MODEL QUESTION PAPER - 2006
NATIONAL CERTIFICATION EXAMINATION
FOR
ENERGY MANAGERS AND ENERGY AUDITORS
PAPER – 3 : ENERGY EFFICIENCY IN ELECTRICAL UTILITIES
Section – 1 : OBJECTIVE TYPE
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22.
The overall efficiency for the coal based power plants in India ranges from -- %
c) 28-35
The standard technical losses in India are around –- %
b) 23
The designed power transformer’s efficiency is in the range of –
a) 96-99 %
The primary purpose of a Capacitor is to reduce –
b) maximum demand
A.C. Motors are –
b) inductive loads
For a given mains frequency of 50Hz and number of poles being 6, its synchronous speed is --- rpm.
c) 1000
The actual value of stray losses of motors up to 200 HP is likely to be -- %
b) 1-3
AC induction motor is a --- motor
b) asynchronous
An induction motor with 4 poles operating at 50Hz, has a slip of 2%. Its actual speed will be --- rpm.
c) 1470
According to BIS standards, a motor will deliver its rated output, if --a) voltage variation is ± 6% and frequency variation of ± 3%
The volumetric efficiency of a compressor --- with increase in altitude of place, where the compressor
is installed
b) decreases
The percentage increase in power consumption of a compressor with suction side air filter and with
pressure drop across the filter of 200 mm Wc is -- %
d) 1.6
The compressor capacity of a reciprocating type is directly proportional to --c) speed
Which of the parameter indicated below is not required to evaluate the volumetric efficiency of the
compressor?
c) power
Multi-step control is employed for --- compressors
b) large
COP of absorption refrigeration system is --b) less than 1.1
For a living room of 12 feet x 12 feet at residence, the required A/C size for comfort- conditions will be
--- TR
a) 0.25
Cascade systems for refrigeration are preferable in the temperature range of --- 0 C
c) -46 to -101
FAD means --b) Free Air Delivery
The other name for Roots compressor is --b) Lobe compressor
Which of the following is not a centrifugal fan type?
a) Vane axial
The ratio of maximum to minimum flow is called ---ratio
b) turn-down
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23.
24.
25.
26.
27.
This type of control for fans gives maximum benefits from energy saving point of view:
d) speed control
Centrifugal blower rotates at -- rpm
a) 15000
Typical design efficiency of aerofoil fan handling clean air is ---- %
b) 80-90
The most efficient method of flow control in a pumping system is -a) speed control
If two similar pumps are operating in series, the --c) head is doubled
28.
In general, water pipe lines are designed with water velocity of --b) up to 2m/s
29.
Changes in impeller diameter in centrifugal forces are limited to reducing its diameter to about --- of its
maximum size
a) 75%
30.
The laws of affinity relating to flow, head and speed are applicable too –
d) all of these
31.
Better indicator of cooling tower performance is --a) approach
32.
L / G ratio in cooling tower is the ratio of --c) water flow rate and air mass flow rate
33.
Nowadays, most of the end-user specification calls for the drift loss in a cooling tower to be --b) 0.02%
34.
Heat release rate to the cooling tower in vapour compression refrigeration system is equal to --- Kcal
per min per ton.
a) 63
35.
Centrifugal fans used in cooling towers are of:
c) FRP
36.
A device that distributes and filters the light emitted from one or more lamps is --b) luminaire
37.
For the same lamp output, 250 W HPMV lamp can be replaced with --c) 150 W HPSV
38.
IS 3646 stipulates --- lux as minimum illuminance for non-working interiors
d) 20
Color
39. rending Index for LPSV is --d) poor
40.
Electronic ballast saves energy up to --d) 35%
41.
Present specific fuel consumption value of DG sets used in Industries is about --- gm per KW
a) 160
42.
The least loss in DG set is -- loss
a) alternator
43.
Auxiliary power consumption of DG set at full load of its operating capacity is about --a) 1 to 2 %
44.
The starting current value of DG set should not exceed -- % of full load capacity of DG set
b) 200
45.
