Classification, Principle, Model Selection and Application, and Troubleshooting of Fan Coils

Fan coil units are mainly composed of a fan, a heat exchange coil, and a casing. According to the external static pressure of the fan coil unit, they can be divided into the standard type and the high static pressure type. According to the number of rows of the heat exchange coil, they can be divided into two rows and three rows. The heat exchange coil generally uses copper tubes strung with aluminum fins. The outer diameter of the copper tubes is 10 to 16 mm, the thickness of the fins is about 0.15 to 0.2 mm, and the spacing is 2.0 to 3.0 mm. The fan generally uses a centrifugal fan with double-inlet forward-curved blades. The motor is a capacitive 4-pole single-phase motor with three-speed gears.

The casing and the condensate water pan are insulated.

With the continuous circulation of indoor air by the fan coil unit, the air is cooled or heated by passing through the coil to maintain the required temperature and a certain relative humidity in the room. The chilled water or hot water used in the coil is supplied by the centralized cooling source and heating source. At the same time, the fresh air centrally processed by the fresh air air conditioning machine room is sent into each air-conditioned room through special fresh air ducts to meet the hygienic requirements of the air-conditioned rooms.

Compared with the centralized system, the fan coil air conditioning system has no large air ducts, only water pipes and smaller fresh air pipes. It has the advantages of convenient layout and installation, small occupation of building space, and good individual adjustment. It is widely used in comfort air conditioning systems with low requirements for temperature and humidity accuracy, a large number of rooms, small rooms, and the need for individual control.

The working principle of the fan coil unit is not as complex as that of the central air conditioning system. In fact, we can imagine the fan coil unit as an electric fan, but the air blown out by this electric fan is at the temperature we need.

The Structure of the Fan Coil Unit
Fan: Driven by a unidirectional multi-speed low-noise induction system motor, the fan speed is changed by adjusting the input voltage, so that the air volume of the fan is divided into high, medium, and low gears, which are controlled by an electrical switch, and the cooling (heating) capacity of the fan coil unit is adjusted accordingly. The fan is the power source for conveying air and also the disturbance source for strengthening the convective heat transfer on the air side (the outer surface of the coil). Together with the motor, it is also the main noise source of the unit.
Coil: It is an air-water heat exchanger made of finned tubes. Chilled water (hot water) flows inside the tubes. Since the temperature of the chilled water is lower than the dew point temperature of the air, there is condensed water on the outer surface of the tubes, presenting heat transfer under wet conditions, which combines heat exchange and mass exchange and improves the heat transfer effect. The coil undertakes most or all of the air conditioning load of the room, and the number of tube rows is generally 3-4 rows.
Condensate Water Pan: Placed under the coil together with the drain connection, its function is to receive the water droplets continuously condensed on the coil and discharge them outdoors through the drain connection.
Air Filter: Placed under the coil together with the drain connection, its function is to receive the water droplets continuously condensed on the coil and discharge them outdoors through the drain connection.
The Working Principle and Refrigeration Operation Process of the Fan Coil Unit
The fan coil unit can be divided into a water circuit and an air circuit. The water circuit is supplied with chilled (hot) water by the centralized air conditioning cooling (heating) source equipment (such as a chiller). Under the action of the water pump, the chilled (hot) water is transported to the inside of the coil for circular flow. The air circuit is that the air is sucked into the room from the return air inlet by the fan, then passes across the coil. After heat exchange with the chilled (hot) water in the coil, the air is cooled and dehumidified, and then sent into the room from the supply air inlet. Through such repeated circulation, the indoor temperature and humidity can be adjusted.

