Corpus sync: neutrality remakes, note hygiene, datasheet relocation, transformer-split cross-refs
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title: Computer Room Air Conditioning Units
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# Environmental and Service Conditions {toc}
−## Precision cooling units shall be selected and rated to maintain the project's environmental envelope at the served equipment air intakes, and to deliver scheduled net sensible capacity at the design entering air condition and the design entering fluid or refrigerant condition of the heat-rejection system. {note}
+## Precision cooling units shall be selected and rated to maintain the project's environmental envelope at the served equipment air intakes, and to deliver scheduled net sensible capacity at the design entering air condition and the design entering fluid or refrigerant condition of the heat-rejection system.
## ASHRAE TC 9.9 Environmental Envelope {toc}
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### The unit fan shall be selected to deliver design airflow against the actual external static pressure of the raised-floor plenum, supply and return paths, filters, and any containment, so that the unit delivers design airflow in the installed condition.
−### Selecting a downflow unit at its free-discharge airflow rather than against the actual raised-floor plenum static pressure is a frequent cause of units that fail to deliver design airflow and rooms that develop hot spots; the plenum depth and floor-tile open area shall be accounted for in the fan selection. {note}
+### Selecting a downflow unit at its free-discharge airflow rather than against the actual raised-floor plenum static pressure is a frequent cause of units that fail to deliver design airflow and rooms that develop hot spots; the plenum depth and floor-tile open area shall be accounted for in the fan selection.
### External static pressure datasheet {toc}
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### EC fans shall be commanded continuously by the unit controller to maintain the controlled air condition over the unit's operating range.
−### Fan airflow shall be modulated on under-floor or supply-air plenum pressure, or on the served equipment delta-T, rather than held at a fixed speed, so that fan energy falls as the IT load falls; fan power varies approximately with the cube of airflow, so a modest airflow reduction yields a large fan-energy saving. {note}
+### Fan airflow shall be modulated on under-floor or supply-air plenum pressure, or on the served equipment delta-T, rather than held at a fixed speed, so that fan energy falls as the IT load falls; fan power varies approximately with the cube of airflow, so a modest airflow reduction yields a large fan-energy saving.
### Variable airflow control datasheet {toc}
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## Total Cooling Capacity {toc}
−### Total cooling capacity defines the unit's combined sensible and latent cooling output at design conditions and shall be reported with the net sensible capacity so that the sensible heat ratio is documented. {note}
+### Total cooling capacity defines the unit's combined sensible and latent cooling output at design conditions and shall be reported with the net sensible capacity so that the sensible heat ratio is documented.
### Total cooling capacity datasheet {toc}
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### Chilled-water coils shall have seamless copper tubes mechanically expanded into aluminum or copper plate fins and shall be circuited for the design entering and leaving water temperature and the design waterside pressure drop.
−### Coil row count shall be selected to deliver scheduled net sensible capacity at the design entering water temperature, because a higher entering water temperature requires more coil surface to reach the same supply-air temperature. {note}
+### Coil row count shall be selected to deliver scheduled net sensible capacity at the design entering water temperature, because a higher entering water temperature requires more coil surface to reach the same supply-air temperature.
### Each chilled-water coil shall be served by a control valve sized for the coil flow and the available differential pressure; a two-way modulating or pressure-independent control valve shall be used on variable-flow distribution, and a three-way valve only on a constant-flow plant.
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# Heat-Rejection Equipment {toc}
−## Heat-rejection equipment shall be the manufacturer-matched remote condenser, water-cooled condenser, or glycol drycooler for the DX unit, selected so that the unit reaches its scheduled net sensible capacity at the design ambient or design entering fluid temperature. {note}
+## Heat-rejection equipment shall be the manufacturer-matched remote condenser, water-cooled condenser, or glycol drycooler for the DX unit, selected so that the unit reaches its scheduled net sensible capacity at the design ambient or design entering fluid temperature.
## Air-Cooled Condenser {toc}
### Remote air-cooled condensers shall be selected for the design summer outdoor dry-bulb so that the condensing temperature stays within the compressor envelope at the design ambient.
−### Air-cooled condensers shall include variable-speed (EC) condenser fans with head-pressure control so that the unit can operate stably across the full outdoor temperature range and reduce condenser-fan energy at low ambient. {note}
+### Air-cooled condensers shall include variable-speed (EC) condenser fans with head-pressure control so that the unit can operate stably across the full outdoor temperature range and reduce condenser-fan energy at low ambient.
