Boiler steam rate "from and at 100 degC"

Mon, 5 Apr 2021 18:07:52 GMT

Hi Mr. Rimbault,

I am working on a boiler project and I came across the John Thompson Europac boiler brochure (attached to this post) and I noticed that boiler manufacturers tend to rate the boiler steam output "from and at 100 degC". What does this actually mean, and how does the steaming rate under typical conditions compare with the "from and at 100 degC" rate?

Thank you

Tue, 6 Apr 2021 12:38:28 GMT

Thank you for your post Amina.

Boiler manufacturers recognise that the operating conditions vary between applications, and so they rate their boilers "from and at 100ºC" in order to level the playing field and allow us to compare the boiler output on a like-for-like basis and to estimate the output under operating conditions. I consider the rating as being more of an indication of the maximum design heat transfer capacity of the boiler.

The term "from and at 100ºC" tells us that the boiler is capable of producing the quoted steaming rate from water at 100ºC into steam at 100ºC (approximately atmospheric pressure). As the feedwater temperature and the steam temperature are the same, there is no sensible heat transfer and all the energy transferred is the latent heat of evaporation. At 100ºC, the latent heat of evaporation is 2256kJ/kg. A boiler with a "from and at 100ºC" rating of 5000kg/hour (model TU500 from the John Thompson brochure) would be capable of transferring 5000 x 2256 / 3600kJ/s = 3133kW of energy.

We can assume that the same maximum energy transfer will take place under all operating conditions which means that we will still transfer 3133kW regardless of the feedwater temperature or the steam pressure.

The John Thompson brochure also gives us a steaming rate "At 1000kPa, feedwater 20ºC" of 4186kg/hour, and we will use these conditions to calculate the expected steaming rate and see how it compares with their figure. We can assume that the 1000kPa is gauge pressure.

Using steam tables for water at 20ºC and steam at 1000kPa, we can calculate the required energy to produce steam under operating conditions as the sum of the sensible heat change in the water and the latent heat of evaporation, this being (781-84)+2000=2697 kJ/kg of steam produced.

As the overall heat transfer is constant, the steam output under operating conditions can be determined by dividing the overall heat transfer by the specific energy required under operating conditions. In this case, the expected steam output would be 3133/2697*3600 kg/hour = 4182 kg/hour, which compares very favourably with the John Thompson figure of 4186 kg/hour.

I have attached an image of a table I created on a whim a few years ago where I repeated the above process for various feedwater temperatures and steam pressures and create a table of correction factors to apply to the "from and at 100ºC" figures. Note that the above calculation gives a ratio of 4182/5000=0.836 which compares well with the correction factor of 0.837 in the table for 20ºC feedwater and 10bar (1000kPa) gauge pressure. The table is useful in giving a quick overview of the impact of feedwater temperature and steam pressure on the quoted boiler ratings.