All
I based my statement about using superheated steam for auxiliaries on the example of the Lima A-1, which was widely reported in the literature of the time as doing this very thing. I happened to have been doing research on this locomotive at the time that I wrote my previous post, and (perhaps rashly) assumed that if the A-1 did it, most of the rest of Super Power did it as well. I've since done some further research and quite frankly can't find any references to tell me if other engines did this or not. So, it seems to me likely that at least some Super Power locomotives used superheated steam for some of their auxiliaries, but it is not necessarily the case that all of them used it for auxiliaries, or that it was used for all auxiliaries. Mea Culpa.
On the subject of the injector, a simple energy calculation can give you at least a ballpark figure for the temperature of the water coming out of the injector. First off, remember that almost all of the energy content of the steam is recovered, including the heat of vaporization. This means that, depending on whether you use superheated or saturated steam in the injector, you will have somewhere in the vicinity of 1,100 to 1,300 BTUs per pound of steam used available to heat and pressurize the water (pressurization takes some energy, too, and the pressure on the outlet side of the injector needs to be the same or slightly higher than the boiler pressure for water to flow into the boiler.) Personally I believe that the posited 10:1 bypass ratio is a bit high, so lets assume a ratio of 7 or so. So, 1,100 BTUs to heat 7 pounds of water from 70 deg. F will give us 157 BTU per pound to heat the water, giving us an outlet temperature of 227 deg. F.
Of course to make this back of the envelope calculation more precise, we would need to calculate the energy needed to pressurize the water, and we would need to account for heat losses to the metal of the pipes and the injector body, but I believe that those would be minor corrections. Note too that this calculation depends rather critically on whether saturated or superheated steam is used (I used saturated in the above example) and on the specific bypass ratio you assume.
Hope this adds some light to the discussion.
Karen Karen Parker karenparker@pobox.com
I recently purchased an infrared thermometer and decided to try it out on the live steamer this weekend. The stated accuracy below 212 F is +/- 4 degrees, above 212 F is +/- 2%. I measured the feedwater pipe from the injector while in operation, and it measured 119 F near the injector, and 135 F near the boiler. Since the boiler measured 345 F (in the ballpark for 100 PSI operation) , I suspect the second reading is probably due to the heating of the feedwater pipe / check valve due to the close proximity to the boiler. This was just a fun experiment with a cheap, non-contact thermometer, so take the information for what it is. It would be interesting to do the same with a modern, superpower locomotive that carries 250 - 300 psi and see what the feedwater pipe temp is while injecting water. One last thing... I was reading an old TRAINS magazine article the other day - from back in the early '60's - and it mentioned a locomotive being modified to have the turrent plumbed into the superheater. I'll try to look back through and see if I can find it again for reference.
Interesting Stuff... - James
Edit: I found the TRAINS article... I was mistaken that it said the turrent, it actually mentioned using superheated steam for auxiliaries. It can be found in the December 1963 issue, page 29. The article is "Gliders, Yellow Jackets, NC&StL and Stripes by David P. Morgan about the J3 4-8-4's. This is the quote from the article: Superheated steam was supplied to such auxiliaries as the air compressor ("They told us it couldn't be done," Darden recalls, "but it was just a matter of using the proper additive oil to lubricate the pump. After all, what better place to use superheated steam than on a cross-compound pump?"). I would assume that superheated steam would have been plumbed to the turrent and would have ran injectors as well, but that may not have been the case. At any rate, this implies that although superheated steam was used for some auxiliaries on this series of locomotives, it was the exception and not the rule.
BigJim So, it seems that Karen's figures on temperature may not be far off, depending on boiler pressure and the only mistake that she has made is in her above post stating that injectors use superheated steam. And let me add, a mistake she did not make in her book. My thanks go out to Mr. Petitjean and to all of your replies for getting me to thinking in the right direction.
So, it seems that Karen's figures on temperature may not be far off, depending on boiler pressure and the only mistake that she has made is in her above post stating that injectors use superheated steam. And let me add, a mistake she did not make in her book.
My thanks go out to Mr. Petitjean and to all of your replies for getting me to thinking in the right direction.
