*3.2.2. Hot water system*

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. Many investigations and case studies were conducted on the efficiency, reliability, and optimization of primary–secondary or primary-only chilled water systems [16–18]. When VFDs are installed on chilled water pumps, how to operate pumps under the maximum

In a chilled water system, as seen in Figure 9, 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

In a chilled water system, as seen in Figure 9, 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 delta T while simultaneously maintaining the minimum water flow requirement for chillers. The secondary pumps are modulated to maintain the loop differential pressure. As the building cooling load reduces, the required chilled water

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 (Δ*P*) or

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

Figure 9 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.

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.

The hot water system delivers the hot water from boilers or heat exchangers to the heating coils of air‐handling units or terminal boxes inside the building. In traditional operation, the

efficiency point for single or multiple pumps are one of the study topics.

decreases. Reduced pump flow results in great pump power savings.

flow decreases. Reduced pump flow results in great pump power savings.

temperature difference (Δ*T*).

176 New Applications of Electric Drives

*3.2.2. Hot Water System*

**Figure 9.** Chilled water and condenser water system

pressure (Δ*P*) or temperature difference (Δ*T*).

The hot water system delivers the hot water from boilers or heat exchangers to the heating coils of air-handling units or terminal boxes inside the building. In traditional operation, the water pumps are running at full speed. The heating valves at the end users are modulated to control the airside temperature set point. Figure 10 shows a hot water system with VFD installed on both primary and secondary pumps. After installing of VFDs, the speed of secondary pump is often modulated to maintain the supply and return temperature difference or loop differential pressure. The speed of primary pump can track that of secondary pump and should be high enough to ensure sufficient water going through boilers. is less so that their lifetime is prolonged. In addition, the compressor power is reduced.Variable Frequency Drive Applications in HVAC Systems

**Figure 10.** Hot water system

#### **3.3. Air Compressors 3.3. Air compressors**

compressor power is reduced.

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 11 shows a schematic diagram of an air compressor system with a VFD installed on each compressor. 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 11 shows a schematic diagram of an air compressor system with a VFD installed on each compressor.

Figure 10 Hot water system

Compressed air has many applications in the manufacturing process. In the HVAC

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, when a VFD is installed, the wear and tear on the compressors is less so that their lifetime is prolonged. In addition, the 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 compressor.

compressor power is reduced.

**3.4. Refrigeration Systems**

Figure 11 Air compressor system

major device where the VFD is installed in a refrigeration loop. The typical applications

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 11 shows a schematic diagram of an air compressor system with a VFD installed on each

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

wear and tear on the compressors is less so that their lifetime is prolonged. In addition, the

**Figure 11.** Air compressor system

#### **3.4. Refrigeration systems** Refrigeration systems are also good candidates for VFD applications. The compressor is the

Refrigeration systems are also good candidates for VFD applications. The compressor is the major device where the VFD is installed in a refrigeration loop. The typical applications include RTUs, HPs, CRAC units, and chillers. include RTUs, HPs, CRAC units, and chillers.
