high temperature heat transfer fluid 1

[CRTDesign]

I have a heat recovery application taking heat out of the exhaust of a turbine (930F) and using the heat in a Rankin cycle to generate power. I need a heat transfer media that can handle the high temperatures in the exhaust stream. My loop between the exhaust heat exchanger and the Rankin cycle equipment only needs 525F but I'm concerned about failsafe conditions and possibility the transfer media could approach the exhaust temp.

Does anyone know of a fluid the can operate up to 921F.

 

[danberry]

The max temp I know of is 750F bulk temp for thermal fluids.

For higher temps you can consider molten salts

 

[unclesyd]

Do you want to recover the heat at 921F or is this a calculated value of the heat recovery fluid?
If this is the temperature needed from the heat recovery system then as mentioned above molten salt is the way to go.

We have large heat recovery systems using molten salt that operate in this temperature range that are used to preheat boiler feed water.

The critical temperature of Diphenyl based fluids is around 927F. This temperature is very critical because if these fluids are heated to this temperature in a closed vessel it will detonate.

We use Thermminol in the vapor state by using gas fired vaporizers that operate at 70 psig and 700F. There are some heat recovery systems that operate using these heating fluids.

 

[CRTDesign]

I don't need the high temperature, only trying to recover as much heat as feasible. My oil transfer fluid loop only reaches 525F. My concern is that I want to leave the heat exchanger in the exhaust stream (take very large dampers to reroute gas stream) and when the heat is not needed the oil could heat up to close to exhaust temp.

Thanks for your response.

 

[danberry]

You can always have backup systems (diesel pumps, alternate heat sinks, etc) on thermal fluid systems. We used thermal fluid in high inertia, biomass fired furnaces. You have to make it failsafe but it can be done.

 

[timbones]

You could use any number of fluids provided you keep it from boiling. You don't have to worry about the fluid approaching the exhaust temperature provided you put in the appropriate shutdowns. If you're worried about high temperature of media, put in a high temp shutdown for the turbine (also put in a shutdown on circulating pump flow failure).

I worked on a project a number of years ago that used triethylene glycol in a HRSG on the exhaust of an LM6000 gas turbine. Worked fine. The system had a bypass stack so that the turbine could run independantly without heating up the coil too much. It wasn't unique in this regard, there are plenty of installations out there with bypass stacks. Lots of steam heat recovery boilers are out there with bypass stacks so that the gas turbine can be run without the steam system.

If you are really adverse to putting in a bypass stack, use steam and a once through style HRSG. Boil the HRSG dry when you don't need the waste heat. High water purity is key with this style boiler (probably cheaper to put the bypass stack than it is to do the water pruification).

Tim

 

[davefitz]

Timbones:

The correct solution is as you indicated, a once thru HRSG. The max gas temp of 921 F implies the HRSG does not need to be incolnel , and a ferritic tubed OTSG will work OK, if the CTG is firing nat gas or LNG.

The water purifiers are likely not as costly as the bypass stack- these mini purifier skids are used in remote areas to supply the OTSG.

 

[daubery]

A thought. Would a heat-pipe system be useful for this application?

If so, it could be an elegant, compact solution. My concern would be the working fluid temperature limit, though.

Des Aubery
(

http://www.adthermtech.com)

 

[davefitz]

heat pipes have a significant problem at temps over 700 F.

Back in 1990 , there was a replacement heat pipe airheater supplied in Maryland. Max fluegas temp = 750 F. The boiler started up after the new air heater was installed . Immediately after startup, the control room operator heard pop - pop- pop etc. The end caps of the individual heat pipe tubes were shot off the tubes like cannon shells, penetrated the casing of the boiler, flying around the plant. The plant was shut down pronto.

It seems the japanes mfr of the heat pipe tubes told the US boiler mfr ( in japanese notes) that these tubes contain water plus a corrosion inhibitor. The inhibitor breaks down to a series of child molecules above 700 F, yielding many more moles of vapor than originally expected, and a mcu higher heat pipe pressure. Unfortunately, the US boiler mfr does not read japanese. The pressure in the heat pipe tubes increased to 2-3 times the original design pressure, and the endcaps failed.

The ASME code then changed it rules, requiring each heat pipe tube to a have a relief valve or burst disk if used in section I or VIII service. That basically ended the US market for heat pipes.

Using a heat pipe results in a "indirect heat exchanger " arrangement, and the effectiveness of this is the product of the effectiveness of teh 2 sections. So if each section is 80% effective, the global effectiveness is 81%. So it is better in HRSG applications to use a direct heat exchanger using steam . Better cycle efficiency can be had if you use a ammonia water mixgture ( Kalina cycle), but it costs big bucks.

 

[davefitz]

typo- each section is 90% effective, yields 81% overall effectiveness.

 

[daubery]

Thanks davefitz. That was very informative. Good thing no-one was injured.

Des Aubery...
(adTherm Technology -

http://www.adthermtech.com

)

 

[moltenmetal]

That's a damned shame: the heat pipe concept is an excellent one, and since a heat pipe is a closed system there was probably no need for the corrosion inhibitor anyway!

 

[Transient1]

Helium gas could operate in this temperature range. The gas fast cooled nuclear reactor's proposed for the future (Gen IV) would make use of Helium coolant at about 850 C which would pass through a rankin cycle to shed its heat.