Did you know that some current boiler designs go back well over 150 years? The modern fire-tube boiler is not all that different in design from the boilers that once powered trains and ships. “Scotch Marine”, and similar fire-tube boilers, consist of a horizontal round steel shell with tube-sheets welded at both ends. The tube sheets are connected inside the shell by a large pipe (aka furnace or Morrison tube) near the bottom of the shell and a series of smaller tubes to the sides and above the furnace. The flame from the burner fires down the furnace in a “first pass” and then returns as hot gasses for a second pass through the smaller tubes. At this point the gasses may exit the stack or make a third or even a fourth pass back through more tubes depending on the fire-tube’s number of passes. The furnace pipe and the rest of the boiler tubes are completely surrounded by water inside the shell. The shell is either 80% full of water – for making steam – or completely filled with water – for generating hot water. They are called fire-tube boilers because the fire travels inside the tubes.
Fire-tube boilers have doors that swing open to expose the inside of the fire-tubes for inspection and cleaning. This was an important feature back when most boilers were fired with coal or wood. The soot that formed inside the tubes from those fuels had to be brushed (“punched”) on a regular basis or the heat transfer efficiency would suffer. Fire-tube boilers also had (and continue to have) hand-holes and manholes for waterside inspections and washouts. A water hose can be directed through these openings to wash out the muck that settles in the belly of these boilers - but otherwise there is no easy or reliable way to mechanically clean the hard scale that will eventually form on the outside of the furnace pipe and the rest of the boiler tubes.
Fire-tube boilers were one of the main engines behind the Industrial Revolution, providing high pressure steam to power factories. Perhaps because they weren’t as closely watched as steam locomotives (where the engineer and fireman were literally right next to the boiler) and not as well understood by those in charge of maintenance, a great number of boiler accidents occurred – usually as a result of low water, human error or automatic safety devices that did not work. How many accidents? Here’s a shocking statistic: In the late eighteen hundreds and early nineteen hundreds, it is estimated that 50,000 people died each year from boiler and pressure vessel related accidents in North America alone. For a detailed understanding of a boiler explosion and a case history of nine boiler accidents, click here.
Organizations such as the ASME, ABMA and NBBI grew out of this crisis and quickly helped turn this trend around; implementing codes and safety standards that are still in practice today. As boilers became safer, the public’s attention naturally waned. But because they played such an important role in industry back then, many steam engineers were not only interested in boilers but keen on improving them. After safety (first!), boiler life and efficiency were two major concerns because (1) boilers were expensive and (2) they used a lot of fuel. Getting back to our fire-tube boiler, the ability to “punch” the tubes on the fireside was a plus (although cleaner burning fuels such as natural gas would make this feature less relevant in later years) but on the negative side - in addition to the lack of waterside access for removing hard scale deposits - there was another problem: expansion and contraction of the vessel itself.
It is one thing to bring a fire-tube boiler up to pressure and leave it there for a long time; it’s quite another if it heats up and cools down on a regular basis. Today we know this as thermal shock or thermal stress cycling. Basically as metal heats up it wants to expand and as metal cools down it wants to contract. If it does this freely without restraint there is no major problem and the metal will be largely unaffected. If not, the metal will work harden and ultimately fail. All materials have a coefficient of expansion. When a straight boiler tube heats up, the longer the tube, the greater the expansion. Fire-tube boilers - with straight tubes, stay-bolts and fixed tube-sheets - are exceptionally rigid, with no provision for the tubes or stay bolts to freely expand and contract. If a fire-tube boiler heats up evenly to a uniform temperature and remains that way (idling when there was no steam demand), no biggie. But if they are heated up and cooled down on a regular basis – as they would be for many businesses and factories trying to save fuel when they weren’t in operation – this cycling would eventually cause the tubes to loosen in the tube-sheets (from repeated creeping at the tube roll), stay-bolts to break (compromising integrity of pressure vessel) and even tube-sheets to crack (more water leaks and potential pressure vessel failure). Compounding this problem was that many boilers in the early nineteen hundreds were fed with cold feed-water, as modern day steam traps and feed-water devices were relatively new and unproven.
Cold feed-water and repeated cold starts were not good for the rigid fire-tube design back then (nor today), however a new boiler design was just around the corner that would address the problem of thermal shock head on.
