Chapter Five. Basic construction Concepts. Materials of Construction.
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ChapterFive Basic construction Concepts
Materials of Construction • The materials of construction used in heat exchangers depend on the fluids or vapor being handled, processconditions; such as pressures, temperatures, etc., and a balance of initial cost against expected life and maintenance requirements. • Any component or the entire unit can be made of materials suchas: Lowcarbon steel, Stainless steel, Copper alloys, Copper, Copper-Nickel, Admiralty, nickel, Hast-alloy, Inconel, Titanium-nickel alloys or other special alloys (drawn and seamless, welded, or brazed). • U-tube designs are specified when the thermal difference of the fluids and flows would result in excessive thermal expansion of the tubes. (drawback). • Selection of materials involves careful evaluation of factors other than the basic cost of possible metals compatible with the application.
Economic Considerations in Heat Exchanger Selection • At the design stage there are usually many variables, such as temperatures, pressures, flow rates, etc…, which can be changed within limits. Later on, these factors become set as fixed quantities and have an influence on the size and cost of heat exchange equipment. •Temperatures of heating and cooling media A higher heating media temperature results in a smaller heat exchanger for a given heating load. Limitations of materials must be kept in mind here. •Pressure drops permitted by the system affect heat exchanger size. The highest allowable pressure drop will result in substantial savings in heat exchanger size. •Length restriction sometimes affect heat exchanger costs There are so many exceptions and limiting conditions that affect the choice of the heat exchanger length and it is not simple to say ‘‘the longer the cheaper.” •Materials of construction Corrosive tendencies and purity requirements of fluids being handled. Often, the choice here is based on reliable data and experience.
The end result of choosing the most advantageous combination of all factors is the most economical value for (A) the effective tube surface area. It is common practice to figure A in square meters (foot) using the outside diameter of the tubing instead of an inside or mean diameter. This is merely for convenience, because the area per unit meter (feet) of given O.D. tube is the same for all wall thickness.
Operation • Be sure the entire system is clean before starting operation to prevent plugging of tubes or shell side passages with refuse. • Open vent connections before starting up. • Start operating gradually (see next tables; two). • After the system is completely filled with the operating fluids and all air has been vented, close all manual vent connections. • Re-tighten bolting on all gasketed or packed joints after the heat exchanger has reached operating temperatures to prevent leaks and gasket failures. Standard published torque values do not apply to packed end joints. • Do not operate the heat exchanger under pressure and temperature conditions in excess of those specified on the nameplate. • To guard against water hammer, drain condensate from steam heat exchangers and similar apparatus both when starting up and shutting down. • Drain all fluids when shutting down to eliminate possible freezing and corroding. • In all installations there should be no pulsation of fluids, since this causes vibration and will result in reduced operating life. • Under no circumstances is the heat exchanger to be operated at a flow rate greater than that shown on the design specifications. Excessive flows can cause vibration and severely damage the heat exchanger tube bundle. • Heat exchangers that are out of service for extended periods of time should be protected against corrosion as described in the storage requirements for new heat exchangers.
Fouling of Heat Exchangers • Fouling in heat exchangers represents a major source of performance degradation. • Fouling not only contributes to a decrease in thermal efficiency , but also hydraulic efficiency. The build up of scale or other deposit increases the overall thermal resistance of the heat exchanger core which directly reduces the overall thermal efficiency. • If build up of a fouling deposit is significant, it can also increase pressure drop due to the reduced flow area in the heat exchanger core. • These two effects combined can lead to serious performance degradation. In some cases the degradation in hydraulic performance is greater than the degradation in thermal performance which necessitates cleaning of the heat exchanger on a regular basis. • Fouling of heat exchangers results in a number of ways. The two most common are corrosion and scale build up.
Shell and Tube Heat Exchangers • There are two distinct types of shell and tube heat exchangers, based in part on shell diameter. Designs from 2” to around 12” in shell diameter are available that feature shell constructions of low cost welded steel, brazed pipe with hub forgings, cast end bonnets and copper tubing rolled or brazed to the tube sheet. • This mass production product has had great success with OEM’s of industrial machinery for oil cooling and water to water applications. • Models of this type generally use 1/4” and 3/8” tubing and are frequently 2 or 4 pass for general industrial use. While roller-expanding tubes to thick tube sheets is regarded as good practice, offering easier service, some manufacturers offer a low cost design that brazes the tubes to a thin tube-sheet. By removing end bonnets or covers, most plant-water cooled heat exchangers can be readily serviced by mechanically cleaning the interior of the tubes. Failed tubes can merely be plugged or replaced, depending on the design. • Some manufacturers offer 1/8” tubes mechanically expanded to fins that fill the shell to greatly increase the effective surface and transfer rate.
