Hot Oil vs Glycol/water as heating medium

[Trond]

Hi all,

We are looking at a new offshore oil & gas facility where we want to install a heating medium system. The heat will come from several WHRU on gas turbin outlets. Operating temperature of the system will be around 170°C (340°F), and the proposal is to go for a glycol/water mixture. I have done some work with hot oil systems before, and would have thought that would have been better suited, but naturally I am biased here

I have done some serching on the web, and some sources indicate that 170°C is too high for a glycol/water system. Others claim it is OK.

The way I see it, hot oil is less prone to fouling/degrading, can be operated at lower pressures, is inherently inert and hence less likely to cause corrosion. On the down side, it is flammable and more expensive to purchase than glycol/water. Then again,I would presume a glycol/water system would be more labour intensive in terms of operation?

Would love to have some feedback from my fellow eng-tips members.

 

[moltenmetal]

I don't know what is typically done in these circumstances but definitely feel that 170 C is way too hot for glycol/water. The primary disadvantage of hot oil is low heat capacity hence a need for quite high flows, large piping etc., as well as high viscosity leading to poorer heat transfer performance per unit pumping energy. At 170 C you're not going to have to worry about viscosity, but if your return oil temperatures are as low as ambient it will affect your choice of oil medium.

 

[Trond]

Thanks for that swift reply, moltenmetal. Very much appreciated!

The return temperature will be at least 30°C (86°F), so I don't anticipate any viscosity issues here.

 

[StoneCold]

Trond
The temperature is a little high for glycol. Glycol works in your car at about 150C but that is because the reservoir is under pressure. If you use a closed system with a bladder for thermal expansion it should work fine. Oil does have less maintenance until the temperatures get really high. I guess I would go with whatever the client wants and just design accordingly.

Regards
StoneCold

 

[Trond]

Hi StoneCold,

thanks for taking the time. The system (if glycol/water) will indeed have to pressurized, but at a much higher pressure than what would be the case for a hot oil system. My dilemma here is that I personally think a HO system would be better in terms of simplicity, lifetime operating cost and possibly also capital cost, but RFQs have already been issued HXs using water/glycol as the heating medium. This despite the fact that the heating medium system has yet to be designed (!).

As hot oil has lower heat capacity than glycol/water, I would expect distribution piping to be larger for the former, and that HXs will have to be slightly larger and thus costlier.

 

[MatthewL]

Trond,

Another reason for using glycol/water is the environmental impact in the event of a spill is negligible. Hot oil typically has a reportable quantity, while some of the glycols do not. Also, if the heat exchangers spring a leak (and they will) and you have tower water on the other side, it will make a mess in the tower. We have to use a two loop system to cool our hot oil - there is a hot oil to intermediate cooling water loop, then an intermediate water to cooling tower loop which has the plates replaced annually.

Regards,

Matt

Quality, quantity, cost. Pick two.

 

[racookpe1978]

150 C - 180 C oil is not "quite" burning (it is below auto-ignition temperature certainly) but it is dangerously hot to personnel and is flammable and is under pressure if something else starts a fire in any of the rooms where the heating system is piped. I think most clients will want to stay away from a hot oil system offshore for that reason as well. There are enough other problem-children systems out there already not want to add another.

 

[Drexl]

Considered steam?