What is bearing wipe, scoring?

Bearing wipe and scoring:
- most common bearing failures due to insufficient or contaminated lubrication

Wiping of bearing: 
- white metal (babbitt) from the bearing surface melts and then hardens in the cooler area of the bearing
- wiping is caused by loss of hydrodynamic oil film, insufficient oil supply 
- wiped bearing is also caused by lesser clearance due to overheating (thermal expansion)

Troubleshooting/Repair of bearing wipe:
- ensure the rotor is properly balanced
- ensure coupling is correctly aligned
- ensure clean and sufficient supply of lubricating oil
- clean all the dirt
- replace the wiped bearing with a new one, increase the clearance


Scoring of bearing:
- scoring in a bearing is the result of metal-to-metal contact
- evidenced by circular scratches or grooves around the circumference of the bearing surface

Causes of Scored Bearing:
- contamination in the lubricating oil
- dirt, foreign particles, wear particles from metal 

Troubleshooting/Repair of scored bearing:
- clean the journal, oil passages, oil filter
- ensure clean supply of lubricating oil
- replace with new journal bearing

Temperature at which steel fails and babbitt melts

STEEL:
Steel is an alloy of iron and carbon.

Temperature Failure of Steel:
400 C (750 F) - structural steel starts losing yield strength
600 C (1112 F) - structural steel is 50% failed, steel expands and twists
800 C (1470 F) - ordinary construction steel is 90% failed
1000 C (1830 F) - carbon steel is 97% failed
1100 C (2012 F) - carbon steel is 100% failed, complete failure


Steel starts to become soft:
538 °C  (1,000 °F)


Melting Point of Steel:
Carbon Steel:     1425 - 1540 Celsius
Carbon Steel:     2600 - 2800 Fahrenheit
Average Steel:     1400-1600 C depending on type of steel 
Steel:         1370 degrees C  (2500 °F)



   
BABBITT:
Babbitt is a metal alloy for bearing surfaces.

Babbitt is an alloy of  89% Tin,  8% Antimony and  3% Copper

Babbit fatigue strength failure temperature is:  130 C  (265 ˚F)

Babbit melting temperature is:  238 C  ( 460 ˚F )

Purpose of Gearbox in Ships

The function of gearbox in the ship:

- convert high speed, low torque rotation of prime mover (gas turbine, steam engine, diesel engine...)
into
low speed, high torque rotation for use by the shaft and propeller

Why lower speed has higher torque?

- torque is the twisting force that causes rotation

TORQUE = FORCE x DISTANCE

- the greater the distance, the greater the torque
- the greater the diameter of the gear, the greater the torque
- the greater the torque, the more power for lifting, moving or towing heavy loads of the ship

SPEED IS INVERSELY PROPORTIONAL TO DIAMETER OF GEAR

- the smaller the gear, the faster it turns
- the bigger the diameter of the gear, the slower it rotates

but from the torque equation:

- the bigger the diameter of the gear, the larger is the torque

therefore:

LOW SPEED HAS HIGH TORQUE

Why are low pressure turbine blades larger

Why low pressure turbines have larger blades?

Low Pressure (LP) turbine blades have longer length so that:
- more force can be extracted --> FORCE = PRESSURE * AREA of blades
- the steam pressure has been reduced after coming from the high pressure (HP) turbine, therefore, to extract more force, the blade area of the low pressure turbine has to be larger
- the more force, the more work
- the more work, the more power output

Why are high pressure turbine blades smaller?

HP turbine have shorter blade length because:
- ANGULAR VELOCITY inversely proportional to RADIUS
- the smaller the turbine blade diameter, the faster it rotates
- the smaller the blades, the faster it turns
- smaller blades can withstand the higher pressure and higher velocity of steam
- smaller blade lengths are more robust against torsional failure and vibrations
- smaller blades can maintain the higher velocity of the steam

FORCE = MASS * ACCELERATION

FORCE = MASS * VELOCITY / TIME

- higher velocity means higher impulse
- higher impulse means higher force
- higher force means higher work done
- higher work means higher energy transferred to the shaft
- higher energy extracted means higher power output
- higher power output means higher thermal efficiency
- higher thermal efficiency means higher savings
- higher savings means higher benefits

