HVAC Systems Main Equipment

In the previous topics, which are as follows:

• Heating Systems Types - Part One
• Heating Systems Types -Part Two 
• Air Conditioning System Configurations-Part One 
• Air Conditioning Systems Configurations – Part Two
• Air Conditioning System Configurations-Part Three

I explained the heating and cooling systems Types and Configurations. Today I will explain the HVAC systems Main Equipment that need electrical power.

In general, HVAC systems have the following Equipments / elements:

1. Equipment to generate heating or cooling: The equipment is selected with a capacity to offset the peak load of the space or spaces to be served.
2. A means of distributing heat, cooling, and/or filtered ventilation air where needed: air, water, or steam.
3. Devices that deliver the heat, cooling, and/or fresh air into the building: registers and diffusers, hydronic radiators or convectors, and fan coil units. 

The electrical engineers will be interested in having a good background about the first point and some about the third one which they include the equipments and elements that need electrical power.


1- Equipment to generate heating or cooling


A- Heating systems main equipments:
The main equipments used in heating systems will include the following:

• Furnace.
• Hot Water & Steam Boiler.
• Heat Pump.
• Local heating Systems.

I explained in detail each type of heating systems main equipments in the following previous topics:

• Heating Systems Types - Part One
• Heating Systems Types -Part Two 

B- Cooling systems main equipment:

The main equipments used in Cooling systems will include the following: 

• Chillers & Compressosr.
• Cooling Towers.
• Air Handling Units & Fan Coil Units.

A- Chillers & Compressors:


The compressor compresses the refrigerant gas, causing it to become much warmer than the outside air. The refrigerant enters the compressor on the "suction side" and after it leaves the compressor, the refrigerant is referred to as "hot gas”.

The Chillers have two principal types of chiller are air-cooled and water cooled. Compared to water, air is a poor conductor of heat and therefore air-cooled chillers are larger and less efficient.

Air cooled chillers are generally located outside the building and reject heat directly to the atmosphere, while water cooled chillers are generally located within the building and use cooling towers located outside the building to reject the heat.

A water chilled system has to be used for larger buildings. In such a system, the entire refrigeration cycle occurs within a single piece of equipment known as a chiller. An electrically driven water chiller uses the same vapor-compression refrigeration as a DX system. But instead of cooling air, it chills water which is pumped to the air handling units.

Types of  Chillers & Compressors:

Different types of chiller are also used depending upon the type of compressor used as part of the refrigeration circuit. The different types of compressor are as follows:


1- Reciprocating Compressors 

Reciprocating Compressors

Reciprocating compressors are driven by a motor and use pistons, cylinders and valves to compress the refrigerant.
Reciprocating compressors are usually used in smaller systems up to 100 tons and are available in hermetic, semi-hermetic or externally driven versions.


2- Scroll compressors 

Scroll compressors

Scroll compressors features two involute scrolls, one stationary and one orbiting around the first. This movement draws gas into the outer pocket and the gas is forced toward the center of the scroll, creating increasingly higher gas pressures. The upper limit of the refrigeration capacity of currently manufactured scroll compressors is 60 tons.


3- Screw compressors

Screw compressors

Screw compressors are based on a mechanism made up of two threaded rotors (screws) that are coupled together. The gas is compressed due to the progressive overlapping of the lobes, causing a reduction in the volume occupied by the gas. Continuous and step-less capacity control is provided by moving a sliding valve toward the discharge port, which opens a shortcut recirculating passage to the suction port.
The refrigeration capacity of twin-screw compressors is 50 to 1500 tons but is normally used in the 200 tons to 800 tons range.



4- Centrifugal compressors 

Centrifugal compressors 

Centrifugal compressors are made up of a rotor located inside a special chamber. The rotor is rotated at high speed, imparting high kinetic energy to the gas, which is forced through the narrow outlet opening, thus increasing its pressure. The characteristics of a centrifugal compressor make it ideal for air conditioning applications because it is suitable for variable loads, has few moving parts, and is economical to operate.
The available refrigeration capacity for centrifugal compressors ranges from 100 to 2,000 tons.


Note: The use variable speed drives greatly enhances energy efficiency. This enables the chiller to match the speed of the compressor to the load at the maximum efficiency. It also allows the chiller to function, without damage, at much lower condenser water temperatures.

