CA1204138A

CA1204138A - Fluid heater

Info

Publication number: CA1204138A
Application number: CA000428393A
Authority: CA
Inventors: Kenneth E. Lehrke
Original assignee: Graco Inc
Current assignee: Graco Inc
Priority date: 1982-06-07
Filing date: 1983-05-18
Publication date: 1986-05-06
Also published as: US4501952A; IT1221434B; EP0110959B1; EP0110959A4; EP0110959A1; DE3377859D1; IT8383395A0; WO1983004465A1

Abstract

FLUID HEATER

ABSTRACT OF THE DISCLOSURE

Apparatus for heating fluids under both static and dynamic flow conditions, including heater temperature sensing circuits for monitoring the temperature of fluid across the cross section of the fluid flow path, and including ambient temperature compensation circuits for monitoring ambient temperature and compensating temperature control circuits for regulating heater temperature as a function of ambient temperature and as a function of static and dynamic flow conditions.

Description

~2~138 Baclcground of the Invention.
_ The present invention reLates to fluid heater apparatus, and more particuLarly ~o an improved Eluid heater wherein compensation in the heating temperature may be made und~r both static and dynamic fluid flow conditions, and through monitoring of the ambient temperature in which the apparatus operates. The invention is particularly directed to the heating of paints, lacquers, varnishes, and other single and plural component materials.
In industrial coating operations it i.s extremely important to contr~l th~ viscosity of the coating materials, particular when those coating materials are applied by some sort of spraying apparatus wheeein it is desirable to create an atomized spray of the coating materials for relatively even application upon a surface to be coated. While there are a number of ways to control fluid viscosity, such as by material and solvent selection, it is particularly convenient to control - such viscosity through the control of the temperature of the material. When such materials are applied by m~ans of a spraying apparatus it is important to control the viscosit~
over fairly narrow ranges. For example, it has been ound that viscosity changes exceeding plus or minus eight percent cause a noticeable change in the spray pattern of the fluid as it is being applied, and it has therefore been desirable to control the temperature of the fluid within plus or minus 3F. The problem is complicated by the fact that spraying devices typically are operated intermittently, resultin~ in both static and dynamic flow conditions in the fluid being fed into the spray device. Thus a fluid heater must have the capability of precise temperature regulation, so as to decrease the amount of heat applied to the fluid under static conditions, and yet increase the amo~nt of heat applied as soon as the liquid fluid flow conditions become dynamic. Further, it has been found that the ambient temperature of the work place affects the quality of the spray pattern if such ambient temperature varies ~2~4~3~3 over significant ranges. Since temperature control in industri~l plants is typically ~oorly regulated, it is not unusual for the work place temperature to vary over as much as 30F - 40F over the -time span of a single work shif t .
When such equipment is used outdoors the problern becvlnes rnore severe, because ambient temperature ranyes may be even broader.
In the past it has been common practice when relatively constant temperature operating conditions are desired to utilize a high mass fluid heater which may be slowly heated to a desirable temperature, and once heated is relatively insensitive to fluid flow conditions. However, such systems are usuaLly operated at a temperature well above the desired fluid temperature, in order to impart enough heat into the fluid durlng dynamic flow conditions. This creates an excessive temperature problem under intermittent operatin~
conditions, particularly when the spray device is turned off for a period of time, for then the fluid in the heater will rise to the temperature of the mass being heated. Attempts have been made to compensate for this problem b~ sensing the fluid temperature rather than the temperature of the mass being heated, and shutting off the heating power to the mass as the fluid temperature reaches the desired level. Similarly, as the fluid temperature drops below the desired level power is applied to heat the mass to return the fluld temperature to the desired level. The problem with this approach lies in the relatively long time it takes to heat and/or cool the large mass through which the fluid passes, resulting in fluid temperature swings both above and below the desired operating range.
It is thus desirable to use a low mass heating element with provision for controlling the temperature of the heating element very rapidly to accommodate fluid flow conditions.
Since it is important to obtain a close control over fluid temperature, and since this temperature is affected to a significant degree by ambient temperature, it is also important to sense ambient temperature ancl use this condition as a further regulation on heating element tempera~ure. ~urther, it is desirable to utilize a low mass heating element haviny the capability of controlling evenly the temperature of all of the fluid contained therein or flowing therethrough.
Summary of the Invent~on The present invention utilizes a low mass heating element in close contact with the Eluid flow path, the flow path being specially constructed to provide continuous mixing and distribution of the fluid within the heater, and a proportional-control, slope-compensated temperature control circuit monitoring temperature conditions ~cross the flow path cross section within the heater as well as m~nitoring ambient temperature.
It is therefore a principal object of the present invention to provide a fluid heater having precise and narrow temperature control capabilities under varying static and dynamic Eluid flow conditions and ambient temperature conditions.
It is another object of the present inventi~n to-provide a fluid heater having the capability to rapidly heat Eluid contained therein to a desired operating temperature.
It is further object oE the present invention to provide a fluid heater having the capability of reducing the watt density of the heating element under conditions of complete static flow to prevent excessive fluid ~emperature rom developing.
Brief Description of the Drawings rrhe foregoing and other objects will become apparent from the following specification and claims, and with reference to the appended drawings, in which:
FIG. 1 shows an exploded view of the invention;
FIG. 2 shows the heating element in partial cross section;
FIG. 3A shows a side cross section view of a portion oE the invention;
FIG. 3B shows a view taken along the lines 3B - 3B of FIG.
3A;

