Overfire Jets on Steam Locomotives
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This page was last updated on July 19, 2021.
Salt Lake City passed its smoke control ordinance on March 30, 1941, and gave all industries including railroads, six months to comply, by October 1, 1941. On April 11th D&RGW advised the mayor that it would have six diesel switchers in service in its yards in Salt Lake City and Roper. UP received five new EMD NW2 switchers in mid-August specifically for service in Salt Lake City.
Research in newspapers of the period suggests that Salt Lake City was among all large cities seeking solutions to smoke pollution affecting their citizens. Throughout the late 1930s and the early 1940s numerous large cities began working with sources of offending smoke, including coal-fired power plants and railroads using coal-fired steam locomotives. The many solutions usually included limiting the time a coal-burning steam locomotive was allowed to sit idle within city limits, modification of coal-burning steam locomotives to insure optimum smoke-free combustion of "soft" coal, and the substitution of diesel switching locomotives in railroad switching yards.
On July 31, 1946, Salt Lake City passed an amendment to its smoke ordinance requiring that any railroad locomotive operating within the city be equipped with an apparatus to minimize smoke. They gave the railroads one year to comply, by August 1, 1947. By June 9th smog in Salt Lake City had been reduced by half over the previous year. The deadline was extended another year upon protest by the railroads and the Utah Coal Association, as well as the railroad unions. The original amendment had been proposed in early May 1946 and would ban all steam locomotives from operating within city limits. The amendment as it was unanimously adopted read: "All railroad locomotives operating within the limits of Salt Lake City, and using coal as fuel, shall be equipped with fire brick combustion arch, steam-operated side induction tubes, and an approved blower and blower valve."
In mid-June 1947 both UP and D&RGW negotiated with the city agreeing to eliminate all steam locomotives used for switching and transfer runs between the city and Roper yard, 2.6 miles to the south. They agreed that all coal-fired steam locomotives operating within the city would be equipped with "induction tubes," and that they would transfer out of the city any locomotives that did not comply. In return, the city agreed to extend the deadline for six months, to January 1, 1948. It was later extended again, to August 1, 1948. The terms "induction tubes" and overfire jets were used interchangeably in trade publications of the period, describing solutions to smoke problems for many large cities, and those same cities' interest in possible regulations limiting smoke.
Overfire jets were usually used in engine terminals where many coal-fired locomotives sat idle for long periods. The railroads' fire watchers would very often add too much coal to a banked fire, and move to the next locomotive in the line, at times up to 20 locomotives. Later all that new coal added to a fire would get hot enough to combust all at once, producing dense clouds of black smoke and noxious gas. With D&RGW's engine terminal right downtown on 4th South, it's no wonder Salt Lake City's residents and city officials complained and passed the ordinance. Overfire jets added fresh oxygen to the lazy fires and greatly improved the draft and clean burning of coal, thus much less smoke.
Overfire jets supplied a secondary source of air above the fire bed to provide more oxygen to help burn the unburned carbon coming off the firebed. The jets themselves operated on the venturi principle. Each overfire jet had a small steam nozzle that passed a jet of steam into a 2" tube installed in the sides of the firebox about 18" above the grates. The steam jet drew in outside air from outside the boiler and injected it into the firebox above the fire. So instead of having to draw all the air needed through the grates, this secondary air inlet helped consume the excess carbon. The overfire jets were operated by a valve on the fireman's side, and was operated when necessary. The operation of overfire jets was designed to be used at station stops when there might be a lot of unburned coal in the firebox and no exhaust available to pull air through the grates and the flues and out the exhaust stack. There were generally four to ten overfire jets on each side of of the firebox depending on the size of the firebox.
Overfire jets were located on the sides of the fire box parallel to the grates, and were offset to each other from right side to left side to create disruptive air flow inside the firebox and above the firebed. This disruptive air flow allowed more of the unburned combustion gases to burn before exhausting through the flues and out the stack. Overfire jets were located specifically above the firebed to bypass drawing air through the fire and eliminate further release of volatile gases which would not be fully burned while the locomotive was sitting idle and stationary, such as at a station stop, or at an engine terminal between runs.
Locomotives that burned oil did not need overfire jets because the flow of oil can be cut back instantly to reduce the amount fire. With a coal-burning locomotive, once a good firebed is made, it is almost impossible to shut it off to reduce smoke. A large active fire is needed to provide sufficient heat to allow the engineer to open the throttle and be able to move the locomotive and train.
D&RGW steam locomotives with overfire jets, based on photo research.
D&RGW 2-10-2 1400-1409 (photos: all except 1403)
D&RGW 4-8-2 1501-1530 (photos: 1501-1504, 1508, 1509, 1528, 1529)
D&RGW 4-8-2 1600-1609 (photos: only 1605)
D&RGW 4-8-4 1700-1713 (photos: 1704, 1705, 1709, 1711, 1713) (removed from 1709 before 1956)
D&RGW 4-8-4 1800-1804 (photos: all)
D&RGW 4-6-6-4 3700-3714 (photos: all except 3709, 3712, 3714)
Instead of overfire jets, Union Pacific used simple non-jet air tubes between the exterior and interior of the firebox, placed above the fire line "overfire" to provide more combustion air. The drawing (dated July 1942) shows that they were made from standard 2-1/4" boiler tubes. The official term was "Secondary Air Openings." The same feature is also shown on drawings as "Combustion Tubes."
