Organic Fluid Boiler question.

[Toymaker]

Rule-of-thumb says that a water tube boiler needs about 5 sqr ft / Hp, and perhaps only 4 sqr ft if the boiler uses small diameter tubing and/or high combustion gas flow rate. But what property or thermodynamic characteristic of water determines the value of 5 sqr ft/HP ? I ask this question because I want to use an organic fluid such as R30 or R234a instead of water, and I don't know how to determine, or even approximate how many square feet of tubing surface area are needed when using a different working fluid instead of water.

 

[abby]

Please explain what R30 actually is , how does it's thermal character compare with water ? When you say "water tube boiler" this can refer to different types including flash steam so please ...more detail !
Dan.

 

[Gedeon Spilett]

many organics are no more allowed to be used for thermal exchange ex: difluoroethane, freon,
I would say it is related to the "mass heat capacity " ie the amount of energy to be provided by heat exchange to raise the temperature of one unit of the mass of a substance by one kelvin

in "joule / kg / k "

.

 

[Toymaker]

Please explain what R30 actually is , how does it's thermal character compare with water ? When you say "water tube boiler" this can refer to different types including flash steam so please ...more detail !
Dan.

In terms of it's thermodynamic properties, R30 is classified as a refrigerant, but its also widely used as a solvent and can be found in paint strippers. R30 is also known as Methylene Chloride and/or dichloromethane, CH2Cl2. You can find a long list of refrigerants and their properties here: Wiki list of refrigerants. R30 has an atmospheric boiling point of 40 C, so it's a liquid at room temperature. R30 critical points are 455 F (235 C) at 881 psi (6,080 kPa).

If I understand correctly, my water tube boiler will be defined as a flash boiler as I intend to keep the "steam" inside the boiler until the pressure is just over 900 psi and temperature is just over 455 F which will keep the working fluid, R30 in this example, in a liquid state. If my design is successful, the hot, pressurized R30 will remain in the liquid state until it leaves the nozzles inside the turbine, whereupon the liquid will "flash" into a vapor thereby converting some of the heat energy into kinetic energy which will increase the velocity of the "steam" as it impacts and passes through the first row of turbine blades.

I hope these are the details you were requesting.

 

[Toymaker]

many organics are no more allowed to be used for thermal exchange ex: difluoroethane, freon,
I would say it is related to the "mass heat capacity " ie the amount of energy to be provided by heat exchange to raise the temperature of one unit of the mass of a substance by one kelvin

in "joule / kg / k "

.

What I'm hoping to find is a mathematical relationship or formula which equates one or more thermodynamic properties of water to the surface area needed to transfer a given amount of power.

I have no formal education in thermodynamics, so I'm not sure where I need to begin my search.

 

[Peter Twissell]

I think the property you are looking for is the heat transfer rate.
There are three phases in the transfer of heat from the combustion gas (fire) to the working fluid (water, or in your case R30).

The first phase is transfer from the combustion gas to the wall of the tube.

The second phase is conduction of heat through the wall thickness of the tube.

The third phase is transfer of heat from the wall of the tube to the working fluid.

The second phase is relatively simple to calculate. The thermal conductivity of the tube material can be looked up online. This value (in ISO units) gives the number of watts of energy which will pass through a square metre of material when the temperature difference across the material is one degree Kelvin per metre thickness. For copper, this value is 386.

Transfer between the combustion gas and the tube wall is dependant on gas velocity and turbulence.
If the gas moves slowly, it will quickly cool, lowering the rate of heat transfer to the tube.
If the gas flow is laminar, the layer adjacent to the tube will cool quickly, while the remainder of the gas remains hot.
The gas will, of course, always lose heat as it passes over the tube - that is the point of the boiler!
For simplicity, we can consider the gas temperature to be the mean of the flame temperature (at the point of combustion) and the exhaust temperature.

Transfer from the tube to the working fluid is similarly governed by velocity and turbulence, but since it is a liquid with a much higher density than the combustion gas, the velocity may be much lower for the same rate of transfer.

In summary, the area needed depends on the tube wall thickness and the temperature difference between the fire and the water (or R30).
The temperature difference is controlled by the velocities of the media, the combustion temperature and the boiler input and output temperatures.
All must work within the limits of the tube material - don't melt the copper!

