A twostage steady state compressor having an interstage cool

A two-stage steady state compressor having an interstage cooler is shown in the figure. It takes in air at 540 R, 14 psia, and compresses it to 280 psia. The cooler then cools the air to 610 R, after which it enters the second stage, which has an exit pressure of 2200 psia. The isentropic efficiency of stage one is 90% and the air exits the second stage at 1140 R. Both stages are adiabatic, and the cooler dumps Q to reservoir at To= 540 R. Assume the air is an ideal gas. The pressure drop across the intercooler is negligible. Find

a) Q in the cooler [Btu/lbm],

b) the efficiency of the second-stage, and

c) the total entropy generated in this process [Btu/lbm-R].

d) Draw T-s state diagram.

Solution

Changeover of fixtures on a machine takes 35 minutes. Following changeover an average of 5 parts must be produced and discarded before correct sitting is achieved. The machine cycle time for each part is 2 minutes and the materials cost is $1.50. The labour cost for a skilled set up person is $45 per hour while the machine operator is paid $30 per hour. Current batch size is 50. Suppose the set up time can be reduced so that the operator can perform the changeover and on average only two parts are required before correct setting is achieved, what should be the new set up time so that a batch size of 20 could be produced with only 90% of the current cost per unit? The cost per unit is the total of material cost, labour of set up and labour cost of running the machine.