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<article article-type="research-article" dtd-version="1.1" specific-use="sps-1.9" xml:lang="en" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">
	<front>
		<journal-meta>
			<journal-id journal-id-type="publisher-id">dyna</journal-id>
			<journal-title-group>
				<journal-title>DYNA</journal-title>
				<abbrev-journal-title abbrev-type="publisher">Dyna rev.fac.nac.minas</abbrev-journal-title>
			</journal-title-group>
			<issn pub-type="ppub">0012-7353</issn>
			<issn pub-type="epub">2346-2183</issn>
			<publisher>
				<publisher-name>Universidad Nacional de Colombia</publisher-name>
			</publisher>
		</journal-meta>
		<article-meta>
			<article-id pub-id-type="doi">10.15446/dyna.v88n217.93354</article-id>
			<article-categories>
				<subj-group subj-group-type="heading">
					<subject>Articles</subject>
				</subj-group>
			</article-categories>
			<title-group>
				<article-title>Analysis of potential waste heat recovery from a stenter in a textile plant</article-title>
				<trans-title-group xml:lang="es">
					<trans-title>Análisis del potencial de recuperación de calor residual de una termofijadora en una planta textil</trans-title>
				</trans-title-group>
			</title-group>
			<contrib-group>
				<contrib contrib-type="author">
					<contrib-id contrib-id-type="orcid">0000-0002-5132-8810</contrib-id>
					<name>
						<surname>Mazo-Restrepo</surname>
						<given-names>Ricardo</given-names>
					</name>
					<xref ref-type="aff" rid="aff1"><sup>a</sup></xref>
				</contrib>
				<contrib contrib-type="author">
					<contrib-id contrib-id-type="orcid">0000-0002-4435-3989</contrib-id>
					<name>
						<surname>Alvarado</surname>
						<given-names>Pedro</given-names>
					</name>
					<xref ref-type="aff" rid="aff1"><sup>a</sup></xref>
				</contrib>
				<contrib contrib-type="author">
					<contrib-id contrib-id-type="orcid">0000-0001-7535-9622</contrib-id>
					<name>
						<surname>Cacua</surname>
						<given-names>Karen</given-names>
					</name>
					<xref ref-type="aff" rid="aff1"><sup>a</sup></xref>
				</contrib>
				<aff id="aff1">
					<label>a</label>
					<institution content-type="original"> Facultad de Ingenierías, Grupo de Investigación Materiales Avanzados y Energía, Instituto Tecnológico Metropolitano, Medellín, Colombia. ricardomazo190223@correo.itm.edu.co, pedroalvarado@itm.edu.co, karencacua@itm.edu.co</institution>
					<institution content-type="normalized">Instituto Tecnológico Metropolitano</institution>
					<institution content-type="orgdiv1">Facultad de Ingenierías</institution>
					<institution content-type="orgdiv2">Grupo de Investigación Materiales Avanzados y Energía</institution>
					<institution content-type="orgname">Instituto Tecnológico Metropolitano</institution>
					<addr-line>
						<city>Medellín</city>
					</addr-line>
					<country country="CO">Colombia</country>
					<email>ricardomazo190223@correo.itm.edu.co</email>
					<email>pedroalvarado@itm.edu.co</email>
					<email>karencacua@itm.edu.co</email>
				</aff>
			</contrib-group>
			<pub-date date-type="pub" publication-format="electronic">
				<day>24</day>
				<month>11</month>
				<year>2021</year>
			</pub-date>
			<pub-date date-type="collection" publication-format="electronic">
				<season>Apr-Jun</season>
				<year>2021</year>
			</pub-date>
			<volume>88</volume>
			<issue>217</issue>
			<fpage>292</fpage>
			<lpage>302</lpage>
			<history>
				<date date-type="received">
					<day>03</day>
					<month>02</month>
					<year>2021</year>
				</date>
				<date date-type="rev-recd">
					<day>20</day>
					<month>04</month>
					<year>2021</year>
				</date>
				<date date-type="accepted">
					<day>04</day>
					<month>05</month>
					<year>2021</year>
				</date>
			</history>
			<permissions>
				<license license-type="open-access" xlink:href="https://creativecommons.org/licenses/by-nc-nd/4.0" xml:lang="en">
					<license-p>The author; licensee Universidad Nacional de Colombia</license-p>
				</license>
			</permissions>
			<abstract>
				<title>Abstract</title>
				<p>The textile sector, an important economic driving force in Antioquia, Colombia, uses great quantities of thermal energy mainly produced by coal combustion, which holds enormous potential for recovery. One of the most common processes in a textile plant is heat setting, which uses a significant amount of thermal energy to adjust the properties of fabrics, such as shrinking, stiffness, pull strength, width, and stretching. In this study, we calculate the mass and energy balances of a stenter and propose a system to recover the energy available in its exhaust gases. The energy recovery potential in this heat setting process is 800.97 kW, which represents 87.2% of the total input energy. Additionally, we evaluate different heat exchangers to recover the available heat and present criteria to select them. Finally, thermosyphons, whose thermal efficiency was theoretically determined here, offer a promising alternative for heat recovery from actual stenters.</p>
			</abstract>
			<trans-abstract xml:lang="es">
				<title>Resumen</title>
