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REFRIGERANTS

General

There is a wide variety of refrigerants used in air conditioning equipment depending on the application. In general the most common refrigerants used in the industry belong to the following three categories -

• CFC - These are the Chloro Fluoro Carbon refrigerants, such as R11, R12, R113, R114, etc. These refrigerants were identified as the most harmful to Ozone layer by the Montreal Protocol, and were phased out in 2000. However they are still being used in the older machines, with precautions to minimize release in accordance with EPA regulations. The most common application of these refrigerant is in the large centrifugal chillers. R12 was also used commonly in the older cars for air condition.

• HCFC - These are the Hydro Chloro Fluoro Carbon refrigerants, such as R22, R123, etc. These refrigerants were identified as slightly harmful to the Ozone layer by Montreal Protocol, and will be phased out by 2030. The R22 refrigerant is commonly used in most reciprocating type of compressors, while R123 is used in centrifugal chillers as a temporary replacement for R11.

• HFC - These are the Hydro Fluoro Carbon refrigerants, such as R134a. These are the new refrigerants that do not harm the Ozone layer, and are being used in the newer machines to replace the CFC and HCFC. R134a is now commonly used as a replacement of R12 and R500, and in all new cars air conditioning systems.

There is extensive research going on to identify new refrigerants that can be used to replace the CFC and HCFC refrigerants. Currently R134a is the most commonly used new refrigerant. The various refrigerants have different characteristics, which make them suitable for a particular application. The pressure and temperature characteristics of the commonly used refrigerants is given in the link below, for a quick reference -

Refrigerants Temperature/Pressure Table for common refrigerants

Other links that provide useful information on refrigerants are also given below -

http://www.refron.com/InfoCenter/Home.asp

http://www.epa.gov/ozone/title6/index.html

Refrigerant Analysis

A periodic refrigerant analysis is important to detect and control contaminants in the refrigerant, which can result in degradation / failure of the various components, and cause inefficient operation of the unit. A log of the periodic refrigerant analysis should be maintained for trending. Refrigerants should be tested for the following contaminants –

·       Moisture

·       Acid

·       Particulate/solids

·       Organic matter – sludge, wax, tars

·       Non-condensable gases

Moisture

Moisture is one of the primary causes of contamination-related problems in a refrigeration system. It also results in formation of some of the other contaminants mentioned above, which in turn cause further damage to the chiller or DX unit. Presence of moisture results in following undesirable effects:

·       Ice formation in evaporator, expansion valve or orifice.

·       Degradation of lubricating oil due to hydrolysis.

·       Acid formation due to hydrolysis of refrigerant in the presence of moisture and high temperature.

·       Corrosion of metals.

·       Copper plating 

The copper plating phenomenon essentially involves carryover of copper ions from exchanger tubes to the steel surfaces. Although the exact mechanism is not completely understood, it involves the following three steps, 1) oxidation of the copper due to contaminants such as air, moisture & acid, 2) solubilization and transport of copper ions by the lubricant, 3) deposition of the copper on hot clean steel surfaces such as bearings. Excessive copper plating can result in a compressor failure. Typically copper plating is a concern in systems with high level of contaminants and high bearing temperatures.

The most common causes for high moisture in the system are:

·       Water leakage in a chiller evaporator, or water cooled condenser.

·       Low pressure side leak resulting in entrance of air carrying moisture (typical problem in negative pressure machines)

·       Improper service procedures, i.e. system left open to atmosphere.

In case of moisture introduction due to improper service procedures, the dryer will eventually reduce the moisture content resulting in a decreasing trend. If the trend is not decreasing then the likely reasons are the first two causes, which require shutting down the chiller for repair. If the chiller cannot be shutdown, it may be possible to temporarily provide on-line cleaning of the refrigerant to maintain the moisture within acceptable limits, depending on the size of the leak. Online cleaning is similar to a kidney function using a portable cleanup unit.

