CR4 - The Engineer's Place for News and Discussion®

Previous in Forum: Where Do I Find Information on Air Craft Refrigeration Anybody Help Me HowitWork   Next in Forum: HVAC
Close

Comments Format:






Close

Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

Close

Rating Vote:







23 comments
Active Contributor

Join Date: Sep 2010
Posts: 10

Gas Jet Sizing - Natural Gas - LPG

10/03/2010 11:03 AM

Can someone spell out the theory behind re-jetting gas burners etc when changing fuel gas (for example modifying a natural gas (methane) cooker to run on LPG or propane). I've enough experience to know it is some combination of maintaining the heat output of the burner, permitting adequate air/fuel ratio under given geometry and maintaining sufficient mixture velocity to keep ahead of the combustion front. A bit of the background physics would be interesting.

Register to Reply
Pathfinder Tags: combustion gas jet LPG Methane
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru
Hobbies - Fishing - New Member

Join Date: Jun 2008
Location: Raleigh, NC USA
Posts: 13317
Good Answers: 459
#1

Re: Gas Jet Sizing - Natural Gas - LPG

10/03/2010 11:18 AM

This pretty much sums it up. It all boils down to the pressure of the gas.

Making changes to an appliance so that it can utilize a different fuel is known as appliance conversion and involves the replacement of gas orifices, burners and/or appliance regulators. These internal connections and gas utilization fittings are designed to work with a particular gas that has a specific pressure. Natural gas is a much lower pressure gas than propane and converting the appliance to one or the other gases requires that the differing pressure is compensated for. In other words, connecting a natural gas appliance to a propane piping system will result in appliance malfunction and possibly danger. The reason for this is because natural gas orifices are larger than propane orifices strictly because of gas service pressure. In this case, the higher pressure gas flowing through a larger orifice will result in more gas through the burner and likely, more flame...an unnecessarily large flame. Conversely, trying to use a propane appliance with natural gas will likely result in a very small flame or no burner flame at all because of the lower pressure gas and the smaller orifice. This is the underlying purpose of an appliance conversion from propane to natural gas or from natural gas to propane. Additionally, appliances cannot be converted from electricity to propane, or vice-versa.

__________________
Proud proponent of "Flea Market" Capitalism!
Register to Reply Score 1 for Good Answer
Guru
Engineering Fields - Electrical Engineering - New Member United Kingdom - Member - New Member Engineering Fields - Electromechanical Engineering - New Member

Join Date: Aug 2010
Location: Under the Major Oak
Posts: 3629
Good Answers: 143
#2
In reply to #1

Re: Gas Jet Sizing - Natural Gas - LPG

10/03/2010 11:34 AM

"Additionally, appliances cannot be converted from electricity to propane, or vice-versa."

Why? Won't the electricity fit through the orifice? Or the gas pass along the cable?

You were due a GA up until that point.

__________________
Pessimists are rarely disappointed.
Register to Reply Score 1 for Good Answer
Guru
Hobbies - Fishing - New Member

Join Date: Jun 2008
Location: Raleigh, NC USA
Posts: 13317
Good Answers: 459
#3
In reply to #2

Re: Gas Jet Sizing - Natural Gas - LPG

10/03/2010 12:16 PM

Actually, there are people that believe this is possible. Replacing gas piping with wire or replacing wire with gas piping and gas burners, but I don't need a GA anyway, since I copied and pasted from here. I knew the info, but it saved me a bunch of typing.

http://www.propane101.com/lpgasapplianceconversions.htm

__________________
Proud proponent of "Flea Market" Capitalism!
Register to Reply
Guru
Engineering Fields - Electrical Engineering - New Member United Kingdom - Member - New Member Engineering Fields - Electromechanical Engineering - New Member

Join Date: Aug 2010
Location: Under the Major Oak
Posts: 3629
Good Answers: 143
#4
In reply to #3

Re: Gas Jet Sizing - Natural Gas - LPG

10/03/2010 12:28 PM

You've just restored my faith in the gullibility of the human race!

__________________
Pessimists are rarely disappointed.
Register to Reply Score 1 for Off Topic
Guru
Hobbies - Fishing - New Member

Join Date: Jun 2008
Location: Raleigh, NC USA
Posts: 13317
Good Answers: 459
#5
In reply to #4

Re: Gas Jet Sizing - Natural Gas - LPG

10/03/2010 12:34 PM

I work in people's homes all the time. You'd be amazed at how little, supposedly smart people, know about the things they use every day. Usually it's nothing, and it's not just women, it is men too, equally.

