9 ms·

I wrote up a brief discussion here on the boiling points of refrigerants https://www.moderndescartes.com/essays/refrigerants/ https://www.moderndescartes.com/es

by brilee 4y ago

I wrote up a brief discussion here on the boiling points of refrigerants https://www.moderndescartes.com/essays/refrigerants/ https://www.moderndescartes.com/essays/refrigerants/ The long and short of it is that if the heat pump works below -20F, then the boiling point of the refrigerant must be below -20F. This, in turn, implies a higher pressurization (as per the Clausius-Clapeyron eq) required in order to achieve a T_hot of 80F (or whatever output temperature you want. The higher pressurizations require more expensive components and compressors.

Scoundreller 4y ago
Maybe I’m dumb, but aren’t compressors pressure-agnostic? They just add pressure, right? Other than needing to be a bit stronger to keep from bursting against the higher delta with the ambient atmospheric pressure like the rest of the components.
- Cerium 4y ago
  In this case there is a higher delta between the system and the atmosphere as well as a higher delta between the hot and the cold side, which is what requires a stronger compressor.
  - Gibbon1 4y ago
    I had a HVAC guy mention that seals are problematic always with non hermetic compressors. Everything larger than a fractional HP system has a shaft seal between the compressor and the motor. Small systems being a pain because there is less refrigerant and no way to monitor the amount.
    - warmwaffles 4y ago
      There is a way to monitor it, but you'd need to put in T-Junctions and add a pressure sensor on the high and low pressure sides. You'd then need to rig up some system to read those pressures. But _you_ would be unfortunately the one that has to do it because I don't think there are any off the shelf solutions.
- mindslight 4y ago
  The ideal model of everything is everything-agnostic. Refrigeration stuff is generally soft copper, and modern refrigerants are already working up in the hundreds of PSI. So I can see that getting expensive or requiring a sea change in materials.
  - eloff 4y ago
    If they’re using copper, changing materials would probably save money. Copper is comparatively expensive. I suspect your premise that they are using mostly copper is incorrect, but then I write software for a living, what do I know.
    - loeg 4y ago
      Copper is (very) effective for conducting heat. Aluminum is also pretty effective but obviously has different material properties than copper. It might be a good fit here; I don't work in the space.
      - kazen44 4y ago
        aluminium also has the problem that it is a dissimaler enough metal from most other plumbing hardware (being either made of brass, copper or steel) that it will result in the buildup of a lot of gunk because the aluminium disolves.
        eloff 4y ago
        That’s galvanic corrosion, but it’s usually an issue in water. I don’t know that it would be a problem with refrigerants, likely not.
      - bluGill 4y ago
        Aluminum generally has fatigue limits that mean it will fail eventually, releasing refrigerant (which is normally an environmental disaster)
        4y ago
- SnowHill9902 4y ago
  Power for compressible fluids is approximately proportional to the relation of pressures not difference.
- zip1234 4y ago
  Even if you do need to make thicker pipes, strength in a metal generally increases with the square of the thickness, so it doesn't cost twice as much in materials to double the strength.
  - londons_explore 4y ago
    This isn't true. For pipes, double the pressure requires double the wall thickness, which requires double the material. However, I don't think raw material cost is a big part of the cost of refrigeration systems anyway.
    - samtho 4y ago
      The “soft” copper tubing they use in refrigerant lines is not only very cheap (it’s not 100% copper) but is very much over built for pressure it is likely going to be subject to. It’s way cheaper, in this instance, to simply sell a line set that works with 99% of systems instead of having dedicated factory lines and consumer confusion from having different grades of registrant tubing.
    - dragonwriter 4y ago
      > For pipes, double the pressure requires double the wall thickness, which requires double the material. Double the wall thickness requires more than double the material.
- lazide 4y ago
  Gearing on the motor driving the compressor, compression ratio, etc. all have a huge impact on wear and tear, power requirements, etc. These all change based on the pressure delta.
raverbashing 4y ago
> The higher pressurizations require more expensive components and compressors Sure, that sounds like an acceptable compromise for those who need the lower operating temperatures And maybe technology can get those compressors at the same price point of current compressors
- jacquesm 4y ago
  But then current compressors would end up cheaper and a difference would still exist.
  - szundi 4y ago
    Or not produced anymore and prices go up for legacy system maintenance. Like try to buy 4MB of EDO RAM
    - abracadaniel 4y ago
      Your point should stand, but it did make me discover that people make new 30pin 4MB simms, which were very difficult to get for years because they were commonly used in ATMs and only available used.
    - jacquesm 4y ago
      I don't think that will happen. The price difference will be substantial due to the changes required and volume will probably not be enough to offset those.
FuriouslyAdrift 4y ago
Scroll compressors have come down in cost a bunch in the last few years. There's really very few reasons not to get a heat pump, now, for new construction.
- bityard 4y ago
  Even in areas where natural gas is much cheaper than electricity?
  - icedistilled 4y ago
    yes. Because Global warming/climate change is real and installing new furnaces bakes in decades of additional natural gas usage. Also, that natural gas price may change.
    - bombcar 4y ago
      If you want to spend the money you can get systems that have heat pump, gas furnace/boiler, and electric. Often used for larger installations that can take advantage of differing prices - and can work best with water heat.
  - dgacmu 4y ago
    A heat pump is likely to have an average efficiency of near 300% - call it 250 if you want to hedge. A high efficiency condensing gas furnace gets about 95%. So on a per-watt-hour basis (equiv., BTUs), your gas must cost less than roughly 1/3 what your electric costs. There are additional savings if your gas furnace is the only gas appliance in your house and you can remove gas service entirely, saving the monthly customer fee.
    - pandaman 4y ago
      And to get to 250% COP in heating on average you'd expect to have, at least, 8.5 HSPF2 rating on your heat pump (it's still just the heating performance w/o de-frosting cycles as far as I understand so the real world COP of a 8.5 HSPF2 will be lower than 2.5)
londons_explore 4y ago
> then the boiling point of the refrigerant must be below -20F This isn't the limiting factor for choice of refrigerant... There is always a low enough pressure that anything boils. The problem is that at very low pressures (think a few millibars), gasses need huge diameter pipes and huge pumps to move even a small number of kilowatts of heat.
- brilee 4y ago
  Another issue is that at pressures below one atmosphere, the system will suck in ambient air, causing condensation to collect inside your refrigeration circuit, which creates ice crystals, etc… nobody does that.
  - londons_explore 4y ago
    Plenty of systems work below atmospheric pressure. For example, Fridge freezers most commonly use isobutane (R600a), and when operating with the freezer at -20C, the pressure will be half an atmosphere (ie. -0.5 bar below atmospheric). The systems are filled using a vacuum pump to ensure there is very little atmospheric air in them, and they also use a 'filter dryer', which is a chemical compound chosen to trap any remaining water and various other common contaminants.
mateo1 4y ago
On top of that, the maximum theoretical COP for a cold source at -30c and a hot side at 35c is 4.7. Real world heat pumps usually get half of that, possibly even less for this extreme delta t, so around 2. Natural gas tends to cost less than half per unit energy compare to electricity, even less typically, so you'll never make back the capital costs. Of course if this technology matures to the extend it's much, much cheaper and you tend to have milder winters and hot summers it's good enough and possibly worth it, but we haven't gotten to maintenance and installation costs yet, as the 35c hot temp assumes underfloor heating or tons of fancoils which have their own issues, the fact that the heat pump must be massively overspectd to output enough heat in peak conditions etc.

