Shell and Heat Tube Exchangers:

Shell and Heat Tube Exchangers: Shell Tube Heat Excanger Double Pass

The objective of this work is to analyze a horizontal conventional double pass shell and tube heat exchanger with a focus on the features as follows: (1) use of distribution pressure not requiring steam regulating valve or steam control valve; (2) Using a higher temperature steam to reduce the heat exchanger size; 3) Providing condensate sub-cooling in a single heat exchanger; 4) Using discharge control valve to flood heat exchanger to modulate capacity; 5) Eliminates/minimizes water hammer because steam is not traveling over water parallel to the water/steam surface; 6) Uses distribution pressure to push condensate back to the steam plant thereby eliminating a separate condensate pump; 7) Eliminates steam flashing from discharged condensate; 8) Provides a closed steam-water system thereby eliminating air and oxygen introduction into the condensate return; and (9) Flood system rather than a conventional drain system.

Introduction

Shell and tube heat exchangers are used throughout the process industry and extensively. The thermal design of a shell and tube heat exchanger includes many interacting design parameters which are those as follows:

Process

1. Process fluid assignments to shell side or tube side;

2. Selection of stream temperature specifications;

3. Setting shell side and tube side pressure drop design limits;

4. Setting shell side and tube side velocity limits;

5. Selection of heat transfer models and fouling coefficients for shell side and tube side. (Edwards, 2008)

Mechanical

1. Selection of heat exchanger TEMA layout and number of passes;

2. Specification of tube parameters -- size, layout, pitch and material;

3. Setting upper and lower design limits on tube length;

4. Specification of shell side parameters -- materials, baffle cut, baffle spacing and clearances; and

5. Setting upper and lower design limits on shell diameter, baffle cut and baffle spacing. (Edwards, 2008)

There are four basic types of heat exchangers including: (1) tubular; (2) plate and frame; (3) Jacketed; and (4) Coil. (U.S. Department of Energy, nd) Tubular heat exchangers are stated to be "...tube bundles that are surrounded by the heated or heating medium. This type of heat exchanger includes finned tube and shell and tube designs." (U.S. Department of Energy, nd) Finned tube heat exchangers are stated to be most often used in heating air of "drying and space heating applications. Shell and tube heat exchangers are often used for liquid heating and evaporation. Since the tube side of shell and tube heat exchangers can be designed to withstand high pressures, sometimes exceeding 1,500 psig, heat exchangers of this type are often used in high temperature and high-pressure applications." (U.S. Department of Energy, nd) Lastly, jacketed heat exchangers are stated to "...use an enclosure to surround the vessel that contains the heated product. A common example of a jacketed heat exchanger is the jacketed kettle." (U.S. Department of Energy, nd) Coil heat exchangers are stated to "...characteristically use a set of coils immersed in the medium that is being heated. Coil heat exchangers are generally compact, offering a large heat transfer area for the size of the heat exchanger." (U.S. Department of Energy, nd)

The work of Bose (2009) states that a shell and tube heat exchanger "...contains a bundle of tubes inside a large pressure vessel which is referred to as the shell. The tubes on each end are attached to tube sheets. Two different fluids run through the shell and the tube heat exchanger, one through the tubes and the other outside the bundle of tubes within the shell. As the fluids flow through the system, transfer of heat takes place from the fluid that is at a higher temperature to that at a lower temperature. The fluid inside the tubes is called the tube side fluid whereas that which is flowing outside the tubes and within the shell is called the shell side fluid. This heat exchange takes place through the tube walls. The fluid that has to be heated or cooled runs through the tubes. The fluid that runs through the shell outside the tube bundle is heated or cooled depending on whether the fluid inside the tubes have to be cooled or heated. Use of a number of tubes instead of one increases the efficiency of the heat exchanger as circular surfaces of all the tubes increases the net area of transfer of heat." The fluids in the single or one phase heat exchanger are either only gas or only liquid. Two phase heat exchangers involve the transfer of heat between the two fluids of two different phases or the exchange of heat between a liquid and a gas. Boilers are stated to be "two phase heat exchangers in which liquids are heated to a gas." (Bose, 2009) The gas is cooled down into its liquid phase in condensers. The U-Tube Heat Exchanger is comprised by the tubes in the bundle bending and forming a U. It is related that the tube ends "open into plenums or water boxes through holes in the tube sheet. There is an inlet plenum and an outlet plenum. The fluid enters through the inlet plenum, runs the entire length of the U-tube and leaves the heat exchanger through the outlet plenum. As the fluid flows inside the U-tube, heat transfer takes place between the fluid in the tube side and that in the shell side. Two phase U-tube heat exchangers are commonly used in nuclear power plants called pressurized water reactors. Using these heat exchangers, water that has been obtained by condensing steam is heated back to steam that is used to turn steam turbines. As the steam turbines rotate, power is generated." (Bose, 2009)

The work entitled: "Heat Exchangers: Design Considerations" states that heat exchangers are "...ubiquitous to energy conversion and utilization. They involve heat exchange between two fluids separated by a solid and encompass a wide range of flow configurations." The following illustration shows Concentric-Tube Heat Exchangers both the parallel flow and counterflow.

