33-most frequently asked questions & answers on air compressors


1-What are the functions of air compressors in power plants?
The functions of the compressor is to take a definite quantity of fluid (air or gas) & deliver it at a required pressure
2-What are the applications of compressed/pressurized air in power plants?
Operating the control valves
Operating the solenoid valves
Operating the dome valves in ash handling system
Air dampers operation in fans
Operating pneumatic tools
3-How does air compressor work? & what are the effects of compression on air?
An air compressor takes in atmospheric air. Compresses it and delivers this air to a storage vessel from which it will be conveyed by the pipe line to the required equipments.
Effects of compression:
Air pressure & temperature increases & moisture content in air increases
4-What are the various types of air compressors used in power plants?
Generally reciprocating, screw & centrifugal air compressors are used.
5-How does the positive displacement compressors work?
These compressors work by increasing the pressure of the air by reducing its volume
5-How does the centrifugal compressors work?
Centrifugal compressors initially increase the air velocity & then pressure at compressor outlet.
6-What are the various types of reciprocating compressors?

  • Single stage & multi stage
  • Single cylinder & multi cylinder
  • Air cooled & water cooled
  • Lubricated & Non lubricated
7-What are single acting compressors?
It is a compressor that has one discharge per revolution of crankshaft.
8-What are double acting compressors?
Compressor that completes two discharge strokes per revolutions of crankshaft
9-What do you mean by compression ratio in compressors?
It is the ratio of volume of air before compression to the volume of air after compression.
10-What do you mean by air compressor capacity?
It is the quantity of air actually delivered by a compressor in m3/minute or m3/sec.
11-What do you mean by Free Air delivery (FAD)?
It is the volume of air delivered by compressor under the compressor intake conditions like pressure &temperatures. FAD is not calculated on standard atmospheric conditions, it is measured in terms of CFM.It means that, amount of compressed air converted back to actual inlet air conditions.
12-What are the various parts of centrifugal compressors?
Impellers, Vanes, Volutes, Suction Eyes, Discharge lines, Diffuser Plates, Seals, Shaft, Casing
13-What do you mean by swept volume?
The volume displaced or swept by piston when it moves between two ends.
14-What do you mean by clearance volume?
 It is the difference between the total volume and the swept volume.
It is the space between the cylinder end and the piston to allow for wear & for easy movement. And also clearance is for valves reception.
Therefore clearance volume should be small as possible, but cannot be reduced to zero since, for mechanical reasons to avoid contact of moving parts in compressors.
15-Compressors efficiency
1. Volumetric efficiency:-
It is the ratio of actual volume of the FAD at standard atmospheric condition in one delivery stroke (Actual air intake) to the swept volume (theoretical air intake) by the piston during the stroke.
2. Isothermal efficiency:-
It is the ratio of isothermal work done to the actual work done
3. Volumetric efficiency:-
It is the ratio of free air delivered to the displacement of the compressor.
It is also defined as the effective swept volume to the swept volume
4. Mechanical efficiency:
It is the ratio of mechanical output to the mechanical input
Mechanical efficiency = Indicated horse power / Shaft horse power
16-Why do the compressors located at higher elevation consume more power as compared to those of lower elevation compressors?
As the altitude increases, air pressure goes on decrease; hence inlet pressure to the compressor decreases. For this compressor has to consume more power to provide required discharge pressure.
17-What is the relation between Relative humidity & moisture carrying capacity of air?
As the relative humidity (RH) increases, Moisture carrying capacity of the air increases
18-What are the reasons for reduction of volumetric efficiency of a reciprocating compressor?

