*2.1.2.1.1 Single-duct conventional systems*

Single-duct systems are generally used for conditioning stores, offices, and industries. A schematic diagram of a single-duct system is shown in **Figure 6**, which is fixed supplied air volume flow rate and variable supplied air temperature type, and thus the required temperature in the zone is attained by the adjustment of heating and cooling coil flow rates and thus their temperature. On the contrary, the required temperature in conditioned zone can also be attained by varying supplied volume flow rate and holding fixed supplied air temperature (**Figure 7**) which is accomplished by simply placing dampers in the single duct; thus supplied air volume rate can be varied, e.g., in summer days, higher supplied air volume rates are provided to the zone to reach required temperature.

It is worth mentioning that single-duct system is used in both single and multizones. Return air can be mixed with the fresh air in the mixing box in appropriate ratio. This will provide energy-efficient running of the system; however care must be taken on the quality of return air as air may have higher percentages of humidity levels [1–5, 8]. This configuration can be seen in **Figure 6** and **Figure 7**; however, in **Figure 7** variable supplied air volume flow rate and fixed supplied air temperature type has new arrangement called side passage flow (green lines) where the air once cooled is directed immediately to the mixing box to accelerate the cooling process.

#### *2.1.2.1.2 Multi-duct conventional systems*

Characteristics the characteristics of multi-duct systems are similar to singleduct systems except that two or more ducts can be used for providing conditioned air to two or more zones. The usage of several ducts in these systems provides flexibilities as long as the required condition can be varied according to the requirement in each zone. These systems are used where control of temperature and humidity in a building zone is required [1–5]. **Figure 8** shows two-duct system of variable supplied volume flow rates with fixed supplied air temperature.

#### **Figure 6.**

*A schematic diagram of single-duct system (fixed supplied air volume flow rate and variable supplied air temperature).*

than large speeds and small ducts through the fan. The only disadvantage of this system is that the fan sound can travel with supply air to the conditioned

*A schematic diagram of draw-through type of air handling unit. Redrawn with modification from [2].*

These systems can be classified into two groups [1–5], namely:

temperature is obtained by varying the supplied air temperature.

2.Low initial cost since the system has simple configuration

which can maintain energy-efficient performance

i. Fixed supplied air volume flow rate with variable supplied air temperature

In these systems the rate of air flow remains constant, and the zone-required

i. Variable supplied air volume flow rate with fixed supplied air temperature

1.Simplicity as the components are of simple configuration and can be found

3.Low running cost as ventilation, return air, and side passage flows can be used,

4.Quite operation as air handing unit including the fan is placed in plant room

5.Centralized maintenance as air handling unit is placed in the plant room

These systems maintain fixed supplied air temperature; however the zone-required temperature is maintained by varying the volume flow rate of

zones [2].

**Figure 5.**

supplied air.

*2.1.2 Types of all-air systems*

*Low-temperature Technologies*

*2.1.2.1 Conventional systems*

A.Classification of conventional systems

B. Advantages of conventional systems

separated from each other

away from zone

**6**

#### **Figure 7.**

*A schematic diagram of single-duct system (variable supplied air volume flow rate and fixed supplied air temperature).*

*2.1.2.3 Constant volume induction system*

*Central Air Conditioning: Systems and Applications DOI: http://dx.doi.org/10.5772/intechopen.89455*

*A schematic diagram of a reheat system.*

**Figure 9.**

Advantages of induction units

can be considered as zone

3.Centralized supplied primary air

2. Simple air duct design

4.Control system simplicity

5.Economic running

*2.1.2.4 Multi-zone unit systems*

**9**

provide comfort quicker with lower running cost [1–5].

6.Possibility of controlling the air ventilation and odor

7.Quite operation as fans are away from the room

In these systems, an induction unit is used either for ventilation or heating purposes. The induction unit usually contains hot water coil, and it induces the zone's air to provide further heating or air ventilation (**Figure 10**). Single or several induction units can be used based on the number of conditioned zones, and hot water is provided to them from boilers through a water circuit which return the water again for reheating. Primary air is usually provided from the air handling unit which is usually responsible for the latent and sensible thermal loads and provides the desired humidity level. In addition, primary air will be provided to induction unit. Induction units are used to meet the increase heating loads and thus can

1.Possibility of controlling room temperature as every room with induction unit

In multi-zone unit systems, the cooling and heating coils are placed parallel to

each other where an amount of air supplied with a constant temperature is

#### **Figure 8.**

*A schematic diagram of multi-duct system (variable supplied air volume flow rate and fixed supplied air temperature).*

#### *2.1.2.2 Reheat systems*

Reheat systems are used in applications of variable loads. In these systems air will be cooled to the zone's lowest required temperature, and then air will be circulated to all zones, and the required temperatures of various zones can be obtained by reheating the supplied air. Electric heater or hot water can be used as reheaters which are located in the terminal units (see **Figure 9**) of conditioned zones [1–5].

In addition, reheaters can be used for reducing the levels of humidity in cold temperature zones by increasing supplied air temperatures. Usually zone thermostat will be used to control the reheater according to required conditions.

