Paper – 2 – Set B SOLUTION
Regn No: _________________
Name: ___________________
(To be written by the candidates)
NATIONAL CERTIFICATION EXAMINATION 2007
FOR
ENERGY MANAGERS & ENERGY AUDITORS
PAPER – 2: ENERGY EFFICIENCY IN THERMAL UTILITIES
Date: 21.04.2007
Marks: 150
Timings: 1400-1700 HRS
Duration: 3 HRS
Max.
General instructions:
o
o
o
o
o
Please check that this question paper contains 7 printed pages
Please check that this question paper contains 65 questions
The question paper is divided into three sections
All questions in all three sections are compulsory
All parts of a question should be answered at one place
Section – I: OBJECTIVE TYPE
(i)
(ii)
(iii)
1.
Marks: 50 x 1 = 50
Answer all 50 questions
Each question carries one mark
Please hatch the appropriate oval in the OMR answer sheet with HB Pencil, as per
instructions
“Turndown ratio” for burners is the ratio of
a) air to fuel
b) maximum fuel input to actual fuel input
c) maximum fuel input over
minimum fuel input
d) maximum air input over minimum air input
2.
A bimetallic strip is used in which of the following traps
a) float trap
3.
b) thermodynamic c) thermostatic d) inverted bucket
Air must be removed from steam line as
a) It reduces partial pressure of steam and decreases thermal resistance to heat transfer
b) It increase partial pressure of steam and decrease thermal resistance to heat transfer
c) It increases saturation temperature of steam and increases thermal resistance to heat
transfer
d) It reduce saturation temperature of steam and increase thermal resistance to heat
transfer
4.
An axial compressor is used in conjunction with which of the following
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Paper – 2 – Set B SOLUTION
a) Back pressure steam turbine
c) Condensing turbine
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b) Gas turbine
d) none of the above
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Paper – 2 – Set B SOLUTION
5.
At the critical point of steam
a) boiling point is 0oC
c) sensible heat is zero
6.
b) enthalpy of evaporation is zero
d) total enthalpy is zero
Boiler Evaporation ratio is the amount of steam generated in kg
a) per kg of fuel burnt
c) per m2 of boiler surface area
7.
Carpet loss occurs in
a) Coal combustion
8.
b) per unit time
d) per kg of makeup water.
b) atomization of oil c) coal storage d) furnaces
Ceramic fibre gives the maximum savings when used in
a) continuous furnace b) induction furnace c) arc furnace d) batch furnace
9.
Chances of NOx formation are least in
a) Chain grate stoker boiler
c) Pulverized coal fired boiler
b) FBC boilers
d) Spreader stoker boiler
10. Co-generation is also known as
a)Re-generation system
c)Combined heat and power system
b)Brayton system
d)Reversible system
11. Concentration of solids in boiler drum is controlled by
a)reducing dosage of chemicals
b) blowdown c) steam venting d)deaeration
12. Deaeration in boiler removes
a) CO2 in flue gas
b) O2 in feed water
c) O2 in fuel
d) O2 in flue gas
13. De-aeration of boiler feed water helps in combating
a) corrosion.
b)TDS c) silica d) hardness
14. Excess air can be derived by measuring percentage of ____________ in exit flue gas
a) CO
b) NO2
c) CO2
d) CH4
15. For coal fired system the flame length is dictated by:
a) moisture
b) volatile matter.
c) ash content.
d) fixed carbon.
16. For equal capacity, the boiler size is the smallest for
a) AFBC
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b) CFBC
c) PFBC
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d) Pulverized coil fired boiler
Paper – 2 – Set B SOLUTION
17. For flash steam calculation, flash steam quantity available depends upon ___
a) condensate pressure and flash steam pressure b) pressure of steam generated in boiler
c) steam enthalpy at atmospheric pressure
d) total heat of flash steam
18. For industrial process indirect heating, the best quality of steam is
a) dry saturated steam b) superheated steam c) wet steam
d) high pressure steam
19. For same inlet conditions of the steam which of the following will generate the maximum
mechanical power
a) condensing turbine
c) extraction-cum-condensing turbine
b) back pressure turbine
b) extraction-cum-back pressure turbine
20. High emissivity coatings are most effective on
a)outer surface of furnace
b)inner surface of furnace
c)furnace charge
d)none of the above
21. In determining the optimal economic insulation thickness for a steam pipeline, thickness
which of the following factors need not be considered
a) annual hours of operation
b) calorific value c) pipe material
d) cost of fuel
22. In oil firing burners
a) primary air is used for creating turbulence and secondary air for completion of
combustion
b) primary air is used for cooling oil and secondary air for completion of the combustion
c) primary air is used for completion of the combustion and secondary air for creating
turbulence
d) Primary air is used for atomizations of oil and secondary air for completion of the
combustion.
