Which of the following is the purpose of fail-safe circuits?
a. fail-safe circuits are designed to provide overload protection for system components
b. fail-safe circuits are designed to protect electric motors against voltage surge
c. fail-safe circuits are designed to maintain constant current in electric motor coils
d. a, b, and c
e. none of the above

Air is to be cooled in the evaporator section of a refrigerator by passing it over a bank of 0.8-cm-outer-diameter and 0.4-m-long tubes inside which the refrigerant is evaporating at -20°c. air approaches the tube bank in the normal direction at 0°c and 1 atm with a mean velocity of 4 m/s. the tubes are arranged in-line with longitudinal and transverse pitches of sl- st 1.5 cm. there are 30 rows in the flow direction with 15 tubes in each row. determine (a) the refrigeration capacity of this system and (b) pressure drop across the tube bank. evaluate the air properties at an assumed mean temperature of -5°c and 1 atm. is this a good assumption?