*Central Air Conditioning: Systems and Applications DOI: http://dx.doi.org/10.5772/intechopen.89455*

**Figure 9.** *A schematic diagram of a reheat system.*

#### *2.1.2.3 Constant volume induction system*

In these systems, an induction unit is used either for ventilation or heating purposes. The induction unit usually contains hot water coil, and it induces the zone's air to provide further heating or air ventilation (**Figure 10**). Single or several induction units can be used based on the number of conditioned zones, and hot water is provided to them from boilers through a water circuit which return the water again for reheating. Primary air is usually provided from the air handling unit which is usually responsible for the latent and sensible thermal loads and provides the desired humidity level. In addition, primary air will be provided to induction unit. Induction units are used to meet the increase heating loads and thus can provide comfort quicker with lower running cost [1–5].

Advantages of induction units


#### *2.1.2.4 Multi-zone unit systems*

In multi-zone unit systems, the cooling and heating coils are placed parallel to each other where an amount of air supplied with a constant temperature is

*2.1.2.2 Reheat systems*

zones [1–5].

**8**

**Figure 8.**

*temperature).*

**Figure 7.**

*temperature).*

*Low-temperature Technologies*

Reheat systems are used in applications of variable loads. In these systems air will be cooled to the zone's lowest required temperature, and then air will be circulated to all zones, and the required temperatures of various zones can be obtained by reheating the supplied air. Electric heater or hot water can be used as reheaters which are located in the terminal units (see **Figure 9**) of conditioned

*A schematic diagram of multi-duct system (variable supplied air volume flow rate and fixed supplied air*

*A schematic diagram of single-duct system (variable supplied air volume flow rate and fixed supplied air*

In addition, reheaters can be used for reducing the levels of humidity in cold temperature zones by increasing supplied air temperatures. Usually zone thermo-

stat will be used to control the reheater according to required conditions.

1.These systems allow separate control of temperature in zones or places that are

considered as separate area as supplied air is provided at the desire

2. It is simple to have the smallest unit size. These systems can either be assembled in plant location or in the factory and fit all requirements.

with various exits and outlets makes the balance process easier.

5.Centralized refrigeration equipment as air handling unit is used.

3.They are easy to switch operation from cold to hot in summer and winter seasons and vice versa as this can be simply done manually from air handling

4.These systems allow easy air distribution and balance. Using only one air duct

6.Centralized maintenance as all air handling unit is placed in the plant room.

Dual-duct systems allow separate control of temperatures in conditioned places and zones. Temperature control is achieved by supplying the mixing box with air from hot air duct and cold air duct; that is hot air and cold air are mixed in mixing box in proper required proportion based on the zone thermostat, and then air can be supplied to the zone to maintain required zone temperatures. These systems are commonly used in multiroom buildings such as offices, hotels, apartments, hospitals, schools, and large laboratories. **Figure 12** shows a dual-duct system that sup-

1.Provide separate control in the temperatures in each zone as cold and hot air

2.Dual-duct systems can be found in the smaller size as the number of served zone by the central system is reduced, whereas the supply air is maintained through the mixing box which contains cold and hot air just at the each zone.

3.Easy switching from hot to cold modes and vice versa. This is accomplished

4.The refrigeration equipment and boilers are placed in one place, and thus electricity, water, and sewage services are located only in plant room but not

6.Centralized outdoor air inlets. This will ensure no winds or rain are likely to

presence at the same time allows rapid change in temperatures.

by the zone or place thermostat which is adjusted once a year.

5.Centralized maintenance and service are accomplished.

temperature.

*Central Air Conditioning: Systems and Applications DOI: http://dx.doi.org/10.5772/intechopen.89455*

unit in plant room.

7.Low-cost operation.

8.Quiet operation.

*2.1.2.5 Dual-duct systems*

plies different zones [1–5].

Advantages of dual-duct systems

in the building parts.

**11**

enter from outdoor environment.

**Figure 10.** *A schematic diagram of an induction unit. Readapted from [5].*

maintained. In these systems, hot air and cold air are mixed in required proportions, and thus the supplied air of fixed temperature and fixed volume will be provided to conditioned zone [1–5]. **Figure 11** shows a multi-zone unit system that supplies air separately to three different zones as dampers can be used.

This system is used and suitable for the following applications, namely:


Advantages of multi-zone unit systems

**Figure 11.** *A schematic diagram of multi-zone unit system. Readapted from [2].*


maintained. In these systems, hot air and cold air are mixed in required proportions, and thus the supplied air of fixed temperature and fixed volume will be provided to conditioned zone [1–5]. **Figure 11** shows a multi-zone unit system that supplies air

i. Buildings that contain a number of small and large zones in which separate

ii. Areas in building of different directions and different internal loads such as

iii. Buildings that have interior zones of varying sizes as in radio or television

This system is used and suitable for the following applications, namely:

separately to three different zones as dampers can be used.

*A schematic diagram of an induction unit. Readapted from [5].*

buildings, e.g., banks

Advantages of multi-zone unit systems

*A schematic diagram of multi-zone unit system. Readapted from [2].*

studios

**Figure 10.**

*Low-temperature Technologies*

**Figure 11.**

**10**

temperature control is desired as in schools and offices.
