Page 63 - Handbook of Energy Engineering Calculations

FIGURE 2 Cycle layout and T-S chart of steam conditions.

Calculation Procedure:

1. Using the steam tables and Mollier chart, list the pertinent steam
conditions
Using the subscript 1 for throttle conditions, list the key values for the cycle
thus:
2
P = 400 lb/in (abs) (2756.0 kPa)
1
t = 700°F (371.1°C)
1
H = 1362.2 Btu/lb (3173.9 kJ/kg)
1
S = 1.6396
1
H = 1178 Btu/lb (2744.7 kJ/kg)
2
H = 1380.1 Btu/lb (3215.6 kJ/kg)
3
H = 1035.8 (2413.4 kJ/kg)
4
H = 69.1 Btu/lb (161.0 kJ/kg)
5
H = 265.27 Btu/lb (618.07 kJ/kg)
6

2. Determine the percentage of throttle steam bled for feedwater heating
(a) Set up the ratio for the feedwater heater of (heat added in the feedwater

heater)/(heat supplied to the heater)(100). Or, using the enthalpy data from
step 1 above, (H − H )/(H − H )(100) = (265.26 − 69.1)/(1178 − 69.1)(100)
2
6
5
5
= 17.69 percent of the throttle steam is bled for feedwater heating.
3. Find the heat converted to work per pound (kg) of throttle steam
(b) The heat converted to work is the enthalpy difference between the throttle
steam and the bleed steam at point 2 plus the enthalpy difference between
points 3 and 4 times the percentage of throttle flow between these points. In

equation form, heat converted to work = H − H + (1.00 − 0.1769) (H − H )
3
2
1
4
= (1362.2 − 1178) + (0.0823)(1380.1 − 1035.8) = 467.55 Btu/lb (1089.39
kJ/kg).

4. Calculate the heat supplied per pound (kg) of throttle steam