Super Heater Tube

A superheater is a device used to convert saturated steam or wet steam into dry steam used for power generation or processes. There are three types of superheaters namely: radiant, convection, and separately fired. A superheater can vary in size from a few tens of feet to several hundred feet (a few metres or some hundred metres).
A radiant superheater is placed directly in the combustion chamber.

A convection superheater is located in the path of the hot gases.

A separately fired superheater, as its name implies, is totally separated from the boiler.

A superheater is a device in a steam engine, when considering locomotives, that heats the steam generated by the boiler again, increasing its thermal energy and decreasing the likelihood that it will condense inside the engine.[1][2] Superheaters increase the efficiency of the steam engine, and were widely adopted. Steam which has been superheated is logically known as superheated steam; non-superheated steam is called saturated steam or wet steam. Superheaters were applied to steam locomotives in quantity from the early 20th century, to most steam vehicles, and to stationary steam engines. This equipment is still an integral part of power generating stations throughout the world.


Locomotive use
In steam locomotive use, by far the most common form of superheater is the fire-tube type. This takes the saturated steam supplied in the dry pipe into a superheater header mounted against the tube sheet in the smokebox. The steam is then passed through a number of superheater elements—long pipes which are placed inside special, widened fire tubes, called flues. Hot combustion gases from the locomotive's fire pass through these flues just like they do the firetubes, and as well as heating the water they also heat the steam inside the superheater elements they flow over. The superheater element doubles back on itself so that the heated steam can return; most do this twice at the fire end and once at the smokebox end, so that the steam travels a distance of four times the header's length while being heated. The superheated steam, at the end of its journey through the elements, passes into a separate compartment of the superheater header and then to the cylinders as normal.

Damper and snifting valve
The steam passing through the superheater elements cools their metal and prevents them from melting, but when the throttle closes this cooling effect is absent, and thus a damper closes in the smokebox to cut off the flow through the flues and prevent them being damaged. Some locomotives (particularly on the London and North Eastern Railway) were fitted with snifting valves which admitted air to the superheater when the locomotive was coasting (drifting). This kept the superheater elements cool and the cylinders warm. The snifting valve can be seen behind the chimney on many LNER locomotives.

Front-end throttle
A superheater increases the distance between the throttle and the cylinders in the steam circuit and thus reduces the immediacy of throttle action. To counteract this, some later steam locomotives were fitted with a front-end throttle—placed in the smokebox after the superheater. Such locomotives can sometimes be identified by an external throttle rod that stretches the whole length of the boiler, with a crank on the outside of the smokebox. This arrangement also allows superheated steam to be used for auxiliary appliances, such as the dynamo and air pumps. Another benefit of the front end throttle is that superheated steam is immediately available. With the dome throttle it took quite some time before the super heater actually provided benefits in efficiency. One can think of it in this way: if one opens saturated steam from the boiler to the superheater it goes straight through the superheater units and to the cylinders which doesn't leave much time for the steam to be superheated. With the front-end throttle, steam is in the superheater units while the engine is sitting at the station and that steam is being superheated. Then when the throttle is opened, superheated steam goes to the cylinders immediately.

Advantages and disadvantages
The main advantages of using a superheater are reduced fuel and water consumption but there is a price to pay in increased maintenance costs. In most cases the benefits outweighed the costs and superheaters were widely used. An exception was shunting locomotives (switchers). British shunting locomotives were rarely fitted with superheaters. In locomotives used for mineral traffic the advantages seem to have been marginal. For example, the North Eastern Railway fitted superheaters to some of its NER Class P mineral locomotives but later began to remove them.

Without careful maintenance superheaters are prone to a particular type of hazardous failure in the tube bursting at the U-shaped turns in the superheater tube. This is difficult to both manufacture, and test when installed, and a rupture will cause the superheated high-pressure steam to escape immediately into the large flues, then back to the fire and into the cab, to the extreme danger of the locomotive crew.

Categories: Economizer, heat exchanger, super heater tube

Economizer

An economiser is a device fitted to a boiler which saves energy by using the exhaust gases from the boiler to pre-heat the cold water used the fill it (the feed water).

Flue gases from large boilers are typically 250 - 350°C. Stack Economizers recover some of this heat for pre-heating water. The water is most often used for boiler make-up water or some other need that coincides with boiler operation. Stack Economizers should be considered as an efficiency measure when large amounts of make-up water are used (ie: not all condensate is returned to the boiler or large amounts of live steam are used in the process so there is no condensate to return.)

It consists of an array of vertical cast iron tubes connected to a tank of water above and below, between which the boiler's exhaust gases are passed. This is the reverse arrangement to that of fire tubes in a boiler itself; there the hot gases pass through tubes immersed in water, whereas in an economiser the water passes through tubes surrounded by hot gases. For good efficiencies, the tubes must be free of deposits of soot.

They are often referred to as feedwater heaters and heat the condensate from turbines before it is pumped to the boilers.

