{"id":2470,"date":"2026-06-19T11:11:11","date_gmt":"2026-06-19T03:11:11","guid":{"rendered":"https:\/\/szfengheng.com\/rdhx-fan-selection-guide-static-pressure-airflow\/"},"modified":"2026-06-19T21:25:00","modified_gmt":"2026-06-19T13:25:00","slug":"rdhx-fan-selection-guide-static-pressure-airflow","status":"publish","type":"post","link":"https:\/\/szfengheng.com\/es\/rdhx-fan-selection-guide-static-pressure-airflow\/","title":{"rendered":"RDHx Fan Selection Guide: Static Pressure vs Airflow for Rear-Door Heat Exchangers"},"content":{"rendered":"<p><script type=\"application\/ld+json\">\n{\n  \"@context\": \"https:\/\/schema.org\",\n  \"@type\": \"TechArticle\",\n  \"headline\": \"RDHx Fan Selection Guide: Static Pressure vs Airflow for Rear Door Heat Exchangers\",\n  \"description\": \"Engineering guide for selecting cooling fans for Rear Door Heat Exchangers in AI data centers, covering static pressure, airflow, fan curve analysis, and N+1 redundancy design.\",\n  \"url\": \"https:\/\/szfengheng.com\/rdhx-fan-selection-guide-static-pressure-airflow\/\",\n  \"datePublished\": \"2026-06-19\",\n  \"dateModified\": \"2026-06-19\",\n  \"author\": {\"@type\": \"Organization\",\"name\": \"Fengheng Technology\",\"url\": \"https:\/\/szfengheng.com\"},\n  \"publisher\": {\"@type\": \"Organization\",\"name\": \"Fengheng Technology\",\"url\": \"https:\/\/szfengheng.com\"},\n  \"keywords\": \"RDHx fan static pressure, rear door heat exchanger fan, AI data center cooling fan\"\n}\n<\/script><\/p>\n<p><script type=\"application\/ld+json\">\n{\n  \"@context\": \"https:\/\/schema.org\",\n  \"@type\": \"FAQPage\",\n  \"mainEntity\": [\n    {\n      \"@type\": \"Question\",\n      \"name\": \"What static pressure do RDHx fans need?\",\n      \"acceptedAnswer\": {\"@type\": \"Answer\",\"text\": \"RDHx (Rear Door Heat Exchanger) fans require static pressure of 300\u2013420 Pa depending on door panel fin density and heat load. For AI GPU racks at 30\u201360 kW, fans delivering \u2265350 Pa are recommended to maintain operating point above the RDHx system resistance curve of 150\u2013300 Pa.\"}\n    },\n    {\n      \"@type\": \"Question\",\n      \"name\": \"How many fans does an RDHx need for a 42U AI rack?\",\n      \"acceptedAnswer\": {\"@type\": \"Answer\",\"text\": \"A 42U AI GPU rack at 30\u201360 kW heat load typically requires 12\u201316 fans in N+1 redundant configuration. Using 92\u00d725mm fans (80 CFM each), 16 fans deliver 1,280 CFM total \u2014 sufficient for 60 kW at 10\u00b0C water-to-air approach temperature. For higher density, 120\u00d738mm fans (140 CFM each) can achieve the same flow with fewer units.\"}\n    },\n    {\n      \"@type\": \"Question\",\n      \"name\": \"What is the difference between 92mm and 120mm RDHx fans?\",\n      \"acceptedAnswer\": {\"@type\": \"Answer\",\"text\": \"92mm fans offer higher rotational speed (up to 8,000 RPM) and fit dense fan arrays in standard 42U doors. 120mm fans deliver more airflow per unit (110\u2013140 CFM vs 80 CFM), lower noise at equivalent flow, and are better for 2U fan positions. 92mm fans (350 Pa) and 120mm fans (380\u2013420 Pa) both meet the static pressure requirement for most RDHx applications.\"}\n    },\n    {\n      \"@type\": \"Question\",\n      \"name\": \"Do RDHx fans need to be 48V DC?\",\n      \"acceptedAnswer\": {\"@type\": \"Answer\",\"text\": \"OCP Open Rack v3 racks use 48V DC power distribution, so RDHx fans integrated into OCP-compliant racks should use 48V DC input. Standard enterprise racks with 12V bus can use 12V RDHx fans. Fengheng Technology offers both 12V and 48V DC versions of all RDHx fan models.\"}\n    },\n    {\n      \"@type\": \"Question\",\n      \"name\": \"How is the operating point calculated for RDHx fan selection?