Shell and Tube Heat Exchangers Using Cooling Water

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Process Engineering Guide: GBHE-PEG-HEA-511

Shell and Tube Heat Exchangers Using Cooling Water Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Shell and Tube Heat

Exchangers Using Cooling Water

CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 3.1 HTFS 3 3.2 TEMA 3 4 CHECKLIST ¾ 5 QUALITY OF COOLING WATER 4 6 COOLING WATER ON SHELL SIDE OR TUBE SIDE 5

7 COOLING WATER ON THE SHELL SIDE 5 7.1 Baffle Spacing 5 7.2 Impingement Plates 5 7.3 Horizontal or Vertical Shell Orientation 5 7.4 Baffle Cut Orientation 5 7.5 Sludge Blowdown 5 7.6 Removable Bundles 5 8 FOULING RESISTANCES AND LIMITING TEMPERATURES 6



9 PRESSURE DROP 6 9.1 Pressure Drop Restrictions 6 9.2 Fouling and Pressure Drop 6 9.3 Elevation of a Heat Exchanger in the Plant 6

10 MATERIALS OF CONSTRUCTION 7 11 WATER VELOCITY 7 11.1 Low Water Velocity 7

11.1.1 Tube Side Water Flow 7 11.1.2 Shell Side Water Flow 7

11.2 High Water Velocity 8 12 ECONOMICS 9 13 DIRECTION OF WATER FLOW 9 14 VENTS AND DRAINS 9



15 CONTROL 9 15.1 Operating Variables 9 15.2 Heat Load Control 9

15.2.1 General 10 15.2.2 Heat load control by varying cooling water flow 10

15.3 Orifice Plates 9 16 MAINTENANCE 11 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 12



0 INTRODUCTION/PURPOSE This Process Engineering Guide is one of a series on heat transfer produced for GBH Enterprises. Many shell and tube heat exchangers use cooling water. There are a number of design criteria/principles, peculiar to the use of cooling water, which should be considered if the best design is to be obtained for such a unit. 1 SCOPE This guide gives good design practice recommendations in the form of a checklist (see clause 4) for shell and tube heat exchangers using cooling water. The contents of the checklist are discussed in more detail in the relevant clauses that follow it. 2 FIELD OF APPLICATION This guide applies to the process engineering community in GBH Enterprises worldwide. 3 DEFINITIONS For the purposes of this guide, the following definitions apply: HTFS Heat Transfer and Fluid Flow Service. A co-operative research

organization, in the UK, involved in research into the fundamentals of heat transfer and two phase flow and the production of design guides and computer programs for the design of industrial heat exchange equipment.

TEMA Tubular Exchanger Manufacturers’ Association. An organization of

(US) heat exchanger manufacturers. Their publication ‘Standard of the Tubular Exchanger Manufacturers' Association’ is a widely accepted industry standard.



4 CHECKLIST This checklist contains design criteria/principles and should be consulted at an early stage in the design process for a shell and tube heat exchanger.



5 QUALITY OF COOLING WATER Properly treated cooling water should be used for shell and tube heat exchangers. Environmental constraints have largely ruled out the use of the synergized chromate systems which were the preferred option before the mid 1980s. Current systems generally involve the use of zinc phosphate, but increasingly tight constraints on discharge are likely to prohibit these also in the future. A water technologist should be consulted for up-to-date advice. Poor quality water can give rise to fouling and/or corrosion problems. If in any doubt, the designer should obtain advice from a water technologist and a materials specialist as to the quality of the water available on the plant in question, and the choice of materials of construction. In many instances it is more cost effective to upgrade the quality of the water than to design to accommodate poor water quality. 6 COOLING WATER ON SHELL SIDE OR TUBE SIDE Cooling water is one of the dirtiest fluids to be found on plants. It is also relatively corrosive, although with careful design and good water treatment this can be controlled. Unless the process stream has worse characteristics, the cooling water should normally be on the tube side because: (a) It facilitates cleaning, either mechanically or by high pressure water jetting. (b) It is possible to inspect individual tubes for signs of pitting corrosion, using

an intrascope. (c) Fewer sedimentation problems occur, because of the simpler flow path.

(Sediments restrict the access of corrosion inhibitors to the metal wall and thus often promote corrosion in cooling water systems, even with proper water treatment.)

(d) Higher velocities are usually possible, which reduce fouling and make it

easier to achieve the required minimum velocity.



