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Propane Burner Designs

07/15/2009 1:54 PM

What is the exact relationship between orifice size, and air inlet, and burner head size in a propane fuelled burner for something like a barbeque or stove? Although I have cleaned, repaired and even rebuilt propane fuelled stoves I am unable to derive an exact relationship that could then be used to design a burner of specific heat output. My internet search produced several sites showing how to build blow torches and high BTU burners for melting and casting metal but nothing along the lines I need. I wish to build a modest size burner of around 3000 - 5000 BTU. My experiments with existing devices have proven disappointing. Where do I go to get exact design data? I have tried going to a propane product dealer but they either want to sell me an expensive and over sized product or tell me only 'authorized' dealer are allowed such knowledge; plus requires a provincial licence which involves several years of trade school attendance.

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#1

Re: Propane Burner Designs

07/16/2009 3:35 AM

The heat output depends upon the propane flowrate and that depends upon the pressure at which the propane is supplied to the burner nozzles. There is a limit, and that is the point at which there is so much propane passing through the burner that the flame detaches from the nozzle and the flame extinguishes itself. Don't go there.

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#2

Re: Propane Burner Designs

07/16/2009 7:53 AM

As PW said a burner jet will deliver a given amount of gas for a given pressure, the tables will relate this volume to so many BTU /hr or min . This volume requires a specific volume of air for 100% combustion, so the induction ports are sized to give this amount as it is not a precise art you have oversize ports with an adjustable sleeve to regulate the air for the best flame. Don't forget your mixing chamber must be long enough to provide intimate mixing or poor flames will result. Also the shape of the chamber should start off narrow and broaden out to where the burner ports are also the ports might need to be distributed in a pattern which will give even burning depending on the design of your mixing chamber.

So there are many points to consider as you have not specified what type of burner you wish to make I cannot offer any other suggestions.

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#3
In reply to #2

Re: Propane Burner Designs

07/16/2009 12:55 PM

Thank you Garth. Now we are getting somewhere. The general statements about mixing fuel and air PW slack and you have mentioned were known to me but you now mentioned 'tables'. Where do I find them? My apparently clumsy google search failed to lotate them, or even hint at their existence. Logic should have suggested there was something already. got a link?

I started with the premise of using the existing pre set regulators found on so many stoves and BBQ sold to the public. I also started with a burner assembly from an Absorption fridge scrounged from a camper. That flame is just a bit small for what I need. No idea how many BTU, but I estimate I needed less than double the BTU.

Leaving the pressure the same, I assumed that by opening the orifice ever so slightly and by increasing the burner (head area?) cross section of holes (?) or whatever; I could increase BTU output by a small amount. I also thought if I could somehow introduce an adjustable flow valve I could go from full to maybe 50% as a fine adjustment of heat. Trying to get a BBQ burner with a huge rectangular burner to just emit flame in one location is not practical. Hence my desire to make a smaller and round shape. Therin lies the challenge.

My reason for using the fridge assembly is because it is equipped with a thermocouple sensing shut off valve and a piezo electric ignitor. The shut off valve for safety and the ignitor for conveneince.

You mentioned 'induction ports'. On the burner I have, the lenght of 'chamber' is maybe one inch and the induction ports are fixed. That makes sense for a fridge with known and fixed parameters. BBQ burnes have 4 - 5" long induction chambers. Since they need variable heat and are manually monitored. My intended application is not monitored 24/7

By way of tooling, I do have micrometers, both watch makers and machinists types. I have fine reamers down to 0.005" diameter and a drill index down to size #80. I have a tapered gauge pin so I can actually measure the inner diameter of jet orifices. In a pinch I have used cats whiskers ( apologies to Del) to measure very fine size holes. A legacy from my hot rod days when I rebuilt carburetors and modified them from factory sizes. I have a watch makes lathe if I have to make new jets from scrach. I also have a pressure gauge suitable for low pressure propane ( a propane test kit) and both water and mercury manometers for differential or absolute measurements.

What I do not have is the information of what size range of holes I have to construct the induction ports with, relative to orifice size at a set pressure and how long the induction chamber must be. This latter is what you just taught me.

TIA

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#6
In reply to #3

Re: Propane Burner Designs

07/16/2009 9:04 PM

Hi elnav.

Just a couple of quick links as have to get to work.

http://www.regoproducts.com/Information/Literature.htm

http://www.regoproducts.com/PDFs/LP-Gas_Servicemans_Manual.pdf

This manual will give you jet sizes. You have all the tools measure the burner jet you have and interpolate to the tables for what you need.