BigJim, I agree with your conclusion about Karen's temperature statements and echo your sentiments about the responses. I tend to think about steam technology from an 1860's - 1890's perspective, whereas Karen's book is clearly focused on Superpower Steam, so my research from the earlier era reflected lower injector feedwater temps than the time period Karen's book is focused on.
I bought and read Karen's book, and I think she did a fine job. It is geared more toward someone who is looking for more information than your basic steam trains book, yet isn't interested in a dry, technical, textbook style manuscript. It has some really good pictures of various aspects of steam locomotive construction - I especially enjoyed seeing the various siphons & circulators in the firebox photos! It is a good book for anyone who is looking for more than just an overview of superpower steam locomotive construction.
- James
After reading all of the above replies, I started to think this whole thing through in a rational manner. Then while reading a hobby magazine, I came across the quote "Many times in this hobby, we assume that we know how things are today or were done yesterday, but do we really?" Hmmmm, somebody is trying to tell me something. This is no coincidence.
Consequently, I sent a letter to Mr. William L. Petitjean, P.E. Mr. Petitjean is the author of "The Thermodynamic Closing of the Great Steam - Diesel Debate", which was a three part article that appeared in the magazine "Locomotive & Railway Preservation" starting with the Jan/Feb 1993 issue. The following is his reply to my temperature question.
First, temperature is a measure of the "quality" of energy contained in water/steam. It is a poor indicator of the "quantity" of energy. The BTU (British Thermal Unit) is the universal measure (in the U.S.) of the quantity of heat (energy) contained in any gaseous or liquid substance. 1 BTU equals the quantity of energy required to raise the temperature of 1 lb of water 1 deg. F at standard conditions. The primary reason the use of a live steam injector tends to "knock the steam pressure down" in a locomotive is the fact that steam is being extracted from the boiler to drive (pump) relatively cold water into the boiler to maintain the proper water level. The circulation of the cool water essentially stops the boiling process and, for a short time the the boiler becomes a water heater rather than an evaporator. An analogy follows. When you have a pan of boiling water on the stove it is evaporating water into steam. When you pour in the frozen corn, the water stops boiling until the corn is brought up to 212 deg. F whence the water begins to boil again. Cold water going into the boiler is just like the frozen corn. This is why a locomotive must be fired much harder when the injector is in use, especially going up a hill when the engine is working hard. More heat release in the firebox is required to replace the steam used to drive the injector, supply the energy required to heat the water to its boiling temperature and supply the energy to supply the "latent heat of evaporation". The latent heat of evaporation is the energy required to separate the water molecules enough so they become a vapor instead of a liquid. At typical locomotive boiler pressures this is the dominant amount of energy required to get the water to boil and is the primary reason steam locomotives have dismal thermal efficiency. When the boiler water stops boiling the steam pressure falls until the evaporation process starts up again. If the locomotive is fired hard enough the heat tranferred to the water is great enough to eliminate this evaporation/water heating/evaporation process and steam pressure can be maintained. The live steam injector was developed well enough for locomotive service by about 1850 that it quickly replaced the crosshead pumps then in general use. It is an amazing device because it has no moving parts and utilizes steam at boiler pressure to inject water into the same boiler, overcoming the pressure used to drive the pumping action. Furthermore, it condenses the propulsion steam in the injected water stream so the boiler always receives heated water instead of cold water straight from the tender. This was a drawback to crosshead pumps in the early 1800's -- no means was provided to heat the water at all. Also, the locomotive had to be moving for the pump to function. From the above we can see that the live steam injector is both a pump and a water heater. The two functions are combined and integral with the device. It turns out the the injector is a relatively poor pump. In other words, it consumes a lot of steam for the quantity of water pumped. However, it is nearly 100% efficient in returning the heat drawn from the boiler back into the boiler. Unfortunately, the returned heat is a much lower quality heat as indicated by the pumped water's temperature being much lower than the temperature inside the boiler. The temperature inside a boiler running at 180 psi is approximately 380 deg. F. The temperature inside a boiler running at 250 psi is about 400 deg. F. The boiling point of water rises with increasing pressure. You can see that the various temperatures of injector feedwater listed in the books and articles are all much lower than the prevailing temperatures required to boil water inside the high