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Steam Passageways - sizes
Posted by: Nigel Westwood (IP Logged)
Date: September 21, 2002 06:57PM
<HTML>Does anyone know a formula, (simple or otherwise) or even a rule of thumb for working out the size required of the steam ports/passages of a cylinder given the diameter, pressure, piston speed and presumably steam temperature?
If you have seperate valves for inlet and exhaust, the inlet could be smaller than the exhaust - but by how much?
Any info gratefully received!</HTML>
If you have seperate valves for inlet and exhaust, the inlet could be smaller than the exhaust - but by how much?
Any info gratefully received!</HTML>
Re: Steam Passageways - sizes
Posted by: Rolly Evans (IP Logged)
Date: September 22, 2002 09:02AM
<HTML>Doble used Port area = Piston area X Piston speed / 10,000.
Herreshoff used Port area S = how wide W = length D = Piston diameter
S = D / 10 & W = D X .75 to .875 Exhaust port area = D / 4</HTML>
Herreshoff used Port area S = how wide W = length D = Piston diameter
S = D / 10 & W = D X .75 to .875 Exhaust port area = D / 4</HTML>
Re: Steam Passageways - sizes
Posted by: George Nutz (IP Logged)
Date: September 22, 2002 12:39PM
<HTML>One old marine engineering reference gives the maximum steam velocity, in feet per minute for the intake ports as 7,000-8000FPM and for the exhaust ports 10,000FPM. As the ports are changing area due to harmonic valve motion use .707 of above for a straight calculation to be on the safe side. It can be seen that the specific volume of the entrance steam and exhaust steam plays an important role in this method.
Best, George</HTML>
Best, George</HTML>
Re: Steam Passageways - sizes
Posted by: Ron Fossum (IP Logged)
Date: October 06, 2002 12:25AM
<HTML>You need to calculate the volume of steam that you will use per MINUTE in CUBIC FEET as well as the WEIGHT of that volume. The figures for this at various pressures and degrees of superheat can be found in any steam tables booklet. Note that these are expressed in psia - pounds per square inch ABSOLUTE (above a perfect vacuum) - and that this figure, therefore, is approximately 15 psi above the normal "gauge" pressure.
INLET steam velocity should not exceed (by very much) 8,000 feet per MINUTE. So, divide your volume by 8,000 and this will give you the cross sectional area - in SQUARE FEET. Divide by 144 to achieve what is (for most of us) the more workable figure of SQUARE INCHES.
EXHAUST steam velocity should not exceed 5,000 feet per minute. As the WEIGHT of the steam being exhausted is the same as that entering the engine (assuming little or no leakage past glands, packing, etc.) it is only necessary to know the PRESSURE against which the exhaust is directed (atmosphere, vacuum, ?) - NOT the pressure it is at in the cylinder when the exhaust valve opens! Again, using the steam tables, find the VOLUME of the steam (based on the WEIGHT) and divid by 5,000 and then by 144 - you may be somewhat surprised by how much larger the passage area is for the exhaust as compared to the inlet.
NOTE: Fluids travel easiest through smooth passages with no bends. If the interior surface is "rough", or there are many or "sharp" bends, the velocity will be decreased (sometimes dramatically). Steam is FLUID - i.e. it consists of molecules of H2O (water). The molecules of H2O do not disassociate until about 2500F.</HTML>
INLET steam velocity should not exceed (by very much) 8,000 feet per MINUTE. So, divide your volume by 8,000 and this will give you the cross sectional area - in SQUARE FEET. Divide by 144 to achieve what is (for most of us) the more workable figure of SQUARE INCHES.
EXHAUST steam velocity should not exceed 5,000 feet per minute. As the WEIGHT of the steam being exhausted is the same as that entering the engine (assuming little or no leakage past glands, packing, etc.) it is only necessary to know the PRESSURE against which the exhaust is directed (atmosphere, vacuum, ?) - NOT the pressure it is at in the cylinder when the exhaust valve opens! Again, using the steam tables, find the VOLUME of the steam (based on the WEIGHT) and divid by 5,000 and then by 144 - you may be somewhat surprised by how much larger the passage area is for the exhaust as compared to the inlet.
NOTE: Fluids travel easiest through smooth passages with no bends. If the interior surface is "rough", or there are many or "sharp" bends, the velocity will be decreased (sometimes dramatically). Steam is FLUID - i.e. it consists of molecules of H2O (water). The molecules of H2O do not disassociate until about 2500F.</HTML>
Re: Steam Passageways - sizes
Posted by: George Nutz (IP Logged)
Date: October 06, 2002 10:43AM
<HTML>In calculating the steam inlet and exhaust velocities take into consideration that with 30% admission(and harmonic valve motion) the time that the valve is open as a percentage of the stroke should be divided into the theoretical velocity based on pounds of steam per hour per admission port/passageway. Also note that most counterflow steam engines the exhaust valves are open for a much greater percentage of the time than the intake, possibly for almost all of the stroke in the case of a Stanley, this will reduce the correction factor for calculating exhaust velocity.</HTML>
Re: Steam Passageways - sizes
Posted by: Pat Farrell (IP Logged)
Date: October 08, 2002 02:38AM
<HTML>I want to aplaud both Ron and George for their wit in their answers. You guys are very educational for me.</HTML>
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