Fan coil unit is not only widely used in hotels, shopping malls, and other central air conditioning systems, but also in the consumption is increasing year by year. However, in practice, the number of engineering quality complaints of fan coil units is increasing year by year. When choosing fan coil units, designers often only consider the performance indicators such as air volume, power, cold / heat, etc., and pay little attention to the condensation problem of the unit. When the unit is installed and used, the consequence and influence caused by condensation problem is more serious. There are two reasons for condensation, one is condensation and the other is secondary condensation.
1. A dew
Generally, fan coil units are equipped with condensate pan. The water condensed by the coil surface cooler should fall in the water pan, and then be collected by the drain of the water pan into the condensate pipe for centralized drainage. However, in practice, condensate water drips out for many reasons
1.1. The condensate pan is small: the area of condensate pan under the unit can not completely include the unit, and the condensed water from the surface cooler will drip outside the panel. In addition, if there is no condensate tray under the manual and electric valves on the connecting pipe between the fan coil unit and the cold water pipe, the condensation water drops easily at the valve handle and valve connection.
1.2. Poor thermal insulation of cold water pipe and connecting air duct: when the thermal insulation of cold water pipe and connecting air duct is not well done, and the insulation material is not close to the surface of the pipe, once contacting with the outside air, condensation will be caused on the pipe wall, and the insulation cotton will be soaked, which will further worsen the condensation. When it reaches a certain amount, condensation drops outside, causing serious consequences. If the thickness of insulation cotton of cold water pipe and connecting air duct is not enough, the surface temperature of insulation cotton will be too low and condensation will be formed.
1.3. The gradient of condensate pipe is not enough: the gradient of condensate pipe is not reasonable, which can not discharge condensate timely and smoothly. As a result, the condensate accumulated more and more in the water pan, and finally overflowed.
1.4. The supports and hangers of air duct and water pipe form a cold bridge: the supports and hangers of air duct and water pipe are in direct and indirect contact with air pipe and water pipe, and the heat preservation is not well treated, resulting in lower surface temperature of supports and hangers, contact with air, and condensation and dripping.
1.5. Condensation of fan coil unit shell: due to the unit itself is not well insulated, resulting in condensation on the shell surface, as shown in Figure 1. According to the national standard, the thermal resistance of thermal insulation layer should not be less than 0.68m2 · K / W, so it is necessary to select appropriate thermal insulation materials.
1.6. There is no heat insulation treatment for the connection between condensate pan and surface cooler, and the screw is not insulated: due to the low surface temperature of the surface cooler, if no corresponding treatment is taken for the connection between condensate pan and surface cooler, condensation and external drip accident is more likely to occur. Some of the screws of the wind disk were not insulated, which also caused the condensation problem.

For a condensation problem, the following measures can be taken to prevent condensation:
The area of the condensate pan shall be large enough to make the condensate of the unit drip into the pan;

The insulation layer of cold water pipe and connecting air pipe shall be closely connected with the pipe to prevent air from entering;

The thickness of the insulation layer should be ensured to prevent the surface temperature of the insulation cotton too low to form condensation;

The condensate pipe shall have a certain slope to facilitate the discharge of condensate

The air duct and water pipe supports and hangers should also be provided with thermal insulation where it is easy to form a "cold bridge";

The fan coil unit shall be internally insulated to prevent condensation of the unit shell;

The connection between condensate pan and unit shall be padded for heat insulation.

2.Secondary condensationAccording to the national standard GB / T19232-2003 fan coil unit, the condensation test conditions are as follows: ambient dry bulb temperature 27 ℃, wet bulb temperature 24 ℃, fan coil unit running at low speed for 4 hours. There should be no condensation water on the external surface of the box of card type and surface mounted unit, and there should be no condensation water dropping from the tuyere; there should be no condensation water dripping on the surface of the box of the concealed unit.
2.1. Use of poor thermal insulation material for condensate pan: the density, thickness, elasticity and other performance indexes of the material are not ideal, that is, the insulation cotton is not easy to recover by pressing gently with fingers. When there is condensed water in the condensate tray, the surface temperature of the insulation cotton with low heat conduction and thermal resistance is also low due to the low temperature of condensate, which causes condensation.
2.2. The edge treatment of condensation water tray is not good: the edge gradient of condensation water tray is not enough, and the heat preservation is not good, resulting in condensation outside drip.
2.3. Improper assembly of fan coil unit and condensate pan: the horizontal installation of condensate pan is not conducive to the smooth discharge of condensate. If a large amount of condensed water is retained in the water pan, on the one hand, it may accumulate more and more, causing overflow; on the other hand, if the heat preservation of condensate pan is not good, it is easy to cause secondary condensation and external drip on the insulation surface, resulting in serious consequences.

2.4. Condensate pipe insulation: in the installation and construction of fan coil unit, attention is often paid to the insulation of cold water pipe, while the insulation of condensate pipe is ignored. When the temperature of condensate water is low, if the heat preservation of condensate pipe is not good, it will also cause secondary condensation and engineering accidents.
For secondary condensation, the following measures can be taken to prevent condensation:
Put an end to inferior materials, the insulation density and thickness shall meet the requirements, and the thermal resistance shall meet the requirements;
The edge of condensate pan should be inclined to the inside of the pan;
The outer edge shall also be provided with heat preservation to prevent condensation, hanging and dripping;
The condensate pan shall be inclined to the outlet;
The thermal insulation of condensate pipe has a certain thickness, and the insulation material should be closely connected with the pipe wall to prevent air from entering.