The largest motor HP rating that can be started with DOL starter is about --- of the KVA rating of the
DG set.
c) 50 %
46.
Typical loss in magnetic type choke for a 40W FTL will be --b) 14W
47.
VFD connected with motor provides --a) variable speed with high efficiency
48.
Occupancy sensors are well suited as --d) noise or movement detectors in a room
49.
Amorphous transformers are –
b) costlier
50.
IPFC eliminates – problems
c) hunting
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Section ii: SHORT DESCRIPTIVE QUESTIONS
S-1 What are the benefits of power factor improvement?
ANS: Power factor improvement has two major benefits:
1. Improvement in performance of the equipments / power network
Reactive component of the network is reduced and also the total current in the system from
the source end.
Transmission and Distribution losses are greatly reduced, due to current reduction.
Voltage profile is highly improved
Utilization of full capacity of the electrical system is made possible
2. Reduction in energy cost
Reduced KVA (MD) charges in the energy bill
Reduced distribution losses with in the plant or industry network
Investment on system facilities like transformers, cables, switchgear etc for delivering load
is reduced.
S-2 What is the thumb rule for installing capacitor to motor terminals?
ANS: Capacitor sizing is influenced by its direct relation to the average power drawn and
trigonometric ratio for the present and desired power factor i.e. KVAR rating = KW [ tanφ1tan φ2 ], where KVAR rating is the size of the capacitor needed, KW is the average power
drawn, tan φ1 is the trigonometric ratio for the present power factor and tan φ2 is the
trigonometric ratio for the desired power factor and φ 1 = Existing (Cos-1 PF1) and φ2 =
improved (Cos-1 PF2)
S-3 Briefly explain how the efficacy of inter and after coolers is an indicators of heat
exchange performance.
ANS: Efficacy is an indicator of heat exchange performance—how well intercoolers and
after coolers are performing.
Intercoolers are provided between successive stages of a multi-stage compressor to reduce
the work of compression, by reducing the specific volume through cooling the air, apart from
moisture separation. Using water of lower temperature reduces specific power consumption.
Inadequate cooling in after-coolers, allows warm, humid air in to the receiver, which causes
more condensation in air receivers and distribution lines, which in consequence, leads to
increased corrosion, pressure drops and leakages in piping and end-use equipment. Periodical
cleaning and ensuring adequate flow at proper temperature of both inter coolers and after
coolers are therefore necessary for sustaining desired performance.
S-4 Briefly explain the alternative Refrigerants for vapour compression systems.
ANS: The common use of CFC is being phased out due to its impact on the environment,
especially to Ozone layer. So, the concerned industry has developed two alternative
Refrigerants: one based on Hydrochloro Fluorocarbon (HCFC) and another based on Hydro
Fluorocarbon (HFC). The HCFCs have a 2 to 10% ozone depleting potential as compared to
CFCs and also, they have an atmospheric life time between 2 to 25 years as compared to 100
or more for CFCs.
S-5 How can the demand on a fan be minimized to save energy?
ANS:
1. Minimizing excess air level in combustion systems to reduce FD fan and ID fan load.
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2. Minimizing air in-leaks in hot flue gas path to reduce ID fan load, especially in case of
kilns, boiler plants etc. Cold air leaks increase ID fan load considerably, due to density
increase of flue gases and choke up the capacity of fan.
3. In-leaks / out-leaks in air conditioning systems also have a major impact on energy
efficiency and for power consumption and need to be minimized.
S-6 What is the effect of over-sizing of a pump?
The system Curves of pumps change according to the physical configuration of system. To
cite an example, the system curves depend upon height or elevation, diameter and length of
piping, number and type of fittings and pressure drops across various equipments like heat
exchanger.
System Curves are basically, a plot of system resistance which is the head to overcome by
the pump versus various flow rates.
S-7 What are the factors affecting cooling Tower performance?