B- Cooling Towers 


A cooling tower is a heat rejection device, installed outside of the building envelope, through which condenser water is circulated. Refrigerant in the refrigeration cycle is condensed in a refrigerant-to-water heat exchanger. Heat rejected from the refrigerant increases the temperature of the condenser water, which must be cooled to permit the cycle to continue. The condenser water is circulated to the cooling tower where evaporative cooling causes heat to be removed from the water and added to the outside air. The cooled condenser water is then piped back to the condenser of the chiller.


types of  Cooling Towers

There are three basic types of towers as follows:

1- Forced Draft Tower 

Forced Draft Tower

In forced draft cooling towers, air is "pushed" through the tower from an inlet to an exhaust. A forced draft mechanical draft tower is a blow-through arrangement, where a blower type fan at the intake forces air through the tower.



2- Induced Draft Tower 

Induced Draft Tower

A second type of tower, induced draft has a fan in the wet air stream to draw air through the fill. The fan located is located at the discharge end, which pulls air through tower.



3- Natural Draft Tower 

Natural Draft Tower

Natural draft tower has no mechanical means to create airflow. Natural-draft cooling towers use the buoyancy of the exhaust air rising in a tall chimney to provide the draft. Warm, moist air naturally rises due to the density differential to the dry, cooler outside air.


C- Air Handling Units & Fan Coil Units


An air handling system is a means of providing conditioned air to the space in order to maintain the environmental requirements.

Types of AHU
AHU’s are available in a wide range of sizes. They come in a variety of forms suitable for different applications.

1- Central system: 

Air Handling Units

Consists of more than one AHU served by the same source of heat and/or cooling. These are usually custom built for particular application.



2- Unitary equipment


Consists of a factory-assembled AHU and cooling compressor contained within a compact enclosure. It is distinguished from a room air conditioner by its capability of being connected to a ductwork. These are further categorized as package terminal air conditioners, rooftop systems and split systems.


a- Packaged Unit


Packaged Unit is a self-contained air conditioner that does not receive hot or cold water from a central plant. It conditions the air and provides it with motive force and is equipped with its own heating and cooling sources. The packaged is always equipped with a DX coil for cooling. Packaged units can be classified according to their place of installation: rooftop and split packaged units. 

a.1 - Roof Top Unit

Roof Top Unit

A roof-top packaged unit sometimes called a penthouse unit is installed on the roof and is completely weatherproof. Rooftop units can be subdivided into:

1- Gas/electric rooftop packaged unit, in which heating is provided by gas furnace and cooling by electric power-driven compressors.

2- Electric/electric rooftop packaged unit, in which electric heating and electric power-driven compressors provide heating and cooling.

3- Rooftop packaged heat pump, in which both heating and cooling are provided by the same refrigeration system using a four-way reversing valve (heat pump) in which the refrigeration flow changes when cooling mode is changed to heating mode and vice versa. Auxiliary electric heating is provided if necessary.
Rooftop packaged units are single packaged units. Their cooling capacity may vary from 3 to 220 tons, Rooftop packaged units are the most widely used packaged units.


a.2- Indoor Packaged Units

Indoor Packaged Unit

An indoor packaged unit is also a single packaged and factory-fabricated unit. It is usually installed in a fan room or a machinery room. A small or medium-sized indoor packaged unit could be floor mounted directly inside the conditioned space with or without ductwork. The cooling capacity of an indoor packaged unit may vary from 3 to 100 tons. Indoor packaged unit usually has its compressors located indoors and condensers outdoors.


a.3- Split Packaged Units

Split Packaged Unit

A split packaged unit consists of two separate pieces of equipment: an indoor air handler and an outdoor condensing unit. The indoor air handler is often installed in the fan room. Small air handlers can be ceiling hung.
The condensing unit is usually located outdoors, on a rooftop or podium or on the ground.
A split packaged unit has its compressors and condenser in its outdoor condensing unit, whereas an indoor packaged unit usually has its compressors indoors. The cooling capacity of split packaged units varies from 3 to 75.



b- Make-Up Air and Recirculating Units

Make-Up Air and Recirculating Unit

A make-up AHU, also called a primary air unit, is used to condition outdoor air entirely. It is a once through unit. There is no return air and mixing box. Recirculating units can have 100% outdoor air intake or mixing of outdoor air and recirculating air.



c- Fan coil unit (FCU)

Fan coil unit

A fan-coil unit is a small-scale air handling unit with circulation fan, cooling and/or heating coil, filter, and appropriate controls. It is essentially a terminal device because it serves only one room or a small group of rooms.

