What’s The Right Heat Exchanger Pressure?
A few years ago, an engineer who works with older New York City buildings started to email me with good questions, mostly about steam heating. I’ve enjoyed hearing from him because he’s not afraid to ask questions, and he’s also not afraid to admit he doesn’t know everything. I like that because I don’t know everything either.
When we first began exchanging emails, he was convinced that big buildings didn’t need much steam pressure. He had come to a seminar I had done in New York City, where I had mentioned that the Empire State Building gets by with about 1 1/2-psi steam pressure on most days. That didn’t seem possible to him at first. It didn’t seem possible to me either when I first heard it. But then, I learned that it’s not the pressure of the steam, but its latent heat that gets the job done. And there’s plenty of latent heat in steam, even at just a few ounces of pressure.
The only reason we need more pressure than that is to overcome the friction the steam meets as it flows through the pipes, and that’s a function of pipe size, not steam pressure. Fortunately, in December 1899 (and in New York City), the members of the heating industry created the 2-psi standard, which allowed them to size pipes so that they would offer roughly one ounce of frictional loss for every 100 feet of steam travel. And this is how the Empire State Building gets by with such low steam pressure, and why just about any steam-heating system will work well with 2-psi pressure or less.
When you lower the steam pressure, you save money on fuel and you usually make the people in the building more comfortable. My engineer friend gets that now and so do I, and that’s why he asked me this question:
“I’ve been pushing 2-psig steam pressure as a maximum in buildings pretty hard but I continue to get plenty of pushback from others. Right now, I have two buildings with steam boilers that also feed steam-to-hot water heat ex-changers. The exchangers heat water in hydronic baseboard loops to take care of the apartments. Typically, the steam pressures are 5 to 7 psig at the inlet to these heat exchangers. Do you have any feel for what the steam pressure should be going to a heat exchanger to make hot water for baseboard heating?”
His question brought me back to my days working for the rep in New York. We sold a lot of shell-and-tube, steam-to-water heat exchangers. I remember how I cringed when I first opened the catalog and looked at all the numbers they showed in their sizing charts. I didn’t know where to begin, so I asked one of the old-timers at the company to walk me through the process.
“What pressure do I need?” I asked.
He smiled and shrugged. “A steam-to-water heat exchanger is sort of like a meatloaf, Dan,” he said.
“A meatloaf?” I said.
“Yes, it just sits there looking delicious, but also waiting to see what you’re going to bring to it.”
“You like meatloaf, Dan?”
“What does it need?”
“Absolutely,” he said.
“And cream corn,” I added, getting into this now.
“And biscuits,” he said. I nodded.
Now that he had my youthful attention, he explained that a shell-and-tube heat exchanger is nothing more than a bundle of copper tubing inside a steel container. There are four places to connect pipe: Steam goes in one; condensate comes out another. The other two tappings are for the supply and return water for the radiators. The exchanger itself doesn’t know what to do until we bring it what it needs.
“The mashed potatoes could be the size of the tube bundle and how many back-and-forth passes the water makes though that bundle,” the old-timer said. “The more square feet there are in the bundle, the longer the water is going to be inside of it, picking up latent heat from the condensing steam.”
“And the cream corn could be what we call the fouling factor,” he said.
“That sounds dirty,” I said.
“It is. This is a dirty business. We gotta size these heat exchanges so they still work once they get filled with gunk,” he said. “And they will get filled with gunk, Dan. It looks sort of like cream corn, but it’s not yellow.”
“How long does it take to foul?”
“It depends,” he said.
“And it means that when the heat exchanger is new and clean, before it meets the cream corn, it’s going to be over-sized.”
“By how much?” I said.
He showed me where to find fouling factors on the sizing chart. “The design engineer usually writes that into his spec,” he said. “Most of those engineers are pretty conservative.”
“OK,” I said. “But what about the steam pressure?”
“That’s the gravy,” he said.
“The pressure is?”
“Yes, it can be anything we want it to be. For instance, let’s say the building is working on district steam that comes in from ConEdison. That pressure will be very high. We’ll reduce it once it’s in the building, but we decide by how much. In some cases, we can work with high pressure to keep the size of the exchanger down.”
“And if we’re locked into low pressure?” I said.
“Then we work with what we have,” he said. “Suppose they’re running a steam boiler at 2-psi, tops. We’ll size that heat exchanger for a very low steam-pressure input. Keep in mind there’s going to be a pressure drop across the steam control valve as well. The lower the pressure, the bigger the heat exchanger is going to be, but only to a point.”
“Can they raise the steam pressure on the boiler to get a smaller exchanger?” I asked.
“Sure,” he said. “They have a screwdriver and a pressuretrol. It’s easy to raise the pressure.”
“How much of a difference will that make?” I asked.
“It depends,” he said again. “Although there wouldn’t be that much of a difference between, say, 2- and 5-psig pressure. It’s the same with jacketed steam kettles in restaurants. The higher steam pressure makes things happen more quickly, but as a wise man once said, a difference to be a difference has to make a difference. Can you heat water to 180° F with steam at 0 psig? Sure you can because the steam is 212° at that pressure. Would the water get hotter faster with 2-psi steam? Yes. And with 5 psi? Yes again, but only marginally because there’s not that much difference in temperature between those two pressures.”
“So would you raise the pressure in a steam system to get a smaller exchanger?” I asked.
“It depends,” he laughed. “Raising the pressure is going to cost more fuel, so what you save on the meatloaf you spend on the biscuits. I would also first look at the type of steam heating that’s in the rest of the building. Let’s say it’s one-pipe steam, for instance, and they’re using the heat exchanger to deal with just some of the building’s heating needs. Maybe it’s for radiators that are in basement apartments below the steam boiler’s waterline. What we have to watch out for here are the air vents on those one-pipe-steam radiators. They won’t vent well if we raise the steam pressure at the boiler too high. The pressure will force the vents closed on the first cycle and they’ll probably stay closed under that higher pressure. That’s not good because if the air can’t get out, the steam will stop moving. That means no heat for some of the people.”
“So then what’s the right pressure for a steam-to-water heat exchanger?” I said.
“The answer is: It depends,” he said. “It always is. And the meatloaf doesn’t care one way or the other. It’s all on you. You’re the one making the decisions. You’re the designer. It’s just going to work with what you bring to it.”
“It’s a meatloaf,” I said.
“You got it,” he said, and then he showed me how to use those charts with all the numbers. Once you decide on what you’re bringing, the rest is pretty easy.
That conversation took place nearly 50 years ago, but it stays with me. I wrote back to my engineer friend who asks good questions, and isn’t afraid to admit he doesn’t know everything. Just like me.
I began by asking him if he likes meatloaf.