Safety Aspects of an A/C System with

Carbon Dioxide as Refrigerant

SAE Subcommittee Safety of Refrigerant Systems

J. Amin1, B. Dienhart1, J. Wertenbach2

Introduction

A carbon dioxide based air conditioning raised the following safety concerns: the possible

exposure to refrigerant under high pressure, to high concentrations of refrigerant, to high

noise levels in the event of component rupture and to flying debris in the event of

component rupture. This paper considers the risk of high CO2 concentration in a vehicle

cabin. Because the refrigerant in a carbon dioxide based air conditioning system is under

high pressure it expands to much larger volume when released into the atmosphere. There

is therefore a risk that displacement of air might pose as health hazard when the refrigerant

is released into a confined space such as vehicle cabin or workshop pit area. Since the safety

thresholds of carbon dioxide are well known, the theoretical and measured carbon dioxide

concentration in a cabin quantify the possible refrigerant charge at a defined cabin space.

Carbon dioxide

Carbon dioxide replaces air, and causes lack of oxygen. At presence of sufficient oxygen,

CO2 has a narcotic effect at stronger concentration. With smaller amounts, CO2 has a

stimulating effect on the respiratory center. Due to the acidic characteristics of CO2, a

certain local irritating can appear, particularly on the mucous membrane of nose, throat and

eyes as well as induce coughing.

The symptoms associated with the inhalation of air containing carbon dioxide are, with

increasing carbon dioxide concentrations. The data, valued for adults with good health, are

as follows (source: AGA Gas Handbook) [2]

2% 50% increase in breathing rate

3% 10 Minutes short term exposure limit; 100% increase in

breathing rate

5% 300% increase in breathing rate, headache and sweating may begin

after about an hour (Com.: this will tolerated by most persons, but it

is physical burdening)

8% Short time exposure limit

8-10% Headache after 10 or 15 minutes. Dizziness, buzzing in the ears,

blood pressure increase, high pulse rate, excitation, and nausea.

10-18% After a few minutes, cramps similar to epileptic fits, loss of consciousness, and shock (i.e.; a sharp drop in blood pressure) The

victims recover very quickly in fresh air.

18-20% Symptoms similar those of a stroke

 

1

 Visteon / Ford

2

 DaimlerChrysler
Carbon dioxide can be lethal at concentration at about 12%, the lowest ever reported lethal

concentration of CO2 is 9% for 5 (!) minutes [3].

Vehicle tests I

In the experiment the spatial and temporal CO2 distribution was measured in the vehicle

cabin. Carbon Dioxide was brought in between evaporator and heater core with a hose via

the water drainage. This configuration allows to simulate a sudden release of the total

refrigerant charge into the compartment. The measuring were made at two medium size

vehicle within a sealed room at room temperature. The induced carbon dioxide mass was

determined by a calibrated volume-flow and the time. The tests were made with various CO2

charges. For measurement, six concentrations of CO2 distributed at different positions in the

cabin were detected. The air flow through the compartment was varied by the blower. At

recirculation mode the air door was sealed with tape in addition.

Results:

Figure 1 shows the CO2

concentration in a closed

vehicle in a closed room

without air circulation for

different escaping CO2

charges. The CO2-volume flow

was kept constant at 200

litre/hr. At a concentration at

4% the reduction rate of

concentration is 0.5% in 10

minutes.

In lowest blower setting in

fresh air mode and a CO2

volume flow of 200 liter/hr.

the concentration is rather

constant between 1 and

1.5 % over the entire time.

When the flow is stopped,

the concentration dropped

soon to normal level.

When the blower ran in recirculated air mode, taking no fresh air from outside the vehicle,

the concentration decayed at steady state, with no difference in concentration between head

level and foot level. With 400 g CO2, the maximum concentration was measured between

4.1 to 4.5%.

With a charge of 600g and with 600 liter/hr the maximum concentration was about 5.3%.