7 COOLING WATER ON THE SHELL SIDE Where it is necessary for cooling water to be contained in the shell side of a heat exchanger, a number of precautions/considerations should be taken into account. These are outlined in sub-clauses 7.1 to 7.6. 7.1 Baffle Spacing Avoid large baffle spacings and large baffle cuts which create low velocity zones where debris may collect; this may result in loss of heat transfer area and increased risk of corrosion. Good design practice usually calls for baffle spacings of between 20-100% of the shell diameter. Baffle cuts are usually between 17 and 35% of shell diameter for optimum performance. Avoid large changes in velocity between cross-flow and window flow. 7.2 Impingement Plates An impingement plate should be fitted at the inlet nozzle if the velocity in the nozzle (or the cooling water supply line to the nozzle) is above 1.5 m/s. It may be necessary to remove tubes from the bundle to give a clearance above the plate of one quarter of the branch diameter. In general, high nozzle velocities should be avoided because they lead to high pressure drops and an increased risk of tube vibration or erosion. On the other hand, it is preferable, but not essential, to avoid nozzles much larger than one third of the shell diameter because they can cause problems in design in complying with the mechanical design codes, and during manufacture in keeping the required shell circularity. 7.3 Horizontal or Vertical Shell Orientation Experience indicates that in general, there will be fewer problems of fouling and corrosion in exchangers with cooling water on the shell side if the shell is arranged horizontally rather than vertically. This is because dirt deposits tend to fall to the bottom of a horizontal shell, away from the tubes, whereas in a vertical shell deposits occur in contact with the tubes on the lower tube plate and on each baffle.



7.4 Baffle Cut Orientation In horizontal shells baffles should be cut vertically (rather than horizontally) wherever possible, to minimize the build-up of sludge deposits. With vertically cut baffles these can largely be swept away by the water flow. 7.5 Sludge Blowdown To install sludge blowdown valves at places where debris may collect is questionable. With a horizontally mounted exchanger with vertically cut baffles, it could be argued that to be fully effective a blowdown valve should be provided at each baffle space. If a close baffle pitch has been used to ensure a reasonable water velocity, this could require blowdown valves every 100 mm, which is clearly impracticable. Experience would suggest that even if installed, their use is unlikely in practice. If sufficient sludge could accumulate to make their use beneficial, then there is a serious fouling problem that should be addressed by other means. 7.6 Removable Bundles If possible provide a removable bundle (U-tube or floating head) with square pitch tube layout to allow regular mechanical cleaning. If this is not feasible, (e.g. single tube pass required on process side for a vertical condenser) arrange for regular sludge blowdown (but see 7.5) in conjunction, if possible, with increased water flowrate, increased level of dispersants and periodic chemical cleaning. It should be remembered that cooling water on the shell side is liable to result in local corrosion at ’dead’ spots near baffles, etc. Avoid the use of bellows in the shell if possible, as they constitute a ’dead’ spot and are prone to corrosion.



8 FOULING RESISTANCES AND LIMITING TEMPERATURES Recommended fouling resistances for treated cooling waters are given in GBHE-PEG-HEA-501. Systems with good water treatment should in general not have surface temperatures in excess of 70°C. Bulk temperatures should normally be kept to lower values, typically 60°C to prevent crystallization. On some plants that have reasonable water treatment, 60°C is the preferred maximum surface temperature, with bulk temperatures limited to no more than 50°C, based on actual fouling observations for water velocities slightly above 1 m/s. Waters with poorer forms of treatment are more prone to fouling/scaling and, if they have to be used, should be limited to lower temperatures. Advice should always be sought from a water technologist. 9 PRESSURE DROP 9.1 Pressure Drop Restrictions An adequate water velocity is essential to avoid severe fouling and potential corrosion problems. If the velocity is limited by pressure drop restrictions, make sure that these are realistic and necessary. In some cases, it may be economical to install a booster pump, particularly where a heat exchanger is mounted high in a structure. Economical designs are obtained by making maximum use of the available pressure drop. Avoid excessive pressure drop in regions of the exchanger away from where the heat transfer is taking place, such as inlet and outlet nozzles. A suitable total nozzle pressure drop is around 5 - 20% of the available pressure drop. 9.2 Fouling and Pressure Drop Allowance should be made for the thickness of the fouling layer when calculating a pressure drop. Pressure drop for flow inside a tube varies as the fifth power of the diameter, so that even a modest fouling layer can have a significant effect.



