Fan Coil unit Selection FAQ

The effect of the use air conditioning, mainly due part in the design selection, and although the regular fan coil air conditioning system is different, but it is also a selection of certain ways, then the fan coil selection we will plant what the problem happen?

First, according to the cooling load selection drawbacks

According to the maximum cooling load of the room air conditioning fan coil unit air conditioning system design chosen common practice, its purpose is to ensure peak load when the room temperature. The air-conditioned rooms actually run most of the time will not be in peak load, so that the excess cooling capacity, switching to low-grade run to reduce the cooling output, thereby maintaining the heat balance of the room. Visible actual output capacity chillers depends on the air conditioning load changes, the nominal cooling capacity of the unit has little relationship. Therefore, the amount of cooling air conditioning is to achieve a necessary condition but not determine the effect of the use of air conditioning. Evaluation of the effect of bad air conditioning, one close to the average degree of room temperature and set temperature; second temperature distribution (gradient) and variations (fluctuations) amplitude. The larger the temperature difference between the air, the fewer the number of ventilators, temperature gradient and the greater the volatility, so the supply air temperature and air conditioning ventilation frequency accuracy and comfort is the main factor influence.

The higher the precision air conditioning required supply air temperature difference is smaller, the more the number of ventilators. Seen by the maximum cooling load selection, only to meet the peak load when the room temperature is not enough, need to meet the appropriate supply air temperature and ventilation rate, in order to ensure comfort requirements of the room.

Second, we can not guarantee a sufficient air supply

Due to air temperature, ventilation rate is to determine the accuracy and air-conditioned comfort of the main factors, it is to ensure adequate air flow to achieve the desired air conditioning effect is a prerequisite. Here, the amount of wind is the actual air supply unit when using it, rather than the catalog nominal amount of wind (GB / T 19232-2003 requirements: nominal air flow in the coil to be unreasonable water, air 14-27 ℃, fan speed high-end, low static pressure on the export of hydrostatic unit without air and filters to 12Pa measured airflow value). The actual use, the dark because they have to add the group installed, return air grilles, filters and short duct, coupled with the effect of water condensate coil surface, dust, clogging of the filter and many other factors, will lead to increased wind resistance, wind decreased, so that the actual air volume is far less than the nominal amount of wind (author through a lot of experiments to prove: Usually low l5-25%). Since the air volume significantly reduced, influence the effect of air-conditioning, mainly following problems:

1) ventilation fewer;
2) low air velocity, air impact jet range;
3) The air temperature is low, the impact of air-conditioned comfort and may cause air grilles or condensation.

On the other hand, for the fan coil unit itself, decrease the amount of air directly affects the heat transfer coil so that the coil cooling capacity decreased, which would form the actual performance of the unit (air flow, cooling capacity) is lower in nominal value of the anomaly. Thus, the nominal amount of wind, cold catalog on selection only as a reference when, and not as a basis for selection. Increase the amount of wind can not only increase the number of ventilators, reduce air temperature, improve air-conditioning effect, but also because the cold will increase correspondingly reduced in volume units. Therefore, to improve the air flow is one of the development direction of the fan coil. Of course, the air volume increase also conditioned by the area allowed wind speed constraints.

To control the supply air temperature difference between the cold air flow and should maintain proper matching relation. Full cooling capacity and air flow (mass flow rate) ratio is the difference between the enthalpy of the air coil import and export, which determines the cooling capacity of the supply air temperature and the size of the unit. From the perspective of controlling supply air temperature, enthalpy difference too negative, while domestic fan coil enthalpy difference and supply air temperature is generally high. Nominal parameters GB / T 19232-2003 shall be calculated enthalpy difference 15.88kJ / kg, supply air temperature of about l2 ℃. In terms of air volume decreased by 20%, the actual enthalpy difference will exceed 19.85kJ / kg, the actual supply air temperature will be up to l5 ℃, clearly beyond the predetermined permissible supply air temperature difference (6-lO ℃), it can not guarantee air conditioning precision and comfort requirements.


Third, ignoring the wind resistance calculation system

General fan coil air-conditioning system, air system a smaller scale, a simple structure, small resistance, within about l5-5OPa range, but this alone is enough to have a crucial resistance on the actual air supply fan coil system Impact. Fan coil units divided into low static pressure and high static pressure units into two categories, the GB / T 19232-2003, for the low static pressure units, with exports of static pressure and air filters, as OPa, and without air filter and other export static pressure 12Pa, that is, air filters, etc. and is composed of resistance 12Pa. US Air Conditioning & Refrigeration Institute standard "room fan coil air conditioner" hRI 440-84 clearly states: the factory without delivery, return air grille or filter fan coil should be under 12.4Pa external static pressure test wind u. Precisely in order to ensure that the actual air volume and the nominal amount of wind matches this provision. The high dust content of the atmosphere, filter clogging Netease, should external static pressure than 12.4Pa high contrast, China's industry standard test conditions specified in rationality open to question. Horizontal concealed in rooms, ceiling air return FCU, for example, additional resistance should at least include return air grilles, return air filters, air supply and air supply grille short tube resistance. If the return air wind speed 1.Om/s, air velocity of 1.5 m / s, this time outside the machine calculated resistance 16Pa, if the choice would certainly have caused low static pressure unit air volume decline, in this case in engineering applications should belong to smaller additional resistance example, and the amount of wind on like this, visible air system FCU additional resistance can not be ignored. Furthermore, for high static pressure units, if not through the resistance calculation, but that the choice of a high static pressure units can meet the requirements of the practice is unreasonable.

High static pressure duct fan coil unit

For example, a building within a horizontal ceiling installation concealed fan coil unit and the corresponding air system, the nominal amount of wind FCU is 750m / h, diffuser throat wind speed 2.5m / s, return air wind speed 1.5m / s, calculated FCU known outside the body of the total resistance of about 61Pa, wherein the diffuser, 80% of the return air filter resistance of the total resistance. In this case, even the use of external static pressure 30Pa machine or 50Pa High static pressure type FCU, air volume will decline about 15%. Thus, in the specific projects proposed in general terms as long as the high static pressure requirements and high static pressure unit does not have to be related air system analysis approach it is not desirable.