UP's combustion tubes were self-regulating due to the physics of a vacuum. Union Pacific's modern steam locomotives were equipped with stokers, so with a locomotive under load pulling a train, the coal dust and fine particles burned immediately much like an oil stream. Under a smaller load, there was less draft pulling air through the grates and the coal bed. Also, due to the distance between terminals, the coal bed got thicker allowing less air to enter the firebox through the grates. With less air flow through the grates, the fire's draft would then pull the needed combustion air through the "overfire" combustion tubes.
Gordon McCulloh, in his book, A History Of Union Pacific Steam, wrote that in 1941, as part of an overall modernization of older locomotives, Union Pacific began equipping coal burning locomotives with secondary air openings in fireboxes as part of an overall program that included labyrinth type front end, multiple jet nozzles, larger stacks, larger injectors, safety valves of increased capacity, and improved sanders. The classes affected included Heavy 2-8-2 Mikados, Heavy 4-6-2 Pacifics, FEF-class 4-8-4 800s (800-834), Simpled 2-8-8-0 3500s, TTT-class 2-10-2 5000s and MT-class 4-8-2 7000s. The secondary air openings were placed in the firebox sides to add over-fire air and improve combustion. Most were removed during General Repairs after 1948. The drawing for secondary air openings shows that the FEF-3 835-844, and the newest 4-6-6-4 Challengers and 4-8-8-4 Big Boys also received the feature. A revision dated August 28, 1948 of that same drawing shows that the openings were removed from all three classes of FEF 4-8-4s, the Jabelmann Challengers and the Big Boys.
As a side note, photos in Life magazine in 1942 showing UP Big Boys being built at the Alco factory, including the photo of the original "Big Boy" inscription, show that UP's Big Boy locomotives were built with combustion tubes in the sides of their fire boxes.
Union Pacific 4-12-2 number 9004 was equipped with muffled overfire jets as a smoke reduction test. Although the drawing for the jets themselves was prepared in 1945, the actual installation did not take place until 1951, and was not deemed as a success. (The Union Pacific Type, Volume 2, pages 258, 259)
On Western Pacific, only those locomotives that burned coal and were assigned to the east end between Elko and Salt Lake City, were subject to Salt Lake City's smoke ordinance. Of WP's total fleet of 208 steam locomotives, this included ten 2-8-2s (301-305, 311-315), and seven 4-6-6-4s (401-407). Photos of 405 and 407 taken in 1949 show that they were equipped with overfire jets. All seventeen locomotives were retired in 1950-1953.
Wes Camp Notes
Wes Camp has written on Trainorders.com about overfire jets:
(used with permission)
Back in the forties, air pollution from steamers was a terrible problem at engine terminals where hundreds of steamers were choking the air with awful amounts of unburned carbon gases as well as tons of soot particulates. So, since there was virtually no sufficient draft to completely burn the released carbon, overfire jets gave some relief to the oxygen starved firebeds. In order to mix the released carbon gases (mostly carbon monoxide), the steam jets were employed to do the mixing above the smoky firebeds.
Overfire jets are very effective. However they are manually operated; somebody must turn them on. They add combustion air above the firebed to aid in complete combustion of the released carbon molecules in the burning gas stream.
I have experimented with them while under way. With a light-gray smoke-stream out the stack, if I opened the overfire jets, even a very small amount, the stack went to 'clear' and the safety valves lifted, all without changing the rate of coal fed by the stoker.
Thus, I could reduce the stoker feed-rate, burn less coal, and maintain a very hot firebox temperature, all by adding more oxygen above the firebed.
You do have to be mindful, and use them with intention. It's a matter of being properly trained.
In practice, they were often applied in response to early, local, air-quality laws, and the railroads grudgingly applied them, issued directives on their use, but failed to follow-up with effective explanations, training and a consistent rewards system for effective compliance that rewarded workers and supervisors, even with token incentives. Then the diesels arrived.
Since the airflow through the firebox of an oil burner is damper controlled, (with both automatically regulated vanes and manually operated vanes) the use of overfire jets on oil-burning locos is superfluous. The jets and/or passive vents are most effectively applied to coal burners.
Ideally, the firebed completely covers the grate area. But the airflow through the bed is severely blocked compared to oil burners. Thus, long flames can consume the restricted oxygen volume that is above the firebed. So, adding-in oxygen above the firebed, and directly into the flames and wind-path does a lot to complete the combustion.
The original intent was to mitigate the smoke produced by a bunch of engines sitting on ready-tracks tended by fire-watchers. The task of fire-watchers was to keep the fresh coal bed from burning-out, and dying. The other duty was to ensure that the water level in the boilers was kept well above the running level --- while readied engines were idle.