The operation process of the central air conditioning system is essentially a process of heat transfer. When the central air conditioning system is cooling, the typical heat transfer process during refrigeration is as follows:
The Advantages and Disadvantages of the Fan Coil Unit with Fresh Air System
The fan coil unit with fresh air system is divided into two parts: the central air conditioning fan coil unit and the fresh air system. The fan coil unit is the terminal equipment of the central air conditioning system, and the fresh air system bears the fresh air load to meet the indoor air quality. The fan coil unit with fresh air system is an important form of the water system air conditioning and is also a commonly used air conditioning form in private buildings.
Advantages of the Fan Coil Unit with Fresh Air System (Compared with the All-Air System)
Advantage 1 of the Fan Coil Unit with Fresh Air System: It has flexible control and the superiority of individual control. It can flexibly adjust the temperature of each room and determine the start and stop of the fan coil unit according to the usage status of the room.
Advantage 2 of the Fan Coil Unit with Fresh Air System: The fan coil unit is small in size, occupies little space, is convenient for layout and installation, and is even suitable for the renovation of existing buildings.
Advantage 3 of the Fan Coil Unit with Fresh Air System: It is easy to achieve system partition control. The cooling and heating loads can divide the system into several regional systems according to the room orientation, usage purpose, usage time, etc., and implement partition control.
Disadvantages of the Fan Coil Unit with Fresh Air System (Compared with the All-Air System)
Disadvantage 1 of the Fan Coil Unit with Fresh Air System: Due to the scattered installation of the units and a large number of units, the workload of maintenance and management is large.
Disadvantage 2 of the Fan Coil Unit with Fresh Air System: The indoor air quality is relatively poor. It is difficult to carry out secondary filtration and there is a high probability of condensate water seepage accidents on the ceiling.
Disadvantage 3 of the Fan Coil Unit with Fresh Air System: It is difficult to solve the fresh air volume problem in the fan coil unit mode itself. Due to the small static pressure of the fan in the unit, the air distribution is limited, and it is suitable for rooms with a depth of less than 6 meters.
The fan coil unit with fresh air system has both advantages and disadvantages. Reasonable design, appropriate equipment selection, and correct construction and installation can reduce the defects brought by the fan coil unit with fresh air system. The above is just an understanding of the advantages and disadvantages of the fan coil unit with fresh air system. For designers, they can learn from each other's strengths and weaknesses; for consumers, they can seek advantages and avoid disadvantages and choose an air conditioning system suitable for themselves.
Key Points of Design and Model Selection of the Fan Coil Unit
The fan coil unit is small in size, flexibly arranged, easy to install, and occupies less building space, making it convenient to cooperate with the interior decoration construction. However, how to select a better fan coil unit according to the different needs of the owners and in combination with the design drawings for application in actual projects should fully consider the following points:
1. Checking of the Cooling Capacity: The products on the market currently are generally nominal cooling capacities, while the actual cooling capacity during operation should be the cooling capacity × the average running time per unit time, that is, changing the running time or the air volume will affect the input cooling capacity of the unit. Therefore, it is not that the higher the nominal cooling capacity, the better. If the unit is selected only according to the high cooling capacity, the cooling capacity will be too large, resulting in a too low operating rate, a decrease in the number of air changes, an increase in the room temperature gradient, an increase in the system capacity and equipment investment, an increase in the air conditioning energy consumption, and a decrease in the air conditioning effect. Therefore, the cooling capacity is only one of the necessary conditions for selecting the equipment, and other factors should also be taken into account.
2. Checking of the Air Volume: The number of air changes is mainly checked according to the quality requirements of the room. The smaller the supply air temperature difference and the more the number of air changes, the better the air quality and the more comfortable it will be. The reason why some air-conditioned rooms have peculiar smells and a stuffy feeling is that the checking of the air volume has not been properly handled.
Since the nominal air volume of the fan coil unit is measured under the working conditions of no water flow and zero differential pressure between the air inlet and outlet, there are some unrealistic factors. Therefore, when actually determining the air volume, the air volume value in this ideal state should be deducted. Through empirical calculation, this supplementary air volume should account for 20-30% of the nominal air volume.
3. Supply and Return Air Modes
The supply and return air modes form the so-called air flow organization, and whether it is reasonable or not directly affects the uniformity and stability of the temperature field and velocity field in the air-conditioned room, that is, the quality of the air conditioning effect. A reasonable air flow organization requires a certain supply air velocity to avoid air flow short-circuiting and ensure a certain jet length. The wind speed depends on factors such as the external static pressure of the unit, the supply air volume, and the supply air outlet. If the external static pressure of the unit is too low, it will lead to a decrease in the air volume and the range, uneven heating and cooling in the room, and a large difference between the designed air flow organization and the actual operation state on the curve chart. Therefore, according to the actual building layout, the structural form of the room, the depth, height, and other conditions, the fan coil unit model should be selected accordingly by choosing the medium-grade air volume and wind speed indicators.
4. Other Factors
a. Control the noise index below 40 dB. For fan coil units with relatively large noise, install a noise reduction treatment device, and the resistance value should not be greater than 10 Pa.
b. Pay attention to the thermal insulation quality, the discharge of condensate water, the slope direction, the pipe fittings joints, and the system cleanliness during installation and construction.
c. The setting mode of the water system, whether it is a horizontal system or a vertical system. In some construction sites, the vertical system is selected, which can better ensure the discharge of condensate water and meet the requirements of the floor height of the room.
The Control Principle and Wiring of the Fan Coil Unit
Simple Control of the Fan Coil Unit: Use a three-speed switch to directly and manually control the three-speed conversion and start/stop of the fan.
Temperature Control of the Fan Coil Unit: Use a thermostat to automatically control the opening and closing of the electric two/three-way valve, the three-speed conversion of the fan, or directly control the three-speed conversion and start/stop of the fan according to the comparison and operation between the set temperature and the actual detected temperature, so as to achieve a constant temperature by controlling the water flow or air volume of the system.
Classification and Parameter Performance of the Fan Coil Unit
According to the form: There are five types, namely horizontal concealed type, horizontal exposed type, vertical concealed type, vertical exposed type, and cassette type;
According to the thickness: Ultra-thin type and ordinary type;
According to whether there is a condensate water pump: Ordinary type and luxury type;
According to the static pressure of the unit: 0 Pa, 12 Pa, 30 Pa, 50 Pa, 80 Pa (external static pressure of the unit);
According to the number of tube rows: Two-tube rows and three-tube rows;
According to the system: Two-pipe system and four-pipe system;
The number of rows mentioned for the fan coil unit refers to the number of rows of the copper tubes of the coil cooler of the fan coil unit. Generally, for the two-row type, there are two rows of copper tubes, with 8 tubes in each row, a total of 16 copper tubes; for the three-row type, there are three rows of copper tubes, with 8 tubes in each row, a total of 24 copper tubes. The more the number of copper tubes, the better the refrigeration effect.
Two-pipe System: In an ordinary fan coil unit, chilled water is used for refrigeration in summer, and hot water is used for heating in winter;
Four-pipe System: It is mostly used in some relatively luxurious places. Hot water and chilled water can flow simultaneously, that is, some rooms can be cooled and some rooms can be heated according to needs.
The Influence of Parameter Changes on Performance
According to statistics, when the water supply temperature rises by 1°C, the cooling capacity decreases by about 10%. The higher the water supply temperature, the greater the decrease, and the dehumidification capacity decreases.
When the water supply conditions are certain and the air volume of the fan coil unit changes, the cooling capacity and the enthalpy difference of air treatment change accordingly. Generally, the cooling capacity decreases, the enthalpy difference increases, and the power consumption of the fan per unit cooling capacity changes little.
When the temperature difference between the inlet and outlet water of the fan coil unit increases, the water volume decreases, and the heat transfer coefficient of the heat exchange coil decreases accordingly. In addition, the heat transfer temperature difference also changes. Therefore, the cooling capacity of the fan coil unit decreases with the increase of the temperature difference between the supply and return water. According to statistics, when the water supply temperature is 7°C and the temperature difference between the supply and return water increases from 5°C to 7°C, the cooling capacity can decrease by about 17%.
Common Fault Phenomena of the Fan Coil Unit
The Fault Performance and Treatment Methods of the Fan Noise
1. Noise caused by damaged bearings; The treatment method is to replace the bearings.
2. Noise generated during operation with the ceiling; The treatment method is to adjust the height of the suspender nuts of the coil, or deal with the friction between the air outlet and the ceiling keel.
3. Noise generated by air in the pipeline; The treatment method is to exhaust all the air in the pipeline at the exhaust valve of the coil, the floor exhaust valve, and the terminal exhaust valve.
The Fault Performance and Treatment Methods of the Fan Not Starting or Running Slowly
1. The temperature switch is damaged; Use an electroscope to measure whether there is electricity at the input end of the temperature control switch and the fan output terminal. If it is determined that the temperature switch is damaged, the temperature switch can be replaced or repaired.
2. Slow running speed:
a. Feel whether the surface temperature of the fan is normal;
b. Manually rotate the fan blades after the machine is stopped and feel whether the rotation is flexible. If there is resistance, replace the fan bearings;
c. If the manual disk operation is normal, replace the starting capacitor;
d. Measure the resistance of the motor coil. If it is not normal, replace the motor.
Poor or No Air Conditioning Effect
1. Open the exhaust valve of the coil and check whether the circulating water temperature of the system is normal;
2. Check the inlet and outlet temperatures:
a. The temperature difference is very small. Open the exhaust valve of the coil and check whether the water temperature is normal. If the water temperature is normal, then check whether the two-way valve is open. If the two-way valve is not open, continue to check whether the power supply of the two-way valve is normal and whether the Y-shaped filter screen is blocked;
b.