Shell and Tube Heat Exchangers -2 • The other major type of shell and tube heat exchanger generally is seen in shell diameters from 10” to over 100”. Commonly available steel pipe is generally used up to 24” in diameter. Above24”, manufactures use rolled and welded steel plate, which is more costly and roundness can become an issue. Heat exchangers of this type are commonly manufactured to the standards set forth by TEMA, the Tubular Exchangers Manufacturers Association. • Although there exists a wide variety of designs and materials available, there are components common to all designs. • Tubes are mechanically attached to tube-sheets, which are contained inside a shell with ports for inlet and outlet fluid or gas. They are designed to prevent liquid flowing inside the tubes to mix with the fluid outside the tubes. Tube sheets can be fixed to the shell or allowed to expand and contract with thermal stresses by having one tube sheet float inside the shell or by using an expansion bellows in the shell. This design can also allow pulling the entire tube bundle assembly from the shell to clean the shell circuit of the exchanger.
Tube-sheets Tube-sheets are usually made from a round flat piece of metal with holes drilled for the tube ends in a precise location and pattern relative to one another. Tube sheet materials range as tube materials. Tubes are attached to the tube sheet by pneumatic or hydraulic pressure or by roller expansion. Tube holes are typically drilled and then reamed and can be machined with one or more grooves. This greatly increases the strength of the tube joint. When the tube and tube-sheet materials are joinable, weldable metals, the tube joint can be further strengthened by applying a seal weld or strength weld to the joint.
Shell Assembly • The shell is constructed either from pipe up to 24” or rolled and welded plate metal. For reasons of economy, low carbon steel is in common use, but other materials suitable for extreme temperature or corrosion resistance are often specified. Using commonly available shell pipe to 24” in diameter results in reduced cost and ease of manufacturing, partly because they are generally more perfectly round than rolled and welded shells. Roundness and consistent shell ID is necessary to minimize the space between the baffle outside edge and the shell as excessive space allows fluid bypass and reduced performance. In extreme cases the shell can be cast and then bored to the correct ID.
Impingement Plate In applications where the fluid velocity for the nozzle diameter is high, an impingement plate is specified to distribute the fluid evenly to the tubes and prevent fluid induced erosion, cavitations and vibration. An impingement plate can be installed inside the shell, which prevents installing a full tube bundle, resulting in less available surface. It can alternately be installed in a domed area above the shell. The domed area can either be reducing coupling or a fabricated dome. This style allows a full tube count and therefore maximizes the utilization of shell space.
Fluid Stream Allocations • There are many tradeoffs in fluid allocation in heat transfer coefficients, available pressure drop, fouling tendencies and operating pressure. 1. The higher pressure fluid normally flows through the tube-side. With their small diameter and nominal wall thicknesses, they are easily able to accept high pressures and avoid more expensive, larger diameter components to be designed for high pressure. If it is necessary to put the higher pressure stream in the shell, it should be placed in a smaller diameter and longer shell.
Fluid Stream Allocations 2. Place corrosive fluids in the tubes, other items being equal. Corrosion is resisted by using special alloys and it is much less expensive than using special alloy shell materials. Other tube side materials can be clad with corrosion resistant materials or epoxy coated. 3. Flow the higher fouling fluids through the tubes. Tubes are easier to clean using common mechanical methods. 4. Because of the wide variety of designs and configurations available for the shell circuits, such as tube pitch, baffle use and spacing, multiple nozzles, it is best to place fluids requiring low pressure drops in the shell circuit. 5. The fluid with the lower heat transfer coefficient normally goes in the shell circuit. This allows the use of low-fin tubing to offset the low transfer rate by providing increased available surface.
TUBE SIDE STEAM CLEANING • If the tube-side of the heat exchanger is to be steam cleaned, it is important that the shell-side remains in operation or flooded and equipped with a relief valve. • This will prevent shell liquid from evaporating and possibly increasing chloride concentration to a unsafe level and cause stress corrosion cracking of the tubes. • Also, if the shell fluid is heated, it is possible for the pressure to rise above design conditions and cause leaks.
test Diagram of Straight-Tube Heat Exchanger, Single Baffles. Diagram of U-Tube Heat Exchanger, Single Baffles.
Plate and Frame Exchangers • Plate heat exchangers consist of a series of thin corrugated formed metal plates. Each pair of plates forms a complex passage in which the fluid flows. Each pair of plates are then stacked together to form a sandwich type construction in which the second fluid flows in the spaces formed between successive pairs of plates. These types of heat exchangers provide a compact and lightweight heat transfer surface. As a result of the small plate spacing and corrugated design, high heat transfer coefficients result along with strong eddy formation which helps minimize fouling. Because of the simple construction, they are easily cleaned and find wide use in food processing applications.
Boilers, Condensers, and Evaporators • A condenser and evaporator are heat exchangers in which a change of phase results. In a condenser, a vapor is converted into a liquid, while in an evaporator (and a boiler) liquid is converted into a vapor. Due to the two phase nature of these devices, design is not as straight forward. Two phase fluid flows are much more complex than their single phase counterparts. • Additional understanding of the phase make up and distribution is required to perform the necessary design calculations. In addition, design correlations for two phase flows can be somewhat complicated.