Barch BS Q&A Papers 1
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Transcript of Barch BS Q&A Papers 1
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MAY/JUNE 2013.
PART-A
1. What do you mean by evao!at"on and #ub$"mat"on%
Evao!at"onis the process of a substance in a liquid state
changing to a gaseous state due to an increase in
temperature and/or pressure. Evaporation is a fundamental
part of the water cycle and is constantly occurring
throughout nature. During Evaporation water changes from a
liquid to a gas or vapor.
&ub$"mat"onis the transition of a substance directly from
the solid to the gas phase without passing through an
intermediate liquid phase.
2. What do you mean by #en#"b$e heat and $atent heat%
&en#"b$e heat 'When an object is heated, its temperature rises as heat is
added. The increase in heat is called sensible heat. imilarly,
when heat is removed from an object and its temperaturefalls, the heat removed is also called sensible heat. !eat that
causes a change in temperature in an object is called
sensible heat.
(atent heat
!eat that causes a change of state with no change in
temperature is called latent heat. "ll pure substances in
nature are able to change their state. olids can becomeliquids #ice to water$ and liquids can become gases #water to
vapor$ but changes such as these require the addition or
removal of heat. The heat that causes these changes is
called latent heat.
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3. )e*ne ton !e+!",e!at"on.
The unit of measure used in air conditioning to describe thecooling capacity of a system. %ne ton of cooling is based on
the amount of heat needed to melt one ton of ice in a &'
hour period. %ne ton of cooling is equal to (&,))) *tu/hr or
+)&' cal/hr.
. )!a the T-# d"a,!am o+ a vao!# om!e##"on y$e
and "nd"ate $ea!$y the va!"ou# !oe##e#.
" Temperature-Entropy diagram, or T-s diagram, is used inthermodynamics to visualie changes to temperature and specic
entropy during a thermodynamic heat transfer process or cycle.
Temperature indicates the degree or intensity of heat present in a
substance. Entropy is a measure of the energy available for wor0.
"lmost all 1efrigeration and "ir-conditioning equipment use vapor-
compression cycles cycle to produce cooling. The 2apor-
3ompression 1efrigeration 3ycle is comprised of four steps.
" wor0ing 4uid #often called the refrigerant$ is pushed through the
system and undergoes state changes #from liquid to gas and
bac0$. The latent heat of vaporiation of the refrigerant is used to
transfer large amounts of heat energy, and changes in pressure
are used to control when the refrigerant e5pels or absorbs heat
energy.
(6 3ompression6
7n this stage, the refrigerant enters the compressor as a gas
under low pressure / low temperature. Then the refrigerant is
compressed and the gas leaves the compressor under high
pressure / high temperature.
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&6 3ondensation6
The high pressure, high temperature gas releases heat energy
and condenses inside the 3ondenser. Thus the refrigerant leaves
as a high pressure / high temperature liquid.
+6 E5pansion # Throttling $6
The high pressure / high temperature liquid refrigerant is pushed
through a E5pansion #throttling $ valve, which causes it to e5pand.
"s a result, the refrigerant is now low pressure / low temperature
liquid.
'6 Evaporation
The low pressure / low temperature liquid refrigerant enters the
evaporator. The refrigerant now cools the air or water surrounding
the evaporator. "s it has pic0ed up the heat the liquid is now
converted to gas. The refrigerant leaves the evaporator as a low
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temperature / low pressure gas and is ta0en into the compressor
again, bac0 at the beginning of the cycle.
. What "# meant by hum"d"*at"on and
dehum"d"*at"on%
!umidication 6 The process of increasing the water vapour /
moisture content of air.
De-!umidication 6 The process of decreasing the water vapour /
moisture content of air.
. ("#t don the advanta,e# o+ +an o"$ heat"n, #y#tem#.
7f multiple smaller ones have to be heated then there are
two methods of doing it.
"ll air heating system 6 We can heat air in one common place
and distribute the hot air to various ones by means of air
distribution syste.
"ll water heating system 6 We can heat water in one
common place, then pump the hot water to various ones
through pipes. These pipes will be connected to each 8an coil
unit installed in each one. The fan coil unit consists of lter,
blower, heating coil and controls. The hot water enters the
heating coil and in turn heats the air coming into contact
with coil.
"dvantages 6
Each one can be controlled individually. 7f a particular one
is not utilied then the hot water will not enter the fan coil
and thus the one is not heated. This will turn ensure that
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hot water production is lowerd based on the usage. Thus
power saving is ensured during part load operations.
This is not possible in "ll air heating system. Elaborate
controls li0e variable air volume and variable frequencydrives will have to be incorporated.
"ll water system do not use ducting which occupies
considerable space above false ceiling. They also don9t need
an air !andling :nit 1oom which again occupies space.
7t is easier to control water than air and during part load
operations the response time for water systems is faster
than air system.
8"; 3%7< =7=7;> ?TE@
8"; 3%7< :;7T
PART-
. 4a5. E6$a"n the o!7"n, o+ E$et!" moto!.
"n electrical motor is an electromechanical device which converts electrical
energy into a mechanical energy. "lmost all the motors used in !2"3
application is 7nduction @otor. The name coming from the fact that the
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current in the moving part is induced, the moving component having no
connection to the source of current.
