Air Conditioning Systems Configurations – Part Three

in the previous topics , Air Conditioning Systems Configurations – Part TwoI explained Semi- centralized systems (packaged systems) plus the first type of Centralized Systems ; Centralized Ducted “All – Air” Systems and today I will continue explaining other types of centralized systems which are:
  • Centralized Fluid Based Hydronic Systems. 
  • Combined (Hybrid) Water and Air Systems. 

In addition to some special types of cooling systems which are:
  • Evaporative Cooling systems. 
  • Central Air Washer or Central Evaporative Air Cooling Plant. 

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

B. Centralized Fluid Based Hydronic Systems 

In an all-water hydronic system, the hot or cold-water is distributed to individual heat transfer devices (terminal units) located in each room of the building. When heating, the terminal units draw heat from the water and when cooling these reject heat to the water. 

ALL-Water Cooling Systems Concept

The biggest drawback of all-water hydronic system is the difficulty in providing adequate indoor air quality. These systems are fairly common in office rooms, hotel rooms, schools, building perimeter control etc.

An All-Water system uses the following basic components: 

1. The use of a chiller (on roofs or plant rooms) to cool the water which would be circulated via circulating pumps to the terminal units (example fan coil units) located in the occupied space.

2. The use of boilers (in plant rooms) to heat the water which would be circulated via circulating pumps to the terminal units located in the occupied space. Hydronic heating systems employ a variety of terminal units that include fan coil units, baseboard radiators, convectors, unit heaters, and radiant floors.

3. The use of Fan coil units:

A fan coil unit is a terminal unit with:
  • Heat exchange coil.
  • A fan motor.
  • A filter.
  • A condensate drain pipe.

 Fan coil units

Fan coil units can be in cased or uncased configurations, vertical or horizontal and with or without fresh air inlet.

The hydronic system piping in All Water Systems can be arranged in 3 basic configurations:

(1) Two-pipe system

Two pipe systems consist of a network of insulated pipes; one pipe supplies chilled water and the second pipe returns it to the chiller. The secondary water is cold in summer and intermediate seasons and warm in winter. With water-changeover, chilled water is circulated during the cooling season and hot water during the heating season. The problems occur during the mid-seasons where cooling can be required part of the time and heating part of the time and no heating or cooling the rest of the time.

(2) Three-pipe system 

In the three pipe system, hot water and chilled water are fed to each fan coil, with a common return. This is somewhat more expensive than 2-pipe system, since a third pipe must be run to each unit. Since the hot water and cold water are mixed in the return, these inefficient systems are seldom installed today.

(3) Four-pipe system


In the four pipe system, the chilled water loop and the hot water loop are completely separate. While these systems are most expensive to install, they are easier to operate in unpredictable climates.

C. Combined (Hybrid) Water and Air Systems 

Hybrid systems use both air and water (cooled or heated in central plant room) distribution to room terminals to perform cooling or heating function. Unlike all-water system, this system ensures ventilation air in the spaces so that indoor air quality is not sacrificed.

Basic Water and Air Cooling Systems 

1- Air Portion of the System 

The air side is comprised of central air conditioning equipment, a duct distribution system, and a room terminal. The supply air, called primary air, usually has a constant volume which is determined by the need for outside (fresh) air for ventilation. When in cooling mode the primary air is dehumidified, to provide comfort and prevent condensation, by a central conditioning unit. In the winter, heating mode, the air is humidified, by the central conditioning unit, to limit dryness.

2- Water Portion of the system 

The water side consists of a pump and piping to convey water to heat transfer surfaces within each conditioned space. The water used can be chilled by direct refrigeration, by using chilled water from a primary cooling system, or by heat transfer through a water-to-water exchanger. Chillers usually supply chilled water anywhere from 35-48°F. Individual room temperature control is by regulation of either the water flow through it or the air flow over it. In the winter, the heating capacity of the coil in a conditioned space must be great enough to heat the space and offset the cool primary air, which is provided. Water Portion of the system which is determined by the need for outside (fresh) air for ventilation. When in cooling mode the primary air is dehumidified, to provide comfort and prevent condensation, by a central conditioning unit. In the winter, heating mode, the air is humidified, by the central conditioning unit, to limit dryness.