__________________
Proud proponent of "Flea Market" Capitalism!
Register to Reply
Guru

Join Date: Dec 2005
Posts: 1552
Good Answers: 17
#17
In reply to #5

Re: Gas Jet Sizing - Natural Gas - LPG

10/04/2010 3:21 PM

I am too old and cynical to be amazed be the lack of knowledge in the "general public". You cannot make anything fool-proof, as bigger and better fools show up all the time!

__________________
Eventually, one needs to realize that it is far less important to be the smartest person in the room than it is to sit next to that person and make friends.
Register to Reply Off Topic (Score 5)
Anonymous Poster
#18
In reply to #5

Re: Gas Jet Sizing - Natural Gas - LPG

10/04/2010 3:34 PM

I'm really surprised at you guys! Do you mean to tell me you never saw water coming out of an electrical ceiling fixture ?

That's why they always use a water example to explain how voltage and current work,right? See, not all guys are a little dense and slow, ya gotta stop pickin on us slow guys.

I just couldn't pass this one up!

Thanks for the great humor, and yes ,some people will suprise you with their theories and explainations. Don't you just love it?

h

Register to Reply
Guru
United Kingdom - Member - Not a New Member Hobbies - Musician - New Member Hobbies - Fishing - New Member

Join Date: May 2006
Location: Reading, Berkshire, UK. Going under cover.
Posts: 8492
Good Answers: 364
#6
In reply to #1

Re: Gas Jet Sizing - Natural Gas - LPG

10/03/2010 7:48 PM

Confess I haven't looked into it, but I don't think it's only about pressure. Some years ago, the UK changed from "town gas" (derived from coal) to "natural gas" (from the North Sea fields). When the changeover happened, every burner on every device (oven, heater etc) was replaced. The pipework wasn't changed. It could've been due to lower pressure supply, but I don't see why the pressure should've been any different.

Think calorific value comes into it somewhere.

__________________
"Love justice, you who rule the world" - Dante Alighieri
Register to Reply
Guru
Engineering Fields - Electrical Engineering - New Member United Kingdom - Member - New Member Engineering Fields - Electromechanical Engineering - New Member

Join Date: Aug 2010
Location: Under the Major Oak
Posts: 3629
Good Answers: 143
#7
In reply to #6

Re: Gas Jet Sizing - Natural Gas - LPG

10/03/2010 10:36 PM

Natural gas is supplied at a higher pressure than town gas. Going back to the UK "change over" there were a lot of worries about the rise in pressure (some of them turned in to real worries with gas leaks). If you look at your meter it has a pressure regulator on it on the supply side. The re-jetting of appliances was to counter the difference in calorific value. The jets were wider, starting more concerns about higher gas usage. Telling Mrs. Brown that she pays for gas by the therm cut no ice with her what so ever.
The rise in pressure was the gas industries "quick fix" to supply more customers without upgrading the grid system.

__________________
Pessimists are rarely disappointed.
Register to Reply
Guru
United Kingdom - Member - Not a New Member Hobbies - Musician - New Member Hobbies - Fishing - New Member

Join Date: May 2006
Location: Reading, Berkshire, UK. Going under cover.
Posts: 8492
Good Answers: 364
#9
In reply to #7

Re: Gas Jet Sizing - Natural Gas - LPG

10/04/2010 1:30 AM

Thanks for the info. As I said, I didn't look into it ...

__________________
"Love justice, you who rule the world" - Dante Alighieri
Register to Reply Off Topic (Score 5)
Guru
Engineering Fields - Electrical Engineering - New Member United Kingdom - Member - New Member Engineering Fields - Electromechanical Engineering - New Member

Join Date: Aug 2010
Location: Under the Major Oak
Posts: 3629
Good Answers: 143
#10
In reply to #9

Re: Gas Jet Sizing - Natural Gas - LPG

10/04/2010 4:06 AM

The reference to Mrs. Brown was no joke, she was a neighbour of my mothers.
Each week she religiously recorded the reading of the gas and electricity meter. Low and behold on conversion her weekly gas readings went up! I tried explaining that gas is billed in BTU's not cubic feet, thankfully for me she turned her wrath on the gas board. Still I suppose a cubic foot is a cubic foot to her, I'm just glad we didn't have domestic water meters then!

__________________
Pessimists are rarely disappointed.
Register to Reply Off Topic (Score 5)
Guru
Hobbies - Fishing - New Member

Join Date: Jun 2008
Location: Raleigh, NC USA
Posts: 13317
Good Answers: 459
#12
In reply to #6

Re: Gas Jet Sizing - Natural Gas - LPG

10/04/2010 7:28 AM

I think your right, but calorific value has to do with air/fuel mixture, which also has to be adjusted. Pressure relates to orifice size.

