assignment thermal uitm

8/10/2019 Assignment Thermal UiTM

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1.0 INTRODUCTION

Thermal engineering is a field of engineering which is deal with the heat transfer; heating and

cooling process. Its become the important field of engineering that give a lot of benefit to

mankind. The basic three laws of thermodynamics is a based foundation in thermal

engineering. Every calculation and solution must obey to these laws.

Heat conduction, heat convection, heat exchanger, and refrigeration cycle are topics

which are discuss in thermal engineering. Heat conduction is heat transfer between the more

energetic particles to the less energetic particle in contact. There are three ways to solve the

problem in heat conduction. There are thermal circuit which is used to solve one dimensional

problem, differential equations and numerical equations.

Next, heat convection is heat transfer between fluids in motion with a surface. Heat

convection is divided into two types which are force convection and natural convection. Force

convection can be divided into two situations; flat surface and pipe. Nusselts Number,

Reynolds Number, Pranatls Number, Grashofs Number, and Reyleigh Number are widely

used in this chapter.

Heat exchanger is a device that facilitates the exchange of heat between two fluids that

are at different temperatures while keeping them from mixing with each other. In other word,

fluid is used to heating or cooling the other fluid. Log mean temperature difference (LMTD)

and effectiveness- NTU method are used in heat exchanger.

Refrigeration cycle is the gas refrigeration cycle in which the refrigerant remains in the

gaseous phase throughout. The main purpose is to drop the temperature from high to low

temperature. This situation will not obey the Zeroth Law of thermodynamics which is heat

will transfer from high temperature to low temperature. Thus, Reverse Carnot Cycle by totally

reversible from Carnot Cycle which consist isothermal and isentropic process invented to

archive this purpose.

Air conditioning system is another chapter in thermal engineering. Air conditioning

process is a series of processes of treating air to simultaneously control the air temperature,

humidity, cleanliness, and distribution to meet the comfort requirement of the occupants in a

space. Generally, the comfort temperature range is 22C to 27C while thecomfort humidity

range is 40%RH to 60%RH.


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Thermal Engineering (MEC551) subject is required us as a student to do the

assignment or case study about air conditioning. We are divided into several groups of 3

persons to complete the task. This task needs us to cooperate and do it as a team. We had

discussed together and find out the solution for this problem to conduct preliminary design

calculations of an air conditioning system. Based on syllabus that we had studied all the topics

mention above in the class, we had referring other thermal books, handbooks, and internet

sources to finish this assessment.


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2.0 APPLICATION IN ENGINEERING PRINCIPLE AND CONCEPT

Refrigeration and air conditioning is used to cool products or a building environment. The

refrigeration or air conditioning system (R) transfers heat from a cooler low-energy reservoir

to a warmer high-energy reservoir.

There are several heat transfer loops in a refrigeration system as shown in Figure 2. Thermal

energy moves from left to right as it is extracted from the space and expelled into the outdoors

through five loops of heat transfer:

i. I ndoor air loop. In the left loop, indoor air is driven by the supply air fan through a

cooling coil, where it transfers its heat to chilled water. The cool air then cools the

building space.

ii. Chi ll ed water loop. Driven by the chilled water pump, water returns from the cooling

coil to the chillers evaporator to be re-cooled.

iii. Refr igerant loop. Using a phase-change refrigerant, the chillers compressor pumps

heat from the chilled water to the condenser water.

iv. Condenser water loop. Water absorbs heat from the chillers condenser, and

thecondenser water pump sends it to the cooling tower.

v. Cooli ng tower loop. The cooling towers fan drives air across an open flow of the hot

condenser water, transferring the heat to the outdoors.

Air-Conditioning Systems

Depending on applications, there are several options / combinations of air conditioning, which

are available for use:

a) Air conditioning (for space or machines)

b)

Split air conditioners

c) Fan coil units in a larger system

d) Air handling units in a larger system


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Refrigeration Systems (for processes)

The following refrigeration systems exists for industrial processes (e.g. chilling plants) and

domestic purposes (modular units, i.e. refrigerators):

i.

