Model Question Paper - 2006
NATIONAL CERTIFICATION EXAMINATION
FOR
ENERGY AUDITORS AND ENERGY MANAGERS
PAPER-2: ENERGY EFFICIENCY IN THERMAL UTILITIES
Duration : 3 Hrs
Max. Marks: 150
General Instructions:
Please check that this question paper contains 65 questions
The question papers is divided into three sections
 All questions in all three sections are compulsory
 All parts of a question should be answered at one place
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
Section-I : Objective Type Questions
(i) Answer all question of this section.
(ii) Each question carries one mark.
(iii) Put a () tick mark on the appropriate box in the answer book.
01
The A grade coals have calorific value
(a) Greater then 6200 kcal/kg
(b) Less then 6200 kcal/kg
(c) Less then 1300 kcal/kg
(d) Greater then 1300 kcal/kg
02
The smallest size of the coal (≈75 micron) is required in
(a) Fluidized bed firing
(b) Pulverized coal firing
(c) Stoker firing
(d) Hand firing
03
Calorific value of liquid and gaseous fuels is generally measured in terms of
(a) kcal/m3 and kcal/N m3 respectively
(b) kcal/N m3 and kcal/ m3 respectively
(c) kcal/kg and kcal/ m3 respectively
(d) kcal/ m3 and kcal/kg respectively
04
Which of the following agro-residues has the lowest gross calorific value
(a) Deoiled Bran
(b) Paddy Husk
(c) Saw dust
(d) Coconut shell
05
Under optimum combustion condition of fuel oil, the percentage of oxygen in flue gas will
range between.
(a) 0 to 0.1%
(b) 2 to 3%
(c) 12 to 13%
(d) 22 to 23%
06
Heat generated in combustion on per kg basis, is the highest for
(a) carbon
(b) hydrogen
(c) sulpher
(d) oxygen
07
Ash fusion temperature of typical Indian coal is
(a) 500ºC
(b) 700ºC
(c) 900ºC
(d) 1100ºC
08
Stoichiometric Volumetric air to fuel ratio for complete combustion of natural gas ranges
between
(a) 9.5 to 10
(b) 14.5 to 15
(c) 18.5 to 19
(d) 22.5 to 23
09
With increase in percentage of excess air for combustion of a given fuel percentage of
oxygen in flue gas.
(a) increases
(b) decreases
(c) remains same
(d) none of the above
10
Permissible limit of TDS in boiler
a) increases for higher pressure boilers
b) decreases for higher pressing boilers
c) is independent of boiler pressure
d) none of the above
11
Radiation losses from a boiler
a) increase with increase in % its loading
b) decrease with increase in % its loading
c) are independent of % its loading
d) none of the above.
12
An evaporation ratio (steam to fuel ratio) of an efficient oil fired boiler is in the range of___.
a) 5 – 6
b) 13 – 16
c) 1-3
d) 7 – 9
13
Name the predominant loss component for furnace oil fed boiler.
a) losses due to radiation and convention
b) loss due to hydrogen in fuel
c) loss due to dry flue gas
d) loss due to moisture in fuel
14
A rise in conductivity of boiler feed water indicates ____ .
a) drop in the contamination of feed water
b) greater purity of feed water
c) rise in the contamination of feed water
d) it has got no relation with the contamination of feed water
15
De-aeration of boiler feed water is referred to as:
a) removal of dissolved gases
b) removal of silica
c) removal of scales by blow down
d) phosphate treatment of feed water
16
F & A (From and At) rating of the boiler is the amount of steam generated from
(a) Water at 0ºC to steam at 100ºC
(b) Water at 27ºC to steam at 100ºC
(c) Water at 77ºC to steam at 100ºC
(d) Water at 100ºC to steam at 100ºC
17
Demineralization in water treatment means
(a) removal of total salts
(b) removal of only hardness salts
(c) removal of alkali salts
(d) removal of non-hardness salts
18
Chemical de-aretion of feed water with sodium sulphate as oxygen scavenger removes
oxygen but
(a) decreases pH of the water
(b) decreases TDS level of the water
(c) increases TDS level of the water
(d) None of the above
19
For industrial process heating, the best quality of steam is:
a) dry saturated steam
b) superheated steam
c) wet steam
d) high pressure steam
20
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
21
Latent heat steam at critical point is
a) Infinite
b) 540 kcal
c) zero
d) none of the three
22
head loss due to flow of steam in a pipe line is proportional to
a) velocity 4
b) velocity –4
c) velocity –2
d) velocity 2
23
Failure mode of bucket steam trap is
a) open
b) closed
c) none of the two
24
Chances of water hammering will be largest in
a) down inclined steam line
b) up inclined steam line
c) horizontal steam line
d) none of the above
25
The temperature maintained in forging furnaces ranges between
a) 700-850ºC
b) 850-1000ºC
c) 1000-1150ºC
d) 1150-1250ºC
26
Radiation Recuperators are used for flue gas temperature of more than
a) 800ºC
b) 600ºC
c) 400ºC
d) 200ºC
27
The pressure drop as the exhaust gases pass through recuperator is of the order of
a) 0.5 to 1.0 mm of H2O column
b) 1.0 to 5.0 mm of H2O column
c) 5.0 to 10 mm of H2O column
d) 10 to 50 mm of H2O column
28
To retain the heat stored in furnace walls it is advisable to run a batch furnace in batch for a
given load.