  • Very high speed of compressor
  • Very large clearance volume
  • Obstruction in inlet valves
  • Malfunctioning of valves
  • Overheating of compressed air
  • Inertia effect of air in inlet pipe
19-Why the FAD is actually less than compressor displacement?
Because

  • The fluid resistance through air intake valve& valves prevent the cylinder being fully charged with air at atmospheric conditions.
  • Losses due to leakages
  • The high pressure air trapped in the clearance space, must expand to a pressure below the atmospheric pressure before the automatic suction valves open
20-What is the significance of inter coolers & after coolers in Heat exchangers?
Intercooler & after coolers are the shell & tube type heat exchangers in which hat is removed from air which has been compressed & its temperature has risen as a result of compression.
The cooler which is placed in between the stages are called Inter coolers & coolers are fitted after the last stage is called after coolers. Generally after coolers are used for removing the moisture from air by reducing its temperature.
21-How do you increase the isothermal efficiency of reciprocating compressors?
Isothermal efficiency can be increased by;
Water jacketing: where water is circulated around the LP & HP cylinders, which helps to cool the air
Inter-cooling: Intercoolers are fitted between the two stages to cool the air coming out from 1st stage. In inter coolers air temperature is maintained around atmospheric temperature.
Adding external fins for smaller compressors
22-Why do you prefer multistage compression rather than single stage?
Because;

  • Air can be cooled between interstages
  • For the same pressure & FAD, multi stage compressors consume less power
  • These are having good mechanical balance
  • The pressure & temperature range can be kept as per requirement. This results into reduced wear & tear & increased volumetric efficiency
  • However multistage compressors with coolers arrangement is more expensive than single stage compressor
23-A single stage reciprocating air compressor takes 1.5 M3/min of air at pressure 1.013 bar & 25 Deg C temperatures & delivers at 6 bar. Calculate the final air temperature
Solution:
Assume law of air compression pVn = Constant
That is pV 1.35 =Constant
P1 = 1.013 bar & T1 = 25 + 273 = 298 K
P2 = 6 bar
We have
T2 / T1 = (P2 / P1) (n-1) / n
T2 = 298 X (6/1.013) (1.35-1)/1.35
T2 =470.36 K
T2 = 470.36 – 273 = 197.36 Deg C
24-Suction pressure of a double stage air compressor is 350 mmwc & discharge pressure 6 kg/cm2 .Then calculate the 1st stage outlet pressure?
P1 = 350 mmwc = (1.03-350/1000) = 0.995 kg/cm2
P2 = √(P1 XP3) = √(0.995 X 6)
P2 = 2.44 kg/cm2
25-What do you mean by perfect cooling in compressors heat exchangers?
The condition, where the inlet temperature of air at each stage of multi-stage machine should be the same as it was at the first stage inlet is called perfect cooling.
26-Is volumetric efficiency of a compressor is less at higher delivery pressure?
Yes, at higher delivery pressure volumetric efficiency of any compressor is less & vice versa
27-What do you mean by loading & unloading in air compressors?
It is the two step control of delivery pressure, where compressor is loaded when there is air demand and unloaded when there is no demand.
During unloading air compressors consume lesser power, say 30 to 40% of full load. Positive displacement compressors consume less power & screw compressors more during unloading.
28-Does incorporating VFDs to reciprocating compressors save energy?
No...Compressors are required to run at full RPM, if run at lower RPM time required for loading will increase. So loading & unloading by using VFDs is not recommended for reciprocating compressors However sometimes it can be applied for centrifugal compressors.
29-Explain the process of loading & unloading in reciprocating air compressors?
In reciprocating air compressors, suction valves of compressors are fitted with un loader. Suction valves get forced open by actuating Solenoid valve (SOV) when discharge goes higher than requirement.
During loading: The SOV de-energies & the suction valves open & close as per the operation of flat springs fitted with SOVs.
During unloading: As the pressure increases more than set pressure, SOV on actuating air line opens to allow air to force open the suction valves. During compression stroke air discharges through already opened suction valves & pressure does not increase during suction stroke.
For 50% unloading 50% of suction valves get forced open & for 100% unloading all the suction valves get opened.
30- Explain the process of loading & unloading in screw air compressors?
Screw compressors are silent in operation & also consume very less power as compared to reciprocating compressors.
Unloading: When the demand of the air reduced, a device known as suction slide valve or suction throttle valve is activated. Then very less quantity of air is allowed to pass through it.
Loading: During loading, suction slide valve remains open 100%.
31- Explain the process of loading & unloading in centrifugal air compressors?
For controlling the discharge pressure of a centrifugal compressor, VFDs are installed. As the air consumption decreases, discharge pressure increases & motor speed decreases accordingly to maintain discharge header pressure.
Inlet guide vanes & Anti surge valves are also used for controlling speed.
Thumb rules on air compressors
Every 4 degree rise in inlet air temperature results in a higher energy consumption by 1%  to achieve equivalent power output
For every 250 mmWC pressure drop in increase across at the suction path will increase the power consumption of air compressor by 2%
An increase 5.5 deg C in the inlet air temperature to the second stage results in a 2% increase in the specific energy consumption.
A reduction in the delivery pressure of a compressor by 1 kg/cm2 would reduce the power consumption by 6-10%
32-What are the functions of air receiver tank?
Functions

