JPS61116292A - Gas-liquid heat-exchanging method and device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION BACKGROUND OF THE INVENTION A wide variety of heat exchange devices have been used to treat exhaust gases from process streams, particularly exhaust gases containing corrosive components such as combustion gases. Such combustion gases typically contain oxides of sulfur and nitrogen that can form highly corrosive acids.

These acids limit the materials used in the heat exchange equipment in the case where the gas is cooled below the dew point.

To compensate for the highly corrosive nature of such gases,
In the past, heat exchange elements have been made of glass, copper, and copper coated with fluorine-containing polymers.

Additionally, very complex devices using fluorine-containing polymer tubing have been suggested, for example in U.S. Pat. No. 3,455,893 to WTT, Hers.

When recovering heat from the exhaust gas from a combustion process, it is always difficult to strike a balance between heat transfer efficiency and pressure drop of the gas stream. Therefore, there has been a continuing need for gas flow heat exchange devices to combine corrosion resistant materials with geometries that provide good heat transfer efficiency with minimal pressure drop.

SUMMARY OF THE INVENTION This invention provides a method for changing the temperature of a gaseous stream by passing the gaseous stream through a heat exchanger maintained at a temperature different from the temperature of the gaseous stream and circulating a heat transfer medium through the heat exchanger. fluoropolymer tubes approximately 3 to 10 mm in diameter in which a heat exchanger is arranged in a row, and the free span between each tube section is approximately 20 to 90 LM, the gas flow being a heat transfer medium. At least five rows of tubes (r
OW), and the tube has a center-to-center distance of approximately 1.25 to 1.25 mm of the tube diameter.
When the gas flow passes through the arrayed tubes, the Reynolds number is approximately 800 to 6000, and the free span to tube diameter ratio is approximately 50.
˜150.

The invention further provides an apparatus for changing the temperature of a gas stream at a predetermined velocity having a flow path and a heat exchanger disposed across the flow path. Approximately 3
~10 m of fluoropolymer tubing, and ca.
Consisting of a free span between each tube section of ~90 cm,
The flow path intersects at least five rows of tubes through which the heat transfer medium is circulated, the tubes being arranged such that the distance from center to center is approximately t25 to 5:0 times the tube diameter, and the free span is Ratio to tube diameter is approximately 50-15
0 and the Reynolds number is selected to be between about 800 and 3000 as the gas flow velocity passes through the arrayed tubes.

DETAILED DESCRIPTION OF THE INVENTION The invention is illustrated in FIG. 1 in which a gas stream 1 passes through a heat exchanger through an inlet 2 and an expansion section 3. The tube 4 of the heat exchanger is stretched between tube sheets 5 and 5A located at each part of the tube. The tubes are separated by spacers for two purposes. One of them is
The purpose is to form a free span in each tube section with a distance of approximately 20 to 9 (1 m) between the tubes; another purpose is to separate successive rows of tubes with a distance of approximately
5~! This is to separate by 1.0 times. After passing through the heat exchanger, the gas stream continues towards outlet 8.

It has been found that a free span of each tube section of about 20 to 905+ causes the tube to undergo low frequency, high amplitude vibrations. This provides the dual benefits of self-cleaning and an increased heat transfer coefficient of the tube. The tube and spacer are shown in more detail in Figure '1Ic2. The spacers 6 are arranged so that a desired free span is maintained between the spacers.
Used with rod 9. The tube passes through an opening 10 in the spacer.

The spacer forms a row of tubes consisting of at least five rows of tubes in the flow direction. It may be arranged in any desired shape, for example square or triangular as shown. In general, a square shape results in less pressure drop in the gas flow, while a triangular shape provides greater heat transfer.

Polymeric tubing may be made from a wide variety of fluorine-containing polymers, which are known for their combination of desirable heat transfer properties, resistance to the corrosive effects of gas flow, and excellent stain resistance. Ta. Particularly suitable are
Tetrafluoroethylene polymer, U.S. Patent No. 2,946
, a copolymer of tetrafluoroethylene and hequinafluoroproherent as described in No. 765,
and copolymers of tetrafluoroethylene and perfluoropropyl vinyl ether as described in U.S. Pat. No. 3,132,123. In addition, other fluorine-containing polymers that can be used in this invention include:
Also included are polyvinylidene difluoride and copolymers of polytetrafluoroethylene and cubrotrifluoroethylene.