Small capacity modular units of the direct expansion type similar to domestic

refrigerators.

ii. Centralized chilled water plants with chilled water as a secondary coolant for a

temperature range over typically 5 oC. They can also be used for ice bank formation.

iii. Brine plants, which use brines as a lower temperature, secondary coolant for typically

sub- zero temperature applications, which come as modular unit capacities as well as

large centralized plant capacities.

iv. The plant capacities up to 50 TR (tons of refrigeration) are usually considered as small

capacity, 50250 TR as medium capacity and over 250 TR as large capacity units.

A large company may have a bank of units, often with common chilled water pumps,

condenser water pumps, cooling towers, as an off-site utility. The same company may also

have two or three levels of refrigeration and air conditioning such as a combination of:

i. Comfort air conditioning (2025 oC)

ii. Chilled water system (80100 C)

iii.

Brine system (sub-zero applications)

Vapour Compression Refrigeration System

Compression refrigeration cycles take advantage of the fact that highly compressed fluids at a

certain temperature tend to get colder when they are allowed to expand. If the pressure change

is high enough, then the compressed gas will be hotter than our source of cooling (outside air,

for instance) and the expanded gas will be cooler than our desired cold temperature. In this

case, fluid is used to cool a low temperature environment and reject the heat to a high

temperature environment.

Vapor compression refrigeration cycles have two advantages. First, a large amount of thermal

energy is required to change a liquid to a vapor, and therefore a lot of heat can be removed

from the air-conditioned space. Second, the isothermal nature of the vaporization allows

extraction of heat without raising the temperature of the working fluid to the temperature of

whatever is being cooled. This means that the heat transfer rate remains high, because the


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closer the working fluid temperature approaches that of the surroundings, the lower the rate of

heat transfer.

Vapour Absorption Refrigeration System

The vapour absorption refrigeration system consists of:

i.

Absorber: Absorption of refrigerant vapour by a suitable absorbent or adsorbent,

forming a strong or rich solution of the refrigerant in the absorbent/ adsorbent

ii. Pump: Pumping of the rich solution and raising its pressure to the pressure of the

condenser

iii. Generator: Distillation of the vapour from the rich solution leaving the poor solution

for Recycling

The absorption chiller is a machine, which produces chilled water by using heat such as

steam, hot water, gas, oil etc. Chilled water is produced based on the principle that liquid (i.e.

refrigerant, which evaporates at a low temperature) absorbs heat from its surroundings when it

evaporates. Pure water is used as refrigerant and lithium bromide solution is used asabsorbent.

Heat for the vapour absorption refrigeration system can be provided by waste heat extracted

from the process, diesel generator sets etc. In that case absorption systems require electricity

for running pumps only. Depending on the temperature required and the power cost, it may

even be economical to generate heat / steam to operate the absorption system.


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3.0 INTEGRATION MATHEMATICAL SOLUTIONS

A. Air-conditioning process

Provide a sketch of the airconditioning processes with the ambient pressur e of 100kpa.

Determine the requi red heat extraction rate at the cooling rate and heating r ate when the

ambient ai r enters at 35C and 70C of relative humidi ty and leave the system at 20C.

Assumptions:

a) This is a steady-flow process and thus the mass flow rate of dry air remains constant

during the entire process.

b) Dry air and the water vapor are ideal gases.

c)

The kinetic and potential energy changes are negligible.

T2= 10

2= 100%

Evaporator

12

Condensat

Condensate

Water

T1= 35C

1= 70%

P = 100 kPa

38,000 cfm

T3= 20C

3


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STATE 1:

From the Psychrometric chart and A-4 table,

h1 = 100.0 kJ / kg dry air

1 = 0.0253 kg H2O / kg dry air

()

v1= 0.9202 m3/ kg dry air

1 cfm = 0.02831 m3/min

38000 cfm = 1075.78 m3/min = 18 m3/s = V

Psat= 5.6291 kPa

STATE 2:

Psat= 1.2281 kPa

h2 = 29.6 kJ/kg dry air

2= 0.0078 kg H2O/kg dry air.

