Application guidelines
Danfoss scroll for refrigeration
LLZ for parallel applications
50 - 60 Hz - R404A - R507
http://cc.danfoss.com
Application Guidelines |
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General overview.................................................................................................................................. |
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Scope............................................................................................................................................................................................................... |
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Benefits........................................................................................................................................................................................................... |
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Oil management concept..................................................................................................................... |
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Active systems............................................................................................................................................................................................. |
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Passive systems........................................................................................................................................................................................... |
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Operating conditions........................................................................................................................... |
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Power supply................................................................................................................................................................................................ |
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Compressor ambient temperature...................................................................................................................................................... |
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Operating envelope.................................................................................................................................................................................. |
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Refrigerants and lubricants.................................................................................................................................................................... |
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Discharge temperature protection..................................................................................................................................................... |
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High and low pressure protection....................................................................................................................................................... |
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Cycle rate limit............................................................................................................................................................................................. |
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System design recommendations....................................................................................................... |
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Essential piping design considerations............................................................................................................................................. |
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Expansion device...................................................................................................................................................................................... |
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Suction accumulator............................................................................................................................................................................... |
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Suction header.......................................................................................................................................................................................... |
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Oil level regulator..................................................................................................................................................................................... |
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Oil separator / oil reservoir................................................................................................................................................................... |
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Oil separator / Oil reservoir for active system............................................................................................................................... |
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Economizer for parallel vapour injection compressors ............................................................................................................ |
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Refrigerant charge limits....................................................................................................................................................................... |
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Installation and service...................................................................................................................... |
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Piping design............................................................................................................................................................................................. |
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Wiring and rotation direction.............................................................................................................................................................. |
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Failure analysis........................................................................................................................................................................................... |
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Sound and vibration management................................................................................................... |
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Running sound level................................................................................................................................................................................ |
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Ordering information......................................................................................................................... |
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Parallel units LLZ013-15-18-24.......................................................................................................... |
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Composition of LLZ013-024 uneven tandem/trio/quadro....................................................................................................... |
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Composition of LLZ013-024 even tandem..................................................................................................................................... |
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Compressor mounting........................................................................................................................................................................... |
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Suction and discharge connection.................................................................................................................................................... |
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Oil equalisation connection................................................................................................................................................................. |
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FRCC.PC.032.A4.02 3
Application Guidelines |
General overview |
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Scope |
The application guidelines describe the operating |
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characteristics, design features and application |
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requirements for LLZ parallel compressors in |
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low temperature refrigeration applications. |
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They apply to both standard and economized |
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compressors. |
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To ensure proper parallel installation and running |
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conditions, the following recommendations must |
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be followed: |
•It is essential to follow all the instructions given in these guidelines, the instruction leaflet delivered with each compressor and the Selection & Application Guidelines for single compressors.
•For additional system components related to specific application requirements, the supplier's recommendations must always be followed.
Benefits |
Parallel compressor installation' refers to a system |
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of interconnected compressors with a common |
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suction line and common discharge line. The |
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technique of mounting compressors in parallel |
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is also referred to as manifolding. In a system |
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with only two compressors, this is referred to as a |
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tandem configuration. |
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The main reason for manifolding is reduced |
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operating cost through greater control of |
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capacity and power consumption. This is |
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achieved by staggering the compressor switch- |
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on sequences that allow the parallel system to |
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match its power with the capacity needed. |
A second reason for manifolding is improved part load efficiency. In a parallel installation the individual compressor(s) can be switched off while the other compressor(s) keep operating at 100% load. Therefore the part load efficiency is very near the full load efficiency. Conventional fixed-speed compressor unloading methods impose a serious penalty on part load efficiency, mainly at low load conditions.
4 FRCC.PC.032.A4.02
Application Guidelines |
Oil management concept |
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Suction gas in a hermetic scroll compressor flows via the oil sump,which makes it more difficult 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
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.
Active systems
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High pressure |
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Low pressure |
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Non return |
Non return |
Non return |
Non return |
Non return |
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valve |
valve |
valve |
valve |
valve |
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Suction |
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header |
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Compressor 1 |
Compressor 2 |
Compressor 3 |
Compressor 4 |
Combined oil |
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separator/reservoir |
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Oil level regulator |
Oil level regulator |
Oil level regulator |
Oil level regulator |
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Oil lter |
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An active system can offer more flexible and efficient oil management. It is highly recommended for manifolding since this
positive system increases the reliability of the manifolding configuration. 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 qualified 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 flow 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.
FRCC.PC.032.A4.02 5
Application Guidelines |
Oil management concept |
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Passive systems |
A passive system is an oil management system |
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Danfoss has qualified only the even tandem |
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without any measurement or control devices |
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(two same compressors) for passive solutions. |
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such as oil level regulators and oil controllers. |
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By contrast, a solution equipped with such |
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measurement or control devices is called an |
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active system. |
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To condenser |
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Non return valve |
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Oil separator with |
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Non return valve |
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oating-ball valve |
Oil equalization |
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line |
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Compressor 1 |
Compressor 2 |
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Oil lter |
Suction header |
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From evaporator |
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6 FRCC.PC.032.A4.02
Application Guidelines |
Oil management concept |
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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.
• 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.
Danfoss has qualified only the even tandem |
• To secure sufficient oil return to the compressor, |
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(two same compressors) for passive solutions. |
Danfoss suggests below dimension requirement |
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Compressor sumps and low-pressure shells are |
of the suction header: H>5D. |
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interconnected. An interconnecting pipe on the |
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lower part of the compressor (installed on the |
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existing oil sight glass) ensures oil balancing. |
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The suction header design is critical, as it |
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ensures a pressure drop balancing and an equal |
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distribution of oil returning from the system |
Main suction |
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when both compressors are running. |
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line |
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The success of such a system relies very much |
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on the sizing and design of the pipe work, as |
Height |
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small differences in sump pressure can result in |
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significant oil level variations. This system needs |
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perfect suction tube balancing. |
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For an LLZ even tandem, oil return can be |
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secured by an oil separator with float ball valve, |
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which will return the oil to the main suction |
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line. The oil equalisation line goes through the |
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oil sight glass with an adaptor on the standard |
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compressor. To avoid refrigerant back flow from |
• The oil equalization tube is recommended to |
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high pressure, it is always recommended to have |
have an outer diameter of ½’’. |
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a non-return valve on the discharge line of each |
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compressor, as well as one non-return valve on |
• If the unit runs in a very cold situation, both |
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the outlet of the oil separator in the system to |
compressors need to be switched on after a |
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prevent refrigerant migration. |
period of single running for better oil circulation, |
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especially in low load conditions. |
The following are recommendations from |
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Danfoss application engineering for connecting |
• Care must be taken to mount all the |
low temperature compressors using an oil |
compressors on the same horizontal level and |
equalisation line system without active control: |
also to provide adequate liquid flood back |
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protection when using this method. |
Danfoss could provide piping drawings for even tandem passive system, please contact Danfoss for more information.
FRCC.PC.032.A4.02 7