a) 48 hrs every six days
b) 8 hrs per day
c) 24 hrs every third day
d) none of the above
29
In large glass industries, the equipment connected with glass melting furnace for preheating
the air is___.
a) recuperators
b) regenerators
c) shell & tube heat exchanger
d) heat wheels
30
The temperature which separates ‘ovens’ from ‘furnaces’ is
a) 170ºC
b) 270ºC
c) 370ºC
d) 470ºC
31
The axis of the oil fired burner in a furnace should be kept:
a) slightly inclined towards the roof
b) more inclined towards roof
c) slightly inclined towards the stock
d) parallel to stock
32
Higher excess air in an oil fired furnace would result in:
a) increased furnace temperature
b) increased heating rate
c) reduced flame temperature
d) none of the above
33
Which of the following is organic insulating material
a) Expanded Polystyrene
b) Calcium silicate
c) Mineral wool
c) None of the above
34
The melting temperature of the pure refractory compounds in highest for
a) Alumina
b) Lime
c) Chromite
d) Meginisia
35
The insulation material suitable for low temperature application is
a) Mineral fibre
b) Fibre glass
c) Silica
d) Polyurethane
36
The unit for thermal conductivity of insulation and refractories is ____.
a) K.cal/m-hr-°C
b) K.cal/ m²-hr-°C
c) K.cal/m²-°C
d) K.cal/ m-°C
37
The insulation which can be used for lining furnaces operating up to 1850ºC is:
a) Alumina
b) Zirconia
c) Dolomite
d) Calcium silicate
38
The material used to control SOx in the FBC boiler is
a) Limestone
b) Alumina
c) Silica
d) All of the above
39
The velocity of fluidizing air in atmospheric fluidized bed boiler is in the range of:
a) Higher than 4.5 m/sec
b) 1.2 – 3.7 m/sec
c) Less than 1.2 m/sec
d) 4-6 m/sec
40
In FBC boiler the combustion is carried out at a temperature
a) closer to steam temperature
b) at adiabatic combustion temperature
c) at and above ash fusion temperature
d) below ash fusion temperature of fuel used
41
Residence time in fluidized bed boiler
a) is equal to conventional grate firing boiler
b) is less then that in conventional grate firing boiler
c) is little more then that in conventional grate firing boiler
d) many times more then that in conventional grate firing boiler
42
The coal size used in AFBC boiler ranges.
a) 1 μm-10 μm
b) 1mm- 10mm
c) 10mm-20mm
d) 10 μm- 100mm
43
The overall conversion efficiency of a PFBC boiler in cogeneration mode is higher by
a) 20-25%
b) 10-12%
c) 5-8%
d) 1-3%
44
An economizer is used to recover waste heat from flue gas to heat
a) stock
b) combustion air
c) feed water
d) room air
45
In a shell and tube heat exchanger, the vapour stream is
a) flown in the tubes
b) contained on the shell side
c) generally not used for heat exchange
d) none of the above.
46
Major advantage of waste heat recovery in industry is:
a) reduction in pollution
b) increase in efficiency
c) both a & b
d) none of the above
47
Ceramic recuperators can withstand temperatures up to:
a) 600 ºC
b) 1300 ºC
c) 1700ºC
d) 950ºC
48
In a combined cycle power plant consisting of gas turbine and waste heat boiler, the exhaust
gas temperature is ____.
a) around 150 °C
b) around 500 °C
c) around 300 °C
d) around 400 °C
49
The overall efficiency of combined cycle cogeneration is of the order of:
a) 69 – 83
b) 90 – 95
c) 70 – 90
d) 55 – 60
50
Heat to power ratio of combined cycle cogeneration is in the range of------------a) 4.0 – 5.0
b) 1.0 – 1.7
c) 2.0 – 10
d) 1.0 – 5.0
Section II: Short Descriptive Questions
(i)
(ii)
S-1
Ans.
Answer all TEN of the following questions.
Each question carries FIVE marks
Explain the significance of the ultimate analysis of the fuels from the combustion point of
view.
