Purdue University

Purdue e-Pubs

International Refrigeration and Air Conditioning

Conference

School of Mechanical Engineering

2000

Allowable Charge Calculation Method for

Flammable Refrigerants

O. Kataoka

Daikin Industries

M. Yoshizawa

Daikin Industries

T. Hirakawa

Daikin Industries

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Kataoka, O.; Yoshizawa, M.; and Hirakawa, T., "Allowable Charge Calculation Method for Flammable Refrigerants" (2000).

International Refrigeration and Air Conditioning Conference. Paper 506.

http://docs.lib.purdue.edu/iracc/506
ALLOWABLE CHARGE CALCULATION METHOD FOR FLAMMABLE 

REFRIGERANTS 

Osami Kataoka, Masahito Y oshizawa, Taku Hirakawa 

Daikin Industries, Ltd. 

1304 kanaoka-cho, Sakai, 

Osaka 591-8511, Japan 

Fax: +81-722-59-9283 Phone: +81-722-50-0900 

Email:osami .kataoka@ daikin.co.jp,masahito. yoshizawa@ daikin.co .jp 

taku.hirakawa@ daikin.co.jp 

ABSTRACT 

Although hydrocarbons and lower GWP HFCs are flammable, they are preferable to high 

GWP HFCs from the perspective of mitigating climate change. Appropriate standards are 

necessary to utilize these refrigerants so that their flammability risks are minimized. Although 

standards for the use of flammable refrigerants exist, some reports indicate that the refrigerants 

can sometimes be ignited even at levels which satisfy the standards. In addition, the theoretical 

background of these standards is unclear. To promote the development of a more appropriate 

standard, a more scientific method for calculating the allowable refrigerant charge was 

established. Because the calculation method was proposed as the foundation for a new 

international standard, a relatively simple formula was developed, based on experimental and 

numerical (CFD) analyses. 

NOMENCLATURE 

CFD; Computational Fluid Dynamics h1; Effective height in 7m2 room [m] 

GWP; Global Warming Potential K; Coefficient for adjusting charge amount 

HFC; Hydrofluorocarbons other than propane 

HCs; Hydrocarbons LFL; LFL in we1gm L kg/m3

] 

UFL; Upper Flammable Limit mmax; Allowable charge [kg] 

LFL; Lower Flammable limit t; Leak duration [minutes] 

A; Floor area [ m2

] x; Height from the floor [ m] 

h; Effective Height [m] y; Concentration of refrigerant [kg!m3

] 

h0 ; Leak height (Installation Height) [m] 

1. INTRODUCTION 

This study was conducted to prepare a foundation for an amendment to IEC 335-2-40; 

"Particular Requirements for Electrical Heat Pumps, Air Conditioners and Dehumidifiers," in 

order to incorporate requirements for flammable refrigerants. Selection of the most appropriate 

refrigerant technology from a range of alternatives, taking into account environmental, safety, 

and other relevant factors, requires consideration of flammable refrigerants. In order to reduce 

the risks due to refrigerant flammability, an appropriate safety standard is necessary. 

Conventional safety standards employ simple safety factors such as four [IJ to calculate the 

Eighth International Refrigeration Conference at 383 

Purdue University, West Lafayette, IN, USA- July 25-28, 2000 
allowable charge in a room. However, several reports indicate that flammable refrigerants which 

are heavier than air can stagnate near the floor and ignite if an ignition source is present, even if 

the charge quantity complies with the conventional safety standard l

2

H31 . Therefore, quantitative 

calculation methods which consider this stagnation phenomena should be developed. 

Propane and HFC-32 are the most important refrigerants to consider, since they have the most 

attractive thermophysical properties for air conditioning. The basic concept for the calculation 

method was first developed for propane. Then, CFD (Computational Fluid Dynamics) analysis 

was conducted on a supercomputer to confirm that the method yielded a sufficient level of safety 

for propane, butane, HFC-152a and HFC-32 and that the approach was neither too restrictive nor 

too relaxed. Finally, based on the results of the CFD analysis, some modifications to the basic 

formula were made. 

If a flammable refrigerant which is heavier than air leaks into a room and stagnates near the 

floor, it can be ignited, resulting in a fire or explosion. This seems to be the most critical 

potential hazard arising from the use of flammable refrigerants . .Therefore, a calculation method 

was developed which limits the refrigerant charge so that even in the case of catastrophic leak, 

the flammable space is very small. 

