# Fan Affinity Laws **URL:** \ **Category:** Factual Questions\ **Created:** [January 29, 2015, 6:33pm UTC](https://boards.straightdope.com/t/fan-affinity-laws/711251 "2015-01-29T18:33:15Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [January 29, 2015, 6:33pm UTC](https://boards.straightdope.com/t/fan-affinity-laws/711251/1 "2015-01-29T18:33:15Z")
I’m hoping other engineers on the SD may come along to answer this. Something doesn’t make sense about the fan affinity laws. When one looks at a fan curve, cfm and dP are inversely related. HOWEVER! The fan affinity laws apply a direct relation, i.e.: as cfm increases, dp would also increase. Am I missing something? 😕 The fan laws may be found [here](http://www.engineeringtoolbox.com/fan-affinity-laws-d_196.html). ---
**Author:** ![newme](https://avatars.discourse-cdn.com/v4/letter/n/e0b2c6/32.png) [@newme](https://boards.straightdope.com/u/newme)\ **Post date:** [January 29, 2015, 8:57pm UTC](https://boards.straightdope.com/t/fan-affinity-laws/711251/2 "2015-01-29T20:57:26Z")
Fan curves plotting cfm versus dP are usually done at constant RPM, but varying constriction on the airflow. Wide open, a fan will move air with little or no pressure drop. When you restrict the airflow, obviously the cfm should decrease, while the pressure will increase. If you completely block airflow, cfm drops to zero, while pressure is maximized. [Understanding Fan Curves](http://aec.us.com/understanding-fan-curves.html) > [@](#): > > The fan “pressure-volume” curve is generated by connecting the fan to a laboratory test chamber. By following very specific test procedures as outlined in the Air Movement and Control Association (AMCA) Standard 210, data points are collected and plotted graphically for a constant RPM, from a “no flow” block off condition to a “full flow” or wide condition.