United States Patent: 3552646

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United States Patent	3,552,646
Hayes	January 5, 1971

BOILER CONTROL USING ROD AND TUBE TYPE TEMPERATURE SENSOR

Abstract

A rod and tube type temperature sensor is provided in a boiler by forming the tube portion of the sensor as an integral part of the boiler heat exchanger tubing through which the fluid being heated flows. The rod member of the sensor is coaxially positioned within the associated tube portion with one of its ends fixed against relative movement with respect to such tube portion. The other end of the rod protrudes through an aperture defined in the boiler tubing. Sealing means is provided encircling the rod where it protrudes out of the boiler tubing to prevent the escape of fluid from the boiler. The rod member and the tube portion of the sensor have different coefficients of expansion such that, as hot gases heat the tubing which transmit such heat to the water flowing inside of the tubing and over the interiorally positioned rod member, expansion and contraction of the tubing relative to the free rod end occurs to provide an output actuating force which may be used for control and indicating purposes.

Inventors:	Hayes; Thomas E. (Goshen, IN)
Assignee:	Penn Controls, Inc. (Oak Brook, IL)
Appl. No.:	04/766,908
Filed:	October 11, 1968

Current U.S. Class: 236/20R

Current International Class: F22B 37/47 (20060101); F22B 37/00 (20060101); F23n 001/00 ()

Field of Search: 236/20,33,102,103

References Cited [Referenced By]

U.S. Patent Documents


1940355	December 1933	Knapp
1977656	October 1934	Vaughn
3112881	December 1963	Hodgson
3368754	February 1968	Solomon

Foreign Patent Documents


841,581	Feb., 1939	FR
267,857	Nov., 1913	DT

Primary Examiner: Michael; Edward J.

Claims

I claim:

1. In a boiler having:

a heating chamber;

fuel burning means for generating hot gases in said chamber;

heat exchanger tubing for conveying fluid to be heated from an inlet to an outlet through said heating chamber;

a control for modulating the fuel fed to said burner;

said control including a rod and tube temperature responsive actuator for actuating said fuel control in response to temperature sensed, characterized in that:

the tube portion of said rod and tube actuator forms a predetermined section of said boiler exchanger tubing near said outlet and through which said fluid flows,

said rod portion of said rod and tube actuator is a rod of predetermined length coaxially positioned in said predetermined section of tubing,

one end of said rod being fixed to said tubing and the other end protruding out of said tubing length for free relative lengthwise motion with respect to said tubing, and

wherein there is provided sealing means at the point of exit of said free rod end from said tubing means for preventing leakage of said fluid out of said boiler tubing thereat.

2. The control set forth in claim 1 wherein said rod portion and said predetermined section of said boiler tubing each have a coefficient of expansion different one from the other for providing relative movement therebetween in response to temperature change.

3. A claim as set forth in claim 1 wherein there is provided means opposing said relative movement between said rod and said predetermined section of exchanger tubing for establishing a set point of operation of said control means.

4. The combination in claim 1 wherein there is provided:

mechanical to electrical transducer means for providing an electrical output signal in response to said relative motion between said rod and said predetermined section of exchanger tubing.

5. A temperature probe for sensing the temperature in a boiler heated by hot gases, said boiler comprising:

a housing defining a heating chamber for said hot gases;

a conduit for conveying fluid to be heated in said chamber from an inlet to an outlet of said boiler;

means for propelling the fluid to be heated through said conduit from said inlet to said outlet;

said probe comprising:

a member of material selected with a coefficient of expansion different from that of said conduit and dimensioned for a loose fit within said conduit;

said member being mounted within a predetermined section of said conduit and fixed against movement at one of its ends relative to said predetermined section of said conduit;

the other end of said member protruding out of said predetermined section of conduit for longitudinal movement relative to said section;

sealing means at the point where said member protrudes out of said conduit section for preventing flow of said fluid being heated out of said conduit;

said construction causing relative motion between said member and said predetermined conduit section due to temperature caused expansion and contraction of said predetermined section of conduit relative to said member;

said boiler conduit is in the form of heat exchanger coils; and

said predetermined section of conduit is a selected one of said boiler heat exchanger coils positioned near said outlet of said boiler for sensing temperature in said heating chamber near said outlet.