With the blower switched off, the concentration at foot level was high and reached nearly

8% and on the seat surfaces front and backseat 4 % were reached. The concentration at

head level was below 1%. CO2-Concentration [Vol-%]

time [ min]

Leakage

Figure 1 CO2 concentration in a vehicle cabin with different

CO2 charges for a simulated evaporator leakage,

Vol.-flow is 200 litre/hr, 100 % recirc.
When the ventilation fan runs in maximum at fresh air mode, taking its supply from outside

the car, the CO2 took less than 1 minute to reduce from 4 % to less than 1%.

Vehicle Tests II

A medium sized car, three doors, without A/C system has been chosen as test vehicle. A

hose leads from the CO2 bottle though the engine compartment into the inlet of recirculated

air of the HVAC. The inlet of fresh air is permanently closed. The mass of spilled CO2 is

measured as difference in weight with a highly sensible scale. Additionally the CO2 volume

flow is measured with a flow meter. The CO2 concentration will be measured with an infrared (IR) sensor. The IR CO2 sensor is a transducer for the measurement of the CO2 partial

pressure in the atmosphere, by the absorption of infra-red radiation. The accuracy is ± 5 %

of the measured value.

The following parameters were regarded :

a) concentration profile inside the passenger compartment,

b) reduction of inner volume due to passengers sitting in the car

c) influence of release time

d) blower off

An instantaneous release of a certain amount of CO2 inside the passenger compartment would

result in a theoretical maximum peak concentration inside the cabin as listed in Table 2.

500g gaseous CO2 were released with 1500 liter/hr into the passenger compartment (see

Figure 2.) The maximum measured concentration of 6.3 % vol. was at the vent center just

before stopping the release. The difference in CO2 concentration on different positions inside

the car was in the range of the statistical error (0.3%). After the spill stopped, the

concentration decreased rapidly. Concentrations lower than 1 % are reached after

approximately 50 Minutes.

The release rate has the most significant influence on the measured CO2 peak

concentration. By decreasing the rate from 1500 liter/hr down to 500 l/h the peak

concentration of CO2 drops from 6.3 % vol. to 3.9 % vol. (see Figure 3).

One test was conducted with 5 Dummies inside the passenger compartment, each with a

volume of approximately 70 liters. The measured peak concentration after releasing 500 g

CO2 with 1500 liter/hr was 6.7 % vol. That’s approximately 0.4 % higher than without

Dummies.

Leaking CO2 inside the car while the blower is switched off leads to the absolutely same

results as with test vehicle I. The peak concentration at the seat level decreases within 10

minutes from 4% vol. to values lower than 2 % vol.

In all cases the opening of the door lead to a very rapid decrease of the CO2 concentration

inside the car. However, with closed doors CO2 disappears with a rate of 1.5 % vol. per 10

minutes. That’s approximately 3 times higher as in test I.
Figure 2: Experimental results of a continuous release of 500 g CO2 with 1500 l/h into the

passenger compartment of a midsize vehicle.

No. Rate

[liter/hr]

Sensor Position Peak

Concentration

Blower step Comment

1 1500 Vent center right 6.3 max Diff. sensor

positions

2 1500 Vent center right 6.7 max With 5 Dummies

3 500 Vent center right 3.9 max

4 1500 Seat level codriver4.1 off

Table 1: Test matrix of vehicle test II. Release of 500 g CO2 inside a passenger compartment of a midsize car with different rates. Ambient temperature 20°C / 68°F

0

2

4

6

8

10

0 10 20 30 40 50

Time [min]

Vent center right

Blower Max / recirc.

Temperature 20°C / 68 F

Opening the door

concentration

 %
Figure 3: Experimental results of a continuous release of 500 g CO2 with 500 l/h into the

passenger compartment of midsize vehicle.

Refrigerant Passenger compartment Volume

[m3]

AC System Charge

[g]

Refrigerant conc.

ppm vol/vol if total

AC charge leaks

[%]

1.5 240 8.9

2.5 225 5

2.5 400 8.9

CO2

4 600 8.3

Notes: Theoretical, for illustration only, actual refrigerant charges and passenger

compartment volume may be different. Assumes a total leakage of refrigerant

charge.