Sequential Throttle Valve - partial arc admission

How does sequential throttle valve governor work? -partial arc admission
- valves lift off their seats in sequence
- valves open in sequential manner
- valves have preset lift clearances between stem nuts and the lift bar so that the length of the clearances are increasing
- valves with smaller clearance will fully open faster than valves with longer lift clearance
- shorter valves are lighter (lesser in weight) so they are raised first and then the longer, heavier valves
- the net effect is that valves are progressively opened to allow a sequential flow from the valves opening in sequenced order beginning with the smallest clearance to the largest clearance
- this design is commonly used for steam chest governor valve


Partial Arc Admission vs. Full Arc Admission - advantages & disadvantages:
- in partial arc admission, the greater the load, more valves will open to admit steam
- in partial arc admission, the lesser the load, fewer valves will open to admit steam
- partial arc admission: disadvantages = uneven heating of high pressure areas of turbine
- partial arc admission: advantages = more control on throttling the valves

- in full arc admission, all valves open at the same time
- in full arc admission, as the load increases, all the valves are more fully opened
- in full arc admission, as the load decreases, all the valves are less opened
- full arc admission: disadvantages = throttling loss in the valves
- full arc admission: advantages = more uniform heating around the high pressure part of turbine


Balance design of throttle governor:
- start with full arc and then switch to partial arc later in the operation process

Purpose of Journal Bearing

Function of Journal Bearings:   
- to support radial loads in shafts, rotors
- to align and guide the rotating shaft
- to reduce friction 
- to cushion radial motion of shaft (journal)


Features, Characteristics, Uses of Journal Bearings:  
- type of plain bearing aka radial bearing, friction bearing, rotary bearing, sleeve bearing
- consists of the journal (shaft) and the bearing surface
- used in high power industrial machinery
- used in high load applications
- used in steam turbines, centrifugal pumps, centrifugal compressors


Purpose of Thrust Bearing:
- is a type of rotary bearing
- to support axial loads to the shaft

Purpose of Turbine Rotor and Stator

Purpose of Turbine Rotor:
- the rotor is the rotating (moving) part of the turbine
- the rotor is attached to the shaft
- the rotor receives the energy from the fluid causing the rotor to spin and the shaft to turn as well
- the function of the rotor is to extract the energy from the fluid and convert it to rotational energy


Purpose of Turbine Stator:
- the stator is the stationary (fixed) part of the turbine
- the stator is attached to the turbine casing
- the stator guides and changes the direction of the fluid from one rotor stage to another rotor stage
- the function of the stator is to redirect the flow of the fluid to the next rotor stage

Curtis turbine - how does it work

The principle of operation of a Curtis turbine:
- Curtis turbine is a type of impulse turbine
- curtis turbine has 2 sets of moving blades separated by 1 set of stationary blade
- rotating (moving) blades are attached to the turbine rotor
- fixed (stationary) blades are attached to the turbine casing

- Curtis turbine arrangement is as follows: (see picture)
  a.  nozzle
  b.  first set (ring) of moving blades ---> M
  c.  set of stationary (fixed) blades ----> F
  d.  second set of moving blades ------> M

Fig. 1.  Curtis wheel turbine having 2 sets of rotating blades with
            1 set of fixed blades in between.  (by Subik Kumar):

- nozzle: converts high pressure energy of the steam into high velocity kinetic energy
- first ring of moving blades: gains momentum force and rotates while steam velocity is reduced
- set of fixed blades: guides the steam to flow with same velocity to the second set of moving blades
- second set of moving blades: rotates while steam velocity is reduced

- steam pressure is reduced through the nozzle only
- steam pressure remains the same after the nozzle as it passes through moving and fixed blades
- steam velocity is reduced after it passes through the moving blades
- steam velocity is constant as it passes through fixed blades

what is epm in boilers, condensers, evaporators, feed water tanks

epm meaning = equivalents per million
- a water analysis reporting method in which the results are on an ion basis
- an ion is an atom or molecule with a net electric charge because of gaining or losing electron(s).
- chemical components of analysis are based on a common denominator which is the chemical equivalent weight
- not a commonly accepted standard, not recommended for use in plants
- advantage of this approach is mainly for simplicity of calculation


ppm meaning = parts per million
1 ppm = 0.0001% which is one ten-thousandth of one percent
one part (by weight) per million parts (by weight)
example: 1 kg of salt is dissolved in 1,000,000 kg of water [ 1 ppm salt in water ]
simpler, reliable, commonly accepted standard for water analyses reports
for feed water analysis in boilers, condensers, evaporators, and other steam plant equipment