It is impossible to eliminate the flammable space completely when a flammable refrigerant 

leaks, because the concentration of the leaking refrigerant is almost 100% at the leak point, while 

the concentration reaches zero at some distant location. The concentration varies continuously 

between these points. Therefore, at some location, the concentration is between the lower 

flammable limit (LFL) and the upper flammable limit (UFL). It is obviously better to have no 

flammable space at all, but some finite flammability risk from refrigeration oil and electric 

circuits has been accepted in air conditioning systems, so it should be acceptable to have a very 

low but non-zero flammability risk from the refrigerant. 

2. LEAK CONDITIONS AND REFRIGERANT CONCENTRATION 

Before the new calculation method was developed, refrigerant dispersion phenomena were 

evaluated experimentally and numerically. Major results of these investigations have already 

been reported in another paper l

31, so they have not been repeated here. 

Numerous parameters affect refrigerant dispersion behavior. Since it is impossible to evaluate 

every parameter in detail, only the most significant parameters were evaluated. The evaluation 

was done under conservative conditions, meaning that parameters other than the one under 

evaluation were fixed at their worst case condition. Table 1 shows these parameters and 

conditions. fu the following analysis, the results under these conservative conditions were 

employed. 

Both vertical and horizontal refrigerant concentration gradients exist. However, the horizontal 

concentration gradient is much smaller than the vertical one, except in the vicinity of leak port. 

The reason for this phenomenon is that if a horizontal concentration difference is present, airflow 

due to natural convection occurs, which reduces the horizontal gradient. This airflow velocity 

can reach approximately 1 meter per second. On the other hand, vertical dispersion occurs due to 

Eighth International Refrigeration Conference at 384 

Purdue University, West Lafayette, IN, USA- July 25-28, 2000 
molecular movement if there is no airflow, at a velocity of a few centimeters per minute. 

Therefore, the horizontal gradient of refrigerant concentration was first neglected. Later, the 

horizontal gradient of refrigerant concentration in large rooms was evaluated through CFD 

analysis. 

Parameter 

Molecular 

Mass 

:D Charge 

<D Amount -..., 

us· Temperature <D 

..., 

Ill 

~ Pressure 

Interaction 

with Oil 

Velocity 

Rate 

r Direction <D 

Ill 

7\ Height 

Location 

Temperature 

Pressure 

:D Tightness 0 

0 

3 Ventilation 

AirFlow 

Shape 

Obstacles 

_....._ 

~ ..--< 6.0 0 

~5.0 0 

-~ 4.0 

c 3.0 <I) 

g 2.0 0 

u 1.0 

0 

Table 1. Leak Parameters 

Evaluated Condition 

Evaluated 

Evaluated 

Neglected 

Neglected 

Neglected 

Low velocity 

Leak takes 4 minutes 

Downward 

Evaluated 

Middle of the room 

Neglected (Used 

approximately 25° C) 

Neglected (Used 1 

bar) 

Tight room except 

door gap 

No ventilation except 

the doorgap effect 

None. 

Rectangular 

None 

20cm 

40cm 

Reason 

Effect does not seem significant. Evaporation of 

refrigerant has a more significant effect. 

Evaluated through leak rate analysis. 

Oil reduces the leak amount, so it is neglected for 

conservatism. 

Conservative. 

As leak rate increases, concentration increases. An · 

appropriate conservative assumption is necessary. 

Conservative. 

A wall close to the leak inhibits mixing, but not 

greatly, so the location was chosen to reduce 

calculation time. 

Lower temperature will generate worse results, but 

the effect is not significant. 

Lower ambient pressure will generate worse results, 

but the effect is adjusted by calculation. 

Conservative, but the smallest opening (30 em") is 

used. 

Conservative but smallest opening (30 em'"') is used. 

Conservative. 

Negligible effect. 

Negligible effect. 

Refrigerant concentration at the floor level 

increases as the leak proceeds, until the leak 

stops. Except for the area directly surrounding 

the leak port, the highest refrigerant 

concentration occurs on the floor. As shown in 

Figure I, the concentration at the floor level 

peaks just after the end of release and then starts 

to decrease. If the refrigerant charge is limited to 

a reasonable amount so that this highest 

200 400 600 concentration on the floor does not reach the 

Time (sec.) 

