Modern HVAC systems are often a complex arrangement of equipment such as chillers, boilers, heat pumps, ductwork, and air handling fans. Since every commercial location is unique and has specific needs, facility managers must carefully consider all aspects of each system. Some equipment recommendations that were once considered optional have become a necessity in order to meet local regulations and code requirements.
One technology which has been around for many years and is now seeing widespread adoption is the variable frequency drive (VFD). The energy-efficiency benefits of a VFD system can be significant. In this post, we will explore four important ways that they can positively impact an operation.
A VFD allows the motor connected to an air handling or chilled water system to operate at partial operating speed. Constant-speed systems operate at only two settings, on or off, and this causes the system to switch between periods of full operation and idle time. Utilizing VFDs in an HVAC system allows operators to match the performance of the system to the actual demands of the environment at any given time.
Variable frequency drives are equipped with control panels and durable graphic overlays. Metalphoto® Overlays and Anodized Aluminum Overlays are a good choice for VFDs, as they offer excellent resistance to harsh environmental conditions such as extreme temperatures and UV, as well as resistance to chemicals, abrasion, and solvents. VFDs should also have durable data plates that include specifications and important operations information, including equipment data and applicable safety warnings.
Now, let’s take a look at four ways VFDs can improve efficiency for commercial HVAC systems.
Water chillers often have the highest energy consumption in an HVAC system, and it is always recommended to start with energy-efficient equipment whenever building a new system or upgrading core equipment. With a static-flow chiller system, the water flow at any given time must be adjusted with valves to modify the delivery of water to the system lines. Choke valves are commonly used to directly reduce the flow or recirculation valves to divert flow and lesson the output to the system. These both have the effect of reduced efficiency through pressure drop and recirculation loss.
Modern energy-efficient chillers often have a helical rotary compressor capable of being operated with a VFD. By using a VFD to regulate the motor operation on the system, the chiller pump can maintain an appropriate flow with no pressure loss or recirculation loss. This is more efficient overall and gives the operator the ability to maximize the use of the chiller system to reduce the operating burden across the other components of the HVAC system. This is often the best way to reduce the overall energy consumption in the system but can vary between sites and equipment.
A VFD has a number of technical benefits that can help protect your drives and extend their useful life. By regulating the amount of power that feeds your system, a VFD eliminates any voltage sag from the incoming power grid. In addition, the VFD can start the system from zero voltage and frequency while requiring an overall lower amount of power to begin operation. This not only helps reduce the wear on motor hardware but also reduces power spikes during peak time operation, which can help reduce energy costs. The use of a VFD provides a smoother operation overall for your motor drives, which can reduce repairs and streamline your maintenance schedules.
In a traditional HVAC system, the blower fans that are utilized in air handling units (AHUs) and roof-top units (RTUs) must operate at full speed in order to provide the right amount of airflow and resistance to keep the entire system in balance. Individual zones are typically controlled with variable-air-volume (VAV) boxes that set how much air is allowed into each area to achieve the desired temperature. When a VFD is installed on these motors, the blower speed can be changed gradually without making major disruptions to airflow throughout the system. This can have the effect of keeping the temperature constant across all the zones while reducing the pressure, and therefore saving energy when the system achieves the desired operating temperature.
While rebate qualifications vary by state and region throughout the U.S., they should not be overlooked when considering VFD as part of your installation. It is possible for a VFD system to help your organization meet qualification requirements and help you to offset a partial cost for your system. According to the American Council for an Energy-Efficient Economy, these types of rebates typically cover 15-35% of the total installed cost. When considering your potential return on investment for a VFD system, those numbers can help you see a return in a very short amount of time.
While VFD components are clearly an important part of many commercial HVAC systems, deciding exactly how to implement them can be a challenge. By comparing the specific site requirements of each location with the potential efficiency improvements of any changes, facility management can continue to make informed decisions about their equipment. Monitoring the performance of all components in an HVAC system over time will always be the best way to find optimal settings. The use of VFDs can provide greater opportunities for regulating the speed of drives within your system and are likely to enable new levels of efficiency.