  • Serves as air reservoir
  • Prevents frequent loading & unloading of air compressor
  • Separates moisture from air
33-What are the different types of air dryers used?
1. Refrigerent type
Adsorption type
a.Desiccant heat less drier
b.Desciccant type hot air drier



Questions Answers on basics of instrumentations for Power plant Mechanical Engineers

1. What is an instrumentation?
Instrumentation is the art of measuring a value of plant parameters.
Or
The technology which is used to measure and control the process system of plant is called instrumentation.
2. What is an instrument?
An instrument is a device that transforms a physical variable of interest (the measurand) into a form that is suitable for recording (the measurement).
3. What is measurand?
A physical quantity, property, or condition being measured. Often, it is referred to as a measured value.
4. What is a sensor?
The portion of a measurement system that responds directly to the physical variable being measured.
5. What are the various variables measured in a power plant?
Pressure, temperature, force, speed, frequency, current, voltage, resistance, time, displacement, acceleration, light intensity, density, pH, conductivity, flow etc.
6. What is calibration of a sensor?
The relationship between the physical measurement variable input and the signal variable (output) for a specific sensor is known as the calibration of a sensor.
7. Define true value.
The desired result of an experimental measurement.
8. Define error of an instrument.
The error of an instrument is defined as the difference between the true value and measured value.
 9. What is accuracy?
An expression of the maximum possible limit of error or the closeness of the agreement between the result of a measurement and a true value of the measurand.
10. What is an actuator?
An actuator is a device that is responsible for moving or controlling a mechanism or system.
11. Define the terms input and output of control system.
Input is an applied signal or an excitation signal applied to a control system from an external energy source in order to produce a specified output.
Output is the particular signal of interest or the actual response obtained from a control system when input is applied to it.
12. What are the various sub systems of instrumentation to control the power plant operation?
The efficient and reliable operation of the power plant requires following sub systems.
Monitoring Instruments: These include electromagnetic and electronic indicators, multi point/multi pen recorders, digital display units, recorders, drum level and flame indicators, ammeters, voltmeters etc. These are very essential for starting, running and shutdown of the units.
Transmitters and Sensors: These include, thermocouples, RTDs, pressure and level transmitters, differential transmitters. Generally transmitters are of two wire transmission principle and provide current signal of 4–20 mA.
Local Instruments or Field Instruments: These include pressure and temperature gauges, level switch, pressure switch, draft gauges, vibration switches etc.
Analytical Instruments: They include measurements required to ascertain chemical condition of process medium at various points. These include pH and conductivity analyzers, dissolved O2 analyzers, silica analyzers, smoke and density meters, SOx and NOx analyzers.
Analog and Digital Control System: The control systems in thermal power plant cover various closed loop control system. It employees sophisticated digital computers for process control. The control systems have
Interconnection with measurement systems, interlocks, protection, sequential control system annunciation and data acquisition system. Programmable Logic Control (PLC) and Digital Control System (DCS) find wide applications in modern power plants.
Control Modules: Control modules are provided to control the equipments.
Protection and Interlock Systems: The operation of the protection is accompanied by audible and visible annunciations to indicate primary causes for operation of protection. Both relay based and logic based methods are adopted.
Control Drives: Include pneumatic dampers, solenoid operated valves, control valves, motorized valves etc.
Power Supply Arrangement: The control and instrumentation devices are provided with power supply sources having back up/standby arrangements. Uninterrupted power supply modules (UPS), battery chargers, unit battery banks, station battery banks, normal/emergency switch gear supply power to the control and instrumentation.
13. What is a control system? What are the various controls used in boiler?
