techology » heat transfer

Compact heat exchangers classification

techicien — Sat, 04 Sep 2010 08:38:03 +0000

Compact elements usually fall into some classes including:

1. Array or matrix surfaces. These are the surfaces that are used in regenerative equipment rotation, such as the burning flue gas – air pre heaters for fossil conventional ovens. In this application, the metal is distributed to its capacity to absorb heat with minimal friction hot fluid while exposed to smoke and to dispense with this cold combustion heat incoming air when it is rotated in the flow of incoming cold air. No designation is used.

2. flattened and circular tubes. These are the simplest form of compact heat exchanger surface. The designation ST indicates flow inside pipes straight (example: ST-1), FRANCE TÉLÉCOM indicates flow inside pipes flattened straight (example: FT-1) and FTD indicates flow inside straight tubes dimpled flattened. Dimple stops the boundary layer, which tends to increase the heat transfer coefficient without increasing the speed of the stream.

3. Plate fin surfaces.

4. Finned-tube surfaces. Circular Tubes with radial spiral fins are designated with the letters CF followed by one or two numbers. The first digit indicates the number offins per inch and the second (ifone is used) refers to the nominal pipe size. With circular tubes with fins continuous, is not used any prefix letter and two numbers have the same meaning as for circular tubes with radial ins spiral. For finned tubes of dishes, do not use any prefix letter; the first digit indicates the fins per inch and the second digit indicates the largest dimension of the tube. When CF don’t appear in the designation of the circular tube with fins radial spiral, the surface shall be presumed to have fins continuous.

5. Surfaces with normal flow smooth tube banks. Unlike low radial fin tubes, smooth ducts are expanded in fins that can accept a number oftube rows, as shown in fig. 11.16a. Holes can be applied in the fin with a hub designed or foot to improve contact resistance or as a spacer between successive fins, as shown, or soldered directly to the pinna with or without a hub. Other types of reduce the resistance to flow out of the tubes through flattened tubes and brazing, as shown in Figure b and c below. Tube plate is done by strips similar to manufacture of circular welded pipes, but is much more subtle and is joined by welding or brazing, rather than welding. The designation considers staggered (S) and in-line (I) agreements oftubes and identifies relationships cross-sectional and longitudinal pitch. The suffix (s) indicates data correlation from steady-state test. All other data were related by a temporary technique. Examples include the surface S1.50-1.25 (s), which is a staggered arrangement with the data obtained via stationary test with of1.50 ratio pitch-a-transversal and longitudinal pitch diameter-to-diameter ratio of1.25. The surface I1.25-1.25 has a provision in line with the data obtained from the transient tests with both of1.25 reports of pitch-a-longitudinal and transverse diameter.

6. Tubular surfaces. These are arrays oftubes ofsmall diameter, from ” 0.9535 down to 0.635 cm, used in service where robustness and cleanup of conventional shell-and-tube heat exchanger are not required. Usually, tubesheets are relatively thin and welding or brazing a tube for a tubesheet provides a suitable seal against interleakage and differential thermal expansion.

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Evaporation

techicien — Thu, 22 Apr 2010 09:41:19 +0000

What is Evaporation ?

Evaporation is the removal of solvent as steam from a solution or slurry. For the vast majority of systems of evaporation of the solvent is water. The goal is usually to focus a solution; Consequently, the vapor is not the desired product and can or cannot be retrieved depending on its value. Therefore, evaporation is usually obtained by spraying a portion to produce a concentrated solution, liqueur often or slurry of solvents.

Evaporation often invades denominated transactions distillation, drying and crystallization. In evaporation, no attempt to separate the components of steam. This distinguishes evaporation distillation. Evaporation is distinguished by drying as the residue is always a liquid. The desired product can be a solid, but the heat must be transferred into the evaporator a solution or solid suspension in a liquid. The liquid can be highly viscous or a leachate. Evaporation differs from crystallization in evaporation takes care to concentrate a solution rather than producing or building crystals.

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component of evaporators

techicien — Mon, 05 Apr 2010 02:54:36 +0000

Concern in the design of the evaporator in three main elements: transfer, steam-liquid separation and energy efficient heat. The units in which heat transfer takes place are named heating units or calandrias. The vapor-liquid separators are called bodies, heads of steam or notes of Flash.