On the shell side, the fouling layer may block the tube to baffle and baffle to shell leakage paths. In extreme cases, this can raise the pressure drop by more than 50%. Unfortunately, the HTFS programs normally used for thermal design do not make allowance for fouling layer thickness when calculating pressure drop. For water on the tube side, the effect can simply be obtained by applying the fifth power law to the fraction of the pressure drop associated with tube friction. For water on the shell side, it is necessary to adjust the clearances to make allowance for fouling. A typical thermal conductivity for cooling water fouling deposits is 1.4 W/m.K and typical fouling layer thermal resistances are 0.0002 to 0.0004 m2.K/W. The corresponding fouling layer thicknesses are 0.28 to 0.56 mm. 9.3 Elevation of a Heat Exchanger in the Plant An allowance should be made for the elevation of a heat exchanger in the plant when estimating permissible feed and return pressures. The exit water pressure on all heat exchangers should be above atmospheric pressure where possible, or difficulty may be experienced in venting air from the water side. The exit pressures on all units have to be compatible with the exit pressure on the most extreme unit (normally the highest on the plant). A computer model of the water network is useful. Where orifice flow meters are installed to measure the water flow, ensure that they are sited at regions of positive pressure to enable impulse lines to be vented properly; it is safest to install them upstream of a heat exchanger for this reason. 10 MATERIALS OF CONSTRUCTION Because of chloride attack (even at ppm levels of chloride) cooling water can be used with ordinary stainless steels only if stringent temperature restrictions are used, and attention is paid to particular details of design. Where there is doubt concerning a particular case, a materials specialist should be consulted. Carbon steel is normally acceptable for cooling water duties. However, most materials are susceptible to corrosion if the water velocity is low (<1.0 m/s), or if there are dead spots where debris can accumulate. This may occur even when treated cooling water is used; the situation may be considerably worse with untreated water.



11 WATER VELOCITY 11.1 Low Water Velocity Water velocities below 1 m/s should be avoided where possible to prevent the excessive deposition of solids that can lead to local corrosion; this may occur even with nominally resistant materials or effective inhibitor systems. Corrosion of carbon steel can occur even in the absence of significant deposits, and with normal levels of treatment chemicals, if the water velocity is low. Where water velocity below 1 m/s is unavoidable, a materials specialist should be consulted. There are several ways of increasing cooling water velocities at the design stage. Increasing the total flow of fresh cooling water to a heat exchanger is not always possible or desirable (see 11.2) but even with a fixed quantity the designer has several options: 11.1.1 Tube side water flow Options include: (a) Increase the length and reduce the number of tubes. This may not be

possible as it may raise the shell side pressure drop above the allowable limit. An increased tube or baffle pitch may counter this problem.

(b) Increase the number of tube passes. This is not always possible as it may

result in too low a value of the ’F’ correction factor to the log mean temperature difference, or even a temperature cross.

(c) The problem can often be overcome by adding shells in series. It may

then be possible to use multi-pass flow on the tubeside of each shell without incurring an excessive ’F’ factor penalty.

(d) Reduce the tube diameter. This increases the ratio of heat transfer surface

to tube cross-sectional area and thus, for a constant heat transfer area, raises the velocity. Note that the minimum diameter for mechanical cleaning is ¾" NS.



11.1.2 Shell side water flow Options include: (a) Minimize baffle spacing. Spacings down to 100 - 150 mm are quite

practicable, and for small heat exchangers even as low as 25 mm can be used.

(b) Keep the tube pitch to a minimum, consistent with mechanical integrity of

the tube tubesheet bond. (c) Reduce tube diameter and thus reduce shell diameter for the same tube

count. (d) Increase tube length and reduce tube count and shell diameter

accordingly. If this results in an excessively long and thin exchanger, consider multiple shells in series on the shell side.

(e) If there is an excessive (>10%) by-passing round the tube bundle or

through pass partition lanes, consider the use of seal strips and seal rods to reduce these streams. If seal strips were not specified in the original design, when the mechanical design is known a check should be made that the actual tube bundle/shell clearance does not lead to an excessive ‘C’ or bundle by-pass stream.

(f) The use of a longitudinal baffle to give two shell side passes (TEMA ‘F’

shell) is sometimes proposed. This design is not generally recommended as it is difficult to prevent thermal or even physical leakage across the baffle, which can lead to inability to meet the design performance. Satisfactory 'F' shell designs have been made where it is possible to weld the longitudinal baffle in place. This will, however, prevent removal of the bundle unless a 4-pass U-tube design is used, arranged so that the U-tubes do not span the baffle.

For either shell side or tube side flow, the use of an auxiliary pump to recirculate water from the exit to the inlet will enable higher velocities to be achieved, without increasing the flow of fresh water. However, there is a penalty in loss of mean temperature difference that should be weighed against the gain in coefficients and lower fouling. This approach is useful as a control scheme (see 15.2.1).