				<p>El sector textil, un importante motor económico de Antioquia, Colombia, usa grandes cantidades de energía térmica, principalmente producida por la combustión de carbón y tiene un enorme potencial de recuperación. Uno de los procesos en una planta textil es el termofijado, el cual usa una cantidad significativa de energía térmica para ajustar las propiedades de la tela, tales como encogimiento, rigidez, fuerza de tracción, ancho y estiramiento. En este estudio, se realizó el balance de masa y energía de una termofijadora y se propuso un sistema para recuperar la energía en sus gases de escape. El potencial de recuperación de energía en el proceso de termofijado fue de 800,97 kW, representando 87,2% de la energía total de entrada. Adicionalmente, se evaluaron diferentes opciones de intercambiadores de calor. Finalmente, los termosifones, cuya eficiencia térmica se determinó teóricamente, ofrecen una alternativa viable para la recuperación de calor de las termofijadoras.</p>
			</trans-abstract>
			<kwd-group xml:lang="en">
				<title>Keywords:</title>
				<kwd>stenter</kwd>
				<kwd>heat setting</kwd>
				<kwd>textile plant</kwd>
				<kwd>waste heat recovery</kwd>
				<kwd>heat pipes</kwd>
				<kwd>thermal energy</kwd>
			</kwd-group>
			<kwd-group xml:lang="es">
				<title><italic>Palabras Clave:</italic></title>
				<kwd>termofijadora textil</kwd>
				<kwd>termofijado</kwd>
				<kwd>planta textil</kwd>
				<kwd>recuperación de calor residual</kwd>
				<kwd>termosifones</kwd>
				<kwd>energía térmica</kwd>
			</kwd-group>
			<counts>
				<fig-count count="4"/>
				<table-count count="12"/>
				<equation-count count="21"/>
				<ref-count count="24"/>
				<page-count count="10"/>
			</counts>
		</article-meta>
	</front>
	<body>
		<sec>
			<title>Nomenclature</title>
			<p><italic>ṁ</italic>
 <sub>
 <italic>waterEC1</italic> 
</sub> = Water mass flow inlet to the air circulation chamber 1 [kg/s]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>Fin</italic> 
</sub> = Fabric mass flow inlet to stenter machine [kg/s]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>AirRTEC1</italic>
</sub> = Air mass flow inlet to circulation chamber 1 [kg/s]</p>
			<p><italic>W</italic>
 <sub>
 <italic>FAE</italic> 
</sub> = Fans and extractors power [kW]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>AirLE1</italic> 
</sub> = Air mass flow outlet from the extraction duct 1 [kg/s]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>WaterLE1</italic> 
</sub> = Water mass flow outlet from the extraction duct 1 [kg/s]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>AirLE2</italic> 
</sub> = Air mass flow outlet from the extraction duct 2 [kg/s]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>WaterLE2</italic> 
</sub> = Water mass flow outlet from the extraction duct 2 [kg/s]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>WaterOC7</italic> 
</sub> = Water in the fabric outlet from the air circulation chamber 7 [kg/s]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>Foutlet</italic>
</sub> = Fabric mass flow outlet from the stenter machine [kg/s]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>AirRTEC7</italic>
</sub> = Air mass flow inlet to the circulation chamber 7 [kg/s]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>OILin</italic>
</sub> = Hot oil mass flow inlet to [kg/s]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>OILoutlet</italic>
</sub> = Hot oil mass flow outlet the heat exchangers set [kg/s]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>WaterEvp</italic> 
</sub> = Evaporated water in the stenter machine [kg/s]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>WaterERTAC1</italic> 
</sub> = Water mass flow inlet to air circulation chamber 1 [kg/s]</p>
			<p><italic>ṁ</italic>
 <sub>
 <italic>WaterERTAC7</italic> 
</sub> = Water mass flow inlet to air circulation chamber 7 [kg/s]</p>
			<p><italic>TFM</italic> = Total fabric meters [m]</p>
			<p><italic>WER</italic>= Weight of each roll [kg]</p>
			<p><italic>V</italic> = Fabric speed [m/min]</p>
			<p><italic>CP</italic>
 <sub>
 <italic>OilIN</italic> 
</sub> = Specific heat of hot oil (inlet) [kJ/kg°C]</p>
			<p><italic>T</italic>
 <sub>
 <italic>OilIN</italic> 
</sub> = Oil inlet temperature [°C].</p>
			<p><italic>CP</italic>
 <sub>
 <italic>FIN</italic> 
</sub> = Specific heat of fabric [kJ / kg ° C] </p>
			<p><italic>T</italic>
 <sub>
 <italic>FIN</italic> 
</sub> = Fabric inlet temperature [°C]</p>
			<p><italic>h</italic>
 <sub>
 <italic>WaterEC1</italic> 
</sub> = Enthalpy of water inlet to the circulation chamber 1 [kJ/kg]</p>
			<p><italic>h</italic>
 <sub>
 <italic>AirRTEC1</italic> 
</sub> = Enthalpy of air inlet to circulation chamber 1 [kJ/kg]</p>
			<p><italic>h</italic>
 <sub>
 <italic>AirRTEC7</italic> 
</sub> = Enthalpy of air inlet to circulation chamber 7 [kJ/kg]</p>
			<p><italic>h</italic>
 <sub>
 <italic>WaterLC7</italic> 
</sub> = Enthalpy of water outlet in the fabric in the circulation chamber 7 [kJ/kg]</p>
			<p><italic>CP</italic>
 <sub>
 <italic>OilO</italic> 
</sub> = Specific heat of hot oil (outlet) [kJ/kg°C]</p>
			<p><italic>T</italic>
 <sub>
 <italic>OilO</italic> 
</sub> = Hot oil temperature outlet [°C].</p>
			<p><italic>CP</italic>
 <sub>
 <italic>FO</italic> 
</sub> = Specific heat of the fabric (outlet) [kJ/kg°C]</p>
			<p><italic>T</italic>
 <sub>
 <italic>FO</italic> 
</sub> = Fabric outlet temperature [°C]</p>