In addition, Rayleigh (Re
Conductive particles can also be incorporated into fluorine-containing polymers to further improve heat transfer properties, as described in U.S. Pat. Particularly preferred are graphite particles.

The tube sheet used at the end of the heat exchange unit is
Wizards (Wit), which is also cited here as a reference.
Hers, U.S. Pat. No. 4,315,740.

In operation of this heat exchange device, water or similar heat transfer fluid is circulated through the heat exchanger while a gaseous stream is simultaneously passed across the heat exchange tubes. The Reynolds number of the gas flow passing through the array tubes is preferably about 100.
It is 0-2000. The Reynolds number, as is known, is based on the minimum cross-sectional area multiplied by the tube diameter and the gas density divided by the gas viscosity. The required Reynolds number can be obtained by adjusting the gas velocity entering the array of tubes, by adjusting the size and placement of the tubes, or by a combination of the two methods.

Why heat transfer is improved in this invention, enough K
Although it is not fully understood, the Reynolds number is approximately 8.
Below 00, even if other structural conditions are met, the gas velocity is too small to cause any movement in the flexible tube, causing it to act as if it were rigid. It is considered that As the air velocity increases, the tube begins to flap back and forth and side to side.

This promotes vortex formation and increases the rate of heat transfer between the tube wall and the gas stream. The energy required to produce this movement is itself expressed as a pressure drop greater than the pressure drop due to the tube bundle over a similar row of rigid tubes.

As the gas flow velocity is further increased, the pressure drop and heat transfer rate increase until the flow becomes turbulent at a Reynolds number of 8000.

It is this oscillation that necessitates the material, size, and speed limitations of this invention, which provide greater heat exchange rates than would normally be expected from a rigid tube heat exchanger.

The gas stream heated or cooled according to the present invention can contain a variety of corrosive components, including oxides of sulfur and nitrogen that can form corrosive acids.

The temperature of the gas stream should be about 80-240°C.

Temperatures above 240°C have an undesirable effect on the fluoropolymer tube, while treating the gas stream below 80°C has little practical benefit. This is because even if the gas temperature is lowered well below this temperature, the natural tendency of the gas to warm up will cause the temperature to rise.

This invention will be explained in more detail by the following specific examples.

EXAMPLES AND COMPARATIVE EXAMPLES A test bundle of approximately 650 hollow flexible tubes made of Nato 2-fluoroethylene-perfluorovinyl ether copolymer was inserted into holes in the top and bottom of a transparent acrylic exhibition box measuring 46 cm high and 46 cpa wide. It was made by stretching the tube group through the tube. These tubes were 4.75 mm in diameter and passed through identical clear acrylic spacers inside the test box. These splices can be removed in position out of the air flow path at the top of the test chamber, and the free span of each tube section is 15 to 46 α along the axial length of the tube. It is arranged so that The tubes were arranged with holes arranged in a square shape.

The heat exchange element was made of 20 rows of tubes.

The tubes were spaced apart by a distance of approximately 2.0 tube diameters. The ends are joined together and the U.S. Pat.
Tubesheets were formed as described in , 315,740.

The module was placed in a wind tunnel and pressure drop measurements of approximately 400
The experiments were carried out at air flow rates corresponding to Reynolds numbers in the range ˜10,000.

After installation in the wind tunnel, a steam supply tube was connected to the upstream tubesheet so that saturated steam at a pressure of up to 30 psig was introduced into the tube bundle to heat the air. Condensed water vapor from the tube bundle was discharged through a drain trap. Pressure, flow and temperature measurements were made so that the overall heat transfer coefficient and pressure drop across the tube bundle could be determined under various air velocity conditions.

Pressure measurement was performed using an inclined manometer (oil density = 829 Kf/ml
) was used. The air velocity across the tube bundle is
Measurements were made with an industrial anemometer calibrated with a DI8A scientific thermostatic anemometer. From the tube to the air.

The heat flow was detected by measuring the temperature of the air stream with a J-type couple before and after the heat exchange module. The heating steam pressure used was 7 paig and 15 paig. Air flow rates were varied over a range spanning laminar, transition-state, and turbulent flow conditions.

