DC HVAC flashcards
DC HVAC · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Sensible heat ratio
- Ratio of sensible cooling to total cooling. In a data hall it is typically 0.90-1.00 because IT equipment releases almost no latent heat.
- ASHRAE A1 envelope
- Recommended IT inlet range of 18-27 C dry-bulb and 5.5-15 C dew point. Operation outside it raises failure rates and can void warranties.
- Dew point control
- Keeping supply air dew point below the coldest surface in the hall. It prevents condensation and avoids simultaneous humidification and dehumidification.
- Air density correction
- Cooling capacity depends on mass flow, not volumetric flow. At 2,000 m altitude, density is about 1.0 kg/m3 versus 1.2 at sea level, cutting capacity roughly 17%.
- Sensible cooling formula
- Q_s = 1.08 x CFM x delta-T in Btu/h, or Q_s = 1.21 x L/s x delta-T in watts. It gives coil sensible capacity from airflow and temperature difference.
- Latent load sources
- In a tight hall, latent load comes mainly from infiltration through doors, people, and open humidifiers. Most halls need humidification only in dry climates.
- CRAC versus CRAH
- CRAC is a self-contained DX unit with a compressor inside the hall. CRAH is a chilled-water coil unit fed by a central plant and has no compressor.
- Redundancy nomenclature
- N is minimum capacity, N+1 adds one unit, 2N duplicates the full path, and 2(N+1) duplicates plus one spare. Concurrent maintainability allows servicing without dropping load.
- Fan affinity laws
- Flow scales with speed, head with speed squared, and power with speed cubed. A 50% speed cut leaves about 12.5% of design power.
- Water-side economizer
- A plate heat exchanger between cooling-tower water and the chilled-water loop. It provides free cooling when outdoor wet-bulb is low enough.
- Cooling tower approach
- Leaving condenser water temperature minus entering wet-bulb temperature. A good design approach is 3-5 K; a larger approach raises chiller kW per ton.
- Three-way versus two-way valves
- Three-way valves keep constant plant flow but waste pump energy. Two-way valves with variable flow save pump energy but need minimum-flow control.
- Air-cooled versus water-cooled
- Air-cooled chillers avoid water and towers but have lower efficiency and depend on ambient dry-bulb. Water-cooled chillers need make-up water and treatment.
- Hot-aisle and cold-aisle layout
- Racks face front-to-front in cold aisles and back-to-back in hot aisles. Servers draw from the cold aisle and exhaust into the hot aisle.
- Bypass and recirculation
- Bypass is cold supply air that never reaches IT intake. Recirculation is hot exhaust that reaches IT intake. Both raise intake temperature or waste cooling.
- Rack Cooling Index
- RCI measures how well rack intakes stay within the temperature envelope. A value of 100% means every rack is within limits.
- Cold-aisle containment
- Enclosing the cold aisle with roof panels and end doors. It stops hot-air recirculation and allows higher supply temperatures and more economizer hours.
- Hot-aisle containment
- Enclosing the hot aisle and returning exhaust air directly to the CRAC or CRAH. It captures hot air at the source and reduces mixing.
- Blank panels and brush strips
- Lowest-cost containment measures. An open 1U slot can leak a large fraction of a rack's cold air, so unused spaces must be sealed.
- Raised-floor plenum velocity
- Plenum velocity should stay under about 2.5 m/s to limit friction loss and turbulence. Cable trays under the floor block flow and should be minimized.
- Ton refrigeration conversion
- One ton of refrigeration equals 3.517 kW and 12,000 Btu/h. A 1 kW IT load needs roughly 0.28 tons of cooling plus overheads.
- Power Usage Effectiveness
- PUE is total facility energy divided by IT equipment energy. Annualized PUE covers 12 months and includes cooling, losses, and lighting.
- Partial PUE
- pPUE scopes a boundary such as one hall or module. It is useful when plant is shared across tenants and full-facility PUE hides local performance.
- DCiE
- DCiE is the reciprocal of PUE expressed as a percentage. A PUE of 1.5 equals 66.7% DCiE.
- Water Usage Effectiveness
- WUE equals liters of water divided by IT kWh. Evaporative cooling can push WUE to 2-5 L/kWh, so it must be read alongside PUE.
- Carbon Usage Effectiveness
- CUE equals carbon emissions divided by IT kWh. It depends on grid carbon intensity and on-site fuel use.
- Part-load chiller efficiency
- Chillers are most efficient at 40-70% load. Oversizing moves operation away from this band, so IPLV/NPLV is a better comparison than full-load COP.
- Supply air temperature reset
- Raising supply air temperature while all rack intakes remain inside the envelope. Each 1 K rise can meaningfully extend economizer hours.
- Economizer high-limit lockout
- A switch or enthalpy comparison that stops outside-air free cooling above a set condition. A failed damper actuator is a common cause of frozen coils.
- Filter differential pressure trending
- Monitoring pressure rise across filters to detect loading. Filters should be changed on pressure rise, and fan curves verified after each change.
- Low delta-T syndrome
- Chilled-water delta-T falls below design while flow rises. Causes include fouled coils, three-way valves, and control valve bypass; plant kW/ton worsens.
- Condenser water reset
- Lowering tower water temperature where the chiller allows, often to 20-24 C, to cut compressor lift and kW/ton, subject to minimum condenser temperature limits.