The savings potential is based on the existing stack temperature, the volume of make-up water needed, and the hours of operation. Economizers are available in a wide range of sizes, from small coil-like units to very large waste heat recovery boilers.
Stack Economizers should be considered as an efficiency measure when large amounts of make-up water are used (ie: not all condensate is returned to the boiler or large amounts of live steam is used in the process so there is no condensate to return) or there is a simultaneous need for large volumes of hot water.

What is a Boiler Economizer?

A boiler economizer is a heat exchanger device that captures the "lost or waste heat" from the boiler's hot stack gas. The economizer typically transfers this waste heat to the boiler's feed-water or return water circuit, but it can also be used to heat domestic water or other process fluids. Capturing this normally lost heat reduces the overall fuel requirements for the boiler. Less fuel equates to money saved as well as fewer emissions - since the boiler now operates at a higher efficiency. This is possible because the boiler feed-water or return water is pre-heated by the economizer therefore the boilers main heating circuit does not need to provide as much heat to produce a given output quantity of steam or hot water. Again fuel savings are the result. Boiler economizers improve a boiler's efficiency by extracting heat from the flue gases discharged.

Systems Equipment Corporation Boiler Economizers are fabricated from uniquely formed tubular elements, similar to a tear drop or diamond shape. Each economizer is specifically designed to match our clients boiler characteristics in order to maximize efficiency and the use of boiler room space. Because Systems Equipment Corporation Boiler Economizers are manufactured from stainless steel the usual corrosion problems encountered by our competitions designs are eliminated.

Categories: Economizer, heat exchanger

Heat exchanger

A heat exchanger is a device that is used for transfer of thermal energy (enthalpy)
between two or more fluids, between a solid surface and a fluid, or between solid particulates and a fluid, at differing temperatures and in thermal contact, usually without external heat and work interactions. The fluids may be single compounds or mixtures.
Typical applications involve heating or cooling of a fluid stream of concern, evaporation or condensation of a single or multicomponent fluid stream, and heat recovery or heat rejection from a system. In other applications, the objective may be to sterilize, pasteurize,
fractionate, distill, concentrate, crystallize, or control process fluid. In some heat exchangers, the fluids exchanging heat are in direct contact. In other heat exchangers, heat transfer between fluids takes place through a separating wall or into and out of a wall in a transient manner.
In most heat exchangers, the fluids are separated by a heat transfer surface, and ideally they do not mix. Such exchangers are referred to as the direct transfer type, or simply recuperators.
In contrast, exchangers in which there is an intermittent heat exchange between the hot and cold fluids via thermal energy storage and rejection through the exchanger surface or matrix—are referred to as the
indirect transfer type or storage type, or simply regenerators.Such exchangers usually have leakage and fluid carryover from one stream to the other.
Heat exchangers may be classified according to transfer process, construction, flow arrangement, surface compactness, number of fluids and heat transfer mechanisms or according to process functions.

Types of heat exchangers

1. Shell and tube heat exchanger
2. Plate heat exchanger
3. Adiabatic wheel heat exchanger
4. Plate fin heat exchanger
5. Pillow plate heat exchanger
6. Fluid heat exchangers
7. Waste heat recovery units
8. Dynamic scraped surface heat exchanger
9. Phase-change heat exchangers

Shell and tube heat exchanger

Shell and tube heat exchangers consist of a series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Shell and tube heat exchangers are typically used for high-pressure applications (with pressures greater than 30 bar and temperatures greater than 260 °C).[2] This is because the shell and tube heat exchangers are robust due to their shape.
There are several thermal design features that are to be taken into account when designing the tubes in the shell and tube heat exchangers. These include:
Tube diameter: Using a small tube diameter makes the heat exchanger both economical and compact. However, it is more likely for the heat exchanger to foul up faster and the small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube diameters can be used. Thus to determine the tube diameter, the available space, cost and the fouling nature of the fluids must be considered.                                                                            watch this video