 

[Rocket Man]

Surface area is what makes a boiler work best. A fire box surrounded by water works best too. Boiler needs to be slightly less than 5 gallons other size its not legal unless it is inspected and certified. I got good information from Allied Boiler supply. I built my boiler to hold 4.9 gallons of water & it worked excellent. I built it with 3/16" steel. I did the math to make water around the tubes and around fire box and above top tube so water was 4.9 gallons. It worked excellent. I welded everything. I think I had about 8 tubes in each row. That was 15 years ago. It powered a 4"x4" factory engines. I sold it all to someone a few years ago.

 

[Toymaker]

I think the property you are looking for is the heat transfer rate.

The second phase is relatively simple to calculate. The thermal conductivity of the tube material can be looked up online. This value (in ISO units) gives the number of watts of energy which will pass through a square metre of material when the temperature difference across the material is one degree Kelvin per metre thickness. For copper, this value is 386.

Transfer between the combustion gas and the tube wall is dependant on gas velocity and turbulence.
If the gas moves slowly, it will quickly cool, lowering the rate of heat transfer to the tube.
If the gas flow is laminar, the layer adjacent to the tube will cool quickly, while the remainder of the gas remains hot.
The gas will, of course, always lose heat as it passes over the tube - that is the point of the boiler!
For simplicity, we can consider the gas temperature to be the mean of the flame temperature (at the point of combustion) and the exhaust temperature.

Transfer from the tube to the working fluid is similarly governed by velocity and turbulence, but since it is a liquid with a much higher density than the combustion gas, the velocity may be much lower for the same rate of transfer.

In summary, the area needed depends on the tube wall thickness and the temperature difference between the fire and the water (or R30).
The temperature difference is controlled by the velocities of the media, the combustion temperature and the boiler input and output temperatures.
All must work within the limits of the tube material - don't melt the copper!

Thank you very much for educating me on the correct terminology, and for also supplying me with a guide to calculating the needed surface areas. This information is exactly what I was looking for.

 

[Rocket Man]

I forgot to mention that you can add 4 do nothing tubes in 4 places around the fire box to reduce water volume to 4.9 gallons. Pick any diameter tube that will fit that you can weld in to take up water space this puts more water near the top where the heat transfer tubes are. Try to get a few inches above the top row of tubes so as water volume changes it never goes below the top row of tubes. The boiler needs 3 valves to check water level, low level valve, maximum run level, and medium level = low run level.

 

[Peter Twissell]

Thanks, glad to be able to help.
When you start doing the calculations, you will find that R30 has a relatively low specific heat capacity.
This means that for a given power throughput, you will need to either run at a higher temperature or higher flow rate of the working fluid, relative to water (or a combination of both).
One of the advantages of water is that the feed pump doesn't need to move very much of it for a given power throughput.
Low boiling point working fluids can be useful when the boiler needs to run at a lower temperature, such as in waste heat energy recovery systems.

 

[HMEL]

Rule-of-thumb says that a water tube boiler needs about 5 sqr ft / Hp, and perhaps only 4 sqr ft if the boiler uses small diameter tubing and/or high combustion gas flow rate. But what property or thermodynamic characteristic of water determines the value of 5 sqr ft/HP ? I ask this question because I want to use an organic fluid such as R30 or R234a instead of water, and I don't know how to determine, or even approximate how many square feet of tubing surface area are needed when using a different working fluid instead of water.

It used to be that a boiler was rated in horsepower. Because they were replacing horses. But as engineers became more skilled they realized that rules of thumb could be generated by how large of a furnace had to be to generate so much steam. It was a marketing tool as much as an engineering specification. The power comes from the properties of what is called the working fluid but basically its a function of the change in pressure from high to low. To calculate that you use thermodynamics. Once you know how much heat is required you then need to use heat transfer to calculate how much surface area is required. Mercury, Ammonia, have been used as working fluids but rarely refrigerants. They fall in line with with heat pumps. So people learned quickly that very few working fluids were safe and economical to use. The organic fluids you mentioned are extremely costly and a few of the older formulations are no longer available to use because they were outlawed to protect the ozone layer. The use of these gases such as R30 are basically closed systems with the power generation device sealed with it. Its been done in some sterling engines but they are very costly and require a great deal of expertise to get them to work. I am not going to say you cant build an organic boiler or power system but unless you willing to spend lots of time I would stick with water.

 

[dazz]

Hi
I am guessing that your flash boiler turbine will run open cycle, meaning that the R30 will be discharged to atmosphere. If so:

• it will cost a lot to run with the R30 consumption,
• if you are only running 1 or a small number of turbine stages, the R30 exhaust gas temp is going to be high,
• as a solvent, exposure to a high temp stream of R30 is a serious injury hazard,
• it is an acute inhalation hazard and is absorbed through the skin,
• It is flammable in air above 100deg C.