~;~4~3~3 - 5 ~

FIG. 4A shows a cross~sectional view of the invention and a portion thereof;
FIG. 4B shows a view taken along the lines 4B - 4B of E'IG.
4A; and FIG. 5 shows a schematic block diagram of the temperature control circuit of the present invention.
Descr.iption of the Preferred Embodiment Referring first to FIG. 1 the fLuid heater of the present invention is shown in exploded view. A housing 10 consisting of a hollow tube formed in the shape of a "U" serves as the fluid passageway for the invention. A fluid mixer 12 is inserted into each of the legs of the "U'l shaped housing, the outer diameter of fluid mixer 12 being sized to relatively snugly fit inside of housing 10. An electric heating element 14 is inserted inside of each of the fluid mixers 12 to a reasonably close diame;tric fit. Heating element 14 may be a commercially available product, such as Model ~323323-5TB5HJO, manufactured by ITT Vulcan Electric Co., Kezar Falls, Maine.
In the preferred embodiment heating element 14`is rated at 2550 watts with 240 volts AC applied, and produces a power density of 58 watts per square inch. Each of the open ends of housing 10 is threaded to accept the complementary threads of a fluid manifold. Fluid inlet manifold 16 is threaded to one end of housing 10 and fluid outlet manifold 18 is threaded to the other end of housing 10. Manifold 16 has a threaded inlet port 17 which is adapted for coupling to a fluid hose or other piping arrangement. Manifold 18 has a threaded port 19 which is adapted for similar attachment as an outlet port. In each case heating element 14 is insertable entirely through the respective maniEolds, and has a threaded lock and sealing nut 15 for threaded attachment to the respective manifolds. A pair of power wires 13 protrude from sealing nut 15, and may be connected to a suitable source of power or energizing heating element 14. The use of a U-shaped tube enables the concentration of a significant heating power into a relatively ~2~ L38 srnall package; iE the heatiny demands of a particular application do not require such heating capabilities other tube shapes could be selected.
Fluid outlet maniEold 18 has an additional port 21 which is threaded to a~cept a seaLing nut 22. A temperature sensing probe 25 is coaxially fitted through sealing nut 22, in a manner to be hereinafter described. A seal 23 is fitted into sealing nut 22, and is held in place by cap 24.
A thermostatic switch 26 is attached to the outside of housing 10, and serves to function as an over temperature safety switch. In the preferred embodiment thermostatic switch 26 is selected to provide an open circuit at a temperature of 180F, and a closed circuit at 200F.
The fluid heater shown in FIG. 1 may be enclosed within an insulated container of suitable size and shape, and the particular shape of hôusing 10 may be varied to meet specific design requirements.
FIG. 2 shows heating element 14 in partial cross section.
Heating element 14 is a hollow tube having therein a heating wire coil 28 which is electrically connected to wires 13.
Wires 13 are fed to the exterior of heating element 14 through an insulation material 30. The outer wall of heating element 14 is preferably constructed of stainless steel or equivalent material.
FIG. 3A shows a portion of the fluid heater in cross-sectional view. Fluid mixer 12 is a helical member, having diametrically opposed, axially extending baffles 32 affixed tilereto. FIG. 3B shows a cross-sectional view of housing 10 taken along the lines 3B - 3B of FIG. 3A. ~affles 32 project inwardly from the outer diameter of fluid mixer 12, and serve to direct the fluid flow path through the device.
The fluid flow path is shown by the arrows in FIGS. 3A and 3B, and is shown to be generally helically following fluid mixer 12, except where baffles 32 are encountered, wherein the flow path is forced inwardly toward the center of the helix. Fluid 4~31~ .

