Basic Classification of HVAC Systems

In industry and commercial world, Air Conditioning is known as HVAC (Heating Ventilation and Air Conditioning) systems.With increasing Global warming and degrading outdoor air quality due to pollution, the demand for Air conditioning is escalating everywhere around the world. Now days HVAC systems has become the most required industry standard for construction of new buildings. And also is an important part of residential structures therefore, we find HVAC systems at almost all places, such as single family homes, apartment buildings, hotels and senior living facilities, medium to large industrial and office buildings such as skyscrapers and hospitals, on-board vessels, and in marine environments, where safe and healthy building conditions are regulated with respect to temperature and humidity, using fresh air from outdoors.

A typical person in modern society spends around 90% of day time indoors. So it is not surprising that providing a healthy, comfortable indoor environment has become a vital factor in current economy. Application wise Air conditioning is either for Human comfort or Process control. It is further classified as Residential, Commercial, Industrial Air conditioning.

  • Residential HVAC System are Commonly used for High/ Low rise Buildings,Individual villas,Dwelling units,Hostels, Lodging.
  • Commercial HVAC Systems are Commonly used for Hotels, Restaurants,Shopping malls, Retail stores,Offices, Hospitals, Clinics,Theatres, Auditoriums,Schools, colleges,Labs, Libraries, Medicals,Gymnasium and Museums.
  • Industrial HVAC Systems are Commonly used for industries like Textile, Paper, Printing,Food processing, Cold storages,Dairy, Milk plant,Pharmaceuticals,IT Data centers,Control rooms and other Precision Mfg.
Before we know the classification of HVAC system one should know about it HVAC Stands for and why do we need HVAC systems for Buildings :-

The HVAC system is used to provide Thermal Comfort like Heating and Cooling Services to Buildings.As we discussed above it became an important part of residential structures and become almost compulsory in number of single family homes, apartment buildings, hotels and senior living facilities, medium to large industrial and office buildings.Where safe and healthy building conditions are regulated with respect to temperature and humidity, using fresh air from outdoors.

Generally humans feel most comfortable at temperatures around 25 degree Celsius and humidity of 50% with certain flow rates of air. This is called as Human Comfort zone. At these temperature humans can feel comfort and able to works at their maximum efficiency.So HVAC is mainly employed to maintain comfortable conditions inside a closed space. And also controls temperature, humidity, air flow rate and the cleanliness of air inside a room.

Basically Air conditioning means processing of air in an indoors environment to maintain Temperature, Humidity, Air Quality (Cleanliness), Air motion and Ventilation. Temperature is controlled either by heating or cooling the air, Humidity is controlled either by removing or adding the moisture to air, Air quality is maintained by filtration which prevents entry of dust and particulate matter and supplies clean air and ventilation is achieved by supply of sufficient fresh outside air. Sometimes Noise levels are also reduced by acoustic liners or sound attenuators.

To achieve all above processes, HVAC systems are available in large number of variations. Even though the objective is common i.e. to control the indoor environment, HVAC systems differ a lot according to different applications and ground conditions. So decision making is very critical function to select right type of HVAC system. HVAC design engineer must consider various factors like application, special requirements of the proc ess, space available, Load variations, operating reliability, Energy conservation and total cost etc. while selecting the equipments.
HVAC systems are broadly classified as :-
  1. All Air systems,
  2. All Water systems (Hydronic),
  3. Air-Water system,
  4. Central systems,
  5. Unitary systems,
  6. Single zone or Multi zone systems.
1.All Air Systems :-

As the name suggests, Air is used as the media in an all air system. Air transports thermal energy from the conditioned space to the HVAC plant.  In these systems air is processed in the A/C plant namely AHU (Air Handling Unit). AHU consists of Dampers, Mixing chambers, Filters, Cooling/ Heating coils, Humidifiers, Fans/ Blowers etc. in a packaged cabinet.This processed air is then supplied to the conditioned spaces through Air Distribution system.
Air Distribution system consists of Ducts, Dampers and Diffusers. This air extracts (or supplies in case of winter) the required amount of sensible and latent heat from the conditioned space. The duct that supplies the air to spaces is called Supply Air Duct (SAD). The return air from the conditioned space is conveyed back to the plant, where it again undergoes the required processing thus completing the cycle. The duct that returns the air from spaces to A/C plant is called Return Air Duct (RAD). Adequate Fresh air is always supplied by AHU to maintain Ventilation and Indoor Air Quality (IAQ).ASHRAE 62.1 standards are followed for ventilation, mostly.Outside Fresh air and Return air are balanced in proportion by rule ‘Supply Air = Return Air + Fresh Air’.Since Fully treated air is supplied by AHU,No further processing of air is required by terminal units in the conditioned space.
All air systems can be further classified into:
1. Single duct, constant volume, single zone system
2. Single duct, constant volume, multi zone system
3. Single duct, Variable volume (VAV) system
4. Dual duct, constant volume system
5. Dual duct, variable volume (VAV) system.

