Page 151 - Handbook of Energy Engineering Calculations

pressure drop means the gas-turbine power output will be higher, while the
boiler surface and the capital cost will be higher, and vice versa. Generally, a
lower gas-pressure drop offers a quick payback time.
If Δ P is the additional gas pressure in the system, the power, kW,
e
consumed in overcoming this loss can be shown approximately from P = 5 ×
−8
10 (W Δ P T/E, where E = efficiency of compression).
e
e
To show the application of this equation and the related payback period,
assume W = 150,000 lb/g (68,100 kg/h), T = 1000°R (average gas
e
temperature in the boiler), Δ P = 4 in water (10.2 cm), and E = 0.7. Then P =
e
− 8
5 × 10 (150,000 × 4 × 1000/0.7) = 42 kW.
If the gas-turbine output is 4000 kW, nearly 1 percent of the power is lost
due to the 4-in (10.2-cm) pressure drop. If electricity costs 7 cent/kWh, and
the gas turbine runs 8000 h/yr, the annual loss will be 8000 × 0.07 × 42 =

$23,520. If the incremental cost of a boiler having a 4-in (10.2-cm) lower
pressure drop is, say $22,000, the payback period is about 1 year.
If steam requirements are not stated for a particular gas inlet condition, and
maximum steaming rate is desired, a boiler can be designed with a low pinch

point, a large evaporator, and an economizer. Check the economizer for
steaming. Such a choice results in a low gas exit temperature and a high
steam flow.
Then, the incremental boiler cost must be evaluated against the additional

steam flow and gas-pressure drop. For example, Boiler A generates 24,000
lb/h (10,896 kg/h), while Boiler B provides 25,000 lb/h (11,350 kg/h) for the
same gas pressure drop but costs $30,000 more. Is Boiler B worth the extra
expense?

To answer this question, look at the annual differential gain in steam flow.
Assuming steam costs $3.50/1000 lb (3.50/454 kg), the annual differential
gain steam flow = 1000 × 3.5 × 8000/1000 = $28,000. Thus, the simple
payback is about a year ($30,000 vs $28,000), which is attractive. You must,

however, be certain you assess payback time against the actual amount of
time the boiler will operate. If the boiler is likely to be used for only half this
period, then the payback time is actually 2 years.
The general procedure presented here can be used for any type of industry

using gas-turbine heat-recovery boilers—chemical, petroleum, power, textile,
food, etc. This procedure is the work of V. Ganapathy, Heat-Transfer