STATE 3:

Since 3 = 2= 0.0078 kg H2O/kg dry air

CALCULATION:

mw= ma (1-2)

ma= V1/ v1

ma = 19.56 kg/s


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mw= 0.3423 kg/s

Qdot.out= ma(h1-h2)mwhw

hw= hf @ Tcondensate

hf= 42.022 kJ/kg at T = 10C

Qdot.out= ]Qdot.out= 1362.62 kW

Qdot.in= ma(h3-h2)

h3= (1.005)(20) + (0.0078)(2537.4)

h3= 39.892 kJ/kg

Qdot.in= 19.56 (39.892-29.6)

Qdot.in= 201.31 kW

The required heat extraction rate at the cooling coil is 1362.62 kWand the heating section is

201.31 kW.

Analyze the cooli ng rate and heating rate when the ambient temperature changes from

28C to 40C i f the exi t temperature wi l l maintain at 20C

Since the value of T2 is greater than the T1, the cooling rate will become smaller than the

value from A(2). It can be proved when the value of h 2> h1on the psychrometric chart, the

value of Qdot.outis increase. Using the equation, we can calculate the newQdot.out. Since the Qdot.out = (-ve) (+ve) sign, the value of Qdot.out will be greater for thecooling rate. So, the cooling cannot be operated.

For the heating rate, the value is good for heating purpose. Since the value of heat transfer is

bigger, the heating rate can be operated.


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B. Refrigeration cycle

Select 2 refr igerants for the system and explain the reasons of selection based on safety and

thermal properties.

First and foremost, r134a do not contain chlorine atom so that it afford to undermine the role

of atmospheric ozone; besides, r134ahas a good safety performance (non-flammable, non-

explosive, non-toxic, non-irritating no rot resistance), in addition, r134ais easier to retrofit

refrigeration system so that the heat transfer performance is closer. Last but no least,

r134aheat transfer performance better than the R12 which can help the amount of refrigerant

greatly reduced.

As a refrigerant, ammonia offers three distinct advantages over other commonly used

industrial refrigerants. First, ammonia is environmentally compatible. It does not deplete the

ozone layer and does not contribute to global warming. Second, ammonia has superiorthermodynamic qualities, as result ammonia refrigeration systems use less electricity. Third,

ammonia's recognizable odor is it's greatest safety asset. Unlike most other industrial

refrigerants that have no odor, ammonia refrigeration has a proven safety record in part

because leaks are not likely to escape detection.

Choose operating conditions for the refrigeration cycle such as the evaporator and

condenser pressure if the surrounding temperature is 35C.


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Analysis:

The T-s diagram of the refrigeration cycle is drawn below.

This an ideal vapor compression refrigerant cycle, and thus the compressor is isentropic and

the refrigerant leaves the condenser as a saturated liquid and enter the compressor at saturated

vapor.

A refrigerator used refrigerant-134a as the working fluid and the assuming design pressures

for the condenser are 0.887MPa while the evaporator is at 1.2MPa. Pressure of evaporator is

assumed below from pressure of condenser to allow heat transfer, from surrounding intothe refrigerant and from refrigerant into surrounding.Assumptions:

1. Steady operating conditions exist

2. Kinetic and potential energy changes are negligible.

From the refrigerant -134a tables, the enthalpies of the refrigerant at all four states aredetermined as below:

State 1: Saturated vapor refrigerant-134a

Assumptions: Pressure = 0.887 Mpa

Temperature = 35 C

Heat transfer efficiently (100%)

By referring to the Refrigerant-134a tables:

To find and we have done the interpolation from table A-11 at (35-34) / (36-34) = (268.57) / (269.49268.57)

(35-34) / (36-34) = (0.91743) / (0.916750.91743)


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State 2: Superheated vapor refrigerant-134a

The chosen pressure is 1.2 MPa due to the pressure on the condenser. We picked this pressure

to allow heat transfer from refrigerant to surrounding. At state 2, it is isentropic process .