## Water-Cooled Condenser {toc}
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### Glycol-cooled units shall be served by an outdoor drycooler and a glycol pump package selected for the design entering fluid temperature, the glycol concentration required for the site's lowest ambient, and the design fluid flow.
−### Glycol concentration shall be selected for freeze protection at the lowest expected ambient at the drycooler; an under-concentrated loop will freeze and split the drycooler coil in winter. {note}
+### Glycol concentration shall be selected for freeze protection at the lowest expected ambient at the drycooler; an under-concentrated loop will freeze and split the drycooler coil in winter.
## Fluid Economizer {toc}
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### Reheat shall not be provided unless a specific room condition requires raising the temperature of dehumidified air, because reheat consumes energy to add back heat that the cooling coil has just removed and is unnecessary in a continuously loaded data center.
−### Providing reheat and humidification on every unit, then allowing dehumidification and humidification to run simultaneously, is a classic data center energy fault; on most projects these functions shall be disabled or omitted on the majority of units. {note}
+### Providing reheat and humidification on every unit, then allowing dehumidification and humidification to run simultaneously, is a classic data center energy fault; on most projects these functions shall be disabled or omitted on the majority of units.
### Reheat datasheet {toc}
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### Humidity control across multiple units shall be coordinated by the group controller so that units do not fight — one unit humidifying while an adjacent unit dehumidifies — which wastes both the cooling and the humidifier energy and destabilizes the room.
−### Humidity shall be sensed and controlled on a common basis (a shared room or supply reference, or a single dewpoint setpoint band) so that all units in a room act on the same target; per-unit return-air humidity control with tight deadbands is the configuration that produces fighting. {note}
+### Humidity shall be sensed and controlled on a common basis (a shared room or supply reference, or a single dewpoint setpoint band) so that all units in a room act on the same target; per-unit return-air humidity control with tight deadbands is the configuration that produces fighting.
# Control Method and Redundancy {toc}
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### The cooling units serving a room shall be configured for the project's redundancy level (such as N+1 or N+2) so that the loss of one unit does not allow the room to exceed its environmental envelope.
−### Standby (redundant) units shall participate in lead-lag rotation rather than sit idle, so that all units share runtime and a standby unit is proven operational before it is needed; a standby unit that has never run is a latent failure. {note}
+### Standby (redundant) units shall participate in lead-lag rotation rather than sit idle, so that all units share runtime and a standby unit is proven operational before it is needed; a standby unit that has never run is a latent failure.
### Redundancy datasheet {toc}
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### Condensate shall drain by gravity to a piped condensate system where the geometry permits; where the drain point is above the unit pan, a condensate pump with an integral high-level safety switch shall be provided.
−### A condensate or humidifier-water leak under a raised floor can reach energized equipment and power distribution; condensate routing and leak protection shall be confirmed for every unit over or near a raised-floor power path. {note}
+### A condensate or humidifier-water leak under a raised floor can reach energized equipment and power distribution; condensate routing and leak protection shall be confirmed for every unit over or near a raised-floor power path.
### Condensate drainage datasheet {toc}
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### Downflow units shall be set on the structural floor with a downflow turning plenum or floor stand that interfaces cleanly with the raised-floor supply plenum, sealed so that supply air is not lost into adjacent chases.
−### The raised-floor plenum depth and the open area of perforated floor tiles shall be coordinated so that the under-floor static pressure delivers design airflow to the cold aisles; raised-floor air bypass through unsealed cable cutouts and gaps starves the cold aisle and shall be sealed. {note}
+### The raised-floor plenum depth and the open area of perforated floor tiles shall be coordinated so that the under-floor static pressure delivers design airflow to the cold aisles; raised-floor air bypass through unsealed cable cutouts and gaps starves the cold aisle and shall be sealed.
## Mounting and Support {toc}
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### The commissioning agent shall verify the airflow at each unit and the under-floor or supply plenum static pressure, and shall verify that the units maintain the design environmental envelope at the served equipment intakes under the available load.
−### Where a load bank or the installed IT load is available, sensible capacity shall be verified at the design environmental envelope rather than assumed from the rating. {note}
+### Where a load bank or the installed IT load is available, sensible capacity shall be verified at the design environmental envelope rather than assumed from the rating.
## Redundancy and Sequence Demonstration {toc}
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## The following spare parts datasheet establishes the spares to be delivered at substantial completion. {note}
+## The spare parts to be delivered to the Owner at substantial completion shall be specified on the spare parts datasheet.
+
```datasheet
label: Spare Parts at Substantial Completion
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## The O&M manual shall include a spare-parts inventory list with manufacturer part numbers and reorder information so that additional spares can be procured during the facility's service life.