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Heating surface calculations
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10092010, 09:11 AM

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Heating surface calculations
I want to figure the HS for my vertical Lookout boiler with a water leg. I have searched on here but have had no luck. Could someone post this formula please. Also the formula for steam required to run a 7 X 7 inch engine running at 250 rpm.
Thanks very much.
Richard

10092010, 11:29 AM


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Re: Heating surface calculations
Richard:
Here's the engine horsepower formula that J. I. CASE Co., used to get the approximate horsepower of a steam engine.
I'll let you calculate the appoximate horsepower of your steam engine, as you learn by doing:
THE HORSEPOWER OF AN ENGINE
The unit of power is a "horsepower" and is defined as the amount of power necessary to raise 33,000 lbs. one foot in one minute. The horsepower of an engine is equal to the total pressure on the piston multiplied by the number of feet it travels per minute and divided by 33,000.
The total pressure on the piston is equal to the area in square inches multiplied by the pressure per square inch, and this pressure is not constant but varies, being nearest boiler pressure during the early part of the stroke and decreasing after the point of cutoff is reached, as the steam expands to fill the space back of the piston, until the end of the stroke.
This pressure can be measured only by means of the steam engine indicator but we can assume a value which approximates the correct one. This we will take to be 50% of the boiler pressure. If we have a boiler pressure of 130 lbs., our average pressure per square inch, or "mean effective pressure," (MEP) as it is called, will be 50% of 130 lbs., or 65 lbs. This multiplied by the area of the piston will give the total average pressure on the piston in pounds. The area of a circle is equal to its diameter multiplied by itself and the product by .7854.
The travel of the piston is equal to twice the stroke, there being two strokes for each revolution, multiplied by the number of revolutions per minute. As the length of the stroke is usually given in inches this product must be divided by 12 to reduce the result to feet per minute.
The following example is an engine with cylinder 9" bore and 10" stroke, speed 250 rpm and boiler pressure 130 lbs. This is the size of the J. I. Case engine rated 15 horsepower.
2 x 10 x 250 / 12 = 416.6 (travel of piston in feet per minute.)
9 x 9 x .7854 = 63.6174 (area of piston in square inches.)
63.6174 x 65 = 4135.131 (total pounds pressure on piston.)
4135.131 x 416.6 = 1,722,695.5 / 33,000 = 52.2 (indicated horsepower)
Taking off 10% for the friction of the engine, .90 x 52.2 = 46.98 or 47 horsepower. This is the power delivered by the engine  brake horsepower as it is called.
Courtesy of Science of Successful Threshing.
Gary K

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10092010, 12:30 PM

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Re: Heating surface calculations
Following Gary K's calculations you will know the Hp requierments of your engine.
To calculate the boiler heating surface required;
Boiler horsepower is used rating a boiler's capacity to deliver steam to a steam engine.
The amount of energy needed to produce 34.5 pounds of steam, per hour, at a pressure and temperature of 0 Psig and 212 deg.F, with feed water at 0 Psig and 212 deg.F. One boiler horsepower is about 33,479 Btu per hour.
Boiler horsepower can be determined from the heat transfer area
PBoHP = A / 17 (1)
where
PBoHP = boiler horsepower
A = heat transfer area (sq.ft.)
Knowing the total heating area, then calculate the surface area of one tube;
A = 3.1416 x tube diameter(inchs)/12 x tube length(ft)
Divide this answer into the total heat transfer area and this will tell you how many tubes you need in the boiler, almost. You need additional heating surface to get the steam from 212 F to the operating temperature of the boiler.
This all sounds complicated, but just start with a pad of paper and work your way through the steps. Don't get carried away with a lot of numbers after the decimal point.
I'm sure that there are some old free ICS course books downloadable from GoogleBooks that can help you.
Max