pressure boiler. The temperature of injector water varies considerably depending on the temperature of the water in the tender, the condition of the injector and the pressure/temperature of the steam used to drive the device. The exhaust steam feedwater heater/pump device was perfected in the early 1900's when rising fuel costs, labor costs and increasing locomotive size and performance justified their application. There are two primary types. They are the exhaust steam injector and the open or closed feedwater heater driven by a steam powered pump controlled from the cab. Open type heaters mingled exhaust steam directly with the water and used two pumps; one to supply water to the heater chamber and one to force the heated water into the boiler. The closed type used a shell and tube heat exchanger so the water and exhaust steam did not co-mingle. Only one pump was required to circulate water through the heat exchanger and force it into the boiler. The steam driven pump exhausts were dumped into the heater chambers so it was not completely wasted. The exhaust steam injector was not as popular but was much simpler and did not require as many separate devices. It essentially used exhaust steam to heat the water and drive it into a separate injector chamber that used live steam from the boiler to force the heated water into the same boiler. Its advantage was the use of exhaust steam to heat the water, thus the live steam section was more efficient and used less steam directly from the boiler than a plain old live steam injector. The use of exhaust steam was an important improvement in steam locomotive efficiency and performance for two reasons. First, they all tended to return condensed exhaust steam to the boiler rather than waste it up the stack. The best exhaust feedwater heaters reduced water consumption around 10% thus pushing more mileage out of a tender load of water. Second, They utilized the waste heat in exhaust to pre-heat feedwater, thus they returned heat to the boiler that otherwise would be wasted up the stack. Even though feedwater heaters delivered hot water at lower temperatures than injectors (because exhaust steam is low pressure, low temperature steam) they increased efficiency and performance in the following way. If we go back to the primary source of heat energy, heat released by the fuel in the firebox, we can see the greatest benefit of the device. All of the energy used by the injector to pump/heat water (remember most of the heat is returned to the boiler as heated water) comes directly from the firebox heat release required to supply the steam used to run the injector. The exhaust feedwater heater requires energy directly from the firebox heat release to pump the water, but these pumps are much more efficient than the injector and use less steam for a given quantity of water delivered to the boiler. Finally all the energy required to heat the water is being recovered from exhaust heat discharged the engine cylinders. This heat has already been charged to the engine cylinders. Therefore, the use of exhaust energy does not require any more heat release in the firebox other than the heat already used by the engine cylinders. On large locomotives feedwater heater/pumps were always on the fireman's side and were the primary means of feeding water to the boiler. The engineer's side was always equipped with a live steam injector for emergencies and while standing still. Engine crews liked exhaust feedwater heater apparatus because they were much easier to control and meter water into the boiler only as fast as it was needed. Small engines and industrial engines still were equipped with injectors on both sides because their small size and low load factors could not justify the high initial cost of feedwater heaters.
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I thought injectors got their supply steam from the turret.
Which indeed they did.
This from the book "Locomotive Feedwater Heating Equipments" by George V. Williamson;
"With an injector operating at capacity, the water is heated by steam from the boiler to a temperature of about 160* F. If then put through a heater the temperature could possibly be raised about 75 degrees higher. With a feedwater heating apparatus, all of the heating is done by exhaust steam, that is, the feedwater may be raised from 60* F to 235* F, or a total of 175 degrees, with heat that would otherwise escape unused."
The subject of injectors is a fascinating one. The sciences involved in their operation can be confusing to many, myself included. They are heat sensative in that water that is too hot will cause them to fail. Pressure, a factor in the ability of water to absorb heat before boiling, is but one variable to be considered in this study. I would like to get to more info so that the truth of the matter of this subject can be brought to light.
Karen, Please excuse the addition of Lamb to your name. My mistake. Your book was next to another author, J. Parker Lamb, that was on my shelf.
The accessories that are run off the turret use saturated steam. Usually the turret steam supply is taken from the back of the steam dome or by direct access through the top of the boiler near the back. Otherwise a pipe would have to run to the superheater header in the smokebox. I thought injectors got their supply steam from the turret.
