The scroll compressor
Refrigeration - Introduction to the basics - The scroll compressor
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 e.g. scroll compressors, screw compressors, rotary piston compressors, turbo compressors and reciprocating compressors. Today we want to focus on the scroll compressors. Scroll compressors are very common in air conditioning applications - their classic application is the chiller. Especially in the heat pump sector, the scroll compressor is increasingly conquering market share. Generally there are scroll compressors but also for normal cooling
Layout and function
On Danfoss scroll compressors, the crankshaft is 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. Through this principle, there are various 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 often. This leads to lower pulsations. For the mechanic, this means that less often a muffler 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. Danfoss scroll compressors have two different methods. In the devices painted in blue, the radial sealing (sealing of the scroll flanks with respect to each other) is achieved solely by the oil film, while flexible metallic seals are used for the axial sealing. The two spirals are not pressed against each other, but run completely stationary, even in the compression mode. The situation is different with the black-painted Danfoss scroll compressors. These work according to the "compliance principle". This means that one of the two spirals is pressed against the other by means of medium pressure from one "pocket" of the scroll set, where 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. If one searches for the term "compliance" in a technical dictionary, one finds "translation" as "translation", while the term is translated in a general dictionary with "agreement". In fact, the truth is probably in between. It is a flexible interplay of the two scroll screws while self-optimization of the interaction in practical operation. The installer should note in this context that for these black compliance compressors (compressor type designation always begins with "H" - for example "HRP" etc.), a certain under-performance may initially occur during initial start-up. This point is usually not really noticeable in normal operation, but is very important for power measurements on test benches. The blue scroll compressors call from the first minute on their full cooling capacity.
Assembly and service
These compressors already include the required amount of oil in the scope of delivery. After a certain period of time after installation, it is advisable to check the oil level via the oil sight glass (in the case of the standard blue scroll compressors) in the lower part of the compressor. The ideal oil level is half the height of the oil sight glass, but 1/4 to 3/4 can also be tolerated. All Danfoss scroll compressors are 100% suction gas cooled. This means that a silencer hood may be fitted where necessary, as the compressor releases all excess heat from the refrigerant flowing through it. The refrigeration connections for these compressors are arranged one above the other - suction side at the bottom (large connection) and pressure side at the top (small connection). Both are designed as Rotolock screw connections or as solder sockets integrated directly in the compressor. For initial installation, Rotolock valves should be used - at least for Rotolock threaded fittings - as these greatly simplify service interventions on the compressor or the refrigeration system and provide the ability to easily install a high and low pressure switch. In this context, it should be noted that the connection closest to the spindle of a Rotolock valve can be shut off (connection option for the service manometer). The other connection can be used for a pressure switch (connection can not be locked). In addition, "SZ" or "SM" scrolls on the compressor housing typically provide three additional, smaller connectivity options: a low pressure port, which is typically not used, and an oil overflow port, which is only needed in compound operation. In combined operation, a 10-gauge copper pipe is attached to the oil overflow as an oil balance to the sister compressor and the suction line to the composite compressors as symmetrical as possible. Additional check valves in the individual pressure lines can be dispensed with, since there is already a non-return valve inside this scroll compressor. A special highlight is the third small connection. If the NPT plug of the oil drain port is replaced with a suitable nipple with a 7/16 UNF gauge port, oil change can be performed without tilting the compressor. For this purpose, 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. This is made possible by a small copper tube inside the compressor which, starting from this connection, runs down into the compressor sump. The assembly of the scroll compressors takes place on rubber buffers. In general, when first contacting with scroll compressors, it should be kept in mind that in this type of compressor, the head, ie the upper 20% of the compressor, has a compression end temperature (hot gas temperature). 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. Especially the compression end temperature is always an issue in scroll compressors. For example, Scroll compressors for air conditioning or heat pump use, if they are operated at the usual evaporation temperatures of 0 or 10 ° C, show no abnormalities in the compression end temperature. This means that the value will seldom rise above 100 ° C - no problem for compressors, bearings, scrollset and refrigeration oil. However, if it happens that such a compressor is driven far below its application limits for a long time with the suction pressure, for example due to a permanent strong throttling of an evaporating pressure regulator - or for other reasons, then excessively high pressure nozzle temperatures can occur. For this reason, it is a very good idea to install or even retrofit a Druckgasendtemperaturüberwachung in systems with scroll compressors, if this is not available. This task can be done by a simple mechanical thermostat with remote sensor (eg "KP 81"). It is not necessary that the temperature setting be extremely close to the operating point, 135 ° C maximum, 120 ° C is here a good practice value.
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 anti-cyclic to the compressor (compressor is running - crankcase heater off, compressor is on - crankcase heater on). The blue scroll compressors are commonly found as three-phase 400 V models in our market. The electrical connection is relatively simple, as the compressors are already internally connected at the neutral point and no jumpers have to be placed in the connection box. There are three connection pins to which the three phases coming from the contactor (or, ideally, the motor protection in the control cabinet) are directly connected. It is now very important that the scroll compressor operates in the correct direction of rotation during operation. 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 everything is in order with the power supply. The phase conductors (measured phase to phase) should always be about 400V. As additional protection against overtemperature and against excessive current load, there are versions of these blue compressors (size "SZ / SM84-110" and "120"), which have a bimetallic protection in the neutral point of the windings. In other words, it can be assumed that the internal motor protection has tripped, if an "infinite resistance" is measured between all three pins during a resistance measurement at the compressor (disconnect supply voltage beforehand). As soon as the compressor has cooled down, the bimetallic protection switches on again. If the electric motor is ready for operation, the three measured resistance values of the pins are approximately equal to one another. The value is between 0.4 and a single-digit ohm value, depending on the capacity of the compressor. In the case of internal motor protection, however, caution is required. For other compressor sizes, such as "SZ / SM115" and "125-185", no internal motor protection is incorporated in the neutral point of the winding. These variants have only one extra bimetallic relFdivease, which switches off at internal overtemperature in the compressor. This bimetallic release is potential-free and must be integrated in the safety chain upstream of the compressor contactor (before "A1"). For the blue Performer compressors, there is even a third engine protection variant. The power ratings "SZ240", "300" and "380" have a "Kriwan INT69" thermistor motor full protection. The contacts "M1" and "M2" on the "INT69" are potential-free and must be integrated into the safety chain upstream of the compressor contactor (before "A1"). This is similar to the connection procedure in the case of the bimetallic release. The "INT69" itself requires a supply voltage of 230 V or 24 V - depending on the version. This information is important for the installer, as not only the usual load cable with the three phases and the protective conductor must be brought to the compressor, but also the "neutral" for the supply voltage of the motor protection module and two wires of the safety chain of the compressor contactor. When the scroll compressor is in operation, the motor full protection permanently checks whether the temperatures in the compressor are OK via thermistors incorporated in the compressor. In the event of overtemperature, it opens the contact between M1 and M2 and thus removes the compressor contactor. The speed of this compressor 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 line frequency (Hz = 1 / s means that 50 Hz alternating current 50 times in the second the current direction is changed).
The content is based on the booklet series of the same name by Danfoss (www.danfoss.de/kaelte), which deals with the basic relationships in compression refrigeration systems and the associated basic components. The series is aimed at refrigeration engineers in service and plant construction, at new entrants in refrigeration technology, at apprentices to the refrigeration plant manufacturer and at all those who would like to gradually incorporate the practice-oriented basic knowledge of the cold again.