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### Note -introduction_of_heat

1. 1. AIR CONDITIONING & REFRIGERATION TECHNOLOGY1. Heat Transfer Terminology 1.1 The difference between heat and temperature. 1.2 The heat and work 1.3 The three modes of heat transfer. 1.4 The following terms as they relate to heat transfer: (a) Heat flux (b) Thermal conductivity (c) Convective heat transfer coefficient (d) Stefan Boltzman coefficient. (e) Overall heat transfer coefficient (u-value) (f) Log mean Temperature Different (LMTD) (g) Number of Transfer Unit (NTU) 1.5 Energy Balance (Equilibrium Temperature) 1.6 Heat Transfer Terminology SummaryDept: HVACR Subject: Heat Transfer & Heat ExchangerSubject Code: FRD 20303 Page: 1 of 8
2. 2. AIR CONDITIONING & REFRIGERATION TECHNOLOGY1.1 The difference between heat and temperature. In describing heat transfer problems, students often make the mistake of interchangeablyusing the terms heat and temperature. Actually, there is a distinct difference between the two. Temperature is a measure the heat intensity or heat level of a substance. It is a relativemeasure of how hot or cold a substance is and can be used to predict the direction of heat transfer.The symbol for temperature is T. The common scales for measuring temperature are the Fahrenheit,Rankine, Celsius, and Kelvin temperature scales. Heat is a form of energy in transit. The transfer of energy as heat occurs at the molecularlevel as a result of a temperature difference. Heat is capable of being transmitted through solids andfluids by conduction, through fluids by convection, and through empty space by radiation. Thesymbol for heat is Q. Common units for measuring heat are the British thermal unit (Btu) in theEnglish system of units, the calorie in the conventional metric system and Kilo Joule (KJ) in SIsystem (International System of Units).1.2 The Work and Heat Work is the transfer of energy resulting from force acting through a distance. Common unitsfor measuring heat are Joule (J). Heat is energy transferred as the result of a temperature difference. Neither heat nor worksare thermodynamic properties of a system. Heat can be transferred into or out of a system and workcan be done on or by a system, but a system cannot contain or store either heat or work. Heat isrelated to the molecular motion or vibration. A molecule is the smallest particles in which anysubstance can be broken down and still retain its chemical identity. As a substance is warmed,molecules move more rapidly. As a substance is cooled, they slow down. If all heat is removed froma substance, all molecular motion stops.Dept: HVACR Subject: Heat Transfer & Heat ExchangerSubject Code: FRD 20303 Page: 2 of 8
3. 3. AIR CONDITIONING & REFRIGERATION TECHNOLOGY1.3 DESCRIBE the three modes of heat transfer. Heat is always transferred when a temperature difference exists between two bodies. Heatalways flows from a warmer substance to a cooler one. What happen is that the faster movingmolecules give out some of their energy so slower moving molecules. Therefore, the fastermolecules slow down a little and the slower ones move a little faster. There are three basic modes ofheat transfer:a) Conduction involves the flow of heat among parts of substance. The flow can also be from onesubstance to another substance in direct contact.b) Convection involves the moving of heat from one place to another place by way of fluid or air.c) Radiation, or radiant heat transfer, where heat transfer in the form of wave motion similar to lightwaves in which energy is transmitted from one body to other body without need of interveningmatter.The three modes of heat transfer will be discussed in greater detail in the subsequent chapters of thismodule. Figure 1: Three methods of heat transferDept: HVACR Subject: Heat Transfer & Heat ExchangerSubject Code: FRD 20303 Page: 3 of 8
4. 4. AIR CONDITIONING & REFRIGERATION TECHNOLOGY1.4 DEFINE the following terms as they relate to heat transfer: a) Heat flux The rate at which heat is transferred is represented by the symbol. Common units for heat ˙Qtransfer rate are Btu/hr. Sometimes it is important to determine the heat transfer rate per unit area, orheat flux, which has the symbol. Units for heat flux are Btu/hr-ft2. The heat flux can be ˙Qdetermined by dividing the heat transfer rate by the area through which the heat is being transferred. Where, q = heat Flux, (W/m2. or BTU/hr.ft2.) Q = Heat flow rate (Watt or BTU/hr) A = Area to the direction of heat flow (m2 or ft2) b) Thermal conductivity The heat transfer characteristics of a solid material are measured by a property called thethermal conductivity (k) measured in Btu/hr.ft.0F and w/m.0C. It is a measure of a substance’s abilityto transfer heat through a solid by conduction. The thermal conductivity of most liquids and solidsvaries with temperature. For vapors, it depends upon pressure. & = kA T1 − T2 → k = Q / A & Q L L(T1 − T2 ) q = L (T1 − T2 )Dept: HVACR Subject: Heat Transfer & Heat ExchangerSubject Code: FRD 20303 Page: 4 of 8
