Designing Mega-AHUs

The advent of fan arrays along with the increased cost competitiveness in the custom air-handling unit (AHU) market has given rise to a new AHU design option: very large AHUs, or mega-AHUs, designed for over 100,000 cfm (50 000 L/s). Here are the advantages and disadvantages of mega-AHUs vs. multiple floor-by-floor AHUs for high-rise buildings and reviews design considerations and options.

Fan Arrays
One of the most significant innovations in air-handling unit design is the concept of using an array of small single-width, single inlet direct-drive plenum fans in lieu of the more conventional design that includes one or two large plenum or housed centrifugal fans. Fan arrays can be used for supply, return, and relief fan assemblies and they offer several significant advantages vs. conventional fan system designs:

• Reduced AHU length;
• Reduced sound power, especially on the discharge side;
• Improved redundancy;
• Reduced fan energy if sound attenuators or system effect are eliminated;
• Easier motor and fan replacement; and
• Easier to install in retrofit applications.

Disadvantages include higher (but ever falling) first costs and increased weight for the fan section. These are minor relative to the advantages.

Custom Air Handlers
Another significant change in the industry has been improved competitiveness of custom AHUs. Until recently, the cost premium for custom AHUs versus modular commercial AHUs was a factor of 5 or so. But improvements in manufacturing processes and lower labor rates in adjacent countries have caused the premium to be reduced to a factor of 1.5 or 2 from about 20,000 cfm (10 000 L/s) to 50,000 cfm (25 000 L/s). This premium can be offset by installation cost savings with the ability to make the AHU almost any desired dimension and aspect ratio and include any desired features. For AHUs above about 50,000 cfm (25 000 L/s), there is no cost premium for custom AHUs—this market sector is dominated by custom AHU manufacturers.

The combination of fan arrays and affordable custom AHUs has also made it practical to design very large mega-AHUs. Our firm has designed several projects with AHUs in the 100,000 cfm (50 000 L/s) to 200,000 cfm (100 000 L/s) range serving large variable air volume (VAV) distribution systems. We have two high-rise office building projects in the design phase that have partially field-built VAV AHUs designed for 240,000 cfm (120 000 L/s) and 585,000 cfm (275 000 L/s), respectively.

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(Article provided by ASHRAE Journal, 04/2018)

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