Anh Văn CN Nhiệt chapter 4

Anh văn Chuyên ngành Nhiệt English for thermal engineering LOGO Chapter Refrigeration and heat pump cycles Contents The fundamentals of refrigeration Air conditioning systems Ventilating system Tài liệu tham khảo Fundamentals of thermal-fluid science, Y A Çengel Fundamentals of thermodynamics (sixth edition), Sonntag, Borgnakke and van Wylen Fundamentals of engineering thermodynamics (Fifth edition), Michael J Moran, Howard N Shapiro 4.1 The fundamentals of refrigeration cycle The reversed Carnot cycle (refrigerator and heat pump) The refrigerant absorbs heat isothermally from a low-temperature source at TL in the amount of QL (process 1-2), is compressed isentropically to state (temperature rises to TH), rejects heat isothermally to a high-temperature sink at TH in the amount of QH (process 3-4), and expands isentropically to state Schematic of a Carnot refrigerator and T-s diagram of the reversed Carnot cycle (temperature drops to TL) The refrigerant changes from a saturated vapor state to a saturated liquid state in the condenser during process 3-4 4.1 The fundamentals of refrigeration The reversed Carnot cycle (refrigerator and heat pump) The coefficients of performance of Carnot refrigerators and heat pumps are expressed in terms of temperatures as: and Schematic of a Carnot refrigerator and T-s diagram of the reversed Carnot cycle 4.1 The fundamentals of refrigeration The reversed Carnot cycle (refrigerator and heat pump) Notes: -Both COPs increase as the difference between the two temperatures decreases, that is, as TL rises or TH falls; -The reversed Carnot cycle is the most efficient refrigeration cycle operating between two specific temperature levels -Processes 2-3 and 4-1 cannot be approximated closely in practice since: + Process 2-3 involves the compression of a liquid–vapor mixture a compressor that will handle two phases; + Process 4-1 involves the expansion of high-moisture-content refrigerant 4.1 The fundamentals of refrigeration The ideal vapor-compression refrigeration cycle Schematic and T-s diagram for the ideal vaporcompression refrigeration cycle 4.1 The fundamentals of refrigeration The ideal vapor-compression refrigeration cycle State - Saturated vapor State - Superheated vapor State - Saturated liquid State - Low-quality saturated mixture This cycle consists of processes as follow: 1-2 Isentropic compression in a compressor 2-3 Constant-pressure heat rejection in a condenser 3-4 Throttling in an expansion device 4-1 Constant-pressure heat absorption in an evaporator An ordinary household refrigerator 4.1 The fundamentals of refrigeration The ideal vapor-compression refrigeration cycle All four components associated with the vaporcompression refrigeration cycle are steady-flow devices, and thus all four processes that make up the cycle can be analyzed as steady-flow processes The steady-flow energy equation on a unit-mass basis reduces to: The P-h diagram of an ideal vaporcompression refrigeration cycle The COPs of refrigerators and heat pumps operating on the vapor-compression refrigeration cycle can be expressed as or 4.1 The fundamentals of refrigeration The ideal vapor-compression refrigeration cycle Example: A refrigerator uses refrigerant-134a as the working fluid and operates on an ideal vapor-compression refrigeration cycle between 0.14 and 0.8 MPa If the mass flow rate of the refrigerant is 0.05 kg/s, determine (a) the rate of heat removal from the refrigerated space and the power input to the compressor, (b) the rate of heat rejection to the environment, and (c) the COP of the refrigerator Solution: From the refrigerant-134a tables, the enthalpies of the refrigerant at all four states are determined as follows: 10 4.2 Air conditioning system B Surface type coolers B4 Variable Refrigerant Volume (VRV) or Variable Refrigerant Flow (VRF) 26 4.2 Air conditioning system B Surface type coolers B4 Variable Refrigerant Volume (VRV) or Variable Refrigerant Flow (VRF) 27 4.2 Air conditioning system B Indirect Surface type coolers B5 Water chiller system * A large, central compressor provides cold water to a heat exchanger - Fan Coil Unit (FCU) -inside apartments * Fan Coil Units (FCU) in apartments contain a fan that draws air into the unit then blows it over a cooling or heating coil The air comes out of the FCU either cooler or hotter than before * FCUs will generally have a chilled water coil for cooling and either a hot water coil for heating or an electric heating element 28 4.2 Air conditioning system B Indirect Surface type coolers B5 Water chiller system Boiler 29 4.2 Air conditioning system B Indirect Surface type coolers B5 Water chiller system A typical AHU (Air Handling Unit) 30 4.2 Air conditioning system B Indirect Surface type coolers B5 Water chiller system 31 4.2 Air conditioning system B Indirect Surface type coolers B5 Water chiller system 32 4.3 Ventilation system A Natural ventilation Relies on natural forces of wind and temperature differences to generate flow of air Advantages: Absence of mechanical components, no plant room needed; Reduction in building energy consumption Disadvantages: Close control not practicable; incoming air can not be filtered; Difficult to exclude external noise; Path for flow of air must form part of architectural building design; Cost saving of mechanical plant may be offset by increased cost of special building components 33 4.3 Ventilation system A Natural ventilation Typical schemes 34 4.3 Ventilation system A Natural ventilation Driving pressure a Wind pressure Where Pw = Wind pressure (N/m2) Cp = Wind coefficient on wall Vw = Wind speed (m/s)  = Density of air (kg/m3) 35 4.3 Ventilation system A Natural ventilation Driving pressure b Temp difference Where Ps = Stack driving pressure (N/m2) i = Density of internal air (kg/m3) g = Acceleration of gravity = 9.81 m/s2 h = Difference in height of inlet and outlet openings (m) Ti = Internal temperature (K) To = Outside temperature (K) 36 4.3 Ventilation system B Mechanical / forced ventilation A building ventilation system that uses powered fans or blowers to provide fresh air to rooms when the natural forces of air pressure and gravity are not enough to circulate air through a building Purposes Mechanical ventilation is used to: -Control indoor air quality, excess humidity, odours; -Contaminants can often be controlled via dilution or replacement with outside air 37 4.3 Ventilation system B Mechanical / forced ventilation 38 LOGO 39 http://blogcongdong.com LOGO 40 ... supermarkets… 25 4. 2 Air conditioning system B Surface type coolers B4 Variable Refrigerant Volume (VRV) or Variable Refrigerant Flow (VRF) 26 4. 2 Air conditioning system B Surface type coolers B4 Variable... The actual vapor-compression refrigeration cycle 14 4.1 The fundamentals of refrigeration The actual vapor-compression refrigeration cycle 15 4. 2 Air conditioning system Method of cooling air... conditioning unit Water cooled condenser, install in roof of buildings 24 4.2 Air conditioning system B Surface type coolers B4 Variable Refrigerant Volume (VRV) or Variable Refrigerant Flow (VRF)