5. 5. AIR CONDITIONING & REFRIGERATION TECHNOLOGY c) Convective heat transfer coefficient The convective heat transfer coefficient (h), defines, in part, the heat transfer due toconvection. The convective heat transfer coefficient is sometimes referred to as a film coefficient andrepresents the thermal resistance of a relatively stagnant layer of fluid between a heat transfer surfaceand the fluid medium. Common units used to measure the convective heat transfer coefficient areBtu/hr.ft2.0F and w/m2.0C. d) Stefan Boltzman A body that emits the maximum amount of heat for its absolute temperature iscalled a black body. Radiant heat transfer rate from a black body to its surroundings canbe expressed by the following equation.σ = Stefan-Boltzman constant 0.1714 x 10-8 Btu/hr-ft2.R4 and = 5.676 x 10-8 W/m2.K4 e) Overall heat transfer coefficient (u-value) In the case of combined heat transfer, it is common practice to relate the total rate of heattransfer ( ), the overall cross-sectional area for heat transfer (Ao), and the overall temperature ˙Qdifference (To) using the overall heat transfer coefficient (Uo). The overall heat transfer coefficientcombines the heat transfer coefficient of the two heat exchanger fluids and the thermal conductivityof the heat exchanger tubes. Uo is specific to the heat exchanger and the fluids that are used in theheat exchanger. Watt w / m2.C. m2 0 CDept: HVACR Subject: Heat Transfer & Heat ExchangerSubject Code: FRD 20303 Page: 5 of 8
6. 6. AIR CONDITIONING & REFRIGERATION TECHNOLOGY f) Log mean temperature difference (LMTD) In heat exchanger applications, the inlet and outlet temperatures are commonly specifiedbased on the fluid in the tubes. The temperature change that takes place across the heat exchangerfrom the entrance to the exit is not linear. A precise temperature change between two fluids acrossthe heat exchanger is best represented by the log mean temperature difference (LMTD), defined inEquation below g) Number of Transfer Unit (NTU) The Number of Transfer Units (NTU) Method is used to calculate the rate of heat transferin heat exchangers (especially counter current exchangers) when there is insufficient information tocalculate the Log-Mean Temperature Difference (LMTD).Method is preferred when only the inlettemperatures are known. The method proceeds by calculating the heat capacity rates (i.e. flow rate multiplied byspecific heat) Ch and Cc for the hot and cold fluids respectively, and denoting the smaller one as Cmin.Dept: HVACR Subject: Heat Transfer & Heat ExchangerSubject Code: FRD 20303 Page: 6 of 8
7. 7. AIR CONDITIONING & REFRIGERATION TECHNOLOGY1.5 Energy Balance (Equilibrium Temperature) In heat transfer analysis, we are usually interested only in the forms of energy that can betransferred as a result of a temperature difference, that is, heat or thermal energy in such cases it isconvenient to write a heat balance and to treat the conversion of nuclear, chemical and electricalenergies into thermal energy as heat generation. The energy balance in that case can be expressed asEnergy Balance for Closed Systems (fixed mass)A closed system consist of a fixed mass. The total energy, E for most systems encountered inpractice consists of the internal energy, U. This case for stationary systems since they don’t involveany changes in their velocity or elevation during a process. The energy balance relation in that caseto reduce towhere we expressed the internal energy change in terms of mass, m the specific heat at constantvolume, CV and the temperature change ∆T oh the system. When the system involves heat transferonly and no work interactions across its boundary, the energy balance relation further reduces toWhere Q is the net amount of heat transfer to or forms the system. This is the form of the energybalance relation we will most often dealing with a fixed mass.For a steady-flow system with one inlet and one exit, the rate of mass flow into the control volumemust be equal to the rate of mass flow out of it. That is When the changes inkinetic and potential energies are negligible which is usually the case, and there is no workinteraction, the energy balance for such a steady flow system reduces toWhere Q is the rate of net heat transfer in to or out of the control volumeDept: HVACR Subject: Heat Transfer & Heat ExchangerSubject Code: FRD 20303 Page: 7 of 8
8. 8. AIR CONDITIONING & REFRIGERATION TECHNOLOGY1.6 Heat Transfer Terminology Summary Heat is energy transferred as a result of a temperature difference. Temperature is a measure of the amount of molecular energy contained in a substance. Work is a transfer of energy resulting from force acting through a distance. The Second Law of Thermodynamics implies that heat will not transfer from a colder to a hotter body without some external source of energy. Conduction involves the transfer of heat by the interactions of atoms or molecules of a material through which the heat is being transferred. Convection involves the transfer of heat by the mixing and motion of macroscopic portions of a fluid. Radiation, or radiant heat transfer, involves the transfer of heat by electromagnetic radiation that arises due to the temperature of a body. Heat flux is the rate of heat transfer per unit area. Thermal conductivity is a measure of a substance’s ability to transfer heat through itself. Log mean temperature difference is the T that most accurately represents the T for a heat exchanger. The local heat transfer coefficient represents a measure of the ability to transfer heat through a stagnant film layer. The overall heat transfer coefficient is the measure of the ability of a heat exchanger to transfer heat from one fluid to another. The bulk temperature is the temperature of the fluid that best represents the majority of the fluid which is not physically connected to the heat transfer site.Dept: HVACR Subject: Heat Transfer & Heat ExchangerSubject Code: FRD 20303 Page: 8 of 8