EVAPORATION 



361 



can leave is that due to the mixture of the steam and the water ; hence the 

 volume occupied by the air is less in the counter-current type, and there is 

 a less necessary displacement in the pump. It follows, too, that, with the 

 counter-current system, the bad effects of water at an elevated temperature, 

 or a limited supply of water, can be more satisfactorily combated by large 

 pump capacity than with the co-current system. 



Quantity of Water required. If ^ be the initial temperature of the cooling 

 water, t 2 the final temperature, h be the total heat of the steam to be con- 

 h - (t 2 + 32) 



densed, then w 



where w is the water required per Ib. of 



steam. For conditions in the tropics it would be well to calculate for water 

 40 times the weight of steam to be condensed, although, with efficient con- 

 densers and air pumps of ample capacity, good results can be obtained with 

 less. 



Inlet and Discharge Pipes to Condensers. The water 

 is in general practice introduced to the condenser by 

 atmospheric pressure from a supply tank, to which the 

 water is pumped, or to which in certain cases it may 

 gravitate, and in the case of low level condensers the 

 tank may be on the ground floor. If 7^ be the excess 

 of the pressure of the atmosphere over the pressure in 

 the condenser, expressed in feet of water, and if h 2 is the 

 height of the condenser inlet above the level of the 

 supply tank, the water enters under a head of h^ h 2 

 feet. Let this head be h. With no loss of head the 

 velocity of the water entering the condenser will be given 

 by the equation v 2 =2 gh, in feet per second, where v is 

 the velocity and g is the acceleration due to gravity or 

 32 ft. per sec. The sources of loss of head are : resis- 

 tance of entry to the pipe from the tank, resistance due 

 to bends and to obstructions, such as valves, and to 

 friction in the pipe. Neglecting friction, these sources of 



loss may be connected by the following equation : h 1 = - 



Considerable uncertainty attaches to the value of these coefficients. For 

 water flowing into a pipe from a tank through a cylindrical orifice, a co- 



efficient of value o 505 is accepted, so that for ^this [influence h 1 = - 



With a bell-mouthed orifice the value of the coefficient may fall to 0-08. 

 The value of the coefficients for bends varies with the radius of the bends, 

 and with right-angle bends of not less than four pipe diameters ranges round 

 about O'i5. The value of the coefficient due to the loss of head caused by 

 the obstruction offered by a valve evidently is entirely dependent on the 

 design of the latter ; it may be guessed as 0-5. A preliminary approximate 

 idea of the velocity of the water entering the condenser can be obtained on 

 these lines : after obtaining the value of h, irrespective of friction, the latter 



*i 



. _ * _ . fN (^*2 



can be allowed for by the equation : h 2 f l, where/ is 0-02 H , 



~ 