What is back pressure in centrifugal pump

Back pressure in centrifugal pump
- area of volute casing is greater at discharge than at suction side
- velocity of fluid at the discharge is lesser than at the suction side
- pressure at discharge side is greater than at the suction side
- volume of fluid at discharge is larger than at the suction side


Analysis of Volute Casing of the Centrifugal Pump:

Similar to the Venturi principle & Bernoulli's principle,

SUCTION SIDE
- smaller area
- greater fluid velocity
- lesser pressure
- smaller fluid volume

DISCHARGE SIDE
- larger area
- lesser fluid velocity
- greater pressure
- larger fluid volume

Important:
1.  At the discharge side of the pump, there is more space for the fluid, thereby causing the fluid to slow down in speed and lose kinetic energy.

2.  First Law of Thermodynamics states that energy is neither created nor destroyed but is transformed or converted to another form.

3.  The fluid at the pump discharge decreased in kinetic energy (speed is reduced) but the reduction in kinetic energy is converted into pressure energy, therefore, the discharge pressure is higher than the suction pressure.


Back pressure of a valve at the discharge side of centrifugal pump:
- back pressure is the pressure behind the flow (at the back of the flow)
- backpressure is the pressure at the inlet side of the discharge valve (which is mounted at the discharge side of the pump)
- backpressure is created by restricting the flow (closing the valve that is mounted at the pump discharge)


Analysis of Valve Fitted at Discharge side of Centrifugal Pump:
When the valve is restricted (slightly closed down),

VALVE INLET
- higher back pressure
- higher volume
- slower fluid speed

VALVE OUTLET
- lower pressure
- lower volume
- faster fluid speed


Benefit of back pressure in centrifugal pumps:
- higher back pressure at the inlet of the discharge valve will decrease the power requirement of the driving motor of the pump


Effects of higher back pressure in centrifugal pumps:
- flow at the discharge valve outlet is reduced
- pressure of fluid at the discharge valve outlet is reduced
- longer time to fill up (tank, container, etc.)
- longer time to empty (tank, container, etc.)


Reasons Why are Centrifugal Pumps widely used in the industries?
- large flowrates handling capacity
- versatile
- simple in construction
- cheaper price
- easier and less expensive to operate, maintain and repair

How does regenerative condenser work

Purpose of regenerative condenser
- to increase thermal efficiency by using some of the steam in the shell of the condenser to reheat the falling condensate to be used for boiler feed water

How does regenerative condenser work:
- baffle plates in a shell and tube condenser are arranged in a configuration which allows most of the steam that enters the shell of the condenser to come in contact with the cooling tubes containing the cooling medium

- majority of the steam then condenses and fall to the condenser hot well

- a portion of the steam, however, is distributed by means of the design of the baffle plates so that it will direct this portion of steam to bypass the cooling tubes and flow directly to the bottom of the shell in order to reheat the falling condensate (the condensate which was the "most of the steam", that is now condensed by means of the cooling medium in the tubes)

- this process will make the temperature of the condensate from the hot well to approach the saturation temperature at condenser pressure

- when the condensate is near the saturation point, the unwanted corrosive non-condensable gases such as oxygen and carbon dioxide will escape

 

Purpose of Staging in Turbines

Why turbines have stages?

What is a steam turbine stage?

- a stage is a pair of rotating and stationary blades (diaphragm, nozzle)
- the rotating blades are attached to the rotor
- the stationary blades (stators) are attached to the turbine casing
- steam pressure and temperature drops as it expands and passes to the stages
- steam expansion means increase in volume meaning larger surface area

Why do steam turbines have multiple stages? Why not just use a single stage turbine?

- one stage extracts only a small portion of the available energy from the steam
- more stages means more energy extracted
- more energy means more mechanical work at the shaft
- more work means more power (shaft horsepower)

Why turbines need higher vacuum (lower pressure)?

