39: Thumb rule for maximum compression ratio for a single stage compression is:
A. 3
B. 4
C. 5
D. 6
B; max. pressure ratio to be between 3 and 4 to keep the discharge temperature within limits, ensuring adequate packing life for reciprocating compressors and to avoid lube oil degradation.
Q40. You are provided with two hourglasses. One of them can measure eleven minutes while the other one can measure thirteen minutes.
If you were to measure exactly fifteen minutes using those two hourglasses, how will you do it ?
Step 1: Start both hourglasses.
Step 2: The moment the eleven minute hourglass is empty, invert it.
Step 3: When the thirteen minutes hourglass becomes empty, invert the eleven minute hourglass.
In step 3, you will have counted thirteen minutes. Since you inverted the eleven minute hourglass in step 2, it started from fresh and was inverted just for two minutes (13-11=2). In this manner, when it is inverted again when the thirteen minute hourglass is finished, it will have two minutes of sand left. This time when the sand finishes, you will have measured fifteen minutes (13+2=15).