mixer 12 ma~ be constructed from a metallic spriny, haviny bafEle members 32 attached thereto.
FIG~ 3B shows a cross-sectional vie~/ taken along the lines 3B - 3B of FIG. 3A. From this cross-sectional view, it is apparent that the fluid flow path throuyh housing 10 is generally circular, with fluid flow forces directed radially inward at the point of baffles 32. This flow pattern produces continuous mixing of the fluid through housing 10, and close contact between the fluid and heating element 14.
FIG. 4A shows a cross section of the invention through outlet manifold 18. Manifold 18 is threaded onto the end of housing 10 so as to provide a fluid tight seal. Similarly, sealing nut 15 is threadably attached to manifold 18 for providing a Eluid tight seal for the attachment of heating element 14. Temperature probe 25 is also sealably attached in manifold 18 by sealing nut 22, reference being made to FIGS. 4A
and 4B for the pertinent construction. Temperature probe 25 comprises a thermistor 34 embedded in a thermistor housing 36, the thermistor having connected thereto a pair~of wires 38.
Temperature sensing probe 25 passes through a seal 23 which is sealably attached by cap 24 to sealin~ nut 22. Sealing nut 22 is threadably attached to manifold 13. Thermistor housing 36 has a conical surface with its pointed end preferably in physical contact with heating element 14. The remaining surface of thermistor housing 36 is exposed to fluid flow within housing 10. Thermistor 34 is a resistance element having a resistance determi.ned by its temperature, and more generally having its resistance determined by the temperature of thermistor housing 36. Since thermistor housing 36 is in direct point contact with heating element 14, the temperature of thermistor housing 36, and therefore thermistor 34, will be directly dependent upon the temperature o~ heating element 14 during static flow condit.ions. However, during dynamic Elow conditions the rèlativel.y large external surface area of thermistor housing 36 becomes primarily affected by the ~ 41~18 temperature oE the fluid flow throuyh housing 10, and therefore the temperature of thermistor 34 is primarily a function of the temperat~re of the fluid flowing through housin~3 10 during dynamic conditions. The net operating effect of temperature sensing probe 25 is to cause it to intec~rate, or average, all of the temperatures present in the fluid flow cross section.
Referring next to FIG. 5, a schematic block diagram of the control circuit of the invention is shown. Alternating current voltage is applied at power lines 40 and ~1. This alternating current voltage may typically be 200/250 volts AC, at 50/60 hertz (llz). AC power line 40 is wired to one of the wires 13 which connects to heating wires 28. For convenience, only a single heating element 28 is shown in the drawing, it being understood that additional heating elements could be connected thereto, either in parallel or series connection. AC power line 41 is connected t~o triac circuit 44. Triac circuit 44 is an AC power switch, of a type well-known in the art. In the preferred embodiment triac 44 is manufactured by Raytheon Company, under type designation TAG 741. Triac~circuit 44 has a control input line 45, the voltage signals appearing on input line 45 causing triac circuit 44 to turn "on" and "off" as a function of these input signals. AC power line 41 also serves as a circuit common or ground connection.
Direct current power to operate the circuit shown on FIG. 5 is obtained through a circuit DC power supply 46, which receives its input power through dropping resistor 48, and a connection to circuit ground (not shown). DC power supply 46 provides a DC voltage on line 47 and other lines not shown, for operation of the circuits to be hereinafter described.
A resistance bridge circuit is formed by resistors Rl, R~, R3, and R4, the function of which will now be described. Resistance Rl is a variable resistance which functions to enable a manual setting of a desired setpoint temperature, and may be set by an operator to any predetermined desired temperature. Resistan~e R2 (thermlstor 34) is the ~Z~4~3~ .

