NCEES practice test problem #510

[Saad85]

Good day all,

would you please explain the mass balance for problem #510 ( heater) of NCEES sample test?

thanks

 

[justin-hawaii]

Hi Saad85,

This problem is an energy balance type.  The energy gained by the hot water(Points 2 to 4) is transferred from the steam (Points 1 to 3).

Points 2 - 4 -> Energy (Btu/min) = 60,000 lbm/min * (375.12 Btu/lbm - 238.95 Btu/lbm) = 8,170,200 Btu/min

This value must equal the change in Btu/min from points 1 to 3.  However, you need to first use the steam tables to find the enthalpy of point 3.  At 110 psia, the saturated liquid enthalpy is 305.8 Btu/lbm

Points 1- 3 -> Energy (Btu/min) = 8,170,200 Btu/min = x lbm/min * (1231.3  Btu/lbm - 305.8 Btu/lbm) 

Solve for X (lbm/min) ->  x = 8,827 lbm/min

The closest answer is 8,800 lbm/min