Properties of Bromotrifluoromethane CBrF3 (Halon 1301) 1

[Sparweb]

Hi
I've forgotten too many things in 20 years since university. I am puzzled by some constants included in a table of fluid properties:

The table is a list of fire-suppression agents that can be used for fire extinguishing/control in aircraft, and can be found in hand-held extinguishers and the built-in automatic fire systems.
Of course many are also found in the fire extinguishers in your kitchen.

Can anyone tell me what "K1" and "K2" represent?
My first thought was Van der Waals equation, but the units of K2 don't work. Real gas VS ideal gas?

For those who want more info, the document can be found at:

http://www.fire.tc.faa.gov/pdf/11-31.pdf

STF

 

[georgeverghese]

k1 is the specific volume of the fluid, in units of m3 / kg. The reciprocal of this is the density in kg/m3. In this case, it looks like it is for the vapor. It appears that this specific volume references a temperature lower than 20degC.

k2 looks like the cubical coeff of expansion for the gas, ie. the rate of change of the gas specific volume per degC change in temp per kg of gas. For example, for a 5degC change, the gas volume changes by 5 x 0.000567 m3 per kg = 0.00285m3 per kg for Halon 1301.

Looks like k2 is a derived parameter; you can derive this value by knowing the mol wt of the fluid. In the case of Halon 1301, looks like MW = 150.4. The mol wt of the fluid can be obtained from the familiar formula PV=zRT; or rho = MW.P/zRT, given that rho = 1/ k1.

From this, I get k2 = 0.000546 m3/kg/degC, assuming z=1, which is close enough to the value listed for k2 (error = 5% approx). The error is probably due to the fact that this fluid is not far from its critical temp at a reference temp of 20degC (ie. dz/dT at constant press at 20degC is not small).

 

[Sparweb]

Thank you George,
I suspected that k1 would be specific volume, but at what conditions? Since the density is given at 20 degrees C on the same table, I would like to know if this represents another temperature.
1/0.14781 = 6.765 kg/m3 which is more dense, so presumably a lower temperature?

If I use the k2 slope as you suggest, then:

k1 @ 20C = 1/6.255 = 0.1599 m3/kg

Delta Temp = (0.1599 - 0.1478) / 0.000567 = 21.3 C

20C - 21.3C = -1.3 C

So the k1 value in the tabe is specific volume at about 0C. If that's right, thanks again! I can use that.

My CRC handbook gives the molecular weight = 148.9. Can you tell me where you found 150.4?
Substances like CBrF3 must be so difficult to make that I wouldn't be surprised if the properties in one source don't agree with another - impurities varying from one manufacturer's process to another etc.

STF

 

[Sparweb]

Is there any way to find out if the specific heat ratio is somewhat close to 1.4, like it is for diatomic gases like air?



STF

 

[georgeverghese]

Yes, your approach would be correct to get the reference temp for the k1 value.

I derived the mol wt from the relation MW = rho.z.R.T/P, assuming z=1 and with the stated rho at 20degC. It was only after posting this reply that I saw your chemical formula for this compound, which would have given the mol wt directly. Using the actual mol wt of 148.9, we now get z=0.99

k2 can be derived from the following
rho = MW.P/ (zRT) or v= specific volume = 1/rho = zRT/(MW.P)

Differentiating this wrt to T, we get
dv/dT = zR/(MW.P) + (RT/MW.P).dz/dT ; assume that dz/dT is 0 in the range of interest.
or
dv = zR.dT/(MW.P)

The nearest I can find for Cp/Cv for this fluid is that for C.(Cl)2. F2 in Perry, which says 1.139 at 25degC, 1atm.
For others pressures / temps, there are some values for Cp for C. F2.H2 ( difluoromethane) in table 2-309 in the 7th edn of Perry. From here, assuming semi ideal behavior, we can use the relation Cp/Cv = Cp/(Cp-R) ; all on mole basis (ie. Cp expressed as kJ/kgmole/degK). Suspect this wouldnt be far from the value for Halon 1301.

 

[Sparweb]

That's a handy relation. Probably something put up on the board in Thermo class, but as I said... a long time ago!
I'll go get a copy of Perry's handbook.

STF