The temperature difference between the inlet and outlet is normal. Check whether the room insulation is normal, such as whether the doors and windows are closed. If the doors and windows are not closed, there will be air convection and the air conditioning will have no effect. An explanation should be given to the customer;
c. The temperature difference between the inlet and outlet is large and the air volume at the air outlet is small. Check whether there is dust on the filter screen of the inlet and whether the rotation speed of the fan is normal.
Unpleasant Odor in the Air Conditioning
1. Check and clean the filter screen of the air inlet;
2. Check whether there is dust on the fins of the coil. If there is dust, a cleaning plan should be implemented;
3. Check whether there are foreign objects, dust, water accumulation, etc. in the air duct, and clean it to remove the odor.
4. Check whether there are foreign objects in the water accumulation pan.

Water Leakage in the Air Conditioning
1. Poor drainage: a. Check whether the water-saving pan is blocked, b. Check whether the drain pipe is blocked, c. Check whether the slope of the drainage pipeline is reasonable.
2. The thermal insulation layer falls off, and the thermal insulation layer should be restored.
3. The exhaust valve leaks water, and the exhaust valve should be tightened.
4. The hose, valve, or pipe fittings leak water. Close the main valve and then replace them.
Emergency Treatment for the Bursting of the Air Conditioning Water Pipe
1. Open the drain valve to drain water and reduce the pressure;
2. Close the air conditioning host, the make-up water valve of the expansion water tank, the booster pump, and the circulation pump;
3. Close the main valve of the area where the pipe bursts;
4. Turn off the power supply of the area where the pipe bursts;
5. When the elevator is endangered, turn off the power supply of the elevator and try to park it on a floor higher than the floor where the pipe bursts;
6. Move the important materials away in time and clean up the accumulated water.