Zoned Air-Conditioning Systems
The air-conditioning system considered thus far provides one source of air with uniform temperature to the whole space, controlled by one space thermostat and one space humidistat. However, in many buildings there's a spread of spaces with different users and ranging thermal loads. These varying loads could also be thanks to different inside uses of the spaces, or thanks to changes in cooling loads because the sun shines into some spaces and not others. Thus our simple system, which supplies one source of heating or cooling, must be modified to supply independent, variable cooling or heating to every space.
When a system is meant to supply independent control in several spaces, each space is named a “zone.” A zone could also be a separate room. A zone can also be a part of an outsized space. for instance , a theater stage could also be a zone, while the audience seating may be a second zone within the same big space. Each features a different requirement for heating and cooling. This need for zoning leads us to the four broad categories of air-conditioning systems, and consideration of how each can provide zoned cooling and heating.
The four systems are
1. All-air systems
2. Air-and-water systems
3. All-water systems
4. Unitary, refrigeration-based systems
System 1: All-air Systems
All-air systems provide air con by employing a tempered flow of air to the spaces. These all-air systems need substantial space for ducting the air to every zone. The cooling or heating capacity, Q, is measured in British Thermal Units (Btu) and is that the product of airflow, measured in cubic feet per minute, (cfm), times the difference in temperature between the availability air to the zone and therefore the return air from the zone.
Q (Btu) a continuing _ mass flow _ temperature difference
Q (Btu) a continuing _ cfm _ (°Fzone _ °Fsupply air)
To change the heating or cooling capacity of the air supply to at least one zone, the system must either alter the availability temperature, °F, or alter the flow, cfm, thereto zone.
Reheat system: the only , and least energy efficient system, is that the constant volume reheat system. allow us to assume that the most air system provides air that's cool enough to satisfy all possible cooling loads, which there's a heater within the duct to every zone.
A zone thermostat can then control the heater to take care of the specified zone set-point-temperature. The system, shown in Figure, is named a reheat system, since the cool air is reheated as necessary to take care of zone temperature. Figure illustrates the essential air-conditioning system, plus ducting, to only two of the many zones. The air to every zone passes over a reheat coil before entering the zone. A thermostat within the zone controls the reheat coil. If the zone requires full cooling, the thermostat will shut off the reheat coil. Then, because the cooling load drops, the thermostat will activate the coil to take care of the zone temperature.
Variable Air Volume (VAV) System: Figure illustrates another zoned system, called a Variable Air Volume system, VAV system, because it varies the quantity of air supplied to every zone.
Variable Air Volume systems are more energy efficient than the reheat systems. Again, assume that the essential system provides air that's cool enough to satisfy all possible cooling loads. In zones that need only cooling, the duct to every zone are often fitted with an impact damper which will be throttled to scale back the airflow to take care of the specified temperature.
In both sorts of systems, all the air-conditioning processes are achieved through the flow of air from a central unit into each zone. Therefore, they're called “all-air systems”.
System 2: Air-and-water Systems
Another group of systems, air-and-water systems, provide all the first ventilation air from a central system, but local units provide additional conditioning. the first ventilation also provides most, or all, of the humidity control by conditioning the ventilation air. The local units are usually furnished with hot or chilled water. These systems are particularly effective in perimeter spaces, where high heating and cooling loads occur. Although they'll use electric coils rather than water, they're grouped under the title “air-and-water systems.” for instance , in cold climates substantial heating is usually required at the perimeter walls. during this situation, a hot-water-heating system are often installed round the perimeter of the building while a central air system provides cooling and ventilation.
System 3: All-water Systems
When the ventilation is provided through natural ventilation, by opening windows, or other means, there's no got to duct ventilation air to the zones from a central plant. this enables all processes aside from ventilation to be provided by local equipment furnished with hot and chilled water from a central plant. These systems are grouped under the name “all-water systems.”
The largest group of all-water systems are heating systems. Both the air-and-water and all-water systems believe a central supply of predicament for heating and chilled water for cooling.
System 4: Unitary, Refrigerant-based Systems
The final sort of system uses local refrigeration equipment and heaters to supply air con . they're called “unitary refrigerant–based systems”. The window air-conditioner is that the simplest example of this sort of system. In these systems, ventilation air could also be brought in by the unit, by opening windows, or from a central ventilation air system. The unitary system has local refrigerant-based cooling. as compared , the opposite sorts of systems use a central refrigeration unit to either cool the airconditioning airflow or to relax water for circulation to local cooling units. the planning , operation and selection of refrigeration equipment may be a huge field of data in itself.
Hope you understood about Zoned Air Conditioning in HVAC System. Please comment for any queries.
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