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

The mixed air state should be mass-weighted based on the dry air mass flow rate of fresh air and return air, and the moisture content and enthalpy should be mass-weighted, and then the temperature and relative humidity should be calculated from the mixed state quantities. The relative humidity percentages of two streams of air cannot be directly averaged.

First unify status and traffic benchmarks

The calculation inputs include the temperature, humidity, local pressure and corresponding flow rate of the two air streams. When the air density is similar, a clearly marked preliminary approximation can be made; when the temperature difference or density difference is large, the mass flow rate should be converted. The mixing process also needs to consider whether there is external heating or condensation, and it cannot be assumed that all situations are consistent with simple mixing.

The moisture content should be weighted according to the two dry air mass flows to maintain moisture balance; the enthalpy should be weighted according to a consistent basis to maintain the corresponding energy balance; for result review, the state should be calculated based on the mixing enthalpy and moisture content, and the rationality should be checked. The three steps correspond to different conservation relationships and cannot just be counted as one temperature. 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?。

Why can’t the relative humidity be averaged directly?

Relative humidity is a function of temperature. When two streams of air of different temperatures are mixed, the temperature changes and the relative humidity changes accordingly. Directly averaging the relative humidity percentages of two streams of air ignores the effect of temperature on the saturated water vapor partial pressure, and the result may deviate from the actual state. Moisture content and enthalpy are conserved by mass in simple mixing and are more suitable as a basis for calculations.

The difference between relative humidity and moisture content, please refer toWhy do we need to consider the temperature when selecting an air-conditioning box based on relative humidity? . The relative humidity after mixing should be recalculated from the temperature and moisture content of the mixed state, rather than the average relative humidity of the two streams.

The calculation steps can be expanded as follows

The first step is to convert the fresh air and return air flow rates into dry air mass flow rates. In the second step, take the moisture content and enthalpy of the two air streams respectively. In the third step, the moisture content and enthalpy are weighted according to the dry air mass flow rate to obtain the moisture content and enthalpy of the mixed state. The fourth step is to find the mixing temperature from the mixing moisture content and enthalpy, and then find the relative humidity from the temperature and moisture content.

The same pressure base and the same enthalpy base should be used in the calculations. If there are any differences in altitude or pressure at the site, this should be noted. Calculation tools or enthalpy-psychrometric diagrams can help with conversion, but input conditions and flow rates must be accurate. Calculation examples only illustrate methods and cannot replace project conditions calculations.

When the length of the mixing section is insufficient, the theoretical value may deviate from the actual value.

The theoretical mixing state assumes that the two streams of air are fully and evenly mixed. If the mixing section length is insufficient, the inlet location is improper, or the air flow is stratified, the actual state of the coil inlet may deviate from the theoretical value. Localized low temperature or high humidity areas may affect coil heat transfer and risk of water entrainment.

How long is the mixing section required? Please refer toHow long is the air mixing distance required in the mixing section of the air conditioning box? . 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? . When designing, it is necessary not only to calculate the theoretical mixing state, but also to check the structure of the mixing section and the representativeness of the measuring points.

The calculated values ​​must correspond to the on-site measurement points.

When the distance between the mixing sections is insufficient or the air flow is stratified, the temperature and humidity at a single point may be biased toward a certain stream of air. The theoretical complete mixing state and the partially measured state should be distinguished; the inlet conditions confirmed by the design should be used for coil selection, and the representativeness of the measuring points should be checked for on-site verification.

Whether the return air measurement point can represent the service area, please refer toCan the air conditioning box return air measurement points represent all service areas? . 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? . The location and number of measuring points should reflect the state of the mixed section.

Calculation data must retain caliber and version

The calculation book should state the status, flow rate, pressure base, enthalpy base and calculation tools of the fresh air and return air. When the design changes, if the fresh air ratio, outdoor conditions, or return air status change, the mixing status needs to be recalculated. Coil selection, fan configuration and control logic should all use the same version of status data.

The air conditioning box is designed to determine whether the inlet air temperature and humidity should be outdoor or return air. Please refer toShould the inlet air temperature and humidity of the air conditioning box be designed to be outdoor or return air? . How to select the type of surface cooler according to the air inlet and outlet conditions, please refer toHow to select the type of air conditioner cooler according to the air inlet and outlet conditions?。

Aili industrial supporting equipment

Aili Industrial can evaluate the functional sections and airflow layout based on the new return air status and flow data; retain the calculated caliber when submitting the data to avoid misuse of coil inlet conditions. 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 structure, coil interface and measuring point position can be matched in the same package; the non-standard mixing section can be adjusted according to the on-site air path and interface 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 status, equipment performance 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

Can the relative humidity of mixed air be directly taken as the average of the two streams of air?
cannot. Relative humidity changes with temperature. The mass should be weighted by moisture content and enthalpy, and then the temperature and relative humidity after mixing should be found.

What is the reason why the calculation of the mixing section is inconsistent with the actual measurement?
It may be that the length of the mixing section is insufficient, the air flow is stratified, or the location of the measuring point is not representative. The mixing structure and measuring points should be checked.

Can Aili Industrial help check the mixing status?
The calculation caliber and equipment configuration can be checked based on the new return air status, flow rate and mixing section structure. The specific plan needs to be confirmed based on the project conditions.

Related articles

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

2. Why does air conditioning box selection depend on relative humidity and temperature?

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

4. How to select the type of air conditioner cooler according to the air inlet and outlet conditions?

Key points for selection and acceptance inspection