Figure 1

Heat Exchangers: Design Considerations, 2009

The following figure illustrates the cross-flow heat exchangers.

Figure 2

Heat Exchangers: Design Considerations, 2009

The following illustration represents the Shell and tube Heat Exchanger. The Baffles are the method by which a cross-flow is established for induction of turbulent mixing of the shell-side fluid. (Heat Exchangers: Design Considerations, 2009)

Figure 3 Shell and Tube Heat Exchangers

Heat Exchangers: Design Considerations, 2009

Baffles are stated to be used for the purpose of establishing a cross-flow and for induction of turbulent mixing of the shell-side fluid, both of which enhance convection. "(Heat Exchangers: Design Considerations, 2009)

Figure 4

One Shell Pass, Two Tube Passes

Heat Exchangers: Design Considerations, 2009

Figure 5

Two Provisions for Thermal

Two Shell Passes, Four Tube Passes

Heat Exchangers: Design Considerations, 2009

Compact Heat Exchangers are stated to be widely used to achieve large heat rates per unit volume, particularly when one or both fluids is a gas. Compact heat exchangers are characterized by "...large heat transfer surface areas per unit volume, small flow passages, and laminar flow." (Heat Exchangers: Design Considerations, 2009) Included are the following:

(1) Fin-tube (flat tubes, continuous plate fins);

(2) Fin-tube (circular tubes, continuous plate fins);

(3) Fin-tube (circular tubes, circular fins);

(4) Plate-fin (single pass); and (5) Plate-fin (multipass) (Heat Exchangers: Design Considerations, 2009)

Figure 6

Five Types of Fins

Heat Exchangers: Design Considerations, 2009

It is related in the Wolverine Tube, Inc. "Engineering Data Book II" that enhanced tubes are used in the refrigeration, air-conditioning and commercial heat pump industries extensively however, the use of enhance tubes in the chemical, petroleum and other industries is "still not standard practice" although it is a practice that is on the increase. The design of enhanced tubular heat exchangers allows for a more compact design than are able to be used in the conventional plain tube units "obtaining not only thermal, mechanical and economical advantages for the heat exchanger, but also for the associated support structure, piping and/or skid package unit, and also notably reduced cost for shipping and installation of all these components" and this results in a cost factor of approximately two to three times that of the exchanger itself in petrochemical applications." (Wolverine Tube, Inc. 2009) It is additionally related that enhanced evaporators and condensers "enjoy much larger tube-side water velocities, which increases the internal heat transfer coefficient and reduces fouling and scale formation." (Wolverine Tube, Inc. 2009)

According to the Wolverine Tube Heat Transfer Data Book entitled: Construction of Shell and Heat Tube Exchangers" Shell and Tube Heat Exchangers are the most widespread and most often used basic heat exchanger configuration in the process industries. Stated to be the reasons for the general acceptance of these are as follows: (1) The shell and tube heat exchanger provides a comparatively large ratio of heat transfer area to volume and weight. It provides this surface in a form which is relatively easy to construct in a wide range of sizes and which is mechanically rugged enough to withstand normal shop fabrication stresses, shipping and field erection stresses, and normal operating conditions; (2) There are many modifications of the basic configuration, which can be used to solve special problems; (3) The shell and tube exchanger can be reasonably easily cleaned, and those components most subject to failure - gaskets and tubes -- can be easily replaced; and (4) Finally, good design methods exist, and the expertise and shop facilities for the successful design and construction of shell and tube exchangers are available throughout the world. (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. 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) Shell and tube heat exchangers require use of the equation as follows for determining the heat load:

Where:

= Quantity of heat energy (kW) kJ/s)

= Secondary fluid flowrate = 7.2 kg/s cp

= Specific heat capacity of the water = 4.19 kJ/kg"C

DT

= Temperature rise of the substance (82-71) = 11°C

= 7.2 kg/s ? 4.19 kJ/kg" C ? 11°C

= 332 kW

(Spirax Sarco, 2009)

Determination of the steam load requires the following equation to be used:

The full-load condensing rate can be determined using the left hand side of the heat balance using the following equation:

