1.0
INTRODUCTION
Thermal engineering is a field of engineering which is deal with the heat transfer; heating and cooling process. It’s 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 cir cuit 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 pi pe. pi pe. Nusselt’s Number, Reynold’s Number, Pranatl’s Number, Grashof’s 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 e xchanger. 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 22◦C to 27◦C while the comfort the comfort humidity range is 40%RH to 60%RH.
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 s yllabus 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.
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.
. In the left loop, indoor air is driven by the supply air fan through a I ndoor air loop
cooling coil, where it transfers its heat to chilled water. The cool air then cools the building space. ii.
. Driven by the chilled water pump, water returns from the cooling Chi ll ed water loop coil to the chiller’s evaporator to be re-cooled.
iii.
. Using a phase-change refrigerant, the chiller’s compressor pumps Refr igeran t loop
heat from the chilled water to the condenser wat er. iv.
. Water absorbs heat from the chiller’s condenser, and Condenser water loop
thecondenser water pump sends it to the cooling tower. v.
. The cooling tower’s fan drives air across an open flow of the hot Cooli ng tower loop
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
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, 50 – 250 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 uti lity. The same company may also have two or three levels of refrigeration and air conditioning such as a combination of: i. ii. iii.
Comfort air conditioning (20 – 25 oC) Chilled water system (80 – 100 C) 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
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.
3.0
INTEGRATION MATHEMATICAL SOLUTIONS
A. Air-conditioning process
Evaporator
T 3 =
T1 = 35 C
T2 = 10
20 C
2 =
3
100%
1 =
Condensat 2
1 Condensate Water
70%
P = 100 kPa 38,000 cfm
Provide a sketch of the air – conditi oni ng pr ocesses wi th th e ambi ent pressur e of 100kpa.
Deter mi ne the requi r ed heat extraction r ate at the cooli ng r ate and heating r ate when the ambi ent ai r ent ers at 35° C an d 70° C of r elative hu midi ty and l eave the system at 20° C.
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.
STATE 1: From the Psychrometric chart and A-4 table, h1 = 100.0 kJ / kg dry air ω1 = 0.0253 kg H 2O / kg dry air
) (
v1 = 0.9202 m3 / kg dry air 1 cfm = 0.02831 m 3/min 38000 cfm = 1075.78 m 3/min = 18 m 3/s = V Psat = 5.6291 kPa
STATE 2: Psat = 1.2281 kPa h2 = 29.6 kJ/kg dry air ω2 = 0.0078 kg H 2O/kg dry air.
STATE 3: Since ω3 = ω2 = 0.0078 kg H 2O/kg dry air
CALCULATION: mw = ma (ω1-ω2) ma = V1 / v1
ma = 19.56 kg/s
mw = 0.3423 kg/s
Qdot.out = ma (h1-h2) – mwhw hw = hf @ Tcondensate hf = 42.022 kJ/kg at T = 10°C 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 kW and the heating section is 201.31 kW.
An alyze the cooli ng r ate and h eatin g r ate when the ambient temper atur e changes f r om 28° C to 40° C i f th e exi t temper atur e wi ll maintain at 20° C
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 > h1 on the psychrometric chart, the value of Qdot.out is increase. Using the
equation, we can calculate the new
Qdot.out. Since the Qdot.out = (-ve) – (+ve) sign, the value of Qdot.out will be greater for the cooling 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.
B. Refrigeration cycle
Select 2 r efr i ger ants for the system and expl ain the r easons of selecti on based on safety and therm al pr oper ties.
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, nonexplosive, 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 superior thermodynamic 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 pr essur e if t he sur r ounding temper atu r e is 35° C.
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 into the 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 are determined 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
(35-34) / (36-34) = (
we have done the interpolation from table A-11 at – 268.57) / (269.49 – 268.57)
(35-34) / (36-34) = ( – 0.91743) / (0.91675 – 0.91743)
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) / (0.9267 – 0.9130) = (h 2 – 273.87) / (278.27 – 273.87) 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
Calcul ate the requir ed refr igerant mass fl ow rate to obtain the desir ed cooli ng eff ect.
̇ ̇ ̇ ̇ ̇ ̇ is obtained from question 1. surrounding.
indicate that the heat transfer rate from refrigerant to
Calcul ate the maximu m COP and actual COP of the cycle if the compr essor eff ici ency is assum ed at 80%.
