Mechanical: Air Handler Systems
Goal: The air handler fan is typically the second largest energy use in the mechanical system, and can even exceed the energy use of the cooling plant in some cases. Optimizing the air handler system for datacenter use, as opposed to relying on traditional air handler design rules developed over years of office system design, is essential to achieve an efficient and cost effective system.
- Objective 1: Minimize fan power requirements. Fan energy is a major operation cost that can often be reduced through design and control.
- Strategies:
- Low Pressure Drop System Design. Use low pressure drop air handlers and ductwork. A face velocity of 250-300 fpm is appropriate for datacenters, which operate 8760 hours a year with continuous loading. Underfloor plenums should be sized to provide space for low pressure drop airflow after accounting for an appropriate level of blockage from utilities, conduits, and other underfloor infrastructure.
More > Central Air Handlers [See Chapter 3 of the Design Guidelines Sourcebook] - Use Redundant Air Handler Capacity in Normal Operation. With the use of Variable Speed Drives and chilled water based air handlers, it is most efficient to maximize the number air handlers operating in parallel at any given time. Power usage drops approximately with the square of the velocity, so operating two units at 50% capacity uses a sum total less energy than a single unit at full capacity.
More > Central Air Handlers [See Chapter 3 of the Design Guidelines Sourcebook]
- Low Pressure Drop System Design. Use low pressure drop air handlers and ductwork. A face velocity of 250-300 fpm is appropriate for datacenters, which operate 8760 hours a year with continuous loading. Underfloor plenums should be sized to provide space for low pressure drop airflow after accounting for an appropriate level of blockage from utilities, conduits, and other underfloor infrastructure.
- Metric: Fan power Watts / CFM under typical conditions. This metric accounts for both the pressure drop of the entire system and the efficiency of the fans used. Typical condition should include operation of the redundant unit(s) where applicable. A baseline system operates at about 1.08 W/CFM, while a good system can achieve 0.30 W/CFM (corresponding to a 1.5 in. w.g. total system pressure drop).
- Metric: Economizer Presence and Lockout Band. An airside economizer is a significant energy saver. Assuming proper design of the humidification control and return air exhaust, a wider band between lockouts indicates greater savings potential.
- Strategies:
- Objective 2: Use an optimized airside economizer. The typical datacenter load profile is ideally suited for cooling with outdoor air during much of the year, particularly at night, in most climates.
- Strategies:
- Implement an Airside Economizer. Datacenters in most climates can significantly benefit from an airside economizer. Datacenters can often be served by using outdoor air during cooler weather and particularly at night when, unlike most office buildings, datacenters still require significant cooling.
More > Economizer [See Chapter 2 of the Design Guidelines Sourcebook] - Design For Medium Temperature Air. Size ducting and layout the datacenter room(s) to allow for cooling with the use of air at 60F or higher temperatures. Set the datacenter temperature setpoint near the top of the ASHRAE TC 9.9 Thermal Guidelines for Data Processing Environments recommended temperature range for the applicable datacenter class (the most stringent datacenter class has an allowable high temperature of 95F and recommended of 77F).
More > Economizer Air Management [See Chapter 2 of the Design Guidelines Sourcebook] - Control to Avoid Unnecessary Humidity Loads. Humidification and dehumidification are very energy intensive. Outdoor air moisture content should be continuously monitored, typically as a drybulb-temperature independent measurement such as dewpoint or absolute humidity. Control to ensure that economization is not resulting in humidity loads in excess of the cooling load reduction it provides. Alternatively, utilize an efficient humidification technology, such as adiabatic humidification driven by waste heat from the return air stream or other source.
More > Economizer Humidification [See Chapter 2 of the Design Guidelines Sourcebook]
- Implement an Airside Economizer. Datacenters in most climates can significantly benefit from an airside economizer. Datacenters can often be served by using outdoor air during cooler weather and particularly at night when, unlike most office buildings, datacenters still require significant cooling.
- Strategies:
- Objective 3: Use large centralized air handlers. Centralized air handlers offer efficiency improvements from larger equipment while accommodating a number of controls and configuration efficiency opportunities.
- Strategies:
- Use Load Diversity to Minimize Fan Power. Use a central air handler system can save energy by running the entire system at a lower pressure drop when areas of the datacenter are loaded below the design assumptions. A distributed system can only realize fan savings on the one or two units serving the lightly loaded area, assuming the smaller units are equipped with Variable Speed Fans.
More > Central Air Handlers [See Chapter 3 of the Design Guidelines Sourcebook] - Optimize Air Handler for Fan Efficiency and Low Pressure Drop. Centralized air handlers are commonly located outside of the datacenter space, in mechanical rooms or on the roof. Since expensive datacenter floor area is not consumed by the units footprint, it is economical to make them moderately larger to reduce the velocity and drop the fan power requirements. Typically, they also allow cleaner entry into and exit from the fan, reducing system effects and allowing the fan to operate more efficiently.
More > Central Air Handlers [See Chapter 3 of the Design Guidelines Sourcebook] - Configure Redundancy to Reduce Fan Power Use in Normal Operation. When multiple small distributed units are used, redundancy must be equally distributed. Achieving N+1 redundancy can require the addition of a large number of extra units, or the oversizing of all units. A central air handler system can achieve N+1 redundancy with the addition of a single unit. The redundant capacity can be operated at all times to provide a lower air handler velocity and an overall fan power reduction, since fan power drops with the square of the velocity. Light loading.
More > Central Air Handlers [See Chapter 3 of the Design Guidelines Sourcebook] - Use Premium Efficiency Motors and Fans. Larger motor sizes are more efficient. Assuming NEMA premium efficiency motors, a 30 HP fan will be 2% more efficient than a smaller 10 HP fan. Larger, lower rpm fans can also be selected for a higher efficiency than most smaller fans.
More > Central Air Handlers [See Chapter 3 of the Design Guidelines Sourcebook] - Control Volume by Variable Speed Drive on Fans Based on Space Temperature. The central air handlers should use variable fan speed control to minimize the volume of air supplied to the space. The fan speed should be varied in series with the supply air temperature in a manner that reduces fan speed to the minimum speed possible before increase supply air temperature above a reasonable setpoint. Typically, supply air of 60F is appropriate to provide the sensible cooling required by datacenters.
More > Central Air Handlers; Air Management [See Chapters 1 and 3 of the Design Guidelines Sourcebook]
- Use Load Diversity to Minimize Fan Power. Use a central air handler system can save energy by running the entire system at a lower pressure drop when areas of the datacenter are loaded below the design assumptions. A distributed system can only realize fan savings on the one or two units serving the lightly loaded area, assuming the smaller units are equipped with Variable Speed Fans.
- Strategies:
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