Refrigerant Handling and Safety
ASHRAE Standard 34-1992, "Number Designation and Safety Classification of Refrigerants," and Standard 15-1994, "Safety Code for Mechanical Refrigeration," have been recently revised to address the classification and safety requirements of alternative refrigerants. With proper installation and the incorporation of a leak detection system with proper ventilation, all alternatives can be used safely.
Standard 34-1992 explains the format for refrigerant numbering and defines Safety Group Classifications for refrigerants according to their toxicity and flammability. Toxicity classifications are based on the Threshold Limit Value-Time Weighted Average (TLV-TWA). This is defined in Standard 34-1992 as "... the time-weighted average concentrations for a normal 8-hour workday and a 40-hour work week, to which nearly all workers may be repeatedly exposed, day after day without adverse effect."
As shown in Figure 18-1, refrigerants are assigned to one of two classes (A or B) based on the following criteria:
• "Class A signifies refrigerants for which toxicity has not been identified at concentrations less than or equal to 400 ppm, based on data used to determine TLV-TWA or consistent indices."
• "Class B signifies refrigerants for which there is evidence of toxicity at concentrations below 400 ppm, based on data used to determine TLV-TWA or consistent indices."
Additionally, refrigerants in each class are assigned a number used to designate flammability. Flame propagation and lower flammability limits (LFL) for each category are also defined in Standard 34-1992.
Group A3 Group B3
Group A2 Group B2
Group A1 Group B1
INCREASED TOXICITY Lower Toxicity Higher Toxicity
Figure 18-1 ASHRAE Standard 34-1992 Refrigerant Safety Classifications.
In low-pressure systems, HCFC-123 is the common replacement for CFC-11 used in centrifugal chillers and exhibits similar pressure-temperature characteristics. According to the standard, it is rated as having higher tox-icity and is not flammable, placing it in a B1 refrigerant class.
In medium-pressure systems, HFC-134a is being used to replace CFC-12 in centrifugal, screw, and reciprocating compression-based systems. HFC-134a has a lower toxicity rating and is not flammable. Therefore, it is classified as an A1 refrigerant and is considered very safe.
In high-pressure systems, HCFC-22 is the primary refrigerant and is used for many applications in reciprocating, screw, and gear-driven centrifugal chillers. It is classified as an A1 refrigerant and is considered safe.
Ammonia, which is classified as a B2 refrigerant, is flammable, toxic in high concentrations, and corrosive and must be used carefully. Precautions include leak detection devices, even though its strong odor serves as a good warning sign. It can be smelled at levels of over 100 times less than when it is harmful. Most construction codes prohibit the use of ammonia in systems where ammonia may leak into a public occupancy area. Ammonia cannot be used with copper and zinc. Instead, more costly, lower heat transfer capability piping, such as aluminum or steel-welded schedule 40 or 80, must be used. However, because ammonia has better thermal characteristics than halocarbons, smaller pipe sizes are required.
ASHRAE Standard 15-1994 requires that "each machinery room shall contain a detector, located in an
Flammability ^
Lower ji Flammability
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