means 38 at a substantially constant temperature. The precise value of this temperature is unimportant except that it must be above the ambient temperature of the atmosphere and below the maximum operating temperature within the drying chamber. For practical reasons, a preferred value would be within the range between 100 and 130 Fahrenheit, since this would place the measured relative humidity within the working range of present relative humidity measuring equipment.

The lower portion of conduit I9 is formed with coils 31 which are immersed in the temperaturecontrolled liquid 36. From the coils 31 the conduit extends to a blower housing 39 which is connected to a discharge duct 40. Housing 39 contains a conventional exhaust blower 49. Preceding housing 39, conduit |9 is formed with an enlarged portion 4| containing a humidity sensing element 42 of the bifilar type, such as is disclosed in United `States Patent No. 2,285,421 to Francis W. Dunmore, issued June 9, 1942, or any other suitable humidity sensing element of the current-modifying type.

Designated at 43 is a conduit formed with coils 44 which are immersed in the temperature-controlled liquid 36 adjacent coils 31. One end of conduit 43 is open to the atmosphere. The other end of said conduit is connected through a blower housing 45 to a discharge duct 46. Housing 45 contains a conventional exhaust blower 41. Preceding housing 45, conduit 43 is formed with an enlarged portion 48 containing a humidity sensing element 49 similar to element 42.

From Figure l it will be apparent that humidity sensing element 42 is exposed to the air exhausting from conduit I9 after passing through the coils 31. Similarly, humidity sensing element 49 is exposed to the atmospheric air exhausting from conduit 43 after passing through the coils 44. Since

coils

31 and 44 are maintained at the same temperature by liquid 36, the air engaging element 42 is at the same temperature as the air engaging element 49, whereby the relative resistances of

elements

42 and 49 provide a direct comparison of the percentage moisture content of the air exhausting from conduit I9 and the atmospheric air exhausting from conduit 43, without requiring any correction for difference in temperature.

Designated at 58 is a control unit containing various electrical components, shown schematically in Figure 2. Referring now to Figure 2, it will be seen that control unit 56 includes the respective constant Voltage transformers i and 52, respective full-

wave rectiers

53 and 54, an ammeter-relay 55, a motor control relay 56, and the respective resistors shown at 51, 58, 59, 60 and 6|. The primary of transformer 5| is connected to the

line wires

32 and 33 by

respective Wires

62 and 63. One secondary terminal of transformer 5| is connected to one terminal of humidity sensing element 42 by a wire 64. The other terminal of the transformer secondary is connected by a wire 65 to one side of the full wave bridge rectifier 53 at 66. The opposite side of the rectiiier is connected through the current-limiting resistor 51 to the remaining terminal of element 42 by a wire 61. The elements of the bridge rectifier 53 are arranged so that the output terminal shown at 68 will be positive in polarity and the output terminal shown at 69 will be negative. Positive terminal 68 is connected by a wire 18 through the resistor 69 to a wire 1 I, which in turn is connected to the positive terminal of the ammeter-relay 55. The negative terminal 69 is connected by a wire 12 to a wire 13 connected to the negative terminal of ammeter-relay 55. The elements of bridge rectier 54 are arranged so that the output terminal shown at 14 will be negative and the output terminal shown at 15 will be positive in polarity. Terminal 14 is connected by a wire 16 through resistor 6| to wire 1|. Terminal 15 is connected by a Wire 11 to one stationary contact 18 of a normally open push button switch 19 located adjacent motor 22. The other stationary contact of the switch, shown at 80, is connected to the wire 13.

The primary of constant voltage transformer 52 is connected by respective wires 8| and 82 to the

wires

62 and 63. One terminal of the secondary of said transformer is connected by a wire 83 to one side of rectier 54. The other secondary terminal is connected by a wire 84 to one terminal of humidity sensing element 49. The remaining terminal of element 49 is connected by a wire 85 through current limiting resistor 58 to the opposite side of rectifier 54.

Ammeter-relay 55 is provided with a pivoted pointer 66 swingable over an arcuate scale 81 in response to a flow of current through the energizing coil of the ammeter-relay. As will be subsequently explained, such a ilow of current takes place when an appreciable potential exists across the output terminals of rectier 53 with switch 19 open, or when switch 19 is closed and the humdity of the air sample from the drying oven contacting the moisture sensing element 42 is greater than the humidity of the atmospheric air adjacent the oven sampled at humidity sensing element 49.