To find h,we have done the interpolation from table A-13 at (0.91709-0.9130) / (0.92670.9130) = (h2273.87) / (278.27273.87)

h2 = 275.1836 kJ/kg

State 2S: Superheated vapor refrigerant-134a h2s

The compressor efficiency is assumed at 80%.

h2s = 273.95 kJ/kg

State 3: Saturated liquid refrigerant-134a

(refer Table A-12)

State 4: Throttling valve


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Calcul ate the requir ed refr igerant mass fl ow rate to obtain the desir ed cooli ng effect.

is obtained from question 1.

indicate that the heat transfer rate from refrigerant to

surrounding.

Calcul ate the maximum COP and actual COP of the cycle if the compressor eff iciency isassumed at 80%.

COP defines the performance of the refrigeration cycle. To calculate COP, we use this

formula

Maximum COP

Actual COP


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Suggest an i nnovative system that can improve the cur rent COP i.e multistages or cascade

refr igeration cycle. Prove your suggestion using analytical analysis.

It is obvious that the lower-temperature unit of the cascade system absorbs less power than the

single stage system. This originates from the fact that the pressure ratio across the compressor

in the lower unit of the cascade system is less than that in the single-stage system for the same

refrigeration capacity. COPs for the lower unit of the cascade system are higher than those for

the single-stage system.

Estimate the cost of runni ng the system (single cycle and mu ltistage or cascade) for a 12

hour operation (based only on the compressor work input) under steady conditions and

actual M alaysian daylight electrical tar if f.

Tariff D - Low Voltage Industrial Tariff

For Overall Monthly Consumption Between 0-200 kWh/month

For all kWh 34.50 sen/kWh


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C. Combustor for the Heat exchanger

Determine the mass flow rate of the hot gases ( )

Combustion equation, (theoretical)

0.75CH4 + 0.1 N2+ 0.07O2+0.05CO2+0.03H2 + ath(O2+3.76N2) = XCO2+YH2O+ ZN2

Balance the equation:

The unknown coefficients x ,y ,z and athare determined from mass balances

C: 0.75 + 0.05 = X N2: 0.1 + 3.76 ath = Z

X= 0.8 Z= 5.5332

H: 0.75 (4) + 0.03(2) = 2Y O2: 0.07 + 0.05 + ath = X +Y/2Y= 1.53 ath = 1.445

Combustion equation for the system is:

0.75CH4 + 0.1 N2+ 0.07O2+0.05CO2+0.03H2+1.445 (O2+3.76N2) = 0.8CO2+1.53H2O+

5.5332N2

Element M, kg/kmol hfo, kJ/kmol h320K, kJ/kmol h298K, kJ/kmol h2273K, kJ/kmol

CH4 16.043 -74850 - - -

N2 28.013 0 9306 8669 74693.2

O2 31.999 0 9325 8682 78279.74

CO2 44.01 -393520 - 9364 117847.46

H2 2.016 0 - 8468 70889.14

H20 18.015 -241820 - 9904 96775.02

0.75CH4 + 0.1 N2+ 0.07O2+0.05CO2+0.03H2

25C

ath(O2+3.76N2)

XCO2+YH2O+ ZN2

2000C

To heater tubes


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( ) ( )Qout = [ 0.75(-74850 + 0 - 0) + 0.1(0 + 9306 - 8669) + 0.07(0 + 9325 - 8682) +

0.05(-393520 + 10186 - 9364) + 0.03(0 + 9100 - 8468)]

[ 0.75( -393520 + 117387.49364) + 1.53( -241820 + 96775.029904) +

5.5( 0 + 746938669)

= -36374.93( -84760.03)

= 48,385.1 kJ/kmol


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The operating cost for 12 hour s if the Natur al Gas cost is RM18.22/mmbtu


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D. Unmixed Cross Flow Heat exchanger for the heating effect

Determine the heat convection coeffi cient in the heater tube and at the outer f low of the

tube.

