Thermal design analysis of a horizontal double pass shell and tube heat exchanger

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

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)

According to the work of Driedger states the simplest way to carry out such a control action analysis is to "...around the loop from the measurement to the controller, to the valve, and back to the measurement. Assume the control action is positive. If the measurement is brought back to the setpoint, everything is OK. If things get worse, reverse the controller action. Modern distributed control systems (DCS) have a built-in option for reversing the signal to the valve. If this is used, it is not necessary to take the failure mode of the valve into account when doing a control action analysis. First think positive then select a reverse output (not control action) if the valve is Fail Open." (2000)

There is a possibility for both valves to be closed according to Driedger and if the outlet valves does not close then the controller senses a low temperature resulting in the output of the controller rising and then closing the bypass. The failure of the valve itself when the signal is good is stated to be unlikely. However, should this possibility be unacceptable then "both valves must be made Fail Open" however, Driedger states that this cannot be done since the same signal drives both valves and their failure modes is opposite due to their opposite processes.

Driedger states that the only method that can be used to solve this problem is to "...have two separate outputs from the controller, one direct, the other reverse. This is not a standard feature, but it can be easily arranged with a DCS. Here is the way to do it: Send the controller output to two calculation blocks. The first doubles the signal so that the range 0 ® 50% becomes 0 ® 100%. Any signal beyond 50% is ignored as the output of the calculation block cannot exceed 100%. The second block subtracts the signal from 100% and then doubles it. The range 50 ® 100% then becomes 100 ® 0%. Any signal below 50% is ignored as the output of the calculation block cannot fall below 0%. The two outputs are then send to their respective valves. Which output goes to which valve depends on the failure mode of the valves. Note that both valves now operate on 0 ® 100% signals. Some models of DCS provide for scaling and linearization in the output modules. If this feature is available, separate calculation blocks are not required." 2000)

There are some advantages to achieving split range action other than the fact that the two valves will have the same failure mode. While an additional output slow is lost as well as a pair of wires and an I/P" the two valves are "self-contained and do not require any special treatment with respect to wiring in series, mounting of I/Ps, or split range calibration." (Driedger, 2000) Driedger states that the second arrangement is one that attempts to keep the pressure drop and therefore keeps the flow as constant as it can across the "entire stroking range of the valves." This can be accomplished through the outlet valve beginning to close the moment the bypass starts to open and being fully closed at the precise moment the bypass fully opens. This is referred to as the "opposite action arrangement." (2000)

Driedger states that a three-way 'flow-splitter or diverter' valve is used quite frequently in the combination of the functions of the two valves into one body. For butterfly valves it is stated that a mechanical link may be installed to join two valves to one actuator so that when one opens the other closes. It is also possible to use two separate valves with either a single I/P, two I/Ps in series, or two separate output signals. The description in the previous section on using calculation blocks to provide opposite control action for the two valves is valid, with one difference. The signal is not doubled and only one block is needed to reverse the signal." (2000)

The figure below labeled Figure 1 shows a typical characteristic of three-way valves with the flow on one side gradually decreasing and the flow through the inlet port stated to be constant ideally "...throughout the entire stroking range of the valve. It is stated that some three-way valves have rather large gap between the two exist ports where both sides have more than 50% flow. Such valves should be avoided as they result in sloppy control because the loop gain near the midpoint is too small." (Driedger, 2000)

The two -- valve arrangement is also given the same consideration and the two valves "should be chosen to have fairly linear characteristics so that the combined flow is approximately constant." (Driedger, 2000) It is related that plotting these two characteristics is useful and adding up the two to check the valve combination is a sound method.

Figure 9

Typical Characteristic of Three-Way Valves

The following illustration shows the simple example in which hot oil is supplied to heat a process stream which should be kept at a constant temperature. There would not be a need to maintain the flow of oil over what is actually needed and it can be throttled to control the temperature. The valve is placed on the exchanger outlet. There is no expectation that the valve will handle a large drop in pressure or the tight shutoff of any specific value and a butterfly valve is stated to be "quite acceptable." (Driedger, 2000) In addition it is stated that its "low pressure drop (high Cv) when wide open is an advantage." (Driedger, 2000)

Figure 10

Medium Flow Tube Side Outlet Throttling

It is stated that the effects of the inlet and outlet throttling "are about the same, so secondary considerations come into play. Inlet and outlet throttling effects are pretty much the same although there are other secondary considerations. It is stated to be a good idea "in general...to keep the pressure on a hot fluid to reduce any chance of dissolved gases bubbling out. A valve on the cooler end may be cheaper and is likely to last longer.

Furthermore, there is less likelihood of leaks "...as the fluid will be more viscous than on the hot side" and for this reason it is preferred that throttling of a heat medium is done on the outlet side. Only in rare cases would a heat changer cooling the process have a reason to use other than bypass control on the process side and it is not desirable in most cases to throttle cooling water since it is highly corrosive and lacking in cleanliness therefore, it is generally "put through the tubes." (Driedger, 2000)

Maintaining water velocity is the best method of improving heat exchange and in avoiding the build-up of deposits and fouling. If the cooling medium must be throttled, the possibility of boiling must be considered and remembering that the cooling medium is the one receiving the heat and making the assumption that boiling is unintended the valve would be placed on the outlet for the purpose of maintaining fluid pressure. It is stated that in some cross exchange applications that it is desirable to recover all of the heat or cold content of the product stream and for it to be transferred to the feed. IN these cases there is not a need for exchanger controls whatsoever.