What Refrigeration Knowledge Do You Need for Cold Rooms?

In the second part of "Refrigeration Knowledge You Need," we will cover how to build a proper cold room. We will focus on the practical challenges and requirements of establishing such an environment.

Design

Several factors must be considered when designing a cold room. The thickness of the wall insulation. In deep-freeze applications, thicker insulation is typically applied. The insulation value of the partition walls is an important aspect. Door frame heaters must be installed in cold rooms where temperatures are below 0°C—including deep-freeze rooms—to ensure the doors do not freeze shut. Additionally, underfloor heating systems need to be installed in freezers and cold rooms with ambient temperatures below 0°C to prevent the floor and the ground beneath from freezing. A certain amount of air exchange must also be ensured. The space should be limited to the required size to avoid unnecessarily cooling new cold room air again, thereby preventing energy waste. Ventilation pressure relief valves or overflow valves should be fitted to balance the pressure between the cold room and the surrounding air (for both positive and negative pressure differentials). If the pressure inside the cold room is unbalanced, the indoor air cools, its volume "shrinks," creating a vacuum. As a result, the cold room door becomes difficult to open, and eventually, the walls and ceiling may collapse. When entering or exiting the cold room, unwanted high air exchange can easily occur. In extreme cases, this air exchange may prevent the refrigeration plant from maintaining the cold room temperature—not to mention the additional energy costs. In situations where frequent use of the cold room is unavoidable, strip curtains or air locks can be used to reduce air exchange.

According to ISO 27000, it is a legal requirement to install alarm equipment in cold rooms with a volume exceeding 10 m³ (rooms that people can freely enter). The alarm must be both visually and audibly perceptible. Even for smaller cold rooms, it must be ensured that personnel can leave the room at any time. This means that even if the cold room is closed from the outside, it must be possible to open it from the inside.

Types of Cold Rooms

For example, refrigeration systems for cold rooms are available as plug-in units. This device integrates the compressor, evaporator, and condenser into a single housing. It is mainly used for cold rooms with a volume of less than 30 m³ and only one temperature zone. Plug-in units are typically mounted on the wall of the cold room. Each plug-in unit includes 3 compressors, 1 condenser, 1 evaporator, and an expansion device.

Cold rooms also come with remote condensing units. This design consists of a condensing unit connected to the evaporator via piping. The evaporator is usually installed on the ceiling of the cold room and is equipped with one or more fans to circulate air within the cold room. This type of cold room is suitable for both indoor and outdoor installations. Cold rooms with remote condensing units each include 1 compressor, 1 condenser, 1 evaporator, and an expansion device. For example, the Optyma Plus™ condensing unit is mainly used for such cold rooms. Major users, such as supermarkets or central warehouses, use centralized refrigeration systems with multiple compressors installed as compressor racks. These units can be installed independently of the cold room and connected via piping to the evaporators of individual cold rooms. Refrigeration controllers can be deployed for solenoid valve control to regulate each evaporator separately. A compressor rack consists of multiple compressors, 1 remote condenser, multiple cold rooms, and in some applications, refrigerated cabinets or display cases.

Temperature Regulation Function

The temperature regulation function is primarily performed by the cold room's refrigeration controller. In its standard version, the refrigeration controller requires at least one control sensor. This sensor measures the room temperature for a standard cold room. Therefore, the sensor is typically positioned to measure the temperature of the return air from the evaporator. The result is then displayed by the refrigeration controller and processed as the actual value. This actual value is always compared with the setpoint of the refrigeration controller. In addition to the setpoint, a differential (hysteresis) can be determined; the sum of the setpoint and the differential represents the upper switch-on value, while the setpoint itself represents the lower switch-off value for temperature control. When the upper switch-on value is reached, the cooling system (compressor or solenoid valve) is activated, and when the setpoint temperature is reached, cooling is switched off again. This is why the room temperature remains within the same range at all times. This temperature control function can be regarded as an important basic function of the refrigeration controller.

Cold Room Zones

The conditions that need to be ensured in a cold room depend on the type of goods stored. The required temperature in a cold room may also depend on the intended storage time and whether fresh products are to be stored and subsequently frozen. The evaporation temperature in cold rooms ranges from -10°C to 0°C, and in freezers, it ranges from -25°C to -30°C.

The type of use (cold room/freezer) and the volume of the cold room are often used as a rule of thumb to simplify the heat input through the walls and thus calculate the refrigeration capacity. This calculation is sufficient in most cases—heat input is related to room temperature, insulation material type/thickness, and air exchange. A more precise calculation can be obtained by considering the nature of the goods and analyzing all heat loads. These factors should be individually incorporated into the cold room/freezer calculation.