23. In pure stochiometric combustion of furnace oil which of the following will be absent in flue
gas ?
a) nitrogen
b) carbon dioxide
c) oxygen
d) sulphur dioxide
24. Increase in feed water temperature by 300C for an oil fired boiler results in a savings of -----% of fuel.
a) 1
b) 4
c) 5
d) None of the above.
25. Magnesite, chrome-magnesite, dolomite are examples of --------- type of refractory
a) acid
b) basic
c) neutral
d) none of the above
26. Major heat loss in an furnace is accounted by
a) radiation .
b) openings.
c) sensible heat in exit flue gas
d) hydrogen in fuel
27. Maximum heat transfer to the stock in a reheating furnace is by
a) conduction
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b) radiation
c) convection
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d) none of these
Paper – 2 – Set B SOLUTION
28. Moisture in combustion air
a) contributes to latent heat loss but not sensible heat loss in flue gas
b) does not contribute to latent heat loss but contribute to sensible heat loss in flue gas
c) does not contribute to latent heat loss and sensible heat loss in flue gas
d) contributes to both the latent heat loss and sensible heat loss in flue gas.
29. Pick the wrong statement. The thermal efficiency of a furnace increases by
a)reducing surface heat loss
c)maintaining high levels of excess air
b)preheating Combustion air.
d)minimising unburnt losses
30. Pre-heating of combustion air by 1000C will save about ------% of fuel.
a)0.5
b)5
c)7
d)None of the above
31. Quality of waste heat in flue gas refers to
a) dust concentration in flue gas
c) moisture in flue gas
b) temperature
d) corrosive gases in flue gas
32. Steam for process heating should always be generated and utilized at
a) same pressure
b) lowest pressure and highest pressure respectively
c) highest pressure and lowest pressure respectively
d) atmospheric pressure
33. Stochiometric air ratio for burning 1Kg of carbon is
a)3.76
b)10
c)11.6
d)None of the above
34. Sulphur percentage in furnace oil
a) sets lower flue gas temperature limit
c) does not add to heat value
b) improves viscosity
d) forms soot
35. The content of solid left in the oven after volatile matter is distilled off (while analysing the
composition of coal in a laboratory) is ___.
a) only sulphur
b) only moisture
c) only ash
d) mix of fixed carbon and ash
36. The factor that influences atomization of fuel oil most is
a) density
b) flash point
c) pour point
d) viscosity
37. The heat recovery device in which high conductivity bricks are used storing heat is
a) heat pipe
b) heat pump
c) thermo compressor
d) regenerator
38. The percentage radiation loss from a boiler will
a) increase with increased loading
c) be independent of loading
b) decreases with increased loading
d) none of the above
39. Velocity of steam in a pipe depends on
a) number of bends
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b) length of pipe c) specific volume of steam d) none of the above
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Paper – 2 – Set B SOLUTION
40. Wetting of coal with water in boiler helps in
a) increasing the calorific value of the coal
b) keeping boiler grate cooled
c) increasing the furnace draft velocity d)stopping coal fines to fall through grate and being
carried away with furnace draft
41. What type of steam is generally used for power generation
a) high pressure steam with super heat
c) dry saturated steam with high pressure
b) dry saturated low pressure steam
d) wet steam with very high pressure
42. When pure hydrogen is burned, with theoretical air, the volume percentage of nitrogen in
flue gas on dry basis will be
a) 79%
b)
100%
c) 21%
d) 0%
43. Which data is not required in calculation of thermal efficiency of boiler by indirect method
a)blow down quantity
b)calorific value of fuel c)excess air level d)flue gas temperature
44. Which fuel uses the lowest amount of excess air during combustion process?
a) pulverised coal
b) bagasse
c)fuel oil
d) natural gas.
