Refrigeration Lecture PDF

REFRIGERATION Dr. Lingyi Liu, FDST, UNL [email protected] 1 Outline Outline & Outcomes ▪ Refrigerants ▪ Components of a Refrigeration System ▪ Vapor compression cycle Pressure-enthalpy diagram and calculations Coefficient of performance Refrigeration capacity vs. load ▪ Processing systems Outcomes ▪ Un...

REFRIGERATION Dr. Lingyi Liu, FDST, UNL [email protected] 1 Outline Outline & Outcomes ▪ Refrigerants ▪ Components of a Refrigeration System ▪ Vapor compression cycle Pressure-enthalpy diagram and calculations Coefficient of performance Refrigeration capacity vs. load ▪ Processing systems Outcomes ▪ Understand the operation of refrigeration, ▪ Name different refrigerants used and their properties, ▪ Describe the thermodynamics of the vapor compression cycle, ▪ Explain the parts of a refrigeration system, and ▪ Solve problems to determine the refrigeration effect, load, capacity, compressor power etc. 2 Refrigeration The process of removing heat from a substance. The science of providing and maintaining temperature below that of the surrounding atmosphere. Consider as a “pump” to transfer heat from a region of low temperature to a region of high temperature. 3 Refrigeration and freezing of food products Quality considerations: Microbial Chemical Physical Nutritional Sensory 4 Refrigeration and freezing of food products Ammonia (NH3) Halocarbons Chlorofluorocarbons (CFC) not manufactured since 1996 CFC-12 or R-12 – dichlorodifluoromethane Hydrochlorofluorocarbons (HCFC) to be phased out in 2030 CFC-22 or R-22 – chlorodifluoromethane 5 Selection criteria for a refrigerant Latent heat of vaporization (how much heat it can absorb) Condensing pressure (the pressure it works under) Freezing temperature (the temperature turning into solid) Critical temperature (the temperature becoming too pressurized) Toxicity Flammability Corrosiveness Chemical stability Detection of leaks (able to detect without too much trouble) Cost (running and maintenance) Environmental impact 6 Components of a Refrigeration System Source: http://www.solarpowerwindenergy.org/how-refrigerators-work-parts-of-a-refrigerator/ 7 Evaporator The liquid refrigerant vaporizes to a gaseous state in the evaporator. The change of state is done by the heat extracted from the surroundings. Source: http://www.solarpowerwindenergy.org/how-refrigerators-work-parts-of-a-refrigerator/ 8 Compressor The refrigerant enters the compressor in a vapor state at low pressure and temperature. The compressor raises the pressure and temperature of the refrigerant. Source: http://www.solarpowerwindenergy.org/how-refrigerators-work-parts-of-a-refrigerator/ 9 Condenser Transfers heat from the refrigerant to another medium, such as air and/or water. The gaseous refrigerant condenses to liquid inside the condenser by removing the heat. Source: http://www.solarpowerwindenergy.org/how-refrigerators-work-parts-of-a-refrigerator/ 10 Expansion Valve It is a metering device that controls the flow of liquid refrigerant to an evaporator by sensing pressure or temperature at another location in the refrigeration system. Source: http://www.solarpowerwindenergy.org/how-refrigerators-work-parts-of-a-refrigerator/ 11 Components of a Refrigeration System 1 evaporator outlet - compressor inlet 2 compressor outlet - condenser inlet C somewhere inside condenser 3 condenser outlet - expansion inlet 4 expansion outlet - evaporator inlet saturated vapor superheated vapor saturated vapor saturated liquid sat. liq. + sat. vap. 12 Pressure-enthalpy diagram Consider a simple vapor-compression refrigeration system, where the refrigerant enters the expansion valve as saturated liquid and leaves the evaporator as saturated vapor. Source: Introduction to Food Engineering, (5th Ed.) by R.P. Singh and D.R. Heldman, Academic Press. 14 (how much pressure is applied to the refrigerant at various points) Pressure-enthalpy diagram (the amount of heat content or energy in the refrigerant） 14 Definitions and calculations Evaporator Evaporator (e - a): constant low pressure (low temperature) evaporation of the refrigerant Accepts heat from surrounding at constant pressure Refrigeration Effect (RE) = Qevap = Ha - He (the amount of heat the refrigerant can remove from the inside of the refrigerator） H is enthalpy of refrigerant at the point (kJ/kg refrigerant) 15 Definitions and calculations Compressor Compressor (a - b): constant entropy for frictionless ideal gas compression Sb = Sa Work done on system: -Ws = Hb - Ha qw=mr(Hb-Ha) mr is refrigerant mass flow rate (kg/s) qw