Shell and Heat Tube Exchangers: Research Paper

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Total Length: 6360 words ( 21 double-spaced pages)

Total Sources: 11

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(Wolverine Tube Heat Transfer Data Book, 2009)

Basic components of Shell and Tube Heat Exchangers include the following basic components although there is a plethora of existing specific features used in design of the Shell and Heat Tube Exchanger. The components specifically are:

(1) Tubes -- "...the basic component of the shell and tube exchanger, providing the heat transfer surface between one fluid flowing inside the tube and the other fluid flowing across the outside of the tubes. The tubes may be seamless or welded and most commonly made of copper or steel alloys. Other alloys of nickel, titanium, or aluminum may also be required for specific applications. The tubes may be either bare or with extended or enhanced surfaces on the outside." (Wolverine Tube Heat Transfer Data Book. 2009) Corrugated tubes have been more recently developed and is stated to have heat transfer enhancement both inside and out as well as "...a finned tube which has integral inside turbulators as well as extended outside surface, and tubing which has outside surfaces designed to promote nucleate boiling;

(2) Tube Sheets -- the tubes are inserted into holes in the tube sheet and are held in place through either expansion into grooves cut into the holes or welded to the tube sheet where the tube protrudes from the surface. The tube sheet is usually a single round plate of metal that has been suitably drilled and grooved to take the tubes (in the desired pattern), the gaskets, the spacer rods, and the bolt circle where it is fastened to the shell. The tube sheet, in addition to its mechanical requirements, must withstand corrosive attack by both fluids in the heat exchanger and must be electrochemically compatible with the tube and all tube-side material. Tube sheets are sometimes made from low carbon steel with a thin layer of corrosion-resisting alloy metallurgically bonded to one side;

(3) Shell and Shell-Side Nozzles. The shell is simply the container for the shell-side fluid, and the nozzles are the inlet and exit ports. The shell normally has a circular cross section and is commonly made by rolling a metal plate of the appropriate dimensions into a cylinder and welding the longitudinal joint ("rolled shells"). Small diameter shells (up to around 24 inches in diameter) can be made by cutting pipe of the desired diameter to the correct length ("pipe shells"). The roundness of the shell is important in fixing the maximum diameter of the baffles that can be inserted and therefore the effect of shell-to-baffle leakage. Pipe shells are more nearly round than rolled shells unless particular care is taken in rolling, In order to minimize out-of-roundness, small shells are occasionally expanded over a mandrel; in extreme cases, the shell is cast and then bored out on a boring mill. In large exchangers, the shell is made out of low carbon steel wherever possible for reasons of economy, though other alloys can be and are used when corrosion or high temperature strength demands must be met."

(4) Tube-Side Channels and Nozzles. Tube-side channels and nozzles simply control the flow of the tube-side fluid into and out of the tubes of the exchanger. Since the tube-side fluid is generally the more corrosive, these channels and nozzles will often be made out of alloy materials (compatible with the tubes and tube sheets, of course). They may be clad instead of solid alloy;

(5) Channel Covers. The channel covers are round plates that bolt to the channel flanges and can be removed for tube inspection without disturbing the tube-side piping. In smaller heat exchangers, bonnets with flanged nozzles or threaded connections for the tube-side piping are often used instead of channels and channel covers;

(6) Pass Divider. A pass divider is needed in one channel or bonnet for an exchanger having two tube-side passes, and they are needed in both channels or bonnets for an exchanger having more than two passes. If the channels or bonnets are cast, the dividers are integrally cast and then faced to give a smooth bearing surface on the gasket between the divider and the tube sheet. If the channels are rolled from plate or built up from pipe, the dividers are welded in place.

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The arrangement of the dividers in multiple-pass exchangers is somewhat arbitrary, the usual intent being to provide nearly the same number of tubes in each pass, to minimize the number of tubes lost from the tube count, to minimize the pressure difference across any one pass divider (to minimize leakage and therefore the violation of the MTD derivation), to provide adequate bearing surface for the gasket and to minimize fabrication complexity and cost;

(7) Baffles. Baffles serve two functions: Most importantly, they support the tubes in the proper position during assembly and operation and prevent vibration of the tubes caused by flow-induced eddies, and secondly, they guide the shell-side flow back and forth across the tube field, increasing the velocity and the heat transfer coefficient. (Wolverine Tube Heat Transfer Data Book, 2009 )

Stated to be the only common safety issues that impacts shell and tube heat exchangers is that of overpressures as they are "pressure vessels and as such are subject to the same codes and practices as other pressure vessels." (Wolverine Tube Heat Transfer Data Book, 2009 ) There must be a provision of pressure relief for both the shell and tube sides. If the pressure source is from upstream then the "relief for that stream is best placed on the inlet." (Wolverine Tube Heat Transfer Data Book, 2009) As well, the heat exchanger's purpose is the transfer of heat from one fluid to another therefore instrumentation must be provided to assure this is occurring. This requires a thermometer being placed at each inlet and outlet. The main problems of heat exchangers is plugging and fouling and this is a diagnosis based on differential pressure. Since these connections "...suffer from the same deficiencies as the ones for thermometers. The solution is the same: Put the right connections in the best place in the piping. A shutoff manifold should be used which has a spare port that can be used for a differential pressure indicator." (Wolverine Tube Heat Transfer Data Book, 2009 )

Theoretical Background

A double-pass heat exchanger is stated to be used generally used when a limitation on the installation of the heat exchanger exists. The double-pass heat exchanger is not as efficient as a single-pass exchanger and is furthermore "subject to internal undetectable leakage across the flow divider in the inlet-outlet water box." (Integrated Publishing, 2009) It is considered best practice to maintain log on the heat exchanger performance in order to inform analysis on the heat exchanger performance. Heat exchanger performance can be monitored through observation of the temperature gradient (AT) between the inlets and outlets of the two fluids." (Integrated Publishing, 2009)

Pilot Operated Temperature Control Valves are those that instead of "operating the control valve head movement directly, these units only control a small pilot device which in turn operates the main valve for throttling of the steam flow." (Spirax Sarco, 2009) This device involves the heat sensitive fluid operating a very small valve; mechanism and this in turn operates the primary throttling device therefore the sensing system is physically much smaller in size.

The work entitled: "Steam Consumption of Heat Exchangers" states the term 'heat exchanger' is applicable strictly to "all types of equipment in which heat transfer is promoted from on medium to another. However the term is more often specifically applied to shell and tube heat exchanges or plate heat exchangers "...where a primary fluid such as steam of used to heat a process fluid." (Spirax Sarco, 2009)

A domestic radiator, where hot water gives up its heat to the ambient air, may be described as a heat exchanger. Similarly, a steam boiler where combustion gases give up their heat to water in order to achieve evaporation, may be described as a fired heat exchanger. A shell and tube heat exchanger used to heat water for space heating (using either steam or water) is often referred to as a non-storage calorifier. Manufacturers often provide a thermal rating for their heat exchangers in kW, and from this the steam consumption may be determined, as for air heater batteries. However, heat exchangers (particularly shell and tube) are frequently too large for the systems which they are required to serve." (Spirax Sarco, 2009).....

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