The body of the term is also used to label the base module of building an evaporator, consisting of a heating element and a camera flash. An effect is boiling a body or bodies at the same pressure. One evaporator multiple effect is a system of evaporator vapor by an effect is used as the heating medium for a subsequent boiling to a lower pressure. The effects can be staged when concentrations of liquids in fact permits; staging is two or more sections operating at different concentrations in a single effect. The evaporator term denotes the whole system of effects, not necessarily a body or an effect.

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what an evaporator does

techicien — Tue, 16 Mar 2010 03:01:23 +0000

As mentioned previously, the object of evaporation can be to concentrate a solution containing the desired product or to retrieve the solvent. Sometimes both may be carried out. Design Evaporator consists of three main elements: efficient use of energy, vapor-liquid separation and heat transfer.

In most cases the solvent is water, heat supplied from condensation of steam and the heat is transferred from the indirect heat transfer through metal surfaces. For evaporators be efficient, selected and used equipment must be able to make several things:

1. transfer of large amounts of heat for the solution with a minimum amount of metal surface. This requirement, more than all other factors, determines the type, size and cost of the evaporator.
2. to achieve the specified separation of liquid and vapor and do so with the simplest of devices available. Separation may be important for several reasons: the value of the product otherwise loss; pollution; fouling equipment downstream they contacted the steam; corrosion of this same downstream equipment. Inadequate Separation can also cause problems of pumping or inefficient operation due to undesired recirculation.
3. to make efficient use of energy. This can take many forms. Evaporator performance often are evaluated on the basis of steam economics-pounds of solvent evaporated per pound of steam used. The heat is needed to increase the temperature of the feed from the specified starting value to the boiling liquid, to provide the energy needed to separate the solvent liquid feed and to vaporize solvent. The largest increase in the economy of energy you can reuse the solvent vaporized as heating medium. This may be achieved in various ways to be discussed later. Energy efficiency can be increased for the exchange of heat between the feed enter and leave residues or condensation.
4. meeting the conditions imposed by the liquid is evaporated or solution be concentrated. Factors that should be considered include the quality of the product, Salting and sizing, corrosion, foaming, degradation of the product, holdup, and the need for specific types of construction.
Today many types of evaporators are in use in a wide variety of applications. There is no set rule regarding the selection of types evaporator. In many fields of different types are used in a satisfactory manner for identical services. The final selection and design often can result from tradition or previous experience. The wide variation of the characteristics of the solution expand operation evaporator and design by heat transfer simple A separate art.

Characteristics of liquid

techicien — Sat, 13 Mar 2010 13:12:36 +0000

Concentration

properties of the feed to an evaporator cannot exhibit unusual problems. However, as the liquor is concentrated, can drastically change the properties of the solution. Density and viscosity may increase with solid content until the heat transfer performance is decreased or the solution becomes saturated. Boiling continues a saturated solution can cause form crystals which often must be removed to avoid deficiencies or soil surface heat transfer. The boiling point of a solution increases significantly as is concentrated.

Blowing

Some materials can foam during steaming. Stable Foams may cause excessive swipes. Foaming may be caused by gases dissolved in liquor, from a loss of air under the level of the liquid and the presence of surfactants or finely divided particles in liquor. Many antifoaming agents can be used effectively. Foams can be suppressed by operating at low levels of liquids, mechanically or hydraulic methods.

Temperature sensitivity

many chemicals are compromised when heated to moderate temperatures for relatively short time. When the evaporation of those materials, special techniques are needed to control the characteristics of the time/temperature evaporator system.

Salting

Salting refers to the growth of a matter that a solubility increases with increasing temperature on surfaces evaporator. Can be reduced or eliminated by keeping the evaporation of liquid in close or frequent contacts with a large surface area of solid crystallized.

Scale

scale is growth or deposition on the surfaces of insoluble or heating has a solubility that decreases with increasing temperature. It may also derive from a chemical reaction in the evaporator. Both in scale and salting liquids usually best are handled in a rotary evaporator not based on boiling for operation.

Fouling

Fouling is the formation of deposits several meeting rooms or scale. They may be due to corrosion, solids entering feed or deposits formed by side heating medium.

Corrosion

Corrosion might influence the selection of the type of evaporator because expensive building materials indicate evaporators offering high rates of heat transfer. Corrosion and erosion are often more serious in evaporators compared to other types of equipment for the high speed of liquid and vapor, the frequent presence of suspended solids and concentrations requests.