__________________
Proud proponent of "Flea Market" Capitalism!
Register to Reply
Guru
United States - Member - Member Hobbies - DIY Welding - New Member

Join Date: Mar 2008
Location: Southeast US of A
Posts: 544
Good Answers: 50
#8

Re: Gas Jet Sizing - Natural Gas - LPG

10/03/2010 11:11 PM

Many gas appliances are supplied with conversion kits, of which different sizes orifices are a part of, which allow rapid conversion from one to the other, by a qualified service person.

__________________
Speak softly and carry a big stick.
Register to Reply
Guru
United States - Member - Member Engineering Fields - Electrical Engineering - Electrical Construction

Join Date: Apr 2010
Location: Mid Western USA - The Corn Belt
Posts: 1447
Good Answers: 58
#15
In reply to #8

Re: Gas Jet Sizing - Natural Gas - LPG

10/04/2010 11:38 AM

"by a qualified service person" - man ain't that the truth.

I bought a NG/LP gas stove recently. I changed out the top burner jets with no problem and completely over looked the oven jet because only four LPG jets were included in the kit.

We used the top burners for about a week with no problems. The wifey went to use the oven for some weekend baking and when she turned it on, POOF - flames were shooting up the insides of the oven.

I went back and re-read the manual and it said to tighten down the oven jet. I turned it down one turn not wanting to strip of damage the threads. I tried the oven and still had too much flame.

My neighbor stopped by that evening and recalled that when he was in high school he worked for an appliance outfit and remembered that the oven jet had to be tightened down completely for LPG. We did this and it worked.

I am glad that I was home when this happened as the wifey was in a complete panic when this happened. Thankfully she was not hurt and the house did not burn down.

__________________
The first 5 days after a weekend are always the hardest................................
Register to Reply
Anonymous Poster
#11

Re: Gas Jet Sizing - Natural Gas - LPG

10/04/2010 5:13 AM

Each jet has a particular amount of BTU's it is capable of passing at a standard pressure,depending on type of gas used.They are normally referenced to 10 to 11 inches of water column(in USA),which is the standard supply pressure from 2nd stage LP regulator or NG lines.Most burners allow adjustment to BTU output by changing jets to a different size.Or course,air feed must be adjusted accordingly to get proper mixture.

A typical gas-pack heat system can vary from 60KBTU/Hr to 100KBTU/hr with same burners,only jet change and air adjustment.

HTRN

Register to Reply
Power-User

Join Date: Apr 2010
Location: Where no man should ever live
Posts: 194
Good Answers: 7
#13

Re: Gas Jet Sizing - Natural Gas - LPG

10/04/2010 8:01 AM

I believe much of the reason for different size orifices in gas burners when different gases are used is flame speed.

I assume the rate of combustion is dependent on bond energy and the number of carbon and hydrogen atoms.

Accordingly, it would seem that to maintain combustion in an atmospheric burner the rate at which the fuel gas carbon and hydrogen atoms combine with the oxygen atoms, in the presence of heat, is dependent on the rate at which the gas is supplied. There may be only two variables in burner design: delivery gas pressure and orifice size, configuration, etc.

I think the balance of primary air and fuel gas is a delicate one, although sometimes seemingly crudely adjusted with air shutters on older burners. The primary air adjustment is now fixed on most new burners, possibly because the delivery gas pressure and the caloric value is more constant than it used to be.

The flames can lift in "overrated" burners, and sometimes "flashback" or "extinction pop" can occur in "underrated" burners.

Interestingly, flashback tendencies are greater in faster burning fuels.

On my way to the bathroom for my morning duty I passed by my bookshelf and, wallah!, my AGA "Fundamentals of Gas Combustion" book jumped out at me! I haven't looked at it in the better part of ten years. This was a good refresher! I am not a combustion engineer, but I think I got it pretty much right!

__________________
Who is so ignorant as not to know that knights-errant are beyond all jurisdiction, their only law their swords, while their charter is their mettle and their will is their decrees? Don Quixote
Register to Reply
Guru
Hobbies - CNC - New Member Canada - Member - Finaly got around to it.

Join Date: Feb 2008
Location: Canada
Posts: 503
Good Answers: 12
#14

Re: Gas Jet Sizing - Natural Gas - LPG

10/04/2010 10:32 AM

Propane is more dense than natural gas. I was told a long time ago if there was a natural gas explosion, duck! If there was a propane explosion, jump!

You need a lager orifice for natural gas than propane due to the different fuel densities, and caloric content.

