8/18/2019 Thermal Engg II Mid II Paper

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1 (a):  Derive an expression for work done by multistage reciprocating air compressor.

(b):  Show that the volumetric efficiency for reciprocating air compressor is given

ηv=1+ K − K ( p2 p1 )1

n

, where all terms having usual meaning.

2 (a): A single cylinder, single acting RAC has a piston displacement of .!m " at ! rpm. #he suction pressure and temp

!bar and $%C respectively and the delivery pressure is & bar calculate 'i( #emperature at the end of compression. 'ii( )sen

rk re*uired if y+ !. 'iii( -olytropic work re*uired if n+ !.$'iv( )sothermal efficiency in each case. Assume R+ " m/

glect clearance.

'b): 2hat do you mean by positive displacement type compressor3 4xplain Roots 5lowers type in detail.

3. (a) A single acting two stage compressor with complete intercooling delivers ! 0g/min of air at !6 bar. #he suction occuand !7 C. #he compression and expansion process are reversible with index n+ !.$7 calculate 'i( #he power re*uired 'i

thermal efficiency 'iii( #he free air delivery 'iv( 8eat transfer in intercooler.

(b): Compare between the centrifugal compressor and axial flow compressor.

4. (a): 2hat do you understand by surging and choking phenomenon3

(b): 4xplain centrifugal compressor and draw the sample velocity diagram at inlet and exit.

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1 (a): Discuss Reversed Carnot cycle for refrigeration and explain why it is not a practical cycle.

(b): )n a standard vapour compression refrigeration cycle, operating between an evaporator temperature of 1!  oC and con

mperature of oC, the enthalpy at the end of compression is $$ 0:/0g. show the cycle on #1s and h1s plot. Also calculate Ccycle, refrigeration capacity 'effect(, Compressor power. Assume mass flow rate of refrigerant as ! 0g/min. -roperty tab

on1!$ is given below=

 P (MPa) hf  (KJ/Kg) hg (KJ/Kg)

1! .$!>! $6.&7 !&".!

.>6% %.7" $".!

2 (a): )n Ammonia based simple vapour compression refrigeration cycle 'S.?.C.R.C( the temperature range in compressor is fr

to 1!7 oC. Assume compression to be isentropic @ no sub cooling of refrigerant takes place. )n wet compression vapour leav

mpressor as dry @ saturated refrigerant. Assuming a Actual C;- of 6$ of the theoretical, calculate the power re*uired to dr

mpressor. #ake mass flow rate !.7 0g/s. Refer the table of 8" given below=

0g(

?apour 

!". >>

!"!>.$$

: 4xplain simple vapour compression refrigeration cycle based on 'i( 2orking -rinciple 'ii( Schematic Diagram 'iii( #1s Diag

4nergy Analysis.

3 (a): refrigeration unit function between 17 oC @ $7 oC. #he working fluid is ammonia has a dryness fraction of .6$ at the empressor. )f the machine has a relative efficiency of 7 calculate the amount of ice formed during a period of $ hours. #he

be formed at oC from water at !7 oC and 6. 0g/min of 8" is circulated having latent heat of ice "67 0:/0g. Refer the

ow=

aturated Temp (oC) !trop" of #$%u$d Sf (KJ/Kg&K) !tha#p" of '$%u$d hf (KJ/Kg) 'ate!t heat (KJ/K

17 .6" !7&.$ !$&.&

$7 !.!$ $>&.> !!6%.!

 (b): 4xplain the effect of sub cooling and superheating on C;- of refrigerator in S?CRC.

4 (a): 2hat do you mean by wet and dry compressions explain with the help of #s and -h diagram.(b): In a Freon-12 based refrigeration cycle with mass ow rate of 0.192 kg/s, the temerat!

ndenser is "0 o# $ e%aorator oerating at -20 o#. &he %ao!r lea%es the e%aorator as dry $ sat!

h no !nder cooling. #alc!late the ower re'!ired refer to the following table

Saturated Temperature Ts (oC) hf 

(KJ/Kg)

hg

(KJ/Kg)

Sf 

(KJ/Kg-K)

Sg

(KJ/Kg-K)

Cp

(KJ/Kg

-20 o# 1(.) 1().*1 0.0(+ 0.(0)2

"0 o#(".+ 20+.0 0.2(1* 0.*)2 0.("