Water vs glycol 1

[Vortech]

I know water is the most eff, but how much loss would there be if one switched to a 33% ethylne glycol instead of water.

we have 2 600 ton absorbers running at full capacity.
just trying to get an idea of what the true "cost" of switch will be.

 

[IRstuff]

I don't have all the curves for EG, but for PG, you'd take a 30% hit on thermal conductivity and about 5% hit on specific heat.

Supposedly, EG and PG are pretty close.

TTFN

 

[quark]

The specific heat of MEG at 30% concentration is about 3.65kJ/kgK and that of water is 4.18kJ/kgK. So to get the same refrigeration effect, you should pump more MEG solution into the system and this equals 4.18/3.65 = 1.145 or 14.5% more flowrate.

Density of MEG at 30% is 1.05kg/cu.m at against 1kg/cu.mtr of water. So there is an increase of 5%. With 14.5% increase in flowrate, the velocity of fluid increases to 1.145² = 1.31 times. For a recirculatory system, the pressure drop can be approximated to the dynamic losses in the system and so your pump has to provide 31% more head.

As the power consumed by a pump is proportional to the mass flowrate of pumped fluid times the head developed, the total increase in pump power can be 1.05x1.145x1.31 = 1.575 or 57.5% increase. I didn't consider here, the efficiency of pump which will go down when you operate it to the right of BEP. You can pick this up from pump performance curve.

Plus, you have to add the cost of MEG. If the purpose of this is to reduce the chilled fluid temperature, there will be derating of the chiller. The chiller manufacturer will give you those details.

BTW, where should I send the bill for consultancy charges

Regards,

 

[25362]

I wonder why Vortech is considering switching from water to an EG solution, cryogenic temperatures ?

 

[Vortech]

Thanks Guys, well to answer your questionas to why, I WAS even thinking about the switch, was due to some EXTREME lack
of proper maintenace/management on all hvac systems,
more to the point
(the winterizing, of chilled water coils combind with phl,rhl,and chl alrams disbaled becuase there a "nusance") we lose one to three coils EVERY winter) and the union style work force, makes personal responsabilty impossable.

seems impossable to switch due to: absorbers at capacity, distribution piping is undersized now.

If i could get approval to bring in a hvac engineering firm, I would not have had to look here, but boy I am glad there is a resource such as this out there.

Vortech

 

[PZas]

quark, I believe that you missed a few things or made a few mistakes. Assuming the same entrance and exit temperatures.

The decrease in specific heat will result in a need for a higher mass flow rate. However the denser mixture will offest the increase in velocity. This and the velocity is not squared when comparing flow rates. If you have a doubling of flow rate you have a doubling of velocity not x4.

that said using your numbers from above.

~9.5% increase in volume flow rate
~9.5% increas in velocity.
~20% increase in frictional losses.

You should find that your pump power requirements are also much lower than calculated above because of these changes.

Basically you forgot to take the 5% increase in density into account when looking at the volume flow rate (velocity) changes.

Is this not correct?

 

[quark]

PZas,

You are right. I first started with actual energy equation but as I have a doubt about the objective of changing the fluid to 30% glycol I simply wanted to make some shortcut calculation. Even my final answer seemed to be unbelievable to me but I presumed that somebody would ask for an explanation. I think 25362 is too confident about me otherwise he could easily have pointed out. However, an error is an error and thanks for the correction.

The mistake about the squaring of velocity is unintentional and I just came to know it after reading your post. Note that I considered the same for the frictional losses so that will be a silly mistake.

So the final approximate answer could be 35% increase in power. Thanks again.

Vortech,

What is the lowest temperature you perceive?

 

[IRstuff]

5% increase in density is probably offset by the 5% decrease in specific heat.

However, on a more general note, EG is considered to be a hazardous material and should be avoided. Even going to PG would be a better alternative, although, PG is not as effective as EG at extremely cold temperatures.

That said, neither EG nor PG is for ANY application, UNLESS there is a requirement for freezing point depression or possibly increased boiling point.

Additionally, EG WILL corrode certain metal systems, so your overall maintenance issues will only get WORSE.

TTFN

 

[25362]

Feeling obliged to add some corrigenda to the pressure drop calculations by both quark and PZas, we shouldn't forget the increased viscosities. These would reduce the Re numbers to about half, and increase the friction factor, f, by about 10-20% (depending on pipe rugosity and Re), say 15%.

Considering the system's friction drop proportional to f[×][ρ][×]v², the expected friction drop would rise by 1.15[×]1.05[×]1.095² ~ 45%.

 

[HVAC68]

Many HVAC companies refuse to deal with MEG, as part of their safety policy. PG would be a better option, just in case you want to use it.

HVAC68

 

[EddyC]

There's an article in one of this month's HVAC magazines that decribed the extensive corrosion that occurred when a Mechanical Contractor installed glycol in the piping system. Instead of installing glycol with the proper necessary additives the contactor installed his special brew of RAW glycol.

 

[IRstuff]

The additives are also very touchy and dependent on the EXACT metal system used.

Even then, I've had TWO water pumps corroded by off-the-shelf anitfreezes; with their "super" corrosion prevention additives.

TTFN

 

[btrueblood]

Vortech,

You say you're already "at capacity" - what kind of delta-T are you getting?