Scroll Rack Application Guidelines - Emerson Copeland Scroll Introduction Manual

Scroll Rack Application Guidelines

1. Introduction
Parallel operation is when several compressors are operating on one common refrigeration system. These installations require a
special design to achieve a maximum possible operating capacity and reliability. Only the system design engineer can make the
decision which configuration of installation is to be used in view of the particular requirements. The concept of scroll compressors
for refrigeration in parallel operation brings several benefits:
• Efficient capacity control
With several compressors in parallel, if one of the compressors is turned off, one can achieve a simple method of capacity control
combined with a maximum possible saving of energy. If the load or ambient conditions change, compressors can be switched on
and off to match capacity requirements. This method of capacity control provides the advantage that the application limitations
are not altered as when the plant is equipped with capacity control. The use of unequal compressors allows many capacity
possibilities.
• Standby operation
If one compressor cannot run for any reason, the load can often be met by the remaining compressors. If the stoppage is caused
by a compressor breakdown, the other compressors could be damaged as well. One has to take immediate action to protect the
whole installation if acid / dirt is generated. The cause has to be detected in order to be able to decide on the urgency of repair.
This is especially required on plants having oil and refrigerant vapour pressure equalization lines, and on plants having no oil level
regulating system with filters in the suction and oil return lines.
• Changing compressors
If a compressor needs to be changed for any reason the small lightweight nature of the scroll makes this a much less costly
operation and causes much less disruption. This is particularly true for the smaller models.
• Matching several evaporating conditions
If there are two or more temperatures to be controlled individual compressors can be matched to each evaporator while working
on a single condenser circuit. This will give power savings compared with an alternative system working at the lowest suction
pressure.
Scroll compressors have no positive displacement oil pump and therefore no oil pressure switch. The high performance Teflon®
bearings provide protection for short periods in case of oil losses. However sustained running at high-pressure differences without
lubricant will fail the bearings. In order to ensure adequate protection it is necessary to observe the following guidelines.
2. Parallel application considerations
If compressors are required to operate in parallel it is normally necessary to stop and start them independently for capacity control
purposes. There are three major considerations in this type of application:
• Oil return
With all capacity controlled refrigeration plant the oil circulation has to be perfectly correct especially under part-load
operation. This means that the maximum possible capacity reduction will be limited by the means of guaranteeing the correct
oil circulation. An adequate oil level at all times is necessary to maintain lubrication of the bearings. Too much oil can result in
inefficient operation and excessive oil carry over to the system.
• Tubing stress
If compressors are mounted close together care must be taken to ensure that there is sufficient flexibility in the tubing. If this
is not done the starting pulse may result in excess stress directed at the mounting positions and may give rise to a leak. Tube
resonance should be avoided.
• Running sequence
A certain logical sequence control is recommended so that each compressor runs for the same amount of time.
34