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10092010, 01:23 PM


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Re: Heating surface calculations
Richard:
Heating surface area of a boiler is usually expressed in square feet (12" x 12" = 144" or 1 sq. ft.), and all too often tubes and firebox sheets are listed in inches.
Example: A 2" diameter tube, 30" long; (2 x 3.1416 = 6.2832" circumference) x (30" length = 188.496 sq. in.) If the boiler has 30 tubes; (188.496 x 30 = 5654.88 sq. in.), and 5654.88 / 144 = 39.27 sq. ft. of tubular heating surface.
If the tube sheet in the firebox is 24" diameter; (24 x 24 x .7854 = 452.3904 sq. in.) Now you must subtract the area of opening for each tube; (2 x 2 x .7854 = 3.1416 x 30 = 94.248 sq. in.) and (452.3904  94.248 = 358.1424 sq. in.) and 358.1424 / 144 = 2.4871 sq. ft. heating surface of the tube sheet.
If the 24" diameter firebox is 20" high above the grates; (24 x 3.1416 x 20 = 1507.968 sq. in.) (If the firebox door opening is 10" x 12" = 120 sq. in.) and 1507.968  120 = 1387.968 sq. in., and this divided by 144 = 9.6386 sq. ft.
Then; 39.27 + 2.4871 + 9.6386 = 51.3957 total sq. ft. heating surface of that boiler.
Gary K
 Post added at 12:23 PM  Previous post was at 11:34 AM 
Richard:
When you mentioned steam required to run your 7" x 7" steam engine at 250 rpm, are you referring to pounds per horsepower per hour (lbs./hp/hr.)?
If so, on page 16 of the CASE Steam Engine Manual, they stated: Amount of Water Required. The amount of water required can not be stated very definitely, but usually it will be sufficiently accurate for calculating purposes to assume that 4 gals. (33.2 lbs.) is required for each brake horsepower developed per hour.
If thats the case, after calculating your engines horsepower, multiply that by 33.2 to find the average pounds of steam required per hour.
Gary K

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09112013, 06:56 PM

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Re: Heating surface calculations
Quote:
Originally Posted by Max Cox
Boiler horsepower can be determined from the heat transfer area
PBoHP = A / 17 (1)
where
PBoHP = boiler horsepower
A = heat transfer area (sq.ft.)

I'm not understand the number 17 here ...
I just purchased the 1902 Toledo steam car engine and am trying to calculate the boiler heating surface necessary.
The calculation I got for the HP matches what the engine was speced as.
It was speced at 6.25 to 7HP depending on what source you look at.
I calculated 7.110 HP at 200PSI and 250RPM using the equations Gary posted above. I just multiplied the HP I got in the end by 2 because it has 2 cylinders. (It's not a compound, so that makes that part of the calculation easy)
But now I'm trying to figure out how much boiler heating surface I will need.
Don't understand the 17...
CB

09112013, 07:39 PM


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Re: Heating surface calculations
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09112013, 09:21 PM

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Re: Heating surface calculations
Is this a newer boiler or older i may have actual info if you tell me more they made so many different verticals it would take to long to scan them all if needed yet let me know

09112013, 10:35 PM

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Re: Heating surface calculations
Quote:
Originally Posted by Glenn Gieszler
Is this a newer boiler or older i may have actual info if you tell me more they made so many different verticals it would take to long to scan them all if needed yet let me know

Glenn,
This is not a traction engine setup  I'm working to make a steam launch(boat).
So the engine is old 1902 (A Toledo steam car engine, 2 cylinder, but not a compound).
The boiler will be a new ASME code boiler built to order that doesn't exist yet. It will be a vertical fire tube (VFT) with water legs.
I'm trying to figure out the heating surface area the boiler will need to satisfy the demand of this engine.
The Engine HP is known, and the calculations that Gary provided work  and they basically match. 67 HP.
I don't understand the number/coefficient 17 in Max Cox example to complete the calculation to get the heating surface area the boiler will need.
CB
 Post added at 10:35 PM  Previous post was at 10:13 PM 
My calculations:
1902 Toledo Steam Car Engine HP  sold as 6.25HP  Other sources Report 7HP.
My calculation based on Gary K's equations (Courtesy of Science of Successful Threshing):
Assumptions:
200 PSI Steam
250 RPM of Engine
Knowns:
Bore: 3"
Stroke: 4"
2 x 4 x 250 / 12 = 166.667 (travel of piston in feet per minute.)
3 x 3 x .7854 = 7.0686 (area of piston in square inches.)
7.0686 x 100 = 706.86 (total pounds pressure on piston.) (100 is the Mean Equivalent Pressure, half of the actual Pressure 200 PSI)
166.667 x 706.86 = 117810 / 33,000 = 3.57 (indicated horsepower)
Taking off 10% for the friction of the engine, .90 x 3.57 = 3.213 HP
This not a compound AKA double expansion engine  But it has two cylinders  So we can multiply that by 2.
3.213 x 2 = 6.42
So this calculation is dead on with how this engine was described as 6.25 HP or other sources say 7 HP.
But I'm really trying to get from that calculation to the heating surface required for the boiler I will commission.
CB

09122013, 09:34 AM

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Re: Heating surface calculations
I know nothing of VFTs, CB, but will take a WAG. The old "rule of thumb" for a hand fed locomotive style boiler (like many steam traction engines) is 10sqft/hp using all heated surfaces. Perhaps the 17 is the similar estimation for a VFT.
HOWEVER....that is a very arbitrary figure. All sorts of things can affect actual heat transfer.
Further, you can figure 3 to 4 brake HP from a boiler HP. After that, then you need to decide, how much reserve capacity you want/ease of steaming etc.
Pulling some assumptions out of thin air (lets say 15sqft: better than 17, but not 10, and 3HP yield) if you're going to be just tooling around at partial throttle, and will be able to plan for some serious firing before and during any extended "balls out" periods: 30 to 40sqft.
But take my advice here as worth even less than you paid for it. No doubt the boiler makers will steer you straight.

