24. June 2019

Scroll compressor for normal cooling

Scroll compressor for normal cooling

Scroll compressor for normal cooling

 

The compressor is the heart of every compression refrigeration cycle. It brings vapor refrigerant from a low pressure level (low pressure suction side) to a high level (high pressure side). Compressors are available with different operating principles. These are, for example, scroll compressors, screw compressors, rotary piston compressors, turbo compressors and reciprocating compressors. Today we focus on scroll compressors for refrigeration. Scroll compressors are very common in air conditioning applications - their classic application is the chiller. The scroll compressor is also gaining more and more market share especially in the heat pump sector. Scroll compressors are also ideal for normal cooling applications.

 

Layout and function

 

Danfoss scroll compressors have the crankshaft standing upright. Above the crankshaft is the scrollset. This scrollset consists of a fixed and an orbiting spiral. These two spirals engage and compress the refrigerant by an orbital motion from the outer part of the scrollset toward the center. Due to this principle, there are different stages of compaction at each stage of the compaction process (different sized "pockets" in which compression is taking place). Thus, compared to reciprocating compressors, smaller portions of refrigerant are ejected more frequently. This leads to lower pulsations. For the fitter this means that a muffler rarely has to be used for pulsation damping. Noise problems or malfunction of pressure switches caused by pulsations are not to be expected in systems with scroll compressors. During operation of the compressor, one of the two spirals is pressed against the other by means of a medium pressure from a "pocket" of the scroll set, in which the complete compaction has not yet been completed. As a result, the two spirals are "retracted", so to speak. This "break-in phase" is completed at the latest after 72 hours of operation. It is a flexible interaction of the two scroll screws while self-optimization of the interaction in practical operation. In this context, the installer should note that MLZ compressors may initially experience a certain under-performance during initial start-up.

 

Assembly and service

All Danfoss scroll compressors are 100% suction gas cooled. This means that it may be possible to mount a silencer hood because the compressor releases all the excess heat from the refrigerant that flows through it. The refrigeration connections for these compressors are arranged one above the other - suction side down (large connection) and pressure side up (small connection). Both are designed as solder sockets integrated directly into the compressor. Inside the "scroll compressor head" is a check valve, which avoids a reverse rotation of the scroll set after switching off the compressor. The assembly of the scroll compressors is done on rubber buffers. In general, one should bear in mind when first contacting with scroll compressors that in this type of compressor the head, ie the upper 20% of the compressor, Compression end temperature (hot gas temperature) has. This is not the case with fully hermetic reciprocating compressors. There are all housing sites on the compressor (with the exception of the pressure port) on the suction side and thus have no high temperatures. In particular, the compression end temperature is always an issue in scroll compressors. For example, "MLZ" scroll compressors, when operated at the usual evaporation temperatures of -10 ° C, will show no abnormality in terms of final compression temperature, which means that in the rarest of cases the value will rise above 100 ° C - for compressors, Bearing, scrollset and refrigeration oil are no problem. that such a compressor, for example, by a permanent strong throttling of an evaporative pressure regulator - or for other reasons - a long time with the suction pressure far below its application limits is driven, then it can quickly come to excessive pressure nozzle temperatures. For this reason, it is recommended to install or retrofit a compressed gas end temperature monitor for systems with scroll compressors, if this is not available. This task can be carried out by a simple mechanical thermostat with remote sensor (eg "KP 81"). It is not necessary that the temperature setting is extremely close to the operating point, 135 ° C maximum, 120 ° C is here a good practice value. then it can quickly come to excessive pressure nozzle temperatures. For this reason, it is recommended to install or retrofit a compressed gas end temperature monitor for systems with scroll compressors, if this is not available. This task can be carried out by a simple mechanical thermostat with remote sensor (eg "KP 81"). It is not necessary that the temperature setting is extremely close to the operating point, 135 ° C maximum, 120 ° C is here a good practice value. then it can quickly come to excessive pressure nozzle temperatures. For this reason, it is recommended to install or retrofit a compressed gas end temperature monitor for systems with scroll compressors, if this is not available. This task can be carried out by a simple mechanical thermostat with remote sensor (eg "KP 81"). It is not necessary that the temperature setting is extremely close to the operating point, 135 ° C maximum, 120 ° C is here a good practice value.

 

Multikältemitteltauglichkeit

"MLZ" Scroll Scroll compressors are approved for the standard refrigerants R404A, R507 and R134a. The application limits for R404A and R507 allow evaporation temperatures from -30 ° C (with limited condensing temperature) to +10 ° C with up to 60 ° C liquefaction. A condensation pressure reduction, as is often the case in modern refrigeration technology, is possible with evaporation from -30 to -10 ° C even up to +10 ° C. This value can not be achieved by many reciprocating compressors. "MLZ" compressors with R134a can be operated at evaporating temperatures between -15 and +15 ° C. Since condensing temperatures up to 74 ° C are possible, the "MLZ" is suitable for seasonal heat pump operation as well as for heat recovery.

 

R404A offers a higher volumetric cooling capacity than R134a. Thus, the compressor with R404A at the same evaporation temperature has a higher cooling capacity than the same model with R134a. These differences in the cooling capacities due to the refrigerant used are also commercially exploitable. An important advantage is the greater flexibility of the customer. If, for example, a new system is being built and the customer does not yet know for sure whether there will be an extension in the next few years, it is advisable to install a multi-refrigerant "MLZ" scroll compressor in operation with R134a It will be possible to add a lot of cooling capacity just by changing the refrigerant to R404A, without, however, replacing the scroll compressor. The customer benefits, because the refrigerant R134a is energetically very good and is R404A (R507) only in terms of universality after. For applications of commercial refrigeration, such as petrol station shops, etc., there are the "MLZ" compressor also already integrated in fully equipped condensing sets "Optyma Plus" with fan speed control, weatherproof housing, compressor contactor, emergency stop, dryer and sight glass. The time savings in on-site assembly is one of the main reasons why such ready-to-use condensing units are used.

