313A Evaporators and Condensers — Questions and Answers
Question 1: What is the temperature difference (TD) across an evaporator and what does it indicate?
- The difference between refrigerant discharge and suction temperatures
- The difference between the air temperature entering the evaporator and the refrigerant saturation temperature; it indicates proper coil sizing and airflow (Correct answer)
- The difference between indoor and outdoor temperatures
- The temperature change of the refrigerant through the compressor
Correct answer: The difference between the air temperature entering the evaporator and the refrigerant saturation temperature; it indicates proper coil sizing and airflow
Evaporator TD is the difference between the entering air temperature and the refrigerant saturation (boiling) temperature inside the coil. A typical TD for comfort cooling is 10°C to 12°C (18°F to 22°F). A higher TD indicates restricted airflow, dirty coils, or an undersized coil; a lower TD may indicate an oversized coil or excess refrigerant.
Question 2: What is the difference between a fin-and-tube evaporator and a plate evaporator?
- They use different refrigerants
- A fin-and-tube uses tubes with attached fins for air-cooled applications; a plate evaporator uses stacked plates for liquid-cooled or direct-contact applications (Correct answer)
- A fin-and-tube is for heating only; a plate is for cooling only
- There is no significant difference
Correct answer: A fin-and-tube uses tubes with attached fins for air-cooled applications; a plate evaporator uses stacked plates for liquid-cooled or direct-contact applications
Fin-and-tube evaporators use copper or aluminium tubes with aluminium fins pressed onto them to increase surface area for air-to-refrigerant heat exchange. Plate evaporators use corrugated stainless steel plates to create channels for refrigerant and the fluid being cooled, offering high efficiency in compact liquid-cooling applications.
Question 3: What causes frost buildup on an evaporator coil and what problems does it create?
- High ambient temperature; it improves cooling
- Operating below the dewpoint of the air combined with coil temperatures below 0°C; it insulates the coil, blocks airflow, and reduces capacity (Correct answer)
- Excess oil in the system; it causes noise
- High refrigerant charge; it causes high efficiency
Correct answer: Operating below the dewpoint of the air combined with coil temperatures below 0°C; it insulates the coil, blocks airflow, and reduces capacity
When the evaporator coil surface temperature is below 0°C and moisture from the air condenses and freezes on it, frost accumulates. Frost acts as insulation, reducing heat transfer, and physically blocks airflow through the coil. This progressively reduces cooling capacity. Heat pump and refrigeration systems use defrost cycles to manage this.
Question 4: What is the approach temperature on a water-cooled condenser?
- The temperature of the water entering the cooling tower
- The difference between the refrigerant condensing temperature and the leaving water temperature; indicates condenser efficiency (Correct answer)
- The outdoor air temperature approaching the unit
- The temperature drop across the water pump
Correct answer: The difference between the refrigerant condensing temperature and the leaving water temperature; indicates condenser efficiency
Approach temperature on a water-cooled condenser is the difference between the refrigerant condensing temperature and the temperature of the cooling water leaving the condenser. A lower approach temperature (typically 3°C to 5°C / 5°F to 10°F) indicates better heat transfer and higher efficiency.
Question 5: What are the three types of condensers used in HVAC systems?
- Plate, shell, and tube
- Air-cooled, water-cooled, and evaporative condensers (Correct answer)
- Horizontal, vertical, and diagonal
- Primary, secondary, and tertiary
Correct answer: Air-cooled, water-cooled, and evaporative condensers
The three types of condensers are: air-cooled (uses ambient air and fans), water-cooled (uses water circulated through a cooling tower or city water), and evaporative (combines air and water spray for enhanced cooling). Air-cooled is most common in residential and light commercial; water-cooled offers higher efficiency in larger systems.
Question 6: Why is it important to keep condenser coils clean?
- Clean coils look more professional
- Dirty coils increase head pressure, reduce system capacity, increase energy consumption, and can cause compressor overheating and failure (Correct answer)
- Clean coils only affect noise levels
- Dirty coils have no significant impact on performance
Correct answer: Dirty coils increase head pressure, reduce system capacity, increase energy consumption, and can cause compressor overheating and failure
Dirty condenser coils restrict airflow and insulate the heat transfer surface, causing the system to operate at higher head pressures. This increases compressor energy consumption, reduces cooling capacity, raises discharge temperatures (risking compressor damage), and shortens equipment life. Regular cleaning is essential preventive maintenance.
313A Evaporators and Condensers
What is the temperature difference (TD) across an evaporator and what does it indicate?