1、Methanation Water CoolersHeat exchangers are mechanical devices designed for the proficient transfer of heat from one fluid matter to another via a solid surface. It is important to note that the fluids themselves never mix but instead are separated by the solid surface. This process has found wide
2、application in the engineering world, but also in everyday household uses such as air conditioning and refrigeration. Probably the most well known heat exchanging device is a cars radiator. Other examples include intercoolers, boilers, condensers, and also pre-heaters. heat exchangers, including a p
3、rocess of convection and conduction to function. In order to properly measure the operation of heat exchangers, both the efficiency as well as the size must be taken into account. Efficiency is most often rated by the measurement of the actual temperature change that both fluids experience, as well
4、as the drop in pressure the heat exchanger evidences. The size of the heat exchanger is determined by the required temperature change forecasted, the speed at which this change is to be accomplished, as well as the allowable pressure drop. A lack of efficiency may point to improper operation of the
5、device, while a sudden drop in efficiency is a clear sign of material failure or input/output strain. Other troubleshooting steps should include proper removal of chemical buildups. Heat Exchangers fall into a number of categories, name parallel-flow, counter-flow, and cross-flow. These classificati
6、ons pertain to the flow definition, ie a parallel-flow heat exchanger allows the fluids to enter the device at the same end and travel through it in parallel mode, exiting at the opposite end. Counter-flow devices force the fluids to enter at opposite ends and also exit opposite from one another. Cr
7、oss-flow exchangers, on the other hand, have the fluids traveling at right angles to one another through the device.一、MethanationA methanation reactor comprising an outer metal casing having a removable lid mounted to the top portion of said casing, said lid having an aperture extending therethrough
8、 for introducing gases to be reacted into the outer casing, an inner casing having cover means and positioned within said outer casing in circumferentially spaced and supported relationship therefrom forming an axially extending vertical gap between said inner and outer casings for the downward vert
9、ical passage therethrough of gases to be reacted, a toroidal catalyst bed positioned in the upper portion of said inner casing, a transverse grid having a plurality of alumina balls located thereon positioned within said casing and supporting said catalyst bed, a heat exchanger assembly mounted with
10、in said inner casing below and in downwardly spaced relationship from said catalyst bed, said heat exchanger assembly including an upper horizontal tube sheet, a lower horizontal tube sheet and a plurality of vertical tubes connected at their respective upper and lower ends to said upper and lower t
11、ube sheets, said heat exchanger assembly also including a plurality of baffles to direct the upward flow of incoming gases within said heat exchanger assembly, said inner casing having a plurality of windows circumferentially spaced at the bottom end thereof below said heat exchanger assembly throug
12、h which gases to be reacted will enter the inner casing from the lower end of the gap formed between the inner and outer casings, insulation means enveloping the outer surface of said inner casing from the top thereof down to the windows therein, a gas outlet tube centrally positioned within the upp
13、er portion of said inner casing, said outlet tube having its lower end mounted to the upper tube sheet of and extending into said heat exchanger assembly and its upper end extending through the beyond said catalyst bed, a central feed tube extending through the aperture in the lid of the outer casin
14、g and through the cover means of said inner casing and terminating at a point just above the upper end of the gas outlet tube for the passage of hot gases into the inner casing of said reactor, and gas discharge means centrally connecting at one end to the bottom of said inner casing and extending t
15、hrough the bottom end of said outer casing to provide an exit for reacted gases from the reactor, whereby the gases to be reached are fed initially through the aperture in the lid into the outer casing and thereafter downwardly through the gap between the inner and outer casings, then through the wi
16、ndows in the lower end of said inner casing and upwardly through the heat exchange assembly and about the tubes for preheating of the gas and thereafter collected in the upper end of the heat exchange assembly and fed upwardly through said gas outlet tube above the catalyst bed and then fed downward
17、ly through the catalyst bed surrounding the gas outlet tube and then into and through the heat exchanger tubes and then downwardly through the gas discharge means in the bottom of the inner casing. 二、Methanation systemDue to the high exothermic character of the methanation reactions the temperature
