by Maryam Hussain
By the time of the Roman Empire, humans
had created such amenities as indoor plumbing, iced desserts, textiles and
leather shoes, dyed clothes, etc. Concrete, chains, solder, and hand tools for
working wood and stone were part of the craftsman’s kit, while horses pulled
light carts and chariots.
In about 5000 B.C., the horse
was employed for domestic services somewhere between Ukraine and the
Caucasus Mountains of central Asia. In about 2000 B.C., humans started to ride
horses. Horseback riding represented a major revolution in technology, with
dramatic effects, particularly in warfare. Horses had to be bred larger to
carry a person, and the practice seemingly grew up first among nomadic tribes
that herded horses in central Asia, including what is now Iran and Afghanistan.
Some of the great conquering peoples of central Asia, including the Huns and
the Mongols, achieved their victories via the horse-mounted cavalry soldier.
Researchers had not only mastered
some basic laws of mechanics to build pulleys and even complex theatrical
equipment but also had developed geometry and various forms of algebra.
Engineers led the building of great monuments, bridges, lighthouses, roads, engines,
automobiles, aircraft, and public buildings. Most research and development of
engines is done using SI units, and this is found in the technical literature.
However, in the non-technical consumer market, English units are
still common, especially with automobiles. Horsepower, miles per gallon,
and cubic inch displacement are some of the English terminology that are still
used.
Mechanically, power is
the product of force times distance divided by time. Power is the measure
of performance of a given amount of work in a given time. Power is the time
rate of doing work and as such is expressed in foot-pounds per minute,
foot-pounds per second, kilogram meters per second, etc. The metric or SI
unit of power is the watt, which is one joule per second.
Horsepower is also the unit of
power. Quantitatively, one unit of horsepower is equal to 33,000
foot-pounds per minute or 550 foot-pounds per second. The kilowatt, used in
electrical work & thermal science, equals 1.34 horsepower; or 1 horsepower
equals 0.746 kilowatts. This horsepower unit of power is also known as
“mechanical horsepower” because of the fact that power is transmitted from
one body to the other is through the shaft of the engines and from the horses
to their respective carts.
There are certain breeds of
horses that are famous because of inherent strength and these breeds also
include Norwegian and German horses. According to researchers, a Norwegian
horse was apparently weaker since a Norwegian hestekraft (also known as a
“metric horsepower”) was only 735.5 W (rarely used today). One Norwegian horsepower (1hp) was the
power exerted by a Norwegian horse to lift a mass of 75 kg up to a distance of
1 meter during a time period of 1 second.
For all engineering and
scientific work, a US hp definition is followed and, in this case, you
would need to lift about 76 kg to get 1 hp (US). A real horse can exert
greater power and it has been measured to be as high as 15 hp for a few seconds
over a few seconds, but if you allow the horse to continue over longer periods,
an average horse gives somewhat less than 1 hp.
The unit of horsepower is still
used in the specifications of most domestic and industrial machines. Whenever
you want to buy an automobile, some machine, or a home appliance that is
coupled with a prime mover then you need to ask the seller about its
horsepower; some examples are:
An automobile car
A water pump either coupled with an engine or motor
A gas or air compressor
An air blower
An engine-driven saw
A wind turbine
A lawn grass-cutting machine
A tractor
A truck
A bus
What is dimensional analysis? What are the dimensions of
Power?
Remember, you can add and
subtract the quantities in an equation only if their units are the same. Let us understand the principle through an
example. Bernoulli’s equation is one of the principal equations of fluid
mechanics. An ideal Bernoulli’s equation is a sum (addition/subtraction) of
the following three quantities; pressure head, velocity head, and potential
head. Although each of these quantities is different from a definition
point of view, these are dimensionally the same; each of these quantities
represents a specific type of energy expressed as the length or height of
fluid.
Pressure head + Velocity head + Potential head = constant
Pressure head = m (dimension of length),
Velocity head = m (dimension of length),
Potential head = m (dimension of length),
Constant = m (dimension of length).
Now we came to the conclusion
point that “If the units of a quantity are the same, it follows that the
dimensions of each term in an equation must be the same”. This conclusion
gives rise to the following rule of dimension analysis: “Every valid
equation must be dimensionally homogeneous: that is, all additive terms on both
sides of the equation must have the same dimensions”.
Now let us analyze the dimensions of Power and horsepower.
Power:
There are many units of power but the commonly employed
units are horsepower & Watt.
Horsepower:
Foot Pound. force per Time = Length. Force. Time-1
Because the Force has a compound
unit, so further dismantling of the force unit, gives
= Length. Mass. Length. Time.Time-1
= Length2. Mass
= Length2. Mass
Watt:
Joule per Time = Joule per Time =
Newton Meter per Time = Force. Length. Time-1
= Or Length. Force. Time-1
Because the Force has a compound
unit, so further dismantling of the force unit, gives
= Length. Mass. Length. Time.Time-1
= Length2. Mass
= Length2. Mass
Observe that whether it is
horsepower a unit of power OR it is Watt a unit of power, the dimensions of
power are constant.
Do the horsepower and Watt
used for the same applications?
Sometimes YES, but not always.
There are rules of thumb for designating the specific unit of power.
Horsepower & Watt both are
used where power is being transmitted through the shaft, i.e., shaft
power but horsepower unit is a preference.
The system shaft may be coupled
with surroundings via shaft-shaft coupling, shaft-chain coupling, shaft-gear
coupling, and shaft-belt coupling.
What are the examples of
equipment/appliances that are integrated with shaft power?
Steam, gas, and water turbines
for electric and mechanical power generation
Gas turbine for turbo &
supercharging in internal combustion engines
Gas compressors
Fans and blowers
Electric motors, AC and DC
Fluid pumps
Internal combustion engines
(cars, trucks, buses, vans, tractors, excavators, motorbikes, auto rickshaws,
etc.).
Watt is used where power
is being transmitted through a fluid, solid, vacuum, and electricity.
What are the examples of
equipment/appliances that have a different mode of power transmission?
In contrast to shaft power
transmission mode, power transmission through liquids, gases, solid, vacuum,
and electricity are the other modes of power transmission; examples of equipment/appliances
that follow this mode are:
The condenser and evaporator of a
fridge, air conditioner, and steam power-generating plant
The boiler of steam power
generating plants and other chemical process plants including petroleum
refineries.
Intercooler of the turbocharger
of a car, truck, bus, jeep, etc.
Lighting bulb, microwave oven,
the heating element of the electrical, room, and kitchen heater.
Solar thermal radiation & photovoltaic radiation-based power harvesting units that are known as collector and photovoltaic cells.
Comments