is rate of heat exchanged in the condenser (kW) H is enthalpy of refrigerant at the point (kJ/kg refrigerant) 16 Definitions and calculations Condenser Superheated vapor cools down Saturated liquid to condensation temperature at Saturated vapor constant pressure (b - c) Temperature remains constant during condensation (c - d) Heat rejected to environment = Superheated vapor - Qc = Hb - Hd H is enthalpy of refrigerant at the point (kJ/kg refrigerant) 17 Definitions and calculations Expansion Expansion (d - e): Saturated liquid Constant enthalpy: Hd = He H is enthalpy of refrigerant at the point (kJ/kg refrigerant) Saturated liquid + vapor 18 Coefficient of Performance (C.O.P.) - - defined as a ratio between the heat absorbed by the refrigerant as it flows through the evaporator to the heat equivalence of the energy supplied to the compressor. Indicator of the efficiency of the system C.O.P. = RE/(-Ws) C.O.P. = (Ha - He)/(Hb - Ha) Refrigeration Effect (RE) = Ha – He -Ws = Hb - Ha 19 Power of compressor Theoretical power: Pth = -Ws. mr = mr (Hb - Ha) mr is refrigerant mass flow rate (kg/s) H is enthalpy of refrigerant at the point (kJ/kg refrigerant) Compressor efficiency: 0.75 < η < 0.95 due to slippage of refrigerant and frictional resistance Real power: Preal = Pth / η 20 Example 1 Problem: The evaporator of a refrigeration unit operates at a pressure of 1.3 x 105 Pa. What are the temperature and the latent energy of evaporation of the liquid refrigerant (R-134a)? What is the entropy of the fluid entering the compressor? Use the Pressure-Enthalpy diagram for R-134a posted on Blackboard to solve the question. P: 1.3 x 105 Pa (or 0.13 MPa) Saturation T (Te): -19°C Enthalpy at e: 175 kJ/kg ∆h = 387 - 175 = 212 kJ/kg Entropy at a: 1.75 kJ/kg.K. 21 Example 2 Problem: The vapor (R-134a) entering the compressor is at -10 °C and H=395 kJ/kg. Find its state, entropy, and pressure? State is at the saturation line :Saturated vapor The entropy corresponding to H=395 kJ/kg: 1.74 kJ/kg.K Pressure: 0.2 MPa 22 Example 3 Problem: At the outlet of a compressor the vapor (R-22) coming out has the following characteristics: H=443 kJ/kg, T=60 °C. What are its condensation temperature, pressure, and entropy? If the compression was adiabatic, what was its temperature at the inlet of the compressor? Solution: (submit as Assignment 1) 23 Refrigeration load Rate of heat removal from a given space (or object) in order to decrease the temperature to the desired level. Factor should be considered in calculating the refrigeration load: Heat of respiration by the food such as fruits and vegetables. Heat transfer through walls, floor, and ceiling. Heat from the warm air entering the room when the door is opened, people entered, use of fork lifts, or when lights are turned on. Refrigeration capacity should be ≥ Refrigeration load Consider unsteady state vs. steady state load 24 Compressor The most common type is the positive displacement compressor which includes reciprocating compressors and rotary compressors. Centrifugal compressors (turbo-compressors) can also be used in particular cases. Rotary vane www.berg-group.com Centrifugal www.globalspec.com Reciprocating piston www.globalspec.com 25 Condenser and Evaporator Heat exchangers Temp. differential of at least 10°C is recommended. Temp. of refrigerant in the condenser should be 10°C higher than ambient temp. Temp. of refrigerant in the evaporator 10°C lower than the temp. of refrigerated area. Condensers Promote releasing more heat Evaporator Promote absorbing more heat 26 Condenser Air-cooled ambient air Water-cooled circulate water (or another coolant) Evaporative air & water 27 Expansion valve controlling how much refrigerant enters the evaporator. Temperature and Pressure Sensing: The valve is equipped with a sensor that monitors the temperature and pressure at the evaporator outlet. This information helps the valve decide how much to open or close. Diaphragm Control Valve: This is part of the valve that physically opens or closes to regulate refrigerant flow. It responds to signals from the temperature sensor. Remote Control Calibration Sensor: Attached to the diaphragm valve, this sensor provides precise control based on the conditions sensed in the evaporator outlet, allowing for adjustments to be made remotely. Return Spring: A component that helps return the valve to its default position, ensuring it can adjust quickly to changes in temperature and pressure 28 Processing systems Air systems Direct immersion Indirect contact Cryogenic freezing (≤ -60ºC) 29