Moisture is normally absorbed in the refrigerant or lubricant, but free-water can also be present. The solubility of water varies with different refrigerants. Generally, lower is the solubility of water in the refrigerant, greater is the potential of free water being present, and lower is the acceptable level of moisture in the system. Water concentration above the maximum solubility level will result in free-water. The maximum water solubility level is different for liquid and vapor phase of the refrigerant, i.e., completely soluble water in liquid phase may transform into free-water in the vapor phase or vice versa depending on the change in solubility from one phase to the other.

The acceptable levels of moisture in new or reclaimed refrigerants are given in ARI 700. These levels are generally more demanding than what is typically feasible and acceptable in an operating system. There is no experimental data available on the maximum permissible moisture levels in an operating system since it is a function of several factors, but ASHRAE has some data on typical levels in a normally operating system. The table below gives a comparison of ARI 700 allowable level and the level typically found in normally operating equipment.

* R113, R114, R134a, R500 data are not available in ASHRAE. Above data is based on similarity with the other refrigerants (R500 is an azeotrope of R12 & R152a).

Testing method for moisture is specified in ARI 700. Based on above discussion and operating experience, the acceptance criteria for moisture should be as follows:

Alert Level Actions

·       Increase frequency of sampling refrigerant to 2x

·       Sample lubricating oil with next sample of refrigerant to check for any signs of degradation

·       Check all potential causes of high moisture, and fix as required.

·       Check moisture indicators rigorously.

·       Check for any signs of lubricating oil degradation

·       Change filter dryers/desiccants as required

Fault Level Actions

·       Re-sample refrigerant to verify results

·       Recycle and clean refrigerant on line

·       Change all filter dryers/desiccants.

·       If trend continues, schedule a shutdown of the chiller and fix leaks.

Acids

A refrigeration system can contain two types of acids, organic and inorganic, depending on the type of refrigerant and oil being used. Organic acids (such as oleic acid) are formed as a result of decomposition of oil at high temperature in the presence of air as the oxidizing agent. These acids are slow to react, soluble in oil, do not vaporize, and typically found in relatively small quantities in the oil sump. Inorganic acids (such as hydrochloric acid and hydrofluoric acid) are formed as a result of decomposition of refrigerants at high temperature in the presence of moisture. These acids are highly reactive, soluble in water, vaporize, and typically found to be the dominant acids that may be present. Therefore, inorganic acids are the real problem in a refrigerant system, which results in degradation of the equipment internals. The major contributors to acid formation in a system are the presence of moisture and abnormally high temperatures around the compressor i.e. bearings, motor windings, terminations, compressor discharge etc. The presence of acids is specially hazardous in case of semi-hermetic and hermetic compressors, since the acid vapor in refrigerant goes over motor windings and can eventually lead to motor burnout. Therefore the amount of acids in a system should be kept to an absolute minimum, and ARI 700 specifications should be followed strictly, i.e., maximum allowable limit for acid in all refrigerants should be 1 ppm by weight.

The acids in a refrigeration system can be kept to a minimum by keeping the refrigerant dry and preventing abnormally high temperatures in the system. Desiccant used in a filter dryer may be capable of removing the acids, but the capacity and efficiency depends on several factors and is difficult to determine.

Testing method for acids is as specified in ARI 700. Based on above discussion and operating experience, the acceptance criteria for acid should be as follows:

Alert Level Actions

·       Increase frequency of sampling refrigerant to 2x

·       Check all potential causes of high acid, and fix as required.

·       Change filter dryers/desiccants as required.

Fault Level Actions

·       Re-sample refrigerant to verify results

·       Recycle and clean refrigerant on line until acid concentration drops to acceptable level.

·       Change all filter dryers and desiccants.

Particulate/solids

The solid contaminants can include metallic particles, chemical compounds or just dirt. The solids found in a system normally result from wear, corrosion and chemical breakdown of the internals, or material left in the system during servicing. The solid contaminants can create problems such as scoring compressor cylinder walls and bearings, damaging motor insulation, plugging lubrication holes, plugging filter/dryers, plugging expansion valves etc. The solid contaminants are removed to a great extent by the filter dryer, but it needs to be sized to handle it without adding too much pressure drop in the system.