Will help if I can later when have time.

Regards Garth.

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#7
In reply to #6

Re: Propane Burner Designs

07/16/2009 11:19 PM

Thanks! That will take a bit of time to digest. Looks like the kind of stuff I was looking for. Much obliged!

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#4

Re: Propane Burner Designs

07/16/2009 2:22 PM

I recently built a gas forge/heat-treating oven, using propane as the fuel source. In order to have sufficient and variable vapor pressure available, I installed a pressure regulator that is adjustable from 0 to 60 psi. The vapor pressure in a full propane tank is around 125 psi and, if you are using a weed burner, that is the pressure that you will get, as there is no regulator in use.

I purchased two monster burners from Rex Price in Georgia. He fabricates several sizes of propane burners for forgers and other users, even NASA. I am quite satisfied with the burners he made for me. I think he makes some relatively small ones that might work for you and he is certainly knowledgeable about the subject. If you 'Google' "Rex Price", you will find his web site and you can see his work and his contact information there.

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#5
In reply to #4

Re: Propane Burner Designs

07/16/2009 2:48 PM

Thank you but that is exactly what I do not want. As I said in OP I did find such websites. Thes flame throwers produce way too much heat and in any case your referenced source is in another country, relative to me. I did work as a gas fitter's helper one summer job, so I am acquainted with the basics. I am now looking for information as a designer, not just an installer.

I want a SMALL burner that would fit inside a 2" diameter housing and probably deliver only 3000 - 5000 BTU of heat, not more.

In case you have not noticed, fortress America is almost closed for business, at least as far as small individuals are concerned. Especially those who lack credit cards and do not have access to banking services. And for those who do, they pay exorbitant transfer fees and it takes two or three weeks for money transfers across the border. This based on my own experience last month.

Which is another reason I want to design my own.

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#8

Re: Propane Burner Designs

07/17/2009 1:36 PM

elnav try http://www.andersonforrester.com/, good site & may help.

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#9
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Re: Propane Burner Designs

07/17/2009 2:39 PM

It looks like the crunch in design comes down to sizing the mixing chamber.

Garth wrote: " the induction ports are sized to give this amount. As it is not a precise art you have oversize ports with an adjustable sleeve to regulate the air for the best flame. Don't forget your mixing chamber must be long enough to provide intimate mixing or poor flames will result. Also the shape of the chamber should start off narrow and broaden out to where the burner ports are. Also the ports might need to be distributed in a pattern which will give even burning depending on the design of your mixing chamber."

While the orifice size is given in tables, no mention is made of induction port sizes. Or their relationship to gas flow input. Most consumer appliances are designed to work with the standard 20lb tank or similar and sealed regulators are also very common. They appear to be interchangable as far as use goes, leading me to think that most consumer RV products are designed to work at 9 - 11 inches of water column pressure. Where this is an exception, the pressure regulator is often included as an integral part of the fuel delivery assembly. Coleman propane stoves that run from the small bottles is the example that comes to mind.

Looking at BBQ replacement burners I see a variety of mixing chambers ranging in size from 4" to 6" Many of them no longer have the adjustable sleeve to fine tune the induction ports. Hmmm?? Does this mean the designs are now optimized or production has been cost reduced and performance has taken second place?

What I was planning to make was a round burner of approximately 1-3/4" to 2" diameter to fit inside a scrap cylinder head from an old air cooled motor. The idea being to heat up the aluminum fins that would then dissipate the heat. The spark plug hole would serve as the 'chimney' outlet and a pipe leads the combustion exhaust outdoors. This approach seem sto be better than just an open flame. I suspect because the radiating fins emit longer infra red wavelenght.

Since the volume of space to be heated is not great and its only intended to take the nip off freezing temps ( green house) I did not want to get something with so much heat output it would require turning on / off. Adding automatic ignition and temp control would needlessly complicate the design.

I found in one table that fridges consume about 3000 BTU per hours. That confirms my estimate of 3000 - 5000 BTU capacity as about right. 10,000 would likely be too much for a small 8 foot green house. Later if we expand I would rebuild it to suit. Now about the mixing chamber and the opening size of the induction ports?

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#10
In reply to #9

Re: Propane Burner Designs

07/17/2009 9:30 PM

Hi Elnav,

I am on my Saturday morning coffee now so a bit more help if I can.

