Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Temperature and pressure
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Temperature and pressure
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Subject: Air Conditioning And Refrigeration
Class: Senior Secondary 3
Term: 1st Term
Week: 7
Theme: Temperature And Pressure
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state the effectsof temperature,pressure and volume on refrigeration and air-conditioningsystem. state Gas laws,Boyies law,Charles law and Dalton law. demonstratecalculationsinvolving gas lawformulae.
Definition: Temperature is a measure of the degree of hotness or coldness of a substance. In the context of refrigeration, it indicates the average kinetic energy of the molecules within a substance, which determines the direction of heat flow. Heat always flows from a region of higher temperature to a region of lower temperature.
Units of Temperature: Celsius (°C): Water freezes at 0°C and boils at 100°C at standard atmospheric pressure. This is the most common unit in Nigeria for daily use. Fahrenheit (°F): Water freezes at 32°F and boils at 212°
F. Kelvin (K): This is the absolute temperature scale, where 0 K represents absolute zero (the lowest possible temperature where molecular motion theoretically ceases). It is crucial for gas law calculations.
Conversion: K = °C + 273.15 (often approximated as K = °C + 273) °F = (°C × 9/5) + 32 °C = (°F - 32) × 5/9 Effects of Temperature on Refrigeration and AC Systems: Refrigerant Boiling/Condensing Points: The boiling point of a refrigerant is directly related to its pressure. Lower temperature in the evaporator allows the refrigerant to boil at a lower pressure, absorbing heat from the refrigerated space. Higher temperature in the condenser allows the refrigerant to condense at a higher pressure, releasing heat to the surroundings.
Heat Transfer Rate: A larger temperature difference between two mediums (e.g., inside a cold room and the refrigerant in the evaporator coil) leads to a faster rate of heat transfer.
System Efficiency: High ambient temperatures (common in Nigeria) increase the load on the condenser, requiring more energy for the compressor to maintain the desired cooling, thus reducing overall system efficiency.
Superheat and Subcooling: Superheat: The temperature difference between the actual temperature of the refrigerant vapour leaving the evaporator and its saturation temperature at that pressure. Adequate superheat ensures no liquid refrigerant returns to the compressor.
Subcooling: The temperature difference between the saturation temperature of the refrigerant liquid leaving the condenser and its actual temperature. Subcooling ensures only liquid refrigerant enters the expansion device, improving efficiency.
Definition: Pressure is defined as the force exerted perpendicularly on a unit area. In refrigeration, it is the force exerted by the refrigerant molecules on the walls of the containing vessel.
Units of Pressure: Pascal (Pa): The SI unit of pressure (N/m2).
Kilopascal (kPa): 1 kPa = 1000 Pa.
Bar: 1 bar = 100,000 Pa = 100 kPa. Commonly used in many parts of the world.
Pounds per Square Inch (psi): A common imperial unit, especially in older equipment or tools.
Atmosphere (atm): Standard atmospheric pressure at sea level (1 atm ≈ 101.325 kPa ≈ 14.7 psi ≈ 1.013 bar).
Types of Pressure: Absolute Pressure (Pabs): Pressure measured relative to a perfect vacuum (zero pressure). This is the pressure used in gas law calculations.
Gauge Pressure (Pgauge): Pressure measured relative to the surrounding atmospheric pressure. Most pressure gauges read gauge pressure.
Vacuum/Suction Pressure: Pressure below atmospheric pressure.
Relationship: P_abs = P_gauge + P_atmospheric (for pressures above atmospheric) P_abs = P_atmospheric - Vacuum (for pressures below atmospheric, where vacuum is a positive value) Effects of Pressure on Refrigeration and AC Systems: Refrigerant Boiling/Condensing Points: Pressure directly influences the boiling (evaporation) and condensing temperatures of refrigerants.
Evaporator (Low Pressure Side): Low pressure allows the refrigerant to boil at a low temperature, absorbing heat from the space to be cooled.
Condenser (High Pressure Side): High pressure causes the refrigerant vapour to condense at a higher temperature, allowing it to reject heat to the warmer ambient air or water.
Compressor Work: The compressor's primary function is to increase the pressure (and thus temperature) of the refrigerant vapour, enabling it to condense. Higher pressure differentials across the system increase the work required by the compressor.
