Air Conditioning System Configurations - Part Two

in the previous topics , Air Conditioning System Configurations-Part One , I explained the different configurations of the air conditioning systems plus the first type of these configurations which was the decentralized systems (Individual Room Systems) and today I will continue explaining other configurations ;semi-centralized and centralized Air Conditioning Systems as follows.

You can review the following previous topics for more information and good following:

2- Semi- centralized systems (packaged systems):

Packaged Systems are similar in nature to decentralized system but serve more rooms or even more than one floor.


The packaged air conditioners are available in the fixed rated capacities of 3, 5, 7, 10 and 15 tons. These units are used commonly in places like restaurants, telephone exchanges, homes, small halls, etc. 

Types of  Semi- centralized systems (packaged systems):

The major types of Semi- centralized systems (packaged systems) are:

  • One piece systems (Unitary Packaged Systems): 
  1. Packaged Air Conditioners with Water Cooled Condenser.
  2. Packaged Air Conditioners with Air Cooled Condensers.
  • Two pieces systems (Split system).

A- One piece systems (Unitary Packaged Systems): 

In the Unitary packaged air conditioners all the important components of the air conditioners are enclosed in a single casing like window AC. Thus the compressor, cooling coil, air handling unit and the air filter are all housed in a single casing and assembled at the factory location. Since all equipment is prepackaged, the installation cost is usually lower, and the performance quality is often higher than field-erected systems.

Depending on the type of the cooling system used in these systems, the unitary packaged air conditioners are divided into two types:

1- Packaged Air Conditioners with Water Cooled Condenser:

In these packaged air conditions the condenser is cooled by the water. The condenser is of shell and tube type, with refrigerant flowing along the tube side and the cooling water flowing along the shell side. The water has to be supplied continuously in these systems to maintain functioning of the air conditioning system.

Packaged Air Conditioners with Water Cooled Condenser 
In the packaged units with the water cooled condenser, the compressor is located at the bottom along with the condenser (refer the figure below). Above these components the evaporator or the cooling coil is located. The air handling unit comprising of the centrifugal blower and the air filter is located above the cooling coil. The centrifugal blower has the capacity to handle large volume of air required for cooling a number of rooms. From the top of the package air conditioners the duct comes out that extends to the various rooms that are to be cooled. 

2- Packaged Air Conditioners with Air Cooled Condensers

In this packaged air conditioners the condenser of the refrigeration system is cooled by the atmospheric air.

Packaged Air Conditioners with Air Cooled Condensers 
There is an outdoor unit that comprises of the important components like the compressor, condenser and in some cases the expansion valve. The outdoor unit can be kept on the terrace or any other open place where the free flow of the atmospheric air is available. The fan located inside this unit sucks the outside air and blows it over the condenser coil cooling it in the process. The condenser coil is made up of several turns of the copper tubing and it is finned externally. The packaged ACs with the air cooled condensers are used more commonly than the ones with water cooled condensers since air is freely available it is difficult maintain continuous flow of the water.

The cooling unit comprising of the expansion valve, evaporator, the air handling blower and the filter are located on the floor or hanged to the ceiling. The ducts coming from the cooling unit are connected to the various rooms that are to be cooled. 

B- Two pieces systems (Ducted Split system)

Ducted Split system 

In this system, the evaporator is separate from the condenser/compressor. These are commonly found in residential and small commercial installations with capacity ranges varying 1 to 50 TR and suitable for an area of 100 – 10000 square feet. The new ductless systems which can be conveniently mounted on the ceiling or wall are in this family.
A split system describes an air conditioning or heat pump system that is split into two sections - an outdoor section and an indoor section. The outdoor unit is comprised of a compressor and condenser and is located outside usually on the ground but sometimes on the roof. The indoor section, usually located in an interior closet or garage, consists of a fan, indoor cooling coil, furnace, and filter.

3- Centralized systems (Central Hydronic systems):

This can be divided to two major types as follows:

  • Centralized Ducted “All – Air” Systems.
  • Centralized Fluid Based Hydronic Systems.
  • Combined (Hybrid) Water and Air Systems.

A- Centralized Ducted “All – Air” Systems :

These are systems in which the primary movement of heat around the building is via heated and cooled air. These systems are the most common in large spaces such as office buildings, common public areas, retail, shopping, manufacturing areas, airports, hotel lobbies etc.

Basic All-Air System
In an ‘All-Air system’, the refrigerant or chilled water is used to cool and dehumidify the air in the air handling unit (AHU). The cool air is then circulated throughout the building thru the ductwork. Heating can also be accomplished either by hot water or electrical strip heaters.

The centralization of these systems allow for better management and system operation. On the other hand, they also require either a mechanical room adjacent to the controlled space for locating the AHU and large ductwork in building space.

