A Review on Air Preheater Elements Design and Testing Akash Modi, Azharul Haque, Bhanu Pratap, Ish Kumar Bansal, Prasoon Kumar, S Saravanan, M Senthil Kumar, C Kumar

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Akash Modi, Azharul Haque, Bhanu Pratap, Ish Kumar Bansal, Prasoon Kumar, et al.. A Review on Air Preheater Elements Design and Testing. Mechanics, Materials Science & Engineering Journal, Magnolithe, 2017, ￿10.2412/mmse.86.90.615￿. ￿hal-01966399￿

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A Review on Air Preheater Elements Design and Testing 1 Akash Kumar Modi1,a, Azharul Haque1, Bhanu Pratap1, Ish Kumar Bansal1, Prasoon Kumar1,b, S. Saravanan1,c, M. Senthil Kumar1,d, C. Ramesh Kumar1,e

1 – VIT University, Vellore, India a – [email protected] b – [email protected] c – [email protected] d – [email protected] e – [email protected]

DOI 10.2412/mmse.86.90.615 provided by Seo4U.link

Keywords: air pre heater, Ljungstrom air-preheater, heating elements, Reynolds number.

ABSTRACT. This review paper is based on theories and complications related to air-preheater & the surfaces used in it. Numerous papers and sufficient amount of literature were gone through to understand how an air- preheater is designed and the performance parameters associated with the same. Air-preheaters (APH) are heat exchangers, which are used to pre-heat the air before any other process takes place. APH finds its wide use in power plants, automobiles and all such areas where there is a need to pre-heat the air and save fuel. A thorough survey was also done on the heating elements or surfaces used in air-preheaters for the transfer of heat between cold and hot fluid. Various types of widely used profiles were identified first and a research was then conducted to find out how the experimental investigation of heat transfer plates can be done. Excerpts have also been provided regarding the design of experimental setup for the same and the dependence of various parameters on one another. The paper is concluded with an opinion on the use of plate profiles for air-preheater.

Introduction. Air-pre heaters are used to pre-heat the air before the commencement of any other process, for ex- in furnace. These are heat exchangers which are widely used in power plants in order to increase the overall efficiency of the boiler. The water inside the boiler has to be converted into steam for which coal is needed as a fuel which undergoes combustion inside the furnace thus generating huge amount of heat. The coal particles inside the furnace require minimum ignition temperature to catch fire and stoichiometrically required quantity of air with proper inter- mixing. So in order to reduce the amount of energy supplied to heat the air so that the coal catches fire, air-pre heaters are introduced thus saving the fuel required to achieve the temperature conditions inside the furnace. Many limitations are also associated with an air-preheater. APH is broadly classified as recuperative and regenerative. Recuperative type exchanges heat between the hot and cold fluid in a continuous process. Under this, there exists tubular type air-pre heater (APH) and plate- type air-pre heater. Regenerative type also exchange heat between the hot and cold fluid alternatively and it is not continuous. Under this, we have rotating-plate air-pre heater (RAPH) and stationary plate APH. Our focus will be on regenerative air-pre heaters as these heat exchangers find a wide use in industries. Many limitations are also associated with an air-preheater. APH is broadly classified as recuperative and regenerative, recuperative is further classified as tubular and plate-type whereas regenerative is categorized as rotating plate APH and stationary plate APH. Tubular type APH - In this type of APH, cold air flows within the tubes while the hot passes over the tubes. The difficulties which generally occur in this case is that the ducting system required

