How long is the air mixing distance required in the mixing section of the air conditioning box?

The distance required for the mixing section of the air conditioning box depends on the position of the new return air inlet, the ratio of the two airflows, speed and temperature difference, as well as the uniformity requirements of subsequent components. There is no fixed length that applies to all units. The low shape of the mixing section does not mean that the air has been fully mixed before entering the coil.

Look at how the airflow enters, rather than just measuring a straight line

When two airflows collide, merge sideways, or have a large speed difference, local high and low temperature zones may be formed. If a small amount of low-temperature new winds rush to a certain area in winter, the normal average temperature does not mean that there is no local low-temperature risk. The inlet distribution, adjustment range and front section status of the coil should be checked, and special analysis or measurement should be carried out if necessary.

The inlet geometry should include the position, direction, area and relative distance of the air outlet; the operation combination should include the maximum and minimum fresh air ratio, speed and temperature difference on both sides; the subsequent requirements should include the wind exposure of the coil, the representativeness of the measuring points and the risk of local low temperature. The three types of conditions correspond to different mixed difficulties, and you cannot just give a length number. How to determine the proportion of new return air to the air-conditioning box based on project requirements, please refer toHow is the proportion of new return air to the air-conditioning box determined by project requirements?。

Changes in the proportion of new return air will change the mixing difficulty

When the proportion of fresh air is small, the fresh air may merge into the return air at a lower proportion. If the inlet position and wind speed are not designed properly, local low-temperature beams may easily form. When the fresh air ratio is close to fresh air, the temperature difference between the two air flows may decrease, but the total air volume and inlet speed change, and the air flow organization in the mixing section may still be different.

For the relationship between fresh air volume and total air supply volume, please refer toHow to calculate the fresh air volume and total air supply volume of the air conditioning box separately? . How to calculate the mixed air state, please refer toHow should the temperature and humidity of the mixed air in the air conditioning box be calculated? . The design of the mixing section should cover the actual operating proportion range of the project, rather than just judging based on the design maximum air volume.

The position of the inlet and the form of the air outlet determine the mixing effect

The relative position of the fresh air and return air inlets, the form of the air outlet and the inflow angle will directly affect the mixing uniformity. The hedging arrangement may accelerate mixing, but may also create local high-speed zones; lateral inflow may form stratification; the top or bottom inlet may produce temperature stratification due to buoyancy effects. If the air outlet area is too small, the inflow velocity will be too high; if it is too large, the effective mixing distance may be shortened.

For the connection between the inlet air outlet and the box structure, please refer toHow to choose between combined and integrated air conditioning boxes? . How to arrange the sequence of function segments, please refer toOn what basis should the sequence of functional sections of the air conditioning box be determined? . The position and length of the mixing section need to be checked with the front and rear functional sections.

Subsequent parts have different uniformity requirements

When the mixing section is followed by a coil, uneven wind speed and temperature distribution at the coil inlet will affect heat exchange and the risk of water entrainment. When a filter is connected afterwards, local high speed may affect the filtration efficiency and dust holding capacity. When subsequent measuring points are connected, the single-point reading may not represent the average state of the cross-section. The higher the requirements for uniformity of subsequent components, the more carefully the distance required for the mixing section or the flow equalization measures need to be checked.

For the influence of wind speed facing the coil, please refer toWhat impact does the coil head-on wind speed have on heat exchange and water entrainment in the air conditioning box? . How to position the supply air temperature sensor to avoid local airflow, please refer toHow should the air supply temperature sensor of the air conditioning box avoid the influence of local airflow? . For the arrangement of the filter section before and after the fan, please refer toWhat needs to be considered when arranging the filter section before and after the fan?。

How to advance a plan when length is limited

Adjusting the air inlet location, improving distribution, or using proven mixing measures can be evaluated, but additional components introduce resistance and maintenance requirements. The plan confirmation should include checking the cross-section and working conditions, and should not just write the words 'fully mixed'. Subsequent temperature probes should also avoid areas where single airflow is concentrated.

How to form internal resistance, please refer toWhat functional segments should be included in the internal resistance of the air conditioning box? . How to establish the fan pressure budget, please refer toHow is the total pressure of the air conditioning box fan determined by the internal resistance and external demand? . After the mixing section length is adjusted, the fan operating point may need to be rechecked.

Aili industrial supporting equipment

Aili Industrial can check the mixing section and the entire machine structure together. Please provide the new return air interface diagram and the main proportions throughout the year before the evaluation to facilitate the judgment of whether the space meets the air distribution tasks. Aili's industrial manufacturing end covers independent R&D and production of motors and fans, control systems, sheet metal, air filters and complete machine assembly. The mixing section structure, air outlet position and coil interface can be matched in the same package; non-standard boxes can be adjusted according to on-site space and air duct conditions.

Product form can be viewedAir handling unit andFan box page; clean air supply package can be combinedFFU fan filter unit andEFU equipment end fan filter unit verification; terminal particle filtration can be combinedHEPA/ULPA filter confirmation; when it comes to gaseous pollution control, you can communicate separatelyChemical filter solutions. The specific mixing effect, applicable conditions and delivery scope still need to be verified based on the corresponding configuration data and on-site working conditions. The actual effect is subject to on-site debugging and performance test results.

Submit project parameters:https://ailigy.com/contactus.html

Related questions and answers

Is there a fixed length standard for the mixing section?
There is no fixed length that applies to all units. The mixing distance depends on the inlet position, air volume ratio, speed and temperature difference, as well as the uniformity requirements of subsequent components.

The length of the mixing section is not enough. Can I just add a baffle?
uncertain. Additional baffles or flow equalization measures will bring resistance and maintenance requirements and should be checked and verified. You cannot just write 'full mixing'.

Can Aili Industrial help check the mixing section length?
The mixing section layout can be checked based on the new return air interface diagram, operating ratio and box space. The specific plan needs to be confirmed based on project conditions.

Related articles

1. How is the proportion of new return air to the air-conditioning box determined by project requirements?

2. How should the temperature and humidity of the mixed air in the air conditioning box be calculated?

3. What impact does the coil head-on wind speed have on heat exchange and water entrainment in the air conditioning box?

4. How should the air supply temperature sensor of the air conditioning box avoid the influence of local airflow?

Key points for selection and acceptance inspection