Leakage: Pressure differences drive refrigerant leakage from the system if seals or connections are faulty.
System Performance: Incorrect operating pressures (e.g., due to overcharge, undercharge, or blockages) significantly reduce system efficiency and cooling capacity.
Definition: Volume refers to the amount of three-dimensional space occupied by a substance, typically a gas or liquid.
Units of Volume: Cubic meter (m3): The SI unit.
Litre (L): 1 L = 0.001 m3 = 1000 cm
3. Effects of Volume on Refrigeration and AC Systems: Compressor Displacement: The compressor's volumetric displacement dictates the volume of refrigerant vapour it can process per unit time.
Refrigerant Charge: The total volume of refrigerant within the system affects its performance.
Expansion Device: The expansion valve controls the flow rate (volume) of liquid refrigerant entering the evaporator, which impacts the system's capacity.
Gas Laws: Volume is a critical variable in the gas laws, describing how gases behave under changing conditions.
Principle: For a fixed mass of gas at constant temperature, the absolute pressure is inversely proportional to its volume.
Formula: P1V1 = P2V2 Where: P1 = Initial absolute pressure V1 = Initial volume P2 = Final absolute pressure V2 = Final volume Explanation in AC/Refrigeration: This law explains the operation of the compressor. As the compressor reduces the volume of the refrigerant vapour, its pressure increases significantly (assuming minimal temperature change during the initial compression phase or considering the overall effect of volume reduction).
Worked Example 1 (Boyle's Law): A compressor takes in 5 litres of R-134a refrigerant vapour at an absolute pressure of 100 kPa. If the vapour is compressed to a final volume of 1 litre at constant temperature, what is the final absolute pressure?
Solution: Given: P1 = 100 kPa V1 = 5 L V2 = 1 L T = constant Using Boyle's Law: P1V1 = P2V2 100 kPa × 5 L = P2 × 1 L 500 kPa·L = P2 × 1 L P2 = 500 kPa / 1 L P2 = 500 kPa Therefore, the final absolute pressure of the refrigerant vapour is 500 kPa.
Understanding temperature and pressure and their governing laws is fundamental to numerous aspects of daily life and industry in Nigeria.
Food Preservation and Agriculture: Application: Cold rooms and refrigerated trucks are essential for preserving perishable goods like fruits, vegetables, meat, and dairy products across Nigeria. Farmers in rural areas often rely on cold storage to extend the shelf life of their produce before transport to urban markets, reducing post-harvest losses.
Connection: The ability to maintain low temperatures inside cold rooms depends on the precise control of refrigerant pressure. Lowering the pressure in the evaporator allows the refrigerant to boil at temperatures below freezing, absorbing heat from the produce. Efficient cold chain management (transport, storage) across varying ambient temperatures (e.g., from Jos to Lagos) directly involves the application of gas laws to manage refrigerant behaviour and system performance.
Comfort Cooling and Health: Application: Air conditioning systems in homes, offices, hospitals, and vehicles provide comfort and enhance productivity in Nigeria's hot and humid climate. In hospitals, controlled temperatures are vital for drug storage and patient well-being.
Connection: The cooling effect of AC relies on the refrigerant absorbing heat at low pressure (evaporator) and rejecting it at high pressure (condenser). Understanding partial pressures (Dalton's Law) is critical for managing humidity, which significantly impacts comfort and the spread of airborne diseases. System efficiency, affected by external ambient temperature (Charles's Law effects on condenser pressure), directly translates to electricity consumption, a major cost for Nigerian households and businesses.
Industrial and Commercial Operations: Application: Industries such as brewing (e.g., Nigerian Breweries, Guinness Nigeria), pharmaceuticals, and even large supermarkets (e.g., Shoprite, Spar) use extensive refrigeration systems for their processes and product storage.
Connection: Maintaining specific temperatures and pressures is crucial for fermentation, chemical reactions, and vaccine storage. Technicians performing maintenance or troubleshooting these large-scale systems use pressure gauges and thermometers constantly, applying their knowledge of gas laws to interpret readings and diagnose faults. For instance, an unexpected pressure drop in a process cooler might indicate a leak (Boyle's Law implication) or a temperature drop (Charles's Law implication).