Heat Transfer Fluid Ignition 1

[tkdhwjd]

I am currently screening a project to circulate heat transfer fluid (Dowtherm HT) to a fired heater for heat recovery purpose. The skin temperature of the exchanger is ~ 200 DegF less than the auto-ignition temperature of the Dowtherm HT. On paper, it does not appear to cause any fire issues in the event of a leak, but Dowtherm HT is not a "fire resistant" fluid. In other words, there is a possibilty that it may catch fire(?).

Do any of you have experiences with heating oil? Any safety issues that I should be considering? Environmental/IH issues?

Thanks in advance for sharing your experience.

 

[IRstuff]

Have you checked the datasheet w.r.t. the vapor pressure at ~516ºF? While it does not boil, if the fluid eventually gets to open atmosphere, there might be some possibility of noxious fumes.


TTFN

 

[quark]

You should check the flash point and fire point of the particular heat transfer fluid. You will have problems if you have a pressurized system to elevate boiling point. Fluid may flash if you have leaks in the system and this will cause accident if exposed to open flames.

I know of one incidence when the fluid(Dowtherm 500) caught fire in the combustion chamber. Once the liquid catches fire, the system temperature may rise to autoignition temperature.

Generally, you will not have much problems as the heaters come with steam smoothering system to suppress accidental fire.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[Mike6158]

I've only seen once instance where leaking fluid ignited but it's definitely something to pay attention to in the design. From my limited experience, it's hard to find a good heat transfer fluid whose autoignition temperature is above operating temperature. Our systems are typically hot oil systems that operate between 550 deg F and 575 deg F. The most common fluid that we use is Thermalane 600 or one of it's competitors (that I can't remember the name of at the moment).

When selecting your heat transfer fluid don't forget about film temperature. Sometimes people (usually to save money ) overlook film temperature and select a fluid that is too close to the design operating temperature.

 

[bchoate]

bchoate
I have worked with heat transfer fluids for years with many applications up to 600 F and some applications above 750F using Syltherm. The risk of fire is low in a properly designed system (mechanical design according to heat transfer fluid manufacturer), appropriate insulation, ventilation, and isolation from ignition sources. Vapor ignition can occur if the following three conditions are satisfied:
1) the liquid temperature is at or above the flash point and the liquid is in contact with air.
2) there is high enough vapor concentration to support combustion
3) an ignition source in the vapor cloud

Leaked fluid cools rapidly in contact with air. Sometimes low volume leaks will produce a gray smoke. The smoke is from rapid oxidation of the hot oil not fire. Leaked fluid cools below the flash point rapidly especially if it is under pressure and is sprayed into an aerosol.
Insulation used on equipment containing thermal transfer fluids should be non-porous. Mineral wool, fiberglas, and CalSil spread the oil over a large surface increasing the risk of fire if sufficient oxygen is present.

The manufacturer's literature gives important thermal properties for heat transfer fluids. Both flash point and fire points are given (fire point being the temperature at which sufficient vapor is present to support a continuous flame). Two fluids that I have used extensively are Multitherm IG-4 and Paratherm NF. IG-4 has a flash point = 440 F and fire point = 490 F. Vapor pressure is 1.6 psia @ 600 F. Paratherm NF's properties are 345 F, 385 F, and 4 psia respectively. Both are hydrocarbon oils rated to 600 F. The autoignition temperature for Paratherm NF is 690 F.
The AIT for multitherm is > 690 F.
Heat Transfer fluids are safe for use and the risk of fire is low.
Bill Choate