*B. Draw-through units*

In this type, the supply fan is placed after cooling coil (see **Figure 5**), heating coil, filter, and humidifier, and thus air is pulled by the supply fan. This system is commonly used as filters and coils which require small air speed and larger ducts

#### **Figure 4.**

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

*2.1.2.1.1 Single-duct conventional systems*

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

*2.1.2.1.2 Multi-duct conventional systems*

**Figure 6.**

**7**

*temperature).*

provided to the zone to reach required temperature.

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

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.

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

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

variable supplied volume flow rates with fixed supplied air temperature.

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

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 zones [2].
