In this article, we’ll explore the importance of enthalpy in Air Handling Units (AHUs) and how understanding it can help save energy. We’ll dive into the concepts of latent heat and sensible heat, breaking down their differences and why these distinctions matter in optimizing energy efficiency. By grasping the role of these thermal properties, you’ll gain insights into how AHUs manage air conditions more effectively, ensuring both comfort and sustainability.
Enthalpy in terms of air treatment expresses the total heat content of the air. This encompasses both the sensible heat, which relates to the temperature of the air, and the latent heat, which is associated with the moisture content or humidity in the air.
Essentially, enthalpy provides a comprehensive measure of the energy within the air, accounting for both its dry air component and the water vapor mixed with it. This measure is crucial in the fields of heating, ventilation, and air conditioning (HVAC), as it allows professionals to analyze and manage energy exchanges during processes like heating, cooling, humidification, and dehumidification.
What is latent heat and sensible heat?
Sensible heat
Sensible heat is the form of thermal energy that directly affects the temperature of the air. When air is heated or cooled, it undergoes a change in temperature that can be readily measured with a thermometer.
This type of heat exchange is straightforward, as it involves no change in the phase of the air’s moisture content.
For instance, raising the temperature of air from a mild to a warm state involves an increase in sensible heat. It’s the type of heat we feel and measure easily, making it a fundamental aspect of thermal comfort in indoor environments.
From a thermodynamics perspective, sensible heat is the energy required to change the temperature of air without changing its phase (i.e., without converting it from liquid to vapor or vice versa).
When you heat or cool the air, you are primarily dealing with sensible heat. This type of heat can be directly measured with a thermometer, as it results in a temperature change that can be sensed (hence the term “sensible”).
For instance, when air is heated from 20°C to 25°C, the increase in temperature represents an increase in sensible heat.
Latent heat
Latent heat, however, delves into the hidden energy exchanges associated with phase changes of water in the air.
Unlike sensible heat, latent heat doesn’t change the air temperature directly. Instead, it involves the energy required to transform water from liquid to vapor or vice versa. This transformation is crucial in processes like condensation and evaporation.
For example, when humid air cools and water vapor condenses into droplets (liquid water), the released energy is latent heat. This type of heat is not detectable by a thermometer because it does not involve a temperature change.
From a thermodynamics perspective, latent heat is the energy required to change the phase of a substance without changing its temperature. In the context of air treatment, this often involves the energy needed to convert water vapor to liquid water (condensation) or liquid water to water vapor (evaporation).
For example, when humid air cools and water vapor condenses into liquid water, the heat released during this phase change is latent heat. This energy exchange is invisible to thermometers but critical in humidity control and moisture management
Why does the difference between latent heat and sensible heat matter?
The distinction between these two forms of heat is paramount in the field of air treatment. Sensible and latent heat together define the total energy content of the air, encapsulated in the concept of enthalpy.
System Energy Efficiency
This holistic measure is indispensable for designing and optimizing HVAC systems. By understanding both components, engineers can better manage energy consumption and improve system efficiency.
For instance, dehumidifying air not only involves cooling it (sensible heat) but also removing moisture (latent heat), highlighting the need for a balanced approach in HVAC operations.
Indoor thermal comfort
Effective management of sensible and latent heat is essential for maintaining indoor thermal comfort. Human comfort hinges on both temperature and humidity levels, which means HVAC systems must adeptly handle these variables.
Controlling humidity through latent heat management prevents issues like mold growth and ensures healthier indoor air quality. This balance directly impacts the effectiveness and energy efficiency of heating, cooling, and ventilation systems.
Psychrometrics
In the broader scope of psychrometrics, the study of air properties, both sensible and latent heat play pivotal roles. Psychrometric charts are valuable tools that visualize these properties, aiding professionals in understanding and optimizing air treatment processes.
Advanced sensors that measure parameters like enthalpy, dew point, and the density of moist air are instrumental in this regard [CS]. These sensors provide precise data that enable real-time adjustments and fine-tuning of HVAC systems, ensuring optimal performance.
HVAC System Design
By grasping the nuances of sensible and latent heat, professionals can design more efficient and effective Air Handling units and other HVAC solutions. This knowledge leads to better energy management, enhanced indoor comfort, and improved air quality, all of which are crucial in modern building environments.