To control means to regulate, to direct or to command. Hence a control system is an arrangement of different physical elements connected in such a manner so as to regulate, direct or command itself or some other system.
  • Drum level control
  • Steam temperature control
  • CBD tank level control
  • Combustion control
  • Deaerator level and pressure control
  • Furnace draught control
  • Soot blower pressure control
14. What is an automatic control system?
Automatic control is to maintain a desired value of a parameter or the condition in a process system by measuring the existing value, comparing it to the desired value and employing the difference to initiate corrective action for reducing the difference.
15. What is control action? What are its different types?
Controller action is to maintain the controlled variables of the process at the desired value set by the operator.
Different Types of Control Actions:
Proportional control action
Integral control action
Derivative control action
16. What are the main components of DCS?
DCS system has following main units:
  • Main servers
  • Operation systems
  • Firewall
  • Controllers
  • Digital input and output modules
  • Analog input and output modules
  • Field terminal assembly
17. What is server?
Server is a master computer whose main purpose is to handle request for data from other peripheral computers.
18. What is redundancy of a system?
Redundancy means supporting simultaneously or back up for any online system. This term is often used in DCS, where redundant server is needed. Redundant server works as main server when main server fails. Similarly we have redundant controller also.
19. What is the difference between consoles and flex stations?
In server Architecture of DCS, there are two types of communication depending on the configuration of the client. There are 2 types of clients – Flex Station and Console station.
Flex station talks to server for data exchange. So, if the server fails, the flex stations do not receive anything and go blind.
The console stations directly talk to controller for IO status, set points, and alarm set points. So, if the server is down, still the plant can be visualized on these computers, dynamic data exchange is possible, you can start/stop the motors, and you can open/close the valves through console stations. (Motors can be started/stopped only if they are connected through hardwired IOs/connected to the controller serially).
20. What are fire wall in DCS?
A firewall is a security device that can be a software program or a dedicated network appliance. The main purpose of a firewall is to separate a secure area from a less secure area and to control communications between the two.
21. What do you mean by controller? What are its functions?
A controller is a comparative device that receives an input signal from a measured process variable, compares this value with that of a predetermined control point value (set point) and determines the appropriate amount of output signal required by the final control element to provide corrective action within a control loop. An Electronic Controller uses electrical signals and digital algorithms to perform its receptive, comparative and corrective functions.
An electronic sensor (thermocouple, RTD or transmitter) installed at the measurement location continuously sends an input signal to the controller. At set intervals the controller compares this signal to a predefined set point. If the input signal deviates from the set point, the controller sends a corrective output signal to the control element. This electric signal must be converted to a pneumatic signal when used with an air operated valve.
22. What are the different types of standard signals used in Industrial Instrumentation measurement?
Standard signals used in instrumentations are 4–20 mA, 0–5 VDC, 1–5 VDC, 24 VDC etc.
23. Why 4 mA and 20 mA are used instead of 0 mA and >20 mA in signal measurement?
For error detection 4 mA is good, If instrument is faulty, it can be easily identified. If the output is 0 mA it will be difficult to identify whether it is a faulty signal or a good signal.
20 mA is used as maximum because the human heart can withstand up to 30 mA of current only. For safety reason 20 mA is chosen as maximum value.
24. What is pressure?
It is the force per unit area.
Pressure = Force/Area
25. What is absolute pressure?
Absolute pressure is the sum of atmospheric pressure and gauge pressure.
Absolute Pressure = Gauge Pressure + Atmospheric Pressure
26. What are the various pressure detectors?
Bourdon tubes, bellows, capsules, diaphragms, strain gauges and manometers.