18、will increase significantly in adiabatic systems. Resultantly, the thermodynamic equilibrium is readily reached but with only limited conversion. To achieve high conversions the temperature must be decreased, ie the reaction heat has to be removed. Typically, this is achieved by internally cooled re
19、actors or by gas recycles as in the commercial processes of eg Haldr-Topsoe and Lurgi. The simplest system, however, comprises a series of (adiabatic) methanation reactors with intermediate heat exchangers. The application of such a system is limited to processes at lower pressures as at higher pres
20、sures the adiabatic temperature increase in the reactors will result in too high temperatures and thermal damage of the catalysts.三、Water CoolersSteam is condensed in a direct-contact heatexchanger from a steamwater mixture on jets of coldwater at a pressure of around 16.0 MPa, with waterbeing heate
21、d to the saturation temperature at the given pressure.Such heat exchangers are being developed for the secondary coolant circuit of the power unit at a nuclear power station (NPS) equipped with a Type BREST-OD-300 lead-cooled reactor. The secondary coolant circuit of this power unit was developed on
22、 the basis of the same thermal scheme as that employed in supercritical-pressure power units at thermal power stations of similar power capacity. However, the temperature of feedwater supplied to the steam generator must not be lower than340C under all operating conditions to prevent lead from solid
23、ifying in the apparatus shell space. A highpressure DCFWH was included in the circuit to meet this requirement. 四、Heat exchanger failureHeat exchangers are commonly used to transfer heat from steam,water, or gases, to gases, or liquids. Some of the criteria for selecting materials used for heat exch
24、angers are corrosion resistance, strength, heat conduction, and cost. Corrosion resistance is frequently a difficult criterion to meet. Damage to heat exchangers is frequently difficult to avoid.The tubes in a heat exchanger transfer heat from the fluid on the inside of the tube to fluid on the shel
25、l side (or vice versa). Some heat exchanger designs use fins to provide greater thermal conductivity. To meet corrosion requirements, tubing must be resistant to general corrosion, pitting, stress-corrosion cracking (SCC), selective leachingor dealloying, and oxygen cell attack in service.Failure:1、
26、Pipe and tubing imperfections2、Welding3、 Fabrication4、 Improper design5、 Improper materials6、 Improper operating conditions7、 Pitting8、 Stress-corrosion cracking (SCC)9、 Corrosion fatigue10、General corrosion11、Crevice corrosion12、Design errors13、Selective leaching, or dealloying14、Erosion corrosionF
27、ailure Analysis:Preventing Fatigue FailureMetal fatigue is caused by repeated cycling of of the load. It is a progressive localized damage due to fluctuating stresses and strains on the material. Metal fatigue cracks initiate and propagate in regions where the strain is most severe.Stress RatioThe m
28、ost commonly used stress ratio is R, the ratio of the minimum stress to the maximum stress (S min /S max ).If the stresses are fully reversed, then R = -1.If the stresses are partially reversed, R = a negative number less than 1If the stress is cycled between a maximum stress and no load, R = zeroIf
29、 the stress is cycled between two tensile stresses, R = a positive number less than 1.Variations in the stress ratios can significantly affect fatigue life. The presence of a mean stress component has a substantial effect on fatigue failure. When a tensile mean stress is added to the alternating str
30、esses, a component will fail at lower alternating stress than it does under a fully reversed stress.Preventing Fatigue FailureThe most effective method of improving fatigue performance is improvements in design:Eliminate or reduce stress raisers by streamlining the part Avoid sharp surface tears res
31、ulting from punching, stamping, shearing, or other processes Prevent the development of surface discontinuities during processing Reduce or eliminate tensile residual stresses caused by manufacturing Improve the details of fabrication and fastening procedures五、Corrosion FailuresCorrosion is chemical
32、ly induced damage to a material that results in deterioration of the material and its properties. This may result in failure of the component. Several factors should be considered during a failure analysis to determine the affect corrosion played in a failure. Examples are listed below:Type of corro
33、sionCorrosion rateThe extent of the corrosionInteraction between corrosion and other failure mechanismsCorrosion is is a normal, natural process. Corrosion can seldom be totally prevented, but it can be minimized or controlled by proper choice of material, design, coatings, and occasionally by chang