Advantages of All Air systems:-
1. Effective Room air distribution and Ventilation is possible under widely varying load conditions
2. All air systems present great opportunities for Energy conservation such as heat recovery wheels.
3. Precise control is possible by use of high class controls. Close range of Temperature (± 0.15ºC) and Humidity (± 0.5 %) can be achieved.
4. Simultaneous cooling and heating can be provided by dual duct system. Also, switching between summer and winter mode is quite easy in All air systems.
5. AHU which is a complete package can be remotely located, well away from conditioned spaces. It helps to reduce noise levels in occupied spaces.
6. Stairwell Pressurization, kitchen ventilation etc. can be easily achieved.

Disadvantages of All Air systems:-
1. Since systems are quite large in size, require separate spaces like AHU Room. It reduces use of effective floor space. Particularly in high rise buildings, it is difficult to provide long runs of ducts, as AHUs are located either on roof or basement.
2. Difficult to install in existing buildings, as large space is required in false ceiling to lay the ducts.
3. Testing, Adjusting, Balancing (TAB) may be very difficult task in case of VAV systems

Applications of All Air systems:-
1. Precision HVAC applications such as IT Data centers, Research Labs, Process industries, Clean rooms, Operation Theatres, Hospitals etc. where accurate and precise control of space environment is required.
2. Theatres, Auditorium, Functional halls, Retail stores etc. where loads are uniform with small external loads.
3. Large corporate buildings with varied requirements of heating and cooling which can be served by Dual duct systems with simultaneous cooling and heating.
4. Multi zone All air systems are suitable for Offices, classrooms, hotels, ships etc. where individual control is required.

2.All Water systems :-

In this system, water is used as working fluid that transports thermal energy between conditioned space and A/C plant. Chilled water is circulated for cooling while Hot water is circulated through coils for space heating. As only water is supplied by HVAC plant to the space, Fresh air has to be supplied by another unit to maintain Indoor air quality and ventilation. These systems are also known as Chilled water systems for cooling applications. Based on number of pipes used in system, they are classified as 2-pipe system and 4-pipe system.
A 2-pipe system can provide either cooling or heating only, but not both at a time. Out of 2 pipes, one is for supply and the other is for return of water to central plant. Chilled water/ hot water circulates through Fan coil units (FCU), or convectors or Radiators located in the each space. Thermal energy is exchanged between water and air in the conditioned space. Flow control valve controls the water flow rate as per load variation. Flow control valve is driven by zone thermostat which gives signal to adjust flow rate as per load. Since The same inside air is circulated repeatedly by terminal units, Fresh air ventilation must be supplied by external units.

A 4-pipe system consists of two supply pipelines – one for cold water and one for hot water; and two return water pipelines. The cold and hot water are mixed in a certain proportion as per the zone load, and the mixed water is supplied to the terminal units in the conditioned space. The return water is split into two streams, one stream flows to the heating coil while the other flows to the cooling coil.

Terminal units used in all water system are of basically three types namely :-

  • Fan coil units (FCU),
  • Convector and
  • Radiator.
Advantages of All Water systems:-
1. All water system needs very less space as compared to all air systems for same capacity. Because, water is used as transport media, which has higher specific heat capacity and density than air. Hence, pipe sizes are very small as compared to large sized ducts. Therefore, system consumes very less space due to absence of big duct works and fans.
2. Less expensive Central A/C plant solution is offered by all water system.
3. It is more suitable for Individual room/ zone control.
4. Simultaneous cooling and heating is possible with 4-pipe system.
5. It can be easily installed in the old buildings without making much changes in the existing space setting.

Disadvantages of All water systems:-
1. Multiplicity of fan coil units means a great deal of maintenance work and cost.
2. Control of space humidity is limited, as only cooling and dehumidification occurs and FCUs do not have humidifiers or reheat coils.
3. Separate ventilation system must be used to supply fresh air, which adds to cost. Also control of
ventilation air is not precise.
4. Condensate may cause problems of dampness in ceiling or walls if condensate drain fails.

Applications of All water systems:-
1. All water systems are suitable for use as low cost central HVAC systems in multi zone High Rise commercial applications.
2. All water system is best solution to replace All air system in High Rise Tall buildings, as it is much
easier to carry water through small sized pipes than air through very big sized ducts across farther
3. It is very effective system in certain applications where space extremely limited, such as narrow ceiling and shafts.i.e. Buildings which were originally not designed for HVAC systems.
4. In case of Individual room control as required in hotels, multi storey apartments, offices etc. all water systems are more famous.