 

lubrication

The compressors already include the required amount of oil in the scope of delivery. After a certain period of time after installation, a check of the oil level via the oil sight glass in the lower part of the compressor is recommended. The ideal oil level is halfway up the oil sight glass, but 1/4 to 3/4 can also be tolerated. In the lower housing area there is an oil drain port, with which oil can be drained without tilting the compressor. For this, it is sufficient to produce a slight overpressure on the suction side of the compressor and to drain the oil from the compressor via this connection and the service manometer. "MLZ" compressors are pre-filled with a PVE (polyvinyl ether) "oil". PVE has the advantage over conventional POE (polyol ester) lubricants that it does not chemically react with water and form acid. Hygroscopy is comparable to POE oil, but moisture in the system can also be easily removed or evacuated by this property. Also interesting is the compatibility with R22. This makes it possible to respond flexibly to export inquiries, such as those from Latin America, because R22 still dominates as a refrigerant there. To ensure optimum lubrication of the compressor internal parts and good oil return, the scroll compressor should not be started more than twelve times per hour and should be switched off for at least one minute after each stop. react flexibly from Latin America, where R22 still dominates as a refrigerant. To ensure optimum lubrication of the compressor internal parts and good oil return, the scroll compressor should not be started more than twelve times per hour and should be switched off for at least one minute after each stop. react flexibly from Latin America, where R22 still dominates as a refrigerant. To ensure optimum lubrication of the compressor internal parts and good oil return, the scroll compressor should not be started more than twelve times per hour and should be switched off for at least one minute after each stop.

 

Electrical connection

For outdoor installation or if low ambient temperatures at the compressor can not be excluded, a crankcase heater should be used. This should always be switched countercyclically to the compressor (compressor is running - crankcase heater off, compressor is on - crankcase heater on). The "MLZ" scroll compressors are commonly found on the market as three-phase 400V models. The electrical connection is relatively simple, since the compressors are internally already connected in the star point and no bridges are to be placed in the connection box. There are three connection pins to which the three phases coming from the contactor (or, ideally, motor protection in the control cabinet) are directly connected. It is important to note that the scroll compressor runs in the correct direction of rotation. If strong mechanical noises occur and the connected service manometer does not set the usual pressure difference between high and low pressure, the scroll compressor will most likely run in the wrong direction of rotation. Remedy can be created by exchanging two phases on the compressor terminal board. With the help of a voltage tester, it can be checked on the compressor terminal box whether the power supply is in order. The phase conductors (measured phase to phase) should always be about 400V. As an additional protection against overheating and against excessive current load, a bimetallic protection is incorporated in the star point of the windings. That is, in general, the internal motor protection triggers when a resistance measurement at the compressor (supply voltage before disconnect) a " infinite resistance "between all three pins is measured. As soon as the compressor has cooled down, the bimetallic protection switches on again. When the electric motor is ready for operation, the three measured resistance values ​​of the pins approach each other. Depending on the capacity of the compressor, the value is in the single-digit range of ohms. The speed of these compressors is at 50 Hz about 2900 U / min, since the electric motor is wound with a pair of poles. At 60 Hz, for example, the compressor would run correspondingly faster (about 3480 rpm) than at 50 Hz, because the rotor of the compressor is based on the corresponding mains frequency (Hz = 1 / s means that at 50 Hz alternating current 50 times in the Second the current direction is changed). the bimetallic protection switches on again. When the electric motor is ready for operation, the three measured resistance values ​​of the pins approach each other. Depending on the capacity of the compressor, the value is in the single-digit range of ohms. The speed of these compressors is at 50 Hz about 2900 U / min, since the electric motor is wound with a pair of poles. At 60 Hz, for example, the compressor would run correspondingly faster (about 3480 rpm) than at 50 Hz, because the rotor of the compressor is based on the corresponding mains frequency (Hz = 1 / s means that at 50 Hz alternating current 50 times in the Second the current direction is changed). the bimetallic protection switches on again. When the electric motor is ready for operation, the three measured resistance values ​​of the pins approach each other. Depending on the capacity of the compressor, the value is in the single-digit range of ohms. The speed of these compressors is at 50 Hz about 2900 U / min, since the electric motor is wound with a pair of poles. At 60 Hz, for example, the compressor would run correspondingly faster (about 3480 rpm) than at 50 Hz, because the rotor of the compressor is based on the corresponding mains frequency (Hz = 1 / s means that at 50 Hz alternating current 50 times in the Second the current direction is changed). The speed of these compressors is at 50 Hz about 2900 U / min, since the electric motor is wound with a pair of poles. At 60 Hz, for example, the compressor would run correspondingly faster (about 3480 rpm) than at 50 Hz, because the rotor of the compressor is based on the corresponding mains frequency (Hz = 1 / s means that at 50 Hz alternating current 50 times in the Second the current direction is changed). The speed of these compressors is at 50 Hz about 2900 U / min, since the electric motor is wound with a pair of poles. At 60 Hz, for example, the compressor would run correspondingly faster (about 3480 rpm) than at 50 Hz, because the rotor of the compressor is based on the corresponding mains frequency (Hz = 1 / s means that at 50 Hz alternating current 50 times in the Second the current direction is changed).

 

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