COP defines the performance of the refrigeration cycle. To calculate COP, we use this formula
̇ Maximum COP
Actual COP
̇
Suggest an i nn ovative system that can impr ove the cur r ent COP i .e mu lti stages or cascade r efr igerati on cycle. Prove your suggesti on u sin g analyti cal anal ysis.
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.
Estim ate the cost of r un ni ng th e system (si ngl e cycle and mu l ti stage or cascade) f or a 12 hou r oper ation (based onl y on th e compressor work in put) u nder steady condi ti ons and actual M alaysian daylight electri cal 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
C. Combustor for the Heat exchanger
̇ ̇ ̇
Determi ne the mass flow r ate of the hot gases (
)
̇
0.75CH4 + 0.1 N2 + 0.07O2+0.05CO2 +0.03H2
25°C
To heater tubes
XCO2 +YH2O+ ZN2 2000°C
ath (O2 +3.76N2)
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 a th are determined from mass balances C:
0.75 + 0.05 = X
N2 :
0.1 + 3.76 a th = Z
X= 0.8 H:
Z= 5.5332
0.75 (4) + 0.03(2) = 2Y
O 2:
0.07 + 0.05 + a th = X +Y/2
Y= 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
o
hf , 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
∑ (̅ ) ∑ (̅ ) 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 + 117387.4 – 9364) + 1.53( -241820 + 96775.02 – 9904) + 5.5( 0 + 74693 – 8669) = -36374.93 – ( -84760.03) = 48,385.1 kJ/kmol
̇ ̇ ̇ ̇ ̇ ̇ ̇ ̇ ̇ ̇ ̇ ̇ ̇̇ ̇ ̇ ̇
̇ ̇ ̇ ̇ Th e oper atin g cost for 12 hour s if the Natur al Gas cost i s RM 18.22/mmbtu
D. Unmixed Cross Flow Heat exchanger for the heating effect
Deter mi ne the heat convection coeffi cient i n th e heater tube and at the outer f low of the tube.
25 mm 15 mm
T2, Ø2
0.5 m Heater Tube 0.5 m Air Conditioning Conduit Heater Tubes Exhaust Combustor T3, Ø3 Example of Heater Tubes
̇̇ Assuming n = 8
̇ ̇ ̇
Layout
Properties of air at 10 °C (1 atm):
⁄ ⁄ ⁄ ̇ ⁄ [* + ] * + [ ] ⁄ (Laminar flow)
Properties of CH4 at 25°C
⁄ ⁄ Calcul ate the over all heat tr ansfer coef fi cient, U (n eglect th e conducti on eff ect in the heater tu be)
⁄
An alyze the requi r ed num ber of heater tu bes f or h eat exchan gin g process usin g L M TD or -NT U method.
̇ ̇ ⁄ ⁄
̇ ( ) Hence the obtained
Effectiveness, ɛ Cross flow for both fluid unmixed
, ,
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, the depletion 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 CO2 or 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 ex ample of blended HFC:
CFC 502 is 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 in use.
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.
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.
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 by manipulated 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.
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
FACULTY OF MECHANICAL ENGINEERING MEC551 – THERMAL ENGINEERING
ASSIGNMENT
Group Members:MOHAMAD NURHAFIZ BIN ANUAR
2012741441
•
AMIRUL HAKIM BIN MOHD SALLEH
2012510131
•
AFIQ AYMAN BIN MOHD PILUS
2012913331
•
UNIVERSITI TEKNOLOGI MARA FAKULTI KEJURUTERAAN MEKANIKAL 40450 Shah Alam, Selangor Darul Ehsan, Malaysia Tel. : 03-5543 6268 Fax: 03-5543 5160 Report Assessment Assignment’s Title
: ________________________________________________________
Groups’ Name
: ________________________________________________________
Leader’s Name
: ________________________________________________________
Member’s Name
: 1) _______________________________________________________ 2) _______________________________________________________ 3) _______________________________________________________
Scale Level
1 Poor
2
3 Acceptable
4
Factor (A)
Criteria [CO1, PO1]
Problem Statement
1
[CO2, PO1]
Application of engineering principles and concepts
4
[CO4, PO3]
Integration mathematical Solutions
4
[CO5, PO9]
Environmental aspects and financial impact
4
[CO3, PO3]
Interpretation of results and discussion
4
[CO4, PO3]
Conclusion
2
[CO1, PO1]
Utilization of resources
1
5 Excellent
Given Mark (B)
Total Marks (100 %)
A x B