			<p><italic>h</italic>
 <sub>
 <italic>AirLE1</italic> 
</sub> = Air enthalpy from extractor 1 [kJ/kg]</p>
			<p><italic>h</italic>
 <sub>
 <italic>AirLE2</italic> 
</sub> = Air enthalpy outlet from extractor 2 [kJ/kg]</p>
			<p><italic>h</italic>
 <sub>
 <italic>Evp</italic> 
</sub> = Latent heat of water [kJ/kg]</p>
			<p><italic>HrelRT</italic> = Relative humidity at room temperature</p>
		</sec>
		<sec sec-type="intro">
			<title>1. Introduction</title>
			<p>Nowadays, the high demand for energy and the effects of climate change have motivated a search for new alternatives that improve the efficiency of industrial processes. In particular, waste heat recovery has been widely implemented to improve energy efficiency. It has been estimated that between 20% and 50% of the energy used in the industrial sector is wasted as heat in the form of hot exhaust gases, cooling water, and heat losses from the surfaces of hot equipment and heated products [<xref ref-type="bibr" rid="B1">1</xref>]. The textile sector presents great opportunities to increase its energy efficiency by using the waste heat from equipment that offer high energy availability, such as boilers, natural gas generators, waste water form dyeing, and stenters [<xref ref-type="bibr" rid="B1">1</xref>-<xref ref-type="bibr" rid="B4">4</xref>]. </p>
			<p>In the textile sector, stenter machines can consume up to 30% of the total thermal energy that is used, and potential energy savings in this process oscillate between 20% and 70% [<xref ref-type="bibr" rid="B5">5</xref>]. Heat setting, part of the finishing process applied to fabrics, aims to provide the fabric with dimensional stability by thermally fixing the fibers. In stenter machines, most power is used to evaporate water in the fabric and heat air [<xref ref-type="bibr" rid="B6">6</xref>]. The power consumption of this process depends on the specific operation and fuel, but it varies between 2.5-7.5 GJ/ton in the drying and 4.0-9.0 GJ/ton in the heat setting stages [<xref ref-type="bibr" rid="B6">6</xref>]. Nevertheless, when optimized, such consumption can be as low as 3.5-4.5 MJ/ton of processed fabric [<xref ref-type="bibr" rid="B5">5</xref>].</p>
			<p>In general, to increase the energy efficiency of heat setting, heat exchangers are implemented to recover the waste energy in the gases it produces. Some of the most common types of heat recovery are recirculation, tube heat exchangers, plate heat exchangers, thermal wheels, heat pipes, run-round coils, and direct contact coolers [<xref ref-type="bibr" rid="B7">7</xref>]. The selection of the type of heat exchanger depends on different criteria such as effectiveness, fouling factor, cost, and payback period. Furthermore, such selection and energy recovery level also depend on the specific operation conditions of the equipment that can provide waste heat [<xref ref-type="bibr" rid="B8">8</xref>].</p>
			<p>Therefore, increasing the energy efficiency in industrial processes by heat recovery is an opportunity to obtain economic and environmental benefits. In this study, we calculated the mass and energy balances in an actual heat setting process in a company in textile sector in Antioquia and determined the amount of thermal energy that could be recovered. Finally, we applied specific criteria to select one of the most commonly used heat recovery technologies for this purpose and determined the thermal energy recovery potential using such technology.</p>
		</sec>
		<sec sec-type="methods">
			<title>2. Methodology</title>
			<p>In this article, we studied a KRANZ stenter which is currently in operation. It has seven air circulation chamber and two hot gas extractor fans. The mass and energy balances were calculated only in the heated chambers because that is where the heat transfer occurs in order to dry and heat set the fabric. <xref ref-type="fig" rid="f1">Fig. 1</xref> presents a scheme of the seven heated chambers. </p>
			<p>
				<fig id="f1">
					<label>Figure 1</label>
					<caption>
						<title>General scheme of the heated chambers in the stenter, the outlet ducts of Chambers 1-4 are connected by Extractor fan 1 (E1). Likewise, the outlet ducts of Chambers 5-7 are connected by Extractor fan 2 (E2). Each chamber is heated using a heat exchanger through which hot thermal oil is circulated.</title>
					</caption>
					<graphic xlink:href="2346-2183-dyna-88-217-292-gf1.png"/>
					<attrib>Source: the authors</attrib>
				</fig>
			</p>
			<sec>
				<title>2.1 Mass and energy balance of the stenter</title>
				<p>This mass and energy balance and the assumptions made here are based on previous studies [<xref ref-type="bibr" rid="B9">9</xref>-<xref ref-type="bibr" rid="B11">11</xref>]. We used the scheme in <xref ref-type="fig" rid="f2">Fig. 2</xref>, which shows each one of the mass and energy inlets and outlets found in the stenter.</p>
				<p>
					<fig id="f2">
						<label>Figure 2</label>
						<caption>
							<title>Scheme for mass and energy balance</title>
						</caption>
						<graphic xlink:href="2346-2183-dyna-88-217-292-gf2.png"/>
						<attrib>Source: the authors</attrib>
					</fig>
				</p>
				<p>Hypotheses for the mass and energy balance:</p>
				<p>
					<list list-type="bullet">
						<list-item>
