INTRODUCTION:

Heat exchangers outline vital part of many processes viz. chemical, fertilizer, petrochemical, nuclear power generation, refrigeration, desalination, and so on. Presently, their impact has enhanced because of the ever growing energy needs. Requirement for better efficiency, strict environmental policies and better cost value, demand reliable materials for heat exchangers, especially for the tubing. A new thrust has been given to this search by the concept of Ultra Supercritical Advanced Power Plants in which, improvements in power plant efficiency can be achieved by increasing the operating temperature and steam pressure [1].

Heat exchangers create various problems in many processes resulting in inferior efficiency and plant availability. The magnitude of problems in steam generators have been highlighted by a report that in the USA alone steam generators accounted for 3.2 % of the lost capacity factor in 1988 [2]. This represents a considerable economic loss to the utilities. It has necessitated repair at a high cost and in some cases has forced premature replacement of the steam generators themselves. The factors that affect material selection and design of heat exchangers are - high heat transfer coefficient, tensile and creep characteristics, corrosion fatigue and creep-fatigue behaviour and high fatigue toughness and impact strength. These factors prevent fast cracking. Moreover, in fewer applications, the nature of the working fluids and thermal conductivity could play an effective role in selecting the materials for heat exchangers [3].

High temperature heat exchangers are subjected to rare material challenges such as reduced strength at higher temperatures, creep, corrosion, and stresses due to thermal shock. As a result, expensive alloys that retain their strength at elevated temperatures are typically the material of choice for high temperature heat exchangers. However, these alloys typically have low heat conductivity, and difficult to manufacture. High temperature heat exchangers that are economical play a key role key to the success of emerging high-temperature, high-efficiency modular power cycles. These are applicable for various energy conversion, power generation and energy/waste heat recovery systems. The plate-fin, plate-and-frame and shell-and-tube type are the most commonly available heat exchangers in the market. The ideal high temperature heat exchangers would typically offer a combination of heat transfer effectiveness, pressure drop, size and weight (indirectly controlling the cost) of the heat exchanger, longevity and reliability requirements [4].

Constant review of the operating record of steam generators worldwide has identified a number of possible mechanisms for unsatisfactory steam generator performance. Through intensive, and continuing, research and development, design of steam generators have evolved which achieve an optimum approach to addressing all the issues. Good tube material selection with appropriate heat treatment has been identified as one of the important design objectives. This optimized design is being used in both current generation steam generators and replacement units. This paper discusses, in general, the materials for heat exchangers and in particular heat exchanger tubing material for the power sector.

MATERIALS SELECTION CRITERIA:

The materials engineer selecting the material for the heat exchanger must know all aspects involved in the construction, operation and maintenance of the heat exchanger. A general procedure [5] used for recognizing the most suitable material for a typical heat exchanger application would consist of the following steps.

· Identify heat exchanger requirements.

· Determine an approach for evaluating suitable materials.

· Categorize suitable materials.

· Evaluate materials in depth.

· Choose appropriate material for the application.

For the first step, the heat exchanger designer must take into account the normal operating parameters (e.g. nature of the fluids on the tube and shell side, flow rate, temperature and pressure), startup and shutdown conditions, special conditions like product purity requirements etc. The heat exchanger designer would also identify the tube fixing methods as this also affects the material selection. If the material selection is being done by someone other than the heat exchanger designer, there must be close consultation between these individuals. While establishing the approach for evaluating suitable materials, the main factors to be considered are cost and reliability. The least cost approach would mean use of less costly materials and solve the problems as they crop up. Highest reliability approach would mean selecting the most reliable material whatever be its cost. Both approaches have to be weighed against initial cost, loss due to possible shutdowns, repair costs etc. In identifying materials, it is desirable to narrow the field to a comparatively small number of materials for more extensive evaluation. To eliminate unsuitable and expensive materials, initial identification and selection procedure has to be done very carefully. This requires operating experience, use of handbook data and literature on advanced materials under development and judgment. The considerations that can influence materials selection include:

Physical properties:

- Heat transfer coefficient (requiring high thermal conductivity for tube material) to be high

- Thermal expansion coefficient to be minimum and attuned as possible with those of the materials used for tubing to provide resistance to thermal shocks.

