CIRO Process Control & Instrumentation 2 — Questions and Answers
Question 1: In SCADA systems used for industrial refrigeration plants, which communication protocol is most commonly used for reading data from PLCs and remote I/O devices?
- RS-232 serial point-to-point
- Modbus RTU/TCP or Ethernet/IP (Correct answer)
- HDMI
- USB 3.0
Correct answer: Modbus RTU/TCP or Ethernet/IP
Modbus RTU/TCP and Ethernet/IP are the dominant industrial communication protocols for SCADA integration with PLCs and field devices in refrigeration plant automation.
Question 2: A high-pressure cutout switch in an industrial ammonia system trips the compressor when discharge pressure exceeds the setpoint. What is the typical action required before the compressor can restart?
- The switch resets automatically when pressure drops below the cutout point
- Manual reset is required after the fault condition has been corrected (Correct answer)
- The PLC automatically restarts the compressor after a 30-second delay
- Refrigerant must be fully evacuated before restart
Correct answer: Manual reset is required after the fault condition has been corrected
High-pressure cutouts are typically manual-reset safety devices; an operator must investigate and correct the cause of high pressure before physically resetting the switch to restart the compressor.
Question 3: What is the purpose of a suction pressure regulator (SPR) valve in a multi-evaporator refrigeration system?
- To boost suction pressure above the compressor's minimum operating range
- To maintain a minimum evaporator pressure regardless of compressor suction pressure, preventing the evaporator from freezing product (Correct answer)
- To regulate oil pressure in the compressor crankcase
- To control condenser head pressure during low ambient conditions
Correct answer: To maintain a minimum evaporator pressure regardless of compressor suction pressure, preventing the evaporator from freezing product
An SPR valve holds the evaporator pressure at or above a set minimum, preventing the temperature from dropping too low and freezing sensitive products while the compressor handles lower-pressure loads from other evaporators.
Question 4: When calibrating a temperature sensor in an ammonia refrigeration system, which reference standard is most appropriate for verifying accuracy near -20°F?
- Ice water bath at 32°F
- NIST-traceable calibrated reference thermometer and dry block calibrator (Correct answer)
- Boiling water at 212°F
- Room temperature dry-bulb thermometer
Correct answer: NIST-traceable calibrated reference thermometer and dry block calibrator
A NIST-traceable reference thermometer combined with a dry block calibrator allows accurate calibration at the actual operating temperature range of the sensor, ensuring reliable process control.
Question 5: An electronic expansion valve (EEV) in an industrial refrigeration system is controlled by a superheat controller. If the measured superheat is too high, the controller will:
- Close the valve further to reduce refrigerant flow
- Open the valve wider to increase refrigerant flow to the evaporator (Correct answer)
- Increase compressor speed to lower suction pressure
- Activate the hot gas defrost cycle
Correct answer: Open the valve wider to increase refrigerant flow to the evaporator
High superheat indicates insufficient refrigerant in the evaporator; the controller opens the EEV wider to admit more liquid refrigerant and reduce superheat back to the setpoint.
Question 6: Which type of flow meter is most suitable for measuring liquid ammonia flow in a large industrial refrigeration system due to its accuracy and lack of moving parts?
- Turbine flow meter
- Coriolis mass flow meter (Correct answer)
- Rotameter (variable area meter)
- Bucket-and-stopwatch method
Correct answer: Coriolis mass flow meter
Coriolis mass flow meters measure mass flow directly with no moving parts, are highly accurate with ammonia, and are unaffected by fluid density variations, making them ideal for industrial refrigeration applications.
Question 7: In a programmable logic controller (PLC) ladder logic program for refrigeration control, a normally-closed (NC) contact associated with a high-temperature alarm will:
- Pass current (be conductive) during normal operation and open (stop current) when the alarm activates (Correct answer)
- Block current during normal operation and close when the alarm activates
- Always remain conductive regardless of input state
- Only conduct during manual override mode
Correct answer: Pass current (be conductive) during normal operation and open (stop current) when the alarm activates
In PLC ladder logic, a NC contact is conductive (passing power) when the associated input coil is de-energized (normal state) and opens to stop current flow when the alarm input energizes, matching real-world NC relay behavior.
CIRO Process Control & Instrumentation 2
In SCADA systems used for industrial refrigeration plants, which communication protocol is most commonly used for reading data from PLCs and remote I/O devices?