313A Evaporators and Condensers 2 — Questions and Answers
Question 1: What is a flooded evaporator and where is it typically used?
- An evaporator with too much refrigerant — a fault condition
- An evaporator where the tubes are submerged in liquid refrigerant, used in large chillers to cool water flowing through the tubes (Correct answer)
- An evaporator damaged by water flooding
- An evaporator with excessive frost buildup
Correct answer: An evaporator where the tubes are submerged in liquid refrigerant, used in large chillers to cool water flowing through the tubes
A flooded evaporator is a shell-and-tube design where refrigerant floods the shell side, submerging the tubes that carry the fluid being cooled (typically water). Liquid refrigerant is maintained at a level covering the tubes, ensuring efficient heat transfer. This design is common in large centrifugal and screw chillers.
Question 2: What is the split between sensible and latent cooling in an evaporator?
- Sensible cooling changes temperature; latent cooling removes moisture — the ratio affects both comfort and coil selection (Correct answer)
- There is only sensible cooling in an evaporator
- Sensible is for heating; latent is for cooling
- The split only matters in commercial systems
Correct answer: Sensible cooling changes temperature; latent cooling removes moisture — the ratio affects both comfort and coil selection
Evaporators perform both sensible cooling (lowering air temperature) and latent cooling (removing moisture by condensing it on the coil). The sensible heat ratio (SHR) describes the proportion of each. In humid climates, more latent capacity is needed; in dry climates, mostly sensible cooling is required. Coil selection and airflow affect this ratio.
Question 3: What is a direct expansion (DX) evaporator?
- An evaporator that uses hot water for heating
- An evaporator where refrigerant expands directly inside the coil to cool air or fluid passing over or around it, with the metering device controlling flow (Correct answer)
- An evaporator with no fins
- An evaporator that uses a secondary coolant like glycol
Correct answer: An evaporator where refrigerant expands directly inside the coil to cool air or fluid passing over or around it, with the metering device controlling flow
A direct expansion (DX) evaporator is the most common type in residential and commercial HVAC. Refrigerant from the metering device expands directly inside the evaporator tubes, absorbing heat from air passing over the finned coil. The metering device controls refrigerant flow to maintain proper superheat.
Question 4: What is the purpose of a condenser fan cycling control?
- To reduce noise during nighttime operation
- To maintain adequate head pressure during low ambient temperatures by cycling condenser fans on and off (Correct answer)
- To increase airflow during peak cooling demand
- To synchronize fan operation with the compressor
Correct answer: To maintain adequate head pressure during low ambient temperatures by cycling condenser fans on and off
In cold weather, the condenser can reject too much heat, causing head pressure to drop too low for the metering device to function properly. Condenser fan cycling controls turn fans off or reduce their speed to maintain minimum head pressure, ensuring proper refrigerant flow and system operation.
Question 5: How does a cooling tower work with a water-cooled condenser?
- The cooling tower heats the water before it enters the condenser
- The cooling tower cools the condenser water through evaporation of a portion of the water, then returns the cooled water to the condenser (Correct answer)
- The cooling tower stores water for emergency use
- The cooling tower filters contaminants from the water
Correct answer: The cooling tower cools the condenser water through evaporation of a portion of the water, then returns the cooled water to the condenser
A cooling tower cools condenser water by spraying it over fill media while ambient air is drawn through. A small portion of the water evaporates, absorbing heat from the remaining water (evaporative cooling). The cooled water collects in the basin and is pumped back to the condenser. This allows water-cooled systems to reject heat very efficiently.
Question 6: What maintenance should be performed on evaporator coils?
- Evaporator coils are maintenance-free
- Regular cleaning of fins and drain pan, checking drain lines, verifying airflow, and inspecting for corrosion or refrigerant leaks (Correct answer)
- Only replace the entire coil when it stops working
- Only the condenser requires regular maintenance
Correct answer: Regular cleaning of fins and drain pan, checking drain lines, verifying airflow, and inspecting for corrosion or refrigerant leaks
Evaporator coil maintenance includes: cleaning fins with appropriate coil cleaner (to remove dust, mould, and biofilm), cleaning and sanitizing the condensate drain pan, flushing drain lines to prevent blockages, checking airflow across the coil, inspecting for corrosion or refrigerant leaks, and replacing air filters regularly to reduce coil fouling.
313A Evaporators and Condensers 2
What is a flooded evaporator and where is it typically used?