Electric fluid heater

An in-line electric heater for heating fluids, such as paint, moving in a conduit has a thermally conductive massive body in which is formed a fluid passage having an inlet port and a outlet port. An electric heating element in the body directly heats only the upstream portion of the fluid passage and heater body. The downstream portion of the fluid passage and heater body is indirectly heated to a substantially lesser temperature by heat conduction from the upstream portion whereby the downstream portion acts as a "thermal accumulator" which damps the cycling, overshoot and undershoot of the temperature of the fluid at the outlet port of the passage. A temperature control means for controlling operation of the heating element is provided and includes a temperature sensor arranged to sense the temperature of the proximate the point in the passage wherein the fluid exhibits its greatest temperature cycling excursion, undershoot and overshoot under constant flow conditions, thereby providing optimum feedback control.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to fluid heaters and more particularly relates to 
in-line fluid heaters for fluids moving in a conduit where the flow rate 
of the fluid is subject to variations, or where the temperature of the 
fluid at the outlet of the heater is subject to cycling variations. 
2. Description of the Prior Art 
Fluid heaters are used in many applications and for many different types of 
fluids. For example, there are heaters for water, thermoplastic materials, 
paints, etc. In the spray coating industry, heating paint or coating 
materials lowers the viscosity of the paint so that paints having high 
viscosities, which could not normally be applied with spray coating 
equipment, can be sprayed. The in-line fluid heater disclosed as the 
preferred embodiment herein was specifically developed for heating paints. 
However, the inventive principles used are equally applicable to fluid 
heaters generally. 
In-line fluid heaters of the past generally comprised a fluid passage in 
heat transfer relationship with a heating element; for example see Krohn 
et al. U.S. Pat. No. 3,835,294. The heating elements in some heaters were 
in direct contact with the fluid, and in others the heating element heated 
the fluid indirectly by heating the structure in which the fluid passage 
was formed, which structure in turn transferred the heat to the fluid in 
the passage. In heaters of past design the heating element was positioned 
with respect to the fluid passage in the heater so as to heat the fluid 
substantially uniformly for the entire length of the passage. 
If the thermal characteristics of the fluid and the flow rate of the fluid 
to be heated were not subject to variations during operation, some heaters 
were designed so that the outlet temperature of the fluid achieved the 
proper value with the heating element having constant power input, and 
there was no need for any control mechanism. However, if the thermal 
characteristics of the fluid or its flow rate were subject to variations, 
then a feedback type control was used to assure that the temperature of 
the fluid being discharged was within a certain allowable range around a 
desired value. A temperature sensor monitored the temperature of the fluid 
being discharged from the outlet of the heater, and a control device 
responsive to the temperature sensor controlled the heating element. 
By use of sophisticated and expensive control devices and heater designs, 
the temperature range could be held to a very close tolerance over a wide 
range of flow rates and/or thermal properties. However, in heaters of 
relatively simple and inexpensive design, certain trade-offs had to be 
accepted. For example, many heaters used a thermostatic type 
sensor/control combination to monitor the temperature of the fluid at the 
outlet of the heater. By "thermostatic type" sensor/control is meant one 
which turns a heater element on or off in response to some preselected 
temperature. In heaters using a thermostatic type sensor, the temperature 
of the fluid at the outlet of the heater, even under constant flow rate 
and thermal characteristics of the fluid, were prone to steady-state 
cycling of the outlet temperature between high and low peak-to-peak 
temperatures. This was due to the on-off cycling of the heating element, 
on/off differential of the temperature sensor, etc. Also in many heaters 
of past design, when the heater was initially started, or when the 
temperature setting was suddenly increased, or when the flow rate of the 
fluid was suddenly reduced, the temperature of the fluid at the outlet of 
the heater would overshoot the high steady-state peak cycling temperature. 
That is, the temperature of the fluid would temporarily exceed the high 
peak temperature which the fluid would reach under steady-state cycling. 
Conversely, when the temperature setting was decreased or flow rate of the 
fluid suddenly increased, the temperature of the fluid at the outlet of 
the heater would undershoot the low steady-state peak cycling temperature. 
The temperature of the fluid would fall below the low peak temperature 
which it would drop to under steady-state cycling. 
The cycling of temperature, overshoot and undershoot is caused at least in 
part by what might be termed thermal lag. This thermal lag is caused by 
the fact that a finite time is required for a body to change temperature 
and hence to react to a temperature change. When the heating element is 
on, the temperature of the fluid is increasing. But when the fluid reaches 
proper temperature, the sensor requires a finite time to respond to this 
temperature. Also the heating element requires a finite time to cool down. 