Even if you try building a closed cycle system, you are going to get R30 leaking out, and most likely, air leaking in. That will require a reasonably complex system to manage. Probably more complex than the boiler and turbine.

It is great that you want to experiment and are asking questions. I suggest you try a more conventional path with water before venturing into such a toxic and hazardous system.

 

[Toymaker]

Hi
I am guessing that your flash boiler turbine will run open cycle, meaning that the R30 will be discharged to atmosphere. If so:

• it will cost a lot to run with the R30 consumption,
• if you are only running 1 or a small number of turbine stages, the R30 exhaust gas temp is going to be high,
• as a solvent, exposure to a high temp stream of R30 is a serious injury hazard,
• it is an acute inhalation hazard and is absorbed through the skin,
• It is flammable in air above 100deg C.

Even if you try building a closed cycle system, you are going to get R30 leaking out, and most likely, air leaking in. That will require a reasonably complex system to manage. Probably more complex than the boiler and turbine.

It is great that you want to experiment and are asking questions. I suggest you try a more conventional path with water before venturing into such a toxic and hazardous system.

I will be running a closed system, and yes, I am a little concerned with the working fluid leaking out around the shaft seals, but I'm willing to accept a small amount of leakage. Good seals used back-to-back should slow any leakage to an acceptable level.
The possibility of air leaking in seems extremely unlikely; just as air conditioning systems always maintain a positive internal pressure, so will my turbine. If you're aware of known issues of air leaking into a closed ORC (Organic Rankine Cycle) system, please let me know so that I can address those problems.

Don't get too focused on my using R30, as this was just an example working fluid that has the thermodynamic properties I want and it's a chemical I can likely find where I live in Thailand. R-514a (also known as XP30) has slightly lower critical points of both temp and pressure which will be more beneficial for my design, but I'm uncertain if I can buy XP30 here in Thailand.

I appreciate your concerns for my safety, but I will be proceeding in very small incremental steps which will either prove the design or point out flaws which need addressed. I'm an electronics engineer with a background in designing FADECs for gas turbine engines. FADEC, or Full Authority Digital Electronic Control is just a fancy name for an onboard computer which I will be using to monitor everything from "steam" temperature and pressure, to RPM of the turbine. The FADEC will have full control of fuel flow, High pressure feed pump, centrifugal air pump, ect. During engine testing, the most important function of the FADEC is it's ability to shut everything down in a microsecond if any parameter exceeds set safety limits.

 

[dazz]

Hi
I am also an electrical/electronics engineer and my experience includes management of the installation of gas and steam turbine power generation.

If you are looking to maximise efficiency and power, then you should be pulling a vacuum on the turbine exhaust through the condenser. There lies the risk of bringing air into the system.

Getting seals to work at the temps and pressures you gave above will be a "challenge".

I suggest you go to the other extreme and look at an engine based on the binary cycle. It includes all the same design elements you are looking for, but at much lower temps and pressures. It is used on Geothermal to generate power from low temp, low pressure steam.

 

[Toymaker]

Hi
I am also an electrical/electronics engineer any my experience includes management of the installation of gas and steam turbine power generation.

If you are looking to maximise efficiency and power, then you should be pulling a vacuum on the turbine exhaust through the condenser. There lies the risk of bringing air into the system.

I was hoping to discuss these details in the "Plans" Forum,... but since we're already here,....

My closed cycle turbine design does include multiple misting nozzles locating inside the "steam" exhaust chamber where cool working fluid will be sprayed into the hot vapor as it exits the final turbine stage, but I doubt very much that this cooling will be enough to pull a vacuum. However, the cooling spay does help achieve two goals, the fist of which is to cool and thereby shrink the exhaust volume into a smaller size so that I can use smaller pipes to transport the vapors to the condenser.

Getting seals to work at the temps and pressures you gave above will be a "challenge".

The second reason for spraying cooling working fluid into the hot exhaust "steam" is to substantially cool it and lower the pressure inside the collection chamber in order to relieve the pressure and temperature seen by the shaft seals. I'm confident that I can drop the temperatures seen by the seals down to 100C or less. As for Pressure, well there I'm not nearly so certain,... this metric will need to be determined experimentally during engine testing.

I suggest you go to the other extreme and look at an engine based on the binary cycle. It includes all the same design elements you are looking for, but at much lower temps and pressures. It is used on Geothermal to generate power from low temp, low pressure steam.