Your refrigerator burner is a good starting reference. If you have checked the jet size of this burner the tables will give the BTU output for this size,look up double the BTUs and the jet size [of about 72 in your case].

Then you can look at the number of burner holes and consider doubling them but keep same hole diameter.

The reason for the narrow throat at the jet position is to give a venturie action to pull in the air then broaden out to slow down and mix the gas. In the case of a BBQ burner yes they are optimised so that the variable flow of gas draws in the correct amount of air, also the mixture is ported into the center of the burner to give even distribution of the gas to the burner ports In your case you have a central rising tube to the burner ports so the gas will distribute itself OK . Also your gas supply is a fixed flow so a movable restriction at the induction ports [ which should be on the sides of your of the rising tube for ease of use, [ block the base off] also you can mount your jet if you wish on a fully threaded adjustable brass rod so you can vary its position relelative to the induction ports] this enables you to adjust the mixture precisley for a nice blue flame.

As to the length of the mixing chamber use your caravan burner for sizeing and you can also make this quite a bit longer as the length is not a problem [within reason] but to short is not good.

Also I think your spark plug hole will be to small, you must have a chimney of a diameter which does not restrict the volume of burnt products, in your case you are heating the air in the greenhouse so you do not realy need a chimney as such, so a shielding enclosure for saftey is all you need.

I do not know how cold it gets in your area but 5000 BTUs might be a bit to much for an 8ft greenhouse, also you must have a small amount of ventillation to avoid using up your oxygen and building up the CO2.

I hope that gives you some help. Regards Garth

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#11
In reply to #10

Re: Propane Burner Designs

07/18/2009 12:36 AM

Thanks Garth! Yes this definitely helps. Especially the bit about how to adjust the size of the outlet holes in the burner.

Actually it gets quite cold here. This year we had a killing frost June 8 and the snow dos not leave the ground until end of April or Mid May. We got stopped by snow drifts in June when out scrounging fro fire wood.

The goal is to add a month at each end of the short summer to give us a true six month growing season. Snow flies by October 31 and the snow blower could be called for anytime after Nov 15 by which time we also have to have snow tires mounted. I built a high peaked roof to give me some snow shedding ability. We typically get 8 feet each winter. If money permits We plan to install that corrugated lexan roof sheeting. But by May we also have 12 - 16 hours of sunlight. Its now just 2130 Hr and the sun has just set. A wood burning stove is too much heat for the size of greenhouse and a small one would not last overnight. Hence the idea of using apropane burner for now.

I was just out playing with a burner head , but was not happy with the result. Now I understand why.

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#12
In reply to #10

Re: Propane Burner Designs

07/18/2009 3:16 AM

Garth here are some numbers.

Original orifice was #76 or #75 Fridge rated at around 3000 BTU/ hr.

Number of burner holes is 8, each is 3mm diameter, spaced around the circumference of a half dome circle of 13 mm diameter. Actual burner head is 20 mm diameter overall including the crimped flange. The air mix tube is tapered from 7 mm diameter at orifice to 12 mm just below burner. Total lenght of air tube is 65 mm but it is bent into a 90 degree right angle. The air induction ports number 4 and each are 4 mm in diameter. It looks like the area of burner holes is 12% greater than the air inlet ports..

Induction ports amount to 50.24 square mm and the burner holes amount to 56.54 square mm.

The orifice is exactly in line with the center point of the inlet holes. The fitting that holds the orifice totally blocks the end of the venturi tube.

One difficulty - you recommended doubling the number of holes but keeping the same size. Given the physical size of the burner head, this is not feasible. There is only 1.5 mm web between each hole at present and there is just under a 10 mm circle inside the existing holes. Not enough to make another circle of holes same size.

Drilling that many holes would effectively punch out the whole burner head. How critical is it that I double the area of the burner outlets. I gather that I do not change the induction ports, only the orifice size.

The burner head and the venturi tube is pressed from two stainless sheet of metal. They are spot welded together and the dome of the burner is popped into place and the edges rolled over slightly to hold it in place. There is a heavy SS mesh in behind the burner holes so the actual open area is reduced. I estimate by about 20%.

I would have to cut away the sheet metal completely, then machine a brass head and drill the new holes but now the head would be much larger than the original. Problem would be in how to fastern the machined head to the sheet metal of the venturi tube.

The original burner head had flames of approximately 12 mm lenght. If I were to reduce each hole diameter, would that mean each flame would be longer? I take it the aggregate area of the total number of new holes has to equal double the area of the original holes.