6. A temperature probe as set forth in claim 5 wherein there is provided:

mechanical to electrical transducer means for providing an electrical output signal in response to said relative motion between said member and said predetermined conduit section.

7. A temperature probe as set forth in claim 6, wherein:

there are provided control means for controlling the amount of said gases applied, and

wherein said control means responds to said electrical output signal of said transducer.

8. A temperature probe as set forth in claim 6 wherein:

said boiler is fueled by gas;

a gas valve for controlling flow of said gas to said boiler;

said gas valve having a housing formed integral with said predetermined conduit section;

wherein said other rod of said member protrudes through said sealing means into said gas valve housing;

said gas valve having valve means for controlling the flow of gas therethrough;

linkage means responsive to said relative motion between said member and said predetermined conduit section; and

said linkage means being connected to said valve means for modulating the flow of gas to said boiler.

9. A temperature probe as set forth in claim 8 wherein:

means are provided for establishing a set point of operation for said valve means in response to actuation by said relative movement.

10. A temperature probe as in claim 9 wherein:

said set point means comprises;

spring means for opposing said relative motion between said member and said predetermined conduit section; and

means for adjusting the force said spring means opposing said relative motion.

11. A temperature probe as set forth in the claim 10 wherein:

said spring means comprises a spring leaf mounted at one end to said predetermined conduit section, and at its other end to said adjusting means;

said adjusting means being carried by said member and being operative for bowing said spring to resist relative motion between said member and conduit section; and

said linkage means operatively connecting said valve means to said spring leaf for actuation by increases and decreases in said bowing of said spring leaf by said adjusting means and in response to relative motion between said member and conduit section.

Description

The invention relates to heater control systems and, more particularly, to fast operating temperature indicators and controls for boilers.

The invention is particularly adaptable to modern day boilers of the type termed "high heat flux" boilers which have a low water content, a low combustion area and mass, all relative to the rate at which fuel is burned for heating. These boilers are designed to heat water at a relatively very fast rate, that is substantially instantaneously as it flows in tubing through the boiler, thus obviating the need to provide a storage of hot water, while providing instant response to demands for heated fluid. In such boilers of low water content, low mass and high heating rate the control of the heating means is critical. This is especially so since, should heat removal (fluid flow) stop because of no demand or due to a malfunction, the temperature rises at a rate much faster than boilers of prior design having larger masses and water content. It is, therefore, desirable to provide improved temperature sensing and control means for such high heat flux boilers.

Most conventional present day boiler temperature controls and limits sense the water temperature directly by means of charge filled bulbs, or bimetallic elements, or rod and tube type probes or thermistor equipped probes, all of which are immersed in the water being heated. Since there is an inherent thermal time lag in heating the water, such conventional temperature sensor equipped controls cause substantial temperature overshoot and undershoot in high heat flux type boilers where full heat is applied and removed suddenly. This prevents efficient operation of such boilers within a narrow proportional band. In addition, as lime and scale accumulate on the interior surface of the boiler tubing, heat transfer to the water is attenuated. With such water immersed type, temperature sensors an increase in heat to the tubing is then required to heat the water. This overheating causes "tube burnout" which today is the predominant cause of failure in boilers.

It is, therefore, an object of the invention to provide an improved system for sensing temperature in a boiler and controlling the flow of fuel thereto.

It is another object of the invention to provide temperature sensing means which anticipate fluid temperature in a boiler.

It is a further object of the invention to provide an improved boiler control with minimum overshoot and operation within a relatively narrow proportional band for a high heat flux type boiler.