 

Types Fan coil unit (FCU)

 

Fan-coil control is typically achieved through control of water flow through the coil using a control signal from the zone thermostat. Further control is sometimes provided by a multi-speed fan option. Occupants can usually adjust supply air louvers to provide some control over air distribution patterns. The most critical performance issue facing an all-water fan-coil system is ventilation air. Fan-coil units installed on an exterior wall can be equipped with an outdoor air connection so that ventilation may be provided. Fan coils installed in interior zones can not easily provide such outdoor air ventilation. An air-water fan-coil system can overcome this constraint. In a fan-coil system, a major system component (the fan coil unit itself) is installed in or adjacent to occupied spaces, requiring that filter changes and maintenance of fans and coils occur in these spaces. Fan noise may be a concern in some critical occupancies. It is most commonly used in hotels, condominiums, and apartments.



d- Induction Units

The inducting system is designed for use in perimeter rooms of multi-storey, multi-room building that may have reversing sensible heat characteristics. It is especially adapted to handle the loads of skyscrapers with minimum space requirements for mechanical equipment. 
In the induction system, ducted primary air is fed into a small plenum chamber where its pressure is reduced by means of a suitable damper to the level required at the nozzles. The plenum is acoustically treated to attenuate part of the noise generated in the duct system and in the unit. The primary air is then delivered through nozzles as high velocity jets which induce secondary air from the room and over the secondary coil.
Induction units are usually installed at a perimeter wall under a window. Some hotel rooms are provide with induction coils.



In the next Topic, I will explain the HVAC System Controls. 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.

Air Conditioning Systems Configurations – Part Three

in the previous topics , Air Conditioning Systems Configurations – Part Two, I 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:

• Electrical Load Classification and Types 
• Heating Systems Types - Part One
• Heating Systems Types -Part Two 
• Air Conditioning System Configurations-Part One 

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.

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:

• Electrical Load Classification and Types 
• Heating Systems Types - Part One
• Heating Systems Types -Part Two 

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.  

Air Conditioning System Configurations

in the previous topics which are as follows:

• Heating Systems Types - Part One
• Heating Systems Types -Part Two 

I explained the different types of the Heating systems and today I will begin explaining the Air Conditioning Systems as follows.

You can review the previous topic, “Electrical Load Classification and Types” for more information and good following.

Air Conditioning System Configurations

The air-conditioning components and equipments may be designed and assembled in hundred different ways but in practice these are broadly classified into three categories:


1- Decentralized Systems (Individual Room Systems): This can be divided to three major types as follows:

• Mini-Split Cooling Units (ductless split Units).
• Window Units.
• Mini- Heat Pump.

2- Semi- centralized systems (packaged systems): This can be divided to two major types as follows: 

• 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 (ducted Split system).

3- Centralized systems (Central Hydronic systems): This can be divided to three major types as follows:

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

The following table will show a comparison between these configurations as follows:

1- Decentralized Systems (Individual Room Systems)

These are systems in which heating and cooling is conducted locally, these systems are relatively common in schools, small hotels, domestic applications, residential homes and small offices.

Decentralized Systems are essentially direct expansion (DX) systems, which operate using direct expansion of refrigerant in the finned tubes across the air path.

In smaller buildings, smaller room air conditioners are less expensive to operate than central units, even though their efficiency is generally lower than that of central air conditioners.


The major types of decentralized systems are:

• Split Cooling Units.
• Window Units.
• Split Heat Pumps.

A- Split Cooling Units

The split systems are individual systems in which the two heat exchangers are separated (one outside, one inside). The split air conditioner can be installed in rooms and offices where you don’t want to disturb the setup of the room and avoid demolitions in your favorite space.

There are two main parts of the split air conditioner. These are:

1) Outdoor unit:

This unit is installed outside the room or office space which is to be cooled and houses important components of the air conditioner like:

The compressor:
External power has to be supplied to the compressor, which is utilized for compressing the refrigerant and increases its pressure before sending it to the condenser. The size of the compressor varies depending on the desired air conditioning load. In most of the domestic split air conditioners hermetically sealed type of compressor is used. In such compressors the motor used for driving the shaft is located inside the sealed unit and it is not visible externally.

Condenser coil:
It is the coiled copper tubing with one or more rows depending on the size of the air conditioning unit and the compressor. The condenser is also covered with the aluminum fins so that the heat from the refrigerant can be removed at more faster rate.

Condenser Cooling Fan:
The condenser cooling fan is an ordinary fan with three or four blades and is driven by a motor. The cooling fan is located in front of the compressor and the condenser coil. As the blades of the fan rotate it absorbs the surrounding air from the open space and blows it over the compressor and the condenser with the aluminum fins thus cooling them. 

