Firing Rate vs Boiler Load

[firefastball]

Hi there,

Is there a graph on how firing rate relates to boiler load?
I'm wondering on how does combustion efficiency relates to boiler efficiency.
On my perspective, a boiler with a burner turndown of 4:1 (25% lowest firing rate) for example, will have 4 different efficiencies (100% to 25%) but it does not take into account the boiler load.

 

[chicopee]

What type of a boiler is it as I do have some specs on FT Cleaver Brooks?

 

[firefastball]

It's a three pass package boiler from Mechmar.
I'm not sure if you are familiar with the brand.

 

[athomas236]

Any reason for not asking Cleaver?

 

[firefastball]

It is more on general question and not specific to any brands.

 

[aliansari]

Firing rate is a function of Boiler Load or in other words, Steam Load demand governs the load of the boiler and fuel firing. If you see the typical (almost the same for all the boiler brands)firing rate logic on the boiler instrumentation line diagram, the signal for firing command is initiated by the steam pressure controller, also called master control.

A chart or graph, you are looking for, is not general in nature but specifically developed for a particular boiler, therefore your vendor is the right source to get one for your boiler

Combustion efficiency is the ratio of fuel energy liberated and the useful heat absorbed by the system. It may be understood from the following relationship

Heat Input = Fuel Fired Kg/h x Lower Heating Value (LHV) KJ/Kg
Losses are Stack Losses (typically 7-10% depending on final stack temperature and Oxygen level) and Radiation Losses (0.5 - 1.0%)
Typical LHV based combustion efficiency comes out to be 89-92%). The easiest method of calculating the combustion efficiency is
1-losses where reliable stack loss charts are available in the literature and vendor's documents may be referred for radiation loss.

Boiler efficiency is slightly different and lower than combustion efficiency since it takes another loss into account, which is blowdown loss, not very significant, but accounts to 0.25-0.5% depending on the prevailing water chemistry.

 

[aliansari]

Further to my previous post, ASME PTC 4 is the code for determination of boiler efficiency and according to my memory, it covers both, direct and indirect methods of efficiency calculations. Generalized indirect equation for boiler efficiency is

Boiler Efficiency = (1 - Stack Losses - Other Losses) x 100 where stack losses include dry and wet losses and other losses include radiation and blowdown losses

 

[chicopee]

See attachment. Turn down ratio in percentages(%) is four. Manufacturer Cleaver Brooks. The material is from a seminar that I attended in 1986 in Lebanon, PA . Please note that CB identifies five types of efficiencies but the chart efficiency is the one that is used to determine fuel cost and these high pressure boilers have four passes.
I don't know anything about Mechmar, however, any respectful boiler manufacturer should have the info. that you seek.