The other variable we need to know is the steam pressure. As an example, one pound of steam contains about 1,203 Btu at 300 psi, and 1,190 Btu at 100 psi. If we assume that each pound of steam contains 1,200 Btu, then 560 pounds of steam per hour contains 672,000 Btu. This is equal to 6.72 therms. One therm is equal to 1 CCF or 100 cubic feet of natural gas.
The boiler that's producing this steam is not 100 percent efficient, so there will be some additional input energy required. If the boiler is 80 percent efficient, then the total gas requirement from the boiler is 8.4 therms per hour, which is equal to 0.84 mcf (thousand cubic feet).
Therefore, at a cost of $10.50/mcf, the cost to supply the 560 pounds of steam per hour is about $8.82/hour.
Steam cost = 0.84 mcf/hour x $10.50/mcf = $8.82/hour
If it is throttled to 25 percent, then the cost would also be roughly 25 percent of rated load cost or $2.20/hour. Note that the efficiency of the boiler and the actual pressure of the steam may vary, and this will affect the amount of natural gas required.
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