Ref Manual Servicing Technicians Unit 4 (Servicing Practices)

Technicians have the important role of making the operation of RAC systems the most energy efficient and decreasing refrigerant emissions. This can only be achieved by the adoption of good practices. The section starts with the evaluation of the problems due to the presence of moisture and contaminants in the system, and how to avoid them through purging and evacuation, charging, and leak testing. The important measurement instruments and tools necessary to achieve good servicing practices are included.

A technician should be should be able to achieve the following servicing operations:

•      Leak detection, purging and evacuation.

•      Charging refrigerants.

•      Identifying the servicing-specific tubing tools and techniques of sizing, un-rolling, cutting, bending, flaring, swaging, piercing, pinching and welding.

•      Identifying the proper use of servicing instruments: manifold gauges, charging scales, and thermometers.

Moisture

Moisture causes several operating problems in RAC systems and understanding the basis of these problems is important. Basically, moisture can be classified as visible and invisible. “Visible” moisture is high concentration of water and can be seen with the eye, and is in liquid form. Occasionally, liquid water is found in systems but this is rather unusual. “Invisible” moisture is water in low concentrations and cannot be seen with the eye, since it is essentially dissolved in the refrigerant. It is important to remember that moisture can easily get into a system but it is difficult to get out.

The main situations and activities that can result in moisture entering the system are:

•      when there is a leak in part of the system that experiences sub-atmospheric pressure (thus, drawing air into the system)

•      during servicing and repair when the system is opened and exposed to the air and evacuation is not carried out properly

•      when filters or lubricant are exchanged

•      during charging with refrigerant and transfer hoses have not been purged properly.

The technician should be particularly aware of the possible introduction of moisture when carrying out these activities.

There will always be some moisture within any refrigerating system, and complete elimination is almost impossible. However, at very low concentrations the moisture is unlikely to cause any significant problem. On the other hand, if the moisture is present in higher concentrations, then a series of problems can arise. The “tolerable” concentration of moisture differs between refrigerants, oil types,

operating temperatures and compressor designs. Nevertheless, in almost all systems – except for ammonia systems – the maximum concentration of moisture should not exceed around 10 ppm.

Above these concentrations, the moisture can have negative effects on the system, such as reactions with the oil, causing the unit to malfunction and accelerating burn-out of hermetic compressors.

Recognising the presence of moisture

The presence of moisture within the system can be recognised by certain observations:

•      The system will stop due to low suction pressure, and proceed to warm up. Since it is the formation of solid ice within the expansion device that has caused the blockage, the warming will result in the disappearance of the ice and thus the unit will work properly again. However, the process will occur again as the ice once again forms at the expansion device.

•       Decreasing pressure, where the suction pressure steadily decreases over several hours – even to a vacuum. Then pressure suddenly becomes normal again. This abnormal cycle will keep repeating.

•      If, during system shutdown, one warms the refrigerant control with a safe resistance heater (hot pad) or radiant heat bulb, the ice will melt. Should the system then begin to work properly, there is definitely moisture in the refrigerant.

These symptoms are explained by the following:

•      The solubility of water in refrigerant reduces with temperature, so as the refrigerant passes through the expansion device, the dissolved moisture may become saturated water.

 

•      If the temperature reaches below 0°C, the saturated moisture

(water) within the expansion device can freeze and subsequently stop the refrigerant flow.

•      As the expansion valve warms, due to the lack of refrigerant, the ice melts and moisture returns to the expansion valve and once more generates an intermittent cooling.

•      Whether or not freezing actually occurs depends primarily upon the amount of moisture (water) and size of the ice particles formed.

Be aware of the risk of corrosion and its impacts

In addition to possible freezing, another serious problem – namely corrosion – can occur within the system due to the presence of moisture. Corrosion can create serious problems because often its effects are not apparent until serious damage has occurred. For example, moisture in the form of water alone can cause rust after a period of time. However, moisture plus the refrigerant create much more corrosion problems. Refrigerant such as R12 containing chlorine will slowly hydrolyse with water and form hydrochloric acids. This acid greatly increases the corrosion of metals.

The corrosion processes can be characterised as follows:

•      Heat increases the rate of corrosion due to acids because at higher temperatures the acid-forming process is accelerated. This acid, of course, attacks all the materials it contacts, the rate of corrosion of the individual materials being determined by their corrosion-resistant qualities. Steel will generally corrode at lower moisture levels than copper or brass.

•      Compressor lubricant presents another problem caused by moisture, particularly in the case of polyol ester (POE) and poly alkyl glycol (PAG) lubricants, used with hydrofluorocarbon (HFC) refrigerants. In fact, these types of lubricant have an affinity for moisture and will absorb it rapidly if left open to the atmosphere. Mineral lubricants do not mix with water in the same range as polyol ester lubricants.