1 © 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

MMSE Journal. Open Access www.mmse.xyz Mechanics, Materials Science & Engineering, December 2017 – ISSN 2412-5954 for cold air and hot air requires space and more supports which eventually enlarges the entire setup. The second problem is that the portion of the tubes, which is exposed to the flue gas, undergoes wear and tear, reason being the fact that flue gas coming out of the furnace is loaded with ash content and dust particles [1]. Regenerative APH - In this, the heating elements are either rotary or stationary. The primary problem here is the wear and tear of the plates as the incoming flue gas is dust-laden having high ash and silica content. The second problem is the leakage of gases from gaps between the rotating and stationary structures, this leakage of gases affects the performance of APH to a great extent. Seals are provided in the APH to prevent the leakage. Another problem is the deposition of unburnt particles on the surface of APH. As the unburnt deposits meet flowing cold air and flue gas, the ignition temperature is reached along with sufficient amount of oxygen due to which the particles start burning. This also sometimes causes explosions inside the APH. Dew Point Corrosion is a problem generally seen in all kinds of air-pre heaters. The flue gas coming out of the furnace is dust-laden containing contaminants such as chlorides, sulphates etc. As the flue gas attains the acid saturation temperature, i.e. the temperature at which condensation of acids takes place, sulphates, chlorides etc. condensate in the form of acids such as sulphuric acid, hydro-chloric acid and other acids. The condensation takes place on steel tubes and plates, the portion of the tubes or plates which is surrounded by acids is less oxygenated, thus serving as anode while the portion which is not surrounded by acids is more oxygenated thus serving as cathode. This anode-cathode leads up to an electro-chemical cell, which in turn corrodes the base material. Amidst all such problems related to APH, the heating elements or plates play a vital role in regulating the performance of the APH. The plates are also changed at regular intervals in order to ensure unin- terrupted performance of the APH. The aim is to find and investigate the dependence of an air-pre- heater on plate profile. As the rotary air-preheater operates at a relatively high temperature, the matrix wall will expand on coming in contact with the flue gas and shrink on contact with cold fluid. At the same time, the inlet gas temperature, flow rate, and thermodynamic properties are varied momentarily; this results in a typical mushroom type deformation [2]. This deformation causes leakage and also results in mechanical friction which seriously affects the thermal efficiency and security of the system [3].

APH Design

Fig. 1. RAPH [47].

As shown in Fig.1, RAPH consists of a central rotor which is connected to the element baskets or rotating baskets. Within the element baskets, the heating elements/plates are kept in stacks. As the rotor rotates, the baskets also rotate with the rotor surrounded by a rotor shell. The area of the RAPH

MMSE Journal. Open Access www.mmse.xyz Mechanics, Materials Science & Engineering, December 2017 – ISSN 2412-5954 is divided into two parts as shown in Fig.2 (bi-sector), 3 parts (tri-sector) 4 parts (quad-sector) etc. In case of bi-sector APH, flue gas passes through one sector while the cold gas passes through the other sector. The rotation of baskets ensures that all the plates are subjected to both hot as well as cold air. The hot flue gas heats up the plates corresponding to its sector; the same heat gets transferred to the cold air by the plates as the basket rotates. Again, the plates get heated up as they come across the sector of flue gas. In case of tri-sector apart from flue gas sector, we have two other sectors for primary air (conveys pulverized coal to the furnace) and secondary air (air for combustion). This type of APH is also known as Ljungstrom air-preheater. Warren [4] published his studies on Ljungstrom air preheater and based on the experimental results, he confirmed a minimum reduction of 10% in power plants fuel consumption. Heidari et al. [5] investigated RAPH in 3-D and treating it as a porous media with the help of fluent software. They highlighted the effect of factors such as rotational speed of the matrix, mass flow rate of fluid, matrix material and temperature of the incoming air on the performance of the preheater. Wang et al. [3] made use of semi analytical method to examine the 3-dimensional heat transfer of tri-sectional rotary air preheater. In this paper, main focus was on the temperature distribution of the matrix. Stationary plate regenerative air-pre heater - The plates or heating elements are stationary in this case as compared to a rotating air preheater. Instead, the ducting system of inlet air and outlet air is made rotary so that all the plates are exposed to both hot and cold air. Such an air-pre heater is also known as Rothemuhle Air-pre heater. Tubular type APH – it consists of a nest of straight tubes that are roll expanded or welded into tube sheets and then enclosed in a steel casing. This casing serves as an enclosure for fluid or gas passing outside the tubes and has got both air and gas inlet-outlet openings. An expansion joint between the floating tube sheet and casing provides an air/gas seal. Intermediate baffle plates parallel to the tube sheets are frequently used to separate the flow paths and eliminate tube damaging flow induced vibration [1]. The most common flow arrangement in tubular APH is the counter flow of fluids in which flue gas passes vertically through the tubes and air passes horizontally through one or more passes outside the tubes. Provisions are frequently provided in the design for the bypass of cold air or recirculation of hot air in order to control cold end corrosion and ash fouling. Pressure drop: in recuperative air-preheaters, frictional loss during flow; inlet and exit shock losses as well as losses during return bends in flow passage contribute towards pressure drop. Pressure drop is directly proportional to the square of the mass flow rate of air. Leakage: recuperative units may begin operation with essentially zero leakage, but leakage occurs as time and thermal cycles accumulate. With regular maintenance, leakage can be kept below 3%. Approximate air heater leakage can be determined based on gas inlet and outlet oxygen (O2) analysis (dry basis). Plugging and Erosion: plugging is referred to as fouling and the closing down of heat transfer flow passage by gas which is enriched with ash particles and corrosion products, whereas erosion is the removal of a material layer because of high velocity dust particles. It usually occurs at the gas inlet where the velocity is high. The consequences of erosion are dangerous such as structural weakening, loss of heat transfer area and perforation of components, which may result in air to gas leakage. Erosion can be controlled by reducing the velocities, removing the affected material, galvanization or by using a sacrificial material. In an APH, the cold end flue gas temperature is designed for acid due point. Once the coal is burnt completely [6], sufficient alkaline is available which can absorb the acid (H2SO4) thus preventing fouling, corrosion of air-preheater and ducting [7]. If ash to sulphur ratio is more than 7:1 then, cold end fouling does not even occur at below 120 ºC temperature.