							<p>There is no heat transfer between the stenter walls and the environment. This assumption is based on the fact there is glass fiber insulation (10 cm thick), and the temperature of the wall measured at 70 random points was 43±2 °C.</p>
						</list-item>
						<list-item>
							<p>The heat setting process is carried out in stationary conditions.</p>
						</list-item>
						<list-item>
							<p>The composition of the outlet gases of the stenter is assumed to be similar to that of air.</p>
						</list-item>
						<list-item>
							<p> Regarding the sign convention, the heat transfers to the system and the work transfer are positive.</p>
						</list-item>
					</list>
				</p>
				<sec>
					<title>2.1.1 Mass balance</title>
					<p>
						<xref ref-type="fig" rid="f2">Fig. 2</xref> presents the scheme for the mass balance of the system, which was established considering different fluids and materials: hot oil, air used to dry/heat set the fabric, water contained in the air, and fabric.</p>
					<p>We used the following equations in the mass balance of the stenter:</p>
					<p>Mass balance of the fabric (1):</p>
					<p>
						<disp-formula id="e1">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e1.png"/>
						</disp-formula>
					</p>
					<p>Mass balance of the oil (2):</p>
					<p>
						<disp-formula id="e2">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e2.png"/>
						</disp-formula>
					</p>
					<p>
						<xref ref-type="disp-formula" rid="e3">Eqs. (3)</xref>-(<xref ref-type="disp-formula" rid="e4">4</xref>) present the mass balance of air. Bearing in mind that Extractor fan 1 is connected to heated Chambers 1-4 and Extractor fan 2 is connected to Chambers 5-7, we have:</p>
					<p>
						<disp-formula id="e3">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e3.png"/>
						</disp-formula>
					</p>
					<p>
						<disp-formula id="e4">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e4.png"/>
						</disp-formula>
					</p>
					<p>
						<xref ref-type="disp-formula" rid="e5">Eqs. (5)</xref>-(<xref ref-type="disp-formula" rid="e8">8</xref>) present the mass balance of water. In general, when polyester/cotton is processed, the fabric is dried in the first three heated chambers and heat set in the other four.</p>
					<p>
						<disp-formula id="e5">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e5.png"/>
						</disp-formula>
					</p>
					<p>
						<disp-formula id="e6">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e6.png"/>
						</disp-formula>
					</p>
					<p>
						<disp-formula id="e7">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e7.png"/>
						</disp-formula>
					</p>
					<p>
						<disp-formula id="e8">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e8.png"/>
						</disp-formula>
					</p>
					<p>The parameters required for the mass and energy balance were experimentally obtained. The mass flow rate of the fabric was obtained with the mass and the time the fabric is exposed in the stenter.</p>
					<p>The stenter was started, but the fabric was kept in place for us to measure the temperature in each area before entering the heated chambers and at the outlet of such chambers. We used a RAYTEK ST20 PRO infrared thermometer with a range between -32 and 400°C ± 1°C and an emissivity of 0.95, which is adequate to measure fabric temperatures [<xref ref-type="bibr" rid="B12">12</xref>]. </p>
					<p>The fabric yield, considering the weight and length of each roll of fabric, was calculated using <xref ref-type="disp-formula" rid="e9">Eq. (9)</xref>.</p>
					<p>
						<disp-formula id="e9">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e9.png"/>
						</disp-formula>
					</p>
					<p>where, with the velocity of the fabric and the yield, the mass flow rate was calculated with <xref ref-type="disp-formula" rid="e10">Eq. (10)</xref>.</p>
					<p>
						<disp-formula id="e10">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e10.png"/>
						</disp-formula>
					</p>
				</sec>
				<sec>
					<title>2.1.2 Mass flow rate of air and exhaust air temperatures</title>
					<p>The mass flow rate of the hot air that is released into the atmosphere was calculated with the volumetric flow rate and density. The volumetric flow rate was measured using a Pitot-type TESTO 485-4 differential pressure meter, which is also equipped with a Type K temperature probe with a measurement range between -60 and 400°C. Such rate was measured considering the requirements described in [<xref ref-type="bibr" rid="B13">13</xref>]. </p>
				</sec>
				<sec>
					<title>2.1.3 Mass flow rate of the water that enters along with air into the stenter</title>
					<p>The moisture of the air that enters the stenter was determined using the psychrometric properties of air and using an online psychrometric calculator that enables users to make corrections for altitude [<xref ref-type="bibr" rid="B14">14</xref>].</p>
				</sec>
				<sec>
					<title>2.1.4 Mass flow rate of evaporated water</title>
					<p>To determine the mass flow rate of water, we measured the fabric moisture before and after the process by taking representative pieces of fabric and analyzing them in a Precisa XM60-HR moisture analyzer. The mass flow rate of evaporated water was calculated using the difference between the inlet and outlet fabric moisture. </p>