Testing method for particulate/solids, and the acceptance criteria should be as specified in ARI 700 for all refrigerants. Any visual presence of dirt, rust or other particulate contamination should be reported as alert condition.

If particulate/solids are found, the refrigerant filter should be replaced. If the problem persists in-spite of changing the filter several times, on-line cleaning of the refrigerant may be required.

Note: Some labs will only give a pass or fail result of this test. If particulate/solids are found, it may be necessary to have the lab give additional details such as size, quantity, color and particle type to provide a better clue on the source. 

Organic matter – sludge, wax, tars

Organic contaminants are typically due to decomposition/degradation of organic materials in the system such as oil, insulation, varnish, gaskets etc. These can circulate in the system and plug small orifices. Organic contaminants dissolved in the liquid refrigerant may precipitate at lower temperature in the expansion device, resulting in plugged capillary tubes or sticky expansion valves. Organic contaminants can also coat heat transfer surfaces resulting in cooling inefficiency. Since heat degrades most organic materials, operating conditions with excessively high temperatures should be avoided. If an organic contaminant is dissolved in the liquid refrigerant, it may not be removed by the filter-dryer.

Testing method for organic matter is specified in ARI 700 for High Boiling Residue test.

ARI specifies 0.01% by volume of high boiling residue for most new or recycled refrigerants. However, this is not practical for operating machines due to miscibility of lubricating oils in refrigerants, i.e. oil carryover. Based on operating experience, the acceptance criteria for organic matter should be as follows:

Alert Level Actions

·       Increase frequency of sampling refrigerant to 2x

·       Change refrigerant filters as required.

                    Fault Level Actions

·       Re-sample refrigerant to verify results

·       Recycle and clean refrigerant on line till levels drop to acceptable levels.

·       Change all refrigerant filters.

 Non-condensable Gases

Non-condensable gases are chemically inert gases, which do not liquefy in the condenser. This contaminant typically results from incomplete evacuation, low side air in-leakage, chemical reactions & decomposition of materials at high temperature.  Typically the first two causes are the primary reasons for high non-condensable gases. These gases reduce cooling efficiency, cause high starting and running currents, and result in higher than normal compressor discharge pressure & temperature, which speeds up undesirable chemical reactions.

 Testing method for non-condensable gases is specified in ARI 700.

The quantity of non-condensable gases that is harmful depends on the design and size of the refrigeration unit and the nature of the refrigerant. ARI 700 specifies a limit of 1.5% of non-condensable gases by volume for most new or recycled refrigerants, which is unrealistic to maintain continuously in an operating system, especially the negative pressure machines. Based on operating experience, the acceptance criteria for non-condensable gases should be as follows:

Alert Level Actions

·       Review operating parameters to confirm high non-condensable gases.

·       Increase frequency of sampling refrigerant to 2x

·       Check purge unit/dehydrator for proper operation

·       Increase purge rate. Caution should be observed to avoid excessive loss of refrigerant due to purge unit inefficiency.

    Fault Level Actions

·       Re-sample refrigerant to verify results

·       If acceptable levels are not achieved, shutdown the machine and repair the leaks or faulty purge operation, as applicable.

·       If the machine cannot be shutdown, recycle and clean refrigerant on line until it reaches acceptable level. 

Oil Analysis

The oil analysis provides a “look inside” a compressor without disassembly. When unacceptable wear conditions develop inside the compressor, a corresponding detectable change in the characteristics of the oil will become evident. The results from oil analysis should be used in conjunction with vibration analysis and bearing temperatures to detect excessive bearing wear. A log of the periodic oil analysis should be maintained to provide the trend.

The oil sample should be tested for the following properties:

·       Metal wear

·       Moisture

·       Acidity

·       Viscosity

·       Solid residue

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Last modified:11/16/2008