MMSE Journal. Open Access www.mmse.xyz Mechanics, Materials Science & Engineering, December 2017 – ISSN 2412-5954

Fig. 2. Rotary APH [47].

The main requirements of an APH are high heat transfer rate, low pressure drop and low sensitivity to fouling. These features mainly depend upon the profile of heating elements used. APH Performance. The performance of a Ljungstrom air-preheater largely depends upon the geometry or profile of the heating elements used in it. The profiles are designed in such a way that it increases the efficiency of the air-preheater along with decrease in fouling rate and pressure drop. Sreedhar Vulloju et al. [8] proved that the performance of a Ljungstrom air-preheater gets affected with the profile of the heating elements used. They conducted the experimental study on two different profiles: FNC (Flat Notched Crossed) and DU (Double Undulated) elements using a wind tunnel setup. The performance characteristics of these elements were evaluated and compared at different Reynolds number and finally the better profile of the two was suggested. The Ljungstrom air- preheater is widely used in power industry compared to any other combustion air-preheater because of its compact design proven performance, reliability and fuel flexibility. Factors contributing towards the deterioration of APH are usually seal leakage, reduction in heat transfer capacity and absorption of heating elements used due to fouling, plugging and corrosion. Ash carry over from hopper can also contribute to degradation of air preheater performance. The expansion joints and ducting also undergo erosion sometimes upstream and downstream of air pre heaters. This erosion also contributes towards the loss of margin in draught system and poor air pre heater performance. The overhauling of an air-preheater consumes plenty of time in overcoming all these obstructions. APH Performance Indices. To know whether the performance of an APH is as per the requirements or not, certain performance indicators have been defined which are used to evaluate the performance of the heat exchanger. A discussion on air-preheater by Ray [9] gives a brief idea about these indices. 1) Air-in leakage: this leakage is assumed to occur entirely between air inlet and gas outlet. It is expressed as a percentage of inlet gas flow. Various seals such as radial seals, axial seals, circumferential seals etc are provided to prevent this leakage as leakage reduces the efficiency of air- preheater. 2) GSE (Gas side efficiency): it is defined as the ratio of gas temperature drop across the air-preheater to the temperature head. Where:

Gas Temperature drop = 푇𝑖푛(flue gas) − 푇표푢푡(flue gas) (1)

Temperature head = 푇𝑖푛(flue gas) − 푇𝑖푛(air) (2)

MMSE Journal. Open Access www.mmse.xyz Mechanics, Materials Science & Engineering, December 2017 – ISSN 2412-5954

