08 PROPERTIES OF STEAM AND AMMONIA 



after ad i aba tic expansion to 60 Ib. is found to be 1139 B.t.u. Hence, if the flow 



is frictionlesB, 



v - 223.7 Vi240 - 1139 - 2250 ft. per sec., approx. 



With 12 per cent loss of energy, the velocity is 



w - 223.7 V(i240 - 1139) X 0.88 - 21 10 ft. per sec. 



Example 9. In case (b) of Example 8, find the quality and specific volume 

 of the .strain in tin- final state, that i-. after expansion to 60 Ib. 



In the frictionlew case the change in / is 1240 1139 = 101 B.t.u. With 

 friction thi> is decreased 12 per cent, leaving 101 X 0.88 = 88.9 B.t.u. Hence 

 in the second state i = 1240 - 88.9 = 1151.1 B.t.u., and the pressure is 60 Ib. 

 From the diagram the corresponding quality is 0.97, nearly. For 60 Ib., v" = 71 s 

 cu. ft., hence the volume per pound is 7.18 X 0.97 = 6.96 cu. ft. 



Example 10. Determine the area of the end section of the nozzle for a dis- 

 charge of 75 Ib. of steam per minute, using the results obtained in Examples 

 8 and 9. 



In the equation of continuity, Fw = Mv, M is given as H Ib. per sec., 

 IT - 21 IO, P = 6.96. Hence the area F is 



60 X fno = ao 4 I 3 **' ft ' = '595 sq. in. 



Example xx. In an ammonia refrigerating machine the ammonia enters 

 the compressor dry and saturated at a pressure of 40 Ib. per sq. in. and is com- 

 pressed adiabatically to 190 Ib. per sq. in. It is then cooled and condensed and 

 in passing through the expansion valve attains the initial pressure 40 Ib. in the 

 brine coils. Required the heat absorbed from the brine, the heat rejected in 

 the condenser, and the heat equivalent of the work per pound of ammonia. 



The solution of this problem requires the values of the heat content i at 

 four points of the cycle. At the beginning of compression the ammonia is dry 

 and saturated at 40 Ib. pressure; from Table 7, or from the ammonia diagram, 

 ii 54 1 - 8 B.t.u. and the entropy is 5 = 1.149. I n tne adiabatic compression 

 the ammonia is superheated and at the end of compression it has the same 

 entropy 1.149 an d a pressure of 190 Ib. From the diagram, or from Table 9, 

 the heat content for this state is is = 639 B.t.u. The ammonia leaving the con- 

 denser is liquid at 190 Ib. pressure, and the corresponding heat content is i% = 

 68.6 B.t.u. The passage through the expansion valve is a throttling process 

 in which i remains constant; hence the heat content of the ammonia as it enters 

 the brine coils is i 4 = 68.6 B.t.u. In any constant-pressure process the heat 

 entering or leaving the medium is given by the change in heat contenl; there- 

 fore during the passage through the brine the ammonia absorbs 541.8 68.6 

 473-2 B.t.u., and in the condenser it rejects to the cooling water 639 68.6 = 

 5704 B.t.u. per pound of ammonia circulated. The work done by the compressor 

 per pound of medium is the difference between these, or 570.4 473.2 = 97.2 

 B.t.u. 



Example 12. With the data of Example n find the refrigerating effect 

 per horsepower-hour. 



The ratio gives the number of B.t.u. absorbed from the brine per B.t.u. 