Pointer 86 is connected by a wire 88 through current limiting resistor 59 to the wire 62. Ammeter-relay 55 is provided on scale 81 with a first adjustable stationary contact 89, located adjacent the zero end of the scale, and with a second adjustable stationary contact 90 located adjacent the high end of the scale. The pointer 86 is engageable with the

respective contacts

89 and 90 as said pointer swings over the scale. Contact 96 is connected by a wire 9| to one terminal of the winding of relay 56. The other terminal of said winding is connected by a wire 62 to wire 63. Relay 56 has an armature 93 normally engaging a stationary contact 94. Armature 93 is connected by a wire 95 to wire 62. Contact 94 is connected by a wire 96 to one terminal of motor 22. The other terminal of the motor is connected by a wire 91 to one stationary contact of a normally closed push button switch 98 located adjacent said motor. The other stationary contact of said switch 98 is connected by a wire 99 to wire 63. It will be seen from Figures 2 and 3, that motor 22 is normally energized by a circuit comprising wire 32, wire 62, wire 95, armature 93, contact 94, wire 96, the motor winding, wire 91, normally closed switch 98, wire 99, wire 63, and wire 33, assuming the main control switch, shown at |00, to be closed.

.mally open push button switch iti.

Varmature Also located adjacent .motor .22.is another nor- One of the stationary contacts of switch is connected by :a wire |02'to the contact 80 of the ammeterrelay 55. The Vother stationary contact of said switch is connected by a wire H33 to one terminal of the winding of a relay ISM. The other terminal of said relay winding is connected to wire 9S.

.Relay |84 has two armatures, .shown respectively at |05 and |06. Both of said armatures are connected to wire E3. Armature |05 normally engages a Contact |01 which is connected by a wire |08 to one Vterminal of the fan and heater unit |15. The other terminal oi said Ifan and heater unit is connected to the wire .62. `From AFigures 2 and 3, itwill be seen that the fan and heater unit .is normally energized through a circuit comprising wire 33, wire B3, |95, contact |01, wire |03, the fan and heater unit, wire E2, and wire 32. The fan :and heater unit becomes deenergized when armature contact relay |04.

gaged by armature |06 when relay |94 becomes energized. Contact Ht is connected by a wire |09 to one terminal or a signal lamp liu. VThe other terminal of said signal lamp is connected by a wire to wire B2. When relay mit becomes energized, lamp ||0 becomes energized by a circuit comprising wire 32, wire |52, wire the signal lamp, wire |69, contact H8, armature IOS, wire |53, and wire 33.

The automatic mixing control device comprising the motor 22 and its associated elements functions to prevent condensation in the sampling conduit |9. If this control device were not provided, condensation would occur when the sampled exhaust air had a dewpoint 'higher than the temperature of the piping and/or the bath 36. The effect of condensation would be to cause inaccurate and unreliable response until all of the condensate had evaporated. The automatic mixing control device mixes the sample of air exhausted from the drying chamber with latmospheric air so as to always keep :the moisture content oi the air in conduit l'below saturation.

Figure l illustrates the starting position of the mixing control vane 2|. In this position, conduit it is fully open for atmospheric air and fully closed for 'drying chamber exhaust air. As soon as the system is placed in operation, .as

.by closing switch itil, motor 22 becomes energized and starts driving vane 2| toward a posi- -tion wherein conduit I9 is Vfully closed for atmospheric air and fully open for drying chamber exhaust air. The vane is driven at a Very low speed because of the speed reducer 25.

In the starting position of the system, both of the moisture sensing elements c2 and l0 are contacted by atmospheric air whose temperature is equalized by the 'bath Bt. The voltage across the output terminals of rectiiier 53 will therefore have a value corresponding to the .percentage moisture content of latmospheric vair at the temperature of equalization, this Value `being substantially below saturation. Pointer 235 6 will therefore swing to a relatively low reading on scale 87.