ANS: THE factors affecting Cooling Tower performance are:
1. Capacity C
2. Range
3. Approach
4. Heat load
Capacity of a cooling tower plays a vital part. For example, a cooling tower sized to cool
4540 m3 per hour through a 13.90 C range might be larger than a cooling tower to cool the
same 4540 m3 per hour through a 19.50 range.
Range is determined not by cooling tower, but by the process it is serving.
Range in 0 C = Heat load in kcals per hour / Water Circulation Rate in LPH.
Heat load imposed on a cooling tower is determined by the process being served. The degree
of cooling required is controlled by the desired operating temperature level of the process.
The size and cost of the cooling tower is proportional to the heat load.
S-8 What are the following?
a) Luminaire b) Ignitor c) Lux d) Colour-rending Index e) Illuminance
ANS:
a) Luminaire is the device that distributes, filters or transforms the light from one or two
more lamps. It consists all parts needed for fixing and protecting the lamps, excepting
the lamps themselves.
b) Ignitor is used to start high intensity Metal Halide and Sodium Vapour lamps.
c) Lux is the illuminance produced by luminous flux of one lumen, uniformly distributed over
a surface area of one square metre
d) Colour rending Index is a measure of a measure of the degree to which the colours of
surfaces illuminated by a given light source confirm to those of the same surfaces under
a reference illuminant, giving suitable allowance for chromatic adaptation.
e) Illuminance is the quotient of flux incident on an element of the surface at a point of a
surface containing the point,by the area of that element. The illuminance provided by an
installation affects both the performance of the tasks and the appearance of the space.
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S-9 State the advantages of accepting Diesel Power Plants.
ANS:
1. Low installation cost
2. Short delivery periods and installation period
3. Higher efficiency (as high as 43-45%)
4. More efficient plant performance under part loads
5. Suitable for different type of fuels, such as low Sulphur and heavy fuel oil, in case of
large capacities
6. Minimum cooling water requirements
7. Adopted with air cooled heat exchanger in areas where water is not available.
8. Short start up time
S-10 How an Occupancy Sensor serves as an Energy Saver?
ANS: Occupancy-linked control can be achieved using infra-red, acoustic, ultrasonic or
microwave sensors, which detect either movement or noise in room spaces. These sensors
switch lighting on when occupancy is detected and off again after a set time period, when no
occupancy movement detected. They are designed to override manual switches and to
prevent a situation where lighting is left on in unoccupied spaces. With this type of system,
it is important to incorporate a built-in time delay, since occupants often remain still or
quiet for short periods and do not appreciate being plunged into darkness, if not constantly
moving around.
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Section iii- LONG DESCRIPTIVE QUESTIONS
Marks 5 x 10 =50
L-1 What are the components of tariff structure?
The tariff structure generally includes the following components:
a) Maximum demand Charges
These charges relate to maximum demand registered during month/billing period and
corresponding rate of utility.
b) Energy Charges
These charges relate to energy (kilowatt hours) consumed during month / billing period and
corresponding rates, often levied in slabs of use rates. Some utilities now charge on the
basis of apparent energy (kVAh), which is a vector sum of kWh and kVArh.
c) Power factor penalty or bonus rates, as levied by most utilities, are to contain reactive power
drawl from grid.
d) Fuel cost adjustment charges as levied by some utilities are to adjust the increasing fuel
expenses over a base reference value.
e) Electricity duty charges levied w.r.t units consumed.
f) Meter rentals
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g) Lighting and fan power consumption is often at higher rates, levied sometimes on slab basis or
on actual metering basis.
h) Time Of Day (TOD) rates like peak and non-peak hours.
i) Penalty for exceeding contract demand
j) Surcharge if metering is at LT side in some of the utilities
L-2 What are the effects of Harmonics on motor operation and performance?
Harmonics increase motor losses, and can adversely affect the operation of sensitive auxiliary
equipment. The non-sinusoidal supply results in harmonic currents in the stator which increases
the total current drawn. In addition, the rotor resistance (or more precisely, impedance)
increases significantly at harmonic frequencies, leading to less efficient operation. Also, stray
load losses can increase significantly at harmonic frequencies. Overall motor losses increase by
about 20% with a six-step voltage waveform compared to operation with a sinusoidal supply. In
some cases the motor may have to be de-rated as a result of the losses. Alternatively,
additional circuitry and switching devices can be employed to minimize losses.
