mec551 (1).pdf

CONFIDENTIAL EM/JAN 2013/MEC551

UNIVERSITI TEKNOLOGI MARA FINAL EXAMINATION

COURSE

COURSE CODE

EXAMINATION

TIME

THERMAL ENGINEERING

MEC551

JANUARY 2013

3 HOURS

INSTRUCTIONS TO CANDIDATES

1. This question paper consists of two (2) parts : PART A (5 Questions) PART B (3 Questions)

2. Answer ALL questions from PART A and two (2) questions from PART B in the Answer Booklet. Start each answer on a new page.

3. Do not bring any material into the examination room unless permission is given by the invigilator.

4. Please check to make sure that this examination pack consists of:

i) the Question Paper ii) an Answer Booklet - provided by the Faculty iii) a Property Tables Booklet - provided by the Faculty iv) a two - page Appendix 1

DO NOT TURN THIS PAGE UNTIL YOU ARE TOLD TO DO SO

This examination paper consists of 8 printed pages

© Hak Cipta Universiti Teknologi MARA CONFIDENTIAL

CONFIDENTIAL 2 EM/JAN 2013/MEC551

PART A

QUESTION 1

A suitable insulation material and thickness for a furnace door is to be determined from the available materials listed in Table Q1. The insulation needs to restrict the heat loss to a maximum of 1000 W/m2. The interior surface of the door consists of two metal sheets; the interior surface is a 10 mm Inconel 600 plate (k = 25 W/m.°C) while the outer surface is a 25 mm stainless steel 316 plate (k = 29 W/m.°C).

Between the metal plates, a suitable insulation is to be placed with the right thickness as a design safety requirement. The effective gas temperature inside the furnace is 1200°C with a heat transfer coefficient of 50 W/m2.K within the inner wall. The heat transfer coefficient between the outer surface of the door and ambient is 12 W/m2.K. The ambient temperature is 25°C.

(10 marks)

Hot gasses

Inconel 600 plate

Ambient

INSULATION Stainless steel plate

Figure Q1: General layout of the system

Table Q1: Available materials for selection

Material

Zirconia powder

Mineral fibre Alumina-silica Vermiculite

Max Operating Temperature

980°C

700°C 1260°C 960°C

Average conductivity (W/m.°C) 300°C - 750°C

0.20

0.12 0.17 0.13

751°C-1250°C 0.26

-

0.27 0.32



QUESTION 2

Glycerin flows over a 40 m long heated flat plate at free stream conditions of 10 m/s and 20°C. If the plate temperature is held constant at 50°C, determine the hydrodynamic and thermal boundary layer thicknesses at the middle of the plate, as well as the total heat flux from the surface per unit width.

(10 marks)

Table Q2: Properties of glycerin

T(°C)

20

35

50

Pr

12500

3711

1630

u(xi(r*m2/s) 11.8

3.5

1.5

k (W/m.°C)

0.286

0.2864

0.2870

P (kg/m3)

1264

1255

1245

QUESTION 3

a) Heat exchanger performance normally deteriorates after a certain period of time. Explain in detail the reason for this phenomenon and how it is represented in heat exchanger analysis. Give an example of that phenomenon in the chemical industry.

(4 marks)

b) A counter-flow double-pipe heat exchanger is used to heat liquid ammonia (cp= 4.43 kJ/kg°C) from 10°C to 30°C with hot water (cp= 4.18 kJ/kg°C) that enters the heat exchanger at 60°C and exit at 30°C. The total area for heat exchanger is 30 m2

and overall heat transfer coefficient is 800 W7m2oC. Calculate the flow rate of ammonia and hot water in this process.