•      Water changed into acid emulsifies with lubricants, the two forming an intimate mixture of exceedingly fine globules. This effect is called “sludging” of the oil and greatly reduces its lubricating ability. Corrosion becomes troublesome from the operating standpoint when the metallic surface is eaten away and a solid, detachable product is formed. This formation is commonly known as “sludge”. Sludge exists as slimy liquids, fine powders, granular solids or sticky solids and causes a variety of problems. They can plug fine strainers, expansion valves and capillary tubes. And because they usually contain acids they corrode whatever they cling to, accelerating damage.

Eliminating moisture problems

To eliminate moisture problems it is necessary to take precautions and actions, which will ensure a moisture-free system. It is important to change the filter drier frequently. The most effective way to eliminate moisture from a system is through the use of a high vacuum pump to create a vacuum deep enough to evaporate and remove this moisture. The recommended level of evacuation is of 1 millibar absolute (100 Pa) to achieve the evacuation of moisture. This level of vacuum must be maintained for 10 minutes without the help of a vacuum pump.

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Non-condensables

Gases in the system which do not liquefy in the condenser are contaminants and reduce the cooling capacity and system efficiency. The quantity of non-condensable gas that is harmful depends on the design and size of the refrigeration system and nature of the refrigerant. Their presence contributes to higher than normal discharge pressures and resultant higher discharge temperature. Higher temperatures speed up undesirable chemical reactions. Gases found in hermetic refrigeration units include nitrogen, oxygen, carbon dioxide (CO2, R744), carbon monoxide, methane and hydrogen.

These non-condensable gases infiltrate sealed systems in the following manner:

•      They are present during equipment manufacture or servicing and remain due to incomplete evacuation.

•      They are desorbed from various system materials or are formed by decomposition of gases at elevated temperatures during system operation.

•      They enter through low side (below atmospheric pressure) leakage points.

•      They are formed from chemical reactions between refrigerants, lubricants and other materials during operation. Chemically reactive gases, such as hydrogen chloride, attack other components in the refrigerating system; in extreme cases, the refrigerating unit fails.

•      They are introduced when connecting refrigerant hoses that have not been properly purged.

Whilst designing, installing and servicing systems, technicians should be aware of these routes to contamination, and adjust their behaviour accordingly.

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Servicing RAC systems

The main concept and procedures of each operation is the same for all refrigeration and air-conditioning systems. They differ only in each system specific connectivity requirements or tools to be used.

Most of servicing activities to RAC systems –dealing with refrigerants – falls within the one of the following main operations:

TEMPERATURE [ °C]

Evacuation

A refrigerating system must contain only the refrigerant in liquid or vapour state along with dry oil. All other vapours, gases, and fluids must be removed. Connecting the system to a vacuum pump and allowing the pump to run continuously for some time while a deep vacuum is drawn on the system can best remove these substances. It is sometimes necessary to warm the parts to around +50°C while under a high vacuum; in order to accelerate the removal of all unwanted moisture, heat the parts using warm air, heat lamps, or water. Never use a brazing torch. If any part of the system is below 0°C, the moisture may freeze and it will take a considerably longer time for the ice to sublimate to vapour during the evacuation process.

The equipment necessary to carry out the evacuation is:

•      vacuum pump

•      manifold gauges

two servicing valves (in the case system is not equipped with servicing valves)

•      vacuum gauge.

It is essential to know that conventional manifold gauges have low sensitivity, particularly at lower pressures. As such, they are ineffective at determining whether or not a sufficient vacuum has been achieved. Therefore it is essential to ensure that a proper vacuum gauge (such as a Pirani gauge) is used.

To understand why system evacuation is very important for moisture elimination, it is useful to remember the concept of vacuum and the relationship between boiling temperature and pressure. For a pure substance, like water, the boiling temperature for a fixed pressure

is called saturation temperature at this pressure, and the pressure at which the water evaporates at a fixed temperature is called saturation pressure at this temperature.

The relationship between these two thermodynamic properties (a natural law) is presented in the figure for water:

It can be seen in this figure that as the pressure reduces, the boiling temperature will be lower. If one wants to remove moisture in vapour phase from a refrigeration system, it is very important to lower the system pressure because this will facilitate the change of the moisture from liquid to vapour phase (through heat transfer from the surrounding environment) making its removal easier.

Always evacuate a system when:

•      replacing a circuit component (compressor, condenser, filter-drier, evaporator, etc.)

•      whilst the system has no refrigerant

•      if the refrigerant is contaminated

•      after the lubricant is charged.