Where the consequences of likely corrosion due to low velocities are unacceptable, consideration should be given to a secondary cooling circuit with a non-fouling, noncorrosive fluid, (such as a closed circuit nitrite dosed water), with a second heat exchanger, (probably a plate type) cooling this secondary circuit with ordinary cooling water. Here also there is a penalty in loss of temperature difference, but this does give a system of high integrity and may be particularly suited to shellside duties where inspection and cleaning is impossible. An alternative solution, which has been used on critical duties, is to use a material of construction that is resistant to cooling water corrosion even with poor water treatment or low velocities, such as Titanium or Hastelloy C. This will not, however, prevent fouling deposits. 11.2 High Water Velocity High water velocities may result in erosion, cavitation and tube vibration. With most alloy/water combinations, velocities of up to 2.5 m/s are safe, and with the more resistant materials and effectively inhibited water, velocities considerably greater than this may be used. A water velocity of 2.5 m/s is, however, too high for copper, and a limit of 1.5 m/s should be applied in this case. For shell side flow, TEMA recommends the use of an impingement plate to prevent damage to the tubes in the entrance region if the product of density and the square of the nozzle velocity exceeds 2250 N/m2; for water this corresponds to a velocity of 1.5 m/s. The safe water velocity is not only dependent on the combination of alloy and water in question, but also on the details of design (e.g. U-tubes) and factors such as the chance of debris etc. being present. It is difficult to generalize, and where it is proposed to operate outside the previously mentioned limits, a materials specialist should be consulted. High velocities combined with large baffle spacings can give rise to tube vibration. This can be very serious, in extreme cases resulting in tube failure within hours of start-up. The main thermal design programs such as the HTFS program 'TASC' have an option for performing a vibration analysis. This should always be done. For meaningful results, the full vibration output option should be selected. If any potential problems are shown up, a more detailed analysis should be performed, and/or the design modified. If in doubt, seek advice.



12 ECONOMICS The costs of cooling water systems and their associated heat exchangers are normally optimized by choosing a high return water temperature from the exchanger, provided the process duties are above 50-60°C. Pollution from the cooling tower plume usually limits the return water temperature to the range 30- 35°C, but often individual items can be beneficially designed with return temperatures above this, if water quality allows. The possibility of using water in series through two exchangers on different duties, where one requires a low temperature and the other does not, should be considered. 13 DIRECTION OF WATER FLOW The cooling water (or any other liquid) should, preferably, flow into the heat exchanger at the bottom and out at the top. This is vital for shell side flow in vertically installed heat exchangers in order to: (a) ensure that air pockets are avoided; (b) discourage recirculation, due to natural convection effects. The lower the pressure drop through the tube bundle (i.e. excluding nozzle losses) the more necessary this becomes. 14 VENTS AND DRAINS The design of the heat exchanger should be examined to ensure that: (a) high points are adequately vented. It may be necessary to provide a vent

within the tubesheet or an internal stand-pipe to ensure this; (b) low points have drains. For cooling water, 1" NS cocks are usually adequate for both duties. However, larger drains may be desirable for units over 100 m2 capacity. No vent/drain branches, with the exception of tubesheet vents, should be smaller than 1" NS; drain cocks should be full bore to reduce the risk of plugging by debris.



15 CONTROL 15.1 Operating Variables Heat exchangers cooled with water are usually designed for maximum plant throughputs with the cooling water inlet temperature at its peak summer value (typically 21-23°C) and the heat exchanger in its anticipated most fouled state. However, the actual operating conditions will vary from these values. In winter the cooling water inlet temperature may be only 10°C or less; when first installed the exchanger can be expected to have a low value of fouling resistance; the plant is required to operate under turndown conditions. On critical duties, performance calculations should be done at the design stage to assess the likely outlet temperatures of the process streams under varying conditions, and their effect on the remainder of the process. 15.2 Heat Load Control In many cases the plant performance is insensitive to the previously stated variations. In these cases the cooling water flow can be set to the design value (which will ensure an adequate water velocity) and left at that value. However, there are occasions when it is necessary to control the heat load on an exchanger (e.g. when the heat load on a partial condenser is being used to control the pressure of a distillation column).

15.2.1 General

Except in very special circumstances, controlling the heat load should not be done by varying the cooling water flow (see 15.2.2). The required range of water flowrates necessary to accommodate changes in throughput, cooling water temperature and fouling resistance is likely to be very great. This results in problems of rangeability of the control valve and also in it being virtually impossible to ensure that the velocity at all times lies within the permitted range.

