ATP Precision Cooling Architecture 2 — Questions and Answers
Question 1: What is the primary operational difference between a Computer Room Air Conditioner (CRAC) and a Computer Room Air Handler (CRAH)?
- CRAC uses chilled water coils while CRAH uses direct expansion refrigerant
- CRAC uses direct expansion refrigerant while CRAH uses chilled water coils (Correct answer)
- CRAC is ceiling-mounted while CRAH is floor-mounted
- CRAC handles humidification while CRAH only provides cooling
Correct answer: CRAC uses direct expansion refrigerant while CRAH uses chilled water coils
A CRAC unit contains its own DX refrigeration compressor, while a CRAH relies on an external chilled water plant and uses water-cooled coils.
Question 2: In-row cooling units are deployed specifically to address which data center challenge?
- Reducing total water consumption
- Delivering cooling close to high-density heat loads with shorter airflow paths (Correct answer)
- Eliminating the need for raised floor infrastructure
- Providing redundancy for perimeter CRAC units
Correct answer: Delivering cooling close to high-density heat loads with shorter airflow paths
In-row coolers are placed between server racks to shorten airflow paths, reducing mixing and improving cooling effectiveness for high-density deployments.
Question 3: Which cooling metric expresses a precision cooling unit's ability to remove heat relative to its total power consumption?
- COP (Coefficient of Performance) (Correct answer)
- SHR (Sensible Heat Ratio)
- EER (Energy Efficiency Ratio)
- CFM per kW
Correct answer: COP (Coefficient of Performance)
COP is the ratio of useful cooling output to electrical energy input; a higher COP indicates a more efficient unit.
Question 4: A Sensible Heat Ratio (SHR) near 1.0 in a precision cooling unit indicates what?
- The unit is removing mostly latent (moisture) heat
- The unit is removing mostly sensible (temperature) heat (Correct answer)
- The unit has failed its dehumidification circuit
- The unit is operating above rated capacity
Correct answer: The unit is removing mostly sensible (temperature) heat
Precision cooling units are designed with high SHR (close to 1.0) because IT equipment generates mostly sensible heat, not latent heat.
Question 5: When sizing a precision cooling system, which factor is most commonly used to express the total cooling load?
- Watts per square foot of floor space
- kW or tons of refrigeration based on IT load (Correct answer)
- BTU/hr per rack unit (RU)
- CFM per kilowatt of power draw
Correct answer: kW or tons of refrigeration based on IT load
Data center cooling capacity is sized in kW or tons directly matched to the total IT power load, since nearly all IT power converts to heat.
Question 6: What is the recommended supply air temperature range for data center cooling according to ASHRAE A1 equipment class guidelines?
- 50–59°F (10–15°C)
- 59–77°F (15–25°C)
- 64.4–80.6°F (18–27°C) (Correct answer)
- 41–59°F (5–15°C)
Correct answer: 64.4–80.6°F (18–27°C)
ASHRAE A1 specifies an allowable inlet temperature range of 59–89.6°F, but recommended supply temperatures generally target 64.4–80.6°F (18–27°C) for energy efficiency.
Question 7: Overhead or ceiling-mounted precision cooling units typically supply cooled air in which direction relative to IT equipment?
- Horizontal, directly into server front intakes
- Downward into the cold aisle or directly onto equipment tops (Correct answer)
- Upward to create positive plenum pressure
- Through perforated floor tiles from below
Correct answer: Downward into the cold aisle or directly onto equipment tops
Overhead cooling units direct conditioned air downward into cold aisles or onto equipment, complementing or replacing raised-floor airflow distribution.
ATP Precision Cooling Architecture 2
What is the primary operational difference between a Computer Room Air Conditioner (CRAC) and a Computer Room Air Handler (CRAH)?