\",\n      \"acceptedAnswer\": {\"@type\": \"Answer\",\"text\": \"The RDHx fan operating point is the intersection of the fan P-Q curve and the system resistance curve. Measure or estimate the RDHx door pressure drop at target airflow (typically 150\u2013300 Pa), then select a fan whose curve passes above this point with adequate margin (\u226520%). Use fan affinity laws to adjust for PWM speed variation.\"}\n    }\n  ]\n}\n<\/script><\/p>\n<h2>Why RDHx Fan Selection Is More Complex Than Server Fan Selection<\/h2>\n<p>Server fans push air through a known chassis resistance \u2014 the system curve is relatively predictable. RDHx fans face a two-part challenge: they must overcome both the inherent resistance of the water-cooled door panel AND compensate for variable server exhaust conditions. Getting this wrong means either insufficient cooling (thermal throttling on GPU workloads) or oversized fans that waste energy and generate excessive noise in the data hall.<\/p>\n<p>This guide provides a systematic approach to RDHx fan selection for AI data center deployments, from measuring system resistance to specifying N+1 redundant arrays.<\/p>\n<h2>Understanding the RDHx System Resistance Curve<\/h2>\n<p>The RDHx door panel creates airflow resistance through its heat exchanger fins. This resistance follows a square-law relationship with airflow \u2014 double the airflow, quadruple the pressure drop. Typical values:<\/p>\n<ul>\n<li><strong>Low-density fins (standard cooling)<\/strong>: 80\u2013150 Pa at rated flow<\/li>\n<li><strong>Medium-density fins (AI server density)<\/strong>: 150\u2013250 Pa at rated flow<\/li>\n<li><strong>High-density fins (maximum heat rejection)<\/strong>: 250\u2013350 Pa at rated flow<\/li>\n<\/ul>\n<p>To find the operating point, plot your fan&#8217;s P-Q curve (pressure vs. airflow) and your system resistance curve on the same graph. The intersection is the operating point. For reliable operation across varying server loads, the fan operating point should remain above the system resistance curve with \u226520% margin even at minimum PWM speed.<\/p>\n<h2>Fan Array Design for 42U AI GPU Racks<\/h2>\n<p>A 42U rack running 8\u00d7 NVIDIA H100 nodes can dissipate 50\u201360 kW. RDHx panel sizing must match this thermal load at the available chilled water temperature (typically 14\u201318\u00b0C supply). Fan array design:<\/p>\n<table>\n<thead>\n<tr>\n<th>Fan Size<\/th>\n<th>CFM\/Fan<\/th>\n<th>Fans Required (N)<\/th>\n<th>N+1 Config<\/th>\n<th>Total CFM<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>92\u00d725mm<\/td>\n<td>80<\/td>\n<td>14<\/td>\n<td>16<\/td>\n<td>1,280<\/td>\n<\/tr>\n<tr>\n<td>120\u00d725mm<\/td>\n<td>110<\/td>\n<td>10<\/td>\n<td>12<\/td>\n<td>1,320<\/td>\n<\/tr>\n<tr>\n<td>120\u00d738mm<\/td>\n<td>140<\/td>\n<td>8<\/td>\n<td>10<\/td>\n<td>1,400<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>N+1 redundancy means one fan can fail without the array dropping below minimum required airflow. For AI workloads with continuous high thermal output, N+2 (two fan failures tolerated) is increasingly specified.<\/p>\n<h2>Static Pressure Requirements by Application<\/h2>\n<p>Not all RDHx installations have the same static pressure requirement. Key variables include chilled water temperature, fin density, door panel depth, and whether an air curtain or plenum is used:<\/p>\n<ul>\n<li><strong>Standard enterprise RDHx<\/strong>: 200\u2013280 Pa required \u2014 92mm fans (350 Pa rated) provide adequate margin<\/li>\n<li><strong>High-density AI GPU racks (30\u201360 kW)<\/strong>: 280\u2013350 Pa required \u2014 120mm fans (380\u2013420 Pa) recommended for comfortable margin<\/li>\n<li><strong>Ultra-high-density (>60 kW\/rack)<\/strong>: 350\u2013420 Pa required \u2014 120\u00d738mm fans or dual-layer fan arrays<\/li>\n<\/ul>\n<h2>PWM