Quality product

quality may require holdup low and high temperatures. Low-holduptime requirements may eliminate the application of certain types of evaporator. Quality of the product may also dictate special building materials.

Other properties of Fluid

other smooth property must be considered. These include: heat solution, toxicity, explosion hazards, radioactivity and ease of cleaning. Salting, resizing and dirty the result steadily heat transfer speed, until the evaporator must be shut down and cleaned. Some deposits can be difficult and costly to remove.?

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evaporator’s improvement

techicien — Wed, 10 Mar 2010 04:52:47 +0000

Many improvements in technology in the last half century evaporator. The improvements have taken many forms, but have served to do the following:

1. greater evaporation capacity through a better understanding of the mechanisms of heat transfer.
2. better economy through a more efficient use of evaporator types of 3. Longer between cleaning Cycles due to a better understanding of salting, scaling, and smudge.
4. more convenient unit costs from modern production techniques and unit size.
5. to reduce maintenance costs and quality of the product better use of the best materials of construction of better understanding of corrosion.
6. more logical application of specific services evaporator types.
7. a better understanding and application of techniques of control and improvement of instrumentation resulted in better product quality and reduced energy consumption.
8. increased efficiency resulting from heat transfer surfaces more and better energy economy.
9. Compressor technology and availability has allowed for the application of mechanical compression.

Heat transfer in evaporators mode

techicien — Mon, 08 Mar 2010 04:49:29 +0000

The literature of heat transfer in general recognize three distinct modes of heat transfer: conduction, convection and radiation. Strictly speaking, conduction and radiation should be classified as heat transfer processes, because only these two mechanisms depend for their operation on the mere existence of a temperature difference. The last of the three, convection leves, is not strictly conform to the definition of heat transfer because it depends for its operation on mechanical mass transport. But because the convection fulfills even the transmission of energy from regions of high temperature regions to lower temperatures, the term “transfer of heat from the convection” has become generally accepted in most situations heat flows not one, but many of these mechanisms at the same time.

Conduction is the transfer of heat from one part of a body in another part of the same body or a body more on physical contact with it, without appreciable particle displacement of the body. Conduction can occur in solid, liquid or gas.

Radiation is the transfer of heat from one body to another, not in contact with it, by means of electromagnetic waves moving through space, even when a gap that exists between them.

Convection is the transfer of heat from one point to another within a fluid, gas or liquid, the mixing of an aliquot with another. In natural convection, the movement of the fluid entirely is the result of density differences resulting from differences in temperature; in convection, the movement is produced by mechanically. Forced when the speed is relatively low, it should be realized that “freeconvection” factors such as the density and temperature difference, can have a major influence.

In solving problems of heat transfer, it is necessary not only for the heat transfer mode which play a role to recognize, but also to determine whether a process is constant or Unsteady. When the rate of heat flow in a system does not vary with the time-when is constant-temperature anywhere not edit conditions prevail and stationary. At steady state conditions, the rate of heat input at any point of the system should be exactly equal to the rate of return of heat and no change in internal energy can take place. Most of the problems of transfer of heat engineering concern steady state systems.

The flow of heat in a system is temporary or unstable when temperatures at various points of the system change with time. Since a change of temperature indicates change internal energy, we conclude that the accumulation of energy is associated with unstable heat flux. Problems of heat-irregular are more complex than are those of steady-state and often can only be solved by methods approximate.

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type of heat transfer operations

techicien — Sat, 06 Mar 2010 04:47:56 +0000

There are two types of heat transfer operation: heat sensitive and phase change. Heat sensitive Operations involve heating or cooling of a fluid in which modify the results of transfer of heat only at a temperature of the fluid. Results of thermal transfer Changeof-phase in a liquid is changed to a steam or vapour is modified into a liquid. Boiling or steaming is the convection process involving a phase change from liquid to vapor. Condensation is the convection process involving a change in liquid-vapor phase. Many applications involve both sensible heat and heat transfer phase change.

Sensible heat transfer Inside Tubes Sensible heat in most applications involves forced convection inside pipes or ducts or convection in external surfaces.