- higher condenser vacuum pressure (lower pressure) means lower exhaust temperature of the steam
- higher vacuum pressure means higher thermal efficiency
- higher efficiency means higher power output
- higher efficiency means lesser power losses

Pressure is inversely proportional to Area

HIGH PRESSURE (HP) TURBINE
- steam is at Higher pressure
- steam is at Higher temperature
- steam is at Higher speed
- turbine has Lesser area (lesser diameter)

LOW PRESSURE (LP) TURBINE
- steam is at Lower pressure
- steam is at Lower temperature
- steam is at Lower speed
- turbine has Larger area (larger diameter)

Pumps in Parallel, Pumps in Series Connection

Two Pumps Connected in Parallel

- purpose: to pump fluid in higher capacities, higher volume, higher amount

- head is constant (total dynamic head is the same, uniform)

- higher discharge volumes

- increase in total pumping capacity

- add flowrates of pumps


Total Flow = Flowrate of Pump1 + Flowrate of Pump2

Q = q1 + q2

Two Pumps Connected in Series

- purpose: to pump fluid in higher elevation, higher height, higher level

- fluid flowrate is constant (the same, uniform)

- higher discharge heights

- increase in total dynamic head

- add heads of pumps 


Total Dynamic Head = Head of Pump1 + Head of Pump2

TDH = H1 + H2

Air Conditioning Heat Load Calculation (Room & Building estimate)

The calculation estimate formulas used here are just rough approximations to simplify the total heat load calculations and quickly select an air conditioning unit capable of providing the necessary cooling of the room or space.


--------------------- QUICK HEAT LOAD CALCULATION ESTIMATE -------------------

Typical Room or Small Office:

Space Heat Load = Cooling Space Dimensions x 4

Space Heat Load = Room Length (ft) x Room Width (ft) x Room Height (ft) x 4

Space Heat Load = L x W x H x 4


Occupant Load = 600 BTU/person


Total Heat Load = Space Heat Load + Occupant Load



------------------- DETAILED HEAT LOAD CALCULATION ESTIMATE ------------------
     
1 Btu (British thermal unit)
1 Btu = energy required to cool or heat 1 pound of water by 1 degree Fahrenheit

Heat Load of People
Heat generated per person = 600 Btu

Units:

1 Btu = 0.3 watt hours
1 Btu = 250 calories
1 Btu = 0.25 kilocalories

1 watt-hour = 3.4 BTU

1 TON of cooling = 12,000 BTU/hour

1 TON of refrigeration = 12000 Btu/hr
1 TON of refrigeration = 200 Btu/min
1 TON of refrigeration = 3.5 KJ/s
1 TON of refrigeration = 3.5 KW  



Total Heat Load:

The total heat load of a room/building is a function of:
1. Size of space to be cooled
2. Area of windows
3. Number of occupants
4. Equipment heat load
5. Lighting heat load

Total Heat Load
= Space Btu + Window Btu + Occupant Btu + Equipment Btu + Lighting Btu
 
   
Example:
Calculate the Total Heat Load and select an Air Conditioning Unit.

Room dimensions:
Length, L = 15 ft
Width, W = 10 ft
Height, H = 8 ft (2.5 m height from floor to ceiling)

Window dimensions:
L = 4 ft
W = 4 ft

Occupants:
2 persons

Equipment wattage:
TV = 100 watts
Radio = 50 watts
Laptop = 60 watts

Lighting Load:
60 watts light bulb


Air-conditioning Heat Load Estimation Formulas:

Total Heat Load
= Space Btu + Window Btu + Occupant Btu + Equipment Btu + Lighting Btu

Space Btu = Room Length (ft) x Room Width (ft) x 32

Window Btu = Total Window Area (sq. ft) x 80

Occupant Btu = Number of Occupants x 600

Equipment Btu = Total Equipment Wattage x 3.4

Lighting Btu = Total Lighting Wattage x 4.3


Total Heat Load Calculation:

Space Btu:
Space Btu = Room Length (ft) x Room Width (ft) x 32
Space Btu = L x W x 32
Space Btu = 15 x 10 x 32
Space Btu = 4800 Btu

Window Btu:
Window Btu = Total Window Area (sq. ft) x 80
Window Btu = Window Length (ft) x Window Width (ft) x 80
Window Btu = L x W x 80
Window Btu = 4 x 4 x 80
Window Btu = 1280 Btu

