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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.
Temperature Regulation Function
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.
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 superheat 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 superheating 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 superheating. 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 dehumifying 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 dehumidification".
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.