Effects of glycol on BTU transfer

ALH_4

> Hot Rod et al said a 50/50 mix of glycol will <BR>
> increase head 50%, and cut BTU transfer by 30% <BR>
> (so did the Munchkin OEM manual by the way). So, <BR>
> if I have a 400,000 BTU Munchkin running 50% <BR>
> glycol, that means I am only going to get 70% <BR>
> output, or 280,000 BTU. Right? If I am only <BR>
> going to get 280,000 BTU, do I now only need to <BR>
> move 28 gpm across the heat exchanger instead of <BR>
> the 40 gpm? Granted, I will have to move 28 gpm <BR>
> at a higher head, but am I right in assuming that <BR>
> a decreased BTU output due to glycol now requires <BR>
> a corresponding decrease in flow, assuming the <BR>
> same delta T?<BR>
> <BR>
> Just trying to clarify this <BR>
> concept in my feeble little brain. <BR>
> Regards, Rocky <BR>
<BR>

Rocky_3

Posts to earlier thread on snowmelt

Hot Rod et al said a 50/50 mix of glycol will increase head 50%, and cut BTU transfer by 30% (so did the Munchkin OEM manual by the way). So, if I have a 400,000 BTU Munchkin running 50% glycol, that means I am only going to get 70% output, or 280,000 BTU. Right? If I am only going to get 280,000 BTU, do I now only need to move 28 gpm across the heat exchanger instead of the 40 gpm? Granted, I will have to move 28 gpm at a higher head, but am I right in assuming that a decreased BTU output due to glycol now requires a corresponding decrease in flow, assuming the same delta T?

Just trying to clarify this concept in my feeble little brain.
Regards,
Rocky

Drew_2

Glycol/BTU

Rocky
Here's something I found that may help. See attachment.

Tom Blackwell_2

This is a misnomer-What is really being said is that the btu capacity is reduced at the same conditions. A reduction in heat transfer capacity will result in higher flue gas temperature and a loss in overall efficiecy due to the increase in btu's going out the stack. Combustion efficiency is not affected. The required velocity through the heat exchanger would not change because the same amount of heat is liberated by the burner, only more of it passes through out the stack.

hot_rod

Could you use a 40 or 45% mix?

that buys you a little more heat transfer room.

Use the freeze and slush tables from the manufacturer of the glycol you plan on buying. They do vary a bit from brand to brand as far as mix percentage and protection rates.

Get the Engineering& Operating Guides from Dow, # 180-01286-1001 AMS. They have a lot of generic great info on EG and PG use.

hot rod

ALH_4

Flow and Temp Rise

What will happen is that the DT will increase proportionally to the flow with a fixed BTU input. You will still have 400,000 BTU. For 50% glycol, GPM=BTU/(500*.85*DT). That will give you the flow rate. Find the head loss at that flow rate, and those are your design operating conditions.

I would design for a larger temperature rise across the boiler in a snowmelt system. A 40°F rise should be acceptable for a dedicated snowmelt boiler. The has head loss numbers starting on page 35 for different sizes of pex, temperatures, and glycol concentrations.

Unknown

CHECK!!

Always do a check with a refractometer on the stuff in the bucket BEFORE you try to use it. It DOES vary quite a bit from maker to maker. I found that Utility Chemical's "No Freeze" was 50/50 IN THE CAN! I found out the hard way, which, in this case, was also the expensive way. Another point of my discussion with them:

Freeze point= the temperature at which the very first ice crystal begins to form below that point the solution turns to slush but the slush will still flow. At this point it begins contracting to provide burst protection until it reaches burst point.

Flow point= The temperature below which the slush will no longer flow.

Burst point: The temperature at which it will begin to re-expand and burst pipes.

When you see the can says, "Protects down to -100*" that's the burst point of whatever is in the can. Flow point is much much higher. On the order of +10*F at 50/50. If I remember correctly.