Table 2 Refrigerant leakage: theoretical concentrations within passenger compartments

Summary

It must be emphasized that CO2 is a refrigerant with low toxicity. The concentrations of CO2

that can lead to any effects are very high and these could only result in the event of a leak

of refrigerant into the passenger compartment under conditions of low fresh air ventilation

rates.

The physiological effects on Carbon Dioxide effects are apparent at exposure limits above

2%. Above 5% the change is marked and the human control means system effects became

apparent. While not like threatening, at such concentrations this reaction may cause further

stress in the subject, who may well be aware of breathing changes, but be enable to detect

any cause. The lowest ever reported lethal concentration of carbon dioxide is 9%.

0,00

1,00

2,00

3,00

4,00

5,00

6,00

7,00

8,00

9,00

10,00

0,0 10,0 20,0 30,0 40,0 50,0 60,0 70,0

Time [min]

Co

nc

ent

rat

ion

CO

2

[%

vol

]

Vent center right

Blower Max / recirc.

Temperature 20°C / 68 F
In the experiment the spatial and temporal CO2 distribution of different cars were measured.

While at an air flow direct from outside only minor concentrations occur the level at

recirculation mode was higher. If the inlet mass of CO2 was small, the concentration in the

compartment was low. A charge between 400g and 600 g cause a CO2 concentration of

about 5 %, this seemed to be acceptable for the medium size vehicle. There was no

influence seen about the concentration level, when the volume flow rise. In case of a non

operating blower fan and open outlet nozzles there were layers of CO2 within the cabin, with

highest concentration at the floor and the lowest in the head region. When the CO2 flow was

interrupted, the CO2 concentration in the compartment declines immediately.

In practice will be a difference of the charge and the escaping charge out of a system. First,

not all CO2 can leak out of a system in a limited time, because some CO2 is solved in

materials, as lubricant, elastomer. Second due to the pressure reduction in the system due

to escaping CO2 the pressure difference to atmosphere falls and the flow rate will be

reduced. As general as design rule it seems to be vise to built systems which contain only an

insignificant charge of carbon dioxide to human health.

Assuming a leak in the evaporator core with a remarkable loss of refrigerant will lead to a

solidification of a certain amount of CO2 while expanding to ambient pressure. That will

stretch the release time of CO2. Additionally the future total charge of the CO2 system will be

lower than 500 g of CO2. Both will effect the hazard of CO2 as refrigerant in a positive

manner.

Important safety reminders for Carbon dioxide:

Good ventilation must be provided. Avoid working with carbon dioxide where it can be collect

in low or confined areas. Leave the premises immediately in the event of large carbon

dioxide release. Before returning to the premises, makes sure that the CO2-concentration is

low enough. The carbon dioxide concentration can be analyzed with CO2 sensitive tubes or

metered instruments.

Carbon dioxide containers must stand upright when gas is being withdrawn. Cold carbon

dioxide can form a very thick mist with moist air. The odor of Carbon dioxide is too weak to

warn of dangerous concentrations.

Persons who succumb to Carbon Dioxide poisoning must be taken into the open air quickly.

Artificial respiration should be administered if the victims stop breathing

Acknowledgement: Special thanks to Mrs. M. Vazan; Mr. Kuenanz and Mr. Pfeil all from

Ford in conducting the leak tests at vehicle II.

References:

[1] R.A.C.E. -Project Final Report; Task: Safety Tests and Evaluation; 1997 (published

within the project)

[2] K. Ahlberg: AGA Gas- Handbook; AGA AB, Lidingo/Sweden

ISBN 91-970061-1-4

[3] Sax, Irvine and Lewis, Richard: Dangerous Properties of Industrial Materials;

7

th Edition, Volume I; VAN NOSTRAND REINHOLD, New York.