{"id":218870,"date":"2025-05-24T14:58:23","date_gmt":"2025-05-24T14:58:23","guid":{"rendered":"https:\/\/learnexams.com\/blog\/?p=218870"},"modified":"2025-05-24T14:58:25","modified_gmt":"2025-05-24T14:58:25","slug":"with-excess-air-determine-the-following","status":"publish","type":"post","link":"https:\/\/www.learnexams.com\/blog\/2025\/05\/24\/with-excess-air-determine-the-following\/","title":{"rendered":"With excess air, determine the following"},"content":{"rendered":"\n

With excess air, determine the following:<\/strong><\/p>\n\n\n\n

Theoretical amount of air needed to burn the fuel oil.
Actual amount of air needed to burn the fuel oil.
Higher and Lower Heating Value of the Fuel.<\/p>\n\n\n\n

The Correct Answer and Explanation is:<\/mark><\/strong><\/p>\n\n\n\n

To determine the theoretical<\/strong> and actual<\/strong> amount of air needed to burn fuel oil, along with its Higher Heating Value (HHV)<\/strong> and Lower Heating Value (LHV)<\/strong>, we must first know the fuel oil\u2019s composition. However, for typical fuel oil (say No. 2 fuel oil<\/strong>, a common distillate fuel), we can approximate its empirical formula as C\u2081\u2082H\u2082\u2086<\/strong> for calculation purposes.<\/p>\n\n\n\n

\n\n\n\n

1. Theoretical Air Requirement<\/strong><\/h3>\n\n\n\n

Combustion reaction of C\u2081\u2082H\u2082\u2086: C12H26+O2\u2192CO2+H2O\\text{C}_{12}\\text{H}_{26} + \\text{O}_2 \\rightarrow \\text{CO}_2 + \\text{H}_2\\text{O}<\/p>\n\n\n\n

Balanced Reaction: C12H26+18.5 O2\u219212 CO2+13 H2O\\text{C}_{12}\\text{H}_{26} + 18.5\\ \\text{O}_2 \\rightarrow 12\\ \\text{CO}_2 + 13\\ \\text{H}_2\\text{O}<\/p>\n\n\n\n

Each mole of O\u2082 needs 4.76 moles of air (21% O\u2082 by volume): Theoretical air=18.5\u00d74.76=88.06 moles of air per mole of fuel\\text{Theoretical air} = 18.5 \\times 4.76 = 88.06\\ \\text{moles of air per mole of fuel}<\/p>\n\n\n\n

Molecular weight of C\u2081\u2082H\u2082\u2086 = (12\u00d712 + 26\u00d71) = 170 g\/mol<\/strong><\/p>\n\n\n\n

So, air-to-fuel ratio (AFR) by mass<\/strong>:<\/p>\n\n\n\n

    \n
  • O\u2082 mass = 18.5 \u00d7 32 = 592 g<\/li>\n\n\n\n
  • Air mass = 592 \u00d7 100 \/ 23.2 \u2248 2552 g (using O\u2082 is 23.2% by mass in air)<\/li>\n<\/ul>\n\n\n\n

    Theoretical AFR = 2552 \/ 170 \u2248 15.01<\/strong><\/p>\n\n\n\n

    \n\n\n\n

    2. Actual Air Requirement (with Excess Air)<\/strong><\/h3>\n\n\n\n

    If excess air is 20%<\/strong>, then: Actual AFR=15.01\u00d7(1+0.20)=18.01\\text{Actual AFR} = 15.01 \\times (1 + 0.20) = \\mathbf{18.01}<\/p>\n\n\n\n

    \n\n\n\n

    3. Higher and Lower Heating Value<\/strong><\/h3>\n\n\n\n

    Typical heating values for fuel oil:<\/p>\n\n\n\n

      \n
    • HHV<\/strong> \u2248 45.5 MJ\/kg<\/strong><\/li>\n\n\n\n
    • LHV<\/strong> \u2248 42.5 MJ\/kg<\/strong><\/li>\n<\/ul>\n\n\n\n

      LHV is lower because it assumes the water formed during combustion remains as vapor (latent heat is not recovered).<\/p>\n\n\n\n

      \n\n\n\n

      Explanation <\/strong><\/h3>\n\n\n\n

      To fully combust a hydrocarbon fuel like fuel oil, it must react with oxygen in air. The theoretical air<\/strong> is the minimum amount of air that provides just enough oxygen to complete the combustion of all carbon and hydrogen in the fuel without excess. In this case, assuming the fuel oil has an empirical formula of C\u2081\u2082H\u2082\u2086, stoichiometry shows it needs 18.5 moles of oxygen per mole of fuel to yield CO\u2082 and H\u2082O. Since only 21% of air is oxygen by volume (or about 23.2% by mass), this translates to a theoretical air-to-fuel mass ratio (AFR) of approximately 15:1.<\/p>\n\n\n\n

      In real combustion systems, a certain amount of excess air<\/strong> is supplied to ensure complete combustion and avoid the formation of pollutants like CO or soot. If 20% excess air is used, the actual AFR becomes about 18:1.<\/p>\n\n\n\n

      The Higher Heating Value (HHV)<\/strong> represents the total energy released when the fuel is burned and the water produced is condensed, recovering latent heat. In contrast, the Lower Heating Value (LHV)<\/strong> excludes this condensation energy, assuming the water remains as vapor. Fuel oils typically have an HHV around 45.5 MJ\/kg and LHV near 42.5 MJ\/kg. The difference is important in engineering calculations for boiler and engine efficiency, especially where the exhaust temperature is too high to recover the latent heat from water vapor.<\/p>\n\n\n\n

      Accurate estimation of air requirements and heating values is essential for efficient combustion, minimizing fuel consumption and emissions in industrial and residential heating applications.<\/p>\n\n\n\n

      \"\"<\/figure>\n","protected":false},"excerpt":{"rendered":"

      With excess air, determine the following: Theoretical amount of air needed to burn the fuel oil.Actual amount of air needed to burn the fuel oil.Higher and Lower Heating Value of the Fuel. The Correct Answer and Explanation is: To determine the theoretical and actual amount of air needed to burn fuel oil, along with its […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"default","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"categories":[25],"tags":[],"class_list":["post-218870","post","type-post","status-publish","format-standard","hentry","category-exams-certification"],"_links":{"self":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/218870","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/comments?post=218870"}],"version-history":[{"count":0,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/218870\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/media?parent=218870"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/categories?post=218870"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/tags?post=218870"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}