Hot Oil Medium Boiling Point

[Sawsan311]

Dear All,

in heating medium application, I observed that some chemicals supplied as heavy heating fluids used a heating medium that the fluid's operating temperature would exceed the reported vendor's boiling point however simultaneously the vapor pressure of that heavy the fluid is too low.

Do you foresee any concerns in operating the fluid above its boiling point knowing that we will also exceed the flash point as well.

Thanks

Regards,

 

[pierreick]

hi,
Yes indeed ! degradation of the hot oil ,coke formation , foaming , etc.
Very sensitive product, don't play with the parameters!
You should refer to vendors' manual.

https://www.chemicalprocessing.com/assets/wp_downloads/pdf/Paratherm_eBook_110324.pdf

https://durathermfluids.com/resources/faq/

Pierre

 

[LittleInch]

sawsan,

You need to supply some data to be clear what we're looking at.

You don't normally run heating medium fluid above its boiling point due to the issue of what happens if you get a leak or drop in pressure.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

If pressurized, it may not boil.

Good Luck,
Latexman

 

[Compositepro]

Heating oil systems are often pressurized with nitrogen, even when operated below the boiling point, to prevent oxidation. Operating above the boiling point is not unusual. The expansion tank will be cool and pressurized with nitrogen. The important factor is thermal stability of the oil, not the boiling point.

 

[Sawsan311]

Thanks all for the great feedback,

I will provide an example of a thermal oil of operating temperature at 280 C while boiling point is 250 C.. what about the film temperature? How can I correlate the boiling temperature to the film temperature.. practically , when the boiling point is exceeded, there will be risk of film boiling on the walls of the metal of either tube/shell side of the heat exchanger.

Typically, exchange vendor will provide selection of tube material based on system's design temperature which is based on the maximum operating temperature. However, not noticing that boiling point is exceeded may impose a latent risk of having bubble/hot spot formation on the walls of the tube with potential coking/fouling as well as reduction in the heat transfer coefficient.. do you agree? Otherwise, do we have to consult the exchanger vendor about the maximum tolerable film wall temperature the tube can accommodate? is there such requirement to be checked?

In summary, if the film temperature of the fluid is not provided, can we prevent any risk by operating always below the boiling point.

Regards,

 

[LittleInch]

sawsan,

Read the posts in reply. No one operates a thermal oil system which is boiling inside the HX. They all pressurise the oil to prevent boiling.

One thing to consider in this system is the requirement in operation to maintain pressure above a certain amount and provide alarms and shutdown / reduce temperature if the pressure falls to near atmospheric.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Only some Dowtherm heat transfer fluids are operated at boiling point.

Hot oils are operated well below boiling point. The hot oil manufacturer will specify in his brochure the max permissible film temperature for continuous operation.

The hot oil heater vendor's thermal design must demonstrate by calculation that this max film temperature is not exceeded at the most arduous normal operating condition: this is usually at the hot end of the heater and at the lowest normal operating flow of recirculating hot oil, coincident with the bulk hot oil temperature at TAH or at normal operating temp. The Owner's engineer or representative is responsible for checking these calcs. Many Owner specs for hot oil heaters fail to ask for this, resulting in premature coking of the hot oil and degradation in actual operation.

 

[Sawsan311]

Thank you all,

I understand that operating the hot oil system above boiling will evidently mandate the presence of padding/blanketing of the expansion vessel not only to prevent the oxidation but also increase the pressure to maintain the fluid in liquid phase.

however, is there any design specification mandated to be spelled out in the data sheet of heat exchangers/reboilers such as skin temperature limitation of the shell and tube side... or the thermal modelling will be looking into the various heat transfer effects such as convective boiling, film boiling, nucleate boiling etc and advise on the zone of danger of hot spots specially under excess surface area conditions during initial clean start-up?

Thanks

Regards,

 

[georgeverghese]

Indeed, you have picked up one more condition that needs to be included in the calculation for max film temp on the hot oil side in the heater:

"this is usually at the hot end of the heater and at the lowest normal operating flow of recirculating hot oil, coincident with the bulk hot oil temperature at TAH or at normal operating temp and with zero fouling on the hot oil side of the tubes"

Excess surface area does not contribute to the hot oil film temp, since the heat input will be controlled to a given max thermal duty. In fact, film temp will drop with excess surface, since heat flux/m2 of surface will be lower.

 

[Sawsan311]

Hi Mr.George, when I was referring to excess area I was actually referring to the clean surface area in absence of fouling resistance. this is envisaged during start-up mandating more bypassing of the hot side of the exchanger. The increase in temperature on the cold side is evidently aggreviated by low flow conditions which also mandate bypassing

 

[georgeverghese]

Either way, it is still excess area, and the effect is the same. The low flow condition on the cold, hot oil side should have been evaluated during design phase- see my previous comment also. One way of alleviating high film temp would be to bypass some of the cold hot oil as you say. Increasing the bypass flow may help to some extent.

Is this a hot oil heater configured as a WHRU sitting on a GT exhaust ?