Variable Frequency Drives (VFDs) have revolutionized the operation and energy efficiency of HVAC systems by enabling precise control of motors and fans. In various HVAC applications such as air handling units, chilled water systems, and air compressors, VFDs optimize system performance by adjusting the speed of motors based on real-time demand, reducing energy consumption and enhancing operational efficiency. From single-zone variable air volume (VAV) systems to complex multi-zone configurations, VFDs play a crucial role in maintaining comfortable indoor environments while reducing operating costs. This article explores the various applications of VFDs in HVAC systems, highlighting their impact on performance, energy savings, and system longevity.
Single-zone Variable Air Volume (VAV) system
Single-zone VAV system is the simplest air system. The VAV system mainly includes outdoor air and return air dampers, filter, heating and cooling coils, and a supply fan. Some units may also have a preheat coil, bypass damper, and return fan. Figure 1 presents a typical single-zone VAV system.
Figure 1
Typical single-zone VAV system
Typically, single zone air handling units are used to control the temperature of only one space. The traditional approach is to integrate cooling and heating valves to control space cooling and heating temperature setpoints. For VFD-equipped supply fans, the fan speed can be adjusted to maintain the space temperature setpoint, while the cooling and heating coil valves are used to control the supply air temperature (SAT).
Since the 2010 edition of ASHRAE Standard 90.1, some requirements have been added for the control of single-zone variable air volume systems. It is required that single-zone AHUs and fan coil units with chilled water cooling coils and supply fans with motors greater than 5 horsepower shall have two-speed motors or inverter-controlled supply fans. Similarly, all ahu and AC units with direct expansion (DX) cooling coils with a capacity ≥ 110,000 Btu/h and serving a single zone shall have supply fans controlled by two-speed motors or variable frequency drives. These requirements are mandatory.
Single-duct VAV system
The single-duct VAV system is the most popular system, which comprises a main AHU, ductwork and a number of terminal boxes. The air-handling units are comprised of an outdoor air damper and return air damper, filter, preheating coil, cooling coil, and safety devices. Figure 2 shows a typical SDVAV system.
Figure 2
Typical single-duct VAV system
In a single duct VAV system, the vfd is mounted on the supply and return fans. Typically, the duct static pressure is maintained at its set point by adjusting the speed of the supply fan. As the system load decreases, the vfd speed decreases to maintain the same setpoint. At the same time, the setpoint does not need to be maintained at a constant value. As the system load decreases, less airflow needs to be delivered to the space. The static pressure setpoint can be reset to meet conditions. This setpoint can be reset based on the inverter speed or supply fan airflow.
For return fans, there are several methods of control:the return fan speed is adjusted to maintain either (a) the return duct static pressure or (b) the building differential pressure. However, these controls are not reliable due to pressure measurements. A new control method is to use the volume tracking method to maintain the airflow difference between the supply and return fans.
Dual-duct VAV system
A dual-duct variable air volume (DDVAV) system handles hot and cold air separately and delivers them through hot and cold ductwork. The hot air and cold air are mixed at the terminal box and then supplied to the space. There are two types of DD system: the single-fan dual-duct system and the dual-fan dual-duct system. The first one has a supply fan delivering the airflow to both hot and cold decks. The second one has a dedicated supply fan in each deck. The cold deck includes a cooling coil, whereas the hot deck is equipped with a hot water or steam coil. Figure 3 shows the schematic diagram of a single-fan DDVAV system.
Figure 3
Single-fan DDVAV system schematic diagram
In a single fan dual duct variable air volume system, install a variable frequency drive on the supply fan. For a two-fan, two-duct VAV system with separate air supply to the hot and cold decks, a VFD is installed on each fan, and if there is also a return fan in this system, a VFD is also installed on the return fan.
Typically, for a single fan dual duct system, the supply fan is adjusted to maintain the cold deck static pressure and the hot deck main dampers are adjusted to maintain the hot deck static pressure set point. For a two fan, two duct system, the speed of each supply fan is adjusted to maintain the respective static pressure set point. Similarly, for single duct variable air volume systems, the airflow difference between the supply and return fans is maintained by adjusting the return fan speed.
Multi-zone system
A multi-zone system serves multiple zones with each zone having its own thermal requirement. Like a dual-duct system, one multi-zone system has cold and hot decks. However, the difference is that the cold air and hot air mixes at the outlet of air-handling unit before delivery to the space, whereas in a dual-duct system the hot air and cold mixes at the terminal boxes. Figure 4 shows the schematic diagram of a typical multi-zone system where a VFD is installed on the supply fan.
Figure 4
Multi-zone VAV system (three zones)
In a multi-zone system, the supply fan speed is modulated to maintain the discharge air static pressure or the temperature in the worst zone at its set point. The zone damper is modulated to maintain each zone temperature set point.
