Optimizing Cold-Storage Environments with Air Curtains

This article explores how temperature and moisture affect air movement, how air curtains work across different applications, and how air curtains improve energy efficiency and working conditions in cold-storage facilities.

The Science Behind Environmental Separation

Maintaining stable conditions within a cold-storage area, such as a walk-in freezer or refrigerated warehouse, fundamentally is about controlling the movement of air and moisture. When a door to a cooled space opens, warmer, moisture-laden air from outside the space rushes in, leading to increased energy consumption, frost formation (especially around the door and on evaporator coils), and temperature fluctuations.

This kind of infiltration is caused by more than a difference in temperature; it is driven by a difference in vapor pressure (based on moisture level). Moisture can move quickly from an area with high moisture content (typically, the warm side of an opening) and a high dew point to an area with low moisture content (typically, the cold side of an opening) and a low dew point. The vapor pressure theoretically creates a suction that directs moisture from the more humid side to the drier, colder side. Put simply, what goes in must come out. So, as warm, humid air enters at the top of a door opening, colder, drier air exits at the bottom.

When warm, humid air enters a cooled, dehumidified space, it can lead to condensation, fog, and frost buildup that triggers increased defrost cycles and reduces a refrigeration system’s efficiency (photos C and D). These disruptions impair the ability to maintain strict—and, often, regulated—environmental tolerances, leading to both product loss and increased operational costs. Although nothing is more effective than a closed, sealed door in terms of protection, doors in cold-storage facilities typically are opened frequently or left open for long periods because of operational needs for fast and easy access.

Strip curtains and high-speed doors provide a passive barrier to infiltration but have limitations. The transparency and effectiveness of strip curtains diminish rapidly from wear and tear from high-traffic use. What’s more, strip curtains often are poorly installed and/or modified, such as when they are tied back or have slits cut into them for visibility. Meanwhile, condensation on high-speed-door panels or leaks from gaps in strip curtains can cause wet floors and ice formation, leading to safety and health concerns.

In contrast, air curtains offer active environmental separation, allowing facility operators to enhance protection, reduce energy use, and improve working conditions without sacrificing access. In existing cold-storage facilities, air curtains can correct for leakage with controlled air streams that support refrigeration systems and reduce defrost cycles.

Air Curtains: Principles and Performance

Typically installed above an entrance or doorway, an air curtain functions by blowing air downward at a high velocity to form an invisible barrier designed to minimize the exchange of air between environments with differing temperatures, moisture levels, and space pressures. This enhances temperature stability and humidity control, helping to extend product life while reducing energy loss.

Effective cold storage is not simply about the amount of air that is moved; it also is about how effectively the air is moved. The objective is to create a stable air barrier that minimizes entrainment, or the unwanted pulling of air from either side of an opening into an air stream. Proper air-curtain design considers both velocity and entrainment. While a narrow air stream can bend more easily and allow a greater amount of leakage than a wide air stream, entrainment is lower. The goal is to find the best balance. Velocity must be calibrated to ensure an air stream reaches the floor or a splash plate, where it forms back pressure off the surface, forming an air barrier to help resist infiltration, wind loading, and vapor pressure.

Space pressurization is a key factor affecting air-curtain performance. The cold side of an opening being under positive or equal pressure relative to the warm side will help reinforce an air barrier, while the cold side being under negative pressure can hinder the formation of an air barrier. With exterior doors, air curtains typically divert about 20 percent of discharged air to the outdoors to help resist wind loading. Internal doorways, depending on cold-side space pressure, vary from 80/20 under positive pressure to 50/50 under more equalized pressure.

Not all air curtains are created equal. Factors such as design configuration, installation, calibration, controls, and integration with other systems can impact performance significantly. For successful application, the right air curtain must be aligned with the unique operational patterns—from door-cycle frequency to product-flow and environmental-control goals—of a facility.

Application-Specific Design

There are four primary applications of air curtains in cold storage, each requiring a tailored air-management strategy:

1. Control of condensation on door, wall, and floor surfaces—The goal here is to direct air across wet surfaces to wick away moisture and allow the moisture to dissipate within the space. Success depends on the amount of moisture that accumulates, the temperature of overcooled surfaces, the dew point, and the relative humidity within the space. Supplemental heat can be used to enhance moisture wicking and change surface temperature enough to prevent the formation of moisture.
2. Use with a door—In this application, an air curtain is most effective when interlocked with door controls, such as dry-contact relays or switches. This moves air away from the door, allowing the air to dissipate within the space. This can improve air distribution and refrigerator conditions. It is important to consider not only the amount of moisture that is visible but the source of the moisture.
3. Use with a high-speed door or a strip curtain (hybrid configuration)—In a high-traffic or continuous-operation area, directional airflow with a hybrid configuration can be particularly effective and aid compliance with hygiene and safety standards. The stand-alone high-speed door or strip curtain provides static resistance while, typically, the air curtain runs continuously to reinforce the barrier. Facility owners should be cautious of removing strip curtains and using only air curtains as a replacement. Removal of a physical barrier changes space pressurization, which can increase infiltration or exfiltration.
4. Use without a door—Where there never was a door, use of an air curtain should reduce infiltration; however, performance can be evaluated only after spaces equalize to a constant air stream. Where a door was removed, it is critical to examine space pressurization and refrigerator-system capacity because of entrainment that did not occur previously. If the system is not already running at maximum capacity, the cooling load could change, and the system could see benefits.

Engineering Considerations

Design of an effective air-curtain system requires in-depth understanding of a facility’s operations, limitations, and environmental challenges. Traffic volume, door-usage patterns, and thermal-load variability all affect a system’s requirements. A unit must be matched to opening type, opening dimensions, available mounting clearance, and exposure duration. A significant consideration should be summer conditions in relation to winter conditions, as summer often sees higher levels of moisture, which works its way to cold-storage openings, where it is more visible than during winter.

Choices such as mounting (vertical or horizontal) and the use of side shields, splash plates, or blank-offs for air-stream containment can greatly affect performance. Integration with high-speed doors or strip curtains can compensate for weak points caused by gaps at floors or between strips, door jambs, and ceilings while reducing the need for frequent manual maintenance of strip panels.

PHOTO A. Air curtain installed on cooler doors.

PHOTO B. Air curtain installed on a high-speed fabric door.

PHOTO C. Humidity infiltration causes significant ice buildup around a doorway.

PHOTO D. Ice buildup from unwanted infiltration increases defrost cycling.