Nominal horse power

Nominal Horse Power (German: nominal horsepower) abbreviated nhp , sometimes also NHP or German PSnom , is a former unit for the power designation of steam engines , later also of other power engines.

details

The origins of the unit go back to the time of James Watts . The nominal horsepower was not a real power calculation, but served as a rule of thumb for the approximate classification of steam engines and was initially determined by Watt from the basic data of the piston diameter and piston speed. In the beginning, the unit served as a real comparative value, especially for stationary machines, but in the course of further development - both the steam engines themselves and the measurement technology and the calculation methods for determining their performance - it has become a calculation formula for statistical and other comparison purposes. The unit for the classification of marine propulsion engines was retained for a very long time, as a number of examination fees from the classification societies were linked to the nhp number.

Conversions

There are numerous formulas for calculating the nominal horsepower. A large part is intended purely for a rough calculation, other, especially late formulas, in some cases go into much more detail with the machines to be calculated. In general, power specifications in PS cannot be converted into nhp or vice versa, but there are still various rough approximate values. The nominal horsepower of heavy, simple stationary machines are therefore multiplied by a factor of two, three or a little more to get the brake horse power output; for lighter, more powerful machines, especially mobile steam machines, factors of six to seven are given. For compound machines that work with higher pressures, the value tends towards seven.

General formulas

Formula according to Hughes / RASE

This formula for simple steam engines, which was also used by the Royal Agricultural Society of England (RASE) around the middle of the 19th century to make it easier for farmers to classify steam engines in agriculture, assumed a steam pressure of 45 psi.

${\displaystyle nhp={\frac {D^{2}}{10}}}$

Conway formula

In this formula for paddle steamers, a piston speed of 129.7 × (piston stroke) ^{1 / 3.35 was} assumed. In order for the nominal horsepower to correspond to the actual power, it is necessary in this formula to estimate the mean cylinder vapor pressure 48 kPa (7 psi) and the mean piston speed between 54 and 75 m / min.

${\displaystyle {\text{nhp}}={\frac {{\text{7}}\cdot A_{K}\cdot c_{m}}{\text{33.000}}}}$

Formula according to Reed (around 1900)

Formula for simple steam engines, for boilers with unknown surface area, assumed piston speed of 220 feet per minute and assumed boiler pressure of 7 psi:

${\displaystyle nhp={\frac {D^{2}}{28}}}$

Special formulas according to Lloyds Register (turn of the century to the 1930s)

Simple steam engines

For boilers with unknown surface and unknown boiler pressure:

${\displaystyle nhp={\frac {D^{2}{\sqrt {S}}}{130}}}$

Composite steam engines

For boilers with unknown surface and unknown boiler pressure:

${\displaystyle nhp={\frac {D^{2}{\sqrt {S}}}{120}}}$

Multiple expansion steam engines

1. For boilers with unknown surface and unknown boiler pressure:

${\displaystyle nhp={\frac {D^{2}{\sqrt {S}}}{100}}}$

2. For boilers with a known surface area and known boiler pressure (below 160 psi ):

${\displaystyle nhp={\frac {P+340}{1000}}\left({\frac {D^{2}{\sqrt {S}}}{100}}+{\frac {H}{15}}\right)}$

3. For boilers with a boiler fan or induced draft, known surface and known boiler pressure (below 160 psi):

${\displaystyle nhp={\frac {P+340}{1000}}\left({\frac {D^{2}{\sqrt {S}}}{100}}+{\frac {H}{12}}\right)}$

4. For boilers with a known surface area and known boiler pressure (greater than or equal to 160 psi):

${\displaystyle nhp={\frac {P+590}{1500}}\left({\frac {D^{2}{\sqrt {S}}}{100}}+{\frac {H}{15}}\right)}$

5. For boilers with a boiler fan or induced draft, known surface and known boiler pressure (greater than or equal to 160 psi):

${\displaystyle nhp={\frac {P+590}{1500}}\left({\frac {D^{2}{\sqrt {S}}}{100}}+{\frac {H}{12}}\right)}$

Steam turbines

1. For steam turbines with known shaft power:

${\displaystyle nhp={\frac {P+340}{1000}}\left({\frac {SHP}{6}}+{\frac {H}{15}}\right)}$

2. For steam turbine systems with boilers with a boiler fan or induced draft:

${\displaystyle nhp={\frac {P+340}{1000}}\left({\frac {SHP}{6}}+{\frac {H}{12}}\right)}$

Diesel engines

1. Formula used until the late 1910s for single-acting four-stroke diesel engines:

${\displaystyle nhp={\frac {N\cdot D^{2}\cdot {\sqrt {S}}}{80}}}$

2. Formula used until the late 1910s for single-acting two-stroke diesel engines:

${\displaystyle nhp={\frac {N\cdot D^{2}\cdot {\sqrt {S}}}{40}}}$

3. Formula used until the late 1910s for double-acting two-stroke diesel engines:

${\displaystyle nhp={\frac {N\cdot D^{2}\cdot {\sqrt {S}}}{20}}}$

4. General formula used in the 1930s for diesel engines:

${\displaystyle nhp={\frac {C\cdot N\cdot D^{2}}{{\frac {S}{D}}+50}}}$

Petrol or paraffin engines

For gasoline or paraffin engines with known braking power:

${\displaystyle nhp={\frac {BHP}{3,5}}}$

Formula symbol

• ${\displaystyle A_{K}}$: Piston area
• ${\displaystyle c_{m}}$: Average piston speed
• ${\displaystyle N}$: Number of working cylinders
• ${\displaystyle D}$: Diameter of the working cylinder in inches
• ${\displaystyle S}$: Piston stroke in inches (on opposed piston machines, half of the total stroke of both pistons)
• ${\displaystyle C}$: Calculated variable (with single-acting four-stroke machines = 5, with double-acting four-stroke machines = 10, with single-acting two-stroke machines = 9, with double-acting two-stroke machines = 18, with single-acting opposed piston two-stroke machines = 16)

literature

• Various years Lloyd's Register of Shipping

Web links

• Richard Carr's Paxman History Pages (English)

Individual evidence

1. ^ WJ Hughes: A Century of Traction Engines. Marshall, London 1959, OCLC 1007607722 .
2. ^ DK Brown: Before the ironclad. Conway Maritime Press, London 1990, ISBN 0-85177-532-2 , p. 188.
3. ^ WH Thorn: Reed's Engineers Hand-Book. Thomas Reed and Co., Sunderland 1864.