Hello All
Have a quick question concerning turndown ratios. It is in regards to a natural gas fired heat exchanger.

I seem to understand the concept of a turndown ratio, as it deals with full capacity versus minimum operating capacity of the gas flow, i.e. 1,000,000 m3 full versus 100,000 minimum equals 10:1.
But what if we talk about percentages? Does 10:1 mean 10 percent of full rated capacity? Or would a 1:20 or 1:30 mean 20/30% full rated capacity respectively?

Don't talk about percentages.

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

Here are some examples:

A 3:1 turndown ratio simply means the boiler is capable of operating at one-third of the nameplate rating. Put another way, a 9 million BTU/hour boiler will operate down as low as 3 million BTU/hour on low-fire before it cycles on and off.

Q: What is turndown ratio and is it different for direct vs. indirect gas heaters?
A: Turndown ratio compares the maximum to minimum heat output. Turndown ratio for direct gas burners is 25:1, which means that the burner can modulate from 4% to 100% of full fire. Indirect gas turndown ratio changes based on the controls. Example based on a single furnace unit (< 400 MBH). 4:1 electric modulation limits the heat output from 25% to 100% of full fire. 8:1 stage limits the heat output from 12.5% to 100% of full fire. See the Indirect Gas-Fired Technical Guide for more information.

http://www.greenheck.com/news/view/256

Star for BigInch. Word for word what I was going to say.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

Thanks David. Ya. No point inventing a new name for a std term that everyone has already settled on and knows what it means.

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

Have to agree to disagree. There are many sources including the Hydraulic Institute that use percentage. There is no master definition of turn down ratio:

Controlled-volume metering pumps with stroke length adjustment typically have a flow rate adjustment range between 10 and 100 percent of rated flow, expressed as a turndown ratio of 10:1. Below the stated turndown, accuracy cannot be guaranteed. The pump should be sized such that the entire flow range falls between 10 and 100 percent of the pump’s flow rate.

http://www.pumps.org/content_detail.aspx?id=4774

A turndown ratio of 3:1 means that the control valve can modulate down to provide "1 over 3", or 1/3 (33 percent) the maximum capacity.

http://books.google.com/books?id=YJfJQjcNficC&pg=PA121&lpg=PA121&dq=%22turndown+ratio%22+or+%25+percentage&source=bl&ots=j2hWNXL-ip&sig=QoS_JYwGPkCC9Bl17Xm2He55JbU&hl=en&sa=X&ei=KkPJT4qqIoLmggeb34HHDg&ved=0CEoQ6AEwADgy

bimr,
Every time I see someone like the Hydraulics Institute do something like that I want to throttle them. 10:1 turndown means something to a huge majority of the population that cares. 10% turndown is just confusing (i.e., is it a good thing or a bad thing? I don't know). Saying that a pump can run between 10% and 100% of rated capacity is clear. Saying the pump has a turndown of 10:1 is clear to most of us. Saying that the minimum capacity of the pump is 10% of the rated capacity is clear. Saying that the pump has a 10% turndown sounds like a really small number that doesn't make any sense.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

Thanks for the responses, big thank you particularly to bimr for his. More or less confirmed what I had thought and clarified it for me. I asked initially because I was dealing with an equipment catalog that used percentages for the turndown of a natural gas heater but a client of mine had used a ratio in his specs.

Obviously you can talk about percent of anything, including "of rated flow". You simply must keep writing until everyone knows what percent you're talking about. If you want to say "Between 10 and 100 percent of rated flow", nothing really wrong about that, but it is a very cumbersome way to say "10:1 turndown ratio".
I'd say a 10% turndown would mean 90% to 100% of rated flow.

ruccolo, your equipment vendor's use of that method is a clear example of why it should not be used. You were confused, which might have resulted in the wrong equipment being ordered and installed.

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

Quite an ample field of applications and of confusing related terms such as rangeability can be found by using the search tool for turndown ratio.

BTW, BigInch what would be a reasonable spanish translation?

Relación de disminución, índice de reducción?

I don't exactly know, but if I had to say something, I would prefer, "Indice de disminución".

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

I'd say campo de regulación or margen de regulación.

You're guessing. Definitely not campo. That's always a playing, or battle field, campground, open field space, in all versions of Spanish I know.
Regulación implies a fixed value, as in holding it.
I'd take that as a +/- margin of error ... for a regulator, for example.

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

So if I understood correctly, a turndown of 20% full rated capacity (0.2) is 5:1, and 30% (0.3) is around 3.35:1? Meaning, the lower the ratio, the smaller the range between max and min? Is it also possible to get 20:1 or better for indirect gas fired? As bimr did state, it can vary according to the controls.

A turndown ratio of
10:1 is range 100% to 10% of full flow.
5:1 is range 100% to 20% of full flow.
2:1 is range 100% to 50% of full flow.

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

As I understand, multistage heating (say, three furnaces, each with TDR=8:1)) offers the possibility of getting overall TDR values of 24:1.

Forgot to mention the source:

www.michiganair.com/newsletters/2006-1/200612.pdf

You'd be lucky if the feeding pipeline had the same capability.

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

If you do the math, if each heater had an 8:1 turndown ratio and you placed them in series the result would be 512:1 instead of 24:1. At least that is the way it works for compression ratios.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

I adhere to BigInch's comment regarding operating practicality.