3) X-Ratio: it is defined as the ratio of heat capacity of air passing through the air-preheater to the heat capacity of flue gas passing through the air-preheater. X-Ratio is dependent upon moisture in coal, air infiltration, air and gas mass flow rates; leakage through the APH; specific heats of air and flue gas. 4) Pressure drop: it refers to the change in the pressure of both air and flue gas as these passes through the APH. Low pressure drop is preferred for better performance of an air-preheater. 5) Temperature drop of flue gas: there is a decrease in temperature of flue gas as it transfers the heat to the heating elements of the air-preheater. 6) Temperature rise of air: as the air being transferred to the boiler comes in contact with the heating elements, the temperature rises as the elements are at a high temperature when compared to in coming air. Shruti et al [10] presented an experimental study on the performance indices of air-preheater. They conducted an experiment based on routine operational data obtained from LANCO-UPCL, Nagarjuna thermal power plant situated in Udupi, Karnataka. The indices were evaluated before and after different adjustments of clearance of radial sector plate and the observations were recorded. The results showed that air leakage gradually decreased after adjusting the radial seal sector plate clearance. It was observed that gas side efficiency gradually increased as the area between air to the gas side between the rotor and the air preheater housing decreases. It was also observed that X-ratio registered an increase with varying adjustments of hot end and cold end radial sector plates. It indicates maximum heat is recovered in the air pre heater. APH Leakage. Leakage is a major problem as far as rotary regenerative or Ljungstrom air-preheater is concerned [11]. It not only reduces the APH efficiency but also increases the overall heat rate of the power plant. Fig. 3 shows the various paths of leakage in an APH.

Fig. 3. Flow passage of air & flue gas.

• Path 1: Flow passage of normal/cold air; • Path 2: Flow passage of gas; • Path A: Ambient air from FD fan directly leaking into APH gas outlet; • Path B: Pre-heated FD fan air flow surpassing or escaping the air-preheater; • Path C: Ambient FD fan air bypassing the air heater; • Path D: Hot gas coming out of the boiler;

MMSE Journal. Open Access www.mmse.xyz Mechanics, Materials Science & Engineering, December 2017 – ISSN 2412-5954

Path 1 is the direct transfer of the cold air coming from the FD (forced draught) fan to the APH and then to the boiler. Path 2 shows the path of flue gas coming from the furnace and being transferred to further components via ID (induced draught) fan. Path A shows that the air coming from the FD fan surpasses the APH from the top and moves out via ID fan. Path B shows the air from the FD fan after getting heated up by the APH moves out of ID fan instead of going to the boiler. Path A and B represents circumferential leakage. Path C and D represents bypass leakage in which both cold air as well as flue gas moves to their respective destinations without coming in contact with the APH. In order to prevent this leakage, seals are incorporated into use. Major types of seals used in power plants are [10] are radial seals, bypass seals, axial seals and circumferential seals. A study conducted by Kumar et al [7] on performance evaluation of air-pre heater at off-design condition provides ample information about air-pre heater leakage and performance improvement of the same. Their paper analyses a simple flow model of the rotary regenerator and classifies fluid leakage into two categories: i) Pressure Leakage: As the highly pressurized flue gas passes through the sealing system [12], a fraction of it gets mixed with low pressurized cold air. So here leakage is by virtue of pressure difference. ii) Carry over leakage: This occurs when a part of gas stream trapped in voids or empty spaces of the rotor element gets carried to the other gas stream during rotation. The carry over leakage can occur in both the direction. Leakage drift [7] - Air-preheaters are designed with a certain percentage or allowance in leakage. Leakage drift refers to the increase in leakage of air to flue gas due to the deteriorating condition of the sealing system used. Many air-preheaters are provided with additional space which allows them to accommodate extra heating elements in case underperformance is observed. But all air-preheaters are not provided with additional space. In such cases, the old heating elements are replaced with a new set of heating elements. Heating elements. Heating elements used in air-preheaters are made up of Corten steel i.e. HSLA (High strength low alloy steel). Corten steel is preferred for heating elements as this material offers high resistance to corrosion, erosion and high thermal conductivity [8]. The profile or geometry of heating elements used also greatly affects the performance of an air-preheater. Based on the profile geometry it is classified into seven types and they are: 1) DU (Double undulated). Undulated means having a wavy form. The common characteristic of DU element is the alternate stacking of undulated element sheets with sheets that contain both undulations and notches. Fig. 4 shows the view of the DU type profile.

Fig. 4. DU Profile [48].

2) DN (Double Notched). These plates offer reduced fouling and better clean-ability when compared with the equivalent DU elements, while maintaining similar pressure drop and heat transfer characteristic. Fig. 5 shows the view of the DN/DL type profile.

Fig. 5. DN / DL Profile [48].

MMSE Journal. Open Access www.mmse.xyz Mechanics, Materials Science & Engineering, December 2017 – ISSN 2412-5954

3) DL (Double notched loose packed). This Configuration is same as that of DN element, however the elements are stacked loosely within the baskets, hence the elements can move back and forth upto 1 inch during soot blowing. 4) FNC (Flat notched crossed). Fig. 6 shows the constructional features of FNC type plates. This type of plates offers higher thermal performance and lower pressure drop than standard DU type element. It is mainly used in low fouling applications such as oil and gas since elements of this profile are extremely difficult to clean.

