Thermodynamic Principles and Refrigeration Systems 01/21 040
Discussion on Temperature Perception and Thermostat Impact
Introduction to temperature perception based on individual experience.
Example of an Eskimo experiencing a temperature transition from -20°F to 32°F.
Concept: Subjective experience of temperature can vary; what is considered warm for one may be cold for another.
Temperature difference: 32°F - (-20°F) = 52°F difference.
Discussion on common domestic thermostat issues.
People often adjust thermostats according to their comfort, leading to conflicts in communal spaces such as homes or workplaces.
Personal experience: Issues encountered at Providence regarding conflicting comfort levels among individuals.
Resulting problems: Equipment short cycling, damaging the compressor and other components.
Solution implemented: A new thermostat was installed, but it was disengaged behind the scenes, allowing adjustments without affecting temperature.
Result: Reduced complaints about temperature and maintained efficient function since thermostat settings did not change actual settings.
Heat Exchange Fundamentals
Introduction to the principles of heat exchange in thermodynamic cycles.
Definition: A condenser is a device that rejects heat to the outside environment.
Key Insight: The temperature at the condenser must be higher than the ambient temperature for effective heat transfer.
Example: If the outside temperature is 95°F, the condenser must be at a temperature higher than this for heat to flow effectively.
Explanation of heat movement: Heat naturally moves from areas of higher temperature to areas of lower temperature.
Comparison to tug of war: Idea of dominance; hotter temperatures (like stronger forces in tug of war) will push against cooler ones, attempting to equilibrate.
Detailed Functions of Components
Condenser: Demonstrates the rejection of heat; must be hotter than surrounding air for cooling to occur.
Evaporator: Absorbs heat from the environment; this process is essential in refrigeration systems.
Pressure and Temperature Drops: Discussion of evaporator function and pressure at 20 psi with corresponding temperature dropping to 15°F.
The warmth of the environment increases the temperature of the evaporator; example: varies from 15°F to about 17°F as it starts to absorb heat.
Evaporator must maintain a temperature lower than the desired environment to allow heat removal effectively.
Thermodynamic Principle: Hot temperatures move towards cool areas, providing the foundation for refrigeration.
Manipulation of Temperature: No external heat is added; instead, heat is constantly removed from a system (e.g., cars, refrigerators).
Refrigeration Systems
Differentiation between Split Systems and Package Units:
Split System: Combination of indoor furnace and evaporator unit, typically used in residential settings.
Package Unit: All components housed in a single outdoor unit, often placed on rooftops.
Understanding Pressure Measurements
Definitions of pressure measurements:
PSIG: Pounds per square inch gauge.
PSIA: Pounds per square inch absolute.
Importance of understanding that PSIG reads pressure above atmospheric pressure, while PSIA incorporates atmospheric pressure.
Clarification regarding understanding pressure readings; e.g., at sea level, the atmospheric pressure is approximately 14.7 psi.
Example of calculating gauge and absolute pressures:
75 PSIG corresponds to 75 plus atmospheric pressure, resulting in total pressure measured in PSIA.
Different numerical representations require clear indicators; e.g., 75 must specify if it’s PSIG, PSIA or degrees.
Practical Applications of Pressure Understanding
Demonstrating how to check gauge pressure by disconnecting hoses to observe zero pressure, confirming the accuracy of gauges before calibration.
Methodology to convert PSIA to PSIG and vice versa, using appropriate arithmetic calculations:
Example for calculation: For 75 PSIA to find gauge pressure, subtract atmospheric pressure (14.7): 75 - 14.7 = 60.3 PSIG.
Vice versa for calculating total pressure from gauge pressure by adding atmospheric pressure.
Example provided with 50 PSIG leads to 50 + 14.7 = 64.7 PSIA chosen as a total pressure measurement.