Control and Fan Speed Optimization<\/h2>\n<p>RDHx fan arrays should use PWM control with tachometer feedback to dynamically match fan speed to actual server heat output. Best practice control architecture:<\/p>\n<ol>\n<li>Monitor server inlet temperature (target: \u226425\u00b0C)<\/li>\n<li>Monitor RDHx supply water temperature<\/li>\n<li>Adjust fan PWM duty cycle to maintain target inlet temperature<\/li>\n<li>Set minimum PWM at 20\u201330% (never below stall speed) for bearing lubrication<\/li>\n<li>Implement alarm on tachometer signal loss (fan failure detection)<\/li>\n<\/ol>\n<h2>Fengheng Technology RDHx Fan Specifications<\/h2>\n<table>\n<thead>\n<tr>\n<th>Model<\/th>\n<th>Size<\/th>\n<th>Speed<\/th>\n<th>Airflow<\/th>\n<th>Static Pressure<\/th>\n<th>Voltage<\/th>\n<th>MTBF<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>FH-9225-48V<\/td>\n<td>92\u00d725mm<\/td>\n<td>8,000 RPM<\/td>\n<td>80 CFM<\/td>\n<td>350 Pa<\/td>\n<td>48V DC<\/td>\n<td>\u226550,000 h<\/td>\n<\/tr>\n<tr>\n<td>FH-1225-48V<\/td>\n<td>120\u00d725mm<\/td>\n<td>5,500 RPM<\/td>\n<td>110 CFM<\/td>\n<td>380 Pa<\/td>\n<td>48V DC<\/td>\n<td>\u226550,000 h<\/td>\n<\/tr>\n<tr>\n<td>FH-1238-48V<\/td>\n<td>120\u00d738mm<\/td>\n<td>4,500 RPM<\/td>\n<td>140 CFM<\/td>\n<td>420 Pa<\/td>\n<td>48V DC<\/td>\n<td>\u226550,000 h<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>All models: 4-pin PWM, tachometer output, dual ball bearing, CE\/RoHS certified, operating temperature -10\u00b0C to +70\u00b0C.<\/p>\n<div style=\"background:#f0f6ff;border-left:4px solid #2E75B6;padding:20px 24px;margin:32px 0;border-radius:4px;\">\n<h3 style=\"margin:0 0 8px;color:#1F3864;\">RDHx Fan Samples Available<\/h3>\n<p>Fengheng Technology offers 92mm and 120mm RDHx fans with 48V DC input for AI data center qualification. Sample lead time: 2 weeks. <a href=\"mailto:fengheng@adc-coolingfan.com\">Contact fengheng@adc-coolingfan.com<\/a> or <a href=\"\/ai-liquid-cooling\/\">view the full liquid cooling fan portfolio \u2192<\/a><\/p>\n<\/div>\n<h2>Related Guides<\/h2>\n<ul>\n<li><a href=\"\/ai-server-liquid-cooling-fans\/\">Do AI Servers Still Need Cooling Fans in Liquid Cooling Systems?<\/a><\/li>\n<li><a href=\"\/cdu-cooling-fan-comparison-92mm-120mm-pwm\/\">CDU Cooling Fan Comparison: 92mm vs 120mm PWM<\/a><\/li>\n<li><a href=\"\/48v-dc-cooling-fans-ai-computing-racks\/\">48V DC Cooling Fans for OCP Open Rack v3<\/a><\/li>\n<li><a href=\"\/gpu-liquid-cooling-auxiliary-fan-mtbf-selection\/\">GPU Liquid Cooling Auxiliary Fan MTBF Selection<\/a><\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Why RDHx Fan Selection Is More Complex Than Server Fan  [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[],"class_list":["post-2470","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"acf":[],"_links":{"self":[{"href":"https:\/\/szfengheng.com\/es\/wp-json\/wp\/v2\/posts\/2470","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/szfengheng.com\/es\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/szfengheng.com\/es\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/szfengheng.com\/es\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/szfengheng.com\/es\/wp-json\/wp\/v2\/comments?post=2470"}],"version-history":[{"count":5,"href":"https:\/\/szfengheng.com\/es\/wp-json\/wp\/v2\/posts\/2470\/revisions"}],"predecessor-version":[{"id":2502,"href":"https:\/\/szfengheng.com\/es\/wp-json\/wp\/v2\/posts\/2470\/revisions\/2502"}],"wp:attachment":[{"href":"https:\/\/szfengheng.com\/es\/wp-json\/wp\/v2\/media?parent=2470"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/szfengheng.com\/es\/wp-json\/wp\/v2\/categories?post=2470"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/szfengheng.com\/es\/wp-json\/wp\/v2\/tags?post=2470"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}