10.4.2 Starting times
The economics of sizing vacuum raising and maintaining plant are described below, using an example with figures taken from a typical power station study. It identifies the possibility of vacuum raising and maintaining plant size being determined by the quick-start requirement referred to earlier. The costs per start per machine incurred by holding the boiler in the start condition whilst waiting for vacuum to be raised are given as £600 per 20 minutes.
Expressed as cost per minute holding = £30/minute
Number of starts anticipated over life of plant (25 years) = 2000
Assuming a typical distribution curve (Fig 4.44), determine the present value (PV) cost of quick-starting over the 25 year life of the plant, for discounted cash flow (DCF) discount rates (OCR) of 5% and 10%.
At discount rates of 5% and 10%, the present value costs of vacuum raising over 25 years is £22 933/min and £10 388/min, respectively.
The options available are pumps А, В and С which have corresponding pumping capacities in the ratio of 3, 2 and 1. Table 4.8 indicates an evaluation which was carried out on the three pump options and their associated costs against pump A, which was chosen as the datum design.
This study suggests that, by reducing the size of the vacuum pumps, and hence the capacity available for quick-starting, the vacuum raising time may be increased without incurring an economic penalty.
Pump C, the lowest capacity pump option capable of dealing with a leakage rate of 40 kg/h, appears to be the best economic choice. However, no allowance is made for the associated increases in turbine-generator, feedheating, and condensing plant costs necessary to ensure that a 40 kg/h air inleakage rate is not exceeded. These costs could offset the apparent cost savings of £134 764.
It would appear that the intermediate vacuum pump size (pump B) can meet the traditional vacuum raising time of 20 minutes, and offers some cost saving, although not as significant as pump C.