Thermostatic or Electronic Expansion Valves

Simple refrigeration circuits can be used for cold rooms. Simple refrigeration circuits are cost-effective and straightforward, but they also have some disadvantages, such as the potential drying out of unpackaged or uncovered products in storage. Thermostatic expansion valves are installed as injection valves in most cold rooms. Electronic superheat controllers offer several advantages for those seeking a better solution. The evaporator should always be filled with refrigerant to achieve an optimal state. Even under conditions of significant capacity fluctuations (i.e., partial load), the refrigerant dose to be injected can be precisely metered. By rapidly transmitting the current superheat in the evaporator—using pressure transmitters and highly sensitive temperature sensors—to the electronic controller, the controller can take measures to control the optimal superheat in the evaporator, ensuring optimal system operation. This adaptive regulation of refrigerant injection allows the evaporator to be optimally utilized, thereby achieving the highest possible evaporation pressure for that particular plant. However, this not only means lower electricity costs for the user—because the temperature difference between evaporation and room temperature is smaller, dehumidification of the room air is reduced, and dryness in the refrigeration equipment is also decreased. The same configuration means, for example, that vegetables stored in a room with an evaporator regulated by an electronic expansion valve remain visually appealing and suitable for sale longer than when using a thermostatic expansion valve. Additionally, the drying rate of refrigerated food is lower. If the evaporator is designed slightly smaller, a larger evaporator can further improve the environment, providing "higher evaporation temperatures" and "reduced dehumidification."

Defrosting

If the temperature of the evaporator is at or below 0°C, frost will form on the surface. Frost accumulation on the evaporator can appear in various forms, such as snow (powdery snow or snowflakes), solid ice, or any other intermediate form.

Frost is caused both by moisture being drawn out of the goods and by humidity in the air flowing through the air cooler. Defrosting refers to the removal of frost accumulated on the evaporator surface.

Defrosting helps prevent excessive frost buildup on the refrigeration surface, thereby ensuring good heat transfer and optimal plant operation. Furthermore, regular defrosting ensures unobstructed air circulation, thus improving air conditioning performance.

The frequency and duration of defrosting depend on factors such as the stored products, moisture content, air exchange, and humidity. How many times a day the cold room door is opened, or how often people enter the room, also plays a significant role. Cold rooms need to be defrosted as often as necessary, and it is important to do it in a timely manner. If the defrosting time is too short and not all the ice melts, ice will accumulate more over time.

The type of defrosting and the defrosting time/frequency are equally important. There are three common defrosting methods: air circulation (cold room defrosting), electric, and hot gas defrosting. If the cold room temperature is above +4°C, natural defrosting via air circulation can be used. The cooling in the cold room is stopped, but the fans continue to run. This process may take longer than other defrosting methods, but it can be accelerated by raising the temperature. This process is energy-efficient because no additional heat is generated that would later need to be removed from the cold room.

Electric defrosting is a common method in cold rooms and is also the simplest defrosting method. The evaporator simply needs to be fitted with electric heaters and connected with cables. From an energy perspective, this defrosting method is often more expensive because it consumes a significant amount of energy. On the other hand, electric defrosting can be well controlled and may be the only feasible defrosting method. Defrosting can be initiated via a real-time clock, time intervals, or manually, and can end based on a preset temperature or after a certain period.

The third defrosting method is hot gas defrosting, which involves drawing gas from the high-pressure side of the refrigeration system for defrosting. In principle, the hot gas defrosting method saves energy. However, hot gas defrosting is a relatively complex defrosting method and is mainly used in large plants with multiple evaporators. The evaporator can operate simultaneously with other evaporators defrosting, allowing for alternating defrosting. Hot gas defrosting requires more valves, and the control system is also more complex than the two common defrosting methods. During hot gas defrosting, it is strongly recommended to install a liquid separator to protect the compressor from liquid return. A pressure regulator can also protect the compressor from high suction pressure. Cold gas defrosting can also be used as an alternative to hot gas defrosting. This simply involves drawing high-pressure refrigerant from the top of the receiver rather than directly from the hot gas line.

However, the defrosting method is not the only important factor; skipping defrost cycles can reduce energy costs, especially when defrosting is not required. Skipping every fifth defrost cycle already provides a significant energy advantage. It is very important that the plant only defrosts at scheduled times. If this is not the case, defrosting can start at an unfavorable time (e.g., while waiting for goods delivery). Equipping the refrigeration controller with on-demand defrosting functionality can itself have a positive impact on the end user's electricity bill.

Conclusion

When designing, assembling, and using cold rooms, there are several considerations to keep in mind—as we have learned in these two issues of "Refrigeration Knowledge You Need." Careful commissioning and regular maintenance ensure smooth plant operation and prevent unnecessary energy waste. Existing plants can also improve energy efficiency by installing electronic expansion valves and refrigeration controllers with on-demand defrosting.