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

The hot gas defrost solenoid valve opens during scheduled defrost cycles to route hot, high-pressure discharge gas into the evaporator, supplying heat energy to melt frost accumulation from the coil.

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

Capacitance level sensors detect the difference in dielectric constant between liquid ammonia and ammonia vapor; as liquid level changes, the capacitance between probe and vessel wall changes proportionally.

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

Gradual rise in discharge temperature over time often indicates reduced condenser efficiency from fouling or scaling, which raises condensing pressure and compressor work, increasing discharge temperature.

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

During compressor startup, oil pressure takes a brief moment to build; the time delay relay prevents nuisance safety trips during the normal startup transient before steady-state oil pressure is established.

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

Dead band is a control range centered on the setpoint where the controller neither turns the system on nor off, preventing short-cycling (rapid switching) that would damage compressors and other equipment.

CIRO - Certified Industrial Refrigeration Operator Practice Test

CIRO - Certified Industrial Refrigeration Operator Practice Test

CIRO Process Control & Instrumentation 3