The pressure drop was calculated as a dimensionless number according to the following formula: where Δ9 is the pressure drop, Pascal N is the number of tube rows in the flow direction, P is the air density, and KIl/m5 is the minimum cross-section within the tube bundle. The air flow velocity, m7sec, based on the overall heat transfer coefficient, UOl, was determined experimentally under each condition. The following formula was then used to back-calculate the external air film coefficient: Q = UOA (LMTD) = = WCpΔT where A is the heat transfer area, m2 LMrD is the logarithmic mean temperature difference, CW is the air flow rate, tbAr g Cp is the air heat capacity, 1°C. ΔT is the difference in air temperature between the inside and outside of the tube bundle, the tube equipment conductivity tw is the wall thickness Do, D1ij is the outer diameter and inner diameter hi of the tube, which are known engineering correlations. A consistent SI unit with internal film coefficients calculated from .

The external thin film coefficient, hOl, is expressed as a dimensionless number Jb.

f' and Jb are shown in Figures 3.4 and 5 as functions of Reynolds number/Re.

-DoVP Re-- Vibration of the μ tube was observed when the Reynolds number of the air flow was about 800 or higher. The amplitude of the vibrations was visibly large, often more than twice the tube diameter.

The results of the tests are summarized in Figures 3-5.

The observed pressure drop and heat transfer rate across the vibrating tube bundle are in the transition state region (Re = 800-5000
), higher than the values predicted by literature correlations based on rigid metal tubes.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of the heat exchange device of the present invention. Second
1 is a perspective view of a tube bundle that can be used in the apparatus of FIG. 1; FIG. FIG. 3 illustrates the relationship between pressure drop and gas velocity in the heat exchange apparatus of the present invention. Figure 4.5 illustrates the relationship between heat transfer and gas velocity in the heat exchanger of the present invention, illustrating the importance of the free span length of the tubes. Outside 2 people Re speed Fig. 4 /? e gas velocity

Claims (1)

[Claims] 1) A method of changing the temperature of a gas stream by passing the gas stream through a heat exchanger maintained at a different temperature than the gas stream and circulating a heat transfer medium through the heat exchanger. comprising fluorine-containing polymer tubes of about 3 to 10 mm in diameter in which a heat exchanger is arranged in a row, and the free span between each tube section is about 20 to 90 cm, and the gas flow is for heat transfer. The medium is circulated through at least five rows of tubes arranged such that the distance from center to center is about 1.25 to 5 times the tube diameter, and the gas flow passing through the arrayed tubes at a velocity such that the Reynolds number is about 800 to 3000, and the free span to tube diameter ratio is about 50 to 150. 2) The method according to claim 1, wherein the fluorine-containing polymer is a copolymer of tetrafluoroethylene and hexafluoropropylene. 3) The method according to claim 1, wherein the fluorine-containing polymer is a copolymer of tetrafluoroethylene and perfluoropropyl vinyl ether. 4) The method of claim 1, wherein the tube further comprises about 5-45% by weight filler particles having a thermal conductivity substantially higher than the fluorine-containing polymer. 5) Claim 4, wherein the conductive filler particles are graphite.
The method described in section. 6) Free span between each tube section is approximately 40-60 cm
The method according to claim 1. 7) The method of claim 1, wherein the tube has a diameter of about 4.0 to 6.5 mm. 8) In a device for changing the temperature of a gas stream having a predetermined velocity, which has a flow path and a heat exchanger disposed across the flow path, the heat exchanger is arranged in a row and has a diameter of about 3 to 10 mm. fluorine-containing polymer tubes and a free span between each tube section of about 20 to 90 cm, the flow path traversing at least 10 rows of tubes through which the heat transfer medium is circulated;
The tubes are arranged such that the distance from center to center is about 1.25 to 3.0 times the tube diameter, and the gas flow passes through the tube array at a velocity such that it has a Reynolds number of about 800 to 3000. , the ratio of free span to tube diameter is about 50 to 150, and the parameters are selected such that the Reynolds number at the velocity of the gas flow through the arrayed tubes is about 800 to 3000. A device that does this. 9) The device according to claim 8, wherein the fluorine-containing polymer is a copolymer of tetrafluoroethylene and hexafluoropropylene. 10) The device according to claim 8, wherein the fluorine-containing polymer is a copolymer of tetrafluoroethylene and perfluoropropyl vinyl ether. 11) The device of claim 8, wherein the tube further includes about 5 to 45% by weight filler particles having a thermal conductivity substantially higher than the fluorine-containing polymer. 12) The device of claim 11, wherein the conductive filler particles are graphite. 13) Free span of each tube section is approximately 40-65 cm
12. The apparatus according to claim 11. 14) The device of claim 11, wherein the tube has a diameter of about 4.0 to 6.5 mm.