Mechanical properties:

- Superior creep strength at the highest temperature of operation and adequate creep ductility to accommodate localized strain at sharp corners.

- Superior corrosion fatigue and creep-fatigue behaviour.

- Superior fracture toughness and impact strength to avoid early fracture.

Corrosion Resistance:

- Minimal corrosion rate to reduce corrosion allowance.

- Superior corrosion resistance, corrosion resulting from leak in upstream heat exchanger.

Manufacture:

Ease of manufacture is an important aspect in selection of heat exchanger materials. Bending of tubes, joining of tubes by welding or rolling, forming of shell geometry and welding of shell plates and shell to nozzle are some of the steps involved in manufacturing of heat exchangers.

FACTORS AFFECTING THE PERFORMANCE OF HEAT EXCHANGERS:

There are numerous factors causing failure of a tube material. Corrosion is one of the major factors and can be categorized into two broad groups, general corrosion and localized corrosion accelerated by an electrochemical mechanism.

General Corrosion:

Corrosion may be categorized into two broad groups, one is general corrosion and other is localized corrosion accelerated by an electrochemical mechanism. Factors inducing corrosion are shown in Figure 1. Hence, these criteria should be taken into consideration while designing heat exchangers. The corroding media is chloride for stainless steel and ammonia for copper alloys. General corrosion is a step by step dissolution of surface metal. Rusting of carbon steel and the wall thinning of copper alloys are examples. General corrosion is a not detrimental. With systematic planning and execution, a heat exchanger can be designed to adapt general corrosion. Heat exchanger commonly adds a “corrosion allowance” to a high-pressure carbon steel feed water heater to allow for a period of 25 year lifetime [5].

Figure 1 Factors inducing corrosion [14]

Electrochemically driven mechanisms are detrimental as leaks can be very unpredictable. These failure mechanisms can be categorized into two stages: (1) an incubation or initiation period and (2) propagation mode. The time of initiation is difficult to determine. Initiation may take a few weeks or take years. Once initiated, the propagation mode can occur rather quickly, driven by the electro potential between the two regions. One of the dominant factors may be conductivity of water. One of the failures occurring due to electrochemical mechanism is pitting. Pitting results from the electrochemical potential set up due to differences in inside and outside regions, commonly referred to as a concentration cell. The oxygen-rich environment in this cell acts as an anode and the metal surface as a cathode, resulting in the metal surface being slowly pitted by the chemical reaction. Another failure occurring due to electrochemical mechanism is crevice corrosion. Crevice corrosion is related to pitting corrosion. Since, tighter crevice allows higher concentrations of corrosion products (less opportunity to flush with fresh water), it is more detrimental than pitting. Crevice corrosion can happen at lower temperatures (30°-50° C lower) than in which pitting occurs in the same environment.

SELECTION OF APPROPRIATE MATERIAL:

The following three factors are considered in selecting the most appropriate tube for heat exchangers:

1- Quality of Water

2- Operation and Maintenance of Heat Exchangers

3 - Design of Heat Exchangers

Each of the above factors can also be considered independently and without interaction among them. Since the failure of heat exchanger tube can be due to the effect of these three factors on the tube, therefore, these three factors are to be examined in detail.

Quality of Water:

Quality of water [3, 5] includes factors like purity, chloride level, dissolved oxygen and sulfide level, residual chlorine and manganese, pH level, temperature and capability of creating sediment. Chloride, Calcium, Magnesium, Sodium, Iron and other ions dissolve in water since they do not form deposits, their concentration will be increased in water boiler coolers. Stainless steel (Type 354) is resistant to corrosion when the chloride content in water is up to 1000 ppm. Alloy steel with 4.5% molybdenum is resistant to corrosion when the chloride content in water is up to 2000 ppm to 3000 ppm. Both alloy steel with 4.5% molybdenum and duplex (a new type of stainless steel) has been applied to sea water, containing 2000 ppm chloride, but corrosion has been seen beneath the sedimentary layer on the tube body. Alloy steel with 6% molybdenum and titanium show better resistance to salt water.

Corrosion is also caused in the converter tubes due to presence of oxygen and sulfate soluble in water. Corrosion preventive substances like sodium sulfite are added to the water, to reduce the amount of oxygen in it. Presence of sulfate in water produces calcium sulfate sediment by which is involved in creating corrosion. Tubes made of copper alloy and stainless steel does not operate well in water containing oxygen of three to four ppm. Copper alloy is found non-resistant to corrosion in dirty waters where oxygen and sulphate are present. Tubes made of stainless steel and titanium has proved an excellent material in such conditions.

