Process Control & Instrumentation Flashcards

Which type of temperature sensor uses the principle that electrical resistance changes predictably with temperature and is commonly used in industrial refrigeration controls?

Answer: RTD (Resistance Temperature Detector)

RTDs use the predictable change in electrical resistance of metals (typically platinum) with temperature, providing accurate and stable temperature measurements in industrial refrigeration systems.

In a PID controller used for refrigeration capacity control, what does the 'I' term correct for?

Answer: Steady-state offset (accumulated error over time)

The Integral (I) term accumulates error over time and eliminates steady-state offset, ensuring the process variable eventually reaches the setpoint even under sustained load conditions.

A differential pressure switch used on an industrial refrigeration compressor's oil system will trigger a safety shutdown when:

Answer: The difference between oil pump discharge and suction pressure falls below the minimum required value

The oil differential pressure switch monitors the net pressure available for lubrication (pump discharge minus crankcase pressure); if this differential drops too low, bearings lose adequate lubrication and the compressor must shut down.

Which instrument is used to measure the liquid level in an ammonia vessel to prevent liquid carryover to the compressor?

Answer: Magnetic float-type level indicator or electronic level transmitter

Magnetic float-type level indicators and electronic level transmitters provide reliable continuous level measurement in ammonia vessels, helping prevent dangerous liquid carry-over to compressors.

In industrial refrigeration, a solenoid valve used as a liquid line solenoid is typically configured to:

Answer: Fail closed on power loss to stop refrigerant flow when the system shuts down

Liquid line solenoids are normally-closed (NC) valves that fail closed on power loss, preventing refrigerant migration and flooding during shutdown to protect the system.

A 4-20 mA analog signal from a pressure transmitter corresponds to a range of 0 to 300 psig. What pressure does a 12 mA signal represent?

Answer: 150 psig

A 12 mA signal is at 50% of the 4-20 mA span (12-4=8, 8/16=0.5), so the pressure is 50% of 300 psig = 150 psig.

Which type of control valve uses a refrigerant-filled power element that senses suction superheat to regulate refrigerant flow into the evaporator?

Answer: Thermostatic expansion valve (TXV/TEV)

The Thermostatic Expansion Valve (TXV/TEV) uses a refrigerant-filled sensing bulb attached to the suction line to detect superheat and throttle refrigerant flow to maintain proper evaporator feed.