Fluid heater and method

A fluid heater and method are disclosed for heating a heat transfer fluid flowing within a heater tube using reradiative and convective heat transfer while avoiding direct flame impingement on the heater tube. The heater tube is separated from the flame by a heat shield that cools the flame temperature and reradiates heat energy to the heater tube. The combustion products are directed around the shield and into contact with the heater tube.

BACKGROUND OF INVENTION AND RELATED ART 
This invention relates to heat exchanger apparatus and methods for transfer 
of heat to fluids and, more particularly, to a fluid heater apparatus and 
method of heating heat transfer fluids. The invention is described below 
with particular reference to heat transfer fluid applications. 
Heat transfer fluids are heated to desired working temperatures for 
transfer of process heat in a wide range of industrial applications such 
as indirect heating of process liquids and polymers, heating and cooling 
in batch processing, energy generation and recovery processes, drying and 
heating bulk materials and gas processing. Typical industrial applications 
include textile, chemical, energy and hydrocarbon processing. These heat 
transfer fluids are typically useful in temperature ranges between about 
350.degree. to about 800.degree. F. There are two types of fluids, the so 
called hot oils derived from petroleum and synthetic fluids such as 
aromatic fluids sold under the designation DOWTHERM by The Dow Chemical 
Company. 
When exposed to high temperatures, such heat transfer fluids undergo 
physical and chemical changes that shorten the useful fluid life and 
result in a loss of useable fluid. Such thermal degradation of the fluids 
may also reduce the heat transfer efficiency and system safety while at 
the same time increasing operating costs and downtime. 
The heat transfer fluids are typically heated as they flow through heater 
or heating tubes of a heat exchanger apparatus. In such cases, the liquid 
flow within the heater tube is characterized by a temperature profile that 
includes a relatively higher film temperature at radially outward regions 
in the heater tube and a relatively lower bulk fluid temperature at 
radially inward regions adjacent the central core region of the tube. The 
film temperature may be 50.degree. to 75.degree. F. higher than the bulk 
fluid temperature. 
The relatively higher film temperature tends to limit the designed upper 
heating temperature and heat flux transfer in such heating apparatus and 
applications. More particularly, the possibility of heat flux variations 
and associated higher operating temperatures require a buffer or safety 
range between the designed operating temperature and the upper thermal 
limit of the heat transfer fluid. For similar reasons, it is desirable 
that the heater tube not be directly impinged by a heating flame since 
this may result in local fluid temperature increases of several hundred 
degrees Fahrenheit and sometimes leave a deposit on the interior wall of 
the heater tube. Such deposits decrease the heat transfer and lower the 
heat transfer efficiency. 
The foregoing difficulties in maintaining a design operating temperature 
substantially at or near the upper thermal limit of the heat transfer 
fluid are further complicated in a heating application requiring a turn 
down capability. In such cases, the required safety margin for heat 
transfer at high operating temperature conditions may result in an 
extremely inefficient or slow response at low operating temperature 
conditions. 
It is also desirable that the heat exchanger be compact since it will often 
comprise an ancillary device or a component of a more comprehensive 
apparatus. Thus, it is not desirable to merely extend the Spatial 
arrangement of the heating flame and the heater tube with variable flame 
operation along the extent of the tube to accommodate turn down. 
Similarly, the prior art use of excess air to suppress flame temperatures, 
reduced BTU input rates, shorter flame lengths and/or separate combustion 
chambers are not satisfactory techniques for avoiding thermal degradation 
of the heat transfer fluid since such techniques also result in increased 
heat exchanger size and inefficient operation and decreased response times 
also result. 
Heating apparatus including a central heat flame and a surrounding heater 
tube array is disclosed in U.S. Pat. Nos. 4,793,800, 4,723,513, 4,473,034, 
4,444,155 and 4,338,888. U.S. Pat. No. 4,679,528 discloses a heating 
boiler having a central heating flame and a surrounding coil heater tube. 
The products of combustion pass radially through the heater tube coil, and 
are discharged through an axially extending vent. 
SUMMARY OF THE INVENTION 
It has now been discovered that reradiative and convective heat transfer 
may be used in combination to efficiently transfer heat to a fluid with 
relatively low levels of heat flux and correspondingly low flame 
temperatures in order to suppress and/or avoid excess fluid temperatures 
and, in the case of heat transfer fluids, thermal degradation of the heat 
transfer fluid. The heat transfer apparatus and process are characterized 
by a uniformity of heat flux distribution and lower maximum operating 
temperatures that enable the use of a heater tube having fins to further 
enhance heat transfer. 
In accordance with the invention, an apparatus and method are provided for 
transfer of heat to a heat transfer fluid using a central burner flame, a 
surrounding heater tube array containing fluid to be heated and a heat 
shield or barrier judiciously positioned between the flame and heater tube 
array. The heat shield reduces the maximum flame temperature and 
reradiates thermal energy to the tube array while preventing direct flame 
impingement and contact. The flow of hot products of combustion from the 
flame is subsequently directed around the heat shield into direct contact 
with the heater tube array for convective heat transfer therewith. 
