Understanding the critical role of **moist air density in air treatment systems** is essential for **optimizing efficiency in HVAC operations.** Moist air density directly affects both the mass and volumetric flow rates, which are crucial for ensuring proper air distribution and energy efficiency. By adjusting volumetric flow based on air density data, ventilation systems can prevent unnecessary energy consumption and improve overall performance, especially when using humid air density sensors for precise mass flow regulation. Let’s dig in.

**Moist air **or **Humid air **refers to the air that surrounds us, containing water vapor within a mixture of various gas components. These free-moving water molecules have a very low molecular mass, making humid air less dense and causing it to rise. This is why in pool areas, humid air tends to accumulate near the ceiling.

The **density of moist air **also depends on atmospheric pressure, which decreases with increasing altitude.

Consequently, the behavior and distribution of humid air are influenced by both its **moisture content **and the **environmental pressure **conditions.

## The Importance of Moist Air Density in AHU: Calculating Mass Flow in Ventilation Systems and Air Handling units

**Mass Flow Rate Vs. Volume Flow Rate**

Let’s dive into the difference between mass flow rate and volume flow rate in Air Handling Units (AHUs) first in order to better understand the basics.

**Mass flow rate **refers to the amount of air mass passing through a point in the AHU system per unit of time. It is usually measured in kilograms per second (kg/s) or pounds per minute (lb/min). This metric takes into account the density of the air, which can vary based on temperature, pressure, and humidity. Therefore, mass flow rate gives a more accurate representation of the actual amount of air being moved, regardless of its physical conditions.

Example: Imagine you’re filling a balloon with air. If the air is cold and dense, the balloon will fill more quickly because more air mass is entering the balloon. On a hot day, the air is less dense, and it will take longer to fill the balloon to the same size. The mass flow rate measures the total weight of the air entering the balloon per second, which remains constant regardless of the temperature.

**Volume flow rate**, on the other hand, measures the volume of air passing through a point in the AHU system per unit of time. It is typically measured in cubic meters per second (m³/s) or cubic feet per minute (CFM). This measure does not account for changes in air density due to temperature, pressure, or humidity. It simply reflects how much space the air occupies.

Example: Think of water flowing through a garden hose. If you measure the volume flow rate, you’re looking at how many liters of water pass through the hose each minute. Whether the water is cold or hot doesn’t change the volume flow rate. Similarly, in AHUs, the volume flow rate indicates how much air (in terms of volume) is being moved, regardless of its density.

For an** AHU expert**, understanding both flow rates is crucial:

**Volume flow rate**is often used in practical applications where the physical volume of air needs to be known, such as duct sizing, ensuring adequate air distribution and indoor air quality.**Mass flow rate**is vital for energy calculations, ensuring that the system is moving the correct amount of air mass to achieve proper indoor air quality, heating, cooling, and ventilation.

**Example in Practice**: In winter, when the air is colder and denser, the volume flow rate might be lower, but the mass flow rate remains consistent because the dense air packs more mass into the same volume. Conversely, in summer, the warmer, less dense air might result in a higher volume flow rate to move the same mass of air. Therefore, modern AHUs need to adjust their operation based on mass flow rate to ensure energy efficiency and optimal performance.

In summary, while volume flow rate measures the space air occupies, mass flow rate measures the actual amount of air by weight. Both metrics are essential for different aspects of AHU operation, ensuring efficient, effective, and comfortable air handling year-round.

### Adjusting the volumetric flow based on Air Density Data

In the building design process, the required amount of fresh air is always calculated under standardized conditions of temperature being **20°C **and an air density of approximately **1.2 kg/m³**.

This also allows us to calculate the** mass flow rate **that must always be ensured for meeting optimal comfort conditions. Modern ventilation systems mostly measure the volumetric flow in the AHUs and use a fixed inserted value for density to calculate the mass flow.

By using a sensor that measures temperature, relative humidity, and air pressure, we can **calculate the air density** and use this data for** more accurate mass flow calculations**. This data is calculated within by the sensor.

By having the precise air density measurement data, AHUs fan will continuously **adjust the volumetric flow **in the AHU to match precise requirements.The effects will be remarkable—ensuring the correct mass flow is always supplied.

The differences between summer and winter can result in up to 20% or more in final savings of airflow quantities.

Thus, the **mass of air delivered will always meet the project’s specifications **and provide the optimal conditions for occupants in the building.

This approach ensures that ventilation systems are more efficient and responsive, leading to improved air quality and comfort, as well as significant energy savings throughout the year.

## Why is it Important to Measure Mass Flow Rate and Not Just Volume Flow Rate?

In this section, we aim to illustrate why it is essential to regulate the actual volumetric intake of outdoor air in ventilation systems based on the measured mass flow rate after heat recovery or regeneration. This information is also crucial when determining the amount of air that needs to be exhausted from a space.

