First │ │ │ │ │ cost │ 640│ 560│ 800│ 800│ 1,500 Emergency│ │ │ │ │ unit. │ │ │ │ │ Renewal│ 550│ 480│ 690│ 690│ 1,290 ───────────┼───────────┼───────────┼───────────┼───────────┼─────────── Total │ 56,134│ 51,314│ 47,821│ 43,963│ 45,800 ───────────┴───────────┴───────────┴───────────┴───────────┴─────────── Type 1. Simple duplex, non-condensing, horizontal. Type 4. Compound condensing low duty horizontal. Type 5. Low duty, triple, condensing, horizontal. Type 6. Cross compound, condensing, horizontal. Type 7. High duty, triple, condensing, vertical. For example, the sewage flow expected at a proposed pumping station is shown in Table 33. The steps involved in the selection of the number and capacity of pumping units to care for these quantities are as follows: (1) Determine the rated capacity of the equipment to be provided. In this case the capacity will be taken as 450 horse-power, which is the maximum load to be placed on the pumps. (2) Select any number of units of such different types and capacities as are available for comparison, and arrange them in different combinations so that each unit will operate as nearly as possible at its rated capacity. The work involved in such a study for 5 units is shown in Table 34. The weight of steam consumed per indicated horse-power hour corresponding to the per cent of the rated capacity at which the unit is operating is read from Fig. 64 or other data. (3) Repeat this step for other numbers and types of units. (4) Prepare a table showing the annual costs of combinations of different numbers and types of units as shown for this example in Table 35. The figures in Table 35 show that the least expensive of the combinations of the units studied is one 200 horse-power unit, and one 250 horse-power unit, with a 250 horse-power unit in reserve. It is to be noted that a reserve unit has been provided in each combination, the capacity of which is equal to that of the largest unit of the combination. CHAPTER VIII MATERIALS FOR SEWERS