Cooling Cycle & Heat Transfer Principles Flashcards
7 cards from real ATP practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Cooling Cycle & Heat Transfer Principles flashcards as text
In a transcritical CO₂ refrigeration system, heat is rejected at the gas cooler rather than a condenser because:
Answer: CO₂ operates above its critical point during heat rejection, so no condensation occurs
CO₂ has a critical temperature of ~88°F (31°C), so in warm climates, heat rejection occurs above the critical point where condensation cannot happen.
Fouling factor in a heat exchanger specification accounts for:
Answer: Thermal resistance added by scale, biofilm, or deposits over time
Fouling factor (Rf) represents the additional thermal resistance from deposits that accumulate on heat transfer surfaces during operation.
The logarithmic mean temperature difference (LMTD) method is used in heat exchanger analysis to:
Answer: Account for the varying temperature difference between fluids along the exchanger length
LMTD provides an effective average temperature driving force when the temperature difference between fluids changes from inlet to outlet.
Which compressor characteristic curve shows the relationship between mass flow rate and pressure ratio at various speeds?
Answer: Compressor map (performance map)
A compressor performance map plots mass flow rate vs. pressure ratio (or head) at different speeds, including surge and choke limits.
In heat transfer, the Biot number (Bi) is used to determine:
Answer: Whether internal conduction resistance or surface convection resistance dominates in a solid
Bi = hL/k; when Bi > 1, internal conduction limits heat transfer.
A refrigeration system's evaporator is said to be 'starved' when:
Answer: Insufficient refrigerant flow causes the outlet to be highly superheated with reduced capacity
A starved evaporator receives inadequate refrigerant feed, resulting in high superheat and reduced active coil area for evaporation, lowering capacity.
When comparing counterflow and parallel-flow heat exchangers with the same inlet conditions, the counterflow arrangement achieves:
Answer: Higher effectiveness because it maintains a more uniform temperature difference along its length
Counterflow maintains a higher and more uniform LMTD throughout the exchanger, enabling greater heat transfer for the same area compared to parallel flow.