James
One thing to remember is that on a modern Super Power locomotive, the injectors, like all the accessories, were running on superheated steam. For example, on the C&O K-4 2-8-4, the boiler pressure was 245 psig, and the rated superheat was 250 deg F, giving you more than 200 BTU/lb more heat content than with saturated steam.
This, of course, means that the injectors are that much more efficient, both in terms of how much they heat the water and how much steam is required to inject a given quantity of water.
- Karen
Karen,
Thank you for taking the time to respond and setting the record straight on the stated feedwater temperatures in your book. I would like to share what information I have found in my research that will shed additional light on the figures.
First, I made a mistake in my previous post, 250 degree F steam would not be 30 psi, it would be 30 psia - absolute pressure - when most of us would be more familiar with it stated as 15 psig, gauge pressure. My copy of McShane's "The Locomotive Up To Date" (second edition, 1920) confirms that there is more than enough latent heat energy from the exhaust steam to heat the feedwater to the temp of the exhaust while using 16% or less of the exhaust steam to do so. At a more moderate back pressure of 5 psig, the feedwater could be heated to 220 degrees F. This is completely in line with the stated 200 - 250 degree F figures as quoted.
The feedwater temp from an injector is a little more tricky. My experience is with boilers that operate at 125 psig or less. At 100 psi, assuming a 10:1 water to steam ratio, there isn't even enough thermal energy to raise 70 degree F makeup water to the boiling point (1 lb of 100 psig steam contains about 1200 btus, that would be 120 btus per pound of water injected, a btu will raise 1 lb of water 1 degree F, so 70 degree F + 120 = 190 degrees F). This doesn't even take into account the energy needed to perform the actual mechanical work involved. Now, for the however part.... HOWEVER, most steam locomotives operate at pressures well above 125 psig, some close to 300 psig (422 degrees F), so there is plenty of thermal energy available. Also, in reading Pochet's book, he is referring to the Giffard injector - the earliest form of injector - in his equations. I believe when he is referring to the temp of the "mixture", it is just that - the point of mixing steam with water within the injector, before passing through the delivery cone. This would be happening in a slight vaccuum, so when the mixture is converted from a high velocity stream to high pressure within the delivery cone, it should increase in temp, just as mechanically pumping water to a higher pressure increases the temp of the water. Pochet gives an example at the end of the book of a different style of injector that is designed to take higher temp feedwater from a condensor (a stationary steam application) and delivers it significantly above the boiling point of water. The feedwater is hot enough that a Giffard style injector would not function with it. Although a steam locomotive is typically non-condensing, a modern injector such as a Chicago or Nathan operating on a high pressure boiler probably would deliver feedwater in the 250 to 300 degree F as stated. The simple injector on my 110 psi boiler probably won't - and now I'll have to measure it the next time it is under steam!
Again, my thanks to Karen for kindly responding to us, and discussions such as this always serve as a good opportunity for me to learn something new!
Well, let's tackle feedwater heaters with exhaust steam first. I don't have a good baseline for exhaust steam pressure, but it has to be above 212 F or it would be water. 250 F would only be about 30 psi, so I think a locomotive, running near wide open throttle, could plausibly have more than 30 psi back pressure in the exhaust and would certainly be passing a very large volume of steam. Hopefully, somebody can chime in with an actual exhaust pressure reading from their experience. An efficient feedwater heater should have more than enough thermal energy available to heat the water to nearly the temp of the exhaust steam. This is just seat of the pants engineering, but I do think that the number is possible. Also, we shouldn't forget that feedwater heaters were typically located in the smokebox area, so it could also receive some thermal energy from combustion exhaust gasses, admittedly not as much energy as received from the exhaust steam, but it would be at a substantially higher temperature.
