Ref Manual Servicing Technicians Unit 2 (Refrigerants)

2.1. Section objectives

This chapter provides a broad overview of most of the issues associated with refrigerants. This includes the criteria normally applied to their selection (for example, thermodynamic properties and safety characteristics), an overview of the various types of refrigerants and how they are identified. Particular attention is paid to the characteristics of refrigerant blends. Although not normally considered to be a refrigerant, the topic of refrigeration oils or lubricants is discussed since it effectively becomes part of the working fluid during system operation and thus requires special attention also.

The information provided here should help the reader to be able to:

•      Identify refrigerant characteristics

•      Recognise the classification of refrigerants

•      State the main refrigerant groups

•      Identify the proper refrigerant for each refrigeration or air-conditioning system

•      State the main characteristics for the most commonly-used refrigerant

•      Identify the suitable lubricant for each refrigerant.

Selecting the refrigerant

Originally when the modern refrigerating system concept was developed in the middle of the 19th Century, a small number of fluids were used as the working fluid, or “refrigerant”. These included ammonia (NH3, R717), carbon dioxide (CO2, R744), sulphur dioxide, methyl chloride and ethyl ether. However, because of the combination of toxicity, flammability and pressure issues, these refrigerants were largely replaced with to a “new” group of fluorinated chemicals which exhibited little reactivity, low-toxicity and no flammability. However, during the 1980s, it was found that these chemicals contributed to the depletion of the ozone layer, which lead to the development of the Montreal Protocol in 1987.

The Montreal Protocol requires the cessation of the consumption and production of all chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) and since its introduction, the refrigeration and air conditioning (RAC) industry has been engaged with the chemical community to establish substitutes for ozone depleting refrigerants. Throughout this time a large number of refrigerants have been introduced worldwide, of which some are long term alternatives, and others are “transitional” substances. With the increasing attention paid to the issue of global warming and climate change, there is now a stronger push towards adopting alternative refrigerants with low or no global warming potential (GWP), as well as zero ozone depleting potential (ODP).

With the continued attention on replacement refrigerants, coupled with the ever growing market for RAC, there are now several hundred refrigerants that are currently commercially available. Such a diversity of refrigerants and their variety of different characteristics can create difficulties in handling and servicing practices for many RAC technicians. This section aims to introduce an overview of refrigerants and their characteristics, classifications, applications, identification and lubricants.

There are usually two situations that necessitate refrigerant selection, the first being for manufacture of systems, and the second being equipment servicing. For manufacturing RAC equipment, the refrigerant selection process is theoretically complex, involving the consideration of huge number parameters.

	SELECTION CRITERIA
THERMODYNAMIC	OPERATING
AND TRANSPORT	PRESSURES
PROPERTIES
	ALTERNATIVES
CHEMICAL	SAFETY
PROPERTIES	CHARACTERISTICS
AND STABILITY

 

Chemical properties and stability

The stability of a refrigerant is linked to the way it behaves in the presence of other substances, particularly within the refrigerating system. It is important that the refrigerant will not react with, or act as a solvent with, any of the materials within the system. These include metals used for pipes and other components, compressor oils and associated additives, plastic motor materials, elastomers in valves and fittings, and desiccants within filter dryers. This should also be considered with respect to the small quantities of contaminants such as moisture and air.

In general CFCs, HCFCs, hydrofluorocarbons (HFCs) and HCs are compatible with most materials (since most components are designed for these refrigerants). However, many components are designed using proprietary mixtures and additives, so there is always a possibility of incompatibility with certain materials if an unspecified refrigerant is used. Carbon dioxide has some compatibility problems with certain elastomers, which is why only dedicated components for R744 should be used with this refrigerant.

Ammonia is not compatible with many materials, such as copper, copper alloys and many electrical wiring insulation materials. Therefore construction metals inside ammonia systems are normally limited to carbon steel and stainless steel.

In all cases, component manufacturers should be consulted to check that their materials are compatible with a non-standard refrigerant.

Operating pressures

It is important to consider the likely operating pressures in both the suction and discharge sides of the system. Ideally, a refrigerant is chosen that will have an evaporating pressure above atmospheric pressure under normal operating conditions, so as to avoid air and moisture being drawn into the system in the event of a leak. Thus, a refrigerant should be chosen with a normal boiling point (NBP) that is lower than the anticipated evaporating temperature. A selected refrigerant should also have a condensing pressure that does not exceed the pressure that the system components are designed for, as this can have safety implications.

Thermodynamic and transport properties

The most important performance criteria for a refrigerating system are cooling (or heating in the case of heat pumps) capacity and efficiency, or coefficient of performance (COP). These performance characteristics are influenced by a number of properties, including:

•      saturation pressure-temperature characteristics

•      critical temperature

•      latent heat

•      density

•      viscosity

•      thermal conductivity

•      specific heat capacity

It is important to consider the likely operating pressures in both the

suction and discharge sides of the system. Ideally, a refrigerant is

chosen that will have an evaporating pressure above atmospheric

pressure under normal operating conditions, so as to avoid air and

moisture being drawn into the system in the event of a leak. Thus, a

refrigerant should be chosen with a normal boiling point (NBP) that

is lower than the anticipated evaporating temperature. A selected

refrigerant should also have a condensing pressure that does not

exceed the pressure that the system components are designed for,

as this can have safety implications.

The capacity and COP are mainly dictated by the design and

control of the system itself (compressor, heat exchangers, piping,

etc), although the properties of the refrigerant play a part in this.

The COP can be affected by the compression ratio (which is

dictated by the saturation pressure-temperature characteristic), heat

exchanger performance and pressure losses around the system,

which are all influenced by latent heat, density, viscosity, thermal

conductivity, specific heat.

For a given evaporating and condensing temperature, the cooling

(or heating) capacity of a system is strongly influenced by the

latent heat and density of the gas entering the compressor. For

conventional systems, a fairly high critical temperature is preferred

(at least 20K above the condensing temperature), unless the

system is specially designed for operation near or above the critical

temperature, such as with R744 systems.

 

Safety characteristics			According to various international and national safety standards,
Refrigerant are classified in terms of two general safety criteria: toxicity and			refrigerants may be allocated one of six safety classifications
			according to its toxicity and flammability. This classification
flammability.
			consists of two alpha-numeric characters (e.g. A2); the capital letter
Toxicity: Both acute (short-term) and chronic (long-term) toxicity			corresponds to toxicity and the digit to flammability. The toxicity
are considered as they affect human safety during handling and			classification is determined by a refrigerants’ TLV-TWA, such that
servicing with refrigerants, and for occupants in refrigerated or air			it may be lower toxicity “A” or higher toxicity “B”. There are no
conditioned spaces. The acute-toxicity exposure limit (ATEL) is			refrigerants that are non-toxic.
the maximum recommended refrigerant concentration intended to			The flammability classification may be no flame propagation “1”,
reduce the risks of acute toxicity hazards to humans in the event of
			lower flammability “2” or higher flammability “3”.
a refrigerant release. For chronic toxicity, the threshold limit value-
time weighted average (TLV-TWA) is the time -weighted average
concentration for a normal 8-hour workday
and a 40-hour workweek, to which nearly all
workers may be repeatedly exposed, day after
day, without adverse effect.
Flammability: Refrigerant flammability can
affect the safety of people and property
mainly during handling and servicing activities,
and it influences the design of equipment.
The flammability of a refrigerant is judged
according to the lower flammability limit
(LFL), which is the lowest concentration of
the refrigerant mixed in air, required for it to
be able to be ignited. Flammability is also considered according to
the refrigerants’ heat of combustion (HOC), which is the amount of

energy released when it burns.