**Example:** Consider a ventilation system with a volumetric flow capacity of 1000 m³/h.

**OUTDOOR AIR:**

- Outdoor air temperature: -15°C
- Outdoor air density: 1.355 kg/m³
- Measurement point for outdoor air intake: external grille
- Mass flow rate: 1.355 kg/m³ × 1000 m³/h = 1355 kg/h
- Enthalpy state: -13 kJ/kg

**INDOOR AIR:**

- Indoor air temperature: 20°C
- Indoor air density: 1.185 kg/m³
- Measurement point for air intake: after regeneration or heat recovery
- Mass flow rate: 1.185 kg/m³ × 1000 m³/h = 1185 kg/h
- Enthalpy state: 38 kJ/kg

When aiming for a desired mass flow rate of 1185 kg/h (to supply air to the space), we would actually be delivering 1355 kg/h into the room—a 14.3% excess in the amount of air supplied.

If we ignore the fact that cold outdoor air will warm to 20°C after regeneration, increasing its volume, we end up supplying too much air to the space, creating an unnecessary surplus. This mistake lowers the energy efficiency of the ventilation system and increases operational costs.

comparison of the enthalpy states between outdoor air and indoor air reveals the required heating power for a volumetric flow rate of just 1000 m³/h:

**Air Density Ratio:** The ratio of outdoor air density to indoor air density is 1.355 / 1.185 = 1.143, or 14.3%, which means that the outdoor air is 14.3% denser than the indoor air.

**Heating Power Calculation:**

Qheating =1000 **m3/h** × 1.335 kg/m3 × 51 kJ/kg / 3600 s = 19.5 kW

The required additional power for the heater is **14.3%**, or the **19.5 kW** capacity is oversized by this percentage, which amounts to approximately **2.7 kW** of extra heating power needed for the hydronic heater in the HVAC unit.

For a medium-sized ventilation system with a flow rate of **10,000 m³/h**, this would mean an additional **27 kW**. For a larger building requiring a flow rate of 100,000 m³/h, this would equate to an unnecessary **270 kW** of additional heating power.

This is the extra thermal energy that needs to be supplied for the operation of the ventilation system—often through heat pumps—resulting in entirely avoidable costs.

**Checking the math “by hand” in the Mollier h,x diagramm: **

**The Unnecessary Increase in Required Transport Energy**

In the example above, the volumetric airflow would be approximately 14% higher. Since the electrical power of the fan varies with the cube of the airflow speed in the duct, this would lead to a significant increase in fan power.

If the duct airflow speed is designed to be 5 m/s, the increased airflow would raise this speed to 5.7 m/s in the scenario described.

The increase in fan power, calculated using the cube of the airflow speed, would be higher for 10-12%

This means that the electrical power required by the **supply air fan **under these conditions would be 10-12% higher. Similarly, the** exhaust air fan **would also need 10-12% more power.

As a result, this unnecessary load also places additional strain on the electrical distribution network, leading to higher connection costs.

**The Importance of Mass Flow Regulation in Ventilation Systems Using Humid Air Density Sensors**

Given the issues highlighted in the example above, it is crucial to program the control system in ventilation units to regulate the** actual volumetric intake of** **outdoor air **based on the measured mass flow rate after heat recovery or regeneration.

If we were to base the volumetric intake solely on the post-recovery or regeneration flow, we would inevitably make mistakes in the amount of air intake.

However, by using sensors that can measure the density of humid air, we can determine the correct volumetric intake required for the ventilation unit from the mass flow rate after heat recovery or regeneration.

The same principle applies to** air exhaust in ventilation systems**: regulate the exhaust volume based on the measured mass flow rate after heat recovery or regeneration.

In summary, it is essential to **regulate the actual volumetric intake of outdoor air based on the measured mass flow rate after heat recovery or regeneration to ensure optimal performance and efficiency.**

**Sensors with Moist Air Density Data Output **

Andivi offers advanced sensors capable of calculating **precise moist air density data** by accurately measuring temperature, humidity, and pressure variables.

These industrial-quality sensors are engineered to be robust and reliable, ensuring consistent performance in various air treatment applications, including AHU units and residential home ventilation systems.

Additionally, they can seamlessly transmit data using **Modbus or BACnet **protocols, integrating easily with modern building management systems.

The ability to provide detailed and accurate air density measurements makes Andivi sensors an essential component for optimizing air quality and energy efficiency in both commercial and residential environments.

**How to Calculate the Air Density of Moist or Dry Air**

The input you need to calculate Air Density of **Moist** **Air **(**kg/m3**) are the following variables:

- Air temperature
- Air pressure
- Relative Humidity or Absolute Humidity

The input you need to calculate Air Density of **Dry** **Air** (**kg/m3**) are the following variables:

- Air temperature
- Air pressure

A simple and quick online **Calculator of Air Density **can be found here.

Czernia, D. and Szyk, B. Air Density Calculator. Available at: https://www.omnicalculator.com/physics/air-density. Accessed: Aug 08, 2024.

.

**Mollier h,x diagram ===>>>>**

– – – – –

This article was conceptualized by Danijel Mursic, a mechanical engineer and thermodynamics expert with over 30 years of experience in the AHU and HVAC sector and former CEO of Menerga Slovenija.