Figure 1. Concentration Variation 

LFL, no other location will ever reach the LFL, 

except directly adjacent to the leak port. The 

refrigerant concentration at higher elevations 

continues to increase for some time after the release ends. However, the concentration at these 

Eighth International Refrigeration Conference at 

Purdue University, West Lafayette, IN, USA- July 25-28,2000 

385 
points will remain below that at the floor level because the dilution of the high concentration on 

the floor is the cause of this increment. 

3.LEAKRATE 

As the refrigerant leak rate increases, the concentration also increases. The leak rate must be 

fixed in order to calculate the refrigerant concentration. Most leaks occur very slowly, but rapid 

leaks have also been reported [

41

• In this analysis, a leak duration of four minutes was chosen to 

represent a catastrophic leak, since it takes about 4 minutes to leak 150g of C02 through the 

capillary tube defined in IEC335-2-24 [SJ. In addition, refrigerant recovery from a room airconditioner generally takes five minutes or more. Therefore, the 4 minutes assumption is 

believed to be sufficiently conservative. 

4. ASSUMPTION OF VERTICAL CONCENTRATION PROFILE 

' \ 

C02 

Release Height 50 em 

\Calculating Curve 

Integrating the vertical concentration (by 

mass) profile curve gives the refrigerant mass at 

a particular height, per unit area. Therefore, the 

total refrigerant amount in a room can be 

calculated as the product of the floor area and the 

integrated profile curve. 

The vertical concentration profile was 

assumed to be parabolic, as indicated in Figure 2. 

o ho 

60 

When refrigerant is released from a height 

substantially above the floor, such as 2 m, the 

refrigerant concentration profiles can be 

Figure 2. Concentration Profile approximated as functions of the 4th power of 

height. If the release point is low, such as 0.1 m, the concentration profiles can be approximated 

as linear functions of height, or functions of the 1.5 power of height. However, the parabolic 

(power 2) function of height was employed for the representative profile curve, since it can 

approximate the profile curve most cases. In addition, it is more restrictive for large charges at 

elevated leak conditions and gives some allowance for small charges at low leak elevations. 

The parabolic curve when the floor concentration reaches LFL is specified by following three 

conditions: 

y = LFL-a·x2 

When x = h, y = 0 

LFL 2 y=LFL---·x h2 

m = fh(LFL- LFL · x 2 ) ·A ·dx max Jo h2 

= 2 ·LFL·A·h 

3 

Eighth International Refrigeration Conference at 

(4.1) 

(4.2) 

(4.3) 

Purdue University, West Lafayette, IN, USA- July 25-28,2000 

• Concentration equals zero at the 

effective height. 

• Concentration on the floor is LFL. 

• Concentration profile curve is 

symmetric with respect to the floor. 

The low concentration above the 

effective height and the high concentration 

near the leak port are neglected. Then, if y is 

the concentration at a given height x, the 

386 
allowable refrigerant charge can be calculated using the (4.1)-(4.3): 

5. EFFECTIVE HEIGHT 

At a certain height, the concentration becomes zero. The estimated height where the 

concentration profile curve crosses the axis is called the "effective height". This effective height 

is affected by most of the parameters in Table 1. However, only the effects of molecular mass, 

LFL, leak height and floor area were evaluated in this analysis. Other parameters such as leak 

velocity and duration were set at conservative levels. 

5.1 Effect of Leak Duration and Velocity on Effective Height 

As discussed in section 3, the leak duration has a significant impact on the concentration 

profile. As the release duration becomes shorter, the highest concentration on the floor increases. 

In this analysis, as indicated in section 3, it was assumed that all the refrigerant was released in 

· four minutes, in order to avoid variation in effective height due to a difference in leak duration. 

The leak velocity was chosen to avoid any dynamic effects. Generally, a catastrophic leak 

occurs at sonic velocities at the leak port. However, if the leak port is surrounded by walls with a 

L6 .--------------,..--~ 

.-.. L4 ,.. "' 

S L2 

;:: 

-~ LO !I) 

::r: 0.8 

-~ 0.6 

0 <8 0.4 '+-< 

~ 0.2 

R32 • 

R290 + 

Rl52ab. 

opening, the velocity can decrease quickly, so a 

minimum velocity was employed in the analysis. 

5.2 Effect of Release Height 

As the leak height becomes higher, the 

effective height also increases. Since the velocity 

of down flow increases as the height increases, 

the concentration of refrigerant is diluted due to 

larger airflow. In addition, release at an elevated 

o.o W<----'--:-----'------'-:---~ position generates mixing through turbulence, 0

·

0 0

·

5 Releas~-~eight (~) 5 2

·

0 while mixing from a low release is caused mainly 

by air and refrigerant acceleration. Therefore, a 

Figure 3. Height Effect on h large volume of the air and refrigerant mixture 

reaches the floor due to dilution, resulting in a higher effective height. This relationship is shown 

in figure 3. 