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09122013, 12:00 PM


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Re: Heating surface calculations
BOILER RATING FACTORS
In the early days of steam power, boilers and engines were coordinated in size through knowledge that the typical steam engine required about 34 1/2 pounds of steam per horsepower per hour.
A typical 100 horsepower steam engine required 3,450 pounds of steam per hour when it was operated at rated load. A terminology was created in the boiler industry to fit these facts. A 3,450 pounds per hour steam boiler was called a 100horsepower boiler. It was sized approximately for a 100 horsepower engine. Thirtyfour and one half pounds of boiler steam per hour became known as a developed boiler horsepower.
1 Boiler Horsepower . . . . . . . . . . . . .Nominally 34.5 pounds of steam per hour.
1 Rated Boiler Horsepower . . . . . . . .10 sq. ft. of effective heating surface (in fireboxtype boilers) (6, 8, and 12 sq. ft. also have been used).
1 Developed Boiler Horsepower . . . . .33,475 Btu absorbed per hour. (34.5 x 970.3 Btu/lb.).
Per Cent Rated Capacity . . . . . . . . .(Developed Horsepower) / (Rated Horsepower).
Factor of Evaporation . . . . . . . . . . . .(Btu absorbed per lb. of steam) / 970.3
Actual Evaporation . . . . . . . . . . . . . .(Pounds steam generated per hour).
Equivalent Evaporation . . . . . . . . . . .(Actual Evaporation) x (Factor of Evaporation).
Kilo Btu per hour, kB . . . . . . . . . . . .(Total Btu absorbed per hour) / 1000.
Mega Btu per hour, MB . . . . . . . . . .(Total Btu absorbed per hour) / 1,000,000
STEAM ENGINE WATER RATES
Approximate steam requirements in pounds per indicated horsepower per hour (lbs./ihp/hr.) of steam engines utilizing saturated steam at 100 pounds per square inch gauge (psig):
Single valve noncondensing . . . . . . .28
Single valve condensing . . . . . . . . . .23
4Valve noncondensing . . . . . . . . . .22
4Valve condensing . . . . . . . . . . . . .16
In 1720 Newcomens steam engine had a "water rate" of 200 lbs./hp/hr., at 0 psi steam pressure . . . . atmospheric pressure. In 1900 a compound operating at 80180 psi steam pressure got from 2030 lbs./hp/hr. In 1950 a uniflow steam engine operating at 200 psi steam pressure, superheated, got its "best performance" at 10 lbs./hp/hr. (I assume it was condensing?)
SURFACE AREA vs TUBE VOLUME
The evaporation rate is greater on a watertube boiler in small diameter tubes than in large diameter tubes. The evaporation rate is the ratio of generating surface area to the volume of contained water. Therefore, the smaller the tube, the greater the ratio of generating surface to the volume of contained water.
(4" tube) Surface area: 4 x 3.1416 = 12.5664 sq. in. Volume: 4 x 4 x .7854 = 12.5664 cu. in. (Ratio 1:1)
(2" tube) Surface area: 2 x 3.1416 = 6.283 sq. in. Volume: 2 x 2 x .7854 = 3.1416 cu. in. (Ratio 2:1)
(1" tube) Surface area: 1 x 3.1416 = 3.1416 sq. in. Volume: 1 x 1 x .7854 = .7854 cu. in. (Ratio 4:1)
 Post added at 11:00 AM  Previous post was at 10:26 AM 
This example of calculating boiler horsepower may be helpful as well?
Q. What is the horsepower of a boiler evaporating 12,000 lbs. of feed water per hour from a feedwater temperature of 170 deg. F. into steam at 150 lbs. gauge? (Total heat of 1 lb. of steam at 150 lbs. gauge is 1195 B.t.u.)
A.
170 – 32 = 138 B.t.u. required to raise 1 lb. from 32 deg. F. to 170 deg. F.
1195 – 138 = 1057 B.t.u. added to each pound of steam generated.
1057 x 12,000 = 12,684,000 B.t.u. added to 12,000 lbs. of steam per hour.
One (1) boiler horsepower = 34.5 lbs. of water at 212 deg. F converted to steam at 212 deg. F.
34.5 x 970.3 = 33,475 B.t.u. (one boiler horsepower).
12,684,000 / 33,475 = 379 horsepower. (answer)
Note: Using this formula, you’ll notice that higher feedwater temperatures produce lower boiler horsepower, as the boiler doesn’t have to work as hard.

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