The motor continues to drive vane .2|, gradually admitting more and more exhaustair from the drying chamber into conduit i9. Since this exhaust air Ycontains an appreciable amount of moisture at the beginning of the drying cycle, pointer 86 begins to swing further up on the scale 8l until it reaches a point corresponding to just below saturation. At this point, pointer 86 engages contact 90. Relay 56 becomes energized through a circuit comprising wire 32,

wire 62, resistor 59, wire 83, pointer 86, contact 90, wire 9|, the winding of relay 55, wire 92, wire E53, and wire 33. This raises armature 93 and opens the energizing circuit of motor 22 at contact 9:3. The motor stops, and remains deenergized until the moisture content of .the air mixture in conduit'l drops below the scale value at which contact 9i] is located, due to the drying of the product in the chamber 'i i. As pointer 86 retreats from this scale value, it disengages from contact 9|), causing relay :55 to become "deenergized, and allowing armature S3 to re-engage contact 04, whereby the motor 22 again becomes energized. The motor starts driving again and opens Vane 2| further, to allow a greater percentage of exhaust air from drying chamber to be present in conduit I9. As the percentage of moisture in the air contactingsensing element i2 reaches the scale value correspending to the location of contact $.10, pointer 86 again engages said contact, stopping the motor. This procedure is repeated over and over again until vane 2| is fully closed for atmospheric air. and fully open for drying chamber exhaust air. At this point, bar ||2 engages the push buttons of the switches $8, |0| and Switch 98 is thereby opened, breaking the motor circuit and stopping the motor. As above explained, switches 'i9 and |i|-| are now closed.

The closure of switch 'i9 connects the output of rectifier 54 across the terminals of ammeterrelay 55 in opposition `to the output of rectiiier `E53. Since the output voltage of rectiiier 513 is controlled by the percentage moisture content in the atmospheric air contacting humidity sensing element 49 at the equalization temperature of bath 35, the position of pointer .8e on scale .8? will show a comparison oi or difference between the moisture content of the subsequent samples of air from the drying chamber with respect to the moisture content of atmospheric air.

The drying of the product continues until the moisture contents of the two air samples respectively contacting the

sensing elements

42 and 40 are equal or have a specied dierence. At this time, pointer engages contact 89. rThis energizes relay |04 through a circuit comprising wire v32, wire t2, resistor 59, wire 88, pointer 8G, contact 89, wire |02, switch |0| in its closed position, wire |03, the winding of relay |04, wire 99, wire 63, and wire 33. Armature |05 is therefore lifted out of engagement with contact |01, deenergizing ythe fan `and heater unit. At the same time, armature Hitl engages contact H8, causing the signal lamp i0 to beenergized and signalling that the drying cycle is at an end.

To reset the system for the next run, reset switch 3e is closed, energizing reset solenoid 2l, and causing the plunger of the solenoid, acting through link 23 and arm 29, to return shaft 24 and vane 2| to the starting position. Bar ||2 is disengaged from the push buttons of switches S8,

7 I I and 19, allowing switch 98 to close and switches IOI and 19 to open.

The values of

resistances

51 and 58 are preferably of the order of I megohm. These resistances are employed to limit the amount of current flowing in the circuits of the

respective sensing elements

42 and 49 to very small values. Resistor 59 is employed to limit the amount of current flowing through the windings of the respective relays 56 and |04 and through the

contacts

89 and 96. Resistor 6| is employed to prevent rectier 54 from acting as a shunt across ammeter 55 when switch 19 closes. Similarly, resistor 60 is employed to prevent rectifier 53 from acting as a shunt across ammeter 55 with respect to the potential available at the output terminals of rectifier 54. Resistors 60 and 6I are preferably of the order of 10,000 ohms.

It should be noted that the system is arranged so that neither of the

humidity sensing elements

42 or 49 may be contacted by saturated air at any time. This prevents damage to the hygroscopic coatings on the elements and substantially reduces the possibility of inadvertently changing the calibrations of these elements.

Figure 3 shows schematically a simplified form of the invention wherein the apparatus is adapted to dry a product in terms of its absolute moisture content. The modified form shown in Figure 3 employs substantially the same arrangement of elements shown in Figures 1 and 2, except that the structure whereby a comparison with moisture content in the atmosphere is obtained is eliminated, that is, the conduit 43, coil 44, sensing element 49, exhaust blower 41, etc., are omitted. The control unit, shown at 50 in Figure 3, includes the constant voltage transformer 5I, the full wave rectifier 53, the ammeter-relay 55, the relay 56, and the

respective resistors

51, 59 and 60, connected in the manner as in Figure 2. Since no comparison switch is required in the system of Figure 3, only the limit switch 98 and the cycle end switch IOI are provided adjacent motor 22. The moisture sensing element 42 is located in the sampling conduit I9 in the same manner as illustrated in Figure 1.

