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

The proportion of new return air to the air-conditioning box should be determined based on ventilation, process exhaust and supplementary air, air reuse permit conditions and system air volume balance. It is not an arbitrary energy-saving setting, nor can it be calculated solely based on the equipment entrance opening area. First clarify which air is allowed to be reused, and then determine the needs of each operating mode.

Determine reuse conditions first, then do quantity balancing

Production exhaust, personnel changes, and service area pollution characteristics may all affect fresh air requirements. Part of the process air should not be returned to the same system, and return air cannot be simply increased in order to reduce the fresh air load. For areas where mixed return air can be mixed, the supply air volume, fresh air volume and exhaust air volume should be provided at the same time, and the building pressure and air leakage factors should be checked.

For normal production, the air flow meter should be established according to the designer, process exhaust and environmental goals; for low-load operation, the minimum ventilation and supplementary air conditions should be checked, and fixed valve positions should not be used directly; for special working conditions, the operation strategy for door opening, maintenance or exhaust changes should be explained. The three types of working conditions correspond to different fresh air needs and cannot be set according to just one ratio. 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?。

Which air is allowed to be reused must be determined first

The availability of return air depends on the contamination characteristics and process requirements of the service area. Air containing high concentrations of process releases, droplets, particles or odors usually requires independent exhaust and should not be returned to the same system. The air that can be reused should meet regional air quality requirements and be filtered or chemically filtered. The target gases that chemical filtration can handle are different from the particles that particle filtration can handle. Please refer toWhat do the chemical filter and particle filter of the air conditioning box deal with respectively?。

After the reuse conditions are determined, the ratio of fresh air volume and return air volume can be calculated. If the non-reusable air is included in the return air, the system air volume balance seems to be established, but in actual operation it may cause cross-contamination or regional pressure imbalance.

Exhaust air volume often determines the lower limit of fresh air volume

Process exhaust, local exhaust and bathroom exhaust all need to be supplemented with fresh air. In areas with large exhaust air volume, the fresh air volume may be controlled by the demand for supplementary air rather than by personnel fresh air or cooling load. When the fresh air volume is insufficient, the room pressure difference may not be maintained and the exhaust effect will decrease.

Please refer to the corresponding relationship between the fresh air tasks of the clean room MAU and the process exhaust air.How does the fresh air task of the clean room MAU correspond to the process exhaust? . The exhaust list should be organized according to equipment, time period and simultaneous operating conditions, and should not just give a total exhaust volume.

Review the processing capacity after changing the ratio

Adding fresh air will change the coil inlet condition and heat and moisture load. If the mixed air unit is changed to operate close to fresh air, you cannot just change the control settings without reviewing the coil, cold source, humidification and other configurations. The actual ratio should be confirmed through corresponding air volume measurements.

After the fresh air ratio is increased, the temperature and humidity at the coil inlet may change. If the surface cooler selection needs to be adjusted, please refer toHow to select the type of air conditioner cooler according to the air inlet and outlet conditions? . 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? . When the proportion of fresh air is high in winter, the preheating and antifreeze requirements must also be rechecked. Please refer toWhat problems do air conditioning box preheating, heating and reheating solve respectively?。

The air valve opening is not equal to the proportion

The opening of the fresh air valve and the return air valve cannot directly represent the proportion of fresh return air. The air volume also depends on the valve characteristics, branch resistance, internal and external pressure difference and fan operating status. Both valves are half open, which does not mean that each of the two winds has half. Proportional control should be based on air volume measurements or verified commissioning results.

For the difference between the air valve opening and the actual air volume, please refer toCan the opening of the fresh air valve and return air valve directly represent the air mixing ratio? . The mixing section requires sufficient air mixing distance, please refer toHow long is the air mixing distance required in the mixing section of the air conditioning box?。

Aili industrial supporting equipment

Aili Industrial can configure the mixing section and related controls according to the new return air data of different modes, first check the air tasks, and then implement the fan, filtration and cabinet interfaces. 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 dampers, air volume measurement points and control logic can be aligned in the same package; the non-standard mixing section can be adjusted according to the on-site air path.

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 proportion, air volume balance 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 system balance results.

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Related questions and answers

Can the proportion of new return air be set at will according to the energy saving target?
cannot. It is necessary to first confirm which air is allowed to be reused, the requirements for exhaust and supplementary air and the regional pressure difference requirements, and then determine the proportion.

If the damper is opened halfway, does it mean 50% fresh air?
no. The air volume depends on the valve characteristics, branch resistance and fan status, and the ratio should be confirmed through air volume measurement or debugging.

Can Aili Industrial help check the proportion of new return air?
The mixing section and control configuration can be checked based on the new return air data of each operating mode. The specific ratio needs to be confirmed based on the project ventilation and air volume balance.

Related articles

1. How to calculate the fresh air volume and total air supply volume of the air conditioning box separately?

2. How does the fresh air task of the clean room MAU correspond to the process exhaust?

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

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

Key points for selection and acceptance inspection