Fan coil units are not only widely used in central air conditioning systems such as hotels, shopping malls, etc., but their usage is also increasing year by year. However, in practical use, the number of engineering quality complaints about fan coil units has been increasing year by year. Designers often only consider performance indicators such as air volume, power, and cooling/heating when selecting fan coil units, and do not pay much attention to the condensation problem of the units. The consequences and impact of condensation problems occurring after the installation and use of the unit are relatively serious. The causes of condensation problems can be divided into two types: primary condensation and secondary condensation.
1.1 One condensation
Generally, fan coil units are equipped with condensate trays. The water condensed by the coil meter cooler should all fall into the water tray, and then be collected by the drainage outlet of the water tray into the condensate pipe for centralized drainage. But in practice, condensation water drips out for various reasons:
1.1.1
The condensate tray is relatively small
The area of the condensate tray under the unit cannot fully cover the unit, and the water condensed by the surface cooler will drip outside the tray.
In addition, if there is no condensate tray below the manual and electric valves on the connection between the fan coil unit and the cold water pipe, condensation water droplets are more likely to occur at the valve handle and valve connection.
1.1.2
Poor insulation of cold water pipes and connecting air ducts
When the insulation of the cold water pipe and the connecting air pipe is not properly done, and the insulation material is not tightly attached to the surface of the pipeline, once it comes into contact with external air, condensation will occur on the pipe wall, soaking the insulation cotton, and further worsening the condensation. When a certain amount is reached, condensation drips out, causing serious consequences.
Insufficient thickness of insulation cotton for cold water pipes and connecting air ducts can cause the surface temperature of the insulation cotton to be too low, resulting in condensation.
1.1.3
Insufficient slope of condensate water pipeline
The slope of the condensate water pipe is unreasonable, making it impossible to discharge the condensate water in a timely and smooth manner. Causing condensate to accumulate more and more in the water tray, eventually overflowing.
1.1.4
Cold bridge formed by air duct and water pipe supports and hangers
The supports and hangers of air ducts and water pipes are in direct or indirect contact with the air ducts and water pipes, and the insulation is not properly treated, resulting in a lower surface temperature of the supports and hangers, which come into contact with air and cause condensation and dripping.
1.1.5
Fan coil unit casing condensation
Due to inadequate insulation of the unit itself, condensation occurred on the surface of the casing, as shown in Figure 1. The national standard stipulates that the thermal resistance of the insulation layer should not be less than 0.68m2 · K/W, so it is important to choose appropriate insulation materials.
1.1.6
The connection between the condensate tray and the surface cooler was not insulated, and the screws were not insulated
Due to the low surface temperature of the meter cooler, if the connection between the condensate tray and the meter cooler is not made accordingly
Handling, forming a cold bridge, is more prone to condensation accidents.
Some of the screws on the wind turbine were not insulated externally, which also caused condensation problems. As shown in Figure 2
The above analysis is about the cause of primary condensation, and there is also a problem of secondary condensation that causes external droplets of condensation. Enterprises and users often attach great importance to the issue of primary condensation and overlook the occurrence of secondary condensation.
1.2 Secondary condensation
According to the national standard GB/T19232-2003 "Fan coil unit", the condensation test condition is an ambient dry bulb temperature of 27 ℃, wet bulb temperature of 24 ℃, and the fan coil unit operates at low speed for 4 hours. There should be no condensation water on the outer surface of the cartridge and surface mounted unit boxes, and there should be no condensation water dripping from the air vents; There should be no condensation on the surface of the concealed unit casing.
In the condensation test, the issue of secondary condensation can be well documented.
1.2.1
The use of inferior insulation materials for condensate trays
To save costs, some companies use inferior insulation materials for their condensate trays. The density, thickness, elasticity and other performance indicators of the material are not ideal. Even if gently pressed with fingers, the insulation cotton is not easy to recover. When there is condensate remaining in the condensate tray, due to the low temperature of the condensate, the surface temperature of the insulation cotton with low thermal resistance through thermal conduction is also lower, causing condensation.
1.2.2
Poor treatment of the edge of the condensation water tray
The inclination of the edge of the condensate tray is not sufficient, and the insulation is not good, resulting in condensation droplets. As shown in Figure 4 and Figure 5
1.2.3
Improper assembly of the fan coil unit with the condensate tray
During installation and use, the fan coil unit must not tilt and the motor shaft must be ensured to work horizontally. However, if the condensate tray is installed horizontally, it is not conducive to the smooth discharge of condensate. If a large amount of condensed water is retained in the water tray, on the one hand, it may accumulate more and more, causing overflow. On the other hand, if the insulation of the condensate tray is not good, it is easy to cause secondary condensation and dripping on the insulation surface, resulting in serious consequences.
1.2.4
Insulation issues of condensate pipes
In the installation and construction of fan coil units, attention is often paid to the insulation of cold water pipes, while neglecting the insulation of condensate pipes. When the temperature of the condensate is low, if the insulation of the condensate pipe is not good, it can also cause secondary condensation and lead to engineering accidents.
Prevention of condensation problems
To prevent condensation, the following measures can be taken:
The area of the condensate tray should be large enough to allow the condensate from the unit to drip into the tray. The length of the water tray should be extended to ensure that the condensed water at the joints of manual and electric valves can be collected and discharged by the water tray;
The insulation layer of cold water pipes and connecting air ducts should be tightly attached to the pipes to prevent air from entering. The thickness of the insulation layer should ensure a certain thermal resistance to prevent the surface temperature of the insulation cotton from being too low and forming condensation;
The condensate pipe should have a certain slope to facilitate the discharge of condensate water. For the convenience of drainage, the condensate discharge pipe can also be set up nearby for drainage, such as connecting to bathroom floor drains, etc;
In areas where air ducts, water pipe supports and hangers are prone to forming cold bridges, insulation should also be added or the supports and hangers should be insulated to prevent condensation;
The fan coil unit should be insulated internally to prevent condensation on the unit casing;
The connection between the condensate tray and the unit should be padded with insulation treatment, and the outer surface of the screws should also be insulated;
To address the issue of secondary condensation, the following measures can be taken to prevent condensation:
Eliminate the use of inferior insulation materials for condensate trays. The density, thickness, and elasticity of the materials should meet certain requirements, and the thermal resistance should meet the standard;
The edge of the condensate tray should have a certain inclination towards the inside of the tray, so that the condensation on the edge can flow into the tray. The outer edge should also be insulated to prevent condensation, hanging, and dripping;
When assembling the unit, while ensuring the level of the unit, the condensate tray should have a certain inclination towards the drainage outlet. Near the drainage outlet, the water tray should be slightly concave, and the drainage outlet should be at the lowest point of the condensate tray to facilitate the discharge of condensate;
The insulation of the condensate pipe should also have a certain thickness, and the insulation material should be tightly attached to the pipe wall to prevent air from entering.