Page 99 - Handbook of Energy Engineering Calculations

and 1000°F (537.8°C) to find throttle-steam properties. Applying the symbols
of the T-S diagram in Fig. 21, we get H = 1474.5 Btu/lb (3429.7 kJ/kg); S =
2
2
1.5603 Btu/(lb · °F) [6.5 kJ/(kg · °C)].

FIGURE 21 Irreversible expansion in reheat cycle.

3. Find the reheat-steam enthalpy
Assume a constant-entropy expansion of the steam from 2000 to 400 lb/in 2

(13,790 to 2758 kPa). Trace this expansion on a Mollier (H-S) chart, where a
constant-entropy process is a vertical line between the initial [2000 lb/in 2
2
(abs) or 13,790 kPa] and reheat [400 lb/in (abs) or 2758 kPa] pressures.
Read on the Mollier chart H = 1276.8 Btu/lb (2969.8 kJ/kg) at 400 lb/in 2
3
(abs) (2758 kPa).

4. Compute the actual reheat properties
The ideal enthalpy drop, throttle to reheat = H − H = 1474.5 − 1276.8 =
2
3
197.7 Btu/lb (459.9 kJ/kg). The actual enthalpy drop = (ideal drop) (turbine
efficiency) = H – H = 197.5(0.8) = 158.2 Btu/lb (368.0 kJ/kg) = W =
3′
e1
2
work output in the high-pressure section of the turbine.
Once W is known, H can be computed from H = H – W = 1474.5 −
3′
3′
e1
2
e1
158.2 = 1316.3 Btu/lb (3061.7 kJ/kg).
The steam now returns to the boiler and leaves at condition 4, where P =
4
2
400 lb/in (abs) (2758 kPa); T = 1000°F (537.8°C); S = 1.7623 Btu/(lb · °F)
4
4