Refrigeration Systems

Thermodynamics, Heat Transfer, Fluid Mechanics and Refrigeration and Air Conditioning.

In this course unit, the program content is discussed with the support of different information and communication tools, as a way to capture students’ interest and encourage more active participation in classes, e.g., profusely illustrated slides, use of software applications and technical catalogs, organization of lectures with experts, problem-solving exercises, and oral presentations by students on topics relevant to the course unit. In particular, topics in theoretical classes are presented through an application-driven approach, which triggers discussion among the audience. “What-if” questions are extensively raised to engage students in the discussion. Students are also encouraged to deepen their understanding through self-conducted research, consulting the recommended bibliography and other documents, including manufacturers’ catalogs and computer applications for the selection of equipment and accessories. Additionally, each student presents specific topics in the classroom, followed by discussion periods. In theoretical-practical classes, problem-solving is promoted to verify the knowledge level already acquired. In practical classes, several technical visits are made to relevant commercial and industrial facilities in the refrigeration sector, as well as visits to the DEM/ISEC air conditioning laboratory to familiarize students with equipment, tools, and accessories used in the air conditioning and refrigeration industry. Students also design a refrigeration plant, individually or in pairs, increasing interactivity in the learning process. In developing the project, computer applications and technical catalogs from refrigeration manufacturers are used, allowing students to become aware of the different practical aspects of selecting and sizing the equipment and accessories used in the plant. Lectures are also organized with professionals and experts in relevant areas to the course unit. Finally, it should be noted that teachers are available to answer students’ questions during office hours or via email.

The primary objective of this course unit is for students to develop engineering skills related to refrigeration systems, learning and selecting available technologies at the domestic, commercial, and industrial levels. To this end, the fundamental principles governing the operation of the various components of a refrigeration system, required for their correct selection and sizing, are addressed. At the end of the course unit, students are expected to be able to: integrate refrigeration fundamentals with refrigeration systems design; describe the different principles and processes used in refrigeration, including main thermodynamic cycles, their variants and intended objectives; describe the principles governing the cold preservation of perishable goods, including cold chain logistics; calculate the heat load of a cold storage chamber; identify the equipment and accessories used in refrigeration, describing their operating principle, selecting and sizing components, and characterizing the refrigerants used.

Introduction: Concepts and units used in refrigeration. History of Refrigeration. Major refrigeration systems:
their classification and application examples. Special examples of application: ground freezing and freeze
drying.
Refrigeration Cycles: Theoretical vapor compression refrigeration cycle. Irreversibilities and the real cycle. Performance coefficients (COP, EER, SEER). Absorption refrigeration cycle. Main factors that influence refrigeration capacity and compression power. Changes to the basic refrigeration cycle: Introduction of a heat exchanger. Expansion vessel for gas-flash removal. Expansion vessel for intermediate cooling. Optimum pressure for intermediate cooling. Installations with multiple compressors and multiple evaporators. Multi-stage cycles and cascade cycles.
Refrigerants: General properties of refrigerants. Effects on the performance of a refrigeration cycle. Traditional, transitional and long-term, natural and synthetic refrigerants: CFCs, HCFCs, HFCs, HCs, HFOs, CO2, NH3. Azeotropic and zeotropic mixtures. Temperature glide. Nomenclature adopted. Safety of refrigerants: flammability and toxicity. Impact on the environment: depletion of the ozone layer and greenhouse effect; replacement of CFCs, HCFCs and HFCs. Refrigerants and moisture in refrigeration plants. Filter-dryers: types, localization, sizing and selection of filter-dryers.
Conservation and transport of perishable products: Cold storage of perishable products. Main methods for cooling, refrigerating and freezing food products. Industrial cold logistics.
Refrigeration compressors: Compressors in refrigeration: Types of compressors and application. Capacity of refrigeration compressors. Volumetric efficiency and adiabatic compression efficiency. Compression ratio. Adiabatic discharge  temperature. Lubrication of compressors. Oil separator: Types, sizing and selection. Compressors of variable capacity; different modes of capacity regulation.
Condensers and evaporators: Types of condensers: air-cooled and water-cooled condensers; evaporative condensers. Determination of the condenser capacity. Types of evaporators. Different methods of feeding refrigerant to the evaporator. Parameters that influence the performance of the evaporator. Calculation of the evaporator capacity.
Control in a refrigeration plant: Expansion valves: float valves, thermostatic valves, and electronic valves; external pressure equalization. Capillary tube; sizing. Thermostats and pressure switches. Other valves and accessories for controlling and regulating a refrigeration circuit. Defrosting methods.
Calculation of refrigeration plants: Thermal load of a cold room. Different components involved: physical envelope, product load, internal equipment, service. Calculation of a cold room: total refrigeration capacity to be installed.
Software for the refrigeration sector: Software for determination of thermodynamic properties of refrigerants, analysis of the performance of a refrigeration cycle, selection of compressors and condensing units.
Legislation in the refrigeration sector: Legislation, standards and regulations for the refrigeration
sector, at EU and national levels.

The evaluation of the course unit comprises 4 components. A minimum grade of 30% is required in each component:
1) Final written exam (35%);
2) Project of a refrigeration plant (40%);
3) Oral presentation of a refrigeration topic (17,5%);
4) Written report of a field visit or invited lecture (7,5%).
Additional notes:
• If there are no lectures and/or technical visits, the respective weight in the final grade is transferred to the final written exam;
• Non-attendance in the lecture(s) or technical visit(s), if duly justified, implies that the respective weight in the final grade is transferred to the final written exam.

NAO