In 1916, a locomotive engineer adapted a boiler to power a tractor that used bent tubes instead of straight tubes. The boiler had to be compact and the bent tubes fit nicely in the small allotted space. Another big plus was that the bent tubes held up very well to the constant “shock” of cold feed-water. This is because bent tubes had the ability to “flex” as they expanded and contracted whereas fire-tube boilers did not. The gasoline engine turned out to be much more practical for powering tractors - but the bent tube boiler lived on. This design was very different from a fire-tube. The bent tubes were (and typically are) installed between an upper steam drum and a lower mud drum with additional down-comers as needed to insure proper water circulation. In this type boiler (aka “bent-tube”, “flex-tube” or “serpentine tube”) the fire (the radiant flame) and resulting hot gasses scour the outside of the tubes as they pass from the combustion chamber, through the tubes and out the stack. Fireside cleaning was more difficult with the bent tube design but as mentioned this was becoming less of a concern with the rise in cleaner burning fuels like natural gas and diesel. There was less heat transfer surface in the bent tube boilers. To compensate for the effects of soot and scale, fire-tube manufacturers had always added more heating surface to make up for the steady decline in efficiency over time. This made the fire-tubes big and bulky compared to this new design that had about half the heating surface (5 square feet as opposed to 10 square feet for a fire-tube). The bent tube boiler was more compact but with all the bends and smaller tube diameters it was also very difficult to remove the hard scale that formed inside the tubes. Minus the ability to mechanically clean the waterside, bent tube boilers had a lot less waterside heating surface to foul (scale) before loss in efficiency and overheating became a problem.
While fire-tube and bent tube boilers could provide high pressure steam (defined as anything over 15 PSI) for most industrial applications, there was a growing demand for low pressure steam - especially for the comfort heating of large buildings and residences in the metropolitan northeastern United States. By now, public perception held that low pressure steam was inherently safer than high pressure steam. Moreover, if the steam pipes were large enough, just a few pounds per square inch of steam pressure could easily heat a tall building. The Empire State Building in New York City, for example, is heated by steam boilers that operate between 1 and 3 PSIG. The ASME Code for pressure vessel design of low pressure steam boilers were (and still are) much less stringent than for high pressure steam. Cast iron was a good metal to make low pressure steam boilers out of for four reasons: It was relatively cheap. It could be cast into shapes. It’s tensile strength was strong enough for it to be used for low pressure steam (although not high pressure steam). And cast iron was more resistant to oxygen corrosion than steel. This last point was important because when any steam boiler cools off it will suck in air due to the vacuum created - and air laden oxygen will lead to waterside oxygen corrosion in boilers. Oxygen scavenging chemicals and other strategies have since been developed to mitigate oxygen corrosion in steam boilers, but back before these chemicals were developed and in general use, cast iron boilers became a widely accepted choice for low pressure steam heat.
Cast iron boilers are made by putting together sections that are bolted and gasketed together with nipples and push rods to form a complete boiler. These boilers are prevalent in the northeastern United States where most cast iron boiler manufacturers are located. Cast iron sectionals can also be hand trucked down into basements one a section at a time, which was something that bent tube and fire-tube boilers were not designed to do. Cast iron is, of course, a far less ductile metal than steel and one good thermal shock can easily crack one or more sections. The sections, albeit cumbersome and heavy, can be replaced although it is very labor intensive. To get to a leaking or cracked section, all the sections in front of it – or behind it - must be disassembled first. While cast iron boilers provided affordable low pressure steam boilers (and low pressure hot water boilers) for the residential/commercial heating market, this design, like the fire-tube and bent tube before it, kept the mud and scale pretty much hidden from view. On the bright side, the dirty side of boilers would not remain in the dark for much longer.
