Boiler Horsepower Realtion to Capacity

[777gne]

How is it that a boiler horsepower can be defined as simply being equal to 34.5 #/hr of water evaporated at 212ºF?

Can this be adjusted to allow for varying feedwater temps and final steam temp/pressure?

 

[dcasto]

the conversion is 33478.8 BTU/hr = 1 boiler HP. The number you have is from old times when there wasn't a complete steam table to get the enthapy for all temperatures and pressures or steam boilers, so it was simplified to your value.

 

[rmw]

The correct term is "from and at 212F".

That takes away any variation in FW temp, etc.

The term was used more in association with fire tube boilers than drum type boilers when I first encountered it in the early '70's. By the time a boiler got big enough to be a drum type, they were usually referred to by steam flow.

rmw

 

[777gne]

Thanks for the responses. The clarification about the "from and at 212F" makes some sense to me. I also noticed that by dividing the 33478 BTU/hr by the latent heat of evap. at 212F (970.3 BTU/#) the result is 34.5 #/hr. Can we use the same logic to determine the steam capacity for other pressures of steam?

For example at ~ 150 psig:
latent heat of evap. ~ 857.3, and it took (338.2 - 180.17) BTU/# to heat the liquid from 212 to 365.5, which is 158.03 BTU's.

So given the definition of boiler hp, would it be correct to say that 1 bhp can produce [ 33478/(158.03 + 857.3) ] #/hr of 150 psig steam? ( = 32.97 #/hr)

I realize we're talking minor differences here, it's more of an academic exercise at this point I suppose.

 

[RossABQ]

Your assumption is correct; at a specific pressure you can derive the equivalent flow rate per BoHP. I find it much better for all concerned to deal in lbs/hr at xxx PSIG saturated (or whatever), just forget the BoHP, it confuses things. Note also that the calculation also needs to correct for altitude/reduced atmospheric pressure, a further complication. At 5300', water boils at 194 deg F, assuming 12.2 psia atmospheric conditions, with virtually the same enthalpy as at 212/sea level.

 

[777gne]

Ross,

Thank you for the reply. Good point about the elevation as well.