D- Special types of Air cooling systems 

1- Evaporative Cooling systems.
2- Central Air Washer or Central Evaporative Air Cooling Plant.

1- Evaporative Cooling systems: 

a- How Evaporative Cooling Works:
When air blows through a wet medium— gel, aspen fibers (excelsior), or treated cellulose, fiberglass, or plastic—some of the water is transferred to the air and its dry bulb temperature is lowered. The cooling effect depends on the temperature difference between dry and wet bulb temperatures, the pathway and velocity of the air, and the quality and condition of the medium.

b- Types of Evaporative Cooling:


1- Direct evaporative cooler
Air is pulled across a thoroughly wetted medium as evenly as possible. Lower speeds give more exposure time to the wetted media, thereby achieving more cooling.

2- Indirect evaporative cooling
“Indirect” evaporative coolers take advantage of evaporative cooling effects, but cool without raising indoor humidity

c- Types of evaporative coolers (swamp cooler, desert cooler and wet air cooler):

1- Portable units:

  • Require standard power outlet.
  • Best positioned near an open window or external door with an opening on opposite side of room.
  • Come with water level gauge, variable fan speed and directional louvres.
  • Suitable for: small rooms (up to 25 m2).

2- Fixed room units:

  • Location—external wall/window.
  • Permanently wired and plumbed.
  • Install towards prevailing summer winds.
  • Suitable for: open areas 30–50 m2.

3- Ducted whole house (central systems):

  • Roof installation.
  • Cool air is ducted into rooms through ceiling outlets.
  • Existing heating ductwork is not suitable.
  • Consider carefully in water restricted areas or when dependent on limited water supply.
  • Suitable for: whole home cooling.

2-Central Air Washer or Central Evaporative Air Cooling Systems 

All the components of the central air washer are enclosed in the box which is also called as the air handling unit. In this unit the water is sprayed from the spray nozzles. There is a large blower that sucks the atmospheric air and blows it over the spray of water. As the air flows over the water, the particles of water absorb heat from the air and get evaporated. Due to loss of heat the temperature of air reduces and it gets humidified. The cool and humidified air leaves the washer and enters rooms to be cooled via the ducts. The whole air washer unit should be kept in open space so that fresh air can be absorbed continuously.

Central Air Washer or Central Evaporative Air Cooling Systems

1) Water Tank

The water tank is located at the bottom of the air washer. It is connected to the water pipe from suitable source for continuous supply of water. The central air washers need large quantities of water, which increases as the number of rooms to be cooled increases. For proper performance of the central air washer one has to ensure abundant supply of water continuously. The water in the water tank can be ordinary water or it can be chilled water. Usually ordinary water is used. 

2) Spray nozzles

The water from the water tank is pumped to the spray nozzles via plastic or mild steel piping. The spray nozzles spray the water inside the central air washer. 

3) Water pump

The water pump pumps the water from water tank to the spray nozzles. It is the heart of the central evaporative cooling system. 

4) Blower

The blower sucks the atmospheric air and blows it over the spray of water. During this process the particles of water get evaporated as they absorb heat from the air. Due to loss of heat the temperature of atmospheric air reduces and its humidity increases. The air at low temperature and high humidity enters the rooms to be cooled via the ducts. 

5) Air filter

Air filter is placed towards the external part of the box of the air washer. This ensures that the air sucked by the blower gets filtered before finally entering into the rooms. 

6) Ducts

Ducts are laid from the air washer to all the rooms of the building that are to be cooled. The ducts end in the diffuser that delivers the chilled air to the rooms. 

In the next Topic, I will explain Main HVAC Equipments that need some attention from the electrical engineer. 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.


  1. Wow that's great explanation on air conditioning systems configurations.Thanks for sharing such a nice explanation with the help of diagrams.
    window air conditioners

  2. this is wonderful article and please keep posting amazing article like this thanks for sharing with us.