(6 marks)



QUESTION 4

Refrigerant-134a enters the compressor of a refrigerator as superheated vapor at 0.14 MPa and -10°C at a rate of 0.08 kg/s and leaves at 0.7 MPa and 50°C. The refrigerant is cooled in the condenser and leaves at 24°C. Then, it is throttled to 0.14 MPa by an expansion valve. Sketch the process on a T-s diagram and determine:

i) the power input to the compressor (hp),

ii) the isentropic efficiency of the compressor (%),

iii) the steam fraction at the inlet of the evaporator (%), and

iv) the COP of the system if it operates as a heat pump.

Note: 1 hp = 0.746 kW (10 marks)

QUESTION 5

Air is heated as it flows inside a 5 cm diameter pipe. The heat is supplied at a rate of 1.32 kW from a condenser. The air condition at the inlet is 200 kPa, 20°C, 100% relative humidity and 10 m/s. Calculate the relative humidity and the temperature of the air at the exit.

(10 marks)



PARTB

QUESTION 1

A natural gas fuel composed of ethane and methane gases of equal volume is supplied to a steam turbine power cycle. The combustion air and fuel is assumed at standard reference states while the combustion products were measured at 1000K. The steam exhaust from the turbine, flowing at 100 litres/min and 227°C, is channelled into a condenser. Water coolant flows into the condenser at 240 litres/min and 20°C. The steam is cooled until fully saturated liquid and its pressure is assumed constant at 1 atm.

The condenser is a typical one-shell pass and 8-tube pass heat exchanger while the copper tubes are 10 cm in diameter with negligible thickness.

Determine:

a) the heat generated per kg of fuel from the combustion process, and

b) the minimal length of the heat exchanger to achieve the required cooling process by strictly using the effectiveness-NTU method.

The overall heat transfer coefficient within the condenser is evaluated at 1400 W/m2.°C.

The density of both working fluids is simplified at 1000 kg/m3, while the specific heat at constant pressure for steam is 4.31 kJ/kg.°C and for cooling water is 4.18 kJ/kg.°C.

The formulation to calculate NTU for this shell and tube heat exchanger is

vi + c2 \ j_ i_ c + vTT?y (25 marks)



QUESTION 2

A cast iron pipe (k = 80 W/m.°C) whose inner and outer diameters are Di = 10 cm and D2 = 10.8 cm, respectively flows steam at TX]=320°C. The convection heat transfer coefficient inside the pipe is hi = 60 W.m2.°C. The pipe is covered with 3 cm thick glass wool insulation with k = 0.05 W/m.°C. Heat is lost to the surroundings at Tx2 =30°C due to convection heat transfer. The pipe is subjected to a cross wind of 3 m/s.

a) Calculate the rate of heat loss for every 1 meter of the steam pipe and also the temperature on the outer surface of the insulation. (You may use properties of air at 40°C as given in the table below)

(20 marks)

b) If you are required to decrease the rate of heat loss, would you increase or decrease the thickness of the insulation? Do you expect the temperature on the outer surface of the insulation to increase or decrease with the change that you propose? Comment on the values of the conduction resistance provided by the insulation and the convection resistance on the outside surface when the thickness of the insulation is increased.

(5 marks)

air \(C J J J — insulation

pipe

Figure Q2

Temperature (°C)

40

i mai Conductivity, k

(W/m.°C)

0.02662

Density, p (kg/m3)

1.127

Dynamic Viscosity, u

(kg/m.s)

1.918 x10"5

Kinematic Viscosity, u

(m2/s)

1.750 X10"5

Prandtl Number,

Pr

0.7255

A ^ = 0.3-0.62R e^Pr^

l+(u.4/Pr)^ K 1+

( Re 28200^


CONFIDENTIAL EM/JAN 2013/MEC551

QUESTION 3

© Condenser

Expansion Valve

®

r3=2o°c

Heating coils

Compressor

Evaporator "®

Refrigerator Working Fluid (R134a)

T, = 38°C T«b,i = 26 °C P = 101.325 kPa

S,i t = 0.5 kg/s

V Heating Section

Air Conditioning Process and Refrigeration Cycle

V Cooling Section

Figure Q3

Figure Q3 shows an ideal refrigeration cycle using refrigerant 134-a as the working fluid and operates between 0.2 MPa and 1.0 MPa. The refrigeration cycle is used to cool an air flow in the cooling section of an air conditioning process. The air enters the cooling section at the rate of 0.5 kg/s and 1 atm of pressure. Measured dry bulb temperature and web bulb temperature is shown at 38°C and 26°C, respectively. The air is cooled until it is condensed at 15°C. The condensed water flows out from this section. Then, the air is heated in the heating section until it reaches 20°C.

