HVAC systems come in several configurations that differ significantly from each other. For example, an air conditioning system based on rooftop units is drastically different from a refrigeration installation. However, HVAC installations have a common denominator: they use electric motors subject to variable loads. Whether these motors are used to drive compressors, water pumps or fans, there is a huge opportunity to save energy by optimizing part-load operation.
In general, running an engine at partial speed is more energy efficient than running it intermittently at full speed.
- Intermittent operation provides only linear savings. For example, a cooling tower fan with an 80% duty cycle consumes 20% less energy than an identical fan operating full time.
- Reducing engine speed provides cubic savings. . On the other hand, slowing down a fan to 80% speed reduces power consumption by almost 50%. The average airflow is the same as a fan running at full power 80% of the time, but savings are significantly increased thanks to speed control.
Speed control can be accomplished with variable frequency drives (VFD), and they have a wide range of applications in HVAC systems due to the cyclic nature of the loads. It's important to note that VFDs are also known as variable speed drives (VSD), adjustable frequency drives (AFD), or adjustable speed drives (ASD).
Fractional horsepower motors in small-scale HVAC systems can also achieve significant savings with speed control. The main difference here is that electronically commutated motors (ECMs) are more economical than VFDs in these applications.
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How Variable Frequency Inverter Works
As explained in electrical engineering textbooks, the speed of an electric motor is defined by the frequency of the power supply and its internal construction. For example, a motor running at 1800 RPM with a 60 Hz power supply in the US would run at 1500 RPM in the UK where the frequency is 50 Hz. Thus, if a motor is suitable for reduced frequency operation, its speed may be reduced during partial load conditions to achieve energy savings. VFDs are connected between the power supply and the motor, adjusting voltage and frequency as a means of speed control.
In applications where only single-phase supply is available, VFDs can be used to integrate three-phase motors and at the same time achieve the benefits of energy efficiency: there are models capable of producing a three-phase voltage from a single-phase input, while adjust magnitude and frequency to control motor speed.
In addition to improving energy efficiency, VFDs allow soft starting of motors by gradually increasing voltage and frequency, rather than directly applying full voltage at 60 Hz. Electric motors draw five to eight times their rated current when started directly , and the voltage drop resulting from the inrush current can damage sensitive equipment.
The main limitation of VFDs is that they produce a phenomenon called harmonic distortion, where high-frequency currents are induced in branch circuits. However, this can be controlled with a properly specified harmonic filter; This device absorbs current distortions at the point of consumption, preventing their propagation throughout the installation.
VFD applications in HVAC systems
In most cases, VFDs are deployed in HVAC systems to control the speed of fans, pumps, or compressors.
Cooling Tower Fans
The main purpose of a cooling tower is to remove heat from a water circuit, which can be used for process cooling or for a cooling plant. Cooling towers use fans to establish airflow, improving convective heat removal. They are open discharge fans, where there are no ducts that create resistance to air flow; therefore, the opportunity for savings through speed control is significant.
When a VFD is deployed in a cooling tower fan, the speed is typically controlled based on the water temperature. Instead of turning the fan on and off, it can be operated at a reduced speed so that the water returning to the cooler or process is maintained at a constant temperature – as explained previously, reduced speed operation is much more efficient than intermittent operation at full speed. speed.
Packaged air handling units and rooftop units
Unlike fans in cooling towers, those used in AHUs and packaged RTUs blow air into a duct system, which offers greater resistance and requires establishing a specific static pressure and airflow. These ducts can have several outlets with variable air volume (VAV) boxes, where the air flow is controlled individually for each zone through a register.
Without a VFD, the fan in the AHU or RTU operates at full speed and each VAV box is individually tuned. However, this is an inefficient approach – if none of the VAV boxes are fully open, energy is being wasted in the form of extra pressure. On the other hand, if the blower is equipped with a VFD, an interesting energy saving strategy becomes possible:
- Fan speed can be gradually reduced while the VAV boxes are gradually opened to maintain constant airflow and temperature.
- The temperature of the different zones is not affected, but the pressure is reduced, saving energy.
- Speed reduction continues until one of the VAV boxes reaches the fully open position. At this point it is not possible to reduce speed further without affecting interior temperatures.
There is also a comfort benefit with this control strategy: the reduced pressure drop translates into less noise, making indoor environments more comfortable.
Speed control for water pumps
Water loops are a key element of many HVAC installations, including those that utilize chillers, boilers, and water source heat pumps. Because HVAC loads are variable, it is often necessary to adjust water flow depending on the total system load. There are three main ways to achieve this:
- Choke Valve -A choke valve is installed in line with the flow and, as its name implies, regulates the flow by partially closing. This control method is simple, but the resulting pressure loss represents a considerable waste of energy.
- Recirculation valve - With this approach, a valve is installed in a parallel recirculation circuit, which is gradually opened to divert part of the water flow, reducing the flow in the main circuit. In this case, however, there is also a significant loss of energy because the recirculated water represents pumping power.
- Pump Speed Control – The most energy efficient approach is to install a VFD on the pump motor. If reduced water flow is required, simply reduce pump speed accordingly. Without pressure or recirculation losses, the savings achieved are significant. This configuration also uses a choke valve for convenience, but it is typically kept in the fully open position as the VFD regulates the flow.
Chillers with variable speed compressors
In air conditioning systems with chilled water, the chiller is by far the equipment with the highest energy consumption. Therefore, it is highly recommended to install the most efficient chiller model possible.
Energy-efficient chillers have helical rotary compressors, capable of adjusting their speed with a VFD depending on cooling needs. Some models may feature multiple individual compressors for greater flexibility and greater part-load efficiency.
When a chiller is highly efficient, the control strategy is typically based on maximizing its capacity rather than operating it at the lowest possible set point. The savings obtained through reducing the load on complementary systems tend to be much greater than the extra consumption in the chiller. There are exceptions, of course, and only monitoring and control can provide the correct answer 100% of the time.
Integrating the entire HVAC system
Equipping all motors in an HVAC system with VFDs is a first step towards energy efficiency, but the best results can only be achieved with a central control system, capable of evaluating building conditions and adjusting HVAC set points in time. real.
The interaction between a chiller and a cooling tower is a great example of how control engineering and VFDs can be applied to HVAC installations:
- Reducing the cooling tower fan speed increases the cooling load on the chiller.
- The reverse also applies: reducing the cooling power in the chiller may require more heat to be rejected by the cooling tower.
If the chiller has high efficiency, the best option in most scenarios is to reduce the cooling tower load. However, only one control system can balance the operation of both components in real time. Ideally, the control system should be able to determine the set point for each individual VFD so that power consumption is minimized at the total system level.
Of course, the performance of an HVAC system begins at the design stage. For best results, be sure to work with licensed and qualified professionals. Installing an efficient, automated HVAC system from the ground up is much cheaper than upgrading an existing installation.