Where:

s

= Steam consumption (kg/s)

hfg

= Specific enthalpy of evaporation (kJ/kg)

= Heat transfer rate (kW)

Rearranging: a 332 kW calorifier working at 2.8 bar g (hfg = 2 139 kJ/kg from steam tables) will condense:

(Spirax Sarco, 2009)

It is related that a shell and tube heat exchanger is comprised by a "...bundle of tubes enclosed in a cylindrical shell. The ends of the tubes are fitted into tube sheets, which separate the primary and the secondary fluids. Where condensing steam is used as the heating medium, the heat exchanger is usually horizontal with condensation taking place inside the tubes. Sub-cooling may also be used as a means to recover some extra heat from the condensate in the heat exchanger. However, if the degree of sub-cooling required is relatively large it is often more convenient to use a separate condensate cooler." (Spirax Sarco, 2009 ) The following figure shows a common design for a steam to water non-storage calorifier or what is known as a one shell pass two tube pass type of shell and tube heat exchanger and is stated to be comprised by a U-tube bundle fitted into a fixed tube sheet.

Figure 7

Schematic diagram of a shell and tube heat exchanger

(Spirax Sarco, 2009)

It is related that the shell and tube heat exchanger is said to "...have 'one shell pass' because the secondary fluid inlet and outlet connections are at different ends of the heat exchanger, consequently the shell side fluid passes the length of the unit only once. It is said to have two tube passes because the steam inlet and outlet connections are at the same end of the exchanger, so that the tube-side fluid passes the length of the unit twice." (Spirax Sarco, 2009) The exchanger header is divided by a pass partition and the tube-side fluid is diverted down instead of straight through the header of the U-tube bundle. It is additionally related that baffles are generally provided in the shell in order to direct the fluid that is shell-side across the tubes which improves the heat transfer rate and supports the tubes. It is necessary that the design consider adequate steam trapping and condensate removal in order to avoid accumulation of condensate in the steam space which can result in: (1) internal corrosion; (2) mechanical stress due to distortion; and (3) noise due to waterhammer. (Spirax Sarco, 2009)

It is additionally stated that steam can be utilized for the evaporation of a liquid in a "...type of shell and tube heat exchanger known as a reboiler." (Spirax Sarco, 2009 )These are stated to be used in the petroleum industry for the purpose of vaporizing a "fraction of the bottom product from a distillation column." (Spirax Sarco, 2009) It is additionally stated that these "tend to be horizontal with vaporization in the shall and condensation in the tubes. An illustration of the same is shown in the following figure.

Figure 8

Kettle Reboiler

(Spirax Sarco, 2009)

The work of Driedger (2000) entitled: "Controlling Shell and Tube Exchangers" published in the Journal of Hydrocarbon Processing states that shell and tube heat exchangers are "among the most confusing pieces of equipment for the process control engineer." Driedger states that the principle of operation of the shell and tube heat exchanger is "simple enough: Two fluids of different temperatures are brought into close contact but are prevented from mixing by a physical barrier. The temperature of the two fluids will tend to equalize. By arranging counter-current flow it is possible for the temperature at the outlet of each fluid to approach the temperature at the inlet of the other. The heat contents are simply exchanged from one fluid to the other and vice versa. No energy is added or removed." (Driedger, 2000)

The heat demands of the process are variable as well as is the heat content of the two fluids therefore the design of the heat exchanger must be "for the worst case and must be controlled to make it operate at the particular rates required by the process at every moment in time." (Driedger, 2000) Furthermore, the heat exchanger is variable and its characteristics are stated to "change with time." (Driedger, 2000) Driedger states that the most common of changes is a "reduction in the heat transfer rate due to fouling of the surfaces." (2000) In order to allow for the fouling, heat exchangers are oversized. The fouling undergoes gradual build up during use until the exchanger can no longer perform. At the time the exchanger is cleaned it is oversized again.

Driedger states that there is only one variable that can be controlled and that is the amount of heat that is exchanged. The heat flux cannot be measured and it is the temperature of one of the fluids that is able to be measured and to be controlled. Both cannot be controlled because the heat that is added to one is taken from the other and this means that the heat temperature must be kept constant. This is usually accomplished by a piece of equipment "somewhere downstream of the outlet of one of the fluids." (2000) Making the assumption that the temperature does not experience variation along the piping, "the measurement may be anywhere within the outlet itself and the point of interest, perhaps at the base of a distillation tower." (Driedger, 2000)

Where the measurement is being taken downstream of a bypass valve, the mixing will be better the further downstream this occurs and will also render a better measurement. However, when the measurement is too far downstream the result is likely to be "process dead time that can make control difficult." (Driedger, 2000) In addition, where the other fluid is the one undergoing manipulation, it is generally enough to take the measurement "directly downstream of the outlet nozzle of the exchanger." (Driedger, 2000)

