A 500 Hp boiler at 33,480 BTU/Hp can put out 16,740 Lb/Hr of steam at full load.

Operation is normally at less than 100% during running time.  This is an important number.  I will assume 80% average loading.

The pounds of steam equal the pounds of condensate made … which is then de-rated by the amount of condensate returned.  In this example 16, 740 Lb/hr is multiplied by 80% average load which gives 13,424 Lb/Hr.  Water is 8.33 Lb/Gallon, and there is 60 minutes in each hour, so the 13,424 Lb/Hr is first divided by 60 Min/Hr and then divided by 8.33 Lb/Gallon to give 26.86 Gallons/Min.  If 70% of this condensate is returned, then 30% needs to be made up.

30% x 26.86 GPM = 8 GPM

The 8 GPM says that the 10 GPM heat exchanger can handle the load.

Now I determine what heating 8 GPM by 20 F is worth if run for 5,694 Hr/Yr.

8 GPM x 60 Min/Hr x 8.33 Lb/Gallon = 3,998 Lb/Hr

BTU/Hr = Lb/Hr x temperature difference x specific heat of 1 BTU per Lb Degree F

BTU/Hr = 3,998 Lb/hr x 20 F x 1 BTU/Lb Degree F

BTU/Hr = 79,968 BTU/Hr

Assuming the heat recovered is displacing steam generated at 80% efficiency, the BTU/Hr is divided by 80% to give 99,960 BTU/Hr of natural gas.

A therm is 100,000 BTU.  I will round the savings above to 1 therm per hour or $0.91/Hr

If the system is running 5,694 Hr/Yr, then the savings would be 5,694 Hr/Yr x $0.91/Hr or $5,181 per year.

Total = 3/years/Payback= $15,544.62