The invention involves providing in a boiler a rod and tube type temperature sensor, the tube portion of which forms an integral part of the boiler heat exchanger tubes. The rod portion of the sensor is mounted in the boiler tube with one end fixed to the boiler tubes against relative movement thereto. The other end of the rod protrudes freely through an aperture defined in the boiler tubing. Sealing means is provided where the rod protrudes from the boiler tubing to prevent leakage of fluid being heated. The rod and tube portions have different coefficients of expansion such that relative motion therebetween occurs in response to the heating gases impinging upon the exterior wall of the exchanger tubing and the temperature of the water which flows inside the tubing over the rod. By utilizing the boiler heat exchanger tubes as the tube portion of the rod and tube actuator, the actuator responds quickly to the heating gases anticipating the temperature transfer function of the copper tubing (the heat exchanger) to the fluid being heated. This compensates for the thermal time lag in heating the fluid, minimizing overshoot in a high heat flux type boiler in which full heat is applied suddenly. Changes in fluid temperature, due to changes in flow rate or different entering temperature of the fluid, are sensed by the tube to provide efficient operation of the boiler within a narrow proportional band, while minimizing "tube burnout" due to overheating of the tubes.

In carrying out the invention according to a preferred embodiment applied to a gas heated, high heat flux type boiler for water, the relative movement between the rod and heat exchanger tube is utilized for controlling the flow of gas to the burner through a linked gas valve.

In another embodiment of the invention the relative motion between the rod and heat exchanger tube is utilized to provide an output electrical signal which may be used for purposes of indication and control.

Features and advantages of the invention will be seen from the above, from the following description of the preferred embodiment when considered in conjunction with the drawings and from the appended claims.

In the drawings:

FIG. 1 is a simplified, pictorial diagrammatic view of a gas boiler system, embodying the invention;

FIG. 2 is a simplified, pictorial view in perspective of a portion of the system of FIG. 1 greatly enlarged and with portions broken away to illustrate the internal mechanism;

FIG. 3 is a graph showing the operation of one tested embodiment of the subject invention as taken by a strip recorder; and

FIG. 4 is a simplified diagrammatic representation of a portion of the rod and tube actuator of FIGS. 1 and 2 modified for providing an output electrical signal.

Referring to FIG. 1 a gas fueled boiler system is shown comprising a heating chamber, generally designated 10, at the bottom of which is mounted a main gas burner 12. Water is propelled through the system by means of a pump 16, connected at its output to a fluid conduit, generally designated 18, which enters the boiler heating chamber 10 near the bottom. Conduit 18 carries water to finned boiler heat exchanger tubes 20 which are coiled inside of chamber 10 so that the water may be heated by main burner 12. The uppermost exchanger coil interconnects with an exiting conduit 22 near the top of the boiler for conveying the water after it has been heated to the place of use (not shown). The water may, if desired, be circulated back to pump 16 in a closed loop system. Water flow is indicated by directional arrows designated WF.

Fuel, such as a gaseous hydrocarbon type from any convenient gas source (not shown), is fed by gas pipe 30 to main burner 12 through a manual rotor cock 32, a multifunction gas control pack, generally designated 34 and a gas modulating and limit control, generally designated 38; gas flow being indicated by directional arrows designated GF.

Manual rotor cock 32 normally remains in open condition, while gas pack 34 controls the flow of gas to main burner 12 subject to modulating control 38. Gas pack 34 may be of any conventional design. For example, pack 34 may be a solenoid 40 actuated valve responsive to the action of a thermostat (not shown) positioned in a space (not shown) being heated by the water circulated from the boiler. Gas pack 34 forms no part of the subject invention, and therefore, will not be described.

In addition to thermostat responsive solenoid 40, multifunction pack 34 is provided with a second solenoid 42 which, as is indicated diagrammatically by dotted line 44, is electrically interconnected to a snap switch 46 mounted on modulating gas control 38 to function as a high temperature limit for the boiler as will be explained hereinafter.