The heating and cooling fluid flowing inside extensions are among the most important processes of transfer of heat in engineering. The flow of fluids in pipes can be subdivided into three flow regimes. These flow regimes are measured in a report called the Reynolds number is an indication of the turmoil of the flow within the duct. The three schemes are: less than 2,100 Transition Flow Reynolds numbers between 10,000 and 2100 turbulent flow numbers greater than 10,000 Reynolds numbers laminare Reynolds

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Flow-Induced Vibration in Evaporations

techicien — Sun, 28 Feb 2010 16:06:21 +0000

Concern for the flow-induced vibration of the tube has become a serious consideration in the design of shell-and-tube equipment. These problems can lead to pipes and tubejoints that loss, increased pressure drop and noise shellside intolerably high. The result is that equipment must be removed from service for repair and modification.

Flow-induced vibrations can damage the tubes in evaporators. All tubes vibrate in all flow conditions! However, we are concerned with the vibrations that cause significant damage to the pipe. As larger evaporators, bring more speed and more shellside become more prevalent, the vibrations of the pipe damage are more likely to occur. N. evaporator design is complete without considering the possibility of damage due to flow induced vibrations.

The damage is most likely to occur with gases or vapors on shellside than with liquids. Flow induced vibrations also occur with liquids on shellside, but the damage is often limited to localized areas of relatively high speed. In severe cases, the pipes can leak in a few days or even hours after the device was placed in service. More often, the damage will appear a year after startup. Give the tube will develop after the initial damage was repaired, but the number and frequency of further damage will diminish with time.

In a number of cases, by a heat exchanger tube failures attributed to vibrations flowinduced consequential damages caused to other devices within a plant. Failures of this type have proven to be the most destructive, most costly, and required the closure of plants longer for the correction.

Currently available methods for the prediction of flow induced vibration damage is not sufficient to predict failures. At best, they identify the equipment that are susceptible to damage. The main reason for this lack of precision is that the flow induced vibrations are extremely complex. We learned a lot, but the probability of its occurrence is not yet known. However, the penalty cost for equipment designed to completely avoid the harmful vibrations is small and is almost always easily justified.

Some problem areas in question, with the prediction of vibration include:

1. the complex pattern of flow through a tube bundle
2. complex fluid mechanics of a bank of vibrating tubes
3. the role of damping
4. rates of wear and fatigue.

However, it is possible to develop design criteria, especially when tempered with experience, to ensure that the material will be safe from harm vibration.

Flow-induced vibration problems in equipment tubulars are commonly regarded as consisting entirely of mechanical failure of the tubes. However, the vibration may increase the pressure drop shellside, sometimes twice. Furthermore, a unit of acoustic vibration can produce an unacceptably high level of noise. With a growing emphasis on controlling noise, vibration noise must be an important consideration in plant design tubular

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Flow-Induced Vibration mechanism

techicien — Sun, 28 Feb 2010 16:02:41 +0000

Induced vibration of any system requires the coupling of some forces moving with a flexible structure. In the case of flow induced vibration, the exciting forces shellside result from the flow of fluid and flexible structure of the beam tubes. Forces oscillate at frequencies of exciting features that increase continuously with flow rate increasing. The tubes vibrate only single frequency response called their natural frequencies. Attack occurs when the exciting frequency match the frequency response and results of vibration of the tube.

The natural frequency of tubes is primarily dependent on their geometry and material of construction. The intensity of vibration is evidenced by the amount of periodic motion, the scope of this peak to peak on the centreline of motion at-rest is called the amplitude of vibration. The energy must be available to excite the tube vibration. The vibrational energy is dissipated by internal and external damping. The exciting force could be the result of:

1. fluid dynamic mechanisms because of the parallel flow through the pipes or
2. pulsation of a compressor or pump
3. mechanical vibrations transmitted through a structure.

Unless amplified by resonance phenomena, the flow of the forces normally enountered in equipment is not sufficient to cause damage. Resonance, which can increase the deformation of the tube in magnitude, occurs when the frequency of a strong cyclical exciting matches the natural frequency of the tube

In order to predict the occurrence of flow-induced vibration, the phenomena that produces the forces exciting and dynamic response of tubes to be understood. The determination of natural frequencies of the tube is relatively straight-forward. However, the determination of the forces created by the exciting shellside flow of liquid is extremely difficult. Shellside flow in a heat exchanger follows a complex path flow. Undergoes a change of direction, acceleration and deceleration. Sometimes, the flow is not perpendicular to the tubes (crossflow), axially along the tubes (parallel flow), or anywhere in between. Phenomena include flow in crossflow vortex shedding, turbulent buffeting and fluid elastic vortex flow phenomena found in parallel flow formation includesaxial Eddy flux