## Safety Aspects of an A/C System with Carbon Dioxide as Refrigerant

SAE Subcommittee Safety of Refrigerant Systems J. Amin 1 , B. Dienhart 1 , J. Wertenbach 2

Introduction A carbon dioxide based air conditioning raised the following safety concerns: the possible exposure to refrigerant under high pressure, to high concentrations of refrig erant, 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 concen tra tion in a vehicle cabin. Because the refrigerant in a carbon dioxide based air con ditioning system is un der high pressure it expands to much larger volume when released into the atmos phere. There is therefore a risk that dis placement of air might pose a s 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 theo retical 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 in duce 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 pres sure) The victims recover very quickly in fresh air. 18-20% Symptoms similar those of a stroke

1 Visteon / Ford
2DaimlerChryslerCarbon dioxide can be lethal at concentration at about 12%, the lowest ever re ported 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 ve hicle 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 me dium size vehicle within a sealed room at room tem perature. The induced carbon dioxide mass was determined by a calibrated vol ume-flow and the time . The tests were made with various CO2 charges. For measurement, six concen trations of CO2 distributed at different positions in the cabin were de tected. 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

concen tration in a closed vehicle in a closed room without air circu lation for different escaping CO2 charges. The CO2-volume flow was kept con stant at 200 litre/hr. At a concentra tion at 4% the re duction rate of concentration is 0.5% in 10 min utes. In lowest blower setting in fresh air mode and a CO2 volume flow of 200 liter/hr. the concentration is rather constant be tween 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 concen tra tion between head level and foot level. With 400 g CO2, the maximum con centration 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 con centration 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 CO 2 bottle though the engine compartment into the inlet of recirculated air of the HVAC. The inlet of fresh air is perma nently close d. The mass of spilled CO 2 is measured as difference in weight with a highly sensible scale. Additionally the CO 2 volume flow is measured with a flow meter. The CO 2 concentration will be measured with an infrared (IR) sensor. The IR CO 2 sensor is a transducer for the measurement of the CO 2 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 instanta neous release of a certain amount of CO 2 inside the passenger com partment would result in a theoretical maximum peak concentration inside the cabin as listed in Table 2

. 500g gaseous CO 2

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 CO 2 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 CO 2

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 CO 2 with 1500 liter/hr was 6.7 % vol. That’ s approximately 0.4 % higher than without Dummies.

Leaking CO 2

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 de creases 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 CO 2 con centration 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 re sults of a continuous release of 500 g CO 2 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 codriver 4.1 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 doorconcentration 

### %Figure 3 : Experimental results of a continuous release of 500 g CO 2 with 500 l/h into the passenger compartment of midsize vehicle.

Refrigerant Passenger compartment Volume [m 3 ] 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 concentra tions 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 com partment under condi tions 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 concentra tions 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 re ported 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 FIn the experiment the spatial and temporal CO2 distribution of different cars were measured.

While at an air flow direct from outside only minor con centrations oc cur the level at recirculation mode was higher. If the inlet mass of CO2 was small, the concentration in the compartment was low. A charge be tween 400g and 600 g cause a CO2 concentration of about 5 %, this seemed to be accept able for the medium size vehicle . There was no influence seen about the con centration 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 de clines 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 CO 2 while expanding to ambient pressure. That will stretch the release time of CO 2 . Additionally the future total charge of the CO 2 system will be lower than 500 g of CO 2 . Both will effect the hazard of CO 2 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 di oxide 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 breath ing

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 thEdition, Volume I; VAN NOSTRAND REINHOLD, New York.