27. Explain the piezoelectric and capacitive methods of pressure measurements in transmitters.
Piezoelectric Method:
These are certain small crystalline materials which generate voltage when they are subjected to forces or stress. Crystalline materials when stressed or forced they develop electric charges, these charges are measured and converted.
Example: Naturally available Quartz, synthetic crystals, Rochelle salts and barium titrate.
Capacitive Method:
In this method the mechanical movement of mechanical material like diaphragm changes the capacitance of an electronic component.
28. What is pressure transmitter?
A pressure transducer, often called a pressure transmitter, is a transducer that converts pressure into an analog electrical signal.
29. What is the difference between pressures switch and pressure transmitter?
Pressure Switch is an electromechanical device which measures the pressure in a system and when the pressure reaches too high or too low of a given set point, the device will switch meaning it will open orclose the circuit that powers a certain devices like creating alarms, start or stop of pump, open or close of valve etc.
Pressure Transmitter is also an electromechanical device which senses the pressure and sends signal to remote device for further measurement and control. For an example continuous measurement of steam pressure, lube oil and control oil pressure measurement etc.
30. What is the difference between pressure transducers and pressure transmitters?
Transmitters are current output devices and transducers are voltage output devices.
31. What are the different types of temperature sensors used in power plant?
RTD, Thermocouples, thermometer, infrared sensors, bimetallic devices.
32. Which effect is used in thermocouple?
See back effect is used in thermocouple.
33. What are active and passive sensors?
Active sensors generate an electric current in response to an external stimulus which serves as the output signal without the need of an additional energy source. Such examples are a diode, piezoelectric sensor, and thermocouple.
Passive sensors require an external power source to operate, which is called an excitation signal. The signal is modulated by the sensor to produce an output signal. For example, a thermistor does not generate any electrical signal, but by passing an electric current through it, its resistance can be measured by detecting variations in the current or voltage across the thermistor.
34. What is the function of thermo-well in thermocouples or temperature sensors?
Thermo-well is used to protect the thermocouple from harmful atmosphere, corrosive fluid, and physical damage.
35. How do you identify J, K and S type thermocouples on their physical appearance?
  • J Type Thermocouple: Positive wire is white and negative wire is red.
  • K Type Thermocouple: Positive wire is yellow and negative wire is red.
  • S Type Thermocouple: Positive wire is red and negative wire is white.
36. What are the different types of RTDs used in power plant?
2 wire, 3 wire and 4 wire RTDs are generally used.
37. What is RTD PT-100?
RTD PT-100 has resistance of 100 ohms at temperature 0 °C.
38. What is the change in resistance at every 1 °C rise in temperature of RTD PT-100?
0.384 ohm resistance will rise/lower at every 1 °C rise/lower in temperature.
39. What are the different types of flow meters used in power plant?
Following are the Flow Meters used in Power Plant:
  • Venturimeter
  • Orifice meter
  • Flow nozzle
  • Turbine flow meter
  • Mass flow meter
  • Magnetic flow meters
  • Rotameters
 40. What are the different types of orifice plates? Explain.
Different Types of Orifice Plates are:
Concentric: These plates are used for ideal liquid as well as gases and steam service. Concentric holes are present in these plates that’s why it is known as concentric orifice.
Segmental: This plate has hole in the form of segment of the circle. This plate is used for colloidal and slurry flow measurement.
Eccentric: This plate has the eccentric holes. This plate is used in viscous and sherry flow measurement.
41. Why negative voltage is used in turbine vibration probes?
The voltage found between the sensor tip and the target is a negative DC voltage. The negative bias voltage used by proximate may be due to its safety. The negative voltage may vary while adjusting the gap that can be checked by the signal conditioner used by the probe.The extreme end of the gap gives a negative DC voltage of 24 and its zero when the gap between sensor and target is almost lapping.
42. What is load cell?
Load cell is transducer which converts a force into an electrical signal. It comprises of strain gauge which is connected to Wheatstone bridge circuit in load cell.
43. What is a control valve? What are different types of control valves?
A control valve is a device capable of modulating flow at varying degrees between minimal flow and full capacity in response to a signal from an external control device. The control valve, often referred to as “the final control element,” is a critical part of any control loop, as it performs the physical work and is the element that directly affects the process.
44. What is the function of positioner?
A positioner is a device put into a valve to ensure that it is at a correct position of opening as per the control signal. An I/P converter only sends the opening/closing request to valve but cannot confirm its position.
                Positioner senses the valve opening through a position feedback link connected to valve stem which is its input signal. I/P converter output is its set point input. The difference between these two is the error signal based on which the positioner positions the valve to correct position to reduce error to zero. Hence positioner is nothing but a pneumatic feedback controller.
45. What do you mean by direct acting and reverse acting control valves?
If the controller output increases when the measurement value rises, it is a direct-acting controller, and if controller output decreases when the measurement rises, it is a reverse-acting controller.
     46. What is the function of an actuator in control valve?
 An actuator is a pneumatic powered device which supplies force and motion to open or close a control valve.
4   47.What is Solenoid Valve? What are its main types?A solenoid is electrically operated valve. It consists of solenoid coil in which magnetic plunger moves. This plunger is connected to the plug and tends to open or close the valve.
There are two types of solenoid valves:
                Normally Open
                Normally closed
Questions & Answers on batteries
48.What are the input and output signals of Turbine governor?
Governor Inputs:
  • Power supply 230 V AC/110 V AC
  • 4–20 mA MW signal
  • Main steam inlet pressure
  • 3rd Extraction pressure
  • 4–20 mA HP and LP governor valve demands
  • Turbine speed
Governor Outputs:
  • Control signals to HP and LP actuators
  • Light fault and heavy faults
  • Electrical and mechanical over speed
Also read basic questions & answers on power plant electrical system

