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scroll for refrigeration LLZ for parallel applications
Application guide
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Application guide [zh]
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Danfoss scroll for refrigeration LLZ for parallel applications Application guide

Application guidelines
Danfoss scroll for refrigeration
LLZ for parallel applications
50 - 60 Hz - R404A - R507
http://cc.danfoss.com
Application Guidelines
Content
General overview .................................................................................................................................4
Benets .......................................................................................................................................................................................................... 4
Oil management concept ....................................................................................................................5
Operating conditions ..........................................................................................................................8
Refrigerants and lubricants .................................................................................................................................................................... 8
System design recommendations ......................................................................................................9
Essential piping design considerations ............................................................................................................................................. 9
Expansion device ..................................................................................................................................................................................... 10
Suction accumulator............................................................................................................................................................................... 10
Suction header .......................................................................................................................................................................................... 10
Oil level regulator .................................................................................................................................................................................... 11
Oil separator / oil reservoir ................................................................................................................................................................... 12
Oil separator / Oil reservoir for active system ............................................................................................................................... 12
Economizer for parallel vapour injection compressors ...........................................................................................................14
Refrigerant charge limits....................................................................................................................................................................... 14
Installation and service .....................................................................................................................15
Piping design ............................................................................................................................................................................................. 15
Wiring and rotation direction ............................................................................................................................................................. 15
Failure analysis .......................................................................................................................................................................................... 15
Sound and vibration management ..................................................................................................16
Running sound level ............................................................................................................................................................................... 16
Ordering information ........................................................................................................................17
Parallel units LLZ013-15-18-24 ..........................................................................................................18
Composition of LLZ013-024 uneven tandem/trio/quadro ......................................................................................................18
Composition of LLZ013-024 even tandem ..................................................................................................................................... 18
Compressor mounting ........................................................................................................................................................................... 19
Suction and discharge connection ................................................................................................................................................... 20
Oil equalisation connection................................................................................................................................................................. 20
3FRCC.PC.032.A4.02
Application Guidelines
General overview
Scope
Benets
The application guidelines describe the operating characteristics, design features and application requirements for LLZ parallel compressors in low temperature refrigeration applications. They apply to both standard and economized compressors.
To ensure proper parallel installation and running conditions, the following recommendations must be followed:
Parallel compressor installation' refers to a system of interconnected compressors with a common suction line and common discharge line. The technique of mounting compressors in parallel is also referred to as manifolding. In a system with only two compressors, this is referred to as a tandem conguration.
The main reason for manifolding is reduced operating cost through greater control of capacity and power consumption. This is achieved by staggering the compressor switch­on sequences that allow the parallel system to match its power with the capacity needed.
• It is essential to follow all the instructions given in these guidelines, the instruction leaet delivered with each compressor and the Selection & Application Guidelines for single compressors.
• For additional system components related to specic application requirements, the supplier's recommendations must always be followed.
A second reason for manifolding is improved part load eciency. In a parallel installation the individual compressor(s) can be switched o while the other compressor(s) keep operating at 100% load. Therefore the part load eciency is very near the full load eciency. Conventional xed-speed compressor unloading methods impose a serious penalty on part load eciency, mainly at low load conditions.
4 FRCC.PC.032.A4.02
Application Guidelines
Oil management concept
Active systems
Suction gas in a hermetic scroll compressor ows via the oil sump,which makes it more dicult to maintain equal pressure in the sumps of parallel compressors. Since oil equalisation usually depends on equal sump pressures, this is a point of special attention. Danfoss Commercial Compressors have developed specially adapted oil management systems which ensure proper
Suction header
Non return valve
Compressor 1
Oil level regulator Oil level regulator Oil level regulator Oil level regulator
Non return valve
Compressor 2 Compressor 3 Compressor 4
oil balancing between the compressors, but it is always recommended to carry out some tests to validate oil balancing in the system.
To ensure suitable oil distribution, both passive and active types of systems are introduced into LLZ compressors.
High pressure
Low pressure
Non return valve
Non return valve
Oil lter
Non return valve
Combined oil separator/reservoir
An active system can oer more exible and ecient oil management. It is highly recommended for manifolding since this positive system increases the reliability of the manifolding conguration. Oil management will be secured mainly by the oil level regulator and the oil separator, which can supply the oil when required. The active system can thus accommodate itself to various oil conditions.
Danfoss has qualied tandem / trio / quadro composition for active systems.
For manifolding with more than two compressors, it is always suggested to use a
suction header. Each compressor will equip the oil level regulator to facilitate the oil level balance.
To avoid refrigerant back ow from high pressure, it is always recommended to have a non-return valve on the discharge line of each compressor, as well as one non-return valve on the outlet of the oil separator in the system to prevent refrigerant migration.
For more details on the oil separator/reservoir and oil level regulator please refer to the “System design recommendation” in this guideline.
5FRCC.PC.032.A4.02
Application Guidelines
Oil management concept
Passive systems
A passive system is an oil management system without any measurement or control devices such as oil level regulators and oil controllers. By contrast, a solution equipped with such measurement or control devices is called an active system.
Non return valve
Oil equalization
line
Compressor 1 Compressor 2
Suction header
Danfoss has qualied only the even tandem (two same compressors) for passive solutions.
To condenser
Non return valve
Oil separator with oating-ball valve
Oil lter
From evaporator
6 FRCC.PC.032.A4.02
Application Guidelines
Oil management concept
This is one of the simplest and cheapest ways of manifolding compressors. It is very popular in air­conditioning applications, but in refrigeration this kind of system needs to be paid special attention due to severe operating conditions.
Danfoss has qualied only the even tandem
(two same compressors) for passive solutions. Compressor sumps and low-pressure shells are interconnected. An interconnecting pipe on the lower part of the compressor (installed on the existing oil sight glass) ensures oil balancing. The suction header design is critical, as it ensures a pressure drop balancing and an equal distribution of oil returning from the system when both compressors are running.
The success of such a system relies very much on the sizing and design of the pipe work, as small dierences in sump pressure can result in signicant oil level variations. This system needs perfect suction tube balancing.
For an LLZ even tandem, oil return can be secured by an oil separator with oat ball valve, which will return the oil to the main suction line. The oil equalisation line goes through the oil sight glass with an adaptor on the standard compressor. To avoid refrigerant back ow from high pressure, it is always recommended to have a non-return valve on the discharge line of each compressor, as well as one non-return valve on the outlet of the oil separator in the system to prevent refrigerant migration.
The following are recommendations from Danfoss application engineering for connecting low temperature compressors using an oil equalisation line system without active control:
• An adequately (generously) sized suction header is needed to provide for equal distribution of returning refrigerant gas and oil to each individual compressor; also the suction header should be installed horizontally.
• To secure sucient oil return to the compressor, Danfoss suggests below dimension requirement of the suction header: H>5D.
D
Main suction line
Height
• The oil equalization tube is recommended to have an outer diameter of ½’’.
• If the unit runs in a very cold situation, both compressors need to be switched on after a period of single running for better oil circulation, especially in low load conditions.
• Care must be taken to mount all the compressors on the same horizontal level and also to provide adequate liquid ood back protection when using this method.
Danfoss could provide piping drawings for even tandem passive system, please contact Danfoss for more information.
7FRCC.PC.032.A4.02
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