Tube thickness: The thickness of the wall of the tubes is usually determined to ensure:
There is enough room for corrosion
That flow-induced vibration has resistance
Axial strength
Availability of spare parts
Hoop strength (to withstand internal tube pressure)
Buckling strength (to withstand overpressure in the shell)
Tube length: heat exchangers are usually cheaper when they have a smaller shell diameter and a long tube length. Thus, typically there is an aim to make the heat exchanger as long as physically possible whilst not exceeding production capabilities. However, there are many limitations for this, including the space available at the site where it is going to be used and the need to ensure that there are tubes available in lengths that are twice the required length (so that the tubes can be withdrawn and replaced). Also, it has to be remembered that long, thin tubes are difficult to take out and replace.
Tube pitch: when designing the tubes, it is practical to ensure that the tube pitch (i.e., the centre-centre distance of adjoining tubes) is not less than 1.25 times the tubes' outside diameter. A larger tube pitch leads to a larger overall shell diameter which leads to a more expensive heat exchanger.
Tube corrugation: this type of tubes, mainly used for the inner tubes, increases the turbulence of the fluids and the effect is very important in the heat transfer giving a better performance.
Tube Layout: refers to how tubes are positioned within the shell. There are four main types of tube layout, which are, triangular (30°), rotated triangular (60°), square (90°) and rotated square (45°). The triangular patterns are employed to give greater heat transfer as they force the fluid to flow in a more turbulent fashion around the piping. Square patterns are employed where high fouling is experienced and cleaning is more regular.
Baffle Design: baffles are used in shell and tube heat exchangers to direct fluid across the tube bundle. They run perpendicularly to the shell and hold the bundle, preventing the tubes from sagging over a long length. They can also prevent the tubes from vibrating. The most common type of baffle is the segmental baffle. The semicircular segmental baffles are oriented at 180 degrees to the adjacent baffles forcing the fluid to flow upward and downwards between the tube bundle. Baffle spacing is of large thermodynamic concern when designing shell and tube heat exchangers. Baffles must be spaced with consideration for the conversion of pressure drop and heat transfer. For thermo economic optimization it is suggested that the baffles be spaced no closer than 20% of the shell’s inner diameter. Having baffles spaced too closely causes a greater pressure drop because of flow redirection. Consequently having the baffles spaced too far apart means that there may be cooler spots in the corners between baffles. It is also important to ensure the baffles are spaced close enough that the tubes do not sag. The other main type of baffle is the disc and donut baffle which consists of two concentric baffles, the outer wider baffle looks like a donut, whilst the inner baffle is shaped as a disk. This type of baffle forces the fluid to pass around each side of the disk then through the donut baffle generating a different type of fluid flow.

Self cleaning

SHEs are often used in the heating of fluids which contain solids and thus have a tendency to foul the inside of the heat exchanger. The low pressure drop gives the SHE its ability to handle fouling more easily. The SHE uses a “self cleaning” mechanism, whereby fouled surfaces cause a localized increase in fluid velocity, thus increasing the drag (or fluid friction) on the fouled surface, thus helping to dislodge the blockage and keep the heat exchanger clean. "The internal walls that make up the heat transfer surface are often rather thick, which makes the SHE very robust, and able to last a long time in demanding environments." They are also easily cleaned, opening out like an oven where any build up of foulant can be removed by pressure washing.

Monitoring and maintenance

Online monitoring of commercial heat exchangers is done by tracking the overall heat transfer coefficient. The overall heat transfer coefficient tends to decline over time due to fouling.

U=Q/AΔTlm

By periodically calculating the overall heat transfer coefficient from exchanger flow rates and temperatures, the owner of the heat exchanger can estimate when cleaning the heat exchanger will be economically attractive.

Integrity inspection of plate and tubular heat exchanger can be tested in situ by the conductivity or helium gas methods. These methods confirm the integrity of the plates or tubes to prevent any cross contamination and the condition of the gaskets.

Mechanical integrity monitoring of heat exchanger tubes may be conducted through Nondestructive methods such as eddy current testing.

Fouling

A heat exchanger in a steam power station contaminated with macrofouling.

Fouling occurs when impurities deposit on the heat exchange surface. Deposition of these impurities can be caused by:
Low wall shear stress
Low fluid velocities
High fluid velocities
Reaction product solid precipitation
Precipitation of dissolved impurities due to elevated wall temperatures

The rate of heat exchanger fouling is determined by the rate of particle deposition less re-entrainment/suppression. This model was originally proposed in 1959 by Kern and Seaton.

Crude Oil Exchanger Fouling. In commercial crude oil refining, crude oil is heated from 21 °C to 343 °C prior to entering the distillation column. A series of shell and tube heat exchangers is typically used to exchange heat between the crude oil and other oil streams, in order to get the crude to 260 °C prior to heating in a furnace. Fouling occurs on the crude side of these exchangers due to asphaltene insolubility. The nature of asphaltene solubility in crude oil was successfully modeled by Wiehe and Kennedy. The precipitation of insoluble asphaltenes in crude preheat trains has been successfully modeled as a first order reaction by Ebert and Panchal[ who expanded on the work of Kern and Seaton.

Maintenance

Plate heat exchangers need to be disassembled and cleaned periodically. Tubular heat exchangers can be cleaned by such methods as acid cleaning, sandblasting, high-pressure water jet, bullet cleaning, or drill rods.

In large-scale cooling water systems for heat exchangers, water treatment such as purification, addition of chemicals, and testing, is used to minimize fouling of the heat exchange equipment. Other water treatment is also used in steam systems for power plants, etc. to minimize fouling and corrosion of the heat exchange and other equipment.

In industry
A variety of companies have started using water borne oscillations technology to prevent biofouling. Without the use of chemicals, this type of technology has helped in providing a low-pressure drop in heat exchangers.

Heat exchangers are widely used in industry both for cooling and heating large scale industrial processes. The type and size of heat exchanger used can be tailored to suit a process depending on the type of fluid, its phase, temperature, density, viscosity, pressures, chemical composition and various other thermodynamic properties.

In many industrial processes there is waste of energy or a heat stream that is being exhausted, heat exchangers can be used to recover this heat and put it to use by heating a different stream in the process. This practice saves a lot of money in industry as the heat supplied to other streams from the heat exchangers would otherwise come from an external source which is more expensive and more harmful to the environment.

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Categories: heat exchanger