In this last issue of the "fitters notes" series we focus on the "cold storage rooms" topic and point out some improvement potential with this application. Regardless of whether this involves a new build, coolant conversion, unsatisfactory product quality with storage, frequent service calls or high electricity bills – these are the most diverse motivations for a system operator to ask the refrigeration system builder for solutions. Frequently it is not easy to have the right answers immediately at hand. The following article provides practical reference points.
Thermostatic expansion valves are installed as injection valves in most cold storage rooms. If the operator now wants to find a better solution, an electronic superheat control offers several benefits at once in this respect. Firstly, the evaporator always contains the optimal refrigerant level. Refrigerant can be precisely injected to suit high capacity fluctuations (i.e. partial loads). This happens because the respective actual superheat in the evaporator is forwarded via a pressure transducer and a very sensitive temperature sensor in real-time to the electronic controller. The controller can now regulate to ensure the lowest possible superheat levels. This adaptive controlling of the refrigerant injection results in optimal use of the evaporator and therefore the highest possible evaporating pressures, related to the specific system. This in turn not only results in higher COP values – it also produces energy savings, because the superheat always adapts to the minimum stable signal (MSS) line of the evaporator, thereby preventing any drifting off into the unstable area. A low temperature difference between the evaporation temperature and the room temperature reduces the dehumidification of the room air and therefore the refrigerated goods. With the same configuration this means, for example, that vegetables stored in a room with electronic expansion valve control of the evaporator stay optically more appealing and saleable for longer than with thermostatic expansion valves. The refrigerated goods also dry out less. If the evaporator values are configured somewhat limited, the “higher evaporation temperature” and “lower dehumidification” effects can be improved even further with a bigger evaporator. Perhaps most appealing though is that as system efficiency is improved, the corresponding electricity usage is reduced, resulting in energy savings!
Variable speed compressors also enable the highest possible evaporation temperatures. Compressors in refrigeration systems are usually sized based on maximum system load, but for approximately 65% of their operating time the systems actually run at partial load, so the compressor is over-sized for most of the time.
Conventional controls for balancing this "capacity excess" include on-off control, pressure-controlled capacity controller or hot gas bypass controller. Compared with these methods a compressor frequency converter package provides increased controllability and a more energy-efficient solution. The refrigerating capacity of a conventional fully hermetic piston compressor is fixed; motor and crankshaft turn at 2,900 RPM (50 Hz, one pole pair). With a Danfoss "VTZ Compressor Drive" by contrast, the speed can be varied in a frequency range of 30 to 90 Hz. Depending on the required cooling load this consequently results in a motor speed of between 1,800 and 5,400 RPM. The compressor is therefore always correctly sized for the refrigeration requirements, which means the “VTZ” provides the same minimum partial load as a 3-piece multi-compressor unit. The control increment is not, however, 33, 66 and 100 % – it is actually continuous, or “stepless”. Together with a pressure transducer the package works similar to a composite controller. The frequency converter is given a pressure set-point, which it tries to keep constant. If the pressure value rises, the compressor speed increases. If the pressure drops, the speed decreases. This controlling achieves a very consistent suction pressure. A compressor connected directly to the mains generally consumes up to eight times its nominal power when starting. With relatively low input powers this can lead to discussions with the power supplier, who will ask for either additional technical measures for power limiting or a higher energy supply price. The “VTZ Compressor Drive” is equipped with an installed soft starter that significantly reduces the compressor’s power peak compared with a direct start. The frequency converter begins the compressor start with a very low frequency and adapts this to the rotor’s actual revolution speed. With a compressor direct start by contrast, 50 Hz direct is generated, even before the rotor turns. This results in start-up power peaks, which do not appear as such in the frequency converter operation. A compressor frequency converter package is therefore the ideal solution for reducing the operator’s energy costs, because it regulates low evaporating / suction pressures and power peaks, which are possible with fixed-speed compressors.
The second most important factor with regard to energy consumption after high evaporating and suction pressures is the defrosting. Quite a bit can be saved here in energy costs. If a refrigeration controller is equipped with an electronic expansion valve, it usually also has a defrost on demand function. The function of the defrost on demand is basically to ensure that unnecessary defrosting is avoided. If, for example, just every fifth defrost were to be skipped, this alone would be a big energy benefit. It is important with on demand defrosting that it only begins at programmed times. If it doesn’t, the defrosting could be started at inappropriate times (e.g. goods loading). So how does the controller recognize if the time is right for a defrost or not? With electronic injection the evaporation temperature falls continuously after defrost in systems with a compressor. At the same time the opening of the electronic expansion valve decreases until a defrosting is performed again. With this value the refrigeration controller can decide whether or not a defrost can be skipped. It’s not so easy for refrigeration controllers without electronic expansion valves. But they can determine if a defrost is necessary or not with the temperature profile on the defrost sensor.
The temperature with “1:1 systems” on the defrost sensor also falls continuously according to when the last defrost was performed. Another option here is to consider the overall refrigeration time. If this constantly rises, then a heavy frosting on the evaporator can be assumed, and a defrost must be started as soon as possible. Just upgrading a refrigeration controller with on demand defrosting alone can mean a very positive energy reduction for the operator.
Electric heaters for defrosting are installed in most cold storage rooms. Hot gas defrosting, however, is clearly more efficient. Hot gas defrosting can also be easily implemented with single evaporator cold storage room systems with the use of a 4-way reversing valve. With the circuit reversal of “1:1 systems”, the evaporator, which has now become a condenser, can be defrosted from the inside. This means that the heat does not have to be brought to the ice in the evaporator by electric heaters, but rather that the hot gas is sent directly through the pipe system on which the ice has settled. This produces excellent defrosting results and is practically unbeatable for defrost time, energetics and specific heat input. For the following description we have the small (pressure) connection pointing upwards and the three other connections pointing downwards. We can see the small pilot magnet valve here with its coil. With a standard 4-way valve there are only two switching positions – no intermediate positions. There is no voltage on the pilot magnet valve’s coil in switching position one. This means that hot gas with high pressure from the pilot line of the small connection (constant pressure side) is introduced from the right into the sliding mechanism chamber. At the same time the pressure on the left side of the sliding chamber can be relieved with the constant suction connection by flowing off on the low pressure side. The slider moves here to the left and opens the main paths from above to the right and down, and left outside to the centre. In switching position two the hot gas finds its way from above to the left, whereby suction gas can flow at the same time from the right to the centre and down. This is achieved when the pilot magnet valve is activated with supply voltage and introduces high pressure from the left into the sliding chamber. The pressure on the right side can consequently be relieved on the centre bottom main connection, which results in a slider movement to the right. If thermostatic expansion valves are to be used in bi-flow operation – i.e. in cooling with default direction and in defrosting in opposite direction – a valve with external pressure balancing must always be selected. This external pressure balancing must always be applied on the permanent suction line between the 4-way valve and the compressor. If this is not observed, the valve cannot work in reverse operation, as high pressure instead of evaporation pressure via the external pressure balancing then closes and literally presses the valve shut. Hot gas defrosting can produce significantly more effective and faster defrosting compared with electric defrosting. This saves on electricity costs and also gives off less external heat into the cold storage room, which means this heat volume no longer has to be dissipated via the refrigeration system after the defrosting.