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Unit Heat Rate vs Output

Unit Heat Rate vs Output

Unit Heat Rate vs Output

(OP)
Ia there a relationship between thermal power plant unit heat rate and the output load ?

What is the shape of the curve between them ?

Thanks

RE: Unit Heat Rate vs Output

Heat rate is BTU/KWh so yes there is a relationship the two. Not sure what you mean by the shape of curve between them

RE: Unit Heat Rate vs Output

The shape of the curve is dependent on the partial load efficiencies of several major pieces of equipment. What type of "thermal plant" are you asking about?

rmw

RE: Unit Heat Rate vs Output

(OP)
I'm take about Steam Power Plant Gas/Oil fired

RE: Unit Heat Rate vs Output

So, your turbine has a curve of efficiency vs. load, as does your boiler, but then too all the components in the water cycle do too, feedwater heaters, condensers, etc. To make a reasonable curve, one would have to consider the effects of partial load on each of them. Your turbine may have a set of heat balances for various load points that would define the turbine efficiency as a combination of the turbine and FWH's, BFP's, etc., but it doubtfully has any accomodation for the boiler's part load efficiencies.

If you have good technical performance data on all your major pieces of equipment, you should be able to develop it in fairly short order. I can remember having to do just that some years ago.

Remember too that the plant's heat rate incorporates the aux loads as well, and some of those do not diminish as load falls off. The circ water pumps, for example, pump just as much water at 50% load as they do at 100% load. So does a lot of other equiment in the plant.

rmw

RE: Unit Heat Rate vs Output

Just a note of caution in evaluating the characteristic curve that you are seeking to develop. There is a common misunderstanding that the greatest efficiency is achieved at full load. For a conventional steam plant, this is usually not true. The best efficiency point is commonly realized at some lesser output rate most often in the general range of 70% to 80% of full load. This has the beneficial effect of having the best efficiency point more nearly match the load range where the plant will operate most of its operating time. A major factor is the adverse thermodynamic effect of superheater attemperator sprays that are required to protect against excessive metal temperatures at higher loads. If your subject plant was designed to operate primarily at or near full load, the it may have been designed so that attemperator sprays would not be needed (or at least not needed as much of the time), but part load operating efficiencies would likely have been more severely compromised.

Typically, at very low loads, the fuel consumption rate will be nearly constant as the load increases. At very low output rates, increases in output are realized primarily from increasing efficiency. At some point, the fuel consumption rate begins to track output more obviously. These low output efficiency characteristics are usually mainly a matter of academic interest because they are most commonly just a part of the burden of startup costs. If, for some reason, the plant must operate for extended periods at very low output rates, then careful attention to the several causes of lower efficiency at lower outputs can lead to operating changes or equipment modifications that can produce significant fuel savings. The details are always unique to the specific plant and its specific main and auxiliary equipment.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

RE: Unit Heat Rate vs Output

You should consider using plant thermal performance software if you need a detailed analysis of this relationship. There are many on the market such as thermoflow, Gatecycle, PEPSE, etc.

RE: Unit Heat Rate vs Output

As long as I know, it depends on many factors, one of them is the method used for regulating turbine load (constant pressure or sliding pressure, and the number of turbine admission valves).

Attached I'm sending a figure taken from the book COMBUSTION, FOSSIL POWER SYSTEMS, by Combustion Engineering -CE, one of the bibles in this field. Hope it helps.

By the way, if your question is about and specific power plant, you can have an idea of its heat rate vs load behavior running a simple "test" during a couple of hours waiting for the unit to be stabilized at some partial loads and measuring fuel flow. But if you want a certified test it would take much more time (and maybe a third party company to take measures and analyze fuel samples), but it can also be done.

Javier Guevara E.
Projects, Mechanical Engineer

RE: Unit Heat Rate vs Output

Hello, As long as I know, it depends on many factors, one of them is the method used for regulating turbine load (constant pressure or sliding pressure, and the number of turbine admission valves).

Attached I'm sending a figure taken from the book COMBUSTION, FOSSIL POWER SYSTEMS, by Combustion Engineering -CE, one of the bibles in this field. Hope it helps.

By the way, if your question is about an specific power plant, you can have an idea about the Heat-rate vs load behavior just running a simple "test", waiting for the unit to be stabilized a partial loads while measuring coal flow. If you want a certified test, it would take much more time (and maybe a third party company to take measures and analyze fuel samples), but it can also be done.

Javier Guevara E.
Projects, Mechanical Engineer

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