In the starting position of the apparatus of Figure 3, conduit I9 is fully open for atmospheric air and fully closed for drying chamber exhaust air. As soon as the system is placed in operation, as by closing switch |00, motor 22 becomes energized and starts driving vane 2| toward a position wherein conduit I9 is fully closed for atmospheric air and fully open for drying chamber exhaust air. Moisture sensing element 42 is contacted by atmospheric air taken in through conduit 20. Pointer 86 will therefore swing to a relatively low reading on scale 81.

As the motor continues to drive vane 2 I, more and more exhaust air from the drying chamber is admitted into conduit I9. Since this exhaust air contains an appreciable amount of moisture at the beginning of the drying cycle, pointer 86 begins to swing further up on the scale 81 until it engages Contact 90. Relay 56 becomes energized and opens the energizing circuit of motor 22 at contact 94. The motor stops, and remains deenergized until the moisture content of the air mixture in conduit I9 drops due to drying of the product. Pointer 86 disengages from contact 90, deenergizing relay 56, whereby motor 22 -again becomes energized, allowing a greater percentage of exhaust air from the drying chamber to be present in conduit I9. As pointer 86 again engages contact 90, the motor is again stopped.

This procedure is repeated over and over again until vane 2| is fully closed for atmospheric air and fully open for drying chamber exhaust air. At this point bar II2 engages the push buttons of switches 98 and IOI. Switch 98 is thereby opened, breaking the motor circuit and stopping the motor. Switch |0| is now closed.

From this point on, pointer 86 indicates on scale 81 the absolute moisture'content of the subsequent samples of air from the drying chamber at the temperature of the bath 36.

The drying of the product continues until the moisture content of the exhaust air decreases to a specified value (determined by test and depending upon the size of the product, the material thereof, the air velocity, etc.) corresponding to the position of contact 89 on scale 81. At this value, pointer 86 engages contact 89, energizing relay |04 and causing the fan and heater unit to be deenergized by the raising of armature |05. At the same time, signal lamp |I0 becomes energized by the engagement of armature |06 with contact I I8, signalling that the drying cycle is at an end.

To reset the system for the next run, reset switch 34 is closed, energizing solenoid 21 and causing vane 2| and bar I|2 to return to their starting positions.

It will be apparent that in either of the systems illustrated in Figures 2 and 3, a moist article may be automatically dehydrated to a desired degree of dryness by placing same in the drying chamber II, closing the main power switch |00, and setting vane 2| to starting position by means of push button switch 34. In the latter stages of the drying cycle, the moisture content of the air leaving the drying chamber will be indicated by pointer 86 on the scale 81 of the ammeter-relay 55. As soon as the article reaches the desired degree of dryness, the fan and heater unit will be automatically deenergized and the signal lamp IIO will become energized, indicating that the article is ready to be removed from the drying chamber.

While certain specific embodiments of automatic drying systems have been disclosed in the foregoing description, it will be understood that V-arious modifications within the spirit of the invention may occur to those skilled in the art. Therefore it is intended that no limitations be placed on the invention except as defined by the scope of the appended claims.

What is claimed is:

1. An automatic drying system comprising a chamber, a source of heated air connected to said chamber, a sampling conduit connected to said chamber, an atmospheric air intake duct connected to said sampling conduit, valve means at the connection of said duct to said conduit regulating the amount of atmospheric air admitted into said conduit, a moisture-sensitive element in said sampling conduit, a control device connected to said source, and a control circuit connected to said control device, said control circuit including the moisture-sensitive element.

2. An automatic drying system comprising a chamber, a source of heated air connected to said chamber, an exhaust conduit connected to said chamber, a sampling conduit connected to said exhaust conduit. an atmospheric air intake duct connected to said sampling conduit, valve means at the connection of said duct to said sampling conduit regulating the amount of atmospheric air admitted into said sampling conduit, a moisture-sensitive element in said sampling conduit, a control device connected to said source, and

'a control circuit connected to said control device and said valve means, said control circuit including the moisture-sensitive element.

3. An automatic drying system comprising a chamber, a source of hea-ted air connected to said chamber, a sampling conduit connected to said chamber, an atmospheric air intake duct connected to said sampling conduit, valve means regulating the amount ofv atmospheric air iadmitted into said sampling conduit from said intake duct, a moisture-sensitive element in said sampling conduit beyond the connection of the air intake duct thereto, means regulating said valve means in accordance with the response of said moisture-sensitive element, control means for the source, and means operating said control means in response to the movement of the v-alve means to a predetermined position.