                 

Basic Questions & Answers on Slop/Spent wash fired Boilers



1-What do you mean by Slop or Spent wash?
Slop or Spent wash also known as Vinasse is an acidic effluent produced in Distillery unit. Slop is formed on fermentation of Sugar cane molasses in distillery.
2-Why do you treat Slop as a Hazardous effluent?
Because it is an acidic effluent having pH around 4 to 5
3-What are the standard parameters of Slop or Spent wash?
  • Colour : Brown
  • Odour : Pungent
  • pH : 4 to 5
  • Solids: 9 to 12%
  • BOD : 45000 to 60000
  • COD : 110000 to 1350000
4-What are the various methods of Slop disposal?
  • Disposal methods:
  • Biomethanation
  • Composting
  • Incineration Technology
5-What is the purpose of using incineration technology for Slop disposal?
Incineration technology method leads to Zero Liquid Discharge, where spent wash is concentrated by heating it in specially designed Boilers.
6-What are the benefits of burning Slop in Boilers?


Benefits:
  • Zero liquid discharge (ZLD) or Zero effluent discharge (ZED)
  • Steam generated in Boilers can be used for Distillery process
  • Power generation to fulfil auxiliary power of distillery, hence reduction in production cost.
  • Ash generated is of potash reach which is good manure


7-What are the challenging situations for designing the Spent wash Boilers?
  • Slop & its ash characteristics
  • Design of combustion system for Slop
  • Design of combustion system for supporting fuel
  • Design of pollution control equipments
8-Why it is being challenging task to burn the spent wash?
Because it is a liquid fuel having very low GCV up to 1800 kcal/kg & high ash & moisture content.
9-What are the various supporting fuels used in Incineration Boilers?
  • Spent wash + Coal (Indian/Imported)
  • Spent wash + Rice husk
  • Spent wash + Bagasse
10-What type of Boilers are used in Incineration Technologies?
  • Travelling grate Boiler: Rice husk & Bagasse
  • AFBC Boiler: Indian & Imported coal
11-What are the features of AFBC boilers used in Incineration Technology?
AFBC Boiler:
  • Bottom supported
  • Single drum
  • Over bed feeding
  • 3-pass flue gas construction
  • Combustor is located in pass-1 & Super heater, Evaporator; Economiser & Super heaters are located in Pass-3

12-Briefly explain the combustor & fuel feeding system in AFBC boilers?
Combustor:As discussed above, Combustor is located in Pass -1, fabricated with high temperature refractory material, enclosed by steel casing and supporting structure. Combustor is divided into various compartments & for all compartments ash drain system is provided.
Coal feeding system: consists of coal bunker, VFD screw feeders to feed the coal. Coal is distributed inside the combustor by pneumatic spreading system.
Spent wash feeding system: Concentrated and pre-heated Spent wash from the storage tank is conveyed to boiler through variable speed pump. Spent wash is sprayed over the fluidising bed at first pass through spray guns.