temperature-variable resistor found in ternperature sensor 25 Its resistance varies inversely with tempera~ure, the resist~nce decreasing as the sensed temperature increases, and increasing as the sensed temperature decre~ses. Resistance R3 is a temperature variant resistor whose resistance values vary inversely with temperature, and whose function is to provide an indication of the room or outdoor ambient temperature. Resistance R4 is a fixed value resis'cance whose function is to provide a resistor balance point for resistance R3. In practice, resistances R3 and R4 are selected so as to be equal in value at a nominal ambient temperature, i.e.
about 80 F, so that the leg of the resistance bridge comprising resistors ~3 and R4 is balanced at a nominal ambient temperature. Resistances Rl and R2 form the other balanced leg of the resistance bridge circuit. Resistor R2 (thermistor 34) varies inversely with the temperature of the fluid within housing 10, and resistance Rl may be manually set to a value corresponding to a desired temperature setting of the fluid within housing 10. Resistances R2~and R3, in the preferred embodiment, are products manufactured by Victory Engineering Corp., Springfield, New Jersey, under type designation VECO T45A35.
The voltage at the junction point 42 of resistances R
and R2 is direct-coupled into bridge amplifier 50, which generates an output signal in response to this voltage. The voltage at the junction point 49 of resistances R3 and R4 is also direct-coupled into bridge amplifler 50 in the same respect. The output signal from bridge amplifier 50 appears on line 51, and is a voltage representative of a signal commanding more or less heat from the heater, i.e., the higher the voltage level on line 51 the longer will be the duty cycle of the AC
power driving the heater, and therefore the ~ore heat will be commanded. This signal is ed into comparator and drive circuit 56 as one of two inputs received by that circuit. The second input into circuit 56 is a signal on line 57, which is a `` ~L2~4~3~

sawtooth voltage riding a DC voltage level. A 60 Hz output signal from circuit 56 will appear on line ~5 at any time ~"hen the input signal on line 57 is at a lower voltage than the input sic3nal on line 51. The signal on line 45 is used as a control signal input to triac circuit ~4, eefectively turning on triac circuit ~ to enable AC power to pass through heating wires 28. FIG. 5 shows several voltage waveforms which may be found at the points indicated on the drawing.
A square wave generator 44 generates a repetitive signal having a period of approximately l l/2 seconds. This square wave sig~al is passed through resistance/capacitance network comprising resistor 58 and capacitor 60. This network produces a sawtooth waveform appearing on line 62 as an input to summing amplifier 64. The sawtooth waveform on line 62 is referenced at a potential of 4 volts having equal portions (+ l l/2 volts) of voltage swings about that voltage. A differential amplifier 52 has an input coupled to junction point 42 via a capacitor 43. Dif~erential amplifier 52 reacts to changes in voltage at junction point 42, and the output of differenti~l amplifier 52 is a signal on line 53 which is a DC voltage representative of the rate o~ change of voltage at point 420 The signal on line 53 is summed with the signal on line 62 by summing amplifier 64, and the output of summing amplifier 64 is therefore a sawtoothed voltage riding a DC level as has been hereinbefore described. It should be noted that the signal received by summing ampli~ier 64 from line 53 is received at an inverting (-) input terminal, whereas the signal received by summing amplifier 64 from line 62 is received at a noninverting (+) input terminal.
The components selected for the control circuit illustrated on FIG. 5 are all standard commercial components which are commonly available. For example, the power supply, comparator, and drive circuits 46 and 56 are in a single integrated circuit manufactured by RCA, under Type CA3058. The components making up bridge amplifier 5~, differential amplifier 52, summing 12~4~3~