3.Air-Water systems :-
Air water system employs best features of all air and all water systems. Combination air-water system distributes both chilled water and /or hot water and conditioned air from a central system to the individual spaces. Terminal units in each zone provide cooling or heating to that zone. One type of air water system uses FCU as terminal units to provide major part of air conditioning and central AHUs are used to supply only ventilation air directly into the space. Another type uses ‘Induction units’ as terminal units. It receives supply air from central AHU, called as primary air and chilled / hot water from central chiller. Primary air is delivered at high pressure in Plenum chamber and discharged into space through high velocity nozzles. This induces room air into induction unit, where it mixes with primary air. Room air is called as secondary air. Supply air is mixture of primary and secondary air. Therefore no fan is required in induction unit. 

Normally terminal unit like FCU keeps on cooling the room air again and again with the help of cooling coil circulating chilled water. Therefore sensible cooling load is handled by water system. Ventilation air is supplied by central AHU of air system. AHU supplies cooled and dehumidified fresh air which can take care of room latent loads. Because of this, there is no condensation problem with cooling coils. Similarly, in winter system heated and humidified fresh air is supplied by AHU and hot water circulates through heating coil of terminal unit.

Advantages of Air-Water systems :-
1. Simultaneous heating and cooling is possible in air-water system.

2. Lot of space is saved, as only ventilation air is supplied through ducts and chilled/ hot water is
passed through pipes.
3. Economic control of individual zones is possible using room thermostats which can either regulate secondary air or secondary water in FCUs.
4. Since there is no latent load on cooling coil, troubles due to condensation do not occur. It prevents marshy or damp regions in the conditioned space.
5. Servicing, Repair, Replacement, Maintenance works etc. are relatively easier than all water systems.
6. Positive pressures can be maintained in conditioned spaces under all varying loads.

Disadvantages of Air- Water systems :-
1. There are a lot of complications in system operation and controls as both air and water have to be handled carefully.
2. Mostly air- water systems find limited applications in perimeter zones.
3. Constant supply of fresh air has to be supplied to all zones, no matter whether spaces are occupied are not. Because control is only for cooling/ heating coil through room thermostat.
4. In case of unusual high latent loads, excessive condensation may cause damp environment and discomfort in the zone.
5. As systems are complicated with greater extent of controls, normally air-water systems are costlier than all water systems.

Applications of Air water systems:-
1. It is much suitable for retrofitting of existing buildings, where there is space restriction.
2. It is very good solution for perimeter zone buildings with large sensible loads.
3. It can be used where close control of humidity in the space is not necessary
4. It serves well for large office buildings, hotels, lodges etc.

4.Central Air Conditioning Systems :-
The Central Air Condition system is suitable for air-conditioning large spaces such as theaters, cinemas restaurants, exhibition halls, or big factory spaces where no sub-division exists. The central systems, are generally employed for the loads above 25 TR and 2500 m3/min of conditioned air. The unitary systems can be more economically employed for low capacity (below 25 TR) units. 

In central system, the equipment such as fans, coils, filters and their encasement are designed for assembly in the field. A central system serves different rooms, requires individual control of each room. A central air conditioner has a primary appliance such as an air handler or furnace ,the condenser, compressor, dampers, heating, cooling and humidifying coils and fan located in an out-of-the-way place such as a basement or attic. The conditioned air is carried to the different rooms by means of supply ducts and returned back to the control plant through return ducts.—often the same system utilized by a forced-air furnace during the heating season. One or more thermostats in the house turn the cooling system off and on as room temperatures rise and fall.
Part of the supply air to the rooms may be exhausted outdoors. Outdoor air enters from an intake which should be situated on that side of the building least exposed to solar heat. It should not be close to the ground or to dust collected roof. The air after passing through damper passes through filters. The filters may be of a mechanical cleaned type, replaceable-cell type or may be electrostatic.

The cleaned air then passes to the conditioning equipment in the following order :

  • Tempering (or preheater) coil,
  • humidifier (Air washer),
  • heating coil and finally fan.
Advantages Of Central System :-
1. Low investment cost as compared to total cost of separate unit.
2. Space occupied is unimportant as competed to a room unit conditioner which must be placed in the room.
3. Bette accessibility for maintenance.
4. The running cost is less per unit of refrigeration.
5. Noise and vibration troubles are less to the people in air-conditioned places as the air-conditioning plant is far away from the air-conditioned places.
6. The exhaust air can be returned and partly reused with obvious saving in heating and refrigeration.
7 . The conditioned air is carried to the different rooms by means of supply ducts and returned back to the control plant through return ducts.

Disadvantages of central system :-
1.The disadvantage of the central system is that it results in large size ducts which are costly and occupy large space.

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