"n "3 motor has two basic electrical parts6 a AstatorA and a ArotorA as shown
in 8igure . The stator is in the stationary electrical component. 7t consists of a
group of individual electro-magnets arranged in such a way that they form a
hollow cylinder, with one pole of each magnet facing toward the center of
the group.
The term, AstatorA is derived from the word stationary. The stator then is the
stationary part of the motor. The rotor is the rotating electrical component. 7t
also consists of a group of electro-magnets arranged around a cylinder, with
the poles facing toward the stator poles.
The rotor, obviously, is located inside the stator and is mounted on the
motorBs shaft. The term ArotorA is derived from the word rotating. The rotor
then is the rotating part of the motor. The objective of these motor
components is to ma0e the rotor rotate which in turn will rotate the motor
shaft. This rotation will occur because of magnetic phenomenon that unli0e
magnetic poles attract each other and li0e poles repel. 7f we progressively
change the polarity of the stator poles in such a way that their combined
magnetic eld rotates, then the rotor will follow and rotate with the magnetic
eld of the stator.
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48!5
#b$. Write notes on
". A"! 9and$"n, Un"t.
"n "ir-!andling :nit #"!:$, is a device used to condition and circulate air aspart of a !2"3 system. The primary function of an "!: is to transmit
processed air from the air conditioning plant to the conditioned space and
distribute it properly within the conditioned space.
"n "!: is usually a large modular metal bo5 containing various sections
li0e 6
8ilter ection with pre-lters
3ooling 3oil ection with copper tubes and aluminium ns.
*lower section with 3entrifugal fans and @otor.
"ccessories include drain pan, dampers , 28D, tarters, 2ibration
isolators etc
The air is rst passed through lters to remove dust particles and then over
to the cooling coils wherein the air is cooled and de-humidied. The blowers
then convey the pressuried air to the ducts which in turn distributes to the
area to be air-conditioned through grilles and diCusers..
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8
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;"T:1"< D1"8T 3%% T%WE1@E3!";73"< D1"8T 3%% T%WE1
;. 4a5. )"#u## "th neat #7ethe# the "ndo tye and
a7a,ed tye ond"t"one!#.
W
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side are separated from each other by an insulated partition
enclosed inside the window air conditioner assembly.'. Window air conditioners are mainly used for residences, small
oces and shops. "dvantages are easy to install and cheaper.
Disadvantages are no fresh air and not suited for large areas. The
units are also noisy.
W7;D%W "713%;D7T7%;E13!E@"T73 T%= 27EW
PA:=A>E) A
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"dvantages are 6
:nits are rugged
@ultiple units can be used for large areas
Easy and fast installation
3an handle long duct runs and higher air quantity requirements
ervices are restricted to the unit room.
Disadvantages 6
@ultiple units for required for larger applications.
8loor space required for locating the units.
="3">ED "3 F 7;D%%1 :;7T
="3">ED "3 F %:T D%%1 :;7T
48!5
#b$. Write note on compressors used in refrigerators.
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The compressors used for 1efrigerators are broadly called as G
!ermetically ealed 3ompressors G. 7n this compressor both the
compressor and motor are conned in a single outer welded steel
shell. The motor and compressor are directly coupled on the same
shaft, with the motor inside the refrigeration circuit.
The main advantages are 6
;o refrigerant lea0.
"ll components are factory assembled hence quality can be
maintained.
;o servicing is possible at site hence chances of meddling at
site is eliminated.
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There are two scrolls wherein one of the scrolls is 5ed, while the
other orbits eccentrically without rotating, thereby trapping and
compressing poc0ets of refrigerant gas between the scrolls.
1E37=1%3"T7;> 1%T"1?
31%
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"dvantages 6 " DH plant is most ecient from the thermodynamic point of
view since the heat transfer is directly between the refrigerant and air. 8irst
cost is lower. impler to operate and service.
Disadvantages 6 Ducts running from one 4oor to another 4oor occupies huge
4oor space. =art load operations are very poor. The plant and "!: have to be
0ept close as the gas cannot travel more than & @eters.
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:ENTRA( )@ P(ANT :ENTRA( :9
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The dimensions of the area to be air-conditioned.
*uilding orientation.
:sage activity of the area to be air-conditioned.
:sage activity of the adjoining areas.
!eight / *eam !eight. 8resh quantity to be provided based on 2entilation
standards.
>lass area and type of glass in all orientation.
E5posed Wall area and type of construction in all orientation.
;on-e5posed # called as partition wall $ and type of
construction.
7f the 4oor below / ceiling above / roof above is non-
airconditioned then heat from these sources have to beconsidered. The type of contruction also has to be ta0en into
account.
%ccupancy.
%utside air
The ne5t step is to nd the air quantity required from this system
to maintain the inside design conditions.
8or this we need to do the following calculations 6
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tep-( 6 E1! is already 0nown.
tep-& 6 8ind the sum of E1! I E1
;ow nd the ratio of tep-( divided by tep-&. This ratio is
called ensible !eat 8actor F !8.
Then from the tables, based on the 1oom temperature,1oom humidity and !8 we can nd the "pparatus Dew
=oint F "D=.
*ased on E1! , 1oom Temperature and "D= we can nd the
"ir Juantity required in 3ubic 8eet per @inute # 38@ $.
8inally we have to select the air-conditioner which satises
both the 3apacity in T1 and "ir quantity in 38@.
7f only the T1 is ta0en into account and not the 38@ then the
inside conditions cannot be maintained.
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