CIRO Process Control & Instrumentation 3

Question 1

What is the primary purpose of a hot gas defrost solenoid valve in an instrumented industrial refrigeration system?

Accepted Answer

To admit hot discharge gas into the evaporator coil to melt accumulated frost during defrost cycles

Answer

To vent high-pressure discharge gas to atmosphere during emergency situations

Answer

To regulate compressor capacity by bypassing discharge gas to suction

Answer

To cool the compressor motor windings during high-load operation

Question 2

A capacitance-type level sensor installed on a high-pressure receiver in an ammonia plant measures liquid level based on:

Accepted Answer

The change in electrical capacitance between the sensor probe and the vessel wall as liquid level changes

Answer

The weight difference between liquid and vapor ammonia above the sensor

Answer

Ultrasonic pulses reflecting off the liquid surface

Answer

Hydrostatic pressure at the bottom of the vessel

Question 3

When using an electronic data logger to trend compressor discharge temperature over time, a gradual upward drift could indicate:

Accepted Answer

Fouled or scaled condenser tubes reducing heat rejection efficiency

Answer

Decreasing ambient temperature around the condenser

Answer

Lower-than-normal refrigerant charge

Answer

Reduced suction superheat

Question 4

An oil pressure differential switch on a screw compressor has a 3-second time-delay relay before tripping. What is the purpose of this time delay?

Accepted Answer

To allow the compressor to fully build speed before oil pressure is monitored, preventing nuisance trips during startup

Answer

To comply with OSHA regulations requiring a 3-second warning before shutdown

Answer

To give operators time to manually override the shutdown

Answer

To allow the refrigerant charge to equalize before the compressor stops

Question 5

In industrial refrigeration control systems, what does the term 'dead band' refer to in a two-position (on/off) controller?

Accepted Answer

A range of values around the setpoint within which the controller takes no action, preventing rapid cycling

Answer

The time period after a safety shutdown during which the compressor cannot restart

Answer

The portion of the control signal that is filtered out as electrical noise

Answer

The pressure drop across a control valve at full open position

Question 6

Which alarm condition should trigger an immediate plant evacuation and activation of the emergency response plan in an ammonia refrigeration facility?

Accepted Answer

High ammonia concentration alarm exceeding 300 ppm in an occupied area

Answer

Suction pressure 5 psig above normal operating range

Answer

Condenser fan motor overload relay trip

Answer

High-pressure receiver level above 80% full

Question 7

A technician notices that the 4-20 mA output of a suction pressure transmitter reads 3.8 mA with the system running. This most likely indicates:

Accepted Answer

A failed or open-circuit transmitter, since valid 4-20 mA signals cannot fall below 4 mA during normal operation

Answer

The suction pressure is at the bottom of the transmitter's range

Answer

The transmitter is at exactly 0% of its measuring range

Answer

Suction pressure has gone into a deep vacuum condition