Why do we need to consider the temperature when selecting an air-conditioning box based on relative humidity?

When selecting an air conditioning box based on relative humidity, you must also know the temperature, because relative humidity represents the ratio of water vapor pressure to saturation pressure at that temperature. The same relative humidity has different corresponding moisture content at different temperatures. It cannot be directly compared which air needs to remove more moisture.

First determine whether it is a change in moisture or temperature.

The air is heated without condensation, evaporation, or mixing. The total amount of moisture does not necessarily change, but the relative humidity usually decreases. In turn, cooling may increase relative humidity. Therefore, 'the humidity number drops' does not necessarily mean that dehumidification has occurred. When using trend recording, you should check the temperature and moisture content or dew point at the same time.

The relative humidity must be paired with the temperature at the same location and at the same time; the moisture content is used to check the actual moisture and mass balance carried by the air; the measurement conditions require checking the probe position, calibration and local temperature effects. The three types of information correspond to different judgments, and you cannot just give a humidity number. How to calculate the dehumidification capacity of air conditioning box from the difference in air moisture content, please refer toHow to calculate the dehumidification capacity of air conditioning box from the difference in air moisture content?。

Relative humidity and moisture content are not the same quantity

Relative humidity is the ratio of the partial pressure of water vapor in the air to the partial pressure of saturated water vapor at that temperature, expressed as a percentage. Moisture content is the mass of water vapor contained per kilogram of dry air, expressed in g/kg or kg/kg. Relative humidity changes with temperature, and moisture content remains constant when heated or cooled but not condensed.

In winter, the outdoor air temperature is low and the relative humidity may be high, but the moisture content is very low. After the air is heated, the relative humidity decreases. If the moisture is not replenished, the room may be dry. The moisture content of air with high temperature and humidity in summer may be high, and the relative humidity rises or even approaches saturation after cooling. You can't tell how much moisture the air actually carries by just looking at relative humidity.

The same relative humidity, the moisture content may be much different

For example, the moisture content of air at 30°C and 50% relative humidity is significantly different from that of air at 20°C and 50% relative humidity. The former carries more moisture and may need to handle more moisture during cooling and dehumidification. If you only look at the relative humidity of both 50%, you will think that the dehumidification tasks of the two air streams are the same, and the selection will deviate from reality.

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? . How the coil water flow corresponds to the design cooling capacity can be referred toHow does the water flow rate of the air conditioning box coil correspond to the designed cooling capacity? . Dehumidification capacity, sensible cooling capacity and latent heat need to be checked separately.

Both humidification and dehumidification depend on the difference in moisture content.

The amount of humidification in winter should be calculated based on the difference between the dry air mass flow rate and the target and inlet moisture content, rather than the relative humidity difference. Summer dehumidification should also be based on moisture content differences. Relative humidity targets can be used as control or acceptance indicators, but should be converted to moisture content or dew point when calculating throughput.

How to determine the amount of humidification according to the wind inlet conditions in winter, please refer toHow to determine the humidification amount of air-conditioning box according to the air inlet conditions in winter? . What role does the supply air dew point play in the selection of fresh air air conditioning boxes? Please refer toWhat role does the supply air dew point play in the selection of fresh air air conditioning boxes? . Dew point and moisture content are both indicators of moisture content and are more stable than relative humidity.

Control measuring points should also be configured in pairs

The temperature and humidity sensor should measure temperature and relative humidity at the same time, or convert the moisture content in the controller. The measuring point should be located away from local hot and cold airflow to ensure that the reading represents the target air condition. If only relative humidity is used to control humidification or dehumidification, temperature changes may cause malfunctions.

How to avoid the influence of local airflow on the air supply temperature sensor of air conditioning box, please refer toHow should the air supply temperature sensor of the air conditioning box avoid the influence of local airflow? . How to determine the installation and calibration requirements for air conditioning box temperature and humidity sensors, please refer toHow to determine the installation and calibration requirements for air conditioning box temperature and humidity sensors? . 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?。

How to avoid giving only one humidity number when making an inquiry

The entrance and exit temperature and humidity and the target temperature and humidity should be written, and the moisture content or dew point should be added if necessary, and the working conditions under which the two parameters need to be met at the same time should be stated. There is only 'reaching a certain relative humidity' without a temperature range. There may be a misunderstanding that the reading is reduced by heating but the moisture treatment task is not completed.

What project information should be prepared before selecting the air-conditioning box? Please refer toWhat project information should be prepared before selecting an air-conditioning box? . What control objectives should be clarified when selecting a constant temperature and humidity air conditioning box? Please refer toWhat control objectives should be clarified when selecting a constant temperature and humidity air conditioning box? . Temperature and humidity targets should be provided in groups, with measurement points and evaluation periods indicated.

Aili industrial supporting equipment

When checking the humidity treatment configuration, Aili Industries can evaluate based on paired temperature and humidity data; unifying the measuring points and units before submitting parameters can help reduce selection ambiguity. Aili's industrial manufacturing end covers independent research and development and production of motors and fans, control systems, sheet metal, air filters and complete machine assembly. Coils, humidification sections, sensors and control logic can be matched in the same package; non-standard working conditions can be adjusted according to the project temperature and humidity 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 humidity treatment 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 it possible to complete the selection of air conditioning boxes by only giving a relative humidity target?
not enough. Relative humidity must be paired with temperature, and moisture content or dew point is also required when calculating throughput, otherwise the actual moisture load cannot be determined.

If the relative humidity drops, does it mean that it has been dehumidified?
uncertain. Heating the air can reduce the relative humidity but keep the moisture content unchanged. To judge whether to dehumidify, you should look at the moisture content or dew point.

Can Aili Industrial help check the humidity parameters?
Coils, humidification sections and control configurations can be checked based on paired temperature and humidity data. Specific plans need to be confirmed based on project conditions.

Related articles

1. How to calculate the dehumidification capacity of air conditioning box from the difference in air moisture content?

2. How to determine the humidification amount of air-conditioning box according to the air inlet conditions in winter?

3. What role does the supply air dew point play in the selection of fresh air air conditioning boxes?

4. What control objectives should be clarified when selecting a constant temperature and humidity air conditioning box?

Key points for selection and acceptance inspection