What's better for heating/cooling? DEG or TEG? 1

[Rosalynn]

My past experience is with DEG/Water solutions for mild heating/cooling applications (40 deg C-150 deg C temperature range). However, many of my colleagues in my new place of work have experience only with TEG/Water solutions for these services. They say it's better because it's more stable at high temperatures (not true, according to physical properties from the Dow website). I say DEG is better because it is less viscous and has a higher heat capacity.
DEG appears to be more toxic to humans and TEG more toxic to aquatic life, according to the few MSDSs I've found on the internet. That inclines me away from TEG (since plant personnel are unlikely to drink their cooling medium!).
I've also heard that TEG costs more than DEG, which again inclines me more toward DEG.

What do you say? Should I push harder for DEG? (I tried the Dow website but they have very little about the specific glycols beyond the obvious physical properties, which I've already examined.)

 

[Yorkman]

Have you considered using Propylene Glycol, the heat transfer is not quite as good but you do eliminate the toxicity issues.


I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[Rosalynn]

Dear Yorkman,
Propylene glycol doesn't quite have the thermal stability we need. While toxicity is important, the application is for the oil industry. Proper controls will be in place to minimize leaks and spills and for their mitigation if they occur.
You're on TV? Cool. Thanks for the suggestion.

 

[Chemeng1122]

What temperature do you need? EG has the best properties for heat transfer, but has a lower degradation temperature than TEG, it is roughly the same as DEG.

EG/DEG can degrade at temperatures >330 F (165 C), TEG degrades over 405 F (207C).

I would use EG ot TEG, not DEG. Of coure it should have an inhibitor package in it as well.

Andrew Lechelt
Technical Support Engineer
Quadra Chemicals

http://www.quadrachemicals.com

 

[Rosalynn]

Hi, Andrew

The maximum process side temperature I have is 140C, in one exchanger in the plant. All others are 130C or less. Based on this, EG/DEG are candidates--please let me know if you disagree. I hadn't considered EG, but I will now--thanks. (I've heard in the past that EG's vapour pressure is too high, causing excessive surge drum losses, but our surge drum will operate at 40C--we should be ok.)

The manufacturers' literature I have gives "onset of initial decomposition" as:
EG: 240C
DEG: 240C
TEG: 240C
The degradation temperatures you've provided are much more useful and I wish they were given in the manufacturers' technical bulletins.

One of my colleagues cautions about availability and price. I would expect price to increase and availability to decrease with the complexity of the molecules (i.e. EG cheapest and most readily available). Can you comment?

I definitely agree with your comment on inhibition.

I appreciate your help.
Regards,
Rosalynn

 

[Chemeng1122]

I would suggest an EG based heat transfer fluid for your system. It sounds like a fairly typical cooling system, so nothing exotic should be required. What do you anticipate your highest glycol temperature being? That would determine your water content.

There are also more advanced corrosion inhibitor packages for EG based fluids. Depending on where you are located, there are different suppliers with differnet products. (Jeffcool, Norkool, etc.).

Inhibition in TEG can be tricky because of inhibitor solubility issues, I have not run across many advanced inhibition packages in DEG either.

Andrew Lechelt
Technical Support Engineer
Quadra Chemicals

http://www.quadrachemicals.com

 

[Rosalynn]

Hi, Andrew

The maximum design bulk process temperature the glycol will see is 140C, in one heat exchanger. This exchanger is in parallel with several other exchangers, all operating at lower temperatures. The hot glycol return header will operate at 100-110C.

The plant design currently includes a standby fired heater to supply heat to the glycol for heat tracing when the rest of the facility is shut down (whether planned or unplanned). I would expect the fired heater to operate for a week or so every couple of years. The design outlet temperature from the heater is only 100C, so if it is operated properly, it should be a less severe service than the heat exchanger I mention above.

Based on this, what water content do you think would be appropriate? I have the vendor brochure and could enter the properties for the appropriate mixture into the process simulation, compare the results with our base case, and determine the savings this alternate design can offer.

Thanks again for your advice on this.

--R

 

[Chemeng1122]

If you are going to be at atmospheric pressure, then you will probably be in the 60-70% glycol range, but if you can run with low pressure, then a 50% solution would be best. These blends are the most common, and in use for most common applications. At 140 C, the equilibrium boiling pressure of a 50/50 EG-Water soultion is about 28 psig.

The more water you have, the better off you will be in terms of heat transfer efficiencies. I would not suggest you go below 50% glycol for freezing concerns in winter (talking in Celsius, you may be in Canada?).

I have seen some plants run a 50/50 solution with temperatures over 160C. These systems do have to be designed with higher opeating pressures in mind (>55 psig).

Andrew Lechelt
Technical Support Engineer
Quadra Chemicals

http://www.quadrachemicals.com

 

[Rosalynn]

Our minimum design operating pressure is 350 kPag (51 psig). This is in our surge drum, which operates at 40C and 350-500 kPa (the pressure is controlled by supply and bleed pressure regulators on the vapour space, using natural gas to blanket the vessel). The hot glycol headers are on the discharge side of the pumps, in the 1200 kPag range (174 psig).

I had looked at the EG properties in the manufacturer's brochure and figured we might need to make up some water once in a while, but our EG losses would be low. I could easily bump up the minimum operating pressure to 400 kPag in the surge drum if necessary. Higher than that may impact flange ratings in the system.

Hope you agree that the existing design is a good 50/50 EG candidate, and thanks again, Andrew.