To begin our new series, “What you need to know about refrigeration”, we will be taking a look at one of refrigeration’s most important applications, namely the cold storage room. As well as explaining the specific features of cold storage we will also explain on the “how” and the “why”. This series is based on a number of complimentary eLearning modules from Danfoss for individual self-tuition on a PC. They are suitable for any type of student, from beginners to refrigeration experts.
Different factors have to be taken into consideration when designing a cold storage room. Thickness of the insulation on the walls. In the case of deep-freezing, a greater insulating layer thickness is usually applied. The insulation value of the isolated walls is an important aspect. A door frame heater has to be installed at cold storage room temperatures below 0°C - in particular with deep-freeze rooms - so that the door doesn't freeze shut. Moreover it is necessary to install an underfloor heating system in deep-freeze rooms and cold storage rooms which have a room temperature below 0ºC in order to prevent ice build-up on and below the floor. A certain air exchange must also be ensured. This should only be as big as required so that new cold storage room air isn't unnecessarily cooled again and thus no energy is wasted. A ventilation pressure relief valve or an overflow valve for equalising the pressure between the cold storage room and the surrounding air (two-way pressure equalisation) should be provided. If the pressure is not equalised in the cold storage room, the air inside cools and the volume "shrinks" thus creating a vacuum. The consequence would be, that the cold storage room walls and ceiling would ultimately collapse. When entering and leaving a cold storage room, it can easily occur that an undesired high air exchange is taking place. In extreme cases, this air exchange can lead to the scenario where the cold storage room temperature can no longer be retained by the refrigeration plant - not to mention the additional energy costs. In cases where frequent use of the cold storage room cannot be avoided, a strip curtain or an air lock respectively, can be used to minimise air exchange.
According to ISO 27000, it is required by law that an alarm device is fitted in cold storage rooms with a volume of more than 10m3 (rooms in which people can walk into). The alarm has to be visually and audibly perceptible. It must be ensured, even for smaller cold storage rooms, that a person can leave the room at all times. This means that even if a cold storage room is closed from the outside, an opening from the inside also has to be possible.
Refrigeration systems for cold storage rooms are available, for example, as clip on devices. Such a device combines the compressor and the registries (evaporator and condenser) in a single housing. It is mainly used for cold storage rooms with a volume of less than 30 m3 and only one temperature zone. The clip on unit can be installed on or next to the cold storage room. Clip on units consist of 1 compressor, 1 condenser and 1 evaporator each.
Another option available for cold storage rooms is the remote condensing unit. This design consists of a condensing unit which is connected to the evaporator by pipelines. The evaporator is normally installed on the cold storage room ceiling and equipped with one or more fans which allow air to circulate in the cold storage room. This type of cold storage room is suitable for use in buildings as well as outdoors. Cold storage rooms with a remote condensing unit consist of 1 compressor, 1 condenser and 1 evaporator each. The OPTYMA PLUS™ condensing unit is intended, for example, for cold storage rooms of this type. Major users such as supermarkets or central warehouses use centralized refrigeration systems with several compressors, which are installed as a compressor network. These plants can be installed separately from the actual cold storage rooms and are connected to the individual cold storage rooms using pipes. Refrigeration regulators can be deployed for solenoid valve control in order to regulate each evaporator individually. A compressor pack consists of several compressors, 1 condenser, various cold storage rooms and usually several refrigeration cabinets or show cases.
The temperature regulation function is mostly undertaken by a refrigeration controller in the cold storage room. A refrigeration regulator always needs as minimum one control sensor in standard version. This sensor measures the room temperature for use in a normal cold storage room. For this reason, the sensor is normally positioned so that it absorbs the temperature coming from the return air flow leading to the evaporator. The result is then displayed and processed by the refrigeration regulator as an actual value. This actual value is always compared with the refrigeration regulator's set point. The set point can be arbitrarily determined in the regulator. Besides the set-point, a difference (hysteresis) can be determined as well. The sum of the set point and the difference gives the upper switching value, whilst the set point itself represents the lower switching value of the temperature control. When the upper switching value is reached, the cooling (the compressor or the solenoid valve) is switched on and when the set point temperature is reached it is switched off again. This is how the room temperature is always retained in the same range. This temperature control function can be viewed as the most important basic function of the refrigeration controller.
The conditions which need to be ensured in a cold storage room are dependent on the types of products that are stored there. The temperature required in the respective cold storage room can also be dependent on the expected storage period, as well as on the question of whether a fresh product is to be stored there and frozen afterwards. The most common cold- or freezer rooms are cold storage rooms. The evaporating temperature in such cold storage rooms ranges between – 10ºC and 0ºC. The type of regulation which is used depends on the type of product which is to be stored as well as the desired quality. The type of use (MBP/LBP) and the volume of the cold storage/freezer room are often used as rule of thumb, in order to perform a simplified calculation of the heat input coming through the walls and thus the refrigerating capacity. This calculation is sufficient in most cases. Heat inputs are associated with room temperature, wall surface and air exchange. A more precise calculation can be obtained, by taking the food properties into account and by analyzing all of the thermal loads. These factors should be individually incorporated into the calculation of the freezer/ cold storage rooms.