Assuming n = 8

Combustor

Exhaust

Heater Tubes

T2, 2

T3, 3

15 mm25 mm

0.5 m

0.5 m

Heater Tube

Air Conditioning Conduit

Example of Heater Tubes

Layout


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Properties of air at 10 C (1 atm):

(Laminar flow)

[

* +

]

[

* +

]


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Properties of CH4at 25C

Calcul ate the overall heat transfer coeffi cient, U (neglect the conduction effect in the

heater tube)

Analyze the requi red number of heater tubes for heat exchanging process using LMTD or

-NTU method.


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(

)

Hence the obtained

Effectiveness, Cross flow for both fluid unmixed

,

- , -


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4.0 ENVIRONMENTAL ASPECTS AND SOCIETAL IMPACT

Mostly refrigerant has a dangerous particle that can affect environmental aspect. Refrigerant

systems are producing HCFC gases that are dangerous to ozone layer and may have other

negative factors. Basically the HCFC gases can reduce the thickness of ozone layer. Thus, thedepletion problem may introduce to other problem such as the radiation light from the sun that

may harmful.

Nowdays, for safety purposes and the awareness of environmental issues, government and

private sector have taken a serious matter. The uses of HCFC gases are recommended replace

by natural refrigerant. These natural refrigerants may help to reduce the affect of ozone layer

thickness. The uses of CO2or R-744 have become an alternative way to replace the HCFC

gases. These gases may able to avoid the negative affect during production. Moreover,

blended HFC also has been introduced. These gases are not natural gases, but it also has quite

same purpose for reducing environmental issues. These are some example of blended HFC:

CFC 502is usually used in low temperature commercial and small industrial cooling

installations (e.g. supermarket frozen food systems, small cold stores and small blast

freezers). In the UK CFC 502 became scarce quite quickly after the 1995 phase out of

CFC production, so it is believed that there are relatively few CFC 502 systems still inuse.

HCFC 22 is a very commonly used refrigerant. It is widely used in commercial,

industrial and air-conditioning systems. It is currently used in many applications that

cannot be manufactured using HCFCs after 1st January 2001. It is also the most likely

refrigerant to be used in the air-conditioning and heat pump applications

CFC 12 is used for a wide variety of refrigeration and air-conditioning applications.

All domestic refrigerators and freezers built before 1994 used CFC 12. Many are still

in use. Similarly CFC 12 is used for many other small hermetic systems such as retail

display cases, icemakers and etc. CFC 12 is used in many medium and large sized

systems in commercial and industrial refrigeration.


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On-road and laboratory experiments with a 2009 Ford Explorer and a 2009 Toyota Corolla

were conducted to assess the fuel consumption penalty associated with air conditioner (A/C)

use at idle and highway cruise conditions. Vehicle data were acquired on-road and on a

chassis dynamometer. Data were gathered for various A/C settings and with the A/C off and

the windows open. At steady speeds between 64.4 and 113 kph (40 and 70 mph), both

vehicles consumed more fuel with the A/C on at maximum cooling load (compressor at 100%

duty cycle) than when driving with the windows down. The Explorer maintained this trend

beyond 113 kph (70 mph), while the Corolla fuel consumption with the windows down

matched that of running the A/C at 121 kph (75 mph), and exceeded it at 129 kph (80 mph).

The incremental fuel consumption rate penalty due to air conditioner use was nearly constant

with a slight trend of increasing consumption with increasing vehicle (and compressor) speed.

A lower fuel penalty due to A/C operation is observed at idle for both vehicles, likely due to

the low compressor speed at this operating point, although the percentage increase due to A/C

use is highest at idle.


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5.0 CONCLUSION

In conclusion, we have achieved the objective of this assignment that is to conduct

preliminary design of an air conditioning. Through the calculations, we also has indicates the

possible value for the correct air conditioning system. The calculations of problem solving

also have teach and improved our fundamental of calculus and thermal principles. Other than

that, we have identified the basic principle and the relationship between theoretical and

practical value of thermal engineering especially in air conditioning.