The effective height of propane in a 7 m2 

room is approximately expressed by formula 5.1 

when the floor concentration is at approximately LFL. As the actual floor area of the test room 

where most tests were conducted is approximately 7m2

, the area 7m2 

was chosen. HFC-32 has a 

higher effective height than propane due to its higher LFL. However, effective heights of 

hydrocarbons are explained here to simplify the formula of the standard. The effective height of 

HFC-32 is discussed in section 6, along with other effects. 

h,=0.63·1zo (5.1) 

Eighth International Refrigeration Conference at 387 Purdue University, West Lafayette, IN, USA- July 25-28, 2000 
5.3 Effect of Molecular Mass and LFL 

For a given amount of refrigerant released, 

a variation in the molecular mass of the 

refrigerant has a minor effect on the effective 

height. However, the refrigerant LFL ·has a 

substantial impact on effective height when 

floor concentrations are close to LFL. Figure 4 

shows this phenomenon. In this figure, the 

accurate formula with adjustment includes 

effects of door gap. As the LFL by weight 

increases, the effective height becomes higher. 

This effect is discussed in section 6. 

5.4 Effect of Floor Area 

2.0.----------------. 

1.8 

s 1.6 

'-' 1.4 

~ 1.2 

~ 1.0 

~ 0.8 

·E 0.6 

~ 0.4 

~ 0.2 ~ 

0. 0 

Accurate Formula _--

0.10 0.15 0.20 0.25 0.30 

LFL w (kgfm3) 

Figure 4. LFL Effect on h 

As the floor area increases, the effective height decreases, because the refrigerant and air 

mixture volume to reach the floor from the unit is almost constant. The floor area and effective 

height appear to have a linear relationship on a log-log scale, as shown in Figure 5. For 

hydrocarbons, this relationship is 

approximated by equation 5.2. 

IOm~~;:n *Leak Hei~t 1.8m g 

~ 

-~ -~;:- (5.2) 

"E ~ l.O~~~~~~mm1~~!! ~ w/o Adjustment ~ This formula gives a constant refrigerant 

~ll;llth Adljulstmll~l~~~ ~ .::_r-- concentration at the proportional distance 

0.1._......_.1-L.LU! IU;II._--'-.1-l..LUIIIwii._......_.I-L.LUw point from the leak port. However, if this 

...... 

~ 

1 10 100 1000 formula is used, the flammable volume and 

Floor Area (m2) 

Figure 5. Floor Area Effect on h 

time integration become large as the floor area 

increases, since the distance between the leak 

port and the representative point increases. 

Therefore, the use of an adjustment coefficient is necessary. Many CFD analyses were conducted 

to confirm that the flammable space is reasonably small. Finally, equation 5.3 was employed to 

obtain a conservative value using a simple calculation method for hydrocarbons. 

to{~)= -05 log(~) 

h = 2.6· A-o.s -~ (5.3) 

This relationship is also affected by LFL, as discussed in section 6. The allowable charge for 

hydrocarbons can therefore be calculated as shown in equation 5.4. 

Eighth International Refrigeration Conference at 388 

Purdue University, West Lafayette, IN, USA- July 25-28, 2000 
2-LFL·A·h mmax =-----

3 

h = 2.6 · A -o.s · h, = 2.6 · A -o.s · 0.63 · h0 

mmax = 3_ · LFL ·A· 2.6 ·A -o.s · 0.63 · h0 :;; 1.1· LFL · h0 · .JA (5.4) 3 

6. COEFFICIENT FOR OTHER REFRIGERANTS 

As mentioned previously, since a simplified calculation formula was established to calculate 

the allowable charge for propane, a correction factor for other refrigerants is necessary to adjust 

for the effect of their different LFL and molecular weight. In addition, the expected ignition 

probability is lower for class 2 refrigerants than for class 3, which is categorized by ASHRAE 34 

1

61 or ISO 5149 [?], due to the higher minimum ignition energy and lower flame speed of the class 

2 refrigerants. Therefore, another correction factor is needed to address the lower safety risks of 

equipment using class 2, rather than class 3, refrigerants. Besides of these factors, for an equal 

flammable volume, the consequence of igniting class 2 refrigerants is different from that ·of class 