Register to Reply
Power-User
United States - Member - New Member

Join Date: May 2008
Location: Arizona
Posts: 346
Good Answers: 4
#16

Re: Gas Jet Sizing - Natural Gas - LPG

10/04/2010 11:56 AM

Good morning chosceng. I think both gases are delivered to the appliance at about the same pressure, i.e., 7"wc. At this pressure, the natural gas has a heat content of ~1000 Btu/cf, vs. 2500 Btu/cf for propane. To produce the same flame, a smaller orifice would be used for the propane.

Register to Reply
Member

Join Date: Aug 2009
Location: Wales
Posts: 9
#19

Re: Gas Jet Sizing - Natural Gas - LPG

10/04/2010 3:46 PM

hi guys

some fact and figures used in the uk for values of gases

natural gas calorific value 1034btu/ft 38.5 mj/m

supply pressure 20mbar pipework (burners may be governed to lower pressure)

specific gravity 0.58

wobbe nuber 1330 btu/ft 49.6 mj/m

volume of air required x 9.75

flammability limits 5 to 15% in air

flame speed 1.12 ft/sec 0.36 m/sec

propane calorific value 2590btu/ft 97.3 mj/m

supply pressure 37 mbar pipework (burners may be governed to lower pressure)

specific gravity 1.5

wobbe number 2140 btu/ft 798 mj/m

air required x 23.8

flammability limits 1.6 to 7.75% in air

flame speed 1.4 ft/sec 0.46 m/sec

wobbe number gives the indication of the heat output from a burner

it is calculated by dividing the calorific value by the square root specific gravity of the particular gas

gas modulus- an expression which relates the heat output from a burner

gas modulus = square root of pressure divided by wobbe index

so to find the pressure at the burner required for a change of gas

square root existing gas pressure divided by wobbe number=

square root new gas pressure divide by wobbe number or

square root new gas =square root old gas pressure x new gas wobbe number divided by old gas wobbe number

to find the new injector size you need to calculate the volume of gas that will pass through it

Q = 0.0467 X A X C X square root of pressure divided by specific gravity

Q = volume of gas in cubic metres per hour

A= area of orice in mm

C= coefficient of discharge (this depends on the shape of injector- usually 0.85 - 0.95)

or in simple terms-

natural gas is at a lower pressure, has a lower calorific value and lower specific gravity, so changing to propane will always mean smaller injectors to get the same heat output at the burner

hope this helps Pete

Register to Reply
Member

Join Date: Aug 2009
Location: Wales
Posts: 9
#20
In reply to #19

Re: Gas Jet Sizing - Natural Gas - LPG

10/04/2010 4:05 PM

A= area of ORIFICE lol

ps

luckily all the hard work is usually done for you by the maker of an appliance by supplying new injectors that will suit the new type of gas, aeration may also have to be adjusted depending on burner type

Pete

Register to Reply
Member

Join Date: Aug 2009
Location: Wales
Posts: 9
#21
In reply to #19

Re: Gas Jet Sizing - Natural Gas - LPG

10/04/2010 4:25 PM

sorry

for the American readers governor = regulator

pressure control device lol

Register to Reply
Active Contributor

Join Date: Sep 2010
Posts: 10
#22

Re: Gas Jet Sizing - Natural Gas - LPG

10/06/2010 11:39 AM

I'm new here so don't know the protocol - I'd just like to say I appreciate the quantity and quality of comments my question provoked... new concept noted...'wobbe index' which reminds me a little of the 'non-dimensionals' we used early in my career when working on gas turbine thermodynamic cycles... not at all non-dimensional of course.

Thanks to all

Register to Reply
Guru
Hobbies - Fishing - New Member

Join Date: Jun 2008
Location: Raleigh, NC USA
Posts: 13317
Good Answers: 459
#23
In reply to #22

Re: Gas Jet Sizing - Natural Gas - LPG

10/06/2010 11:56 AM

Hi chosceng,

Protocol here seems to be, that questions do get answered, but usually with a little off topic craziness thrown in for good measure. If you can deal with that you'll be fine. Welcome aboard.

__________________
Proud proponent of "Flea Market" Capitalism!
Register to Reply
Register to Reply 23 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Anonymous Poster (2); chosceng (1); Coldspot (1); flyinghigh (1); Icarus (1); JohnDG (2); KJK/USA (1); kramarat (5); peteincardiff (3); Ried (1); standarded (1); TonyS (4)

Previous in Forum: Where Do I Find Information on Air Craft Refrigeration Anybody Help Me HowitWork   Next in Forum: HVAC

Advertisement