I've actually read through quite a few articles on Geothermal power generation, along with waste heat power, solar heat driven turbines, and how to select a working fluid. One design feature all ORC (Organic Rankine Cycle) systems share is the need to use far fewer turbine stages to extract all the heat energy from the working fluid. Most ORC turbines use between one to three stages, which is huge benefit in keeping size and weight down. (I've actually considered building a smaller scale ORC electric power generator powered by solar heat pipes,...but that's a discussion I'll leave for another day.) The primary goal of this current project is to maximize power output while minimizing size and weight of the overall system, along with developing the best fuel efficiency possible.

 

[dazz]

The primary goal of this current project is to maximize power output while minimizing size and weight of the overall system, along with developing the best fuel efficiency possible.

I think you will quickly find that the auxiliary systems (air extraction, pumps, condenser, safety devices etc) will convert a fundamentally simple cycle engine into a complex set of interacting machinery.

I wish you luck. Keep us updated with your progress. Most of all, keep it safe.

 

[Toymaker]

I think you will quickly find that the auxiliary systems (air extraction, pumps, condenser, safety devices etc) will convert a fundamentally simple cycle engine into a complex set of interacting machinery.

I wish you luck. Keep us updated with your progress. Most of all, keep it safe.

Thanks for the good wishes,...I may need them

I just started a new thread in the, "A Work in Progress" forum where I have started by describing how I started, what I've already done, and what still needs to be completed. I'll add more pics and videos as I have time, and I will try to keep everyone posted on my progress as well as any failures.

 

[TSutrina]

I was going to do the analysis which I did one around 2000 for the total time I practiced engineering. However when I looked up the properties of R134a and R32 (couldn't fine R30) the boiling point at atmosphere pressure is not practical and for a hobby boiler you wouldn't want a pressurized system and quite high. Couldn't attach https://www.researchgate.net/figure/Properties-of-refrigerants_tbl1_327403452 The boiling point of water at atmospheric pressure is 100C, ( H2O Molar mass 18.016 gm/mole). Best option on the table is R123 (CP3-CHCl3 Molar mass 152.9 gm/mole) boiling point 27.9C that is about 8C about room temperature. R21 boiling point is 8.73C not useable. Even worse is R32 boiling point -51.7 C and R134a boiling point -26.8C.

Better candidates are listed on this table: Ethanol 78.4C. Benzene 80.1C, Cylohexane 80.7C and Acetic Acid 117.9C.
https://1.bp.blogspot.com/-vNaYPRzW...wyMTtFPB-if7DljGRqBqwySUQCLcB/s1600/molal.jpg

 

[Toymaker]

I was going to do the analysis which I did one around 2000 for the total time I practiced engineering. However when I looked up the properties of R134a and R32 (couldn't fine R30) the boiling point at atmosphere pressure is not practical and for a hobby boiler you wouldn't want a pressurized system and quite high. Couldn't attach https://www.researchgate.net/figure/Properties-of-refrigerants_tbl1_327403452 The boiling point of water at atmospheric pressure is 100C, ( H2O Molar mass 18.016 gm/mole). Best option on the table is R123 (CP3-CHCl3 Molar mass 152.9 gm/mole) boiling point 27.9C that is about 8C about room temperature. R21 boiling point is 8.73C not useable. Even worse is R32 boiling point -51.7 C and R134a boiling point -26.8C.

Better candidates are listed on this table: Ethanol 78.4C. Benzene 80.1C, Cylohexane 80.7C and Acetic Acid 117.9C.
https://1.bp.blogspot.com/-vNaYPRzW...wyMTtFPB-if7DljGRqBqwySUQCLcB/s1600/molal.jpg

Thanks for the ideas,...they're always welcome.

Wikipedia has a fairly complete list of refrigerants here: Wiki list of refrigerants which includes R-30 which has an atmospheric boiling point of 40.6C and a molar mass of 84.9, so a good possible candidate as a working fluid,...Except that at temperatures above 500C it can decompose into Phosgene Gas. So,...NOT a safe choice.

Although I like the room temperature boiling points of Ethanol, Benzene, and Cyclohexane, I really don't like using a highly flammable working fluid in an experimental boiler.

R-123 looks attractive but was banned in 2020,...however, it will likely still be sold for many years yet, and its already been used in several turbine designs.

R-514a (aka XP30) has nearly identical properties as R-123 and is being sold as a "drop-in replacement for R-123.

I like both R123 and R514a as working fluids. Room temperature boiling point = 82F (27.7C), & Vapor pressure at 35C (95F) is a mere 4.3 psi. I think my shaft seals can handle can easily handle 10 or even 20 psi without substantial leakage.

What's your opinion on these two?