**3.1. Air systems**

1 1 2 2 *Q N*

1 1 2 2 *P N P N* æ ö <sup>=</sup> ç ÷ ç ÷ è ø

1 1 2 2 *W N W N* æ ö <sup>=</sup> ç ÷ ç ÷ è ø

specific control strategy rather than a fixed low-speed operation without modulation.

**2.3. Variable speed compressors**

170 New Applications of Electric Drives

of variable speed drives in HVAC systems [7–9].

2

3

Equation (3) clearly indicates how the speed change affects the power change. For example, 50% speed reduction results in 87.5% power reduction theoretically. There are several types of air and water systems including the single-duct variable air volume (SDVAV) system, dualduct variable air volume (DDVAV) system, single-zone system, multi-zone system, primary and/or secondary chilled water system, and the hot water system. Each type of system requires

Variable speed compressors or variable frequency compressors are compressors equipped with VFDs. In the HVAC industry, there are several types of compressors associated with the refrigeration systems: reciprocating, screw, scroll, and centrifugal. All of them are viable components for a VFD application. The commonly used working media includes air and refrigerant. Air compressors are frequently used in industrial processes as a power source for pneumatic control systems. Refrigerant compressors are typically used in air conditioners, unitary air-handling units (AHU), or chillers in residential and commercial buildings. Signif‐ icant progresses on the theoretical study and applications on variable speed compressors were made during the past thirty years. As early as 1982, Itami et al. [5] conducted an experimental study on the performance and reliability of a rotary compressor and reciprocating compressor combined with frequency inverters. In 1996, Qureshi and Tassou [6] presented a review of application of variable speed capacity control on refrigeration systems. It pointed out that VFD applications on compressors were largely restricted to small-capacity air-conditioning units thus far, with only a small amount of applications to medium-range capacity units. More research work was still required. Since then, dramatic studies were conducted on applications

Unlike other types of compressors, centrifugal compressors have similar energy performance as centrifugal fans and pumps. A scroll compressor is especially suitable for a VFD due to its intrinsic structural design. Scroll compressors and reciprocating compressors are the most commonly used types on rooftop units (RTUs) and heat pumps (HPs) units. For the majority

*Q N*<sup>=</sup> (1)

(2)

(3)

## *3.1.1. 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 4 presents a typical single-zone VAV system.

typical single‐zone VAV system.

Figure 4 Typical single‐zone VAV system **Figure 4.** Typical single-zone VAV system

Typically, a single‐zone air‐handling unit is operated to control the temperature of only one space. The conventional method is to integrate the cooling and heating valve to control the space cooling and heating temperature set point. For a VFD‐equipped supply fan, the fan speed can be modulated to maintain the space temperature set point, whereas the cooling and heating coil valves are used to control the supply air temperature (SAT). Typically, a single-zone air-handling unit is operated to control the temperature of only one space. The conventional method is to integrate the cooling and heating valve to control the space cooling and heating temperature set point. For a VFD-equipped supply fan, the fan speed can be modulated to maintain the space temperature set point, whereas the cooling and heating coil valves are used to control the supply air temperature (SAT).

Since the 2010 edition of ASHRAE Standard 90.1 [12], some requirements were added for single‐zone VAV system control. It required that single‐zone AHU and fan coil units with chilled water cooling coil and supply fan with motor greater than 5 hp shall have supply fans controlled by two‐speed motors or VFD. Similarly, all the AHUs and AC units with direct‐expansion (DX) cooling coil and a capacity ≥110,000 Btu/h that serve single zones shall have their supply fans controlled by the two‐speed motor or VFD. These requirements are mandatory. Since the 2010 edition of ASHRAE Standard 90.1 [12], some requirements were added for single-zone VAV system control. It required that single-zone AHU and fan coil units with chilled water cooling coil and supply fan with motor greater than 5 hp shall have supply fans controlled by two-speed motors or VFD. Similarly, all the AHUs and AC units with directexpansion (DX) cooling coil and a capacity ≥110,000 Btu/h that serve single zones shall have their supply fans controlled by the two-speed motor or VFD. These requirements are manda‐ tory.

There are many successful case studies on VFD applications for single‐zone units in projects. Li et al. [13] applied the VFD technology to a dozen single‐zone systems by installing a VFD on each of them. They demonstrated that installing VFDs on supply fans in a series of single‐zone units can save much more energy than running half of the units at constant speeds and shutting off the remaining half. There are many successful case studies on VFD applications for single-zone units in projects. Li et al. [13] applied the VFD technology to a dozen single-zone systems by installing a VFD on each of them. They demonstrated that installing VFDs on supply fans in a series of singlezone units can save much more energy than running half of the units at constant speeds and shutting off the remaining half.

#### *3.1.2. Single‐Duct VAV System 3.1.2. 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 5 shows a typical SDVAV system.

In a single-duct VAV system, VFDs are installed on the supply fan and return fan. Typically, the supply fan speed is modulated to maintain the duct static pressure at its set point. As the system load is reduced, the VFD speed decreases to maintain the same set point. Meanwhile, the set point need not be kept at a constant value. With the system load reduced, less airflow is needed to deliver to the space. The static pressure set point could be reset to meet the condition. This set point can be reset based on the VFD speed or supply fan airflow [14].