Heavy fouling deposits can be expected during turndown conditions, which will not necessarily be flushed away under conditions of higher flowrate, unless the water velocity is maintained above 1 m/s at all times. Premature failure can be expected from the resulting corrosion.



In general, it is better to control the process exit temperature of coolers by means of a bypass on the process side. However, problems can occur if the process fluid becomes very viscous or freezes at temperatures approaching the water inlet temperature. If by-passing of the process fluid cannot be allowed and it is required to control the heat load, then one of two methods is recommended as follows:

(a) An auxiliary pump recirculating the exit water back to the inlet, with

a controlled makeup of fresh cooling water and a bleed back to the cooling tower. The control system allows the temperature level of the water in the circuit to float.

(b) A secondary cooling circuit with properly treated non-fouling

coolant. This is cooled in a secondary heat exchanger, designed for constant (high) cooling tower water velocity. The temperature of the secondary coolant is controlled by by-passing it round the auxiliary exchanger. The system has a high integrity, but is expensive and may not be justifiable.

15.2.2 Heat load control by varying cooling water flow

It may be possible to vary the water flow without problems provided that a minimum stop is put on the valve such that the velocity is never less than 1m/s. However, when designing such a system, remember that the water pressure drop will rise for a constant flowrate as the exchanger fouls. This means of control has worked successfully in various locations that have used non-chromate treated water for several years.

Where the methods outlined in the previous paragraph or in 15.2.1 are not adopted and heat load control is to be by varying the cooling water flow, then it is imperative that the heat exchanger be regularly inspected (if fabricated from material that could corrode). On critical duties this inspection should include thickness monitoring. The frequency of the inspection will depend on the quality of the cooling water, but as a guide, it is likely to be every two years. A materials specialist should be consulted for advice. Because of the costs of inspection and the risks of failure, it may be found to be more economic to install a heat exchanger made from resistant material (e.g. Hastelloy ’C’).



15.3 Orifice Plates It is often desirable to fit restriction orifice plates in a cooling water system, to balance the flows through different units. Although the isolation valves associated with the exchanger can be used for this purpose, a fixed restrictor is generally preferable to using an isolating valve because: (a) it is more reliable than a manual setting of a valve; (b) the isolation valve can be opened fully, which is an unambiguous

operation; (c) the risk of erosion damage to the valve, with consequent leakage during

isolation, is reduced. Against this, as exchangers foul in service, it may be necessary to make adjustments to maintain the required flow through all units. Orifice plates (or control valves), if fitted, should be in the exit line from the heat exchanger to reduce the risk of air degassing and venting problems in the heat exchanger. 16 MAINTENANCE Heat exchangers are classified as pressure vessels, and as such are subject to regular inspection. In addition, there is often a requirement for cleaning. If the water is on the tube side, mechanical cleaning can often be performed without removing the exchanger from its berth. The use of TEMA ’A’ or ’C’ front end type and ’L’ or ’N’ rear head type enables this to be done without disconnecting the water pipework. However, these head types are more expensive than the ’B’ or ’M’ types. With cooling water on the shell side, mechanical cleaning can only be done with a removable bundle. The plant layout should allow room for rodding through on the tube side, or removing the bundle if necessary. Mechanical cleaning can be performed by rodding, brushing or high-pressure water jetting. It is generally possible to clean the inside of the U-bend region for tube sizes down to ¾" NS if the contractor is specifically asked to do so.



In order to reduce the shutdown time associated with cleaning and inspection a spare heat exchanger is sometimes provided to replace that which is being maintained. Consideration should be given to the storage of the spare after cleaning. Chemical cleaning cannot be guaranteed to remove all cooling water deposits, especially on the shell side. The remaining material is difficult to dry out completely, and acts as a potential source of corrosion during storage. The alternative to dry storage, which is to store the exchanger filled with water heavily dosed with treatment chemicals, presents problems of disposal of the water before re-installation. Techniques are now available to measure local wall thickness of the tubes in an exchanger without having to remove them. A materials specialist should be consulted for further details. DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: GBH Enterprises PROCESS ENGINEERING GUIDES GBHE-PEG-HEA-501 Fouling Resistances for Cooling Water

(referred to in clause 8) OTHER DOCUMENTS TEMA Standard of the Tubular Exchanger Manufacturers Association

(referred to in clause 3, 11.1.2, 11.2 and clause 16). While every effort has been made to ensure the accuracy of the references listed in this publication, their future availability cannot be guaranteed.

Shell and Tube Heat Exchangers Using Cooling Water

Technology

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