				</sec>
			</sec>
			<sec>
				<title>2.2 Energy balance</title>
				<p>To calculate the energy balance, we took into consideration the mass flow rates of the fluids in the process and the fabric, as well as the power of the fans. <xref ref-type="disp-formula" rid="e11">Eq. (11)</xref> presents the energy balance we implemented.</p>
				<p>
					<disp-formula id="e11">
						<graphic xlink:href="2346-2183-dyna-88-217-292-e11.png"/>
					</disp-formula>
				</p>
				<p>The properties of the fluids and the fabric were determined using tables of thermodynamic properties [<xref ref-type="bibr" rid="B15">15</xref>]. The instantaneous power of the fans was measured using a Fluke 435 power quality analyzer. </p>
				<p>
					<table-wrap id="t1">
						<label>Table 1</label>
						<caption>
							<title>Selection criteria and assigned values.</title>
						</caption>
						<graphic xlink:href="2346-2183-dyna-88-217-292-gt1.png"/>
						<table-wrap-foot>
							<fn id="TFN1">
								<p>Source: authors’ elaboration</p>
							</fn>
						</table-wrap-foot>
					</table-wrap>
				</p>
			</sec>
			<sec>
				<title>2.3 Selecting a heat exchanger to recover heat from the stenter</title>
				<p>Since different types of heat exchangers can be used to recover heat in the textile industry, we defined specific selection criteria to find the best alternative (see). Such criteria, taken and adapted from [<xref ref-type="bibr" rid="B7">7</xref>], were assigned values that depend on the system requirements.</p>
				<p>Once we applied the values of the selection criteria to each exchanger, we obtained the results in <xref ref-type="table" rid="t2">Table 2</xref>. Additionally, considering that higher values represent better options, <xref ref-type="table" rid="t3">Table 3</xref> presents a more detailed analysis of each type of exchanger that can be used to recover waste heat from the stenter.</p>
				<p>
					<table-wrap id="t2">
						<label>Table 2</label>
						<caption>
							<title>Application of selection criteria to several types of heat recovery.</title>
						</caption>
						<graphic xlink:href="2346-2183-dyna-88-217-292-gt2.jpg"/>
						<table-wrap-foot>
							<fn id="TFN2">
								<p>Source: authors’ elaboration</p>
							</fn>
						</table-wrap-foot>
					</table-wrap>
				</p>
				<p>
					<table-wrap id="t3">
						<label>Table 3</label>
						<caption>
							<title>Technical analysis of heat exchangers applicable to heat recovery from stenters.</title>
						</caption>
						<graphic xlink:href="2346-2183-dyna-88-217-292-gt3.png"/>
						<table-wrap-foot>
							<fn id="TFN3">
								<p>Source: authors’ elaboration</p>
							</fn>
						</table-wrap-foot>
					</table-wrap>
				</p>
			</sec>
			<sec>
				<title>2.4 Methodology to design the heat exchanger</title>
				<p>Based on the methodological analysis described above, heat pipes are a viable solution to recover heat in this case. The exchanger was designed using an iterative process to find the best arrangement of the thermosyphons. This process was completed following the decision-making map in <xref ref-type="fig" rid="f3">Fig. 3</xref>. The equations to find the value of each variable in this map were taken from [<xref ref-type="bibr" rid="B15">15</xref>-<xref ref-type="bibr" rid="B19">19</xref>]. The constraints to define the final geometry of the exchanger included:</p>
				<p>
					<fig id="f3">
						<label>Figure 3</label>
						<caption>
							<title>Decision-making map to design the thermosyphon bank.</title>
						</caption>
						<graphic xlink:href="2346-2183-dyna-88-217-292-gf3.png"/>
						<attrib>Source: authors’ elaboration.</attrib>
					</fig>
				</p>
				<p>
					<list list-type="bullet">
						<list-item>
							<p>A gas outlet temperature of 100°C ± 2°C </p>
						</list-item>
						<list-item>
							<p>A maximum bank width of 1300 mm (The ducts of the stenter are 750 mm. This constraint seeks to avoid abrupt transitions at the inlet, which can result in the fluid only going through the middle and not coming into contact with the thermosyphons located on the sides.)</p>
						</list-item>
					</list>
				</p>
				<sec>
					<title>2.4.1 Working fluid and thermosyphon material</title>
					<p>The working fluid and the material of the thermosyphons were selected considering the temperature range of the gases and the recommendations in [<xref ref-type="bibr" rid="B20">20</xref>]. </p>
				</sec>
				<sec>
					<title>2.4.2 Hydraulic design of the heat recovery system</title>
					<p>The pressure drop was calculated using <xref ref-type="disp-formula" rid="e11">Eq. (11)</xref> [<xref ref-type="bibr" rid="B15">15</xref>]</p>
					<p>
						<disp-formula id="e12">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e12.jpg"/>
						</disp-formula>
					</p>
					<p>The electric power required to move the fluids through the exchanger depends on the pressure drop (ΔP), and it was calculated with <xref ref-type="disp-formula" rid="e12">Eq. (12)</xref>
					</p>
					<p>
						<disp-formula id="e13">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e13.png"/>
						</disp-formula>
					</p>
				</sec>
			</sec>
		</sec>
		<sec sec-type="results">
			<title>3. Results</title>
			<sec>
				<title>3.1 Mass balance</title>