The uniformity of the heat flux distribution and lower maximum operating 
flame temperature enable the use of a heater tube array having fins to 
further enhance the heat transfer. In the absence of the heat shield to 
reduce heat flux and temperature as well as direct flame impingement, 
conventional finned heater tube coil arrangements will be damaged and/or 
induce hot spots at which the temperature exceeds the thermal degradation 
temperature of the heat transfer fluid. Thus, an important aspect of the 
invention is the realization that a temperature reducing and thermal 
energy reradiating heat shield avoids extremes in the amount of heat flux 
transferred to the heater tube and the corresponding temperature extremes 
so as to enable the use of a finned tube array to enhance heat transfer at 
lower temperatures and thereby better maintain a relatively high overall 
heat transfer efficiency. 
An important aspect of the invention is the achievement of a relatively 
compact heat exchanger or fluid heater apparatus. Compactness is achieved 
by recognition that a net decrease in the heat exchanger size may be 
effected by the trade off between (1) the decrease in heat exchanger size 
due to an increase in heat transfer with the use of a finned heater tube 
array and (2) the increase in heat exchanger size due to the lower more 
uniform heat flux and correspondingly lower temperature difference driving 
the transfer of heat. In other words, from the view point of relative heat 
exchanger size, the benefits in total heat transfer achieved by the use of 
reradiation and fins exceed the losses in heat transfer due to lower heat 
flux and flame temperature, and thereby enable compactness.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIG. 1, a heat exchanger or fluid heater apparatus 10 includes 
as its main elements or components a burner 12, a heater tube array 14 and 
a heat shield or barrier 16 mounted within a housing 18. The housing 18 
may be heat insulated with a suitable material such as fibrous insulation 
18a and enclosed within a metallic cabinet wall 18b. 
The housing 18 has a generally cylindrical shape extending about a 
longitudinal axis "A". The housing 18 includes a sidewall 20 which is 
closed at its opposite ends by a proximal end wall 22 and a distal end 
wall 24. The walls 20, 22 and 24 cooperate to define a substantially 
closed combustion and heat transfer chamber 26. The walls 20, 22 and 24 
may be formed of a suitable heat resistant metal such as 304 stainless 
steel. 
The burner 12 is mounted adjacent the proximal end wall 22 of the housing 
18. The burner 12 comprises a powered burner arranged to burn a supplied 
fuel such as natural gas. The burner 12 includes a nozzle arrangement 28 
adapted to provide a flame 30 extending along the longitudinal axis "A" of 
the housing 18. The burner 12 is preferably a swirl flame burner that 
provides the flame 30 with a swirl pattern as indicated by the arrows "F". 
The flame 30 should contact the heat shield 16 along a major portion or 
substantially all of its longitudinal length, but the flame 30 should not 
impinge upon the distal end wall 24 of the housing 18, and preferably 
should not extend beyond the longitudinal length of the heat shield 16. 
The burner 12 should be sized for the particular heating application, and 
typical heater input ratings may range from about 50,000 to about 500,000 
Btu/hour. 
The heat shield 16 has a generally cylindrical shape, and it is 
concentrically disposed about the longitudinal axis "A" within the housing 
18. The heat shield 16 comprises a relatively thin wall 32 formed into an 
open-ended cylinder having an inner wall surface 32a and an outer wall 
surface 32b. The heat shield wall 32 is continuous and substantially 
imperforate, and it may be formed of a suitable flame shielding and 
reradiating material such as 304 stainless steel. 
The heat shield wall 32 surrounds the burner nozzle 28 and substantially 
restricts or confines the flame 30 so as to prevent flame impingement of 
and/or contact with the heater tube array 14. To that end, the heat shield 
16, and more particularly the wall surface 32a thereof, provides a conduit 
shaped, flame receiving region 34 surface 32a. The heat shield 16 and wall 
32 are arranged to provide the region 34 with sufficiently large 
dimensions to enhance radiation heat transfer to the wall surface 32a 
since radiation is proportional to the length of the radiation beam. For 
example, the region 34 may be about 12 inches long and have a diameter of 
about 9.5 inches to provide a region volume of about 850 in..sup.3 for a 
burner 12 having a rating of 150,000 Btu/hour. During operation, the wall 
32 is heated to a red glowing temperature with the highest wall 
temperatures adjacent the proximal end of the wall. 
The heater tube array 14 has a cylindrical coil configuration including a 
heat transfer tube 36 arranged in a spiral pattern around the heat shield 
16. The tube 36 has an inlet end connected to fluid inlet 38 and an outlet 
end connected to fluid outlet 40. The fluid inlet 38 is positioned 
adjacent the proximal end wall 22 of the housing 18 and the fluid outlet 
is positioned adjacent the distal end wall 24 of the housing 18 so that 
the fluid flows in a generally upward direction (as shown in FIG. 1) 
toward the distal end wall 24 of the housing 18. 