Ultimate analysis of the fuel besides its moisture and ash content gives percentage of
various elemental chemical constituents of the fuel such as carbon, hydrogen, oxygen,
sulpher etc. It helps in estimating the quantity of stoichiometric air required for
combustion and volume and composition of combustion gases. This also helps in
estimating the flame temperature, flue gas duct design and calorific value of the fuel.
S-2
Explain why stack temperature in boilers can not be reduced below 160-170ºC
Ans.
For fuels containing sulpher low temperatures of stack can lead to sulpher dew point
corrosion of chimney air preheater and economizer.
S-3
List out the data required for calculation of boiler efficiency using ‘indirect method’.
Ans.
The data required for calculation of boiler efficiency using indirect method are:
• Ultimate analysis of fuel (H2, O2, S, C, moisture content, ash content)
• Percentage of Oxygen or CO2 in the flue gas
• Flue gas temperature in ºC (Tf)
• Ambient temperature in ºC (Ta) & humidity of air in kg/kg of dry air.
• GCV of fuel in kcal/kg
• Percentage combustible in ash (in case of solid fuels)
• GCV of ash in kcal/kg (in case of solid fuels)
S-4
List the characteristic of steam which make it most popular for carrying energy
Ans.
a)
b)
c)
d)
e)
Highest specific heat and latent heat.
Highest heat transfer coefficient both at generation and use point.
Easy to control and distribute.
Cheap and Inert.
Source (water) is easily available in the quantities required anywhere any time.
S-5
Explain why furnaces should operate at slightly positive pressure?
Ans.
It is important to operate furnace at a slightly positive pressure. Negative pressure lead to
air ex-filtration affecting air fuel ratio and furnace temperature thus increasing fuel
consumption. Excessive positive pressure leads to infiltration resulting in leaking out of
flames, overheating of furnace refractories, reduced brick life and other associated
problems.
S-6
List advantages of ceramic fibre insulation?
The advantages are
 Low Down time
 Increased productivity
 Low maintenance cost
 Longer service life
 Higher thermal efficiency
 Faster response
S-7
Calculate rate of heat loss from a furnace whose external surface (area 20 m2) is at
temperature 70ºC placed in a room at 35ºC
Ans.
Rate of heat loss per unit area = [10+(70-35)/20] [70-35] =411.25 kcal/m2-m
Rate of heat loss from furnace = 20 x 411.25 = 8225 kcal/hr
S-8
What is the principle of CFBC (circulating fluidized bed combustion) boiler?
Ans.
CFBC technology utilizes the fluidized bed principle in which crushed (6 –12 mm size)
fuel and limestone are injected into the furnace or combustor. The particles are suspended
in a stream of upwardly flowing air (60-70% of the total air), which enters the bottom of
the furnace through air distribution nozzles. The balance of combustion air is admitted
above the bottom of the furnace as secondary air. While combustion takes place at 840900oC, the fine particles (<450 microns) are elutriated out of the furnace with flue gas
velocity of 4-6 m/s. The particles are then collected by the solids separators and circulated
back into the furnace. This combustion process is called circulating fluidized bed (CFB).
S-9
Discuss the advantage and disadvantage of Reciprocating engine (Internal combustion
energy) in the cogeneration mode.
Ans.
Reciprocating engine cogeneration systems
Also known as internal combustion (I. C.) engines, these cogeneration systems have high
power generation efficiencies in comparison with other prime movers. There are two
sources of heat for recovery: exhaust gas at high temperature and engine jacket cooling
water system at low temperature (see Figure). As heat recovery can be quite efficient for
smaller systems, these systems are more popular with smaller energy consuming facilities,
particularly those having a greater need for electricity than thermal energy and where the
quality of heat required is not high, e.g. low-pressure steam or hot water.
Though diesel has been the most common fuel in the past, the prime movers can also
operate with heavy fuel oil or natural gas. These machines are ideal for intermittent
operation and their performance is not as sensitive to the changes in ambient temperatures
as the gas turbines. Though the initial investment on these machines is low, their
operating and maintenance costs are high due to high wear and tear.
S-10
Briefly explain the principle of ‘thermo compression’.
Ans
In many cases, very low pressure steam is reused as water after condensation for lack of
any better option of reuse. In many cases it becomes feasible to compress this low
pressure steam by very high pressure steam and reuse it as a medium pressure steam. The
major energy in steam is in its latent heat value and thus thermo compressing would give
a large improvement in waste heat recovery.
The thermo compressor is 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. It is typically used in evaporators where the boiling
steam is recompressed and used as heating steam.
Section II: Long Descriptive Questions
(i)
(ii)
L-1
Ans.
Answer all FIVE of the following questions.
Each question carries TEN marks
List various energy conservation opportunities in a boiler.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Heat recovery from flue gas through air preheating.
Heat recovery from flue gas through an economizer for feed water heating.
Excess Air Control.
