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
Scope...............................................................................................................................................................................................................	4
Benefits...........................................................................................................................................................................................................	4
Oil management concept.....................................................................................................................	5
Active systems.............................................................................................................................................................................................	5
Passive systems...........................................................................................................................................................................................	6
Operating conditions...........................................................................................................................	8
Power supply................................................................................................................................................................................................	8
Compressor ambient temperature......................................................................................................................................................	8
Operating envelope..................................................................................................................................................................................	8
Refrigerants and lubricants....................................................................................................................................................................	8
Discharge temperature protection.....................................................................................................................................................	8
High and low pressure protection.......................................................................................................................................................	8
Cycle rate limit.............................................................................................................................................................................................	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

FRCC.PC.032.A4.02 3

Application Guidelines	General overview

Scope	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:

•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
	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 configuration.
	The main reason for manifolding is reduced
	operating cost through greater control of
	capacity and power consumption. This is
	achieved by staggering the compressor switch-
	on sequences that allow the parallel system to
	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

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

					High pressure
					Low pressure
	Non return	Non return	Non return	Non return	Non return
	valve	valve	valve	valve	valve
	Suction
	header
	Compressor 1	Compressor 2	Compressor 3	Compressor 4	Combined oil
					separator/reservoir
	Oil level regulator	Oil level regulator	Oil level regulator	Oil level regulator
				Oil lter

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
Passive systems	A passive system is an oil management system					Danfoss has qualified only the even tandem
	without any measurement or control devices					(two same compressors) for passive solutions.
	such as oil level regulators and oil controllers.
	By contrast, a solution equipped with such
	measurement or control devices is called an
	active system.
										To condenser
										Non return valve
										Oil separator with
				Non return valve
										oating-ball valve

Oil equalization
line
Compressor 1	Compressor 2
	Oil lter
Suction header
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.

• 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,
	(two same compressors) for passive solutions.	Danfoss suggests below dimension requirement
	Compressor sumps and low-pressure shells are	of the suction header: H>5D.
	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	D
	ensures a pressure drop balancing and an equal
	distribution of oil returning from the system	Main suction
	when both compressors are running.
		line
	The success of such a system relies very much
	on the sizing and design of the pipe work, as	Height
	small differences in sump pressure can result in
	significant 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 float 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 flow from	• The oil equalization tube is recommended to
	high pressure, it is always recommended to have	have an outer diameter of ½’’.
	a non-return valve on the discharge line of each
	compressor, as well as one non-return valve on	• If the unit runs in a very cold situation, both
	the outlet of the oil separator in the system to	compressors need to be switched on after a
	prevent refrigerant migration.	period of single running for better oil circulation,
		especially in low load conditions.

The following are recommendations from
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
	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