Normally, gas flows through modulating control 38 to burner 12 where it is ignited in any convenient manner, as by a standing pilot flame (not shown), to provide a main burner flame FL. Hot gases generated by the main burner flame flow upward in heating chamber 10 over the finned exchanger coils 20 and out the flue exit 48 at the top of the heating chamber. The hot gases heat the boiler heat exchanger tubes 20 which transmit the heat to water flowing in the tubes, heating the water.

Gas modulating control 38 is provided with a temperature probe for sensing the temperature of the water being heated. The probe comprises a rod and tube type actuator, generally designated 50, and is shown along with modulating control 38 in more detail in FIG. 2.

Gas enters gas modulating control 38 through pipe 30 (FIG. 1) at inlet 38A (FIG. 2) and exits at outlet 38B to flow to main burner 12, as is indicated by directional arrows GF. Hot gases generated by burning fuel (flame FL) are indicated as HG and flow upward over the finned surfaces of boiler heat exchanger coils 20.

Gas flow through gas modulating control 38 is subject to a butterfly valve 52 mounted in inlet 38A. Valve 52 is pivotally mounted on pins 54 (only one of which is shown) projecting from the interior wall of inlet 38A. Valve 52 is biased clockwise towards normally opened position by a coil spring 56 connected at one end to a tab 58 formed on butterfly valve 52 and at the other end to a tab 60 projecting from the interior wall of inlet 38A. Valve seat surfaces 62 are cut into the interior wall of inlet 38A to cooperate with valve 52 in controlling gas flow. In the embodiment shown inlet 38A is formed as part of a top plate 64 fastened by screws 66 onto a boxlike control housing 68, a gasket 71 being provided as a gas seal between top plate 64 and housing 68.

Butterfly valve 52 is connected to actuating mechanism by linkage, generally designated 70. Such actuating mechanism includes rod and tube temperature sensor 50, comprising a tube portion 72 and a rod portion 74. Tube portion 72 is formed of a length of a heat exchanger coil 20, preferably of a coil near the exit of the boiler. Tube portion 72 is provided with a tubular extension 72B upon which is mounted control housing 68 in any convenient manner, such as by brazing. Rod portion 74 is coaxially mounted in tube portion 72 with its left end fixed to tube portion 72, as by brazing to the boiler tube 20 at 80. The free end of rod 74 protrudes through tube extension 72B and an aperture 82 in the left wall of housing 68 of the gas modulating control 38. Sealing means, such as an "O" ring 84 are provided in aperture 82 encircling the free end of rod 74 to prevent water from boiler tubes 20 escaping into control housing 68 and, conversely to prevent the flow of gas from housing 68 into boiler tubes 20.

The free end of rod 74 is threaded at 74B into a cooperating aperture formed in the left sidewall of a yoke 83 of rectangular cross-sectional shape. Yoke 83 is slidably supported on a stepped portion 68B of the bottom wall of gas pack housing 68 for horizontal sliding movement within the confines of housing 68. An adjusting stud 90 with a knurled knob 92 formed on its right end protrudes through an aperture 94 formed in the right wall of modulating control housing 68. An "O" ring 96 encircles stud 90 at aperture 94, sealing the opening against gas escape.

The left end 90A of stud 90 is threaded through a stud receiving aperture 83B formed in the right sidewall of yoke 83. Stud end 90A abuts a sliding block member 98. Sliding block 98 is provided with a "V" notch 100 in its left side surface for receiving the knife edge shaped right end of a flat spring leaf member 104. The left end of spring leaf 104 is positioned in a similar "V" notch 106 formed on the side of a pin 108 protruding from the back vertical wall of housing 68, the spacing between block 98 and pin 108 being adjusted by stud 90 so as to "bow" spring leaf 104 upwards.

