Fundamentals of Air Conditioning Systems Flashcards
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Read the first 7 Fundamentals of Air Conditioning Systems flashcards as text
What causes a compressor to 'short cycle' in a residential air conditioning system?
Answer: Oversized system or faulty thermostat causing rapid on/off cycling
Short cycling typically results from an oversized system that satisfies the thermostat setpoint too quickly or a malfunctioning thermostat, leading to excessive starts and stops.
Which type of metering device maintains a constant evaporator outlet superheat regardless of load changes?
Answer: Thermostatic expansion valve (TXV)
A TXV uses a sensing bulb to modulate refrigerant flow and maintain a nearly constant superheat at the evaporator outlet across varying load conditions.
A residential condensing unit's condenser fan motor is running but the compressor is not. Which safety device has most likely opened?
Answer: High-pressure switch or internal thermal overload
A high-pressure switch trips when discharge pressure exceeds the set point, or an internal thermal overload opens when the compressor windings overheat, both shutting off the compressor.
What is the primary difference between a single-stage and a two-stage compressor in residential AC?
Answer: Two-stage operates at full and partial capacity; single-stage runs at full capacity only
A two-stage compressor can run at a lower capacity stage during mild conditions and switch to full capacity during peak demand, improving efficiency and comfort.
What does an elevated wet-bulb temperature at the evaporator inlet indicate?
Answer: High indoor humidity or high sensible heat ratio
A higher wet-bulb temperature at the coil inlet indicates more moisture (latent heat) in the air, increasing the total load on the evaporator.
During a nitrogen pressure test on a refrigerant system, the technician should use a:
Answer: Dry nitrogen cylinder with a pressure regulator set to the test pressure
Dry nitrogen with a regulator is the approved method for pressure testing because it is inert, dry, and will not leave residue or introduce moisture.
When replacing a TXV, the technician must ensure the sensing bulb is:
Answer: Clamped firmly to the suction line and insulated from ambient air
The TXV sensing bulb must make good thermal contact with the suction line and be insulated so it accurately senses refrigerant vapor temperature, not ambient temperature.