45. Which is not a property of Ceramic fibre insulation
a) low thermal conductivity b) light weight c) high heat storage d) thermal shock resistant
46. Which of the energy saving measures will not be applicable for a heat treatment furnace
a) complete combustion with minimum excess air b) waste heat recovery from the flue
gases c) optimum capacity utilization d) heat recovery from furnace openings
47. Which of the following is not true of steam ?
a) highest specific heat and latent heat
c) easy to control and distribute
b) low heat transfer coefficient
d) cheap and inert
48. Which of the following will require minimum excess air for combustion
a) fluidized bed boiler
c) pulverized coal fired boiler
b) spreader stoker boiler
d) manually fired boiler
49. Which one of the following cannot be used as fuel for the gas turbine
a) naphtha
b) LPG
c) LSHS
d) natural gas
50. With increase in excess air for combustion which of the following will result in flue gas
a) % Oxygen decreases
c) % Oxygen and CO2 decreases
b) % CO2 decreases
d) % Oxygen and CO2 increases
----------------------End of Section - I----------------------
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Paper – 2 – Set B SOLUTION
Section – II: SHORT TYPE QUESTIONS
(10 Questions x 5 Marks Each )
S-1
•
•
•
•
•
S-2
(Total 50 marks)
List five benefits of condensate recovery in a process plant
For every 60C rise in the feed water temperature, there will be approximately 1%
saving of fuel in the boiler. So financial benefits.
Reduction in Water charges
Minimising effluent temperature and hence adhering to effluent restrictions
Maximises boiler output
Better boiler feedwater quality
In a natural gas fired boiler the air to fuel ratio is maintained at 10Nm 3/Nm3 of
gas. An air preheater is installed to recover the waste heat, which brings down
the exit flue gas temperature from 220oC to 170oC. If inlet air temperature to air
preheater is 30ºC, find out the exit air temperature. Assume that the specific
heat of flue gas and ambient air is equal.
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Paper – 2 – Set B SOLUTION
Ans.
Ratio of volume flow rate flue gas to that of combustion air = 11/10
Va.a.Cpa (Toa-Tia) = Vf.f. Cpf. (Tof-Tif)
Ratio of density of flue gas to that of combustion air = (273+30) / (273+220) = 0.6146
Toa = Tia +
Vf . f
. (Tof – Tif) = 30 + 11/10 x 0.6146 x (220-170)
Va . a
 63.8ºC
S-3
Explain the importance of draft in a reheating furnace
High negative pressures leads to air infiltration- affecting air-fuel ratio control,
problems of cold metal and non-uniform metal temperatures.
High positive Pressure leads to Ex-filtration -Problems of leaping out of flames,
overheating of refractories,burning out of ducts etc.
Hence the optimum condition is to maintain a slightly positive pressure
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Paper – 2 – Set B SOLUTION
S.4
Discuss the role of three T’s in efficient combustion process
Ans
3 T’s of Combustion
The objective of good combustion is to release all of the heat in the fuel. This is
accomplished by controlling the "three T's" of combustion which are (1) Temperature
high enough to ignite and maintain ignition of the fuel, (2) Turbulence or intimate
mixing of the fuel and oxygen, and (3) Time sufficient for complete combustion.
For sustained combustion the temperature of fuel/air mixture must be at temperature
above ignition temperature. Air contains 21% O2 and 79% N2 only O2 takes part in
combustion process. For combustion each fuel molecule must be in contact with at
least required number of molecules (theoretical) of O 2. This is only possible when fuel
and O2 is mixed on molecule to molecule basis. This complete mixing will require
turbulence in fuel in gaseous from and air. To ensure that each gaseous molecule
meets the O2 molecule, the fuel air mixture must stay for sufficient longer period in
region where temperature is more than the ignition temperature (Furnace chamber).
S-5
A fluidized bed boiler generates saturated steam at 15 atmosphere absolute
pressure (hg = 666 kCal/kg). If the feedwater temperature is 60ºC (h f = 60
kCal/kg), for evaporation ratio of 5 for a particular fuel (GCV of fuel = 4200
kCal/kg), estimate the boiler efficiency.
Ans.
=
Q(h g  h f )
q GCV
x 100 =
5 * (666 – 60)
* 100
4200
=72.14%
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Paper – 2 – Set B SOLUTION
S-6
Draw a schematic diagram of a combined cycle power plant
S-7
A boiler is generating steam at 5500 kg/hr. The maximum permissible limit of
TDS in the boiler is 3000 ppm. If the make up water is 40% at a TDS level of
200 ppm, calculate the blowdown percentage and blow down rate.
Ans. Blow down percentage =
200 * 40
= 2.67%
3000
Blow down rate = 5500 x .0267 = 146.85 kg/hr.
S-8 Explain the working of a float trap with a sketch.
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Paper – 2 – Set B SOLUTION
or
Float traps operate in a very similar way to a ball cock. A float contained within the trap
body is raised or lowered by the volume of condensate delivered to the trap. As
increasing levels of condensate raise the ball float, the mechanism lifts a valve allowing
condensate to discharge thus lowering the level of condensate within the trap. The trap
eventually closes preventing the further passage of steam.