				<p>
					<xref ref-type="table" rid="t4">Table 4</xref> presents the results of applying the equations described in the methodology of the mass balance and the values of the experimental properties.</p>
				<p>
					<disp-formula id="e14">
						<graphic xlink:href="2346-2183-dyna-88-217-292-e14.jpg"/>
					</disp-formula>
				</p>
				<p>
					<table-wrap id="t4">
						<label>Table 4</label>
						<caption>
							<title>Properties of the fluids involved in the system.</title>
						</caption>
						<graphic xlink:href="2346-2183-dyna-88-217-292-gt4.jpg"/>
						<table-wrap-foot>
							<fn id="TFN4">
								<p>Source: authors’ elaboration</p>
							</fn>
						</table-wrap-foot>
					</table-wrap>
				</p>
				<p>Thus, the mass flow rate of the oil is</p>
				<p>
					<disp-formula id="e15">
						<graphic xlink:href="2346-2183-dyna-88-217-292-e15.png"/>
					</disp-formula>
				</p>
				<p>The energy flow that enters and leaves along with each fluid in the heat setting process is presented in <xref ref-type="table" rid="t5">Table 5</xref>.</p>
				<p>
					<table-wrap id="t5">
						<label>Table 5</label>
						<caption>
							<title>Energy flows in the heat setting process.</title>
						</caption>
						<graphic xlink:href="2346-2183-dyna-88-217-292-gt5.png"/>
						<table-wrap-foot>
							<fn id="TFN5">
								<p>Source: authors’ elaboration</p>
							</fn>
						</table-wrap-foot>
					</table-wrap>
				</p>
				<p>The energy balance shows that the total transferred heat is 918.75 kW, which are supplied by the thermal oil to dry and heat set the fabric, in addition to 40.01 kW of electrical energy that are transformed into work to move the air in the heated chambers and extract it from the stenter. <xref ref-type="fig" rid="f4">Fig. 4</xref> is a Sankey diagram that represents the thermal energy in this process. </p>
				<p>
					<fig id="f4">
						<label>Figure 4</label>
						<caption>
							<title>Sankey diagram of the thermal energy used in the stenter.</title>
						</caption>
						<graphic xlink:href="2346-2183-dyna-88-217-292-gf4.png"/>
						<attrib>Source: authors’ elaboration</attrib>
					</fig>
				</p>
				<p>The energy that can be potentially recovered, leaving through the extractor fans of the stenter in the form of hot air, amounts to 800.97 kW, which represents 87.19% of the total energy used in the process. The heat setting process of the fabric selected in this study consumes 11.01 GJ/ton in total, which is 22.00% higher than the maximum values in the literature [<xref ref-type="bibr" rid="B5">5</xref>] and may be caused by the age of the equipment and its original design. However, thanks to our analysis, we identified an opportunity to reduce its energy consumption and increase the energy efficiency of the process by recovering heat from the exhaust gases of the stenter to preheat entering air.</p>
			</sec>
			<sec>
				<title><italic>3.2 Design of the heat exchanger</italic></title>
				<p>
					<xref ref-type="table" rid="t6">Table 6</xref> presents the summary of the results of Heat Exchangers 1 and 2 following the methodology described in <xref ref-type="fig" rid="f3">Fig. 3</xref>. (As the stenter has two extractor fans, the values were calculated for one exchanger per extractor fan.) The table shows a difference in the area of the two exchangers, which is due to the fact that Heat Exchanger 2 has more stages because the fluid enters the equipment at a higher temperature, modifying the convective heat transfer coefficient. Furthermore, thanks to the larger number of stages, its effectiveness is also higher. </p>
				<p>
					<table-wrap id="t6">
						<label>Table 6</label>
						<caption>
							<title>Results of heat Exchangers 1 and 2.</title>
						</caption>
						<graphic xlink:href="2346-2183-dyna-88-217-292-gt6.jpg"/>
						<table-wrap-foot>
							<fn id="TFN6">
								<p>Source: authors’ elaboration</p>
							</fn>
						</table-wrap-foot>
					</table-wrap>
				</p>
				<p>Our effectiveness results are in line with those in [<xref ref-type="bibr" rid="B21">21</xref>], which reported an effectiveness between 37% and 65%, and [<xref ref-type="bibr" rid="B22">22</xref>], in which an effectiveness between 15% and 75% was found. The following section details the configuration and working fluids of the thermosyphons. </p>
				<sec>
					<title>3.2.1 Working fluid and material of thermosyphons</title>
					<p>The working fluid selected for the thermosyphons was water because the outlet temperatures of the stenter gases are 144.97°C and 180.18°C. </p>
					<p>The material of the thermosyphons was Type M copper, which is a kind of economical commercially available tubing in Colombia, where it is mainly used for hot water installations. Furthermore, copper offers high thermal conductivity and is compatible with water. </p>
				</sec>
				<sec>
					<title>3.2.2 Operating temperature and pressure of the thermosyphons</title>
					<p>
						<xref ref-type="table" rid="t7">Table 7</xref> presents the calculated operating temperature and corresponding pressure for saturated water of the thermosyphons in Exchangers 1 and 2.</p>
					<p>
						<table-wrap id="t7">
							<label>Table 7</label>
							<caption>
								<title>Operating temperature of the thermosyphons.</title>
