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1.Difference between diesel powered refrigeration unit and engine driven refrigeration unit
Diesel powered refrigeration unit: Independent and non-independent refrigeration units are divided according to power source. Independent refrigeration unit (Diesel powered refrigeration unit) means that the refrigeration unit has a separate power source and the unit itself has an independent power source.
Generally speaking, diesel powered reefer unit is integrated structures, but there are also modified split structures (this structure is generally hoisted on the beam of the reefer truck). This type of unit itself has an engine and a generator that provide power and electricity to the refrigeration system Device. It can work independently without being restricted by the working state of the truck engine.
Non-independent refrigeration unit (Engine driven refrigeration unit😞 As the name implies, it is a refrigeration unit that cannot work independently. This type of unit has no power unit and must rely on the truck engine to drive the compressor to work.
The engine driven refrigeration unit can also be divided into an integral structure and a split structure from the structure. The split structure also includes three types of condenser front type, condenser top-mounted type and condenser bottom-mounted type (bottom-mounted or concealed); but the condenser front-mounted structure is the most common structure in split non-independent refrigeration units.
2.Advantages and disadvantages of diesel engine refrigeration unit and engine driven refrigeration units
Independent refrigeration unit
Advantages: The advantage of diesel engine refrigeration unit is stability and reliability, which is also the most needed for refrigerated trucks. Since the diesel engine refrigeration unit has a separate source of power output, the refrigerator can still work when the refrigerated vehicle is turned off or the refrigerated vehicle has an engine failure, so as to ensure that the goods in the refrigerated vehicle will not deteriorate.
Integrated independent refrigeration unit
Disadvantages: The disadvantage of independent refrigeration units is that they are expensive and maintenance is more complicated than non-independent refrigeration units. Due to the addition of a set of power systems, independent refrigeration units are more complicated than non-independent refrigeration units in terms of maintenance, and independent refrigeration units are relatively more technically complex and more expensive.
Non-independent refrigeration unit
Advantages: The advantages of non-independent refrigeration units are low price, simple structure and easy maintenance.
Disadvantages: The disadvantage of the non-independent refrigeration unit is that it relies too much on the power source of the refrigerated vehicle. When the refrigerated vehicle engine stops working, the non-independent refrigeration unit loses power support and the refrigeration system stops working. If the door is frequently opened when the vehicle is turned off, the large loss of cold air in the refrigerated box causes the temperature in the box to rise, or the refrigerator will stop cooling when the vehicle fails (engine failure), and the cargo in the vehicle is easily lost.
3.Choice of independent refrigeration unit and non-independent refrigeration unit
From the price point of view: If your budget is relatively sufficient and have strict temperature requirements (the required temperature should be at least minus 10 degrees below) ) It is recommended to use a diesel powered refrigeration unit.
In terms of the length of transportation distance: For short-distance distribution within the city or between districts and counties, it is recommended to use non-independent units, which not only lower the procurement cost, but also easy to repair even if there is a failure; For long-distance transportation, it is recommended to use independent refrigeration units, because Even if the vehicle engine stops working during long-distance transportation, the refrigerator can still cool normally to ensure the safety of the transported items.
In terms of vehicle models: Generally speaking, light trucks with a body less than 7 meters are recommended to use non-independent units. For trucks and semi-trailers above 7 meters, independent refrigeration units are better.
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Truck refrigeration system consists of four parts: compressor, condenser, expansion valve and evaporator.
transport refrigeration units
The compressor is the power of the refrigeration cycle and is driven by an electric motor to continuously rotate. It can not only extract the vapor in the evaporator in time, maintain low temperature and low pressure, but also increase the pressure and temperature of the refrigerant vapor through the compression function, creating conditions to transfer the heat of the refrigerant vapor to the external environment medium. In other words, the low-temperature and low-pressure refrigerant vapor is compressed to a high-temperature and high-pressure state, and room temperature air or water is used as a cooling medium to condense the refrigerant vapor.
The function of the compressor is to compress low-pressure steam into high-pressure steam, thereby reducing steam volume and increasing pressure. The compressor sucks the working medium vapor with lower pressure from the evaporator, and sends it to the condenser after the pressure increases. The high-pressure liquid is condensed in the condenser. After being throttled by the throttle valve, it becomes a low-pressure liquid and is sent to the evaporator. In the evaporator, it absorbs heat and evaporates into low-pressure steam, which is then sent to the compressor inlet to complete the refrigeration cycle.
The condenser is a heat exchange device that uses the ambient cooling medium (air or water) to take the heat of the high temperature and high pressure cooling steam from the cooling compressor, so that the high temperature and high pressure refrigerant vapor is cooled and condensed into a high pressure normal temperature refrigerant liquid. It is worth mentioning that in the process of transforming refrigerant vapor into refrigerant liquid, the pressure of the condenser is constant and still at high pressure.
3.Throttle element (namely expansion valve)
The high-pressure and normal-temperature refrigerant liquid is directly sent to the low-temperature scale evaporator. According to the corresponding principle of saturation pressure and saturation temperature, the pressure of the refrigerant liquid is reduced, thereby reducing the temperature of the refrigerant liquid. The high-pressure and normal-temperature refrigerant liquid passes through the throttling element of the decompression device to obtain a low-temperature and low-pressure refrigerant, which is sent to the evaporator to absorb heat and evaporate. In daily life, capillary tubes are often used as throttling elements for refrigerators and air conditioners.
The evaporator is also a heat exchange device. The low-temperature and low-pressure refrigerant liquid evaporates (boils) into steam after being throttled, absorbs the heat of the material to be cooled, reduces the temperature of the material, and achieves the purpose of freezing food. In the air conditioner, the surrounding air is cooled to cool and dehumidify the air. The lower the evaporation temperature of the refrigerant in the evaporator, the lower the temperature of the cooling substance. In the refrigerator, the evaporation temperature of the general refrigerant is adjusted to -26°C to -20°C, and in the air conditioner, it is adjusted to 5°C to -8°C.
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transport refrigeration systems
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