Expansion Valve:
The expansion valve is usually a copper capillary tubing with several rounds of coils.The high pressure and medium temperature refrigerant leaves the condenser and enters the expansion valve, where its temperature and pressure drops suddenly.



2) Indoor unit:

The most common type of the indoor unit is the wall mounted type though other types like ceiling mounted and floor mounted are also used. the indoor unit produces the cooling effect inside the room or the office and houses

The evaporator coil or the cooling coil:
The cooling coil is a copper coil made of number turns of the copper tubing with one or more rows depending on the capacity of the air conditioning system. The cooling coil is covered with the aluminum fins so that the maximum amount of heat can be transferred from the coil to the air inside the room.

Cooling Fan or Blower:
The blower sucks the hot and unclean air from the room and supplies cool and clean air back. The shaft of the blower rotates inside the bushes and it is connected to a small multiple speed motor, thus the speed of the blower can be changed. When the fan speed is changed with the remote it is the speed of the blower that changes.

The drain pipe:
The drain pipe helps removing dew water collected inside the indoor unit.

Louvers or Fins:
The cool air supplied by the blower is passed into the room through louvers. The louvers help changing the angle or direction in which the air needs to be supplied into the room as per the requirements.

There are two types of louvers: horizontal and vertical. The horizontal louvers are connected to a small motor and their position can set by the remote control and the vertical louvers are operated manually and one can easily change their position as per the requirements

Air filter:
It removes all the dirt particles from the room air and helps supplying clean air to the room. The air filter in the wall mounted type of the indoor unit is placed just before the cooling coil.

Operation Method:
In the outdoor unit there is a fan that blows air over the condenser thus cooling the compressed Freon gas in it. This gas passes through the expansion coil and gets converted into low pressure, low temperature partial gas and partial liquid Freon fluid, after passing from the expansion coil, the chilled Freon fluid enters the cooling coil. The blower sucks the hot, humid and filtered air from the room and it blows it over the cooling coil. As the air passes over cooling coil its temperature reduces drastically and also loses the excess moisture. The cool and dry air enters the room and maintains comfortable conditions of around 25-27 degree Celsius as per the requirements.

Controls:

The temperature inside the space can be maintained by thermostat setting.

Note: These days, multi-split air conditioners are also being used commonly. In units for one outdoor unit there are two indoor units which can be placed in two different rooms or at two different locations inside a large room.

B- Window Units 

• Window-mounted air conditioners cool the individual conditioned spaces. 
• A window unit is an encased assembly designed primarily for mounting in a window, through a wall, or as a console. 
• A window unit produces cool air on one side and rejects hot air on the other and provides delivery of conditioned air to a room without ducts. 
• A window unit has a low initial cost and is quick and easy to install. 
• A window unit requires outside exposure for heat rejection and cannot be used for interior rooms 

A/C Window Unit Construction 

Window Unit Construction:

• Condenser (outdoor coil).
• Condenser fan.
• Hermetic compressor.
• Capillary tube.
• Evaporator (indoor conditioning coil).
• Evaporator fan.
• Controls: The controls for window units are simple and inbuilt, with a rotating switch marked with a hot-cold scale with no temperature settings. Most units will heat as well as cool. Fresh air control, if it exists, is normally adjusted by a manual lever. 

C- Mini- Heat Pumps

• A cooling only system cools the indoor air but a heat pump provides cooling in summer season and heating in winter season. 

• DX system operating in reverse vapor compression cycle is classified as Heat pump. Through an addition of a special four-way reversing valve, heat flow in mechanical refrigeration loop can be reversed so that heat is extracted from outside air and rejected into the building. 

• Such a facility is required during winter season to heat the indoor using the same physical components. Due to added heat of compression, the efficiency of heat pump is higher compared to the cooling cycle. 

• A supplementary electric resistance heater may also be used to assist the heat pump at lower outdoor temperatures. In colder climates, heats pumps require defrost period. During defrost times the electric heater is the only means of heating the interior of the building. 

• Split heat pumps are Air-to-Air types and they are the most common type of heat pumps. It is particularly suitable for factory-built unitary heat pumps, and has been widely used for residential and commercial application. Air is used as the heat source and heat sink. Extended surface, forced convection heat transfer coils are normally employed to transfer the heat between the air and the refrigerant. When selecting or designing an air-source heat pump, two factors in particular must be taken into consideration: 

1. The variation in temperature experienced in a given locality. 
2. The formation of frost.

In the next Topic, I will explain other types of air conditioning systems configurations " Semi- centralized systems and centralized systems air condition". 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.