The Point of No Pressure Change

In 1994, I wrote a book called, Pumping Away, which I began by remembering the late, great Gil Carlson. Here's what I wrote:

When I knew Gil Carlson, he was never far from the cigarettes he so loved. On the days when I was fortunate enough to be able to sit and listen to this giant of the hydronics industry, it was always through a cloud of bluish smoke. I think the man used only one match a day.

The blue haze of those days seems almost poetic to me now. Gil was an engineer and he was brilliant. I was neither. Gil saw, I believe, the mechanics of hydronics in his mind's eye as Einstein must have seen the Universe. It all flowed together in a very visual way for him. It didn't for me.

But as a young man, I was lucky enough to have the chance to listen to him when he'd visit from Bell & Gossett's Illinois plant. Gil was B&G's Director of Technical Services for many years. He traveled the country and lectured to large groups of engineers about things hydronic.

When he'd come to New York, I'd sit in the smoke and attempt to see what he saw. I'd stare into the depths of my confusion and try to imagine the mechanical movements he described by way of charts and formulas as real things. I knew he saw them, but try as I might, I couldn't. Not at the time, anyway. On most days, I'd walk to the subway in total confusion and frustration.

I remember one January day when the wind blew cold in Manhattan and we sat together in an office perched high above Fifth Avenue. We drank hot coffee. Gil was a quiet man who rarely spoke unless asked a question. I struggled to find one that wouldn't reveal too much of my ignorance.

Finally, and more to break the silence than for any other reason, I asked him how he'd come up with this idea that the location of the circulator matters in a closed hydronic system. I knew that this principle, which Gil had proposed in an early-1960s paper, had radically changed the way professional engineers design hydronic systems. It's one of the cornerstones of hydronic heating, but it had never occurred to me to ask him this question before.

"When did you first get the idea about 'the point of no pressure change?'"I asked.

He turned from me and stared out the window for a long moment, and then he smiled a silly grin and said, "Oh, it's just something I've always known."

"'Oh," I echoed, like an empty canyon.

Of course, his answer struck me as pretty bizarre. The principle was obviously something he'd arrived at along the way, but apparently, he'd realized it so long ago that he couldn't remember ever not knowing it.

"You mean you figured it out so long ago that you don't remember when?" I asked.

He smiled that silly grin again. "No," he said, "I've just always known."

"But why did you write the paper when you did?" I persisted.

He took a long drag on his cigarette and exhaled, adding to the thick haze that already filled the room. "I wrote the paper then, because that's when they asked me about it." He looked at me as though I should have known that. "You see," he continued, "I always knew about it, but they just never asked me before. That's why I wrote the paper when I did. Because they asked me." He smiled that Carlson grin.

"Uh huh," I said. "I see," I said. I let my empty head bob up and down. "I understand."

But, of course, I didn't. I'll tell you this, though. The moment burned itself into my memory, and as I think back on it now, I remember this neat quote I read somewhere: "Talent is what you possess; genius is what possesses you."

And that was that. I got the memory out of my system and we sold thousands of copies of Pumping Away.

We recognize Gil Carlson as the person who first pointed out the importance of a circulator's location in a closed hydronic system. In a paper he submitted to ASHRAE back in the '60s. He declared the compression tank, "the point of no pressure change" in any hydronic system, and changed the way we pipe.

This is the essence of what he said:

  1. A closed hydronic system works with the same water over and over again.
  2. When you heat that water, it will expand, and you must make provisions for this.
  3. A closed compression tank is the best way to handle the water's expansion and contraction.
  4. Within a closed hydronic system, a circulator will impart velocity to water and create pressure differential rather than pressure.
  5. Since it operates in a closed system, the circulator can neither add water to, nor remove water from, the compression tank. For the circulator to add water to the tank, it would have to first remove it from the system piping. That would leave a blank space in the system piping, a place void of water, and that can't possibly happen. And if the circulator were to remove water from the compression tank, it would have to stuff it into a piping system that’ already filled. That's not possible because water’s not compressible.
  6. Because it can't add water to, or remove water from, the compression tank, the circulator, can't change the pressure within the compression tank. This is why we call it the point of no pressure change.
  7. The circulator will respond to the tank's location, raising or lower the differential pressure based on that location. The tank is a hydronic bookmark.
  8. If you pump away from the compression tank, the circulator will add its differential pressure to the system's static fill pressure.
  9. If you pump toward the compression tank, the circulator will remove its differential pressure from the system's static fill pressure.
  10. If you pump toward the compression tank and the pump's differential pressure exceeds the system's static fill pressure, the pressure at the pump's suction will be below atmospheric and air will enter the system. At that point, you’re in trouble.

During 1968 and 1969, Gil wrote a series of articles that he called, "Hydronic Systems Analysis and Evaluation." These appeared in the ASHRAE Journal. There's a footnote in those articles stating, "The author and his company are generally given credit for developing the 'point of no pressure change' thesis. It should be pointed out, however, that R.C. Chewning and R. Peterson of J. Donald Kroeker & Assoc., Portland, OR, and C.L. Boyer of Carnahan & Thompson, Oklahoma City, OK, separately and independently came to the same conclusion at the same time."

But now let me take you back to 1930.

Just before he passed away, my dear friend, "Professor" Fremont Lobbestael of Ann Arbor, MI, sent me a photocopy of pages 385 and 386 from the 1930 edition of The American Society of Heating and Ventilating Engineers Guide. Accompanying some drawings are these words:

"In most central heating systems it is necessary to use an expansion tank. Such a tank is generally installed near the circulating pump and on its suction side; the pump being located in the return line near the heater. With such an arrangement, the pressure in the main remains constant at the suction side of the pump; it rises sharply in the pump and then decreases gradually along the main, as indicated in Fig. 13. In this figure, the line 1, 2, 3, 4, is a base line; the line 5, 6, 7, 8, is a line parallel to the base line drawn so that the distance 1 to 5 represents the pressure in the main when the system is not in operation. This pressure is determined by the elevation of the expansion tank and must be such that sufficient pressure is maintained in the highest radiator to prevent boiling when the water is heated to the maximum temperature for which the system was designed.

"The line 9, 10, 11, 12, 5, is drawn so that the vertical distances from its points to the corresponding points of the base line show the pressure in the respective parts of the main when the water is being forced through the main by the circulating pump, neglecting the slight reduction in pressure head caused by he velocity head. And inspection of Fig. 13 shows that, with the expansion tank located at the suction end of the pump, the pressure in the building is considerably higher than necessary. It also shows that if the expansion tank were located at the line 3, 7, 11, the pressure at that point would remain constant and the pressure would fall below the line 7, 8, 5, and above the line 7, 6, 5. In that case, the expansion tank would need to be at a higher elevation than if located at the pump. This comparison explains the relation which exists between the location and the required elevation of the expansion tank."

That is "The point of no pressure change" thesis. Gil Carlson was eight years old when some unknown Dead Man wrote it down in the 1930 ASHVE Guide. It doesn't appear again in any of the later issues of the Guide, which is very strange.

"When did you first get the idea about 'the point of no pressure change?'" I asked.

He turned from me and stared out the window for a long moment, and then he smiled a silly grin and said, "Oh, it's just something I've always known."

"Oh," I echoed, like an empty canyon.


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