Fig. 6. FNC Profile [48].

5) NF (Notched flat). As shown in Fig. 7 this consists of a notched flat plate followed by a completely flat plate. Most common configuration of NF element are NF6 which has large, open spaced notches and is ideal for high ash coals and NF3.5 for low fouling fuels with high acid dew points.

Fig. 7. NF Profiles [48].

6) NU (Notched Undulated). Fig. 8 shows NU type profile. These are similar to NF series but carry an undulated sheet instead of a flat sheet.

Fig. 8. NU Profile [48].

7) CU (Corrugated Undulated). This profile is typically used only in natural gas fired units. Most appropriate when used with the low density flue gases produced when firing gaseous fuels like natural gas. Fig. 9 shows the features of CU profile.

Fig. 9. CU Profile [48].

The plates are provided with corrugations so as to increase the heat transfer surface area [13]. There are different geometries available in corrugated plate such as cross corrugated surface, corrugated undulated, cross wavy surfaces etc. The cross wavy (CW) and especially the Cross Corrugated (CC)

MMSE Journal. Open Access www.mmse.xyz Mechanics, Materials Science & Engineering, December 2017 – ISSN 2412-5954 surfaces show superior performance over others thereby giving a small volume and weight of the heat transfer matrix [14]. However, as the CC surface is well documented in the literature and probably it is easier to manufacture with small passage dimensions, this should be the first choice for further studies by manufacturers [15]. Testing of Heating elements. The testing of heat transfer surfaces of various profiles is done so as to find out the profile which optimizes the heat transfer characteristics and efficiency of an air- preheater. It must be emphasized that regarding the need to determine the heat transfer coefficient the most important parameters are the identification of the geometry of the thermal element and specifically the hydraulic diameter of the flow channels as well as the mass of a single element [16]. Software Analysis. Dilip et al. [17] conducted a software analysis on heating elements on different plate profiles. The profiles were first developed using Pro-E software and were then imported to ANSYS CFX to conduct the CFD analysis. A 70 MW Unit of KLTPS (Kutch Lignite ) of GSECL (Gujarat state electricity corporation limited) situated in Panadhro, Kutch was under observation to get the operational data for the experiment. The APH of this 70 MW Unit was under observation to collect data such as inlet-outlet temperature of air and flue gas, inlet-outlet pressure of air and flue gas, composition of flue gas and properties of flue gas. The profiles of the heating element used were NF, ACE (Advanced clear element), FNC, NU and CU. The boundary conditions were applied and CFD analysis was conducted on each plate profile so as to obtain the temperature profile of flue gas outlet temperature. The efficiency of APH was then calculated for each plate profile and results were then compared. The results showed the heat transfer was dependent on the type of plate profile used in air-preheater and NF profile was the most efficient among all the profiles studied. Experimental Testing. In the paper presented by Vulloju et al. [8] two methods have been identified to determine the performance of heat transfer elements namely Residual time test and Cold flow studies which were explained bellow: 1) Residual Time test: It is the time taken by air to travel from one side of the element to another. Residual time is directly proportional to the length of air travel which in turn is directly proportional to the surface area of the plates. High length of air travel signifies high surface area which means high heat transfer as heat transfer takes place through convection in case of heating elements. Higher the residual time, higher is the heat transfer coefficient of the plates. The study was conducted on FNC and DU plate profiles and concluded that FNC elements have more residual time than DU elements.

푅푇 훼 퐼푇 훼 퐴 (3)

Where RT – residual time;

lT – Length of path of air travel; A – Contact of air surface area through element. But h ∝ A, where h – heat transfer co-efficient. 2) Cold flow studies: This involves the construction of an experimental setup on the footsteps of a Wind tunnel. The setup is then used to determine the performance parameters for various plate profiles. Dariusz Butrymowicz et al. [16] suggested a method for the measurement of heat transfer coefficient of matrices or heating elements used in rotating regenerative heat exchangers. The so- called single blow technique is thought to be an efficient method used to experimentally determine the average heat transfer coefficient α in regenerative heat exchangers composed of thermal elements. Heat transfer coefficient is based on the actual surface area of the thermal elements and takes into account convective heat transfer between gas and the thermal element on the wall surface. In the discussed method the average heat transfer coefficient ‘α’ to be found was determined by means of the comparison of the actual temperature, profile of gas (that is heated or cooled in the tested matrix)