Stud 90 is threaded in and out of yoke 83 to move block 98 horizontally to increase or decrease the "bow" of spring member 104 to a desired initial position. Such bowing of spring 104 is transmitted to butterfly valve 52 by linkage 70. Linkage 70 includes a horizontal lever 110 pivoted intermediate its ends onto a pin 112 protruding from a bracket 114 fastened on housing 68. A link 116 pivotally connects the right end of lever 110 to butterfly valve 52 to transmit lever motion to the valve. A vertically disposed pin 118 protrudes loosely through an aperture 120 formed in upper horizontal wall of yoke 83. Pin 118 has an upper necked down end 118A which rests loosely in a hole 110B formed in the left arm of lever 110 and a lower necked down end 118B resting in a hole 104A formed in the midsection of bowed member 104, thereby, interconnecting bow member 104 to horizontal lever 110.

The high limit control snap switch 46 is also actuated by movement of horizontal lever 110 in response to bowing of leaf spring 104. Such movement is transmitted to the switch mechanism by means of a vertically disposed pin 124 slidably protruding through a sealed opening 126 in top plate 64 of control housing 68. The lower end 124A of pin 124 is necked down and confined in an aperture formed in the left arm of lever 110. The upper end of lever 124 is similarly necked down at 124B and confined in an aperture formed in a horizontally disposed bellcrank arm 128. Horizontal arm 128 is formed integral with a vertical bellcrank arm 130 which has bent over tabs 132 rotatably attaching the bellcrank by means of a pin 136 to a block 134 on which switch 46 is mounted on top plate 64. An adjusting screw 138 is threaded through an aperture formed in vertically disposed arm 130 in position to engage switch actuator 46A of switch 46 upon movement of arm 130 about pin 136 counterclockwise. It may be noted that there is sufficient mass provided by adjusting screw 138 and horizontal arm 128 to bias high limit actuator (128, 130, 138) clockwise to maintain its engagement with the shoulder of neck down portion 124B of pin 124.

In one tested embodiment of the invention the rod and tube temperature sensor 50 was formed with a section of boiler exchanger tube 20 of copper material providing the tube member 72, while the rod member 74 was constructed of invar material which has a substantially low coefficient of expansion. It should be understood nevertheless that all that is required for proper operation is that materials having different coefficients of expansion be used for the tube portion 72 and the rod portion 74 in order that relative motion therebetween occurs in response to temperature change. Additionally, the tube portion 72 need not be the usual boiler heat exchanger tubing, nor need it be of cylindrical configuration. All that is required is that the tube portion 72 of the actuator 50 form a portion of the fluid conduit 20 conveying the fluid through the boiler heating chamber, thereby acting as a heat exchanger, and that the rod 74 be immersed in the fluid inside the heat exchanger. In this manner the heating gases act upon the exterior wall of the tube 72, which then acts as a heat exchanger, transferring the heat from the gases to the fluid conveyed within it, in which fluid the rod 74 is immersed. It is preferable to install the rod and tube actuator 50 as a section of the boiler tubing 20 near to the exit point of the fluid from the boiler heating changer 10, as is shown in FIG. 1. In this manner the temperature sensed is substantially the exit temperature of the fluid from the boiler.

In the tested embodiment, the length of invar rod 74 (FIG. 2) remained substantially stable within the temperature range of the boiler such that substantially all relative motion between the rod 74 and tube 72 was provided by expansion and contraction of the tube portion 72. This relative motion was utilized to modulate the flow of gas to main burner 12 through gas control 38, as will now be described.

In operation assume that gas at main burner 12 is ignited, gas flowing through gas control inlet 38A, past butterfly valve 52. Initially, knob 92 is rotated to bow spring member 104, sufficiently to place through linkage 70 butterfly valve 52 in a predetermined opened position for the condition of sensor 50, allowing a certain amount of gas to flow to main burner 12, thereby establishing an operating point for the boiler. In making such adjustment, block 100 is moved to the left, increasing the bow of spring 104 and causing horizontal lever 110 to rotate clockwise, thereby moving butterfly valve 52 counterclockwise towards its closed position. Conversely, moving sliding block 100 to the right by means of adjusting stud 90 decreases the bow of spring member 104, allowing horizontal lever 110 to rotate counterclockwise as butterfly valve 52 is rotated clockwise by its biasing spring 56 towards fully opened position. Adjusting stud 90, therefore functions to establish a setpoint for gas modulating control 38.