Fresh air is drawn into the building through the intake louver, mixed with return air, heated or cooled to a controlled temperature, circulated around the building and provided to the occupied space. Local temperature control is provided by a terminal reheat unit attached to a temperature controller within the occupied space. Exhaust air is extracted from the space and dumped to the outside. In general, the majority of the return air is recycled via the return air duct.

System Construction:

Basic Components of All-Air System

 The individual components of this system are:

1. Air Handling Unit: This is a cabinet that includes or houses the central furnace, air conditioner, or heat pump and the plenum and blower assembly that forces air through the ductwork.

Basic Air Handling Unit 
2. Intake louvers: These are the external louvers through which supply air is drawn into the building. Intake is generally equipped with volume control damper to regulate the amount of fresh air and economizing the quantity of outside air during favorable outside conditions.

3. Filters: These are used to remove particles of dust or dirt from the supply air.

4. Heating coils: These heat up the incoming airstream using coils through which hot water is passed or banks of electric heating elements.

5. Cooling coils: These cool the incoming airstream using coils through which refrigerant or water is passed.

6. Supply fans: These are used to circulate the air through the network of ductwork.

7. Ductwork: It is a branching network of round or rectangular tubes generally constructed of sheet metal, fiberglass board, or a flexible plastic and wire composite material located within the walls, floors, and ceilings. The three most common types of duct material used in home construction are metal, fiberglass duct board, and flex-duct.

8. Supply Ductwork: These carry air from the air handler to the rooms in a house. Typically each room has at least one supply duct and larger rooms may have several.

9. Return Ductwork: These carry air from the conditioned space back to the air handler. Most houses have only one or two main return ducts located in a central area.

10. Supply and Return Plenums: These are boxes made of duct board, metal, drywall or wood that distribute air to individual ducts or registers.

11. Terminal reheat heating coils: These use hot water coils or electric heating elements to heat up the air being supplied to one part of the building according to the temperature in that space.

12. Supply and extract grilles : These are the points at which the air is either supplied into or extracted from the space, and may be ceiling-mounted or wall-mounted. Also called diffusers or registers.

13. Boots: These connect ductwork to registers.

14. Extract fans :These are used to extract the air from the space and discharge it to outside.

15. Return air duct: These are interconnections between inlet and outlet ductwork sections, which let a controlled amount of air recirculate around the air conditioning system when full fresh air is not required.

16. Exhaust louvers: These are the external louvers through which extract air is discharged from the building

Types of "All -Air" Systems:

There are three major types of Centralized Ducted Air Systems:

  • Constant Volume Systems.
  • Dual Duct Systems.
  • Variable Volume Systems.

1- Constant Volume Systems (CAV):

Constant volume systems deliver a constant volume of air and responds to changing thermal loads by varying air temperature. The air volume is usually based on the design cooling load for the given zone. The ducting and air handling system are sized to match the heat gain from equipment, lights, exterior, and people. Typical applications include:-

  • Space with uniform loads (small office buildings, manufacturing plants, retail etc.).
  • Small spaces requiring precision control.

Constant Volume Systems (CAV) Configurations

a- Local AHU: Each space is supplied by its own specific unit. 

Local AHU 

b- Single zone AHU: Air is supplied to several spaces by a single AHU. 

Single zone AHU 

c- Multi zone AHU: each space has its own separate supply air duct and the conditions of the supply air are adjusted by mixing the cool and warm air.

Multi zone AHU 

2- Dual Duct Systems:

Dual Duct Systems 

Dual or double duct system is an air conditioning system in which cold and warm air is circulated throughout a building via two parallel ducts. Hot air flows within one duct, cold air within the other. The proportion of hot and cold air delivered to each room within the building may be controlled by thermostatically operated dampers on the ducts outlet. The system is well suited to provide temperature control for individual spaces or zones in applications include:
  • Office Buildings.
  • Institutional.

3- Variable Volume Systems

Variable air volume (VAV) systems use variable volume terminal units or boxes to vary the airflow in each zone or space in accordance to the thermostat signals within the space.

Variable Volume Systems 

Heating is turned on when the air flow reaches a predetermined minimum. If more cooling is required, more cold air is introduced into the space.

In the best implementation of these systems, the central air handling unit fan speed is controlled to maintain a constant duct pressure. An interlock is arranged between the supply and extract fans. Variable air volume systems are very common in larger office buildings.

In the next Topic, I will continue explaining other configurations of cenetreliazed air conditoining systems ; Centralized Fluid Based Hydronic Systems & Combined (Hybrid) Water and Air Systems. so, please keep following.

Note: these topics about HVAC in this course EE-1: Beginner's electrical design course is an introduction only for beginners to know general basic information about electrical HVAC Power loads. but in other levels of our electrical design courses, we will show and explain in detail the HVAC Loads Estimation and HVAC equipment Capacities with examples for different types of buildings.  


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