How To Select The Model of Fan Coil Unit

As the core terminal equipment of air conditioning system, fan coil units exhibit significant regional differences in their technological development. 

In the design practice of air conditioning systems, selecting fan coil units based on the maximum cooling load of the room is a common practice, and its core value lies in ensuring temperature stability under peak load conditions. However, actual operating data shows that about 90% of the operating time of the air conditioning system is in a non full load state. At this time, excessive cooling can cause the unit to frequently switch operating modes. A case study of a commercial building shows that the proportion of operating time in the medium and low gears is as high as 78%. This phenomenon reveals a strong correlation between the actual cooling capacity of the unit and the dynamic load. The nominal cooling capacity parameter only reflects the performance indicators under specific testing conditions and cannot accurately characterize the actual operating efficiency.

The air conditioning performance evaluation system includes two key dimensions: temperature control accuracy, which refers to the deviation between the average room temperature and the set value (usually requiring ≤± 0.5 ℃); The second is the uniformity of the temperature field, involving room temperature gradient (≤ 1.5 ℃/m) and fluctuation amplitude (≤ 1 ℃/h). Research has confirmed that the synergistic effect of supply air temperature difference and air exchange rate has a decisive impact on indoor environmental quality - when the supply air temperature difference increases from 6 ℃ to 12 ℃, the air exchange rate needs to be increased by three times to maintain the same level of comfort.

Ensuring sufficient air volume is a prerequisite for achieving the expected air conditioning effect, as the temperature difference and air exchange rate are the main factors determining the accuracy and comfort of air conditioning. The air volume referred to here refers to the actual air supply volume during the use of the unit, rather than the nominal air volume in the product sample (GB/T 19232-2003 stipulates that the nominal air volume must be measured at a static pressure of 12Pa for low static pressure units without air vents and filters when the coil is not filled with water, the air temperature is 14-27 ℃, the fan speed is high, and the outlet static pressure is 12Pa). In actual use, concealed units are affected by various factors such as the addition of return air grilles, filters, and short air ducts, as well as condensation, dust accumulation, and clogged filter screens on the coil surface, which can increase air resistance and decrease air volume, resulting in actual air volume much lower than nominal air volume (the author has proven through extensive experiments that it is generally 15-25% lower). Due to the significant reduction in air volume, it affects the air conditioning effect.

In the design and operation of fan coil units, air volume parameters have a decisive impact on heat transfer performance. When the actual air volume is lower than the design value, the contact efficiency between the coil and the air decreases, resulting in synchronous attenuation of sensible heat and latent heat exchange capacity. Research data shows that a 20% reduction in air volume can lead to a decrease of over 17% in actual cooling capacity, while the energy efficiency ratio of the unit is significantly reduced due to underutilization of the heat exchange area. This phenomenon reveals the deviation between nominal parameters (such as air volume and cooling capacity annotated in the sample) and actual operating conditions, indicating that dynamic operating parameters must be combined for correction during selection.

In the design of fan coil air conditioning systems, the influence of air system resistance is often underestimated. Although the wind resistance of such systems is usually only 15-50Pa, this value is sufficient to significantly change the actual air supply volume. It is worth noting that if a high static pressure unit is directly selected without resistance calculation, its performance cannot be guaranteed. A case study of an office building shows that a 30Pa static pressure unit still experiences a 15% decrease in wind volume under a total resistance of 61Pa. This indicates that the design of the wind system needs to focus on three dimensions:

① the deviation between the actual air volume of the coil and the nominal value (usually 15-25% lower); 

② Accumulated resistance effect caused by filter blockage; 

③ The matching relationship between static pressure parameters and pipeline resistance avoids selecting high static pressure units based solely on experience and neglecting dynamic resistance calculation.