Energy Content of Steam: Variation with Pressure

[rollingstein]

I was playing with an online steam calculator provided by TLV, one of the bigger steam equipment vendors & the steam cost it calculates ($/ton) seems lower at high P than low Pressure.

http://www.tlv.com/global/TI/calculator/steam-unit-cost.html

e.g. The $/ton of 200 bar steam seems 6.09 vs 7.17 for 10 bar steam. Does this make sense? I always intuitively thought there's a whole lot energy content in high pressure steam than low pressure. So why would the cost per ton of low pressure steam be higher?

For now I'm ignoring the relative efficiencies of low and high pressure boilers, differential losses etc.

I assumed saturated steam for both cases and feed water at 90 C which seem reasonable assumptions.

Coal I assumed was $70/ton with a calorific value of 5600 kcal/kg. i.e. Energy unit cost of 0.0125 $/Mcal

Do any of my assumptions seem unreasonable? Or a bug in my calculations?

 

[gmax137]

Check your steam tables for the vapor enthalpy at the two pressures.