-----------+-----------+-----------+-------------- Single. | Double. | Triple. | Quadruple. -----------+-----------+-----------+-------------- 95 | 150 | 220 | 300 -----------+-----------+-----------+-------------- There is naturally a limit beyond which the capital cost of the machine neutralizes the advantage of steam economy, and it is seldom that octuple effects are used. There are probably more triple effects in use than any other machine. An essential and important part of the modern evaporator is the "condenser," in which the vapour from the last effect is conducted into water (jet condensers) or over cooled surfaces (surface condensers), with a view to producing and maintaining the vacuum. A lasting vacuum cannot be maintained without an air-pump, as air is often introduced (1) with the steam, having entered the boiler dissolved in the feed water; (2) by leakage from the atmosphere into the condenser and the connected vacuous spaces; and (3) in jet condensers, in solution with the circulating condenser water. That from the first two sources may be reduced, but the third is beyond control: hence if high vacua are necessary, surface condensers are to be preferred. Dissolved air is usually 5-20 per cent. of the water volume, and is least for sea-water. It should be noted that water leaving a surface condenser is in a very air-free state, and therefore particularly suitable for boiler supply. Apart from the capital cost of a condenser the chief cost of maintaining a vacuum is in pumping the circulating water, of which up to 70 lbs. is usual per lb. of steam condensed. If W = weight of steam condensed (lbs. per hour); Q = weight of cooling water circulated (lbs. per hour) T{i} = inlet temperature (° F.) of cooling water; T{o} = outlet temperature (° F.) of cooling water; then T{o} = T{i} + 1050(W/Q) It will be understood that for high vacua, low temperature of cooling water (T{i}) is more important than copious supply (Q/W). It is advantageous, however, to choose a site yielding plenty of cold water, such as a river or canal side. Otherwise it is often necessary to use cooling towers or spray nozzles. The cooling is by evaporation (= 60 to 80 per cent. of W), cold water replacing that evaporated, and yielding water 75° to 80° F. If T{i} = 80° F. and Q/W = 70°, a vacuum of 28.34" is possible, but the 0.34" should be allowed for the partial pressure of the air, determined exactly by the air entering and by the displacement of the air-pump.