An air-conditioning system provides cooling, ventilation and humidity control of a place.
Purpose of air-conditioning system is to provide a favorable indoor comfort condition or a specific condition in required place. The required place can be a residential building or office building, hotel, hospital, inside transportation vehicles such as airplanes, automobiles or trains and also certain production places like electronic chips production, pharmaceutical and hygienic. For this purpose various systems are designed specifically for each location.
air-conditioning components are divided to four parts:
- Source: provider of primary cooling and heating effects that includes chillers, boilers, cooling towers and equipments like them.
- Distribution part: its duty is to transfer heating and cooling effects from the source to places that need ventilation and includes ductwork, fans, piping and pumps.
- Delivery part: its duty is to enter heating and cooling effects to ventilated places and includes diffusers, radiators, fan-coil units, and other terminal devices.
- Control elements: their duty is to adjust operation of equipments and devices for comfort, safety and optimization of energy usage.
In central systems these components can be placed all over the building while in local systems these components gathered in a case.
One of the most important steps of designing an air-conditioning system is the selection of systems type. For selection of an air-conditioning system the following must be considered:
- Comfort condition or special condition providing method
- Operation method of system
- Amount and level of desired comfort
- Capacity of system
- Placement situation of system
- Initial costs
- Operating costs such as gasoil, water, power and maintenance
- Reliability of system
- Flexibility of system
- The possession and use of space
Before any action it is necessary to obtain general information about the amount of budget, desired comfort level and future expansion. For optimal selection of an air-conditioning system, designer must have proper information about different types of air-conditioning systems.
Air-conditioning system types include:
- All air systems
- Air-water systems
- All water systems
- All refrigerant or direct expansion (DX) systems
All air systems transfer hot or cold air from central unit by ducts and distribute air by a number of grills or diffusers to rooms. All air systems have the lowest cost for its equipment but its installation is too difficult and expensive because of required ducting size and installation cost. Also its may be difficult to control temperature properly and energy efficiency of the system. All air system is in second rank among other air-conditioning systems for required energy consumption to achieve required result.
Important types of all air system are water coolers, air washers and air handling unit (AHU).
All air systems provide sensible and latent cooling capacity only from transferring cooled air to ventilated place. No auxiliary cooling is transfered from cooling resources to places and no cold water flows into places. Heating can also be provided through the same air flow, so that heating source may be placed at a central facility or near the zone.
Benefits of all air systems:
- Main equipment focused centrally in a service place that allows proper maintenance in non-residential spaces.
- Main equipment that produces sound placed centrally in a sound-proof case that reduces noise level.
- It’s no need to condensate discharge piping and power wiring in residential areas.
- This system is suitable for air-side economizer, heat recovery, winter humidification for providing large amounts of outside air.
- All air systems are the best choice for controlling temperature and humidity of a zone.
- Normal all air systems are good choice for applications where air quality of indoor is very important.
- Usage of all air systems with smoke control systems is appropriate.
- Seasonal changes are simple in all air systems.
- All air systems usually can be able to provide heating and cooling for different zones.
All air systems can be divided into two categories:
- Constant-air-volume systems (CAV systems): in CAV systems, controlling of heating and cooling is done by changing of provided air temperature and input air volume in space is fixed.
- Variable-air-volume systems (VAV systems): VAV systems, controlled heating and cooling by keeping constantly input air temperature and air volume is changed.
Air- water systems are doing air-conditioning by simultaneous distribution of ventilated air and water to terminal units that are installed in spaces. Air and water are heated or cooled in central mechanical room. Provided air is called primary air that is determined by recirculated air or secondary air.
Advantages and disadvantages of the air-water systems
Because of higher thermal capacity and much higher density of water compared to air, cross-section of pipes for a heating or cooling capacity is much smaller than ducts. Because of a large amount of heating or cooling capacity of place is provided by the water part of system, the need of distribution ducts in this system is significantly lower than all air systems and save more space in buildings. If the system is designed to provide primary air equally with desire ventilation or exhaust amount, we can eliminate back air system.
Air handling system in an air-water system is smaller than all air system, while creating a positive ventilation. Different zones can be controlled individually and we can be confident about independent providing of their heating and cooling needs. At the right time or in the non-residential hours it can provide heating needs just by water part of system and without central air system. When all primary air is provided from outside, you can prevent movement of contaminants in room.
Design of the air- water system is very critical for the transition seasons. Seasonal changes can be difficult and it requires skilled operators. Controlling of air-water systems is more complicated than all air systems, and complete control of humidity is not possible. Terminal units and fan coil units need repair and maintenance.
In all water systems, cooling or heating provided by circulated hot or cold water that comes from central cooling system or boilers to the terminal units in ventilated rooms. Heat transfer from or to room is done by natural or forced convection. All water systems can be used for heating and cooling. Heating water can be piped from the same network that cooling water is used it in summer or separate piping system can be provided.
Benefits of all water systems
- Requires less space for distribution components.
- Due to good performance in distribution, are suitable for modernization.
- Requires a little space or without any space need for central fan room.
- Ability of individual controlling of rooms and preventing movement of contaminants between rooms.
- Due to the possibility of using low temperature water for heating, all water systems are suitable for solar heating or heat recovery applications.
Disadvantages of all water systems
- There is need to maintenance and terminal repairing must be at residential areas.
- There is need to condensate discharge trays, in addition they must be cleaned periodically.
- Providing or controlling of air-conditioning is not central and it`s often done by opening windows or entering outside air to each terminal units, thereby providing good indoor air quality is an important issue.
- In summer, the relative humidity of a space is high, especially if cold water regulator valves are used to regulate the room temperature.
In direct expansion coil systems (DX coil systems) or all refrigerant systems, required air for cooling of a space is provided directly by the refrigerant in cooling coils of cold air-handling systems. Since air is cooled directly by refrigerant, cooling efficiency of DX systems is higher. Refrigerant Piping are not always possible for distance locations and direct expansion central cooling systems or DX are used in small and single floor buildings. Important types of direct expansion systems are such as air-conditioners and spliters.