Curve Balls…

Back where I grew up, we had “Rascoe’s Curve,” where Dean Rascoe plowed his ’68 Saab into a telephone poll on Old Shirley Road back in 1977. Miraculously Deano didn’t get hurt, but the Saab was destined for the junk yard.

I like curves.  I like driving through them.  I like racing go-karts on road courses (was faster when I was lighter — that old “thrust to weight” ratio ain’t what is used to be!).  I even like looking at them (nudge, nudge, wink, wink!).

The most famous curve of all, no doubt, is Dead Man’s Curve.  There are tons of Dead Man’s Curves in the US, but this one in Cleveland might be the original, and the scariest (zoom in on the Innerbelt Freeway). It was in 1964, however, that Beach Boy wannabe’s Jan & Dean made Dead Man’s Curve a teen-aged classic…

Click here for a Google Map of the route the Sting Ray and Jag XKE take in the song.  The song says “he passed me at Doheny then I started to swerve. But I pulled her out and there we were, at Dead Man’s Curve..” Even though it’s a bit of a hike, I’d say Dead Man’s Curve is where Sunset Boulevard takes a sharp right near Greenway Drive, just after North Whittier. That would make it about an 8 mile drag race, which is a lot for a 2:43 song.

Ironically, Jan Berry of Jan and Dean had a near fatal accident in his own Sting Ray in 1966, not far from Dead Man’s Curve.  He was in a coma for months, had severe brain damage and was partially paralyzed.  Amazingly, he worked his way back, and was able to perform again with Dean Torrence in the 70′s, all the way into the 90′s.

And speaking of curves…

We’ve talked a lot about pump curves in the blog (see this post, this post and this post for reviews), and how pump curves and system curves dictate where and how a heating system will actually perform.

But where do these curves come from?

Let’s start with pump curves.  You see these charts printed up by all the pump manufacturers showing the performance capabilities of their circulators. How do we know?  Well, the process is fairly simple.

You start with a big tank of water.  Out of the water comes a stand-pipe.  Connected to the pipe is a circulator, a pressure gauge and a flow-control valve.  The other end of the pipe drains back into the water tank.

Initially, the flow-control valve is completely closed.  Then we turn on the circulator.  Since the valve is closed, there’s no flow.  But if we check the pressure gauge, the pressure should be at the highest level the circulator can produce.  We jot down that PSI reading, multiply by 2.31, and determine what’s called the “shut-off” head pressure. That’s the pressure differential the circulator can create when it’s not moving any water.

Take a closer look at a pump performance chart.  You see at the bottom, on the X-axis, is GPM, or gallons-per minute.  The Y-axis on the left hand side is Feet of Head.

Let’s say we’re plotting the performance curve of a Taco 005 (The bright green line, labelled #2).  At zero GPM, the pressure gauge reading is 3.8 PSI.  Multiply 3.8 PSI times 2.31 (1 PSI = 2.31 feet of head pressure), and you get a shut-off head of roughly 8.8 feet.

We can plot that performance point – zero GPM at 8.8 feet of head on the chart.  That’s the starting point for our pump performance curve.

Next, open the flow-control valve until the circulator is moving one gallon-per-minute.  Check the pressure, multiply by 2.31 to find the head pressure, and plot that point.  Continue to open the valve  - 2 GPM, 3 GPM and so on – and read the pressures at those different flow rates.  Those flow and head pressure points are all plotted on the pump curve chart.  When the valve is fully open, you can see the flow is high but the pressure is low.  At this point, the circulator is just moving fluid, but not creating much of a pressure differential.

Connect the dots, and – presto – you have a pump performance curve.

Pump curves always go down and to the right.  The higher the flow rate created by the circulator, the lower the pressure differential.

As  you can see from this Taco “00″ series pump curve chart, different circulators have different performance curves.  We’ll discuss why next time.

One Response to “Curve Balls…”

  1. Its so simple, who knew…

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