The trap will remain closed and partially flooded unless there is a sufficient level of
condensate within the trap. At start up any air ahead of the steam and condensate will
not therefore be vented. Consequently it is necessary to incorporate an air cock or a
balanced pressure device (as described above) to release air in the trap.
S-9
For a oil containing 9% hydrogen (GCV = 10,200 kCal/kg), estimate the
percentage of sensible and latent heat loss due to evaporation of water formed
due to hydrogen in the fuel, if the flue gas temperature is 180ºC and combustion
air temperature is 40ºC.
(latent heat of vapours = 584 kCal/kg, specific heat of vapours = 0.45 kCal/kg/0C)
Ans. For 0.09 kg of hydrogen per kg of fuel, water generated is = 9 x 0.09 = 0.81 kg
Percentage Sensible Heat Loss = mCp (Tf-Ta)/GCV =
0.81* 0.45 * (180-40)
= 0.5 %
10200
Percentage Latent Heat Loss =
0.81* 584
= 4.6 %
10200
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Paper – 2 – Set B SOLUTION
S-10 How does a thermo compressor work? Explain briefly with a sketch.
In a thermocompressor low pressure steam is converted to medium pressure steam by
using high pressure steam. The thermocompressor is a simple equipment with a
nozzle where HP steam is accelerated into a high velocity fluid. This entrains the LP
steam by momentum transfer and then recompresses in a divergent venturi.
----------------------End of Section -II----------------------
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Paper – 2 – Set B SOLUTION
Section – III: SHORT TYPE QUESTIONS
(5 Questions x 10 Marks Each )
(Total 50 marks)
L-1 A steam pipeline of 100mm diameter is not insulated for 100 metre length,
supplying steam at 8 kg/cm 2. Find out the fuel savings if it is properly insulated
with 50mm insulating material. Assume 8000 hours of operation per year.
Given:
Boiler efficiency
: 75%
Cost of coal
: Rs.2500/tonne.
Gross Calorific value of fuel
: 4000 kCal/kg
Surface temperature without insulation : 1500C
Surface temperature after insulation
: 500C
Ambient temperature
: 300C
Existing Heat Loss:
Surface heat loss S= 10+(Ts –Ta)/20 x(Ts-Ta)
Where Ts =Hot surface temperature0C
Ta =Ambient temperature0C
S =Surface heat loss in k.Cal/hrm2
Substituting values S= 10+(150 –30)/20 x(150-30)
=1920 k.Cal/ hr-m2
Modified System :
After insulating with 50mm insulating material the surface temperature has reduced to
500C
Substituting values S= 10+(50 –30)/20 x(50-30)
=220 k.Cal/ hrm2
Calculation of Fuel savings:
Pipe dimension
Surface area existing
=100 metre length and 100mm diameter
=3.14x0.1x100 =31.4 m2
Surface area after insulation
=3.14x0.2x100 =62.8 m2
Total heat loss in existing system =1920x31.4 = 60288 k.Cal/hr
Total heat loss in modified system
Reduction in heat loss
No of operating hours per annum
Savings in heat per year
Annual savings in fuel
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=220 x 62.8 =13816 k.Cal/hr
=60288 -13816 = 46472 k.Cal/hr
=8000
=46472 x 8000 = 371776000 k.Cal/year
=371776000 /(4000x0.75)x1000 =123.93 Tons
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Paper – 2 – Set B SOLUTION
Monetary savings per annum
=123.93 x 2500 =Rs. 3.098 lakhs
L-2 Compare the cost of power generation from a gas turbine installed with a 10 TPH
waste heat recovery boiler vis-a-vis grid supply
The operational data are as under:
Capacity of gas turbine
Auxiliary power consumption
Operating hours per annum
Plant load factor
Heat rate
Calorific value of natural gas
Cost of gas
Cost of capital of gas turbine
and boiler and maintenance
of W.H.R charges per annum
Cost of electric power from grid
(Demand and energy charges)
: 3000kW
: 1% of electricity generated
: 8000
: 90%
: 3050kCal/kWh
: 9500kCal/sm3
: Rs.6000/1000 sm3
: Rs.400 lakhs
: Rs.4.5/kWh
Power generation from Cogeneration
plant
3000x(90/100)x8000 =216 x105 kWh
Auxiliary consumption
Net generated power
0.01 * 216 x 105 = 216000 kWh
21600000 – 216000 = 21384000 kWh
Natural gas required for above generation
(216*105) x 3050/9500
6934737 sm3
Cost of fuel per annum
6934737 x 6000/1000
416.08 lakhs
Cost of capital and operation charges
Total cost of power from Cogeneration
plant
Rs 400 lakhs
416.08 + 400
816.08 lakhs
Cost of Cogenerated power
Rs 816.08 / 213.84 =Rs. 3.82/kWh
Cost of grid power
Rs 4.50/kWh
Cost of cogeneration power is cheaper by Rs 0.68/ kWh
L-3 An oil fired boiler uses furnace oil with ultimate analysis of Sulphur 3%,
Hydrogen 11%,Carbon 85% and oxygen 1% and gross calorific value of 10,268
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Paper – 2 – Set B SOLUTION
kCal/kg. The boiler furnace is operated with 15% excess air at 27ºC and humidity
ratio 0.0175. If the flue gas temperature is 270ºC, determine the dry flue gas
losses and latent heat loss due to evaporation of water.