4. An automatic drying system comprising a chamber, a source ci heated air connected to said chamber, an exhaust conduit connected to said chamber, a sampling conduit connected to said exhaust conduit, an atmospheric air intake duct connected to said sampling conduit, electrically operated valve means at the connection of said duct to said sampling conduit regulating the amount of atmospheric air admitted into said sampling conduit, a current-responsive control device, a moisture-sensitive resistance element in said sampling conduit, a control circuit including said resistance element connected to said current-responsive control device, first switch means associated with said current-responsive device, circuit means connecting said electrically operated; valve means to said first switch means, a control circuit associated with said source, second switch means associated with said currentresponsive control device, and circuitr means connecting said second switch means to said latter control circuit.

5. An automatic drying system comprising a chamber, a source of heated air connected to said chamber, an exhaust conduit connected to said chamber, a sampling conduit connected to said exhaust conduit, an atmospheric air intake duct connected to said sampling conduit, electrically operated valve means at the connection of said duct to said sampling conduit regulating the amount of atmospheric air admitted into said sampling. conduit, a current-responsive control device, 'a moisture-sensitive resistance element in said sampling conduit, a control circuit including said resistance element connected to said current-responsive control device, first switch means operated by saidcurrent responsive device in response to a relatively high current therein, circuit means connecting said electrically operated valve means to said iirst switch means, a control circuit associated with said source, second switch means operated by said current-responsive device in response to a relatively low current therein, iand circuit means controlled by said electrically operated valve means at times connecting said second switch means to said latter control circuit.

6. An automatic drying system comprising a chamber, a source or heated air connected to said chamber, an exhaust conduit connected to said chamber, a sampling conduit connected to said exhaust conduit, an atmospheric air intake duct connected to said sampling conduit, electrically operated valve means at the connection of said duct to said sampling conduit, means controlling the temperature of said sampling conduit, a current-responsive control device, a moisture-sensitive resistance element in said sampling conduit, a control circuit1 including said resist-V ance element connected to saidcurrent-responsive control device, iirst switch means operated bysaid current-responsive device in response to a relatively high current therein, circuit means connecting said electrically operated valve means to said first switch means, a control circuit associated with said source, second switch means operated by said current-responsive device in response to a relatively low currentV therein, and circuit means controlled bysaid electrically operated valve means attimes connecting said second switch means to said latter control circuit.

'7. An automatic drying system comprising a chamber, a source of heatedV air connected to said chamber, an exhaust conduit connected to said chamber, a sampling conduit connected to said exhaust conduit, an atmospheric air intake duct connected to said sampling conduit, electrically operated valve means at the connection ofl said duct to said sampling conduit regulating the amount of atmospheric air admitted into said sampling conduit, a current-responsive control device, a nrst moisture-sensitive resistance element in said sampling conduit, a second moisture-sensitive resistance element exposed to atmospheric air adjacent said chamber, respective energizing circuits including the respective moisture-sensitive elements connected in opposition to said current-responsive control device, means associated with said valve means controlling the energizing circuit containing the second resistance element, first switch means operated by said current-responsive device in response to a relatively high current therein, circuit means connecting said electrically operated valve means to said first switch means, a control circuit associated with said source, second switch means operated by saidcurrent-responsive control device in response to a relatively low current therein, and circuit means controlled by said electrically operated valve means at times connecting said second switch means to said'latter control circuit.

8, The structure of claim 10, and wherein an additional atmospheric air duct is provided adjacent the chamber, the second moisture-sensitive element being located within said additional atmospheric air'duct, and wherein means are providedmaintaining the sampling conduit and said additional atmospheric air duct at a common temperature.

