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

In a system with return air, the total supply air processed by the unit is usually composed of fresh air and return air; in the fresh air condition, indoor air is not reused. When calculating, the same boundary and consistent flow caliber must be used, and leakage of the unit and air duct must be considered. The fresh air volume cannot be directly regarded as the total air volume of all models.

First draw the calculation boundary

If the fresh air entering the mixing section is 1000m³/h and the return air is 4000m³/h, in a simplified example that ignores density changes and leakage, the mixed air volume is about 5000m³/h. This example only illustrates the quantitative relationship and does not mean that the proportion of fresh air in any room should be the same.

The fresh air volume is the amount of air entering the calculation boundary from the outside; the return air volume is the amount of air returned from the service area and allowed to be reprocessed; the supply air volume is the amount of air delivered to the backend from the selected air supply interface. The three quantities correspond to different boundaries and cannot be mixed together. 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?。

Quantity relationship under mixed conditions

For a mixed air unit with return air, the total air supply volume is approximately equal to the sum of the fresh air volume and the return air volume, but the actual calculation must take into account unit air leakage, air duct leakage and air density changes. The fresh air volume plus the return air volume should be equal to the mixed treated air volume, and the treated air volume minus leakage and loss is equal to the actual supply air volume. If only the fresh air volume and the total supply air volume are given, the return air volume is equal to the total supply air volume minus the fresh air volume, but only if the two use the same air volume caliber and the same status benchmark.

The opening of the fresh air valve and return air valve cannot directly represent the ratio. Please refer toCan the opening of the fresh air valve and return air valve directly represent the air mixing ratio? . 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?。

There is no return air volume under the new air condition

The total air supply volume of the fresh air unit is equal to the treated fresh air volume, excluding return air. At this time, the fresh air volume is the air volume processed by the unit, but air supply pipeline leakage and terminal distribution need to be considered. Under fresh air conditions, the selection of coils and filters should be calculated based on fresh air conditions, and the parameters for mixed air conditions cannot be used.

How the fresh air tasks of the clean room MAU correspond to the process exhaust air, please refer toHow does the fresh air task of the clean room MAU correspond to the process exhaust? . The clean room may require fresh air treatment or a larger proportion of fresh air, which should be determined based on process exhaust and pressure difference requirements.

Exhaust air and fresh air constitute room air volume balance

The fresh air volume is not only determined by the air conditioning box, but also affected by the room exhaust volume, personnel fresh air and pressure difference requirements. In areas with a large exhaust air volume in the room, the fresh air volume may need to be increased to supplement the exhaust air. Maintaining the pressure difference also requires a certain amount of fresh air. When designing, the room air volume should be balanced, the lower limit of fresh air volume should be determined, and then matched with the processing capacity of the air conditioning box.

The clean room pressure difference is qualified but external gas may still enter. Please refer toIf the clean room pressure difference is qualified, why can external gas still enter? . Whether the chemical filter section needs to be re-selected when the fresh air volume changes seasonally, please refer toDoes the chemical filter section need to be reselected when the fresh air volume changes seasonally?。

Different measurement positions cannot be mechanically subtracted.

The air volume measured at the inlet, outlet and end of the unit may be affected by leaks, branches and air density. When it is necessary to judge the relationship between the new return air and the new return air, you should first confirm that the measurement conditions are consistent; if it is used for mass balance, the unified density benchmark or dry air mass flow rate is preferred.

How to convert the air volume unit of air conditioning box in m³/h, L/s and CFM, please refer toHow to convert the air volume unit of air conditioning box in m³/h, L/s and CFM? . 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?。

Aili industrial supporting equipment

When submitting the selection form to Aili Industrial, it is recommended to separate the fresh air, return air and total supply air, and indicate the interfaces and representative working conditions, so that the whole machine can be configured according to the actual air volume. 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 fresh air inlet, return air outlet and air supply interface can be corresponded in the same set of supporting materials; non-standard air volume can be adjusted according to the air volume balance of the project.

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 volume value, interface form 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 balance and performance test results.

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

Related questions and answers

Is the fresh air volume the total air supply volume?
no. Only under fresh air conditions, the total air supply volume is approximately equal to the fresh air volume; under mixed air conditions, the total air supply volume also includes return air volume.

How to calculate return air volume?
Under mixed air conditions, the return air volume is usually equal to the total supply air volume minus the fresh air volume, but it should be calculated under the same air volume caliber and status benchmark, and leakage should be considered.

Can Aili Industrial help check the fresh air volume and total air supply volume?
The equipment configuration can be checked based on the system air volume balance and interface conditions. The specific values ​​must be confirmed by the system design.

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

4. How to convert the air volume unit of air conditioning box in m³/h, L/s and CFM?

Key points for selection and acceptance inspection