The post World War II building boom in the United States created an unprecedented demand for boilers - especially hot water heating boilers. In the east and mid-west, the older designs remained the tried and true choice for boilermakers and installers who’d cut their teeth on them. But out west a new hydronic heating boiler design was quickly catching on. Far less complicated than previous designs, this new design took a pair of rectangular header boxes with tube-sheets, spaced them apart, and connected them with a bank of water tubes. With a large combustion chamber underneath and the tubes pitched at an inclined angle to promote natural circulation, these boilers were so ridiculously simple that there had to be something wrong with them. Had to be! But there wasn’t. First and foremost, these boilers finally solved the scale removing problem completely. With removable head-plates and straight tubes, this new design allowed 100% simple, fast and easy waterside access for cleaning. For the first time ever, a boiler that scaled up - no matter how many times or how bad – could be restored to its original efficiency in less than a day. Secondly, with two opposing header boxes, water naturally had to flow through all the tubes to get from the return to the supply. Because fire-tube, bent tube and cast iron boilers were originally designed for steam, they had to be adapted to hydronic heating. This created problems because the flow of water often short circuited and bypassed heating surfaces in boilers that were originally designed with a fixed water line to make steam. And third, by simply increasing the size of the header boxes and adding a bank of drying (“superheated”) tubes at the top, the horizontal Rite water-tube boiler was easily modified to produce steam as well. With far less mass (water content) than fire-tubes, the Rite boiler are able to generate steam much faster from a cold start. In addition, the drying tubes produce high quality saturated steam with less than 1% moisture content. This type of boiler eventually found acceptance across the rest of North America and has never looked back. Rite has manufactured this type boiler since 1952. It would be reasonable to ask how a Rite Boiler overcomes thermal shock with straight tubes. The answer is we added an expansion joint to one of the header boxes so that it is allowed to slide back and forth as the tubes expand and contract. Rite was actually the first boiler manufacturer to offer a 25 year thermal shock warranty because of this and we have yet to process a claim. Rite’s horizontal inclined water-tube boiler is also one of the most versatile boiler designs ever: equally at home as a hot water boiler, a low pressure steam boiler, a high pressure steam boiler. Rite Boilers are also available in two knock down versions, as weatherproof boilers, and can fire on most fuels including #2 oil (diesel) and biogas.
Along with the post war building boom came residential swimming pools. Lots of them. And in the 1950’s and 60’s, people liked their drinks cold and their pools warm. Large hotels with central boiler plants would use heat exchangers to heat their pools. This was not practical for single family residences that needed something small, non-ferrous and direct fired. The development of the integral copper fin tube allowed for the mass production of small, lightweight residential pool heaters. By now most newer buildings were being heated with water rather than steam and as the pool and spa market matured, pool heater manufacturers simply lengthened their heat exchangers (up to a maximum input of about 4 million BTU’s) and entered the hydronic heating market in the mid 1970’s.
Copper fin tube boilers can be opened up and inspected just like a Rite boiler but there is a very big difference between the two heat exchangers. While Rite has a bank of steel tubes absorbing both radiant and convective heat, copper fin tube boilers rely on just one row of tubes to do the same job. The amount of heat absorbed is roughly the same for both designs - the supercharged copper fin exchanger’s rapid one-row heat absorption about equal to the more gradual absorption by 7-8 rows of steel tubes in a Rite. Compared to Rite, and most other designs, copper fin tube heat exchangers are the hares of the boiler world. This is because copper fin tubes have a far greater amount of heating surface per square inch on the hot gas side than other designs. All that concentrated energy must be removed – and very quickly – or the copper fin tube(s) will overheat and fail. Fire a copper fin tube boiler that is full of water and the tubes will burn up. Why? Because just having water in the boiler cannot remove the energy absorbed quickly enough. It must have flow. A Rite boiler can fire with no water flow at all - and no damage will occur. It will simply come up to temperature and turn off. That is a very forgiving design in a world where mechanical systems that rely on moving parts can and do fail. Rite steam boilers – in fact almost all steam boilers - operate with a fixed water line with no water flow. Copper fin tube boilers can’t do this because they must have flow. And not just any flow: 7 feet per second is recommended, so an extra hefty pump usually separate from the system pump is required. The reason for 7 feet per second? Less flow and the tubes may scale; more flow and the tubes may erode on the inside, all due to the mineral hardness found in water – the same minerals that scale up most boilers and water heaters. The copper fin tube heat exchanger can work well as long as everything around it works perfectly. Unfortunately mechanical things with moving parts don’t work forever. Sooner or later a flow switch paddle sticks or a pump coupling breaks and before you know it you have a building with no heat. For most boilers that won’t be a problem. You call a plumber, he fixes the problem and you have heat again. If the same scenario happens to a copper tube boiler, the results are more likely to be summed up this way: “You need a new boiler”.