by brilee 4y ago

Scoundreller 4y ago

Maybe I’m dumb, but aren’t compressors pressure-agnostic? They just add pressure, right? Other than needing to be a bit stronger to keep from bursting against the higher delta with the ambient atmospheric pressure like the rest of the components.

Cerium 4y ago
In this case there is a higher delta between the system and the atmosphere as well as a higher delta between the hot and the cold side, which is what requires a stronger compressor.
- Gibbon1 4y ago
  I had a HVAC guy mention that seals are problematic always with non hermetic compressors. Everything larger than a fractional HP system has a shaft seal between the compressor and the motor. Small systems being a pain because there is less refrigerant and no way to monitor the amount.
  - warmwaffles 4y ago
    There is a way to monitor it, but you'd need to put in T-Junctions and add a pressure sensor on the high and low pressure sides. You'd then need to rig up some system to read those pressures. But _you_ would be unfortunately the one that has to do it because I don't think there are any off the shelf solutions.
mindslight 4y ago
The ideal model of everything is everything-agnostic. Refrigeration stuff is generally soft copper, and modern refrigerants are already working up in the hundreds of PSI. So I can see that getting expensive or requiring a sea change in materials.
- eloff 4y ago
  If they’re using copper, changing materials would probably save money. Copper is comparatively expensive. I suspect your premise that they are using mostly copper is incorrect, but then I write software for a living, what do I know.
  - loeg 4y ago
    Copper is (very) effective for conducting heat. Aluminum is also pretty effective but obviously has different material properties than copper. It might be a good fit here; I don't work in the space.
    - kazen44 4y ago
      aluminium also has the problem that it is a dissimaler enough metal from most other plumbing hardware (being either made of brass, copper or steel) that it will result in the buildup of a lot of gunk because the aluminium disolves.
      - eloff 4y ago
        That’s galvanic corrosion, but it’s usually an issue in water. I don’t know that it would be a problem with refrigerants, likely not.
    - bluGill 4y ago
      Aluminum generally has fatigue limits that mean it will fail eventually, releasing refrigerant (which is normally an environmental disaster)
      - 4y ago
SnowHill9902 4y ago
Power for compressible fluids is approximately proportional to the relation of pressures not difference.
zip1234 4y ago
Even if you do need to make thicker pipes, strength in a metal generally increases with the square of the thickness, so it doesn't cost twice as much in materials to double the strength.
- londons_explore 4y ago
  This isn't true. For pipes, double the pressure requires double the wall thickness, which requires double the material. However, I don't think raw material cost is a big part of the cost of refrigeration systems anyway.
  - samtho 4y ago
    The “soft” copper tubing they use in refrigerant lines is not only very cheap (it’s not 100% copper) but is very much over built for pressure it is likely going to be subject to. It’s way cheaper, in this instance, to simply sell a line set that works with 99% of systems instead of having dedicated factory lines and consumer confusion from having different grades of registrant tubing.
  - dragonwriter 4y ago
    > For pipes, double the pressure requires double the wall thickness, which requires double the material. Double the wall thickness requires more than double the material.
lazide 4y ago
Gearing on the motor driving the compressor, compression ratio, etc. all have a huge impact on wear and tear, power requirements, etc. These all change based on the pressure delta.