MMSE Journal. Open Access www.mmse.xyz Mechanics, Materials Science & Engineering, December 2017 – ISSN 2412-5954 measured at the outlet of the tested matrix with the predicted one on the basis of the theoretical model. The agreement between the experimental temperature profiles and theoretical prediction depends on the heat transfer coefficient ‘α’ that is applied in the theoretical model of heat transfer. CFD is a branch of science which can be helpful for analysing fluid flow, heat transfer, chemical reactions etc by solving complex mathematical equations with the help of numerical analysis. It is potentially helpful in designing a heat exchanger system from scratch as well as in troubleshooting or optimization by suggesting design modifications. Some of the commercial CFD codes frequently used are FLUENT, CFX, STAR, CD, FIDAP, ADINA, CFD2000, PHOENICS and others [15]. The model became the basis for many further modifications and is still being developed by removing the numerous simplifications assumed by the author in order to obtain an analytical solution of the model equations. Shekoor et al [15] did an investigation on the design optimization of corrugated surface heat exchangers using CFD. A quantitative examination of the thermal performance of the corrugated surface heat exchanger [19] was carried out with various modifications in pitch-height ratio and different corrugation angles [20]. Design optimization of cross corrugated surface with different corrugation angles [21] such as 300, 450, 600 and 750 was investigated in this paper. Experimental Apparatus. Experimental analysis of APH mostly involves test setup which includes a wind tunnel [8]. As shown in Fig. 10, it consists of a converging section, test section in the middle and a diffuser section. A wind tunnel is a tool used for aerodynamic research. The object or specimen is kept in the middle and the fluid is made to pass over it. A wind tunnel reverses the real life situation in which the fluid is stationary and the object moves through it. Inside the tunnel, the object is stationary and the fluid moves over it. The driving unit consists of a Fan, Blower or a Compressor connected to an electric motor. The location of the driving motor depends upon the type of the tunnel. Compressor or blower or the fan creates a flow of air which gets settled in a large chamber known as the settling chamber, this chamber is equipped with wire gauges and a set of honeycombs so that the flow can be straightened and irregularities be removed. In cases of very low velocities, near stagnation conditions exist in the settling chamber. This chamber supplies the flow to the converging section located downstream. This is carefully designed to accelerate the flow from the settling chamber to the test section velocity with minimum disturbance. The converging section or the nozzle feeds the test section with a jet of uniform velocity. The model to be tested is fixed here with suitable supports. A transparent window is also provided on the side- walls of the test section so that the test specimen and the measuring instruments can be monitored properly. The diffuser takes the flow from the test section and discharges it into the atmosphere at a relatively high pressure.

Fig. 10. Wind tunnel setup [8].

The plates or heating elements are kept in the middle test section. The flue gas is made to pass first thus heating the elements. The blower or compressor attached to the diffuser then creates a suction

MMSE Journal. Open Access www.mmse.xyz Mechanics, Materials Science & Engineering, December 2017 – ISSN 2412-5954 effect, which passes cold air through the heating elements. The overall idea is to simulate the condi- tion of a Ljungstrom air-preheater through the use of this wind tunnel. After the heating, cold air passes through, heat gets transferred from the plates to the air hence a temperature rise of cold air is noticed. The velocity of cold air is measured for a range of Reynolds number based on which the fanning friction factor, pressure drop and heat transfer coefficient of the plate profiles are calculated. In the experiment conducted by Vulloju et al [8], the velocity measurement was done using a TSI Velocity meter. TSI Velocity meter is an advanced electronic model of a Hot Wire Anemometer. A hot wire anemometer consists of a tiny wire (d=0.005 mm) held between two prongs; it is heated (hence the term Hot-wire) to a given temperature by passing an electric current through it. When such a wire is introduced into the flow of a gas it cools down the hot wire to a lower temperature due to convective heat transfer from the wire element to the gas. Higher the heat transfer from the wire, higher is the heat transfer coefficient of the fluid which is directly proportional to the velocity of the fluid and so the velocity can be calculated.