Thermostatic or electronic expansion valves
Simple refrigeration circuits can be used for cold storage rooms. Simple refrigeration circuits are cost-effective and not complicated, however, they do have some disadvantages, for example, the risk that stored products, which are not packed or covered, will dry out. Thermostatic expansion valves are installed as injection valves in most cold storage rooms. An electronic super-heat controller offers several advantages for anyone who might be looking for a better solution. The evaporator should be filled with refrigerant at all times for optimal use. Even in the event of strong capacity fluctuations (that is, part-loads), the amount of refrigerant, which is to be injected, can be dosed accurately. This is done by promptly transmitting the current super-heating in the evaporator - using a pressure transmitter and a very sensitive temperature sensor - to the electronic regulator. The regulator can now take measures to reach ideal small super-heating. This adaptive regulation of the refrigerant injecting leads to optimal use of the evaporator and thereby, to the highest possible evaporation pressures which are feasible in this specific plant. However, this not only means a reduced electricity bill for the user. Due to the lower temperature differences between the evaporation and room temperature, there is a reduced dehumidify of air in the room and with it less dry out of the refrigerated goods. The same configuration means that, for example, vegetables, which are stored in a room with an evaporator with electronic expansion valve regulation, remain visually presentable and fit for sale for a longer period of time than with thermostatic expansion valves. Furthermore, the refrigerated goods dry out less. If the design of the evaporator is somewhat too small, a larger evaporator allows you improve the conditions even further with the effects "higher evaporation temperature" and "less de-humidification".
Frost will build up on the surface of an air cooler if its temperature is at or below 0°C. Frost which builds up on the evaporator can arise in different forms, that is as snow (powdery snow or snowflakes), as solid ice or in any other intermediate form respectively.
Frost is caused by the withdrawal of water from the goods as well as from the atmospheric humidity (air which flows through the air cooler). Defrosting is understood as the removal of frost which has built up on the surface of the evaporator. Defrosting can be carried out by feeding electrically generated heat, hot / cold gas on the high-pressure side, warm air out of the cold storage room or warm water respectively combined heating media.
Defrosting can help prevent the excessive built up of frost on the refrigeration surfaces, which reflects in a good heat transfer and an optimal operation of the plant. Moreover, regular defrosting brings about an unobstructed circulation of air, whereby the air cooler performance improves.
How often, and the length of time for which defrosting is carried out is dependent on, among other things, the stored products and their moisture content as well as air exchange and humidity. How many times a day the door to the cold storage door is opened or somebody enters the room also plays a major role. The cold storage room needs to be defrosted as often as required and above all in a timely manner. If the defrosting period is too short and not all of the ice melts then even more ice will build up with time.
The type of defrosting is just as important as the defrosting time/frequency. There are three customary defrosting methods. Air circulation (thawing out in the cold storage room), electric, and hot gas defrosting. Natural defrosting (air circulation) with the help of air is possible if the temperature in the cold storage room is higher than +4°C. Cooling of the cold storage room is stopped but the fan continues to run. This process can take longer than the other defrosting methods, however, defrosting can also be sped up by increasing the temperature. This process is energetically advantageous since no additional heat is produced which has to be removed from the cold storage room again later on.
Electric defrosting is the most common and at the same time a simple defrosting method for cold storage rooms. The air cooler just has to be installed with electric heaters and connected to electrical cables. This tends to be a more expensive defrosting method from an energy point of view because it uses a lot of energy. On the other hand, electric defrosting can be well controlled and represents perhaps the only feasible defrosting option. Defrosting can be started using a real-time clock, time intervals or manually and ended according to a pre-set temperature or after a certain period of time.
The third defrosting method, namely hot gas defrosting, involves gas being diverted from the high-pressure side of the refrigeration plant to be used for the defrosting. In principle, energy is saved through the hot gas defrosting method. However hot gas defrosting is a relatively complicated defrosting method and is mainly used in large plants with several evaporators. Evaporators can be operated at the same time as the air cooler which is to be defrosted, thus defrosting alternately. More valves are required for hot gas defrosting and the control system is also more complicated than both of the other common defrosting methods. When hot gas defrosting, it is highly recommended that a liquid separator is built in, in order to protect the compressor. A pressure regulator can also protect the compressor from high suction pressure. Cold gas defrosting can also be an alternative to hot gas defrost. This simply involves extracting the high-pressure refrigerant from the suction dome instead of directly out of the hot gas line.
However it's not only the defrosting method that is important; energy costs can be reduced by skipping a defrosting operation, especially if it isn't needed. Skipping every fifth defrosting process is already a huge energetic advantage. It's very important that the plant is only defrosted at the programmed times. If this is not the case, then defrosting could be started at unfavorable times (e.g. when awaiting delivery of goods) Fitting a refrigeration regulator with defrost on demand can be positively reflected, by itself, in the user's electricity bill.
There are certain points that have to be taken into consideration when designing, assembling and using cold storage rooms - we learnt more about this in these two editions from "what you need to know about refrigeration". Careful commissioning and regular maintenance ensure that the plant runs smoothly and that energy is not wasted unnecessarily. Energy improvements are also possible for existing plants by fitting electronic expansion valves and refrigeration regulators for defrost on demand.