Understanding the Psychrometric Chart: A Vital Tool for Cold Storage

In the cold storage industry, maintaining the right environmental conditions is crucial for preserving the quality of stored goods. The psychrometric chart is an indispensable tool in achieving this. This graphical representation of air properties under varying conditions of temperature and humidity is a cornerstone for designing and managing efficient cold storage systems.

 

What is a Psychrometric Chart?

A psychrometric chart is a detailed diagram used in HVAC (Heating, Ventilation, and Air Conditioning) that represents the thermodynamic properties of air in a graphical form. It plots various properties of air at different temperatures, including humidity, dew point, and wet bulb temperature, providing essential insights for understanding and controlling air conditions in various environments.

Download a Copy of our Psychrometric Chart Here

 

Uses of a Psychrometric Chart in the Cold Storage Industry

Here are some common uses of a psychrometric chart in the cold storage industry:

• Assessing Desired Storage Conditions: Identify the ideal temperature and humidity for the specific goods stored. For instance, in a fruit storage facility, psychrometric charts can be used to optimise temperature and humidity, significantly reducing spoilage. A meat storage company could use the chart to determine the right conditions to prevent freezer burn and dehydration of products.
• Adjusting Conditions: If the current conditions deviate from the ideal, use the chart to determine the necessary adjustments required. For example, if the air is too humid, the chart can show how much cooling is needed to reach the desired dew point, or if condensation is occurring, you can use the chart to determine the correct amount of heat required to add to eliminate the risk.
• Energy Efficiency Planning: By understanding the air properties, cold storage operators can optimise their systems for energy efficiency, a key consideration given the energy-intensive nature of cold storage.
• Preventing Condensation and Frost: The chart helps in maintaining conditions that prevent condensation and frost, which can damage stored goods and the storage infrastructure.

 

How Does the Psychrometric Chart Work?

 

The chart works by correlating several physical properties of air. Here are the main ones you need to know:

• Dry Bulb Temperature (DBT): This is the air temperature measured by a regular thermometer, representing the heat in the air. This is represented along the horizontal axis.
• Wet Bulb Temperature (WBT): Measured by a thermometer covered in a water-soaked cloth. It reflects the lowest temperature air can reach through evaporative cooling. This is indicated by lines that slope slightly downward from left to right.
• Relative Humidity (RH): Indicates the percentage of moisture in the air compared to the maximum it can hold at that temperature. Curved lines represent RH in percentages. The highest curve, following the chart’s top contour, represents 100% RH (saturated air).
• Absolute Humidity: These are horizontal lines indicating the amount of moisture in the air.
• Enthalpy: In the context of air and HVAC systems, enthalpy is used to describe the total energy (both sensible and latent) contained within air. Lines of constant enthalpy run almost parallel to the wet bulb temperature lines.

By intersecting any two values, the chart enables the determination of air properties crucial in cold storage, like moisture content and dew point.

 

Example – How To Calculate the Dew Point Using a Psychrometric Chart

Let’s look at this scenario, the room temperature is 10°C and showing at 70% humidity, and I want to know the dew point, using the chart. How do I do this?

Firstly, why do I want to know the dew point? The dew point is the temperature at which air becomes saturated (100% RH) with moisture and condensation begins. Knowing the dew point will allow you to prevent condensation build-up and potentially ice build-up in your cold storage area. Find out why this is important here.

Steps to Calculate the Dew Point:

1. Locate the Initial Point: On the psychrometric chart, find where the line for a 10°C dry bulb temperature intersects the 70% RH line. This point represents the current state of the air.
2. Trace the Line of Constant Humidity Ratio: From the identified point, move horizontally to the left, along the line of constant specific humidity. This line shows the moisture content of the air remaining constant.
3. Find the Saturation Curve Intersection: Keep moving left until you reach the saturation curve, which represents 100% RH on the chart.
4. Determine the Dew Point Temperature: The temperature at this intersection point, where the line of constant humidity ratio meets the saturation curve, is the dew point temperature. This is the temperature at which the air becomes fully saturated, and moisture will start to condense.

The Impact of Adding Heat and Dehumidification

Understanding how the chart reflects the addition of heat or the process of dehumidification is key:

Adding Heat: This moves the chart point right, indicating an increase in temperature without moisture change.

• Relative Humidity: As you add heat and move right, the relative humidity decreases because warmer air can hold more moisture.
• Wet Bulb Temperature: This may increase slightly, but not as much as the dry bulb temperature.
• Enthalpy: The total heat content of the air increases as you move to the right.

Adding Dehumidification: Moving down along a vertical line shows moisture reduction at a constant temperature.

• Decrease in Specific Humidity: As you move down, the specific humidity (moisture content of the air) decreases.
• Relative Humidity Changes: As the air becomes drier, its relative humidity changes. The impact on RH depends on the starting condition and the extent of dehumidification.
• Enthalpy Reduction: Removing moisture generally leads to a slight decrease in the total heat content (enthalpy) of the air.
• Wet Bulb Temperature: The wet bulb temperature also decreases slightly during dehumidification, as it is influenced by the moisture content.

 

Why we use Psychrometric Charts at Ulti Group

At Ulti Group, we integrate the use of psychrometric charts into our sales process to ensure that our clients receive the most effective and efficient solutions tailored to their specific needs. These charts are pivotal in helping us understand and articulate the unique environmental conditions of each client’s facility. By analysing the intricate relationship between temperature, humidity, and air properties, we are able to recommend and customize our products, like the Ulti HCR Air Door, to perfectly align with your specific requirements. We can visually illustrate how our products will perform in your environment, offering you peace of mind and confidence in our solutions. This detailed analysis ensures that we not only meet your immediate needs but also anticipate and plan for future environmental changes, guaranteeing long-term satisfaction and success in managing your controlled environments.

Ready to optimise your cold storage area? Contact us today to discover how we can help transform your cold storage environment for the better and protect your profits.