							</caption>
							<graphic xlink:href="2346-2183-dyna-88-217-292-gt7.jpg"/>
							<table-wrap-foot>
								<fn id="TFN7">
									<p>Source: the authors</p>
								</fn>
							</table-wrap-foot>
						</table-wrap>
					</p>
					<p>To ensure the copper tubing would endure the operating pressures, its minimum thickness was </p>
					<p>
						<list list-type="bullet">
							<list-item>
								<p>0.103 [mm] in Exchanger 1 and</p>
							</list-item>
							<list-item>
								<p>0.192 [mm] in Exchanger 2.</p>
							</list-item>
						</list>
					</p>
					<p>These two minimum thicknesses are below that of the copper tubing selected in this study (i.e., 0.889 mm), which can therefore be used ensuring an appropriate operation and minimizing the thermosyphons’ risk of breaking.</p>
				</sec>
				<sec>
					<title>3.2.3 Casing material</title>
					<p>Since the exhaust gases of the stenter contain water vapor, we selected galvanized steel for the casing material. The properties of the latter are presented in <xref ref-type="table" rid="t8">Table 8</xref>.</p>
					<p>
						<table-wrap id="t8">
							<label>Table 8</label>
							<caption>
								<title>Characteristics of galvanized steel.</title>
							</caption>
							<graphic xlink:href="2346-2183-dyna-88-217-292-gt8.jpg"/>
							<table-wrap-foot>
								<fn id="TFN8">
									<p>Source: authors’ elaboration</p>
								</fn>
							</table-wrap-foot>
						</table-wrap>
					</p>
				</sec>
				<sec>
					<title>3.2.4 Insulation material</title>
					<p>The heat exchangers were covered with thermal insulation blankets made of mineral wool, whose properties are detailed in <xref ref-type="table" rid="t9">Table 9</xref>. The conductivity of this material is low, which is why it is considered adequate for heat exchanger insulation. </p>
					<p>
						<table-wrap id="t9">
							<label>Table 9</label>
							<caption>
								<title>Characteristics of the thermal insulation blanket.</title>
							</caption>
							<graphic xlink:href="2346-2183-dyna-88-217-292-gt9.jpg"/>
							<table-wrap-foot>
								<fn id="TFN9">
									<p>Source: authors’ elaboration</p>
								</fn>
							</table-wrap-foot>
						</table-wrap>
					</p>
				</sec>
				<sec>
					<title>3.2.5 Geometric arrangement of the tube bank of the thermosyphons in Exchanger 1</title>
					<p>The thermosyphons were placed in a triangular (rotated) staggered configuration with both flows crossing the thermosyphons. Such geometric array was selected so that the (hot or fresh) air comes into contact with the thermosyphons in its way in the second row, thus avoiding possible outlet ways without maximum contact with the thermosyphons. </p>
					<p>
						<xref ref-type="table" rid="t10">Table 10</xref> details the initial values that were randomly assumed and the final values after the iterative process was completed following the decision-making map.</p>
					<p>
						<table-wrap id="t10">
							<label>Table 10</label>
							<caption>
								<title>Measurements of the thermosyphon bank in Exchanger 1 and constraints.</title>
							</caption>
							<graphic xlink:href="2346-2183-dyna-88-217-292-gt10.jpg"/>
							<table-wrap-foot>
								<fn id="TFN10">
									<p>Source: authors’ elaboration</p>
								</fn>
							</table-wrap-foot>
						</table-wrap>
					</p>
				</sec>
				<sec>
					<title>3.2.6 Equivalent thermal conductivity of a thermosyphon</title>
					<p>Finally, the total thermal resistance and conductivity of a thermosyphon are, respectively,</p>
					<p>
						<disp-formula id="e16">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e16.png"/>
						</disp-formula>
					</p>
					<p>
						<disp-formula id="e17">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e17.png"/>
						</disp-formula>
					</p>
					<p>This conductivity, a result of the total thermal resistance that was found, is 851 higher than that of copper (average copper conductivity: 400 W/m²°C). Zohuri (2016) described a thermosyphon, with water as working fluid, that has an effective thermal conductivity in the order of 1x105 W/m²°C; in addition, depending on design and operating conditions, it is common to find values around 4x105, which is 1000 times higher than that of copper. Other studies [<xref ref-type="bibr" rid="B20">20</xref>,<xref ref-type="bibr" rid="B23">23</xref>,<xref ref-type="bibr" rid="B24">24</xref>] support this result and associate it with high heat transfer coefficients in boiling and condensation processes. </p>
				</sec>
				<sec>
					<title>3.2.7 Operating limits of the thermosiphon</title>
					<p>
						<xref ref-type="table" rid="t11">Table 11</xref> presents the operating limits obtained for the thermosyphon designed here. The volume of liquid used to calculate the dry-out limit was 50% of the evaporator, which is a volume of 0.000140679 m3 of water. Considering that Exchanger 1 is composed of 1012 thermosyphons, which are subjected to a heat flow of 118802 W, and if each thermosyphon transfers the same amount of energy, as a result, we have that each thermosyphon will be subjected to a maximum heat flow of 117.4 W. This value is significantly lower than those found for each one of the operating limits, which indicates that the design proposed here is adequate. </p>