Tubes made of copper - nickel or stainless steel alloys at high pH are preferred to admiral tubes (71 percent copper, 28 percent brass and 1 percent tin) or tubes made of aluminium-brass alloy which get corroded at alkaline pH. Stainless steel tubes operate well at less than 5 and above 9 pH. Manganese and iron are of the substances that cause water discoloration during deposition. Stainless steel (Type 304) tubes are not resistant to fresh water containing significant amount of manganese. However, copper alloy are found to be more resistant and have operated well in such waters. Sediment or mass absorption level has shown an important effect on thermal transfer in heat exchangers. Creating sediment in internal surface of the tube not only reduces thermal transfer and the flow section level, but also adds to the resistance against thermal transfer. This resistance is so called fouling factor. The fouling process is considered to be the net result of two simultaneous sub processes: a deposition process and a removal process as shown in Figure 2. Table 1 contents in water and the problems caused by these contents are shown. If the water contains mineral acids, higher pH level, sulfate or chloride ions may lead to corrosion. Oxygen and silicate lead to formation of fouling deposits.

Figure 2 Overall fouling process [14]

Operation and Maintenance of Heat Exchangers:

Operation and maintenance procedures [13, 14] having specific and determined program have an important effect on heat exchanger longevity. Although both factors have much influence on product cost, on the other hand, selecting appropriate alloys also leads to maintenance cost reduction.

How to Operate and Maintain:

Stop time of the stagnant water in the heat exchanger tubes should be taken into account during operation. If heat exchanger is supposed to be stopped for two or three days for some reasons, its water would better be replaced with pump once a day and if it is supposed to be out of service for more than one week, its water must be completely evacuated. The evacuation and water replacement operation has an important influence on the prevention of corrosion in tubes.

Table 1: Water contents and its drawbacks

Contents in water	Problems generated
Free and mineral acids	Corrosion.
Bicarbonate oxide	Corrosion occurrence especially in condensers and steam lines.
PH level	Induces acidity or alkalinity levels in water leading to corrosion.
Sulfate ion	It combines with calcium and forms calcium sulfate sediment.
Chloride ion	Increases corrosion rate of water.
Na	It combines with OH in water and causes corrosion in boiler tubes and other parts.
Silicate	It causes deposition in boilers and cooler systems .
Oxygen	It involves in deposition occurence in boilers.
Hydrogen sulfide	It causes bad smell (smell of rotten eggs) and corrosion.

Because when the water is inside the converter, it would get deposited and provide the appropriate environment for the growth of bacteria. Thus, no converter should ever be left with stagnant water. Keeping water containing or even moist converter leads to its corrosion.

Cleaning Schedule:

The cleaning schedule of heat exchanger tubes, depend on the fluid type. The stickier the fluid, the sooner it gets deposited and also creates defects in the operation of the unit and operation of the converter. Thus, converters must be opened continually and scheduled, their water be drained and be brushed so that the sediments would be removed in the internal surface of tubes. Thus, corrosion will be prevented and thermal load of converter will be increased as well. Converter which has water containing high biological materials and sediments should be cleaned weekly. Although tube cleaning seems beneficial, on the other side, it should be noted that warm water produces a protective film on the surface of the tubes and frequent mechanical cleaning may cause loss of this protective film. Thus the critical optimum period should be considered for tube cleaning. It is best suggested that tubes made of copper alloy or stainless steel should be cleaned monthly or once every three months for all types of waters. However, it should be acted carefully. If sediment remains in the tubes for more than three months, corrosion occurs in the tubes.

Design of Heat Exchangers:

The performance heat exchanger as shown in Figure 3 are influenced by these factors: fluid velocity, tube diameter, converter tube shape (U or cross shape), converter layout order (horizontal or vertical), venting valve, material of tube sheet and channel and location order of input channel [8].

The following are the design constraints to be considered during exchanger design:

· Efficiency – should operate most efficiently with minimum loss of energy transfer and minimum drop in the fluid pressure.

· Materials – should be built from materials that are compatible with the process and with optimum cost.