Exhaust air system
An exhaust air system is often associated with one air-handling unit, make-up unit, or fresh air unit. An exhaust air system is applicable for several types of facilities, such as kitchens, cafeterias, and laboratories in the hospital, just to list a few. They require enough fresh air and associated exhaust air. Proper exhaust airflow should be provided to satisfy the building or space pressure requirement. As the airflow delivered by air-handling unit is variable, the exhaust airflow is adjustable accordingly. Figure 5 shows an exhaust air system where a VFD is installed on the exhaust fan.
In this exhaust air system, the VFD is modulated to maintain the suction air pressure set point, or the differential airflow between the supply and exhaust air to maintain the required building pressure.
Figure 5
Exhaust air system
Water systems
The major water systems in HVAC system include chilled water system, condenser water system, and hot water system. Each system has dedicated pumps circulating water through a closed or an open loop. VFDs can be installed on these systems, which could reduce the pump energy consumption at partial load conditions.
1.Chilled water system and condenser water system
Chilled water system and condenser water system are two independent systems in the chiller plant. Figure 6 shows a typical chiller plant comprising these two loops. A chilled water system includes one or more chillers, chilled water pumps, and cooling coils. The cooling coils are usually located in the AHUs or fan coil units. There are two types of pumping system: primary-only system and primary–secondary system. In a primary-only system, the chilled water pump circulates the chilled water through the evaporator of chillers and cooling coils. In a primary–secondary system, there are two loops. The primary pumps circulate chilled water through the chiller only, while the secondary pumps circulate the chilled water through buildings. Usually, there is one bypass pipe, which connects the primary and secondary water loops.
In a chilled water system, as seen in figure 6, the cooling load of each coil varies at different zones and times, making the required chilled water flow variable. The primary pumps are modulated to maintain the loop differential pressure while simultaneously maintaining the minimum water flow requirement for chillers. The secondary pump speeds are equal to the primary pump speeds. As the building cooling load reduces, the required chilled water flow decreases. Reduced pump flow results in great pump power savings.
In a condensing water system, the condensing water pump circulates the condensing water through the condenser of chillers and cooling tower. When a VFD is installed on the condensing water pump, the pump speed is adjusted to maintain the loop differential pressure (ΔP) or temperature difference (ΔT).
Figure 6
Chilled water and condenser water system
Furthermore, the VFDs could be installed on the fans of cooling tower. The fan speed is optimized to maintain the condensing water leaving temperature from the cooling tower.
Hot Water Systems
A hot water system delivers hot water from a boiler or heat exchanger to the heating coils of an air handling unit or terminal box in a building. In conventional operation, the pumps run at full speed. A heating valve at the end user is adjusted to control the airside temperature set point. Figure 7 shows a hot water system with inverters installed on both the primary and secondary pumps. With the inverter installed, it is often necessary to adjust the speed of the secondary pump to maintain the supply and return temperature differential or loop pressure differential. The primary pump speed tracks the secondary pump speed and should be high enough to ensure that enough water passes through the boiler.
Figure 7
Hot water system
Air compressors
Compressed air has many applications in the manufacturing process. In the HVAC industry, air compressors can be used to generate the pressurized air to drive the pneumatic actuators for dampers and valves in air-handling units. The compressed air is stored in a pressurized tank, which serves as an air source to the end users. Traditionally, the pressure of tank is maintained by the on–off control of one or multiple air compressors. Figure 8 shows a schematic diagram of an air compressor system with a VFD installed on each compressor.
Figure 8
Air compressor system
Typically, staging control is used to maintain the compressed air pressure. When the end users require less compressed air and the compressed air pressure is higher than the set point, the compressor will shut off. On the contrary, one more compressor starts when the end user utilizes more compressed air and the compressed air pressure drops down below the set point. This inefficient control causes frequent compressor start–stops, which definitely shortens the lifetime of the compressor. However, if a VFD is installed, the wear and tear on the compressors is less so that their lifetime is prolonged. In addition, the compressor power is reduced.
The integration of Variable Frequency Drives into HVAC systems has significantly improved their flexibility, efficiency, and sustainability. Whether in single-zone or multi-zone air handling systems, water systems, or air compressors, VFDs provide the ability to fine-tune motor speeds, ensuring optimal performance while minimizing energy consumption and wear on equipment. By reducing unnecessary motor operations and allowing for dynamic system adjustments, VFDs not only contribute to lower energy bills but also extend the lifespan of critical HVAC components. As buildings increasingly strive for energy efficiency and operational reliability, VFDs remain an essential technology for modern HVAC solutions.
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