Can the opening of the fresh air valve and return air valve directly represent the air mixing ratio?

The opening of the fresh air valve and the opening of the return air valve usually cannot directly represent the proportion of fresh return air. The air volume also depends on the valve characteristics, the resistance of the two circuits, the internal and external pressure difference, and the operating status of the fan; if both valves are opened half way, it does not mean that each of the two winds has half.

Separate actuator position from air measurement

The actuator feedback indicates where the valve shaft rotates, not the amount of air passing through. Even if the valve position is fixed, dust accumulation on the filter, changes in outdoor wind pressure or system pressure may change the branch flow rate. When the amount of fresh air needs to be controlled, measurement or proven debugging methods should be configured according to system requirements.

The opening feedback should be used to confirm the mechanical action and stroke and cannot replace the air volume measurement; the branch conditions should check the pressure difference between the front and rear valves, the air duct resistance and the external pressure changes; the debugging results should measure and record the actual new return air volume under fixed working conditions. The three types of information correspond to different verification links and cannot just be given an opening percentage. 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?。

Valve characteristics determine the relationship between opening and flow

Different valves have different opening-flow characteristics. The flow rate of linear characteristic valves is approximately proportional to the opening. The flow rate of equal percentage characteristic valves changes slowly when the opening is small and changes quickly when the opening is large. Butterfly valves, split multi-leaf valves and grille valves also have different characteristics. For the same 50% opening, the corresponding flow area and flow rate may be different on different valves.

Therefore, the valve opening percentage cannot be directly regarded as the flow rate percentage, nor can the fresh air valve opening and return air valve opening be directly regarded as the new return air ratio. When selecting a damper, you should understand its flow characteristics and adjustable range.

The two circuits have different resistances and the same opening and different flow rates.

The resistance of the fresh air branch and the return air branch are usually different. The fresh air branch may contain rainproof louvers, filters, ducts and insulation, and the return air branch may contain return air filters, ducts and ceiling spaces. Even if both valves open to the same degree, the flow rate in the branch with less resistance may be greater. The difference between outdoor wind pressure and indoor pressure will also affect the fresh air branch flow.

The difference in resistance between the fresh air and return air branches should be confirmed by air volume measurement during commissioning. The length of the mixing section and airflow organization will also affect the mixing effect, please refer toHow long is the air mixing distance required in the mixing section of the air conditioning box? . What functional segments should be included in the internal resistance of the air conditioning box? Please refer toWhat functional segments should be included in the internal resistance of the air conditioning box?。

Changes in system operating status will change the actual proportion

Changes in fan speed, dust accumulation on filters, changes in outdoor air pressure, and changes in room pressure difference will all change the actual flow rates of fresh air and return air. Even if the valve opening remains unchanged, the proportion of new return air may change. Systems requiring constant fresh air volume may require air volume measuring devices or pressure-independent dampers.

When the total air volume is insufficient, check the measurement first or increase the speed first. Please refer toIf the total air volume of the air conditioning box is insufficient, should you check and measure it first or increase the speed first? . 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?。

The linkage curve must be supported by operating conditions

The relative opening curve can be used for control and adjustment, but it should be checked under representative working conditions. Fully open, minimum air volume and common modes are verified separately, which is more reliable than copying the valve positions of other projects. When the control target changes, the flow rate should also be rechecked rather than just modifying the percentage.

For control point table and signal definition, please refer toWhat point tables need to be provided before the air-conditioning box is connected to the building automation system? . The difference between BACnet interface and dry contact, please refer toWhat is the difference between the BACnet interface and dry contact linkage of the air conditioning box? . How to change the debugging sequence from stand-alone to system linkage, please refer toHow to change the debugging sequence of air conditioning box from stand-alone unit to system linkage?。

How to verify the actual air mixing ratio

During verification, the fresh air volume, return air volume and total air volume should be measured under stable working conditions, or the mixed air state should be measured to back-calculate the ratio. The measurement section should avoid elbows and valves, and the air volume unit should be unified. Valve opening, fan speed and filter pressure differential should be recorded simultaneously to facilitate subsequent comparison.

For air volume unit conversion, please refer toHow to convert the air volume units of air conditioning box in m³/h, L/s and CFM? . How to calculate the temperature and humidity of mixed air, please refer toHow should the temperature and humidity of the mixed air in the air conditioning box be calculated?。

Aili industrial supporting equipment

Aili industrial control package can distinguish between valve position feedback and air volume feedback, and confirm the logic based on on-site measurement conditions to avoid taking execution instructions as performance results. Aili's industrial manufacturing end covers the independent research and development and production of motors and fans, control systems, sheet metal, air filters and complete machine assembly. The mixing section air valve, air volume measuring point and control logic can be matched in the same package; the non-standard mixing section can be adjusted according to the on-site air path and measurement requirements.

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 air mixing ratio, measurement method 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 air volume measurement and performance test results.

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Related questions and answers

If the fresh air valve is opened 50%, does that mean 50% fresh air?
no. The opening is only the valve position, and the actual air volume also depends on the valve characteristics, branch resistance and fan status.

If the two valve openings are the same, will the new return air volume be the same?
uncertain. The fresh air and return air branches have different resistances, and the flow rate of the branch with lower resistance may be greater.

Can Aili Industrial help check the air mixing ratio?
The proportional control method can be checked based on the mixing section structure, air valve characteristics and measurement conditions. The specific plan needs to be confirmed according to the 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. How to calculate the fresh air volume and total air supply volume of the air conditioning box separately?

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

Key points for selection and acceptance inspection