Fire watching is a some-time thing. A whole roundhouse full of idled engines can be maintained by a single watcher. You go to each engine, add water via the injector, throw on a layer of green coal, then go to the next engine, until all are refreshed. Then you find a warm place to go to wait for the next round.
Too often that warm place worked its magic, and the fire-watchers dozed undisturbed through the wee-hours of the morning.
Thus, was born the low-water alarm --- a screaming, cab mounted, steam whistle that was activated when the boiler water was just barely out of sight --- below the bottom of the visible water glass. That alarm would alert the midnight shift foreman to go and poke the fire-watch guy to wake up and do another round of fire-tending of all the engines in the roundhouse, and those sitting on the ready tracks.
So, the fires would be nearly dead by this time, the water very low and that would lead to a lot of coal being added across the firebed and to revive the nearly dead fire, and then flooding of the boiler with water to stop the screaming low-water alarms. Sometimes, several low water alarms were all screaming at once, in a roundhouse full of parked steamers.
So, a little later, all that green coal hastily thrown onto the cold firebed, would warm-up and ignite, all at once, like after about 20 minutes of "cooking time." Now you have a fully involved grate of fresh coal, a very low draft and dense clouds of smoke hanging over the cities and towns, choking everybody.
When dense smoke, with high levels of CO, accumulates in the the coal fired locos, the overfire jets, when used on parked locos, brought cold fresh air into the firebox, over the coal bed without the need to increase the draft up the stack, using the steam-jet, artificial-draft steam piping (the so-called "blower").
[Using the Blower --- the ring of steam jets below the stack-base in the loco's smokebox --- provides a means to pull a lot of air through the grates, typically while standing, idling and at fire-up times. You also use the blower whenever you need to keep the smoke out of the cab. When fire-tending, typically at night, you don't want to burn the coal to boil more water. You only want to add coal to replenish the burned-out carbon fuel, to keep the fire alive, across the grate. For this stand-by service, the blower adds way too much airflow through the fire and burns your coal way too-fast. So, while fire-tending you only keep the blower valve barely cracked open, just enough to keep the smoke and fumes from polluting the cab where you're sitting. The idea is to have an even, level, bed of glowing coals, almost no flames, and very little coal consumption. Keeping the fire ready for the operating crew that will take-over and run the engine, until the next nightly lay-over. Steam powered over-fire jets will add-in and mix the fresh air into the the excessively carbon-rich gases lingering in the firebox. The fresh air immediately combusts, adding oxygen, to complete the reduction of the carbon to CO2, and producing greater heat release.]
The overfire jets were the result of early localities enacting "anti-smoking" laws. The overfire jets worked very well. When properly used they could consume the high -carbon smoke, without pulling more air through the firebed, while clearing-up the stack from emitting dense clouds.
You gotta remember that large railroad yards in towns across America were choking in polluted air, and poor management of the many idling steamers overnight, was a significant cause which was a real choking and breathing hazard in cities, especially with half-awake fire watchers, peacefully dozing through the night.
(During WW2, the pollution problem (dense smoke) was aggravated by great traffic demands and the hiring of substitute, young engine-watchers, hired to replace the drafted workers. The newbies were poorly trained and inexperienced in all aspects of steam locos..)
Oil burners could be managed much better, since you don't need a whole grate area to be glowing, and the coal burners might be needed on very short notice. The short-notice Fire Up!! orders were another cause of dense, black-smoking steamers common on the thousands of coal fueled engines.
On the larger overfire jets, I suspect that the large canisters carried sound-suppression media (non combustible, fibrous material) to absorb the broad-band, audio, high frequencies emitted by the jets. If several equipped locos are parked in a roundhouse, or are clustered together outside, all with the overfire jets blowing, the noise could be painful. So the "muffled" jets were intended more for bunches of engines, together, than for individual instances. In the cab, the noise of the roaring jets is barely noticeable (the stoker jets are louder), but between several locos at a service area, it made hearing conventional conversations nearly impossible.
The simple jets are covered with "silencing-cans", packed with loosely formed, fireproof noise-deadening material [rock wool] to adsorb the higher frequency noises from the the Venturi jets.
In my experience (with unmuffled) overfire jets, you don't have to set them for more than a gentle breeze, to get the benefits of fresh oxygen into the flames.
Loud, roaring jets are not any more effective. Their most common use is in engine terminal areas where many engines are sitting with fires at varying levels of fire intensity. The common practice with engine watchmen was maintaining live locos --- like 20 or more -- sitting with banked fires.
Sometimes the fires would get very low, and watchmen would apply a new, heavy charge of green coal, then go to the next engines. After a while, that 'heavy charge' would would combust all at once, producing dense clouds of freed black carbon and CO gas.
Overfire jets were used to add oxygen at lazy rates into the smoke and combustible carbon released into the flames lingering over the calm firebed.
There was little or no training provided to the fire-watch crew members on the intent and use of the jets. So, jet use was spotty at best. Folks, including the firemen, didn't know how best to manage the science of more complete combustion of low-draft fires.... when sitting idle.