Regarding the spark plug hole. There are also two side ports one on either side of the cylinder head. It was an old husquarna chainsaw in a better life. So if I just use a 3" diameter sheet metal sleeve and starting above the cylinder head fins, that should do it. On the other hand plants use Co2 gas, so it probably would not harm them. We would just have to remember to open door and ventilate the space before a person enters. Or I could ream out the hole to allow a 1" diameter pipe for a chimney. Its only aluminum.

I once saw a home made DIY cabin heater on a boat, built like that. It used the cylinder head of an OMC lawnmower and the burner of a primus stove. It was fuelled by kerosene but the principle is essentially the same. The marine chandles want over $1000 for such a factory made heating appliance. That is more than I make in a month. Simply unaffordable.

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#13
In reply to #12

Re: Propane Burner Designs

07/19/2009 10:17 PM

Hi Elnav,

Sorry I did not realise [I misinterpreted ] you were to use the piston head as a burner I thought you were placing the piston over the burner you were going to make. Thats why I mentioned restricting the burnt products.

Re the burner holes if you double the amount of gas which will double the amount of air your burner holes will need to be doubled [ if you wish to have the best flame ] or you will get some back draught. So you could shoot for a 50% increase regarding the head size. The size of the burner holes are suited to the velocity and volume of the mixture , if you increase the diameter and reduce the number they will pass that volume of gas but flames will be longer and somewhat sloppy due to the lower velocity. You will see even on much bigger burners the holes are not a lot larger just more of them.

The mesh behind the burner head is to stop burn-backs which is not a good idea behind a fridge in a caravan.

If you are going to use a piston head the gudgeon holes need to be blocked only air introduced at the Venturi ports is valid. Also you would fit a cast iron or brass disk in the head for the burner[ have not seen brass used before the holes might erode certainly don't think aluminium will last long ] The disk can be fixed on to the head by a single center nut and bolt not a screw.

If you fit a tube up into the piston to carry your jet assy and induction ports, I would suggest a 3/4" tube about 2" long sealed into the base of your piston.[ or fit into a 2" length of tube 11/2"dia and fit your burner disk on top of that if you dont want to use the piston, no need to weld it on ] . drill 3 X 5/16" air ports around the jet fit a spring sleeve around them so you can increase or decrease the amount of air.

A No 70 drill will give you about 6000BTUs/Hr at the recommended pressure.

If your greenhouse i 8X8X8= 512Cu ft 6000BTU will raise the temp 11.7F in one hour allowing for poor insulation very approximate answer neglecting external temp etc.

The chimney over the burner will help shooting the hot air round your greenhouse.

I wish you good luck and good tomatoes.

Garth

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#14
In reply to #13

Re: Propane Burner Designs

07/19/2009 11:34 PM

Garth you had it right the first time. The burner is separate from the cylinder head. The only reason for the cylinder head is to have the heat dissipating fins at work. The burner is smaller than the cylinder head and serves to heat it up much like a pot or frying pan. In turn the fins re-radiate the heat from the burner.

We often use the same teichique on a boat by placing a clay or ceramic flower pot over the propane stove top burner. The pot seem to dissipate the heat in a nicer manner than just having the flame.

Thanks to your mentoring I think I have the concept of how jet orifice, induction ports and burner outlet holes relate to each other.

I am making one assumption. RV propane systems have interchangable botles and regulators seem to be standard. At least none of them seem to be application specific. Therefore I am concluding they are all set for nearly the same pressure of 9 - 11 inches of water column. Otherwise they would not be so interchangable.

As part of my self education I made a point of scrounging several old BBQ burners and regulators. from the junk yard. I can check the orifice size against the induction tube length and cross section area and then the burner head outlets. The waffle shaped insert between the two halves pf the big rectangular burner head does make this a bit of a challenge. The side plate heater with a round burner is easy by comparison. I have also noticed that regardless of the marketing brand found on most BBQ they all have Marshall brand regulators.

All of a sudden burner design doesn't seem as much of a black art or mystery any longer. Not that I feel I'm an expert but at least now it makes sense. .

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#15
In reply to #14

Re: Propane Burner Designs

07/20/2009 6:48 AM

Hi Elnav,

Thats good also when you have your maximum output burning OK if you wish a needle valve in the inlet line will enable you to turn down the flame if things are getting to warm.

Garth

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