*3.1.5. Exhaust air system*

In a single-fan dual-duct VAV system, a VFD is installed on the supply fan. For a dual-fan dual-duct VAV system with separate supply fans for the hot and cold deck, a VFD is installed on each fan. If there is a return fan in this system as well, a VFD is also equipped on the return

Typically, for a single-fan dual-duct system, the supply fan is modulated to maintain the cold deck static pressure, whereas the hot deck main damper is modulated to maintain the hot deck static pressure set point. For a dual-fan dual-duct system, each supply fan speed is modulated to maintain its own static pressure set point. Similarly, with single-duct VAV system, the return fan speed is modulated to maintain the airflow difference between the supply and return fans.

Typically, for a single‐fan dual‐duct system, the supply fan is modulated to maintain the cold deck static pressure, whereas the hot deck main damper is modulated to maintain the hot deck static pressure set point. For a dual‐fan dual‐duct system, each supply fan speed is modulated to maintain its own static pressure set point. Similarly, with single‐duct VAV system, the return fan speed is modulated to maintain the airflow difference between the

The energy savings of a dual-duct VAV system are often derived from the fan speed control and duct supply air temperature reset. Liu and Claridge [15] presented the models for the maximum potential energy savings by optimizing the hot deck and cold deck reset schedules,

The energy savings of a dual‐duct VAV system are often derived from the fan speed control and duct supply air temperature reset. Liu and Claridge [15] presented the models for the maximum potential energy savings by optimizing the hot deck and cold deck reset

A multi-zone system serves multiple zones with each zone having its own thermal require‐ ment. 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 7 shows the schematic diagram of a typical multi-zone system where a VFD is installed

A multi‐zone system serves multiple zones with each zone having its own thermal requirement. Like 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 7 shows the schematic diagram of a typical multi‐zone system

Figure 7 Multi‐zone VAV system (three zones)

where 75% in potential savings can be expected.

where a VFD is installed on the supply fan.

**Figure 7.** Multi-zone VAV system (three zones)

schedules, where 75% in potential savings can be expected.

*3.1.4. Multi-zone system*

*3.1.4. Multi‐Zone System*

supply and return fans.

174 New Applications of Electric Drives

on the supply fan.

fan.

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 8 shows an exhaust air system where a VFD is installed on the exhaust fan. variable, the exhaust airflow is adjustable accordingly. Figure 8 shows an exhaust air system

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. where the 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

**Figure 8.** Exhaust air system

#### **3.2. Water Systems 3.2. 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. *3.2.1. Chilled Water System and Condenser Water System* 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.

Figure 8 Exhaust air system

#### Chilled water system and condenser water system are two independent systems in the *3.2.1. Chilled water system and condenser water system*

required building pressure.

chiller plant. Figure 9 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 system, the chilled water pump circulates the chilled water through the evaporator of Chilled water system and condenser water system are two independent systems in the chiller plant. Figure 9 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: primaryonly 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 efficiency point for single or multiple pumps are one of the study topics.

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 decreases. Reduced pump flow results in great pump power savings. 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 temperature difference (Δ*T*). 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

Furthermore, the VFDs could be installed on the fans of cooling tower. The fan speed is **Figure 9.** Chilled water and condenser water system

*3.2.2. Hot Water System*

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

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

Figure 9 Chilled water and condenser water system
