{"id":23719,"date":"2026-10-27T09:16:38","date_gmt":"2026-10-27T08:16:38","guid":{"rendered":"https:\/\/mag.ebmpapst.com\/en\/?p=23719"},"modified":"2026-10-27T09:16:39","modified_gmt":"2026-10-27T08:16:39","slug":"the-formula-of-the-hydraulic-diameter","status":"publish","type":"post","link":"https:\/\/mag.ebmpapst.com\/en\/insights\/the-formula-of-the-hydraulic-diameter_23719\/","title":{"rendered":"The formula of the hydraulic diam\u00adeter"},"content":{"rendered":"
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Michael Strehle, Flow Solu\u00adtion \u2014 Aero\u00addy\u00adnamic Devel\u00adop\u00adment (Image | ebm-papst)<\/figcaption><\/figure>\n<\/div>\n\n\n

The hydraulic diam\u00adeter (dh<\/sub>) is a calcu\u00adlated para\u00admeter. It allows non-circular flow cross-sections \u2013 such as rectan\u00adgular or square chan\u00adnels \u2013 to be compared to an ideally circular pipe. The formula is surpris\u00adingly simple: It relates the cross-sectional area A through which the air flows as a ratio of the wetted perimeter U \u2013 that is, the perimeter of the duct in contact with the air. This is based on a funda\u00admental prin\u00adciple of physics: The larger the flow area rela\u00adtive to the wall area, the more easily air can flow. This is because as soon as air flows through a duct, fric\u00adtion occurs at the wall surfaces \u2013 resulting in pres\u00adsure losses and, conse\u00adquently, energy losses. The geom\u00adetry thus deter\u00admines the effi\u00adciency or loss of the entire system <\/p>\n\n\n\n

A large hydraulic diam\u00adeter means favor\u00adable flow condi\u00adtions: The air encoun\u00adters little resis\u00adtance, and little energy is required to achieve the desired effect. A small hydraulic diam\u00adeter means more contact with the walls, greater fric\u00adtion, deflec\u00adtion, and loss. A simple illus\u00adtra\u00adtion makes this clear: In a wide, even stream, water flows calmly and effi\u00adciently. In a shallow, winding stream, on the other hand, the water is slowed down, swirls, and loses energy. The same is true for airflows. Why is this impor\u00adtant for centrifugal fans like the RadiPac 3? <\/p>\n\n\n\n

These fans are designed to accel\u00aderate air evenly and very effi\u00adciently. However, in prac\u00adtice, when the air exits the fan, it often encoun\u00adters rectan\u00adgular hous\u00adings, tight instal\u00adla\u00adtion spaces, or other\u00adwise unfa\u00advor\u00adable duct config\u00adu\u00adra\u00adtions. We\u2019ve addressed this by signif\u00adi\u00adcantly reducing the fric\u00adtion losses of air flowing from the centrifugal to the axial direc\u00adtion in the new Radi\u00adFlow impeller. The result: The effi\u00adciency of the entire system remains optimal for a very long time, even with a small hydraulic diam\u00adeter \u2013 that is, with unfa\u00advor\u00adable duct shapes (see diagram). As a general rule, it is always worth\u00adwhile to ensure flow-friendly geome\u00adtries, smooth tran\u00adsi\u00adtions, and balanced cross-sections in the airflow path to fully exploit the perfor\u00admance capa\u00adbil\u00adi\u00adties of modern fans. The hydraulic diam\u00adeter makes these rela\u00adtion\u00adships visible and calcu\u00adlable.<\/p>\n\n\n\n

<\/div>\n\n\n\n
If the ratio of the hydraulic diam\u00adeter dh<\/sub> to the fan diam\u00adeter D (x-axis) decreases, a corre\u00adsponding correc\u00adtion factor must be applied to the effi\u00adciency (y-axis). (Graph | ebm-papst)<\/figcaption><\/figure>\n\n\n\n

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

The hydraulic diam\u00adeter (dh) is a calcu\u00adlated para\u00admeter. It allows non-circular flow cross-sections \u2013 such as rectan\u00adgular or square chan\u00adnels \u2013 to be compared to an ideally circular pipe. The formula is surpris\u00adingly simple: It relates the cross-sectional area A through which the air flows as a ratio of the wetted perimeter U \u2013 that [\u2026]<\/p>\n","protected":false},"author":45,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"wp_typography_post_enhancements_disabled":false,"footnotes":""},"categories":[16,4,9],"tags":[],"class_list":["post-23719","post","type-post","status-publish","format-standard","hentry","category-fans","category-insights","category-refrigeration-ventilation"],"acf":[],"_links":{"self":[{"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/posts\/23719","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/users\/45"}],"replies":[{"embeddable":true,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/comments?post=23719"}],"version-history":[{"count":7,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/posts\/23719\/revisions"}],"predecessor-version":[{"id":23729,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/posts\/23719\/revisions\/23729"}],"wp:attachment":[{"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/media?parent=23719"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/categories?post=23719"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/tags?post=23719"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}