· Maintenance – should be easily cleaned and repaired.

· Ease of construction

· Cost of heat exchanger

Fluid Velocity:

Velocity: High fluid velocity increases the heat transfer coefficient, but is associated with pressure drop. If fluid velocity is very high it could prevent particle deposition but it would cause corrosion of the tube. A plastic cover is often applied to the inside of the tube to lessen corrosion rate. However, high fluid velocity prevents from particles deposition or tube fouling and increases thermal transfer coefficient. On the other hand, it increases pumping costs. Thus, fluid velocity would be determined considering pumping cost, tube longevity and cleaning cost against thermal load score.

Diameter of Heat Exchanger Tube:

Tubes of the 8/5 to 2 inches range are generally used. Diameters of less than 8/5 inches are preferred for higher thermal loads. Tubes of larger diameters can be easily cleaned and are more appropriate for fluids which form sediment.

Exchanger Order of Orientation:

Converters, and especially condensers, are usually installed horizontally and water flows within the tubes. In such converters, copper alloy and stainless steel tubes have operated well. In conditions in which water must flow in the crust and sediment deposition cannot be prevented in the tubes at the bottom around baffles, corrosion occurs under the sediments in these locations. Even when it is necessary for completely smooth water to flow in the crust, tubes should be made of corrosion resistant alloys. Condensers are sometimes located vertically and water cooler flows within the shell for the thermal transfer rate to increase. This location of condenser causes non-condensation gases to be gathered on the top of tubes and consequently the tube wall temperature becomes equivalent to the inlet gas temperature, which is supposed to be condensed, and consequently, it will evaporate the water and sediment will be deposited on the hot surface of the tube causing corrosion.

Venting Valve:

Converters should be equipped with venting valve. Especially when water contains chlorine, condensers should be equipped with gas venting valve to discharge the chlorine and air collected at the top of the condenser. If chlorine gas or other corrosive gases are not discharged, they will cause corrosion in the tubes.

Tube-Sheet Material:

Tubes of heat exchangers are connected to two sheets called tube sheets at the two ends or in other words they are welded to these two sheets. Thus, it is important to choose an appropriate material for tube sheet which does not cause polarization. Carbon steel, copper alloy, Metal Munoz (61 to 58 percent copper, 1 percent lead on the rest zinc), Admiral Brass etc. are some of the materials. Galvanic protection of this type of tube against tubes made of copper alloy does not remove corrosion at the inlet and outlet of tubes, but it is on an acceptable level.

Carbon stainless steel is rarely used with a tube made of copper alloy. Since stainless steel acts as cathode when used with copper alloy, if tube sheet is made of copper alloy and tubes are of stainless steel or titanium, corrosion occurs quickly. Since stainless steel acts as a cathode against copper alloy, if tube sheet is made of copper alloy and tubes are of stainless steel or titanium, they will get polarized easily, so that the entire level inside the tube, which is cathodic, will be located against anode which is copper alloy. Cathodic protection flow should be established on the tube sheet of the copper alloy to prevent from or improve such a situation, i.e. the issue of anode – cathode.

Figure 3 Heat Exchanger [4]

Materials for High Temperature Exchangers:

Following classification of high-temperature materials that are promising for different applications are mentioned below: [2, 6]

High-temperature ferritic steels: (particularly oxide- dispersion ferritic steels)

High-temperature ferritic steels as shown in Table 2 work effectively at very high temperatures. They exhibit good performance at temperatures around 750°C. They exhibit good potential for compatibility with lead/bismuth under appropriate composition.

Table 2: Thermal properties of ferritic steels

Material	Composition %	Thermal conductivity at 20^oC (W/m-^oC)	Coefficient of thermal expansion (µm/m-^oC)
410	Ni-0.75; C-0.15; P-0.04; S-0.03; Mn-1; Si-1; Cr-13.5; Fe- Balance.	25	12
430	Ni-0.50; C-0.07; P-0.04; S-0.03; Ti-1.10; Mn-1; Si-1; Cr-18; Fe- Balance.	25	10.4-11.5
430Ti / 439 / 441	Ni-0.50; C-0.12; P-0.04; S-0.03; Mn-1; Si-1; Cr-19; Fe- Balance.	25	11.5
434 / 436 / 444	Mo-0.5; Si-1; Ni-0.50; C-0.12; P-0.04; S-0.03;Cr-18; Fe- Balance.	23	10.5-11.5

High temperature nickel-based Inconel alloys:

High temperature nickel-based Inconel alloys as shown in Table 3, exhibit better mechanical and corrosion resistance properties at higher temperatures than iron based materials. These materials can operate at temperatures as high as 816^oC.