Condensing boilers first appeared in Europe and have become commercially accepted in the United States only in the last 10-15 years. They are strictly for domestic and hydronic water heating – no steam - and generally cannot fire on fuels other than propane or natural gas. They are called condensing boilers because when the hot gasses from the products of combustion strike cold water backed heating surfaces, the moisture in these gasses condenses out, much like hot breath against a cold pane of glass. When this happens, latent heat is released and this increases the efficiency of the boiler. With the current glut of “green” technology, LEEDS certified buildings, new government efficiency regulations and rebate programs for “more efficient” equipment – it should come as no surprise that condensing boilers have become the new darling of the hydronic boiler market. But are condensing boilers truly revolutionary and a big step forward?
Until condensing boilers came around, condensing a boiler was the last thing you wanted to do. Condensing – or “rain inside the boiler” was bad news for a number of reasons: 1) The acids in the condensation would destroy ferrous and even non-ferrous metals like copper rather quickly. 2) Condensing was bad for combustion, often leading to wet soot that lowered efficiency and plugged up the gas side of the heat exchanger. 3) Condensing could not only destroy the boiler but the stack as well. 4) Lower stack temperatures could reduce the natural draft to where the products of combustion could spill back into the boiler room.
Then again, there were very few true “condensing” water temperature applications in the U.S. hydronic heating market to begin with. That is because most heating coils are designed to operate between 140 and 200 F. Since this is above the condensing temperature range, condensing boilers were largely ignored since they were too expensive and wouldn’t condense anyway. The few true condensing hydronic heating loads were generally on the small side: snow melt, swimming pools, water source heat pump systems and radiant floor heat primarily - and there were not enough of these to drive the condensing boiler sales we have seen in the last few years. So what changed?
A combination of things. First, some heating systems have been over-sized on the principle that too much heat is better than not enough. Between oversized heating coils and oversized boilers, some buildings were getting too hot. Coils on oversized systems that were designed to operate at 160 to 180 F could operate at lower (condensing or near condensing) temperatures and still provide all the heat the building needed. This would be a legitimate application for a condensing boiler retrofit. Another factor was the marketing of condensing boilers to up-and-coming engineers, architects and energy wonks as another solution to the green revolution. 99% Efficiency! The Age of Aquarius was back! Condensing boilers would pay for themselves in just a couple of years! Never mind that most condensing boilers were being installed in systems that rarely, if ever, condensed – they were 99% fuel efficient! Right? That is true, unless your heating system liked to operate at higher than 60 degrees for some strange reason. Raising the temperature in a condensing boiler and watching the efficiency fall from 99% down to 85 - 87% is a good lesson in “There is no free lunch… and if it sounds too good to be true it probably is”.
Some new buildings are being designed that can take advantage of condensing boiler temperatures but there is a tradeoff: lower the operating temperature of the boiler and you must increase the number or size of the heating coils to compensate. Changing a system from 180 F supply water to 120 F means the heating coils will have to be increased by a factor of about 3 times to get the same heat into the building. This is more like borrowing from Peter to pay Paul than something truly revolutionary.
Is that the only trade-off? Consider this: Because the heat exchangers of condensing boilers must be made from more exotic metals, be prepared to pay anywhere from 30% to 60% more for a condensing boiler than a conventional boiler like Rite. Installation costs will be higher because the venting system will need to be upgraded. An acid neutralization system will be required before the condensation is allowed to go down the drain. The maintenance cost will go up as the condensing boiler will require more frequent cleaning due the acidic conditions inside the unit. Most parts in a condensing boiler proprietary, so be prepared to pay dearly for them down the line. The expected life of condensing boilers in the U.K. (where they have a longer history than in the US) is 10-15 years according to UK Energy Saving. Compare that to many Rite Boilers that are 60 years old and still going strong. And finally, take a look at the heat exchanger. There are as many different heat exchanger designs as there are condensing boiler manufacturers, so it would take too long to comb through each one here. But take a close look at how easy it is to access the waterside – or whether it can be opened up at all. A scaled heat exchanger is an inefficient heat exchanger – no matter what the efficiency was when it was brand new. It don’t mean a thing if it can’t be cleaned. And if someone tells you they have a “self cleaning” boiler, well, good luck. Maybe someday… The history of boilers is still being written. For a related article on condensing boilers, click here.
Thanks for reading this brief history of boilers. The history of burners, combustion and controls were omitted because they are relatively the same for all boilers. This history is obviously slanted toward our horizontal inclined water-tube design - one that we believe in and feel strongly about. If you have a boiler of a different design, if nothing else perhaps some of the information above will help you understand and maintain it better.