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.