3 substances. To adjust for these differences, a coefficient is necessary. I 

(

0.00061)6 

.!_ ( 7) LFL h = l.45·h0 • LFL4 • A (6.1) 

I 

(

0.00061)6 

2 . LFL . A . ho .!. ( 7 ) LFL 

m = ·1.45· LFL4 • -

~ 3 A (6.2) 

= 1.1· K · LFL·h0 -JA (6.3) 

Based on the results of the CFD analysis, equation 6.1 was derived to maintain the flammable 

volume and time integration at a level less than 15-m3

-minutes for refrigerants other than 

propane. Equation 6.2 then gives the allowable charge calculation. However, equation 6.2 is too 

complicated to be used as a standard. Therefore, a simplified equation and correction coefficient 

was proposed, as shown in equation 6.3 that is derived from equation 5.4. 

20 I I I I I I 

Accurate Formula 

~ 

...- .... _ _, .... 1-- k=2 ~,...,."""' .... .... 1--" 1c=l..;.5 ........ .... .... 

... ... k=l.O 

7 1 0 20 40 70 100 

Floor Area (m2) 

Figure 6. Detailed Calculation and K 

Eighth International Refrigeration Conference at 

Figure 6 shows the results of this detailed 

calculation and simplified formula with some 

coefficients for HFC-32. In order to maintain the 

allowable charge of HFC-32 using the simplified 

formula to be equivalent to or less than that 

derived from detailed calculations, a 

conservative value of K = 1. 7 is appropriate, as 

shown in Figure 6. To compensate for the 

different probability and consequence of ignition 

for HFC-32, larger value of K seems to be 

appropriate, but this most conservative approach 

was chosen for all refrigerants other than 

389 Purdue University, West Lafayette, IN, USA- July 25-28, 2000 
propane. If. the K is fixed by this minimum value, K can be calculated by following formula 

because most critical condition appears when the room size is the minimum. Finally, formula 6.5 

is obtained by using this K value. 

K = 2.33 · 1J LFL (6.4) . 

h=K·h0 

~ 2.33 · 'J LFL · 1.~ h, ~ 3.82 · WJf · h, 

mmax = 2 . LFL. A. ho · 3.82 · ~ s 2.5 · LFL ~ · h

0 • JA 

3 -vA 

(6.5) 

Since this formula 6.5 is obtained by evaluation of propane, butane, R152a and R32, this is 

applicable to HCs and fluorocarbons that has larger molecular weight than 44 because larger 

molecular weight gives higher effective height than this calculation. 

7. CONCLUSION 

A new method for calculating allowable charge for flammable refrigerants has been 

developed based on experimentation and numerical analysis. This formula was proposed to the 

IEC SC 61D and ISO TC86 SCl WGl joint working group to assist in establishing a standard 

for using flammable refrigerants in electrical heat pumps, air conditioners and dehumidifiers. 

Although the key coefficient (K value) is still under discussion at the time of writing this paper, 

the basic concept was accepted in the working group. 

8. ACKNOWLEDGEMENT 

The authors greatly appreciate the assistance of the Tsukuba Advanced Computing Center, a 

part of the Japanese Agency of Industrial Science and Technology, for providing the large 

computing capacity necessary for the CFD analyses used in this study. 

9. REFERENCE 

1. ANSI-ASHRAE 15-1994, "Safety Code for 

Mechanical Refrigeration" 

2. "Study of diffusion of propane and isobutane in 

rooms respecting the standardized practical 

limit", D. Clodic, February 1997, ASHRAE 

annual meeting seminar 3.8. 

3. "Experimental and numerical analyses of 

refrigerant leaks in a closed room", 0. Kataoka 

et al, ASHRAE SE99-19-02, July 1999 

4. DOE/CE/23810-92, "Risk assessment of HFC32 and HFC-32/134a (30170 wt.%) in Split 

Eighth International Refrigeration Conference at 

Purdue University, West Lafayette, IN, USA- July 25-28, 2000 

System Residential Heat Pumps", W. Goetzler et 

al, April 1998 

5. IEC335-2-24, 1997, "Particular requirements for 

refrigerating appliances and ice-makers" 

6. ANSI-ASHRAE 34-1997, "Designation and 

Safety Classification of Refrigerants" 

7. ISO 5149-1993, "Mechanical refrigerating 

systems used for cooling and heating - Safety 

requirements" 

390