The Terminal of Central Air Conditioning System

A central air conditioning system typically includes a source of heating and cooling, a distribution system, air handling equipment, and terminal devices. Terminal devices, in a narrow sense, refer to the final components of the system that deliver heated or cooled air into the room. The heating and cooling source provides the necessary energy, commonly referred to as the main unit (outdoor unit). The distribution system transports the heated or cooled air or water to the required locations, known as the duct system or piping system. Air handling equipment, such as air conditioning boxes and fresh air units, produces the necessary air. Terminal devices, which include radiators, various air outlets, fan coil units, and floor radiant heating/cooling, represent the final step in delivering heated or cooled air into the room. In a broader sense, air handling equipment and terminal devices are collectively referred to as terminal equipment. Professionally, terminal equipment encompasses the combination of devices used to cool, heat, humidify, dehumidify, and filter air.

Fan Coil Units and Fan Coil Air Conditioning Systems

Fan coil systems, which typically serve the peripheral areas of buildings, represent a widely applicable and economical system. They can handle independent control areas with different load characteristics within a building. Compared to other centralized air handling systems, they require minimal space for equipment placement and are simpler to install in existing buildings. For detailed information on fan coil units, please refer to relevant articles what is a fan coil unit? .

Below is a brief introduction, including a description of switchable two-pipe systems, four-pipe systems, and two-pipe systems with auxiliary electric heating. The features of these systems are listed for designers to choose, evaluate, and apply in basic or varied system forms.

Structure of Fan Coil Units

1. Fan: Fans come in two forms – centrifugal multi-blade and cross-flow types.
2. Fan Motor: To meet noise requirements, capacitor circuits are generally used, allowing the motor speed to be adjusted by changing the input voltage, thereby altering the air volume.
3. Coil: Typically, coils are made of copper tubes with aluminum fins, usually arranged in two or three rows.
4. Air Filter: Filters are generally made of coarse pore foam plastic or fibrous fabric.
5. Control Device: Units generally feature three-speed control (high, medium, and low), with an adjustable air volume range of about 50%.

Basic Parameters of Fan Coil Units

Air volume, cooling capacity, heating capacity, static pressure, noise level, water resistance, input power, water flow rate, and the temperature difference between inlet and outlet water.

Fan Coil Air Conditioning System

The system consists of fan and motor, coils (cooling and heating), filters, casing, condensate pan, optional control valves, room or in-unit temperature sensors, and optional fresh air valves. Auxiliary options are added to enhance specific functions. A switchable two-pipe system features one supply pipe and one return pipe connected to the terminal equipment’s coil, used in temperate regions with low thermal loads. The system requires seasonal switching, resulting in periods when indoor comfort may not be optimal. Adding an auxiliary electric heating coil resolves this issue, allowing the system to maintain cooling until heating is necessary. A four-pipe system includes two supply pipes (hot and cold) and two return pipes, enabling simultaneous heating and cooling, thus maintaining indoor comfort regardless of load changes.

Design Steps for Fan Coil Central Air Conditioning System

1. Define control zones.
2. Calculate peak cooling and heating loads.
3. Select terminal equipment type and size.
4. Choose control type for terminal equipment.
5. Design chilled water piping layout.
6. Design condensate drainage piping.
7. Select chiller unit.
8. Select auxiliary equipment.

Air Handling Units and Selection

Air handling units, also known as cabinet fan coil units, cabinet air conditioning units, variable air volume air conditioning units (with control boxes), fresh air units (for fresh air conditions), air handlers, and air conditioning boxes, consist of low-noise fans, cooling coils, frames, panels, and air filters. They are classified based on installation modes: ceiling-mounted, horizontal, vertical, jet direct blow, and exposed direct blow.

The function of air handling units is to cool, dehumidify, heat, humidify, and filter air, delivering treated fresh air through ducts to air-conditioned spaces.

1. Basic Components of Air Handling Units: Centrifugal fans, panels, filters, casings, cooling coils.
2. Select Fan Type: Axial flow or centrifugal fans.
3. Drive Types: External rotor, belt-driven, direct-driven, or without casing.
4. Same Diameter Fan Types: Forward-curved, backward-curved, or without casing.
5. Fan Parameters and Characteristics:
   • Air Volume (Q): The amount of air flowing through the fan per unit time (m³/s, m³/min, m³/h).
   • Air Pressure (H): Total energy per cubic meter of air provided by the fan (kg·m), which includes static pressure (Hs) and dynamic pressure (Hd). Ht = Hs + Hd.
   • Shaft Power (P): The effective total power of the fan, also known as air power.
   • Efficiency (η): The ratio of the effective power remaining after removing losses from the shaft power, indicating fan efficiency.

The fan similarity theory involves complex equations relating flow rate, operating pressure, and shaft power to rotational speed, adjusted for environmental changes. Engineering applications typically use fan performance curves for approximate parameter calculations.

Combination Air Conditioning Boxes

Combination air conditioning boxes use water as a cooling medium and water or steam as a heating medium. They function as modular units that perform air mixing, filtration, purification, sterilization, cooling, heating, dehumidification, humidification, and silencing. These systems are explained, including basic classifications, function segment roles, coil placement, and fan type selection.

1. Types and Applicable Locations: Comfort units, process units, medical units, textile air conditioning.
2. Function Segments: New return air mixing, fresh air segment, return air segment, primary filtration, intermediate segment, new return exhaust, secondary return air, cooling, heating, spray, water baffle, humidification, total heat recovery, new air blower, exhaust blower, supply blower, return blower, silencing, sterilization, supply air, medium efficiency supply.

Inducers and Air Curtains

1. Inducers: Semi-central air conditioning systems with end duct inducers. These systems use high-speed nozzles to induce secondary air from the room, mixing it with primary air before delivering it into the room. Inducers also provide cooling or heating, handling room loads with air and water.
2. Air Curtains: Air curtains use linear air distributors to create air curtains at certain velocities and temperatures to seal doors, entrances, passages, openings, and counters, preventing external air intrusion and maintaining indoor conditions.
3. Selection Principles for Air Curtains: Design parameters (temperature and airflow), regional climate characteristics, surrounding environment, and special requirements. Frequent door openings in large stores should have air curtains:
   • Cold regions should have hot air curtains if vestibules or anterooms cannot be set up.
   • In cold regions, if deemed reasonable after technical and economic comparison, hot air curtains can be installed.
   • Large stores with air conditioning should have air curtains, while small stores may optionally have them.