19. June 2019

Energy optimization of refrigeration systems

Energy optimization of refrigeration systems

Potential for improvement in cold rooms

We are dedicated to the topic of "cold rooms" and show the potential for improvement of this application. Regardless of whether it is a new building, refrigerant conversion, unsatisfactory goods quality during storage, frequent service calls or a high electricity bill: There are many different motivations for a system operator to ask the refrigeration system manufacturer for solutions. Often it is not easy to have the right answer ready. The following article gives practical hints.

 

 

Thermostatic or electronic expansion valves

In most refrigerated rooms thermostatic expansion valves are installed as injectors. If the operator is looking for a better solution, an electronic overheating control offers several advantages. The evaporator is always optimally filled with refrigerant. Even with strong power fluctuations (ie partial load cases), the amount of refrigerant to be injected can be precisely metered. This is done by passing the current overheating in the evaporator via a pressure transmitter and a very sensitive temperature sensor to the electronic controller in a timely manner. The controller can now take measures to achieve optimally small overheating. This adaptive control of the refrigerant injection leads to an optimal use of the evaporator and thus to the highest possible evaporation pressures, which can be realized in this specific plant. This in turn not only leads to higher COP values, but also to energy savings, as overheating always adapts to the evaporator's minimally stable signal (MSSLine), thus avoiding drifting into the unstable region. But not only the electricity bill of the operator sinks.

 

 

Due to the lower temperature difference between evaporation and room temperature and the dehumidification of the room air and thus the refrigerated goods is reduced. This results in the same configuration that, for example, vegetables when stored in a room with electronic expansion valve control of the evaporator longer visually attractive and salable than with thermostatic expansion valves. In addition, the refrigerated material dries less. If the size of the evaporator is a bit short, the effects of higher evaporator temperature and less dehumidification can be further improved with a larger evaporator.

 

Variable speed compressors

Even speed-controlled compressors make it possible to realize the highest possible evaporation temperatures. Normally, refrigeration compressors are designed exclusively for maximum system load. In fact, the systems run at 65% of their operating time at part load, so the compressor is oversized over long periods of time.

 

Conventional regulations to compensate for this "excess power" are on-off control, pressure-controlled power controller or hot gas bypass controller. Compared to these methods, a compressor drive package offers superior control performance and is the more energy efficient solution. The cooling capacity of a conventional fully hermetic reciprocating compressor is constant, engine and crankshaft rotate at 2900 revolutions per minute (50 Hz, one pair of poles). With a Danfoss "VTZ Compressor Drive", however, the speed can be varied in a frequency band from 30 to 90 Hz. Depending on the necessary cooling load thus results in an engine speed between 1800 and 5400 U / min. Therefore, the compressor is always properly sized in terms of cooling requirements.

The regularity is not 33, 66 and 100%, but stepless. Together with a pressure transducer, the package works much like a compound regulator. The frequency converter receives a pressure setpoint, which it tries to keep constant. If the pressure increases, the compressor speed is increased. If the actual pressure value drops, the speed is reduced. With this regulation, a very constant suction pressure can be achieved. A compressor connected directly to the mains generally consumes up to eight times its rated current during startup. This can lead to discussions with the energy supplier even at relatively low power levels, which will require either additional technical measures to limit the current or an increased energy supply price.

The frequency converter starts with a very low frequency when starting the compressor and adapts it to the actual rotational speed of the rotor. In the case of a direct start of a compressor, however, 50 Hz are applied directly, even if the rotor has not yet started to move. This leads to starting current peaks that do not occur in frequency converter operation in this form. Thus, a compressor-frequency converter package is the solution to reduce the operator's energy costs by consistently avoiding too deep suction pressures and evaporation temperatures, as well as current spikes, which can occur with fixed speed compressors.

 

defrost

The second major aspect in terms of energy consumption in addition to high evaporation or suction pressures is the defrost. Here you can save a lot of energy costs. If a refrigeration controller is equipped with an electronic expansion valve, it usually also has a demand defrost function. Basically it is the task of the defrosting to ensure that no unnecessary defrosts are initiated. For example, if only every fifth defrost can be skipped, this is already a great advantage energetically. Important in case of demand defrost is their exclusive initiation at programmed times. If this is not the case, the defrost could be started at unfavorable times (eg loading of goods). How does the controller recognize whether the time is right for a defrost or not?

 

With electronic injection, in systems with a compressor, the evaporation temperature drops steadily after defrosting. At the same time, the degree of opening of the electronic expansion valve continues to decrease until a new defrost is performed. With this value, the cold store controller can decide whether a defrost can be skipped. Refrigeration controllers without electronic expansion valves are not that easy. But you can also determine by the temperature profile at the defrost sensor, whether a defrost is necessary or not. Here, too, the temperature in the case of "1: 1 systems" at the defrost cooler recedes more and more, the longer the last defrost lasts. Another option is to consider the total cooling time. Is this steadily increasing, then it is assumed that the evaporator evaporates heavily and a defrost is initiated at the earliest possible time. For an operator, the retrofitting of a refrigeration controller with demand defrost can already have a significant positive impact on the electricity bill.

 

4-way valves for hot gas defrosting

In most refrigerated rooms electric heaters are mounted for defrosting. However, the hot gas defrosting is clearly more efficient. Hot gas defrosting can be easily implemented in evaporative cold room systems through the use of a 4-way reversing valve. When the "1: 1 plants" are reversed, the evaporator, which has now become a liquefier, can be defrosted from the inside. This means that the heat does not need to be brought to the ice in the evaporator by electric heaters in the evaporator package, but the hot gas is sent directly through the pipe system on which ice has previously set. This leads to excellent defrosting results and is hard to beat in terms of defrost time, energetics and targeted heat input. For the following description we introduce ourselves, the small (pressure) port is facing up and the other three ports are pointing down. Here we see the small pilot solenoid valve with its coil.

 

With a standard 4-way valve, there are only two switching positions - no intermediate positions. In switching situation one, no voltage is applied to the coil of the pilot solenoid valve. As a result, high-pressure hot gas is introduced from the pilot line of the small port (permanent pressure side) from the right into the slide mechanism chamber. At the same time, the pressure on the left side of the valve chamber can be relieved via the permanent suction port by outflow to the low pressure side. Thus, the slider slides to the left and opens the main paths top to bottom right and left outside to the center. In switching situation two, the hot gas finds its way from top to left, whereby at the same time suction gas can flow from the right to the middle downwards. This is achieved

The pressure on the right side can thus be relieved to the middle, lower main connection, which leads to the slide movement to the right. If thermostatic expansion valves are to be used in biflow operation - that is, in the case of cooling with standard and in defrosting in the opposite direction of flow - a valve with external pressure equalization must always be selected. This external pressure compensation must always be attached to the permanent suction line between the 4-way valve and the compressor. If this is not heeded, the valve can not work in the backward operation, because then high pressure instead of evaporation pressure via the external pressure equalization closes the valve and formally compress. Hot gas defrosting can result in much more effective and faster defrosting than electric defrosting. This saves energy costs and at the same time brings less external heat into the cold room. This eliminates the additional removal of this amount of heat through the refrigeration system after defrosting.

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