TABLE OF CONTENTS
Any industrial plant or commercial area that require refrigeration will have, with a few minor exceptions, one or more compressors installed. These devices are one of the cornerstones of any refrigeration plant, as they generate the necessary work to move the circuit’s gas from a lower-pressure zone to a higher-pressure one, thus creating two differentiated areas, but how exactly do they work?
Generating cold consists, in simple terms, in absorbing a room’s heat to reduce its temperature and, subsequently, evacuating it. To achieve this, we usually resort to refrigerants, fluids that are circulated through a closed circuit to create high and low pressure zones, which are separated by the expansion valve. There, the saturated (or subcooled) refrigerant loses its pressure, causing an isoenthalpic pressure drop that provokes that the refrigerant become a two-phase flow that consists of liquid and vapour (80% and 20%, approximately). Finally, once the mixture enters the evaporator, it absorbs heat energy from the surroundings.
In the case of those machines that base their functioning on the refrigerants’ change of state, the compressor must increase their pressure so that outside temperature is enough to trigger this change.
Once this is understood, it is possible to realise that compressors are crucial for any industrial or commercial refrigeration system. In particular, its maintenance is essential, among other things, to:
Consequently, it is not advisable to opt for a corrective kind of maintenance in cases in which the installation must be available most of the time and/or in which this equipment’s failure could put production and/or personal safety at risk. In these contexts, the organisation should consider taking preventive or predictive maintenance actions, although the latter is the most common one among Industry 5.0 companies. Thanks to the latter, not only do they manage to anticipate possible errors, but they also have the capability to know the state of their installation at all times, which helps to design much more effective maintenance plans.
We could mention numerous techniques aimed at monitoring the variables that have a major impact on the compressor’s performance; however, experts recommend the following maintenance tasks.
Predictive maintenance vibration analysis is a type of action that can be applied to multiple components. However, it has proven to be especially useful to analyse the compressors’ status, as they are part of a closed circuit and this kind of test allows us to know, without accessing the inside of the circuit, if the play between pieces has increased, if there is any component that has loosened, if the system has been overstressed, etc.
Moreover, predictive maintenance vibration analysis takes into account those characteristics of the machinery that may affect their vibration (such as rotational speed, type of support or type of gear) to detect anomalies and diagnose them. Vibrometers (the devices that indicate the vibrations’ value) can be used to identify them, but, nowadays, sensors can be programmed to collect this same information in real time and on a continuous basis. This provides more reliable data, as it is always extracted from the same point, the sensors are always calibrated and there is no human intervention.
Different methods can be used for this purpose, such as applying a fast transformation algorithm, creating a digital reconstruction of the signal (thanks to the installation of accelerometers) or calculating the expected vibration spectrum. However, the most common ones are listed below.
Noise analysis is particularly useful for industrial plants, since, like vibration analysis, it allows to know the machinery’s state without halting production and avoiding the manipulation of the devices. Ultrasound analysis makes it possible to detect noises that may be generated both by the compressor or by its components, which may be caused by leaks, problems in the electrical circuit or by mechanical malfunctions, among others.
These systems combine machine learning, IoT, algorithms and big data analysis to obtain information remotely and in real time. Therefore, sensors must be installed in the machinery in order to study the sound waves generated as a result of the compressor’s operation, which have certain characteristics. Then, the monitoring systems record these ultrasounds and, when their acoustic characteristics change, this variation is analysed to identify the potential problems that may be behind it.
In combination with vibration analysis, this system allows technicians to carry out a highly effective predictive maintenance on equipment that, due to its function and/or price, should not be manipulated and/or cannot be stopped frequently.
Generally speaking, the analysis of lubricants is aimed at ensuring that they be in good condition, which has a direct impact on the product’s own lifespan and, therefore, on costs savings and on the company’s environmental footprint. Indirectly, however, this kind of maintenance also helps to preserve the compressor’s components.
Some of the variables that are measured are: the lubricant’s status and the presence of pollutants and of contamination produced by worn components. This is possible thanks to the assessing of parameters such as the presence of water and nonferrous particles, the dielectric constant, the viscosity, etc.
Some of the most common techniques for assessing these parameters are listed below.
These predictive maintenance techniques can also be applied to some of the compressors’ most important components, such as the electric motor that powers them or their bearings. For this reason, now we will offer an overview of industrial electric motor preventive maintenance and bearing maintenance.
These devices, which are used to convert electrical energy into mechanical power, have many components (spools, poles, switches, etc.), so there are numerous tests available to assess their functioning.
The functioning of the bearings is subject to the effect of multiple variables. The most important of these is mechanical stress, which means that proper lubrication is key, as we have explained above. However, there are other factors that must be also taken into account to ensure their effectiveness.
Firstly, they should be inspected to make sure there are no crackings. These can be provoked by stress, corrosion or wear, so it is advisable to manufacture the bearing-race from stress free materials. In addition, their coating should be improved to resist spallation and be non-porous.
Although it may seem obvious, it is also important to check that the shafts are properly aligned. Alignment faults can be detected by noise and vibration analyses, as well as by monitoring energy consumption.
Carrying out heat measurements, cleaning the bearings and monitoring the lubrication are also extremely advisable. Furthermore, it is important to look for evidence of electrical discharge machining, which are microscopic wholes caused by the discharge of shaft voltage through the bearings. Finally, a microscope can be used to identify grey lines around the bearing, so that it is possible to know whether these are provoked by mechanical processes or by electrical discharge machining.