Table 3: Thermal properties of inconel alloys

Material	Composition %	Temperature range ^oC	Thermal conductivity at 20^oC (W/m-^oC)	Coefficient of thermal expansion (µm/m-^oC)
Alloy 600	C -0.15; Mn-1; Si-0.5;Cu-0.5; Cr-17; Ni (plus cobalt)-72; Fe-Balance.	-150 to 900	12.5-27.5	10.9-16.4
Alloy 601	Al-1.7; C-0.1; Mn-1; Si-0.5; Cu-1; Cr-25; Ni-63; Fe-Balance.	20 to 1000	11.2-27.8	13.75-17.82
Alloy 617	Al-1.5; C-.15; Mn-1; Si-1; Ti-0.6; Cu-0.5; Mo-10; Co-15; Cr-24; Ni-44.5; Fe-Balance.	20 to 1000	13.4-28.7	11.6-16.3
Alloy 625	Co-1; Mn-0.5; Al-0.4; Ti-0.4; Nb-4.15; Cr-23; Mo-10; Ni-58; Fe-Balance.	-157 to 1093	7.2-25.2	12.8-16.2
Alloy 718	Co-1, Mo-3.3; Ni-5.5;Ti-1.15; Al-0.8; Cu-0.3; Cr-21; Ni-55; Fe-Balance.	-195 to 1093	11.4	13
Alloy 740H	C-0.03; Mn- 0.10; Ti-0.87; Mo-1.56; Co- 20.67; Nb- 0.65; Al-1.44; Cr- 24.37; Ni- 50.31; Fe-Balance.	23 to 1150	10.2-27.9	12.38-15.81

Advanced Carbon and Silicon Carbide Composites:

Excellent mechanical strength at elevated temperatures of 1000^oC are exhibited by advanced carbon and silicon carbide composites. Carbon composites are one of the lighter and stronger materials. Carbon composites are five times stronger than steel and one third its weight. They are ideal for any application that requires strength, durability, and light weight. On the other hand, SiC composites exhibit excellent creep resistance and thermal stability even at temperatures above 1000^oC.

Properties that are commonly used for high temperature materials are listed in Table 4 below.

Table 4: Selected properties of high temperature materials

Material of heat exchanger	Temperature range ^oC	Strength of heat exchanger	Thermal conductivity at 20^oC (W/m-^oC)	Coefficient of thermal expansion (µm/m-^oC)
Metallic Ni alloys (Inconel 718)	1200-1250	Strength-adequate but limited due to creep and thermal expansion.	11.2	13
Ceramic oxides of Al, Si, AlN, B4C,BN etc..	1500-2500	Strength-not adequate.	0.05-300	0.54-10
Carbon-carbon composite	1400-1650	Strength-poor, oxidation starts at 300^oC.	80-240	0.6-4.3
Carbon fiber-SiC composite	1400-1650	Strength –highest due to carbon fiber and SiC	1200	-0.26(-1.5) (Londitudinal) +25 (Transverse)

CONCLUSIONS:

Cost effective high temperature heat exchangers are key to the success of emerging high-temperature, high-efficiency power cycles for diverse applications of energy conversion, power generation and energy/waste heat recovery applications. The ideal high temperature heat exchanger would offer an optimum balance among heat transfer effectiveness, pressure drop, size and weight (indirectly controlling the cost) of the heat exchanger, while meeting longevity and reliability requirements. For elevated temperatures, most heat-resistant superalloys that can withstand the required temperature suffer from low thermal conductivity and high cost. Therefore, innovative design, materials, and manufacturing techniques are crucial to successful development of such heat exchangers. Nickel-based alloys and composite based materials are found to be the best choice for heat exchanger tubing materials [7, 13].

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Received on 10.08.2020 Accepted on 12.09.2020

Research J. Engineering and Tech. 2020;11(3):125-132.

DOI: 10.5958/2321-581X.2020.00022.7