What is in this article?:
- The devil is in the details
- Choose the lowest horsepower fan and vary the speed
- Eliminate the need for reheat
Just as today’s cars are designed and built much differently than cars from the 1970s, so too are air-delivery systems in commercial buildings. Sadly, 70s-style air systems that look and behave like the gas guzzlers of the past are still being designed and built today.
This article documents how today’s high-performance air systems (HPAS) make use of more efficient application engineering, equipment, and control logic. According to a committee within AMCA International—consisting of AMCA member companies and non-members—have defined HPAS as follows:
- Apply modern design approaches with leading-edge products and technology to optimize energy efficiency, comfort, and indoor air quality.
- Integrate the strategies of right-sizing, free-cooling using outside air, and small thermally similar control zones and minimize static pressure, system leakage, and system effects.
- Technologies include high-efficiency fans with variable-speed capacity controls, low-leakage outside-air economizers, low-leakage ducts and fittings, low-pressure-drop components (filters, coils, ducts and diffusers), diagnostic monitoring, and digital controls.
High performance air systems are not a product—they’re not purchased off the shelf as a complete package. HPAS are a result of industry professionals thinking differently about air systems in new construction and substantial renovations to deliver better comfort, lower energy use, and competitive installed cost.
The Devil Is in the Details
In practical terms, HPAS follow the design guidelines listed below. Actually, this list is truncated so we can fit it all into this short article. To really understand how these guidelines combine to deliver a better system without driving construction costs over tight budgets, please visit the AMCA Website.
These guidelines are in accord with ASHRAE’s “Advanced Energy Design Guide for Small to Medium Office Buildings: Achieving 50 % Energy Savings Toward a Net Zero Energy Building.”
Select the Lowest-HP Fan and Vary the Speed
Every HPAS varies fan speed with a change in load—even single-zone systems. Selecting the right fan, minimizing power at design conditions, and maximizing savings at part-load depends on how you control and how you apply the fan. For example:
- Right-size the fan, selecting as close to peak efficiency as capacity controls will allow.
- Use high-efficiency airfoil centrifugal fans. Maximum fan power should not exceed 0.72 W per cfm.
- Use a more efficient scroll type when noise and space limits allow. Use plenum fans when space is constrained.
- Use electronically commutated motors and speed controls where possible (now available up to 15 hp).
- Use variable-speed drives (VSD) on fans with induction motors.
- Control the VSD from a static pressure sensor located close to the last variable air volume (VAV) terminal in the duct run. Use multiple sensors for ductwork with multiple branches. (See “Specify Controls for Low-Energy VAV” below). This capacity-control approach enables fan selections closer to peak efficiency.
- Use the lowest-pressure-drop air system possible (See “Design for the lowest possible pressure drop in the VAV system”).
- Specify that no balancing damper shall be installed before the last VAV terminal or the last diffuser (alternatively that the balancing damper before the last VAV terminal or last diffuser shall remain open) so that the system will be balanced at the lowest possible fan speed.