Based on the result and calculations, it is proved that high CCOP value can reduce the work in

needed for the system. In air conditioning application the electrical source can be reduced bymanipulated the COP value. The selection of air conditioning product also must take in care

because there is quite different between the energy consumption needed. Always prefer to

select the product that least energy consumption. For the old refrigerant and air conditioning

system, we need to regularly maintenance for better performance by cleaning and service the

coil of cooling and heating. Thus, Thermal Engineering has been proven to teach us on how to

apply the knowledge on our daily life. It also can be reference and guidance for us to reduce

the uses and sources of natural environment.


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6.0 UTILIZATION OF RESOURCES

1. MEC551, Thermal Engineering, 2013, First edition, Mc Graw Hill Education

2.

Walter T. Grondzik, Air-conditioning system Design Manual, 2007, Elsevier

3. http://www.gsa.gov/portal/content/101297

4. http://www.epa.gov/iaq/schooldesign/hvac.html

5. http://www.engineeringtoolbox.com/ansi-steel-pipes-d_305.html

6. http://www.vesma.com/tutorial/hr_principles.htm

7. http://environment.nationalgeographic.com/environment/green-guide/buyingguide/

air-conditioner/environmental-impact/

8.

http://www.alternet.org/story/37882/air-conditioning%3A_our_cross_to_bear

9.

http://www.engineeringtoolbox.com/flow-velocity-water-pipes-d_385.html
http://www.gsa.gov/portal/content/101297http://www.gsa.gov/portal/content/101297http://www.epa.gov/iaq/schooldesign/hvac.htmlhttp://www.epa.gov/iaq/schooldesign/hvac.htmlhttp://www.engineeringtoolbox.com/ansi-steel-pipes-d_305.htmlhttp://www.engineeringtoolbox.com/ansi-steel-pipes-d_305.htmlhttp://www.vesma.com/tutorial/hr_principles.htmhttp://www.vesma.com/tutorial/hr_principles.htmhttp://www.alternet.org/story/37882/air-conditioning%3A_our_cross_to_bearhttp://www.alternet.org/story/37882/air-conditioning%3A_our_cross_to_bearhttp://www.engineeringtoolbox.com/flow-velocity-water-pipes-d_385.htmlhttp://www.engineeringtoolbox.com/flow-velocity-water-pipes-d_385.htmlhttp://www.engineeringtoolbox.com/flow-velocity-water-pipes-d_385.htmlhttp://www.alternet.org/story/37882/air-conditioning%3A_our_cross_to_bearhttp://www.vesma.com/tutorial/hr_principles.htmhttp://www.engineeringtoolbox.com/ansi-steel-pipes-d_305.htmlhttp://www.epa.gov/iaq/schooldesign/hvac.htmlhttp://www.gsa.gov/portal/content/101297


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FACULTY OF MECHANICALENGINEERING

MEC551 THERMAL ENGINEERING

ASSIGNMENT

Group Members:-

MOHAMAD NURHAFIZ BIN ANUAR 2012741441

AMIRUL HAKIM BIN MOHD SALLEH 2012510131

AFIQ AYMAN BIN MOHD PILUS 2012913331


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UNIVERSITI TEKNOLOGI MARA

FAKULTI KEJURUTERAAN MEKANIKAL

40450 Shah Alam, Selangor Darul Ehsan, MalaysiaTel. : 03-5543 6268 Fax: 03-5543 5160

Report Assessment

Assignments Title : ________________________________________________________

Groups Name : ________________________________________________________

Leaders Name : ________________________________________________________

Members Name : 1) _______________________________________________________

2) _______________________________________________________

3) _______________________________________________________

Scale 1 2 3 4 5

Level Poor Acceptable Excellent

CriteriaFactor

(A)

Given Mark

(B)

A x B

[CO1, PO1] Problem Statement 1

[CO2, PO1]Application of engineering principles

and concepts4

[CO4, PO3] Integration mathematical Solutions 4

[CO5, PO9]Environmental aspects and financial

impact4

[CO3, PO3] Interpretation of results and discussion 4

[CO4, PO3] Conclusion 2

[CO1, PO1] Utilization of resources 1

Total Marks (100 %)

assignment thermal uitm

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