34、ing the environment. Various types of metallic and nonmetallic coatings are regularly used to protect metal parts from corrosion.六、Heat Exchanges Fluid ComtibilityWhen selecting a heat exchanger technology, coolant compatibility with wetted surfaces must be considered. A copper fluid path is compati
35、ble with water and most common coolants used within heat exchangers. A copper fluid path is compatible with water and most common coolants used within heat exchangers. A copper fluid path is compatible with water and most common coolants used within heat exchangers.七、Method for detecting leaks in he
36、at exchangers In oil or chemical process equipment wherein process fluids under pressure are cooled by indirect heat exchange with cooling water which is subsequently recycled through a water cooling tower, leakage of process fluidA simple method for detecting such leaks and spotting the responsible
37、 heat exchange equipment consists of passing a sample stream of the cooling water, taken ahead of the cooling tower, through a testing vessel wherein the liquid flow is slowed sufficiently to permit entrained process fluids to separate while a constant water level is maintained in the testing vessel
38、, and visually or instrumentally determining the presence and, if desired, the identity of contaminants separated in the testing vessel. s into the cooling water is objectionable and potentially dangerous. The method of detecting leakage of water-immiscible, liquid or gaseous process fluids which co
39、mprise at least one member of the group consisting of hydrocarbon liquids having a lower density than water and hydrocarbon vapors into cooling water which has passed through at least two pieces of indirect heat exchange equipment wherein the process fluids are at a higher pressure than the cooling
40、water, which comprises continuously withdrawing a sample stream of cooling water downstream from said heat exchange equipment, passing the sample stream through a testing vessel wherein the liquid flow is slowed sufficiently to permit entrained process fluids to separate while a constant water level
41、 is maintained, and analyzing the immiscible fluid separated in said testing vessel to determine its identity in order to locate the source of leakage.八、Heat exchanger foulingAlthough heat exchangers were developed many decades ago, they continue to be extremely useful in many applications requiring
42、 heat transfer. While many improvements to the basic design of heat exchangers have been made over the course of the twentieth century, there still exist tradeoffs and design problems associated with the inclusion of heat exchangers within commercial processes. One of the most problematic aspects as
43、sociated with the use of heat exchangers is the tendency toward fouling. Fouling refers to the various deposits and coatings which form on the surfaces of heat exchangers as a result of process fluid flow and heat transfer. In the case of corrosion, the surfaces of the heat exchanger can become corr
44、oded as a result of the interaction between the process fluids and the materials used in the construction of the heat exchanger. The situation is made even worse due to the fact that various fouling types can interact with each other to cause even more fouling. Fouling can and does result in additio
45、nal resistance with respect to the heat transfer and thus decreased performance with respect to heat transfer. Fouling also causes an increased pressure drop in connection with the fluid flowing on the inside of the exchanger. One type of heat exchanger which is commonly used in connection with comm
46、ercial processes is the shell-and-tube exchanger. In exchangers of this type, one fluid flows on the inside of the tubes, while the other fluid is forced through the shell and over the outside of the tubes. Typically, baffles are placed to support the tubes and to force the fluid across the tube bun
47、dle in a serpentine fashion. Fouling can be decreased through the use of higher fluid velocities. In fact, one study has shown that a reduction in fouling in excess of 50% can result from a doubling of fluid velocity. The use of higher fluid velocities can substantially decrease or even eliminate th
48、e fouling problem. Unfortunately, sufficiently high fluid velocities needed to substantially decrease fouling are generally unattainable on the shell-side of conventional shell-and-tube heat exchangers because of excessive pressure drops which are created within the system because of the baffles. Al
49、so, when shell-side fluid flow is in a direction other than in the axial direction and especially when flow is at high velocity, flow-induced tube vibration can become a substantial problem in that various degrees of tube damage may result from the vibration.九、Heat exchanger development:The development of the current heat exchanger basic trend is: to continue to improve the efficiency of heat transfer equipment, and promote the compactness of the
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