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| A Working Guide to Process Equipment by Norman Lieberman |
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Basic
Terms and Conditions
Work I was told in school that work is force times distance. I
still have no idea what this means. To me, work is carrying bricks up a hill.
If I carry 50 lb of bricks up a 100-ft hill, I have done 5,000 ft-lb of work.
Power This is how fast I work. If I carry 50 lb of bricks up a
100-ft high hill in an hour, then my power output is 5,000 ft-lb per hour. If I
do the job in 30 min, my power output has doubled.
Amperage Amps are a form of electric work. Motor amps are controlled
by what the motor is driving, not the horsepower rating of the motor.
Kilowatts Watts are a form of electric power.
Latent
Heat This is the number of BTUs needed to change 1 lb of liquid
into 1 lb of vapor. For water, this is about a 1000 BTU per pound; for diesel
oil, about 100 BTU per pound.
Sensible
Heat This is the heat associated with the temperature of the
material. This does not include latent heat.
Specific
Heat This is the number of BTUs needed to heat 1 lb of liquid
1°F; for water, 1.0 BTU, for diesel oil, 0.5 BTU.
Enthalpy This is sensible heat plus latent heat.
Friction A
fluid moving through a pipe loses pressure. The lost pressure is converted to
heat by friction inside the pipe. Friction converts work into heat. Converting
heat back into work is much
more
difficult.
Acceleration
Energy It takes more energy to make a fluid move faster than to
keep it moving at a constant speed.
Momentum An important term momentum is mass times velocity. Alice
weighs 100 lb and has the same momentum as Alan, who weighs 200 lb. If Alice is
running at 8 miles per hour, how fast is Alan running? (Answer: 4 miles per
hour).
Water
Hammer If I hammer a nail into a brick wall, the nail gets very
hot. This is an example of converting the momentum of the hammer into sensible
heat of the nail. When water flowing through a pipe is suddenly stopped, the
resulting bang is called “water hammer”. It’s the conversion of the momentum of
the water into pressure.
Mole
of Gas I have a box full of air. The air is at atmospheric
pressure and 60°F. The size of the box is such that it contains 379 ft3. The
box size was selected arbitrarily a long time ago. The number of
molecules
of air inside the box is called a “mole” (mol) of air. The number of molecules
is about 10^23. A really big number.
Molecular
Weight If I weigh the air in this box, it will weigh 29 lb. Therefore
the molecular weight of air is 29 lb/mol of air. If the box contained hydrogen,
it would weigh 2 lb. Therefore the molecular
weight of
hydrogen is 2. If I displace the hydrogen with propane from an LPG cylinder,
the box weight would be 44 lb. The molecular weight of propane is 44 lb.
Gas
Laws As I double the absolute pressure of a gas, its volume is
cut in half. As I double the absolute temperature of a gas, its volume doubles.
Compressibility Heavier gases are easier to compress than lighter gases.
The compressibility factor for hydrogen is 1.0. For propane or butane, it’s
about 0.90.
Compression
Ratio The compressor discharge pressure (in absolute pressure)
divided by the suction pressure (also in absolute pressure). If I’m compressing
air, the suction pressure is atmospheric pressure, and the discharge pressure
is 29.4 psig, what is the compression ratio? (Answer: 3).
Heat
of Compression Gases get hot
when they are compressed. The hotter they get, the less efficiently the
compressor is working. Bigger compression ratios also increase the temperature
of the gas as it is compressed.
Expansion
Cooling You would suspect that when we depressure gas it would
cool, if the gas gets hotter upon compression. Except for pure hydrogen, that
is correct. Steam, air, and fuel gas all cool when we let the pressure down
across a control valve.
Head A tank with 23 ft of liquid has a head of 23 ft regardless
of the liquid density.
Head
Pressure A tank with 23 ft of water has a head pressure of 10 psi. A
tank with 23 ft of gasoline has a head pressure of 7 psi, because gasoline is
less dense than water. Its specific gravity is 0.70.
Specific
Gravity Water has density of 62 lb/ft3. This density has been
arbitrarily defined as a specific gravity—1.00 s.g. Since gasoline has a
density that is 70 percent of water, its specific gravity is 0.70 s.g.