During this time energy continues to be applied to the fluid. This causes 
the temperature of the fluid to increase beyond the desired to set 
temperature. When the heating element has been off and the fluid 
temperature decreases below the desired temperature, a finite time is 
required for the sensor to react to this situation and to energize the 
heating element. The temperature of the fluid continues to decrease before 
the heating element heats up and causes the temperature of the fluid to 
increase. 
It is an object of the present invention to reduce the steady-state cycling 
of the feedback controlled fluid heaters as well as their overshoot and 
undershoot characteristics. Through the present invention these reductions 
can be achieved in simple inexpensive heaters using thermostatic control, 
as well as in heaters using more sophisticated control means, and without 
adding undue cost to the heater. 
SUMMARY OF THE INVENTION 
The present invention is an improved in-line paint heater having feedback 
control of the fluid temperature, wherein the heating element operates 
directly on only the upstream portion of the fluid passage in the heater 
body. The downstream portion of the fluid passage and heater body is 
indirectly heated, and therefore heated substantially less than the 
upstream portion. This downstream portion acts as a "thermal accumulator" 
which damps the cycling, overshoot and undershoot of temperature. This 
integral downstream "accumulator" portion of the heater body has a 
substantial thermal mass (specific heat times mass) and fluid passage 
surface area. It is insulated sufficiently from the ambient conditions so 
that it does not merely cool the fluid passing through. Heat is taken up 
by the "accumulator" portion when it is colder than the fluid, and given 
off to the fluid from the "accumulator" when it is hotter than the fluid. 
Thus this accumulator portion of the heater damps cycling, overshoot or 
undershoot of the temperature of the fluid at the outlet port of the 
heater. The effect is more pronounced as flow rate increases.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Generally, the heater comprises a heater body consisting essentially of a 
heater core 1 and cover 2, a heating element 7, a temperature sensor 10, 
and a control box 16. 
The heater core 1 is an elongated cylindrically shaped piece of aluminum of 
substantially uniform construction and cross-sectional dimension along its 
elongated length. The core has an elongated length of approximately 340 
mm, a cylindrical radius of 38 mm, having three bores or cavities 4, 5 and 
6 open from one end, and having a groove in its outer cylindrical surface 
which spirals circumferentially around the heater core 1. The groove is 
rectangular in cross section, having a depth of 11 mm and a width of 6.35 
mm. The wall thickness between successive adjacent portions of the groove 
is 4.94 mm. Because of the dimensions and material of the core 1, it has a 
substantial thermal mass. 
The core 1 is threadedly attached to the control box 16 at the upper (in 
the FIGURE) or outlet end of the heater core 1. 
A cylindrical, plated steel cover 2 having an inside diameter of 0.08 to 
0.20 mm greater than the outside diameter of the heater core 1 girds the 
core 1 for at least the whole extent of the spiraled groove. The groove on 
the heater core 1 combines with the cover 2 to form a spiraled passage 3, 
the surface of which is in heat exchange relationship with fluid in the 
passage 3. Because the cover 2 is larger in diameter than the core 1, 
there is a gap 15 between the cover 2 and core 1. The gap 15 between the 
inside of the cover 2 and the outside of the heater core 1 is maintained 
under 0.20 mm so that the fluid to be heated spirals around the core 1 
rather than passing directly across the gap 15. The cover 2 is sealed to 
the core 1 by means of O-rings 12, 13 beyond each end of the spiraled 
passage 3. The cover 2 is held in place by a steel retaining ring 14 at 
the lower end, and a hose connection fitting 20 at the upper end. An inlet 
fluid passage 18 and an outlet fluid passage 19 both located interiorly of 
the heater core 1 each communicate one end of the spiraled passage 3 to 
the exterior of the core 1. These inlet and outlet passges 18, 19 are each 
adapted to terminate in a suitable hose connection fitting. 
The three cavities 4, 5, 6 in the heater core 1 are cylindrical, having 
their cylindrical axes parallel to the cylindrical axis of the heater core 
1 itself. Each of the cavities 4, 5, 6 is open to the exterior of the 
heater core 1 through the end of the core 1 closest to the fluid discharge 
passage 19. One of the cavities, the heating element cavity 4, is located 
centrally of the heater core 1 and houses a cylindrically shaped heating 
element 7. This central cavity 4 has a cylindrical diameter of 12.7 mm and 
extends into the core 1 such that the bottom or lower extremity of the 
cavity 4 is radially opposite the most upstream part of the spiraled 
passage 3. The remaining two cavities 5, 6 are located radially between 
the central cavity 4 and the outer surface of the heater core 1. A sensor 
cavity 5, houses a temperature sensor element 10, and the remaining cavity 
6 houses a heat limiter 9 which is optional. 