Flow cross Sectional area 퐷 = 4 × (4) ℎ Wetted perimeter 1.293 ×273 푘푔⁄푚3 휌푎𝑖푟 = (5) (273 +푡푎푣𝑔)

Reynolds number is calculated by using formula:

휌푉 퐷 푅 = 푎 ℎ (6) 푒 휇 where ρ – Density of air in kg/m3; µ - Viscosity of air in pa/sec. Friction factor is calculated by the following formula

∆푝 푓퐿푉2 = 푎 (7) 휌 2퐷ℎ

Where Δp – Pressure difference in mm of water column; L – Length of test, section in wind tunnel in m; f – friction factor;

Dh – hydraulic diameter of the plate profiles.

푁푢 Colburn coefficient (푗 factor) = (8) 푅푒.푃푟1⁄3 where Nu – Nusselt number; Pr – Prandtl number. Single blow technique [16] can also be used to find the heat transfer coefficient of the matrices of a rotary regenerative air-preheater.

MMSE Journal. Open Access www.mmse.xyz Mechanics, Materials Science & Engineering, December 2017 – ISSN 2412-5954

Fig. 11. The idea of heat transfer measurement by single-blow technique [16].

The experimental part of the single blow method is carried out in a wind tunnel as shown in Fig.11. Because of limitations of the test chamber of the tunnel, usually the entire heat exchanger cannot be tested so the test involves only a matrix composed of thermal elements used in a real heat exchanger. Temperature and pressure measurement points are placed at the inlet and outlet of the tested matrix. Static pressure difference is measured in order to determine the frictional flow resistance [22]. In addition, the velocity of gas flow through the matrix should be also measured. It may be suggested that the temperature of gas flowing through the matrix should be raised above the ambient temperature by approximately 10–20 K. An electrical heating coil made of resistance wires may be applied for this purpose. Due to the required velocity homogeneity, the heating coil should disturb the flow as little as possible. The structure of the coil also affects the gas temperature profile obtained after switching it on or off which is important from the point of view of the theoretical model applied to draw up the measurement results. Due to the homogeneity of velocity profiles required in theoretical models, it is recommended to place a flow straightener (in the form of a honeycomb matrix) before the test section [23]. In order to eliminate the influence of disturbances generated at the tunnel outlet geometry, a similar flow straightener should be also placed at the outlet of the test section. The fol- lowing conditions required before the measurement: • Gas flow is steady state (constant velocity) • The temperature in the measurement section is constant and equal in the axial and radial directions. The measurement of pressure differences between the inlet and the outlet of the tested matrix, includ- ing gas velocity measurement, can be used to determine the frictional resistance as a function of Fanning coefficient f versus Reynolds number, Re.

Fig. 12. The complete measurement apparatus [16]. 1 – Packages of investigating elements, 2 – pressure measuring point, 3 – electrical heater for heating air, 4 – flow rectifier 5 – fan, 6 – pile up elements DEBIMO, 7 – diffuser.

MMSE Journal. Open Access www.mmse.xyz Mechanics, Materials Science & Engineering, December 2017 – ISSN 2412-5954

The difference here as shown in Fig.12 is that instead of using flue gas to heat the elements, an electric heater is provided just before the heating elements to heat the incoming air, which later transfers the same to the plates. Dependence of parameters on Reynolds number. Experimental characteristics such as fanning friction factor, pressure drop and heat transfer coefficient vary with Reynolds number (Re). The below formula shows the relation between the friction factor and Reynolds number.

−0.446 푓 = 1.1835 푅푒푎𝑖푟,𝑖푛 (9)

The experimental graph given by Dariusz Butrymowicz [13] is given below in Fig. 13:

Fig. 13. Graph showing the variation of colburn-j factor with Reynolds no. [16].

Fig. 14. Variation of friction factor with Reynolds no. [16].

From the analytical as well as graphical results, it was found that there was a decrease in friction factor as shown in Fig.14, as well as in colburn-j factor with the increase in Reynolds number. Summary. After going through numerous study material, research papers and experimental work done in the field of air-preheater and heating elements, the fact got consolidated that the heating elements forms the heart of the air-preheater and the performance of the APH gets significantly affected with the change in geometry of the plates. Although when it comes to the experimental investigation of heat transfer characteristics of heating elements used in air-preheater, less but significant work has been done in the field so we couldn’t find much research papers on this topic. It is possible to determine the heat transfer characteristics of the plates using a wind tunnel like setup which aims to simulate the conditions of an air-preheater. However, we feel that an appreciable amount of difference in the readings may arise when we compare the simulated readings with the

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