AE 310 Fundamentals of Heating, Ventilating, and Air-Conditioning Chapter1 Chapter 1. Review of Thermodynamics and Heat Transfer 1.1 Introduction 1.2 Basic concepts of Heat Transfer 1.3 First Law of Thermodynamics 1.4 Second Law of Thermodynamics 1.5 Ideal Gas Readings: • M.J. Moran and H.N. Shapiro, Fundamentals of Engineering Thermodynamics,3rd ed., John Wiley & Sons, Inc., or Other thermodynamics texts 1.1 Introduction 1.1.1 Thermodynamics Thermodynamics is the science devoted to the study of energy, its transformations, and its relation to the status of matter. energy  the first law (conservation of energy) entropy  the second law (quality of energy) every naturally occurring transformation of energy is accompanied somewhere by a loss in the availability of energy for future performance of work 1.1.2 System and Surroundings System: an object, any quantity of matter, any region of space, etc. selected for study Surroundings: the rest Basic system types: Closed system (control mass) and Open system (control volume) 1.1.3 Property, State, Process and Equilibrium A property is the observable characteristic of a system such as temperature, pressure and density. A state is the condition of the system defined by properties. A process is transformation of the system from one state to another. A thermodynamic cycle is a process that begins and ends at the same state. 1 AE 310 Fundamentals of Heating, Ventilating, and Air-Conditioning Chapter1 Pressure-volume-temperature surface for a substance that expands on freezing 2 AE 310 Fundamentals of Heating, Ventilating, and Air-Conditioning Chapter1 1.1.4 p-v-T Relationships Thermodynamic properties: tables, graphs, equations and programs. 1.2 Basic Concepts The engineering discipline of heat transfer is concerned with methods of calculating rates of heat transfer. These methods are used by engineers to design components and systems in which heat transfer occurs. (1) Temperature: Degree of molecular movement S.I. oC, K I-P: oF, oR (2) Energy: E (Btu, J) Capacity to do work. Examples: • Thermal • Light • Mechanical • Electrical • Chemical 1 Btu=1055 J Work (W) is an action of a force on a moving system. (3) Heat (Thermal energy): Q (Btu, J) Heat is energy transferred across the system boundary by temperature difference (∆T). 3 AE 310 Fundamentals of Heating, Ventilating, and Air-Conditioning Chapter1 (4) Specific heat: C (Btu/lboF, J/kg K) p Heat needed to raise temperature of 1 lb (1 kg) material for 1 oF (1 oC). (5) Heat capacity: (Btu/ft3 oF) and thermal mass (Btu/oF) Ability to store energy Heat capacity = density x specific heat = ρ C p Thermal mass = density x volume x specific heat = ρ V C p (6) Energy change (heat transfer): Q (Btu) Q=ρ V C ∆T p where ρ - density, V - volume, ∆T - temperature difference. (7) Heat flow (heat transfer rate or energy change rate): Q& (Btu/h) Energy transferred per unit time. 4 AE 310 Fundamentals of Heating, Ventilating, and Air-Conditioning Chapter1 1.3 First Law of Thermodynamics The first law of thermodynamics expresses principle of conservation of energy. 1 Forms of energy: KE = mv2 Kinetic (motion of the system) 2 PE = mgz Potential (position of the system) U = U(T) Internal (stored in matter) System total energy (E) = sum of all forms of energy Energy transfer mechanisms are work (W) and heat (Q), which are not properties of the system. Conventions: Work done by a system is positive. Heat transfer to a system is positive. 1.3.1 First Law for the Closed System Heat (Q) – Work (W) = Change in Total Energy (∆E) Closed system with process between states “1” and “2”:  v2 −v2 g(z −z ) Q1−2 −W1−2 = m(u2 −u1)+ 22g 1 + 2g 1    C C Example 1 Constant volume heating of a cylinder filled with a gas. 1st Law: Q = m ∆u = m c ∆T 1-2 V weight V=const. p=const. Q Q Example 1: Constant volume heating Example 2: Constant pressure heating 5 AE 310 Fundamentals of Heating, Ventilating, and Air-Conditioning Chapter1 Example 2 Constant pressure heating of a cylinder filled with a gas. 1st Law: where i = u + pv is enthalpy. Enthalpy is a property that combines ∆u and pv work that are only forms of energy change in many processes. c =c +R P V where R - specific ideal gas constant, and c >c because constant input does work. P V 1.3.2 First Law for the Open System (Control Volume Formulation) Heat Transfer (Q& ) – Net Rate of Work (W& ) = Net Rate of Energy Flow (across the system boundary) dE cv dt E E in out - + + - Q& W&   v2 gz    v2 gz  dE Q& CV −W& CV = ∑m&i+ 2g + g -∑m&i+ 2g + g + dtcv out   C C  in   C C  6 AE 310 Fundamentals of Heating, Ventilating, and Air-Conditioning Chapter1 Special case of the 1st law is for steady-state flow that has constant flow across the boundary and no mass or energy change in CV. For a typical HVAC system two assumptions are valid: dE dm& 1) Steady-state ( cv =0, cv =0) flow across the system boundary dt dt 2) KE and PE terms usually small Therefore, for the typical HVAC system: Q& −W& = ∑m& ⋅i-∑m& ⋅i CV CV out in Example 3 A house/building is a thermal system and its envelope is the boundary. Let us consider some energy transfer in a single family house. Q& =150 Btu/h Q& = 200 Btu/h infiltration conduction Q& =300 Btu/h occupant Q& = 350 Btu/h ac Q& =500 Btu/h solar Q& =100 Btu/h ground The thermal mass of the house is assumed to be 700 Btu/oF. (a) Is the system in equilibrium? 7 AE 310 Fundamentals of Heating, Ventilating, and Air-Conditioning Chapter1 Since Q& = Q& , the system is in equilibrium. The heat flows are steady state. The temperature in out will not change. (b) What will happen if the A/C is shut off? 8 AE 310 Fundamentals of Heating, Ventilating, and Air-Conditioning Chapter1 1.4 Second Law of Thermodynamics All processes obey the 1st Law of Thermodynamics. However, some 1st law processes never occur. For example, heat transfer from cold reservoir to hot reservoir or flow from low pressure to high pressure. The 2nd Law of Thermodynamics defines: • direction of change for processes • final equilibrium for spontaneous processes • criterion for theoretical performance limits of cycles • quality of energy Energy changes and transfer involves both conservation principle and degradation in quality. Therefore, the thermal efficiency of all heat engines must be less than 100% due to dissipative effects. Processes occurring in a system such as heat engine are irreversible since either the system or its surroundings cannot be returned to their initial states. A reversible process is an idealization. Heat engines (heat pumps) are closed systems, which operates continuously, or cyclically, and produce (use) work while exchanging heat across its boundaries. 1.4.1 Heat Engine Work produced while heat extracted from high temperature (T ) reservoir and rejected to H low temperature reservoir (T ). L T H Q& H W& Q& L T L 9 AE 310 Fundamentals of Heating, Ventilating, and Air-Conditioning Chapter1 1.4.2 Heat Pump Work used to extract heat from low temperature reservoir (T ) and reject to high L temperature (T ) reservoir. H T H Q& H W& Q& L T L 1.4.3 Performance evaluation of cycles Performance evaluation of cycles: comparisons with the ideal Carnot heat engine that is a totally reversible heat engine or pump. Q& ∝Tof reservoir (absolute scale) (1) “Efficiency” (η) of a Heat Engine W Q& −Q& Q& η = = H L =1− L , 0< η<1 Q& Q& Q& H H H 10