					<p>
						<table-wrap id="t11">
							<label>Table 11</label>
							<caption>
								<title>Operating limits of the thermosyphons in Exchanger 1.</title>
							</caption>
							<graphic xlink:href="2346-2183-dyna-88-217-292-gt11.png"/>
							<table-wrap-foot>
								<fn id="TFN11">
									<p>Source: authors’ elaboration</p>
								</fn>
							</table-wrap-foot>
						</table-wrap>
					</p>
					<p>It can be concluded that the thermosyphons operate without a problem because the maximum heat flow is never above any of their operating limits.</p>
				</sec>
				<sec>
					<title>3.2.8 Mechanical design</title>
					<p>The parameters to calculate the differential pressure are presented in <xref ref-type="table" rid="t12">Table 12</xref>:</p>
					<p>
						<table-wrap id="t12">
							<label>Table 12</label>
							<caption>
								<title>Parameters to calculate the differential pressure.</title>
							</caption>
							<graphic xlink:href="2346-2183-dyna-88-217-292-gt12.png"/>
							<table-wrap-foot>
								<fn id="TFN12">
									<p>Source: authors’ elaboration</p>
								</fn>
							</table-wrap-foot>
						</table-wrap>
					</p>
					<p>The differential pressures for the evaporation and condensation sections are</p>
					<p>
						<disp-formula id="e18">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e18.png"/>
						</disp-formula>
					</p>
					<p>
						<disp-formula id="e19">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e19.png"/>
						</disp-formula>
					</p>
					<p>The power needed to move the hot and fresh air through Exchanger 1 is</p>
					<p>
						<disp-formula id="e20">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e20.png"/>
						</disp-formula>
					</p>
					<p>
						<disp-formula id="e21">
							<graphic xlink:href="2346-2183-dyna-88-217-292-e21.png"/>
						</disp-formula>
					</p>
					<p>This additional power needed to install the heat exchange system represents 8.16% of the total power recovered with Exchanger 1 and 11.2% of that with Exchanger 2. These values are low considering the global size of the equipment.</p>
				</sec>
			</sec>
		</sec>
		<sec sec-type="conclusions">
			<title>4. Conclusions</title>
			<p>We found that, in the process of fabric drying and heat setting in a stenter, the main heat transfer occurs in the heated chambers. The mass and energy balances showed that the thermal consumption of the stenter amounted to 918.65 kW, 95.82% of which are supplied as thermal energy by the hot thermal oil and 4.18% as electricity. Out of the total thermal energy, 800.97 kW are available in the hot air used in the process. </p>
			<p>The heat setting process of the stenter analyzed in this study consumes 11.01 GJ/ton in total, which is 22% higher than the maximum values in the literature and may represent an opportunity for improving its energy efficiency. Finally, using this heat recovery system with thermosyphons, the thermal energy consumption of the stenter can be reduced by 30.1%.</p>
		</sec>
	</body>
	<back>
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				<label>R. Mazo-Restrepo, </label>
				<p>received the BSc. Eng in Electrical Engineering in 2013, from the Universidad de Antioquia, Medellín, Colombia, the MSc. in Industrial Energy Management in 2020, it is finished and awaiting degrees from the Instituto Tecnológico Metropolitano in Medellín, Colombia. From 2010 to 2021 he has worked in the textile industry where he has developed energy use projects. His research interests include: simulation of thermal systems, heat transfer and combustion, energy audits, renewable energy and heat transfer systems. ORCID: 0000-0002-5132-8810</p>
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			<fn fn-type="other" id="fn2">
				<label>P.N. Alvarado, </label>
				<p>is BSc. in Chemistry and received his PhD. Sc. in Chemistry in 2017, from the Universidad de Antioquia, Colombia. He is currently a professor of the Instituto Tecnológico Metropolitano (ITM), in Medellín. ORCID: 0000-0002-4435-3989</p>
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			<fn fn-type="other" id="fn3">
				<label>K. Cacua,</label>
				<p> received the BSc. Eng. in Chemical Engineering in 2006, from the Universidad Industrial de Santander, Bucaramanga, Colombia, the MSc. in Engineering in 2010, from the Universidad de Antioquia, Medellin, Colombia, and the PhD in Engineering Science and Technology of Materials in 2019, from the Universidad Nacional de Colombia, Medellin, Colombia. From 2011 to 2021, she has been worked for Instituto Tecnológico Metropolitano in Medellín, Colombia. Currently, she is a professor in the Electromechanics and Mecatronics Department, Facultad de Ingenierías, Instituto Tecnológico Metropolitano. Her research interests include: nanofluids for heat transfer and combustion, engines, thermal systems simulation, energy audits, renewable energies and heat transfer systems. ORCID: 0000-0001-7535-9622</p>
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				<label>How to cite: </label>
				<p>Mazo-Restrepo, R., Alvarado, P. and Cacua, K., Analysis of potential waste heat recovery from a stenter in a textile plant.. DYNA, 88(217), pp. 292-302, April - June, 2021. </p>
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