raverbashing 4y ago

> The higher pressurizations require more expensive components and compressors Sure, that sounds like an acceptable compromise for those who need the lower operating temperatures And maybe technology can get those compressors at the same price point of current compressors

jacquesm 4y ago
But then current compressors would end up cheaper and a difference would still exist.
- szundi 4y ago
  Or not produced anymore and prices go up for legacy system maintenance. Like try to buy 4MB of EDO RAM
  - abracadaniel 4y ago
    Your point should stand, but it did make me discover that people make new 30pin 4MB simms, which were very difficult to get for years because they were commonly used in ATMs and only available used.
  - jacquesm 4y ago
    I don't think that will happen. The price difference will be substantial due to the changes required and volume will probably not be enough to offset those.

FuriouslyAdrift 4y ago

Scroll compressors have come down in cost a bunch in the last few years. There's really very few reasons not to get a heat pump, now, for new construction.

bityard 4y ago
Even in areas where natural gas is much cheaper than electricity?
- icedistilled 4y ago
  yes. Because Global warming/climate change is real and installing new furnaces bakes in decades of additional natural gas usage. Also, that natural gas price may change.
  - bombcar 4y ago
    If you want to spend the money you can get systems that have heat pump, gas furnace/boiler, and electric. Often used for larger installations that can take advantage of differing prices - and can work best with water heat.
- dgacmu 4y ago
  A heat pump is likely to have an average efficiency of near 300% - call it 250 if you want to hedge. A high efficiency condensing gas furnace gets about 95%. So on a per-watt-hour basis (equiv., BTUs), your gas must cost less than roughly 1/3 what your electric costs. There are additional savings if your gas furnace is the only gas appliance in your house and you can remove gas service entirely, saving the monthly customer fee.
  - pandaman 4y ago
    And to get to 250% COP in heating on average you'd expect to have, at least, 8.5 HSPF2 rating on your heat pump (it's still just the heating performance w/o de-frosting cycles as far as I understand so the real world COP of a 8.5 HSPF2 will be lower than 2.5)

londons_explore 4y ago

> then the boiling point of the refrigerant must be below -20F This isn't the limiting factor for choice of refrigerant... There is always a low enough pressure that anything boils. The problem is that at very low pressures (think a few millibars), gasses need huge diameter pipes and huge pumps to move even a small number of kilowatts of heat.

brilee 4y ago
Another issue is that at pressures below one atmosphere, the system will suck in ambient air, causing condensation to collect inside your refrigeration circuit, which creates ice crystals, etc… nobody does that.
- londons_explore 4y ago
  Plenty of systems work below atmospheric pressure. For example, Fridge freezers most commonly use isobutane (R600a), and when operating with the freezer at -20C, the pressure will be half an atmosphere (ie. -0.5 bar below atmospheric). The systems are filled using a vacuum pump to ensure there is very little atmospheric air in them, and they also use a 'filter dryer', which is a chemical compound chosen to trap any remaining water and various other common contaminants.

mateo1 4y ago

On top of that, the maximum theoretical COP for a cold source at -30c and a hot side at 35c is 4.7. Real world heat pumps usually get half of that, possibly even less for this extreme delta t, so around 2. Natural gas tends to cost less than half per unit energy compare to electricity, even less typically, so you'll never make back the capital costs. Of course if this technology matures to the extend it's much, much cheaper and you tend to have milder winters and hot summers it's good enough and possibly worth it, but we haven't gotten to maintenance and installation costs yet, as the 35c hot temp assumes underfloor heating or tons of fancoils which have their own issues, the fact that the heat pump must be massively overspectd to output enough heat in peak conditions etc.