Ans. For 100 kg of oil
Combustion of carbon C + O2 = CO2
Theoretical O2 required =
CO2 produced =
44
12
32
12
x 85 = 227 kg
x 85 = 312 kg
Combustion of hydrogen 2H2 + O2 = 2H2O
Theoretical O2 required =
H2O produced =
36
4
32
4
x 11 = 88 kg
x 11 = 99 kg
Combustion of sulphur S + O2 = SO2
Theoretical O2 required =
SO2 produced =
64
32
32
32
x 3 = 3 kg
x 3 = 6.0 kg
Theoretical O2 required = 227 + 88 + 3 = 318 kg
O2 in fuel = 1%
Actual O2 required = 318 – 1 = 317 kg
Excess O2 required = 317 x 0.15 = 47.55 kg
Total O2 required = 317 + 47.55 = 364.55 kg
Accompanied N2 = 364.55 x
77
23
= 1220.45 kg
Accompanied moisture = (1220.45 + 364.55) x 0.0175 = 27.74 kg
Dry flue gas loss = [(312 + 6 + 47.55 + 1220.45) x 0.23 + (270-27)
= 88642 kcal
Latent heat loss = 99 x 584 = 57816 kcal
L-4 List 10 energy saving measures in a steam system
1.
Monitoring Steam Traps
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Paper – 2 – Set B SOLUTION
2.
Avoiding Steam Leakage
3.
4.
Providing Dry Steam for Process
Proper Utilisation of Directly Injected Steam
5.
Miminising Heat Transfer Barrier
6.
Proper Air Venting
7.
Condensate Recovery
8.
Insulation of steam pipe lines and hot process equipments
9.
Flash Steam Recovery
10.
Reducing the work to be done by steam
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Paper – 2 – Set B SOLUTION
L-5 Explain briefly the principles of operation of
a) Heat pipe
b) Radiation Recuperator
c) Plate Heat Exchanger
a) Heat pipe
The Heat Pipe comprises of three elements – a sealed container, a capillary wick
structure and a working fluid. The capillary wick structure is integrally fabricated into
the interior surface of the container tube and sealed under vacuum. Thermal energy
applied to the external surface of the heat pipe is in equilibrium with its own vapour as
the container tube is sealed under vacuum. Thermal energy applied to the external
surface of the heat pipe causes the working fluid near the surface to evaporate
instantaneously. Vapour thus formed absorbs the latent heat of vapourisation and this
part of the heat pipe becomes an evaporator region. The vapour then travels to the
other end the pipe where the thermal energy is removed causing the vapour to
condense into liquid again, thereby giving up the latent heat of the condensation. This
part of the heat pipe works as the condenser region. The condensed liquid then flows
back to the evaporated region.
b) Radiation Repuperator
A metallic radiation recuperator consists of two concentric lengths of metal tubing. The
inner tube carries the hot exhaust gases while the external annulus carries the
combustion air from the atmosphere to the air inlets of the furnace burners. The hot
gases are cooled by the incoming combustion air which now carries additional energy
into the combustion chamber. Radiation recuperator gets its name from the fact that a
substantial portion of the heat transfer from the hot gases to the surface of the inner
tube takes place by radiative heat transfer.
c) Plate Heat Exchanger
Ans.
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Paper – 2 – Set B SOLUTION
A plate type heat exchanger consists of a series of separate parallel plates forming thin
flow pass. Each plate is separated from the next by gaskets and the hot stream passes
in parallel through alternative plates whilst the liquid to be heated passes in parallel
between the hot plates. To improve heat transfer the plates are corrugated.
Hot liquid passing through a bottom port in the head is permitted to pass upwards
between every second plate while cold liquid at the top of the head is permitted to pass
downwards between the odd plates. When the directions of hot & cold fluids are
opposite, the arrangement is described as counter current.
----------------------End of Section -III----------------------
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