The tube 36 includes fins 42 radially extending from the exterior tube 
surface for enhancing convective heat transfer to fluid flowing within the 
tube. The fins 42 may be integrally formed with the tube 36 or 
subsequently applied thereto. The cross-section of the tube 36 may have an 
outside diameter of about 0.75 inches and an outside fin diameter of about 
0.875 inches. In the illustrated embodiment, a total of about 12 wraps of 
the tube 36 about the heat shield 16 are used to provide a heat exchange 
area of about 1550 in..sup.2 or 10.76 ft..sup.2 The tube 36 and the fins 
42 may be formed of 304 stainless steel as an integral construction, and 
satisfactory finned tubes are commercially available. 
The coil formed by the tube 36 is centrally positioned with an annular 
passageway 44 formed between the outer shield wall surface 32b and the 
sidewall 20 of the housing 18. The passageway 44 is sized to receive the 
coil of the tube 36 with about 0.125 inches of clearance on each side of 
the adjacent extremities of the fins 42. (For clarity of illustration, the 
clearance is exaggerated in FIG. 1.) 
The distal end wall 24 operates to direct the hot products of combustion 
from the region 34 around the distal end of the wall 32 and into the 
passageway 44 for direct contact and convective heat transfer with the 
heater tube array 14 as shown by the arrows "P". A vent 46 is provided 
adjacent the proximal end of the passageway 44 for discharge of combustion 
products from the chamber 26 to the atmosphere. 
The tube array 16 is heated by reradiation of thermal energy from the outer 
wall surface 34b and convective counter current heat transfer from the 
products of combustion as they contact the tube 36 and fins 42 during flow 
through the passageway 44. 
In the illustrated embodiment, the heat exchanger apparatus or fluid heater 
10 has a generally cylindrical shape, the housing 18 being about 15.5 
inches in diameter and 13 inches long. The combustion and heat transfer 
chamber 26 is centrally disposed in the housing 18 about the longitudinal 
axis "A" and it has a diameter of about 11.5 inches and a length of about 
13 inches. The burner 12 is mounted to the proximal end of the housing 18. 
The heat shield 16 and tube 36 are concentrically mounted about the 
longitudinal axis "A" of the housing 18. The heat shield 16 or wall 32 has 
a diameter of about 9.5 inches and a length of about 12 inches. The distal 
end of the heat shield 16 is spaced about one inch from the distal end 
wall 24 of the housing 18. 
The annular passageway 44 defined between the outside wall surface 32b and 
housing sidewall 20 has a radial dimension of about 1.125 inches. The tube 
36 wrapped with fins 42 has a diameter of about 0.875 inches. Assuming the 
cross-sectional area of the tube 36 and fins 42 to block the fluid flow 
through the passageway 44, the available flow area through the passageway 
44 is about 8.13 in..sup.2 In comparison, the flow area through the region 
26 is about 67.17 in..sup.2 so that the reduction of flow area is in a 
ratio of about 8:1, that is the flow area is reduced by about 88%. This 
reduction of flow area substantially accelerates the combustion products 
passing from the region 34 through the passageway 44 and thereby increases 
the coefficient of heat transfer and the amount of heat transferred to the 
fluid flowing within the tube 36. 
The heat exchange apparatus 10 was used to heat Dowtherm J heat transfer 
fluid from an inlet temperature in the range of from about 300.degree. to 
450.degree. F. to an outlet temperature of from about 450.degree. to about 
465.degree. F. in a single pass through the apparatus at flow rates of 
from about 3.5 to about 8.0 gal/min. The maximum heat transfer fluid film 
temperature is 650.degree. F. In this application the burner 12 is 
operated at a maximum fuel input of 125,000 Btu/hour with a turn down 
ratio of 1 to 5 to accommodate varying thermal energy requirements as 
determined by the required fluid temperature increase and fluid flow rate. 
The hydraulic resistance to the flow of the thermal fluid is about 4.2 
psi, and the pressure drop of the combustion products is about 1" H.sub.2 
O. The foregoing applications were achieved with no significant thermal 
degradation of the heat transfer fluid. Further, a thermal efficiency of 
about 80% was attained without condensation of the products of combustion. 
Referring to FIG. 2, a comparison of the heat flux to the heat transfer 
tube 36 with and without the use of a heat shield 16 in the fluid heater 
10 is graphically shown. As indicated, the elimination of the heat shield 
16 results in significantly higher and lower levels of heat flux and 
temperature extremes at spaced locations along the length of the tube 
axis. The higher heat flux locations give rise to an increased risk of 
thermal degradation of the heat transfer fluid due to the higher 
temperatures associated with such higher heat flux levels. The lower 
levels of heat flux resulting without the use of the shield may be 
considered to be unacceptable, less efficient locations of heat transfer 
as compared with the lowest levels of heat flux attained with the use of 
the heat shield. Accordingly, the use of the heat shield 16 results in a 
more uniform heat flux distribution characterized by a decreased risk of 
thermal degradation and relatively acceptable levels of heat transfer 
efficiency at all locations along the axial length of the tube 36.