Instability can also occur due to the interaction between the motor and the converter. This is
especially true of motors of low rating, which have low inertia. Harmonics can also contribute to
low power factor.
L-3 Explain the simple method of capacity assessment of air compressors
Explain the simple method of capacity assessment of air compressors.
 Isolate the compressor along with its individual receiver being taken for test from main
compressed air system by tightly closing the isolation valve or blanking it, thus closing the
receiver outlet.
 Open water drain valve and drain out water fully and empty the receiver and the pipe line.
Make sure that water trap line is tightly closed once again to start the test.
 Start the compressor and activate the stop watch.
 Note the time taken to attain the normal operational pressure P2 (in the receiver) from initial
pressure P1.
 Calculate the capacity as per the formulae given below :
Actual Free air discharge
Q
P2  P1 V
 NM 3 / Min.
P0
T
Where
P2
=
Final pressure after filling (kg/cm2 a)
P1
=
Initial pressure (kg/cm2a) after bleeding
P0
=
Atmospheric Pressure (kg/cm2 a)
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V
=
Storage volume in m3 which includes receiver,
after cooler, and delivery piping
T
=
Time take to build up pressure to P2 in minutes
The above equation is relevant where the compressed air temperature is same as the ambient
air temperature, i.e., perfect isothermal compression. In case the actual compressed air
temperature at discharge, say t2 0C is higher than ambient air temperature say t1 0C (as is usual
case), the FAD is to be corrected by a factor (273 + t1) / (273 + t2).
L- 4 In an automobile industry one compressor of rated capacity of 1000 cfm is operated
to evaluate leakage quantity in the plant during a holiday when no equipment was using
compressed air. FAD test was also carried out before conducting leakage test and found
that the compressor is delivering out put of 90% of rated capacity.
The observations on leakage test are:
a) Compressor was on load for 08 minutes
b) Compressor was unloaded for 48 minutes
c) Compressor was consuming 150 kW
Evaluate
a) Free air delivery b) Specific power consumption
c)
% leakage in compressed air system d) Leakage quantity
d) Power lost due to leakage
Compressor capacity
a.
b.
c.
:
1000 cfm
Operating capacity (FAD)
:
90% of rated capacity
:
0.90 x 1000 cfm
:
900 cfm
Actual power consumption
:
144 kW
Actual output
:
900 cfm
Specific power consumption
:
900/144
:
6.25 cfm / kW
:
08 minutes
Specific power consumption
% Leakage in the system
Load time (T)
Un load time (t)
:
48 minutes
% leakage in the system
:
Tx 
x 100
(T  t )
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d.
e.
Leakage quantity
Power lost due to leakage
:
08
x 100
(08  48)
:
14.2%
:
0.142 x 900
:
127.8 cfm
:
Leakage quality
Specific energy
:
127.8
6.25
:
20.45 kW
L-5 Why variable torque loads offer greatest energy savings? Explain electronic methods
of
speed controllers.
In variable torque applications, Affinity law holds good which defines the relationships between
speed, flow, torque, and horsepower illustrated below:
1.
Flow is proportional to speed
2.
Head is proportional to square of the speed
3.
Torque is proportional to square of the speed
4.
Power is proportional to cube of the speed
Variable torque loads include centrifugal pumps and fans which make up the majority of large
plant loads.
Thus, theoretically, for 50% of full load requirement the power input needed would be less than
13% of full load power. In a centrifugal pump when the flow discharge is reduced to 70% of the
rated discharge a savings of 65 percent in power is theoretically possible.
To achieve this savings, Solid state electronic VSDs are used to control the voltage and
frequency delivered to the motor to change its speed. However, in practice the savings will
depend upon the type and efficiency of variable speed drive employed and other system losses.
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