The second thing that must be considered is which stream will be manipulated and there may be complication due to the fact "that the exchangers have four ports and involve two different fluids, either of which may change phase." (Driedger, 2000) Driedger states that the two streams can be labeled as the 'process' side and the 'heat exchange medium' side and that a "complete tabulation of the possibilities" is as follows:

a - Process side, outlet throttling.

b - Process side, inlet throttling.

c - Process side, bypass with outlet restriction.

d - Process side, bypass with inlet restriction.

e - Medium side, outlet throttling.

f - Medium side, inlet throttling.

g - Medium side, bypass with outlet restriction.

h - Medium side, bypass with inlet restriction. (Driedger, 2000)

Driedger states that among these various alternatives "some must be better than others" and the best choice is always dependent on the particular situation as "there are a number of varieties of the basic shell and tube exchanger that can be controlled along similar lines: (1) Plate exchangers consist of thin sheets of corrugated metal. The corrugations are formed to produce passages so that the two fluids pass in opposite directions on opposite sides of each sheet. The "shell" side and the "tube" side are essentially interchangeable; and (2) Aerial coolers, sometimes called fin fan coolers, are similar to shell and tube exchangers except that they are all tube. The air blowing past the tubes can be considered to be in an extremely large shell. (2000)

Driedger states that it is erroneous to try to control the process temperature by throttling the inlet or the outlet of the process fluid as the "desired process flow rate is set by other requirements and these would be interfered with by manipulating the process flow." (2000) In addition, the temperature would change a little due to the residence time of the fluid being increased by the flow reduction and as well the outlet temperature will be close to the inlet temperature of the medium. Also, the variations in process flow which results from external influences is one of the primary reasons for variation in temperature and is the reason why some other parameter must be changed to maintain a temperature that is constant.

According to Driedger "...process temperature can be controlled by manipulating process flow if a bypass is installed. As the outlet temperature rises (assume this is a heater), more fluid is bypassed around the ex-changer without being heated. As the two streams are blended together again, the correct temperature is achieved." (2000) Bypass manipulation, while sounding quite simple, does have a few tricks and stated are first that "there are two ways of arranging the valve controls: (1) the attempt can be made to minimize pressure drop at all times; or (2) the attempt can be made to keep the pressure drop constant. (Driedger, 2000)

In either of these cases it is important not to interrupt the total flow. The likeliest choice to minimize pressure drop is a butterfly valve although Driedger states that "a wide open butterfly has some pressure drop. It may be greater than that of the heat exchanger itself." (2000) Driedger states that this means that when the valve is wide open only half of the flow or even less will bypass the exchanger. To accomplish a greater degree of bypass, a restriction must be placed on the flow through the exchanger. The restriction should be adjustable since conditions change and we do not want more restriction than necessary. The easiest way to do this is with a hand valve. Since these valves are often in relatively inaccessible places, remote actuators may be added. Once that is done it becomes an obvious matter to arrange automatic controls so that once the bypass is fully open, the restriction valve starts to close, and vice versa." (2000)

Driedger states that this is "a split range. The valve positioners, or I/Ps, are calibrated so that 0 ® 50% signal opens the outlet and 50 ® 100% signal closes the bypass valve. With this arrangement, at least one of the valves is fully open at all times and the effective Cv ranges from 100% to 200% of that of a single valve. It should be noted that by arranging for minimum pressure drop we must accept that the pressure drop, and consequently the flow, will vary as the valve positions change. Note also that there must be either a single I/P tubed to both valves, or two I/Ps (or positioners) must be wired in series. Either way, special care must be taken during construction and maintenance." (2000)

According to Driedger making the assumption that the Fail Open valve is in the bypass and the process stream is heated "...the failure modes of the two valves is such that a signal failure to either or both" will result in less heat reaching the process stream and the medium failing to be cooled. Driedger states that the opposite failure response can be arranged easily and depends upon the choice made. One these choice is made the controller's control action is a matter of deduction.

Driedger states as follows: (1) Assume that the process stream outlet is too hot. That is, it is above the setpoint; (2) Then the deviation of the controller is positive; (3) Assume the controller action is positive. This produces a rising valve output signal that will tend to open the outlet and close the bypass; (4) That would raise the temperature of the process steam. Wrong! The situation is getting worse; (5) This controller must be configured to be reverse acting; (6) Now a rising outlet temperature will cause a falling valve signal; (7) That will open the bypass and close the outlet; (8) This will lower the temperature of the process stream thus bringing the measurement back to the setpoint." (Driedger, 2000)