amplifier 64, and square wave generator 54 are all found in a single integrated circuit manufactured by National SeMiconductor Company, as Type LM124. In the preerred embodiment resistor 58 has been selected to have a value of 150 kilohms (150 K), and capacitor 60 has been selected to have a value of 10 microfarads (uf). Similarly, capacitor 43 has a value of 11 uf, capacitor Cl has a value of 50 uf, and resistor R4 has a value of 47 K. Resistor R1 is a variable potentiometer having a nominal range of values from 0 to 45 K.
In operation, resistor Rl is nominally set by the operator to a setting representative of the desired fluid temperature AC power is then applied to the circuit, and the heater begins operating. If the fluid temperature is initially lower than the temperture setting of Rl, a positive voltage will be present at point 42, which will be amplified by bridge amplifier 50 resulting in a positive voltage on line 51. Since the positive voltage on line 42 is initially unchanging, the output of differential amplifier 52 is initially zero, and the output of summing amplifier 64 is therefore a s~wtooth waveform riding about a 4 volt level. To the extent the voltage on line 57 is lower than the voltage on line 51, circuit 56 will generate a 60Hz output signàl on line 45. The signal on line 45 will trigger triac circuit 44 to cause AC power to pass through heater wires 28. Initially, it may be presumed that the signal on line 57 is lower than the voltage on line 51 during almost the entire sawtooth period, resulting in a 60 Hz continuous output signal from circuit 56, and therefore resulting in a triggering of triac circuit 44 and applying AC
power to heater wires 28. This causes the heating temperature to rapidly develop in the fluid heater, and brings about an increase in temperature sensed by resistance R2. Thereforel the voltage at point 42 begins dropping at a rate consistent with the rise injtemperature. Bridge amplifier 50 develops an output signal on line 51 which follows the change of voltage at point 42, but capacitor Cl sh~nts any rapidly changing ~Z~ 38 voltage, effectively desensitiziny bridge amplifier 50 during times when the rate of change oE temperature is rapid.
Conversely, the rate oE change of voltage at point 42 is sensed by differential amplifier 52 to generate a negative voltage on its output Line 53. The voltage on line 53 is sumMe~ ~"ith the sawtoothed waveform on line 62 to generate a less negative-riding sawtoothed voltage on line 57. As the temperature within the fluid heater reaches the nominal setpoint temperature, the voltage at point 42 becomes more negative and the output from bridge ampllfier 50 becomes more negative. This results in the signal on line 51 dropping in magnitude and thereby decreasing the drive signal from circuit 56. This decreased drive signal results in a lowered duty cycle operation of triac 44, and gradually lowers the amount of AC power fed into heater coils 28. As the amount of AC power applied to heater coils 28 diminishes the rate of change of increase of heat sensêd by resistor R2 diminishes and differential amplifier 52 generates a less negative output signal, tracking this rate of change. This causes the output signal on line 53 to decrease towards zero, and summing amplifier 64 produces an output on line 57 which is a sawtooth voltage riding a DC level approaching the 4 volt bias line.
This effectively removes differential amplifier 52 from the circuit and causes summing amplifier 64 to pass a sawtooth voltage to comparator and triac drive circuit 56. At the nominal temperature setpoint the rate of change of voltage at point 42 becomes zero or near zero, differential amplifier 52 generates a zero or near zero output signal on line 53, and summing amplifier 64 generates a sawtoothed waveform on line 57 ~hich is referenced about the bias voltage reference. This is compared with the signal on line 51, resulting in approximately a 50 percent duty cycle operation of triac switch 44.
When temperatures within the heater are at or near the nominal setpoint temperature the voltage on line 51 tracks these minor disparities, increasing or decreasing slightly the - ~2~38 duty cycl.e of triac switch 44 to lncrease or decrease AC po~ler applied to heater wires 28 by the small additional amount needed to compensate for the temperature disparity. Under these conditions bridge amplifier 50 functions as a high gain a~plifier, and small voltage changes at junction 42 produce significallt corrective voltages at line 51. When wide disparities exist between the temperature setpoint and the actual temperature the circuit permits rapid heater buildup by tracking the rate of change of actual temperature versus setpoint temperature, thereby permitting the heater to develop full power until actual temperature approaches nominal temperature settings. The output from bridge amplifier 50 is effectively desensitized by capacitor Cl, causing it to react wi.th high sensitivity toward slowly varying temperture disparities, and with lower sensitivity towards rapidly varying temperature disparities. Conversely, the output from differential amplifier 52 causes it to react only toward rapidly varying temperature disparities, to control the triac switch 44 duty cycle when such temperature disparities exist.
The present invention may be embodied in ot~er specific forms without departing from the spirit or essential attributes thereof, and it is therefore desired that the present embodiment be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrically operated fluid heater and control circuit comprising:
a) an elongated hollow tube adapted for insertion into a fluid flow line;
b) an electrically operated heater in said hollow tube;
c) a helical coil arranged about said heater in said hollow tube, said coil creating a helical fluid flow path in said hollow tube and around said heater;
d) temperature sensing means for detecting the temperature of fluid in said tube, said sensing means comprising a conical housing having its apex contacting said heater and a conical surface area increasing in a direction away from said heater but in said fluid flow path, and a temperature responsive resistance element in said housing, and electrical conductors connected to said resistance element and projecting external said housing; and e) temperature control circuit means connected to said electrical conductors and to said electrically operated heater, for controlling said electrically operated heater in response to the temperature of the fluid flowing through said flow path as sensed by said resistance element.