Power lines 21 to the heating element 7, and the control lines 22 from the 
temperature sensor 10 enter a chamber in the control box 16 and are 
connected to a control mechanism (not shown). 
The heating element 7 is a cartridge type heating element and can be one 
sold under the Trademark "Firerod" manufactured by Watlow Electric 
Manufacturing Company. It is located in the central cavity 4 and is 
shorter than the elongated length of the part of the heater core 1 having 
the spiraled groove. The heating element 7 has a close tolerance fit to 
the central cavity 4 so that heat will pass readily from the heater 
element 7 radially into the portion of the heater core 1 radially adjacent 
to the heating element 7. The heater core in turn heats the fluid in the 
passage. 
When the heater core 1 is threaded onto the control box 16 a hollow 
aluminum tube 23 through which the power lines 21 to the heater element 7 
pass, is urged by a control mechanism housing 8 in the control box 16 
against the end of the heating element 7 so as to hold the heating element 
7 into the bottom or lower part of the central cavity 4. The tube 23 has 
annular dimensions such that its end will abut against the top of the 
heating element 7 and such that the power lines 21 to the heater element 7 
can pass through its hollow center portion. Thus, the heating element 7 is 
positioned so as to be radially opposite to and effectively directly heat 
only the upstream portion (or in the figure, the bottom portion) of the 
spiraled fluid passage 3. The spiraled passage 3 continues downstream 
beyond the location where the heating element 7 is radially proximate the 
spiraled passage 3. In this embodiment the heating element 7 is proximate 
the spiraled passage 3 for approximately 165 mm, and the spiraled passage 
3 continues for approximately another 41 mm of heater core length. This 
downstream 1/5 of the fluid passage 3 is substantially unheated by direct 
radial action of the heating element 7. 
The temperature sensor 10 is located in the sensor cavity 5 radially 
between the heater cavity 4 and the cylindrical outer surface of the 
heater core 1. This sensor can be a type sold as a Model 102 by Essex 
International Co., Controls Division. The sensor 10 is a low pressure 
averaging type sensor, of elongated cylindrical configuration. It has a 
4.degree. on/off differential. That is, it is effective to turn the heater 
element 7 on at 4.degree. F. lower than it is to turn the heating element 
7 off. The sensor 10 senses temperature along substantially its full 
length, and its output is related to the average of the temperatures 
sensed. 
The temperature sensor 10 actually responds to the temperature of the 
heater core 1. However, this temperature to which the sensor responds is 
primarily influenced by or associated with the temperature of the fluid in 
the part of the passage 3 radially proximate the sensor 10. Because the 
sensor 10 is a low pressure type and the fluid is under a higher pressure 
than the sensor 10 can withstand it is not positioned to sense the actual 
temperature of the fluid in the spiraled fluid passage 3. However, the 
cavity 5 for the sensor 10 is positioned such that the sensor 10 will be 
as close as possible to the spiraled fluid passage 3, while still leaving 
enough wall thickness between the spiraled passage 3 and the sensor cavity 
5 to safely withstand the pressures to which the fluid may be subjected. 
This wall thickness may vary depending on the fluid pressures and the 
heater core material. 
The averaging center 11 of the temperature sensor 10 is located radially 
opposite the most downstream point 24 of the heating element 7 (the top of 
the heating element 7 in the figure). This location generally corresponds 
to the point along the spiraled fluid passage 3 which will experience the 
greatest temperature cycling excursion, overshoot and undershoot. Sensing 
the temperature at this location provides optimum feedback control. 
An averaging type sensor 10 is used for the sake of economy. A point 
sensor, which responds to the temperature at a specific location or point, 
could be used. If a point sensor were used, the sensor cavity 5 need only 
extend into the heater core 1 to a point radially adjacent to the top of 
the heating element 7, and the sensor would monitor the temperature of the 
core 1 at the bottom of this shortened cavity 5. 
The output of the temperature sensor 10 is operatively connected to a 
control mechanism 8 which responds to the sensor output so as to energize 
and de-energize the heating element 7 to maintain the desired fluid 
temperature. 
Because temperature control mechanisms are relatively well known in the 
art, the control mechanism need not be discussed in detail here. In 
general, the control mechanism responds to the temperature sensor 10 so as 
to energize the heating element 7 when the temperature sensed by the 
sensor 10 is below some desired preset value, and to de-energize the 
heating element 7 when the temperature sensed is greater than a desired 
preset value. 
It is to be noted that the amount of damping to the steady-state 
peak-to-peak cycling, overshoot and undershoot of temperature is not the 
same at all flow rates. The damping is more pronounced at the higher flow 
rates. Additionally, it is to be noted that the steady-state peak-to-peak 
cycling, overshoot and undershoot are not necessarily damped by the same 
respective amounts.