methanol properties in calculations

Discussion in 'Geothermal Loops' started by milkweed, Mar 9, 2016.

  1. milkweed

    milkweed Member

    Two questions.
    The design guide that was supplied with the our heat pump (previous owner installed) has a few values that I want to confirm.

    What is that viscosity value of a 20% by volume of methanol?
    The design guide I have says 3.5 centipoises (cP) at 25 degree F.
    I ask because they have the wrong value for polypropylene glycol.
    I have given up searching online, as I have only found values for 100% methanol.

    When calculating the feet/head loss, the design guide I have says to use a multiplier of 1.25 for 20% methanol @ 25 degree F.
    My question here is whether that 1.25 multiplier is specific to my heat pump?
    ... because I found at http://www.waterfurnace.com/literature/envision/IM1006WN.pdf
    page 33 shows a value of 1.197 at 30 degree F, but there is no mention of which heat pump model those values apply to, as though the values are universal.
     
  2. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    The increase in viscosity is independent of the heat pump.

    For the purpose of flow, all the heat exchanger is is a flow restrictor.
     
  3. milkweed

    milkweed Member

    So is the multiplier (when figuring the ft.hd loss calculations) a function of the thermal transfer ability of the antifreeze solution?

    Our heat pump manual says the minimum flow rate for water only is 5 GPM, and 7 GPM for salt or potassium acetate solution, but no reference to methanol or ethanol. I found a manual for a newer model heat pump by the same manufacturer that has the same ratio of GPM flow that lists 15% methanol solution. I am wondering then if 5 GPM * 1.25 = 6.25 GPM would be required for our heat pump, but use 7 GPM when looking at ft.hd loss values in the table to use in calculating the pressure drop of the system...?
     
  4. frankz

    frankz New Member

    The viscosity of coolant is really a separate issue from the heat transfer capacity. If you have a minimum flow specified for water, you would have to divide that by the heat capacity factor (typically 0.9XX) to give the (sightly increased) GPM you would need to get the same heat transfer. Then you’d have to know the feet of head needed for that flow rate for water, and you’d multiply that by the viscosity factor to get the pressure needed for a methanol/water mixture. In other words, in computing the required feet of head, you have to compensate for both the lower heat capacity and the higher viscosity of methanol/water, both of which are intrinsic properties of the mixture and independent of the specific heat pump.
     
  5. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    In general we account about 3% for a loss in heat transfer when antifreeze is used. But you cannot just make this up with increase in flow (e.g. set the pump higher) and think that now the heat transfer will be the same, your delta T will decrease now too. Just because you change the flow, you will not change the heat transfer capacity of the fluid, or make up for it.

    Not sure why this is an issue. You need to be in the ballpark of the flow to keep the heat pump happy and running efficient. To be off by 10-20% does not matter very much for the heat pump to run well. We purposely reduce flow any times to optimize our system, because the pumping power reduction is significantly more than the performance penalty on the heat pump due to the reduced flow, significantly reducing the overall energy.

    One could be a bit cynical here, and state that the current rating system incentives a rating game to be played, where the pumping power penalty is so minimal on paper, but it make the heat pump performance look so great. In reality, pumping power is more significant.

    So most of our system we design with 75% flow to have them run more efficient.......:)
     
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  6. milkweed

    milkweed Member

    docjenser wrote:
    I don't follow because I don't understand all of that; sorry I wish I could say that I did. I get that an antifreeze solution does not have same capacity/ability to transfer heat as pure water. But why then do all of the heat pump manuals from different manufacturers specify a minimum flow rate for the loop that is faster when the fluid is an antifreeze solution than for pure water?


    docjenser wrote:
    Does that result in the heat pump running longer?
    Or you oversize the heat pump capacity in order to match the load at 75% flow?
    I wonder if your 75% remark is related to the performance charts I see in the manuals of newer model heat pumps.


    frankz wrote:
    Where could I find that heat capacity factor?
    Is the "Antifreeze Correction" table on page 33 of http://www.waterfurnace.com/literature/envision/IM1006WN.pdf a list of heat capacity factors?
    ... so 20% methanol during heating source ( 30 F ) = 0.924 ???
     
  7. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Because pure water is considered an open system, which usually is at or above 50F, thus it has much more heat in it, and the water flow does not have to be as much. So the heat pump takes 10F out of it instead of 5F, due to less flow. That way you save pumping power.


    No, the heat pump capacity remains the same (almost), thus the runtime is almost the same. Thus no need to oversize. It just means that the heat pump takes more heat out of lesser water, but the overall heat extraction from the water remains (again almost) the same. The the capacity remains the same. The 75% remark was for the water flow only.
     
  8. milkweed

    milkweed Member

    docjenser wrote:
    So the heat pump is given more time to exchange heat with the same interval of water?