Understanding and leveraging these thermal dynamics is key to advancing HVAC technology and achieving sustainable, comfortable indoor climates.
Moisture Control
Excess moisture in the air can lead to mold growth, structural damage, and poor indoor air quality. Managing latent heat is essential for controlling humidity levels and preventing these issues.
The Importance of Enthalpy in Free Cooling for Air Handling Units (AHUs)
Free cooling in AHUs is a smart and sustainable method that optimizes the use of natural outdoor air to maintain indoor thermal comfort. It requires precise control and monitoring of air properties to be effective, highlighting the importance of advanced sensors in achieving this balance.
For professionals in the HVAC industry, understanding and leveraging free cooling can lead to significant energy savings, reduced operational costs, and enhanced sustainability in building management. We have written extensively about the importance of enthalpy in free cooling in this article: The Importance of Enthalpy in Free Cooling for Air Handling Units (AHUs).
Room Sensors with enthalpy measurement for indoor use
A building can harness free cooling effectively only if it employs sensors that measure enthalpy, not just temperature.
Relying solely on temperature is inadequate for determining the optimal use of free cooling because it doesn’t account for humidity levels.
There are instances where indoor air might have lower humidity than outdoor air, resulting in a lower overall energy content despite a higher temperature. By measuring enthalpy, sensors provide a comprehensive understanding of the total heat content, ensuring that free cooling is applied under the most favorable conditions.
A prime example of an industrial-grade sensor is the Andivi indoor Temperature, Humidity, Dew Point, and Enthalpy Sensor, which features Modbus or BACnet communication.
This advanced sensor can relay real-time enthalpy data directly to the DDC controller of the AHU unit or to the Building Management System (BMS), ensuring precise and efficient monitoring and control of indoor air quality and energy management.
What physical parameters are required to calculate enthalpy?
To accurately calculate the enthalpy of air, several key physical parameters must be considered. Each of these variables plays a crucial role in determining the total heat content of the air.
First and foremost, temperature is a fundamental parameter. It represents the sensible heat component of enthalpy, reflecting the amount of thermal energy in the air due to its temperature. When air temperature changes, it directly impacts the sensible heat, thereby affecting the overall enthalpy. Temperature sensors are essential in capturing this variable accurately.
Next, humidity, either expressed as relative humidity or humidity ratio, is vital for understanding the latent heat component. This parameter indicates the moisture content in the air. The amount of water vapor present has a significant impact on the total heat content because it includes the energy required to evaporate or condense the water vapor. Hygrometers and other humidity sensors are used to measure this parameter precisely.
Pressure, although often assumed constant at standard atmospheric conditions, can influence enthalpy calculations, especially in varying environments such as different altitudes or controlled spaces. Accurate pressure measurements ensure that the calculations account for the actual conditions of the air, rather than relying on approximations. Barometers and pressure sensors provide the necessary data for this parameter.
The specific heat capacity of air is another critical factor. This value represents the amount of heat needed to change the temperature of a unit mass of air by one degree. It accounts for the intrinsic properties of air, including its composition and temperature-dependent characteristics. Understanding the specific heat capacity is essential for precise enthalpy calculations.
Finally, the enthalpy of vaporization is a key parameter that reflects the energy required to convert water from liquid to vapor. This latent heat component is crucial when dealing with processes involving humidification and dehumidification. Knowing the enthalpy of vaporization helps in accurately calculating the total heat content related to moisture changes in the air.
Each of these parameters—temperature, humidity, pressure, specific heat capacity, and enthalpy of vaporization—must be accurately measured and considered to calculate the enthalpy of air. Advanced sensors like the Andivi Modbus Enthalpy Sensor, that in addition also measures temperature, humidity, pressure and dew point, is an indispensable tool in capturing these variables, ensuring precise and reliable enthalpy calculations for various applications in air treatment and HVAC systems.
The Unit for Enthalpy
The unit used for measuring enthalpy in air treatment is typically kilojoules per kilogram (kJ/kg). This unit denotes the amount of energy per unit mass of air, including its moisture content. Using this unit allows for a standardized and precise way to quantify and compare the energy content of air in various HVAC applications, facilitating effective energy management and system optimization.
While kilojoules per kilogram (kJ/kg) is more typical, joules per kilogram (J/kg) can also be used, especially in more detailed or specific scientific calculations. One kJ equals 1,000 J.
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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.