Temperature
Correction of Specific Gravity Hydrocarbon
density drops by 5 percent for each increase of 100°F. For water it’s 1 percent
for each 100°F.
Viscosity Expressed in centipoises it is a fluid property measuring
how much a fluid in a pipe drags along the walls.
Centipoises
or Centistokes Both terms have about the same value. A high viscosity
fluid has 100 centipoises (cP) such as cold, heavy crude oil. A low viscosity
fluid has 1 or 2 cP such as water or kerosene. Going from 2 to 20 cP would
about double the pressure drop in a pipeline. Liquids over 100 cP or
centistokes (cSt) are not easily pumped by a centrifugal pump. Increased
temperature reduces viscosity.
Thermal
Conductivity This is the
ability of a material to let heat pass. Metals, water, and materials that are
good conductors of electricity have a high thermal conductivity. Air, rubber,
and materials
that are
bad conductors of electricity have a low thermal conductivity. High viscosity
hydrocarbons are bad conductors of heat.
Thermal
Expansion Railroad tracks grow longer in the heat of the sun. The hot
tubes in an exchanger grow more than the cold shell. Hence, we have a floating
head in the tube bundle to accommodate differential rates of thermal expansion
between the tube bundle and the shell.
Auto
Ignition This is the temperature at which materials burn without a
source of ignition. Gasoline autoignites at 450°F. Asphalt autoignites at 320°F
methane at 1000°F. The heavier the hydrocarbon,
the lower
the autoignition temperature. Iron sulfide (pyrophorric iron) burns at room
temperature when it is dry.
Flash
Temperature Hold a flame over a cup of
diesel fuel; it will start to burn at its 160°F flash temperature. Gasoline’s
flash temperature is below room temperature. Jet fuel is 110°F. The lighter
the
hydrocarbon, the lower the flash temperature.
Vapor
Pressure This is a key concept. As we heat a liquid, the molecules
in the liquid try to escape into the vapor phase. The hotter the liquid, the
harder they try to escape. The pressure that the
molecules
of liquid create as they push out into the vapor space is the liquid vapor
pressure. More volatile liquids such as LPG, have a higher vapor pressure, than
less volatile diesel oil.
Boiling
Point Heat a liquid and its vapor pressure increases. When the
liquid’s vapor pressure equals the pressure in the vessel, the liquid starts to
boil. The temperature at which this boiling starts is the
liquid’s
boiling temperature.
Relative
Volatility Divide the vapor pressure of a lighter material by the
vapor pressure of a heavier material. The bigger the resulting number, the
larger the relative volatility. It’s easier to separate two
components
in a distillation tower if they have a larger relative volatility.
Dynamic
Machine Centrifugal pumps and centrifugal compressors are dynamic
machines. They convert velocity imparted by the impeller to the fluid, into
fluid pressure.
Positive
Displacement Machine A
reciprocating compressor or gear type pump is an example of a positive
displacement machine. It increases pressure by squeezing or pushing the fluid
into a region of greater pressure.
Cavitation When the pressure of liquid flowing into a centrifugal pump
gets too low, liquid boils inside the pump case and generates bubbles. The
discharge pressure and flow become erratically low.
NPSH
(Net Positive Suction Head) The net positive suction head required to keep a
centrifugal pump from cavitating. Cooling a liquid in a pump’s suction line
increases the pump’s available NPSH, as does increasing the liquid level in the
suction drum.
Draft A
draft is the pressure difference inside a heater firebox and the surrounding
air at the same elevation. The draft causes combustion air to flow through the
burners and up the stack.
Convective
Heat At the top of a firebox there is another section of the
heater packed with tubes. Usually these tubes have fins or studs. This is the
section of the heater where convective heat transfer occurs. About 30 percent
of heat recovered in a process heater is due to convective heat transfer. A
sauna is an example of convective heat transfer.
Thermo-Siphon
Circulation (or Natural Circulation) partially vaporizing a liquid causes it to flow to a higher
elevation. The driving force is the density difference between the denser
liquid and the less dense, partly vaporized effluent.
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