    Ahh, so is the published heat pump flow requirements a different perspective then, 'here is the flow rate required for antifreeze, but if you are using water only then the flow rate is less' ??? in my case 7 for antifreeze then 5 if water only
    Would I assume then for my heat pump to use 20% methanol it would be 7 GPM?
    ... cause if I understood what frankz wrote, and if I found an appropriate heat capacity factor, then it would be 6 GPM
     
  9. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Yes, lesser flow means higher delta T, it has more time to pull more heat out of lesser water, but the total amount of heat pulled out remains similar.
    Again, flow rate is independent from antifreeze concentrations, but if you mix with antifreeze, the water mix contains lesser heat at the same temp than water. But this does not influence the capacity of your heat pump.

    Flowrate is less with water since it is assumed that water is warm enough that you don't need antifreeze, thus there is more heat in the water.
    Manufacturers publish flow rates for heat pumps, and reduce the recommended flow rate in open loop, water only applications. For example, open systems are rated at 50F, closed system are rated at 32F. Thus there is more heat in the water at 50F and you can slow down the water flow to save pumping costs.
     
  10. geoxne

    geoxne Active Member Forum Leader

    I posted this on another thread. I think it might help. Antifreeze correction factors are not manufacturer or model specific.
    AntifreezeCorrection.JPG
     
  11. Rogerthat

    Rogerthat New Member

    New Member Here

    Methanol – Water Solution Properties?
    Maybe not a major thing but I too am looking for properties of methanol and water. Say viscosity (cp or cs) and gravity at 40F, concentrations of 10 vol% and 20 vol%. Freeze Point I have at 23F and 8F but Burst Point ??? Notice Dow provides Burst Point for their Glycols. Anything out there for Methanol and/or Ethanol?

    Antifreeze Mechanical Protection?
    Equipment protection as relates to burst point in the heat pump source heat exchanger (lowest temperature point in loop).

    Personally see little advantage to operate subfreezing and freeze up the ground loops. Would seem that is just an indication the ground loops are undersized. But maybe at the 10% level for burst protection due to heat pump flow problems in the loop.

    Thanks for information and thoughts.
     
  12. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Freeze point and burst point is pretty close together with methanol, if it freezes it freezes solid quick, which is in contrast to glycol, which forms a slush first for a while.

    Let me be clear here, and I have stated that before: By definition, the loops cannot freeze up! They are not the coldest point. It is always, ALWAYS, the tail of the source heat exchanger which is the coldest part, where the water is the coldest. The heat exchanger will freeze from the outer lumen first and ice will build up on the metal between water and refrigerant, which is much colder than the loop water. So even if you have the loop water coming in at +35F it will leave at subfeeze temps of 30F or below. That is normal, you need to create a temp differential to transfer heat.
    Because of that, open water systems without antifreeze, usually do freeze below 38F leaving water temp, because thereat exchanger is already way below freeze point. You need the antifreeze not only at sub freeze loop temp, but already at much warmer loop temps.
     
  13. Rogerthat

    Rogerthat New Member

    New Member Here

    Methanol – Water Solution Properties?
    Maybe not a major thing but I too am looking for properties of methanol and water. Say viscosity (cp or cs) and gravity at 40F, concentrations of 10 vol% and 20 vol%. Freeze Point I have at 23F and 8F but Burst Point ??? Notice Dow provides Burst Point for their Glycols. Anything out there for Methanol and/or Ethanol?

    Antifreeze Mechanical Protection?
    Equipment protection as relates to burst point in the heat pump source heat exchanger (lowest temperature point in loop).

    Personally see little advantage to operate subfreezing and freeze up the ground loops. Would seem that is just an indication the ground loops are undersized. But maybe at the 10% level for burst protection due to heat pump flow problems in the loop.

    Thanks for information and thoughts.
     
  14. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Alcohols/Glycols... there are graphs and tables all over the internet for this information.

    But years ago, I went down to the public library and photocopied the tables from the CRC Chemistry and Physics handbook.

    All part of my "OneNote" technical manual now.
     
  15. Rogerthat

    Rogerthat New Member

    Doc, thanks for your answer. Doing some “digging” here as lay out the rest of the system. At this point, have loops in place. North of Seattle here and ground temperatures in the mid 40’s. While loops not deep (3-3-1/2’) are in clay and always wet. Six 800ft loops ¾”, about half in coils and half in straight runs to and from valve box at house. Five ton unit, flows 15-18 gpm. My thought is to monitor temperature (in and out) and flow with heatpump shutdown on either. Was thinking 35F-out, without antifreeze, maybe too close and should bump that up or go with 10% methanol. Not a large volume, but a pressurized system and hard to maintain. For flow, are you familiar with Universal Flow Meter (flowmeter.com)? They have a PCV vortex shedding meter (low pressure drop) with 4-20ma transmitter in the $500 range. My thought is to couple that with a PLC from entertron.com. Not inexpensive but not terrible either. With some amps, would then have the above shutdowns and real-time COP.
     

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