2. The apparatus of claim 1, wherein said temperature control circuit means further comprises means for controlling said electrically operated heater in response to the resistance of said resistance element and in response to the rate of change of resistance of said resistance element.

3. The apparatus of claim 1, wherein said helical coil further comprises at least one baffle longitudinally arranged along said coil and in flow disturbing relation to said fluid flow path.

4. The apparatus of claim 1, wherein said elongated hollow tube has a fluid flow inlet and fluid flow outlet, and further comprising a manifold attached to said fluid flow outlet, said manifold having means for insertion of said electrically operated heater therethrough into said tube and said manifold containing said temperature sensing means.

5. The apparatus of claim 1, further comprising ambient temperature sensing means for detecting temperature outside said tube, and said temperature control circuit means further including means for controlling said electrically operated heater in response to said ambient temperature sensing means.

6. The apparatus of claim 4, wherein said temperature sensing means further comprises a housing having a conical external surface shape and having a pointed tip contacting said heater.

7. The apparatus of claim 5, wherein said temperature control circuit means comprises means for controlling said electrically operated heater in response to the temperature of said temperature sensing means and in response to the rate of change of temperature of said temperature sensing means and in response to the temperature of said ambient temperature sensing means.

8. The apparatus of claim 7, wherein said helical coil further comprises at least one baffle longitudinally arranged along said coil and in flow disturbing relation to said fluid flow path.

9. The apparatus of claim 1, wherein said elongated hollow tube further comprises a generally U-shaped tube, and said electrically operated heater further comprises first and second heater elements in respective legs of said U-shaped tube.

10. The apparatus of claim 9, further comprising ambient temperature sensing means for detecting temperature outside said tube, and said temperature control means further including means for controlling said electrically operated heater in response to said ambient temperature sensing means.

11. The apparatus of claim 10, wherein said temperature control circuit means comprises means for controlling said electrically operated heater in response to the temperature of said temperature sensing means and in response to the rate of change of temperature of said temperature sensing means and in response to the temperature of said ambient temperature sensing means.

12. The apparatus of claim 11, wherein said helical coil further comprises at least one baffle longitudinally arranged along said coil and in flow disturbing relation to said fluid flow path.

13. The apparatus of claim 12, wherein said temperature sensing means further comprises a housing having a conical external surface shape and having a pointed tip contacting said heater.

14. The apparatus of claim 13, wherein said helical coil further comprises first and second helical coil sections respectively arranged about said first and second heater elements.

15. The apparatus of claim 14, wherein said elongated hollow tube has a fluid flow inlet and fluid flow outlet, and further comprising a manifold attached to said fluid flow outlet, said manifold having means for insertion of said electrically operated heater therethrough and said manifold containing said temperature sensing means.

16. The apparatus of claim 15, wherein said temperature sensing means further comprises a housing having a conical external surface shape and having a pointed tip contacting said heater.

17. The apparatus of claim 16, wherein said temperature control circuit means further comprises means for controlling said electrically operated heater in response to the temperature of said temperature sensing means and in response to the rate of change of temperature of said temperature sensing means.

18. The apparatus of claim 17, wherein said helical coil further comprises at least one baffle longitudinally arranged along said coil and in flow disturbing relation to said fluid flow path.