13-Where do the Secondary air & spent wash nozzles are provided in AFBC Boilers?
Secondary air system is provided at 2 to 3 locations at various heights at furnace. And spent wash nozzles provided just below the secondary air nozzles
14-What are the various online cleaning mechanisms are provided for pressure parts?
Mechanical rapping systems for super heaters
Soot blowers for Evaporators
Sonic soot blowers for Economisers
15-How do the spent wash concentration (Brix) & its calorific values are related?
GCV of the spent wash increases with increase in its concentrations
Sl No.
Brix (%)
GCV (Kcal/kg)
1
40
850
2
45
1200
3
50
1400
4
60
1800

16-How do you increase the Brix of Spent wash?
Spent wash Brix can be increased by heating
17-What parameters of Brix are recommended for efficient burning?
  • Brix: 55 to 60%
  • Moisture: 40 to 50%
  • Ash: 18 to 20%
  • GCV: 1550 to 1750 kcal/kg


18-Write down the ultimate analysis parameters of Spent wash
  • Ultimate analysis report:
  • Carbon: 18.5 to 23%
  • Hydrogen: 1.5 to 2.5%
  • Oxygen: 12 to 15%
  • Nitrogen: 1.5 to 2%
  • Sulphur: 0.5 to 0.7%
19-What are the various contents of spent wash?
Ash contents analysis by weight is as follow
  • Silica (SiO2) : 2.3 to 2.8%
  • Iron Oxide (Fe2O3) : 2 to 2.5%
  • Calcium Oxide (Cao) : 12 to 16%
  • Magnesium oxide (MgO) : 6 to 7%
  • Sulphate, SO3: 15 to 17%
  • Phosphate (P2O5) : 8 to 12%
  • Potassium Oxide (K2O) : 45 to 50%
  • Chlorides: 0.8 to 1.2%

20-What is the quantity of steam generated per square meter of heating surface in Spent wash AFBC boilers?
It is around 8 to 10 kg of steam is generated per square meter of heating surface
21-What is the quantity of steam generated per square meter of heating surface in Spent wash Biomass (Bagasse, rice husk etc) fired boilers?
It is around 10 to 12 kg of steam is generated per square meter of heating surface.
Note: For incineration:AFBC Boilers have more heating surface as compared to Biomass fired Boilers
22-Why do the Spent wash Boilers have taller furnaces?
Because;
  • To maintain higher residence time in furnace up to 8 seconds
  • To maintain optimum flue gas temperature at super heaters inlet to avoid coils fouling due to alkali ash property and to avoid corrosion of super heater coils due to chlorides (1%) in ash
23-What is the design configuration of APH in spent wash fired Boilers?
In spent wash Boilers APH are designed for air flow through tubes & flue gas outside to avoid choking of tubes
24-What is the flue gas temperature that we get at APH outlet for spent wash fired Boilers?
It is around 180 to 200 Deg C
25-What is the size & density of refractory materials used in Spent wash AFBC Boilers?
Size of refractory materials is around 0.8 to 2.5mm. And density is around 1000 to 1100 kg/m3
26-What are the main compositions of Refractory materials?

Questions & Answers on AFBC Boilers
  • Silica 55 to 60%
  • Alumina 35 to 40% & small traces of Feo, Mno, Cao, Mgo, P2O5 etc
27-What is the optimum temperature of spent wash at the nozzle inlet?
It is around 70 to 80 deg c
28-What is the maximum efficiency that we can get for spent wash & Rice husk fired Boilers?
It is around 72%




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80-Interview questions & answers for fresher Mechanical Engineers

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