9. An automatic drying system comprising a chamber, asource of heated air connected to said chamber, an exhaust conduit connected to said chamber, a sampling conduit connected to said exhaust conduit, an atmospheric air intake duct oonnectedfto said sampling conduit, a constant temperature bath, a portion of the sampling conduit beyond the connection thereof to said atmospheric air intake duct being immersed in said bath, electrically operated valve means at the connection of said duct to said sampling conduit regulating the amount of atmospheric air admitted into said sampling conduit, a current-responsive control device, Va rst moisturesensitive resistance element in said sampling conduit beyond the portion immersed in the bath, an additional atmospheric air duct also having a portion thereof immersed in the bath, a second moisture-sensitive resistance element in said additional duct beyond the immersed portion thereof, respective energizing circuits including the respective moisture-sensitive resistance elements connected in opposition to said current-responsive control device, means associated with said valve means controlling the energizing circuit containing the second resistance element, first switch means operated by said current-responsive device in response to a relatively high current therein, circuit means connecting said electrically operated valve means to said first switch means, a control circuit associated with said source, second switch means operated by said current-responsive control device in response to a relatively low current therein, and circuit means controlled by said electrically operated valve means at times connecting said second switch means to said latter control circuit.

10. An automatic drying system comprising a chamber, a source of heated air connected to said chamber, an outlet conduit connected to said chamber, a constant-temperature bath, a sampling conduit connected to said outlet conduit, a substantial portion of said sampling conduit being immersed in said bath, a moisturesensitive element in said sampling conduit, a control device connected to said source, and a control circuit connected to said control device, said control circuit including the moisture-sensitive element,

11. In an automatic drying system, the combination of a chamber, a source of heated airconnected to said chamber, an exhaust conduit communicating with the atmosphere and connected to said chamber, a relatively small sampling conduit connected to said exhaust conduit, an atmospheric air intake duct communicating with the atmosphere and connected directly to said sampling conduit, a moisture-sensitive element in said sampling conduit beyond the connection of the air intake duct thereto, and means regulating the amount of atmospheric air admitted into said sampling conduit in accordance with the response of said moisture-sensitive element.

l2. In an automatic drying system, the combination of a chamber, a source of heated air connected to said chamber, an exhaust conduit communicating with the atmosphere and connected to said chamber, a relatively small sampling conduit connected to said exhaust conduit, means controlling the temperature of said sampling conduit, an atmospheric air intake duct communicating with the atmosphere and connected directly to said sampling conduit, a moisture-sensitive element in said sampling conduit beyond the connection of the air intake duct thereto, and means regulating the amount of atmospheric air admitted into said sampling conduit in accordance with the response of said moisture-sensitive element.

13. In an automatic drying system, the combination of a chamber, a source of heated air connected to said chamber, an outlet conduit connected to said chamber, a sampling conduit connected to said outlet conduit, a constanttemperature bath, a substantial portion of said sampling conduit being immersed in said bath, an atmospheric air intake duct connected directly to said sampling conduit ahead of said constant-temperature bath, a moisture-sensitive element in said sampling conduit beyond the connection of the air intake duct thereto, and means regulating the amount of atmospheric air admitted into said sampling conduit in accordance with the response of said moisture-A sensitive element.

14. In an automatic drying system, the combination of a chamber, a source of heated air connected to said chamber, an exhaust duct comi municating with the atmosphere and connected to said chamber, a sampling conduit connected to said exhaust duct, an atmospheric air intake duct communicating with the atmosphere and connected directly to said sampling conduit, a moisture-sensitive element in said sampling conduit beyond the connection of the air intake duct thereto, and means regulating the amount of atmospheric air admitted into said sampling conduit in accordan-ce with the response of said moisture-sensitive element.

15. In an automatic drying system, the combination of a chamber, a source of heated air connected to said chamber, an exhaust conduit communicating with the atmosphere and connected to said chamber, a sampling conduit connected to said exhaust conduit, an atmospheric air intake duct communicating With the atmosphere and connected directly to said sampling conduit, valve means regulating the amount of atmospheric air admitted into said sampling conduit from said duct, a moisture-sensitive element in said sampling conduit beyond the connection of the air intake duct thereto, and means operating said valve means in accordance with the response of said moisture-sensitive element.

ROBERT E. HADADY.

References Cited in the iile of this patent UNITED STATES PATENTS Number Name Date 1,167,740 Carrier Jan. 11, 1916 1,467,306 Carrier Sept. 11, 1923 1,539,230 Anderson May 26, 1925 1,707,547 Adams Apr. 2, 1929 2,037,695 Brownlee et al. Apr. 21, 1936 2,080,059 Peebles May 11, 1937 2,333,000 Gayring Oct. 26, 1943 2,347,601 Jackson Apr. 25, 1944 2,548,939 Brion Apr. 17, 1951