Occupant Btu:
Occupant Btu = Number of Occupants x 600
Occupant Btu = 2 persons x 600
Occupant Btu = 2 x 600
Occupant Btu = 1200 Btu

Equipment Btu:
Equipment Btu = Total Equipment Wattage x 3.4
Equipment Btu = (TV watts + Radio watts + Laptop watts) x 3.4
Equipment Btu = (100 watts + 50 watts + 60 watts) x 3.4
Equipment Btu = (100 + 50 + 60)  x 3.4
Equipment Btu = 714 Btu

Lighting Btu:
Lighting Btu = Total Lighting Wattage x 4.3
Lighting Btu = 60 watts x 4.3
Lighting Btu = 60 x 4.3
Lighting Btu = 258 Btu


Total Heat Load
= Space Btu + Window Btu + Occupant Btu + Equipment Btu + Lighting Btu
= 4800 + 1280 + 1200 + 714 + 258
= 8252 Btu

Select 8000 Btu to 10,000 Btu Air Conditioning Unit


Commercial Air Conditioner Unit Selection & Prices (July 2013):

5000 Btu Room Air Conditioner
150 sq. ft. bedroom (15 ft x 10 ft) or 15 sq. meter (5 m x 3 m)
Price: $100


8000 Btu Room Air Conditioner
300 sq. ft. master bedroom (20 ft x 15 ft) or 35 sq. meter (7 m x 5 m)
Price: $200


10,000 Btu Room Air Conditioner
450 sq. ft. living room (30 ft x 15 ft) or 50 sq. meter (10 m x 5 m)
Price: $400


See also:
1. Basics of Refrigeration and Air Conditioning
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#RX#

Why motors have large starting current

Reasons why motors have big starting current or inrush current:

- running motors produce back emf (electromotive force) in their coils

- back emf is caused by magnetic field which resists current (flow of electrons) through the armature winding

- back emf functions like a resistor which prevents current flow

- no back emf exists when motor is not running

- no back emf when motor is at rest

- at start up, resistance is zero (which is a short circuit condition), therefore current is large because no resistance to flow of current

- when a motor is running, the armature windings produce a resisting voltage (pressure) which opposes current flow as long as it is running

- a motor at rest acts like a transformer having secondary winding short circuited because the rotor windings in a squirrel cage induction motor is short circuited (closed circuit)

- starting currents (in-rush currents) for induction motors generally are 8 to 10 times the rated full load current

- a motor at rest has large inertia, therefore larger current is required to start it (this is similar to pushing a cart from stationary position, it's harder to push at the beginning, you need to exert a larger force and power, but as soon as the wheels start rolling, it is easier to push the car once it is moving)

What is air locking in pumps

Air Locking
  
- An air lock is trapped air or dissolved gases inside the pump impeller.
- This air or gas is compressed and becomes high pressure as the pump runs.


Causes of Air Lock:

- Leaks in the suction side
- Switch failure
- Entrained air mixed (dissolved) in the liquid


Dangers of air lock:

- overheating
- pump inability to start
- reduced flow of liquid
- pump unable to move fluid


How to prevent air lock in pumps:

- air relief valves
- repair leaks
- replace worn gaskets and seals
- operate within allowable limits

How does an airplane, boat and ship propeller work?

Spinning of a Twisted Airfoil-Shaped Blade

Blade sections near the center have Lower Speed

Blade sections near the Tip have Higher Speed

Movement of Blade Draws Fluid (Air, Water, Liquid, Gas)

Speed is Inversely Proportional to Pressure (Bernoulli's principle)

Fluid near blade center have Lower Speed but Higher Pressure

Fluid near blade tip have Higher Speed but Lower Pressure
  

Drawing of Airfoil (Aerofoil) shape of Propeller Blade. 

  

Fluid flows from Higher Pressure to Lower Pressure

Every action has an equal and opposite reaction (Newton’s Third Law of Motion)

Propeller exerts force to the Water --> Water pushes the Ship, Boat forward

Propeller exerts force to the Air ------> Air pushes the Airplane, Aircraft forward

Pressure Differential creates Forward Force called Thrust --> Ship, Boat, Airplane

Pressure Differential produces Upward Force called Lift ----> Airplane, Aircraft


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