Next assume that, while relatively cold water is being pumped through exchanger coils 20 at a certain rate, a demand for heat causes gas pack 34 (FIG. 1) to feed a maximum quantity of gas through gas modulating valve 38 to main burner 12 for quickly heating the water to a desired temperature. As hot gases HG (FIG. 2) flow over exchanger coils 20, the temperature of the copper coils increases rapidly. Such temperature increase is transferred by the tubing to the water flowing within, but with a slight attenuation due to the temperature transfer function between the tubing and water. The water is, thus heated towards the desired temperature with an inherent thermal time lag due to its mass and the aforementioned attenuation. Normally, the water temperature tends to approximate the tubing temperature, but with a major time lag.

The hot gases heat tube portion 72 of rod and tube actuator 50, causing expansion of tube portion 72, while rod 74 remains substantially at the same length. As tube 72 expands, modulating gas control housing 68 is carried by it to the right with respect to rod 74, increasing the bow of spring member 104, since yoke member 83 is attached to rod 74 and does not move, while pin 108 moves with housing 68 to the right.

As spring member 104 is bowed a greater amount with increases in temperature acting upon tube 72, valve linkage 70, as was previously described, moves butterfly valve 52 counterclockwise towards its closed position, decreasing gas flow to burner 12 to provide an amount of fuel sufficient to maintain the water at the desired preset temperature.

A record of the operation of one tested embodiment of the control with full heat on demand is shown in the graph of FIG. 3. Curve WT shown therein represents the record of boiler water output temperature made by strip recorder. During the recorded operation water was pumped through coils 20 at a 6 gallons per minute rate and at an entering temperature of approximately 145.degree. F. The set point of the modulating gas control was adjusted by knob 92 (FIG. 2) to provide approximately 175.degree. F. water at the boiler output. The graph of FIG. 3 has a horizontal coordinate T in seconds of time elapsed and a vertical coordinate .degree.F in degrees farenheit of the water temperature at the boiler output. At time 0 the exiting water temperature, as is shown by curve WT, is approximately 145.degree. F. At time 70 seconds later full gas was allowed to flow through gas modulating control 38 to burner 12. Such full flow of gas immediately caused a sharp rise in the temperature of the output boiler water flowing through conduit 22 (FIG. 1) that is an increase at a rate of approximately 5.degree. per second while water flowed through conduit 22 at 6 gallons per minute. The output water temperature rose from approximately 145.degree. F. to 187.degree. F. in about 8 seconds. In the next 65 seconds, boiler output temperature decreased from the initial maximum overshoot of 187.degree. F. to 178.degree. F. Operation stabilized within an operating band of from 172.degree. F. to 179.degree. F, (a remarkable 7.degree. F. proportional band) in just 90 seconds after the application of "full heat on" to maintain the desired 175.degree. F. output boiler water temperature within a 7.degree. F. proportional band. It should be noted that such "full heat on" operation is the harshest operation that can be demanded of a modulating control. A less harsh operation is one in which the water flow rate is varied. In such a case, as the water flow rate increases through exchanger tubes 20 and thus through actuator tube 72, the temperature acting on the interior wall of tube 72 decreases, causing the tube to contract. Such action causes the bow of spring member 104 to decrease slightly, causing butterfly valve 52 to move clockwise towards its open position, increasing the flow of gas to main burner 12 to maintain the water temperature at the desired temperature. Conversely, a reduction in water flow rate increases the heat transferred to the water and acting on tube 72, thereby elongating the tube to cause valve 52 to decrease gas flow.

