Subscribe to our 

A lesson learned sixty years ago. When I was a little boy in the late 1940’s my father drove a 1939 Ford coupe, new cars were not available because of the war effort. The Ford coupes were good cars for the day however the windshield defrost system was almost nonexistent. Everyone added a small rubber bladed fan to the dash that blew air across the windshield acting as a defroster.

Most fans were powered by 6 volt systems as 12 volt systems were not yet standard on today’s cars. Seatbelts were not even used in race cars let alone in passenger cars and kids road in the front seat when there was room. Many families only had one car which dad drove to work leaving mother home to take care of the house and kids. Many women didn’t even have driver licenses as there was no need. However, women whose husband was fighting in the war did drive and work outside the home to support the family and war effort.

Enough of history let me get back to the lesson. One cold winter day the family was going to town on Saturday for the weeks shopping trip and I got to ride in the front seat. Standing on the seat between mother and dad I was told not to put my fingers in the fan. Being a typical little boy what do you think I did? Of course I put my finger in the fan, now remember it was rubber so I didn’t get hurt only a soar finger. Years later I related that experience to NPSH.

How does a soar finger relate to NPSH? The amount of energy I used to push my finger into the fan. NPSHr (required) is like the energy it takes to push liquid into the impeller eye of the impeller and just past the vane tips. NPSHa (available) is the energy in the piping system that is used to push the liquid into the impeller eye of the impeller and just past the vane tips. That is how a soar finger years ago relates to NPSH.

So for non-engineers you now have an explanation of NPSH that you can use to explain NPSH to an engineer.