Ensurance complete combustion
Reduction in Radiation and Connection losses from boiler shell surface.
Reduction in scale and soot losses.
Reduction in boiler steam pressure.
Variable speed control of fans, blowers and pumps
Automatic Blow down control.
Proper Boiler scheduling.
Replacement of inefficient boiler.
L-2
Calculate the theoretical air requirements for CH4 on mass basis and percentage volume of
CO2 (on dry basis) in flue gas. If the actual measured value of percentage of CO2 in flue
gas is 10%, estimate excess air ratio
Ans.
CH4 + 2(O2 + 3.76 N2)
= CO2 + 2H2O + 2 x 3.76 x N2
For 16 kg of CH4 mass of required air = 2 x (32 + 3.76 x 2.8) = 274.56 kg
For 1 kg of CH4 mass of
= 17.16kg.
Moles of CO2 in flue gas
=1
Moles of N2 in flue gas
= 2 x 3.76 = 7.52
% moles of CO2 in flue gas (dry basis)= (1/8.52) x 100 = 11.7 %
= % volume of CO2 in flue gas
(as per Avegadro’s law)
Excess air = [theoretical CO2 %/ actual CO2% -1] x 100
= [11.7/10-1] x 100 = 17%
L-3
Describe various consideration involved in retrofitting of FBC systems to conventional
system.
Ans.
Retrofitting fluidised bed coal fired combustion systems to conventional boilers has been
carried out successfully both in India and abroad.
The important aspects to be considered in retrofit projects are:
a) Water/steam circulation design
b) Furnace bottom-grate clearance
c) Type of particulate control device
d) Fan capacity
e) Availability of space.
Retrofitting of a fluidised bed combustor to a conventional stoker fired water tube boiler
may involve:
a) The replacement of grate by a distributor plate with short stand pipes for admitting air
from the wind box located underneath.
b) Installations of stand pipes to remove ash from the bed.
c) Provision of horizontal hairpin tubes in the bed with a pump for forced circulation from
the boiler drum.
d) Modification of crusher to size the coal/limestone mixture for pneumatic underbed
injection of the mixture.
It may be emphasised that conversion of a conventional coal fired system to a fluidised
bed combustion system can be accomplished without effecting major changes, after
making a cost-benefit analysis. Oil fired boilers can also be converted to coal fired
fluidised bed combustion systems. However it has to be examined on a case-to-case basis.
L-4
Ans.
For an extraction-cum-condensing turbine from the given diagram, evaluate (i) heat load
on cooling tower (ii) power generation in MW.
P=0.6MPa
T=250ºC
Q=5 MT/h
h=2957.2 kJ/kg
P=0.2 Mpa kJ/cm2
T=150ºC
Q=50MT/h
h=2768.8 kJ/kg
P=4.5MPa
T=500ºC
Q=72MT/h
h=3439.6 kJ/kg
P=10 kPa
T=45.8ºC
Q=17MT/hr
hf=191.8 kJ/kg
CONDENSOR
Input heat to turbine
: 72,000 x 3439.6 = 2.477 x 108 kJ/hr = 68,792 kW
Output heat at different streams
1st extraction
: 5000 x 2957.2 = 0.148 x 108 kJ/hr = 4,107kW
2nd extraction
: 50,000 x 2957.2= 1.384 x 108 kJ/hr = 38,456kW
Condenser heat load
: 17,000 x(2768.8-191.8) = 0.438 x 108 kJ/hr = 12169 kW
(i) Heat load on cooling tower : 12169kW
(ii) Heat equivalent to power generation: Input-Output
= 68792-(4,107+ 38,456 + 12,169)
Power generation
= 14,060 kW
L-5
What are waste heat recovery boilers? Explain their need and benefits?
Ans
Waste heat boilers are ordinarily water tube boilers in which the hot exhaust gases from
gas turbines, incinerators, etc., pass over a number of parallel tubes containing water. The
water is vaporized in the tubes and collected in a steam drum from which it is drawn off
for use as heating or processing steam. Because the exhaust gases are usually in the
medium temperature range and in order to conserve space, a more compact boiler can be
produced if the water tubes are finned in order to increase the effective heat transfer area
on the gas side. The pressure at which the steam is generated and the rate of steam
production depends on the temperature of waste heat. The pressure of a pure vapor in the
presence of its liquid is a function of the temperature of the liquid from which it is
evaporated. If the waste heat in the exhaust gases is insufficient for generating the
required amount of process steam, auxiliary burners which burn fuel in the waste heat
boiler or an after-burner in the exhaust gases flue are added. Waste heat boilers are built
in capacities from 25 m3almost 30,000 m3 / min. of exhaust gas.
Typical applications of waste heat boilers are to recover energy from the exhausts of gas
turbines, reciprocating engines, incinerators, and furnaces.