It can be, thus, seen that the subject control minimizes overshoot or overheating of the boiler tubes 20, obviating boiler tube burnout, while providing efficient operation of a high heat flux type boilers within an efficient narrow proportional band. This is effected by utilizing the boiler heat exchanger itself, that is the boiler tubing 20 (which convey heat from the heating gases to the fluid being heated) as the tube portion of the rod and tube temperature sensor 50. In this manner the temperature of the fluid which is being heated is anticipated one step or transfer function ahead in the direct chain of heat convection to such fluid, providing close and anticipated control of the output fluid temperature. Such anticipation minimizes overshoot from the desired operating temperature, especially in high heat flux type boilers where full heat is turned on and off on demand.

It may be noted that with the aforementioned prior art immersion type sensors, sensing lags the actual water temperature by one time lag, whereas the subject sensor anticipates the water temperature by one time lag.

It should be noted that bowed spring member 104 responds nonlinearly to temperatures sensed by sensor 50. Similarly, the flow response of gas valve 52 is nonlinear. The movements of bow spring member 104 and valve 52 are preferably selected to compensate for each others nonlinearality, thus providing a linearly operating gas modulating control 38.

An additional feature provided by the subject modulating control is the provision of a high temperature limit for the boiler. This is provided by actuation of switch 46 upon a certain maximum high temperature being sensed by rod and tube actuator 50. Switch 46 may be electrically connected to control solenoid 42 (FIG. 1) in main gas pack 34 to shut off the flow of gas to modulating control 38 when the high temperature limit is exceeded. Such temperature is sensed by modulating control 38 through rod and tube sensor 50, as has been previously described. Should the temperature sensed exceed the desired limit, the expansion of tube 72 (FIG. 2) is sufficient to cause horizontal lever 110 to rotate clockwise to where vertical pin 124 actuates adjustable screw 138 clockwise into engagement with switch actuator 46A. Switch 46, upon actuation, through any conventional electrical circuit (indicated generally by broken line 44 in FIG. 1) energizes solenoid 42 to prevent gas flow through modulating control 38 to burner 12, until the temperature is reduced. A high limit control is, thus, incorporated in modulating gas control 38, which high limit also utilizes rod and tube actuator 50.

It may be noted that, since the subject sensor, responds to boiler tube temperature rather than to water temperature for high temperature limit control, liming and scaling on the tube interior will not cause overheating and tube burnout as with prior art immersion type sensors.

Although the invention has been described with respect to a gas boiler it should be noted that it is just as applicable to any type of heating means in which heated gases or air flow over heat exchanger conduits of which rod and tube actuator 50 is an integral part.

In addition, it should be noted that the relative motion provided in response to temperature by rod and tube actuator 50 may not only be utilized to control directly a gas valve but also may be utilized for control and indicating purposes by means of other transducers, such as mechanical to electrical transducers. For example, in FIG. 4 is shown an embodiment in which rod and tube actuator 50 is provided with a rheostat wiper W attached to tube portion 72 by an insulator 140 for movement therewith along a rheostat R mounted by insulator 142 onto rod 74. Rheostat R is connected to two electrical lines, designated L1 and L2, to control equipment (not shown). Relative motion between tube 72 and rod 74 causes movement of wiper W along rheostat R, providing a change in resistance or proportional output electrical signal over lines L1 and L2. Such resistance change or signal may be utilized to control a solenoid, or motor, or relay, or any other means of controlling the application of hot gases to fluid heat exchanger tubes 20.

As changes can be made in the above-described construction and many apparently different embodiments of this invention can be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown on the accompanying drawings be interpreted as illustrative only and not in a limiting sense.

* * * * *

Current U.S. Class:	236/20R
Current International Class:	F22B 37/47 (20060101); F22B 37/00 (20060101); F23n 001/00 ()
Field of Search:	236/20,33,102,103