How Psychrometric Charts Win the War Against Ice and Humidity in Cold Storage

When you think of a large cold storage facility, your mind probably goes straight to temperature—a massive effort to keep pounds of product below zero. But what if the real threat isn’t the cold? What if it is the invisible enemy riding on every molecule of air? That enemy is moisture, and if it’s not precisely engineered, it quickly turns into costly problems. Slick ice on walkways, and suffocating frost on product and cooling coils are some examples. It’s time to stop fighting temperature alone and start using the ultimate roadmap—the psychrometric chart—to win the war on moisture. The psychrometric chart is the single most vital tool for engineers, transforming a complex process into a quantifiable, engineered one that ensures food safety, improves worker safety, maximizes energy efficiency, and protects the bottom line.

Why Humidity is Devastating at Low Temperatures

The temperature in cold storage is usually kept between 0°F and 40°F. A cold storage facility does not create humidity; rather, the low temperature and difference in moisture between the inside and outside environments are the primary drivers that cause moisture. Having too much humidity in a cold storage can eventually become hazardous for workers, equipment, and products.

The Three Major Humidity Hazards:

1. Coil Frosting: Warm, moist air entering the room deposits frost on the cooling coils, acting as an insulator. This drastically reduces cooling capacity and requires energy-intensive defrost cycles.
2. Critical Safety Hazards (Icing & Fogging): High dew point (DP) air causes rapid flash-freezing on floors, loading docks, and pedestrian walkways when warm air infiltrates. This leads to dangerous slip-and-fall incidents, poor visibility from fogging, and obscured signage, all of which are major workplace safety issues.
3. Energy Waste: The latent heat removal (converting water vapor to ice) and the frequent defrost cycles consume disproportionate amounts of energy.

There are several variables that describe the airflow and moisture level in a cold storage facility. Relative humidity (RH) refers to a measure of how much water vapor is in the air compared to the maximum amount it can hold at its current temperature. Dry-bulb temperature is the ambient air temperature measured by a standard thermometer. The most critical component of measuring a cold storage facility is the Dew Point, which is the temperature to which air must be cooled, at constant pressure, for water vapor to condense into liquid water. We have gone over the definitions of these terms, but how can you read them on a Psychrometric chart?

Decoding the Engineer’s Map: A Guided Tour of the Psychrometric Chart

So, what is a psychrometric chart? A psychrometric chart is a visual representation of the thermodynamic properties of moist air. There are several parts of this chart to help ventilation engineers tackle any air complications. 

• Deconstructing the chart:

  • Dry-bulb temperature: Located on the x-axis; this is the standard air temperature we feel. 
  • Relative humidity: These are the curved lines. It is the amount of water vapor in the air relative to the maximum it can hold at a given temperature.
  • Wet-Bulb Temperature: Shown as the slanted lines on the chart; it is the temperature a parcel of air would have if cooled to saturation by evaporating water into it. It is also important to note that this is a crucial part of indicating heat stress. 
  • Specific Humidity/Moisture Content: Located on the Y-axis; this is the mass of water vapor per unit mass of dry air.
  • The Saturation Line: The curved line on the left, representing 100%RH and the dew point curve.

The critical point is finding the air state point (DP+RH) and tracing horizontally left to the saturation line. The temperature at this intersection is the dew point.

Dew Point is especially critical in cold storage facilities because if any surface in the cold room (coils, walls, floors) is at or below the air’s DP, condensation or frost will form. It is especially important for the safety of your workers. Taking care of the humidity in the cold storage facility can prevent harm and danger from happening. The job of the desiccant dehumidifier is to drive the air’s DP below the coldest surface temperature. These are important to eliminate frost and ice hazards. 

Psychrometric charts are used by engineers to model and analyze various air conditioning and ventilation processes, which are represented as paths or lines on the chart. Understanding your system’s psychrometric chart allows industrial engineers to address worker safety concerns. 

Applying the Psychrometric Chart to Real-World Cold Storage Processes 

The scenario we will use to demonstrate real-world use of a psychometric chart is that we want to determine the moisture load on a cold storage area from outside air infiltration.  We start with the conditions of the outside air.  For our scenario, we will assume the outside air has a dry bulb temperature of 94°F and a wet bulb temperature of 78°F.  Drawing lines on the chart to where they intersect shows that the outside air has a moisture content of 120 gr/lb of air. 

We will assume that the cold storage area in our scenario maintains a temperature of 35°F.  Condensation will form in our cold storage area at 100% RH. Therefore, we want to keep the moisture level at or below 35°F wet bulb temperature.   Starting at the 35°F wet bulb temperature on The Saturation Line of the psychrometric chart, we draw a line across to the right and see that the maximum moisture content that we can have in the air inside the cold store area is 30 gr/lb of air.