Sketch the temperature-entropy diagram and pressure-enthalpy diagram for the refrigeration cycle. Using the psycrometric chart to assist in analysis, and assuming there is no entropy generated in the vapour-compression process, calculate:

a) the rate of heat rejection from the cooling section (kW),

b) the condensed water flow rate if the condensed water is 15°C (kg/s),

c) the exit relative humidity, 03 and the heating capacity in the heating section if the exit temperature is 20°C,

d) the refrigerant mass flow rate (kg/s),

e) the rate of heat rejection into environment from the refrigeration cycle (kW),

f) the Coefficient of Performance (COP) of the refrigerator, and



g) if the air inlet temperature is changed (increase or decrease) at the constant relative humidity and mass flow rate, how can the exit condition be maintained?

END OF QUESTION PAPER


CONFIDENTIAL APPENDIX 1(1) EM/JAN 2013/MEC551

REFERENCE FORMULA'S IN THERMAL ENGINEERING ANALYSIS

Q = -k-A AT Ax Q = h-AAT

Q = s-(7-A-T4 <7 = 5.67x10 -8 w m2-K*

1 &L a dt

(d2T d2T d2T} • + -

dxl dyz dz1 k

a dt ~ r dr\ dr) r2 dG2 8z2 k

j_ar_j_ e_ a dt r2 dr

{ 2dT\ 1 r + •

( . ndT\ 1 d2T q'

V dr J r'sinO d0{ dO ) rlsin' 0 df k

R = — , # = —,J? hA kA

lnl S . h) R_ Tx-T2

2nL-kx ' <J-£-F^2(T*-T24)

UA IX hL Kt = v_±=e^A,^J_A

M v

S = 5-x

S,= ' 1.026

•Pr •X

0.382 x

Re 1/5 A = 4- A.

NuL = 0.664 Re05 PrI/3 0.8 T , _ ] / 3 NuL = 0.037 Reu 8 Pr

^ = ( 0 . 0 3 7 Re -871)Pr .1/3 GrL- -2

Ra = GrYx K = - m P-A


CONFIDENTIAL APPENDIX 1(2) EM/JAN 2013/MEC551

• AT Q = — = UAAT

R

_AT2-AT,

+ Q = rhc-Cpc-(TC0Ut-Tcin)

-Q = ™h-Cph-(Thin-Th0Ul)

z-' max

NTU=A''U

r

m AF= mr

mjuel

fi=*c=I(V/)-ZW)

7 _K-\

m P a = ̂ = 0.622^-

™0 ^a

h-ha+ ahg

a _cfi2+Tx) + co2hfgi

K~hh

mal+ma2=ma3

co}ma] + a2ma2 = co3mai

™aA+ma2h2 =ma3h3

Q = U-AATm

^Tm=F-ATLMrD, counter-flow

r c= mln

r max

b =c (T ~T ) i imax min V hjn c,m)

-L.-L.-Ml. y> P V N

m m m

AF (fy _ actual

AF stoic

H = Y,Ni(h°f+K-h°)i

w —- — h2-\ m

COP = S-

mv Pv o>P

mg Pg (0.622 + co)Pg

mf -ma{co2 -a\)

0.622Pe ft)9 = —

P -P

ma\ _ a>2-co3 _h2-h3

ma2 a^-CDy h^-hx


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