The book I am reading is "Steam Injectors: Their Theory and Use" by Léon Annet Pierre Pochet, which dates from 1877. I have been trying to find information on injector design and operation in order to build my own from scratch someday, but this type of information is scarce as hen's teeth. Keeping in mind that the information in the book is 133 years old and only a generation removed from the original work of Giffard, I'm sure that newer injectors at higher pressures would be more efficient. Nonetheless, Pochet lists 80 degree C feedwater at 8.55:1 ratio and 60 degree C at 12.61:1, making your 10:1 assumption look entirely realistic for the 70 degree C feedwater (ratio is calculated by weight, 1kg of dry steam to X kg of water). Disclaimer: This is assuming that I am interpreting his tables and calculations correctly! If anyone sees a problem in this, please post a correction for me... I just want to learn and understand to the best of my ability. As far as experience, I have never applied any instrumentation to an injector to see what type of ratio (or even feedwater temp) can be expected from a commercial unit. I have dealt mainly with smaller injectors, 1" NPT and smaller. I have always just repaired them, which is a much simpler proposition than design from scratch!
Does anyone else have thoughts / experience in these areas?
I hope this helps, Anthony. Let me know if this information seems reasonable or not.
JamesPAccording to Leon Annet Pierre Pochet, theoretically the injector ceases to function when the temperature of the mixture (steam and water, the feedwater) reaches 100 degrees C. He claims in practice, it ceases when the feedwater temp reaches 70 degrees C - that would be 158 degrees F. My experience with injectors is that neither the injector body or the feedwater pipe exceed the boiling point of water when operating, so the feedwater could not be above 100 degrees C. Note that I said when operating... everybody who has been around steam has probably witnessed an injector that got overheated with steam from a leaky boiler check, won't pick up, and some poor fireman is dousing it with water to make it function! I was running a steamer today, if I had seen this earlier, I could have stuck a thermometer on the feedwater pipe from the injector! But, I will take Pochet's word on it. - James
According to Leon Annet Pierre Pochet, theoretically the injector ceases to function when the temperature of the mixture (steam and water, the feedwater) reaches 100 degrees C. He claims in practice, it ceases when the feedwater temp reaches 70 degrees C - that would be 158 degrees F. My experience with injectors is that neither the injector body or the feedwater pipe exceed the boiling point of water when operating, so the feedwater could not be above 100 degrees C. Note that I said when operating... everybody who has been around steam has probably witnessed an injector that got overheated with steam from a leaky boiler check, won't pick up, and some poor fireman is dousing it with water to make it function! I was running a steamer today, if I had seen this earlier, I could have stuck a thermometer on the feedwater pipe from the injector! But, I will take Pochet's word on it.
James:
Before my last post I ran some numbers for steam at 300 psi. I am not sure how, but I arrived at feedwater temperatures approaching 250F. Rerunning the numbers (using a feedwater-to-steam mass flow rate ratio of 10, which I believe is reasonable) the estimated feedwater temperature is in the 160 to 170F range, which is more in line with your last post. Based on your experience, how do the flow rates of the feedwater and steam compare?
Also, I have read accounts of feedwater heated as high as 250 F using feedwater heaters. This doesn't seem possible given that exhaust steam temperatures are so relatively low. Is this possible, and if it is, what am I missing about steam locomotive plumbing?
Thanks
Anthony V.
BigJimAnthonyVWhat does she say about them? Without getting too involved, basically, she says the injector heats the water "commonly to between 250 F and 300 F". Then she states that the feedwater heater heats the water "only to between 150 F and 200 F". I think she got her degrees mixed up. lbeamlicker I was so offended I vowed never to buy another one of her books. I should have expected as much from a TLC book.
AnthonyVWhat does she say about them?
Without getting too involved, basically, she says the injector heats the water "commonly to between 250 F and 300 F". Then she states that the feedwater heater heats the water "only to between 150 F and 200 F". I think she got her degrees mixed up.
lbeamlicker I was so offended I vowed never to buy another one of her books.
I was so offended I vowed never to buy another one of her books.
I should have expected as much from a TLC book.
It sounds plausible to me. The injector uses steam from the boiler at a high temperature (350-400F) while feedwater heaters use exhaust steam which is somewhere in the range of 212F.
BigJimIn the book "How a Steam Locomotive Works" by Karen Parker Lamb, does anyone take any exception to her explanation of injectors and feedwater heaters regarding their heat transfer ability?
In the book "How a Steam Locomotive Works" by Karen Parker Lamb, does anyone take any exception to her explanation of injectors and feedwater heaters regarding their heat transfer ability?
I don't have the book.
What does she say about them?
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