Figure 5 Typical single‐duct VAV system **Figure 5.** Typical single-duct VAV system

units may also have a preheat coil, bypass damper, and return fan. Figure 4 presents a

Figure 4 Typical single‐zone VAV system

Typically, a single‐zone air‐handling unit is operated to control the temperature of only one space. The conventional method is to integrate the cooling and heating valve to control the space cooling and heating temperature set point. For a VFD‐equipped supply fan, the fan speed can be modulated to maintain the space temperature set point, whereas the cooling

Typically, a single-zone air-handling unit is operated to control the temperature of only one space. The conventional method is to integrate the cooling and heating valve to control the space cooling and heating temperature set point. For a VFD-equipped supply fan, the fan speed can be modulated to maintain the space temperature set point, whereas the cooling and heating

Since the 2010 edition of ASHRAE Standard 90.1 [12], some requirements were added for single‐zone VAV system control. It required that single‐zone AHU and fan coil units with chilled water cooling coil and supply fan with motor greater than 5 hp shall have supply fans controlled by two‐speed motors or VFD. Similarly, all the AHUs and AC units with direct‐expansion (DX) cooling coil and a capacity ≥110,000 Btu/h that serve single zones shall have their supply fans controlled by the two‐speed motor or VFD. These requirements are

Since the 2010 edition of ASHRAE Standard 90.1 [12], some requirements were added for single-zone VAV system control. It required that single-zone AHU and fan coil units with chilled water cooling coil and supply fan with motor greater than 5 hp shall have supply fans controlled by two-speed motors or VFD. Similarly, all the AHUs and AC units with directexpansion (DX) cooling coil and a capacity ≥110,000 Btu/h that serve single zones shall have their supply fans controlled by the two-speed motor or VFD. These requirements are manda‐

There are many successful case studies on VFD applications for single‐zone units in projects. Li et al. [13] applied the VFD technology to a dozen single‐zone systems by installing a VFD on each of them. They demonstrated that installing VFDs on supply fans in a series of single‐zone units can save much more energy than running half of the units at constant

There are many successful case studies on VFD applications for single-zone units in projects. Li et al. [13] applied the VFD technology to a dozen single-zone systems by installing a VFD on each of them. They demonstrated that installing VFDs on supply fans in a series of singlezone units can save much more energy than running half of the units at constant speeds and

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

In a single-duct VAV system, VFDs are installed on the supply fan and return fan. Typically, the supply fan speed is modulated to maintain the duct static pressure at its set point. As the system load is reduced, the VFD speed decreases to maintain the same set point. Meanwhile, the set point need not be kept at a constant value. With the system load reduced, less airflow is needed to deliver to the space. The static pressure set point could be reset to meet the condition. This set point can be reset based on the VFD speed or supply fan airflow [14].

and heating coil valves are used to control the supply air temperature (SAT).

coil valves are used to control the supply air temperature (SAT).

typical single‐zone VAV system.

172 New Applications of Electric Drives

**Figure 4.** Typical single-zone VAV system

mandatory.

tory.

speeds and shutting off the remaining half.

*3.1.2. Single‐Duct VAV System*

shutting off the remaining half.

5 shows a typical SDVAV system.

*3.1.2. Single-duct VAV system*

For the return fan, there are several control methods: modulating the return fan speed to maintain (a) return duct static pressure or (b) building differential pressure. However, these controls are not reliable due to the pressure measurement. A new control method is using a volume tracking method to maintain the airflow difference between the supply and return fans. the supply fan speed is modulated to maintain the duct static pressure at its set point. As the system load is reduced, the VFD speed decreases to maintain the same set point. Meanwhile, the set point need not be kept at a constant value. With the system load reduced, less airflow is needed to deliver to the space. The static pressure set point could be reset to meet the condition. This set point can be reset based on the VFD speed or supply fan

In a single‐duct VAV system, VFDs are installed on the supply fan and return fan. Typically,

#### *3.1.3. Dual-duct VAV system* airflow [14]. For the return fan, there are several control methods: modulating the return fan speed to

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 dualduct 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 6 shows the schematic diagram of a single-fan DDVAV system. maintain (a) return duct static pressure or (b) building differential pressure. However, these controls are not reliable due to the pressure measurement. A new control method is using a volume tracking method to maintain the airflow difference between the supply and return fans. *3.1.3. 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

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 return fan in this system as well, a VFD is also equipped on

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

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

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

Figure 6 Single‐fan DDVAV system schematic diagram **Figure 6.** Single-fan DDVAV system schematic diagram

return fan.

supply and return fans.

*3.1.4. Multi‐Zone 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 fan. Typically, for a single‐fan dual‐duct system, the supply fan is modulated to maintain the

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. 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.

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

#### *3.1.4. Multi-zone system 3.1.4. Multi‐Zone System*

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 on the supply fan. 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)

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

where a VFD is installed on the supply fan.

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.
