NATE Low-GWP flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

GWP definition

Global Warming Potential: the 100-year integrated radiative forcing of 1 kg of a substance relative to 1 kg of CO2, whose GWP is defined as 1. Always cite the IPCC assessment report used.

ODP versus GWP

ODP measures ozone-depletion potential relative to CFC-11; GWP measures climate forcing relative to CO2. HFCs have zero ODP but high GWP; HFOs have near-zero GWP and zero ODP.

R-410A GWP

R-410A, a 50/50 blend of R-32 and R-125, has a 100-year GWP of about 2088 (AR4). It is an A1 refrigerant being replaced by lower-GWP A2L options.

R-32 GWP

R-32 (difluoromethane) is a single-component HFC with a 100-year GWP of about 675 (AR4). It is classified A2L and has higher volumetric capacity than R-410A.

R-454B composition

A zeotropic blend of 68.9% R-32 and 31.1% R-1234yf with GWP near 466 (AR4). It is A2L and used as an R-410A replacement; charge as liquid.

R-1234yf properties

An HFO with GWP below 1 (about 4 by AR5), classified A2L, with volumetric capacity close to R-134a. Used in automotive and small commercial systems; mildly flammable.

R-1234ze(E) properties

An HFO with GWP below 1, classified A2L, with lower volumetric capacity than R-134a. Common in chillers and foam blowing; often used with larger displacement compressors.

A2L classification meaning

ASHRAE 34 designation for lower toxicity and lower flammability. Burning velocity is low (about 5-10 cm/s), ignition energy is high, and flammable concentrations are narrow, so charge limits and ventilation apply.

A3 classification meaning

ASHRAE 34 designation for lower toxicity and higher flammability, such as R-290 and R-600a. Burning velocity and flame speed are much higher than A2L, so strict charge limits and ignition-source controls apply.

R-290 GWP and class

Propane: GWP 3, ODP 0, ASHRAE A3. It has excellent thermodynamic efficiency and high latent heat but is highly flammable, so many codes cap system charge.

R-717 GWP and class

Ammonia: GWP 0, ODP 0, ASHRAE B2L. It is toxic and slightly flammable, has high efficiency, is incompatible with copper, and requires ventilation and leak detection.

R-744 critical point

CO2 has a critical temperature of 31 C (87.8 F) and critical pressure of 73.8 bar (1070 psia). Above that temperature a system runs transcritical and efficiency depends on gas-cooler approach.

R-744 triple point

CO2 triple point is -56.6 C at 5.18 bar. Below this pressure solid dry ice can form, so systems must not be vented or pumped down below the triple point.

Zeotropic blend definition

A mixture whose components boil at different temperatures at a given pressure, producing temperature glide. Charge as liquid and account for glide in superheat and subcooling measurement.

Temperature glide

The difference between bubble and dew temperatures of a zeotropic blend at constant pressure. R-454B glide is about 1.5 K; R-410A about 0.1 K, effectively near-azeotropic.

Kigali Amendment purpose

An amendment to the Montreal Protocol that schedules phase-down of HFC production and consumption worldwide, using CO2-equivalent baselines and stepped reduction targets by country group.

AIM Act

The US American Innovation and Manufacturing Act of 2020 authorizes EPA to phase down HFCs, manage allowances, and set use restrictions and leak-management rules for high-GWP refrigerants.

EPA SNAP program

Significant New Alternatives Policy: EPA lists acceptable refrigerant substitutes by end use, with use conditions such as charge limits, A2L requirements, and required leak detection.

CO2 equivalent calculation

Charge mass in kg multiplied by GWP gives CO2-equivalent mass in kg. Example: 10 kg of R-410A at GWP 2088 equals 20,880 kg CO2e, or about 20.9 metric tons.

A2L charge limit basis

Safety standards such as ASHRAE 15 and IEC 60335-2-40 set maximum A2L charge from room volume, lower flammability limit, and installation height; larger charges require mitigation.

Lower flammability limit

The minimum refrigerant concentration in air that will sustain a flame. For A2L refrigerants it is typically above 100,000 ppm, much higher than for A3 refrigerants such as propane.

Minimum ignition energy

The smallest spark energy that can ignite a flammable mixture. A2L refrigerants require far more energy (millijoules) than A3 refrigerants, which is central to their lower risk profile.

Burning velocity comparison

A2L refrigerants burn at roughly 5-10 cm/s versus about 40 cm/s for propane. Lower burning velocity reduces overpressure risk and underpins the relaxed A2L requirements.

R-452B composition

A blend of R-32, R-125, and R-1234yf (67/7/26) with GWP near 676. It is A2L and a near-drop-in R-410A replacement with similar capacity.

Transcritical CO2 cycle

A cycle where heat rejection occurs above the CO2 critical temperature, so the condenser becomes a gas cooler and pressure is controlled for best efficiency rather than fixed by saturation.

TFA from HFO breakdown

HFO refrigerants degrade in the atmosphere to trifluoroacetic acid, which deposits in rain and water bodies. This is a separate environmental concern from GWP.

Flammability of A2L

A2L refrigerants will burn only when concentration is within the flammable range and a sufficiently energetic ignition source is present; leaks in occupied spaces must be diluted below the LFL.

R-134a replacement options

R-1234yf is the common low-GWP replacement for R-134a in automotive and small systems, with similar capacity and pressure but A2L classification requiring updated service practice.

R-404A GWP

A blend of R-125, R-143a, and R-134a with GWP about 3922 (AR4). It is a high-GWP A1 refrigerant targeted for phase-out in commercial refrigeration.

Cascade system

Two refrigeration circuits with separate refrigerants coupled by a heat exchanger. Common with CO2: a high-stage HFC or HFO circuit condenses the low-stage CO2, avoiding transcritical operation.

Refrigerant recovery requirement

Refrigerant must be recovered rather than vented during service, especially HFCs and HFOs. Recovery reduces CO2-equivalent emissions and is required by EPA Section 608 rules.

Section 608 leak rules

EPA rules set leak-rate thresholds and repair timelines by appliance type and refrigerant GWP. Higher-GWP and larger-charge systems face lower thresholds and stricter reporting.