Equipment Operation Flashcards

An air handling unit's mixed air temperature drops below freezing on a cold morning and the unit shuts down. Which device most likely tripped?

Answer: Low-limit freezestat

The low-limit freezestat trips when air leaving the preheat or downstream of the mixing section falls near freezing to protect coils.

Which measurement best indicates that an air filter bank in an AHU needs replacement?

Answer: Pressure drop across the filters reaching the manufacturer's final resistance

Differential pressure across filters shows actual loading, and replacement is due when it reaches the rated final resistance.

A three-phase motor is running hot and drawing unequal current on each leg. What is the most likely cause?

Answer: Voltage imbalance between phases

Even a small voltage imbalance causes much larger current imbalance and heating in three-phase motors.

When starting a chilled water system, which sequence is generally correct?

Answer: Establish chilled water and condenser water flow, then start the chiller

Chillers require proven water flow through evaporator and condenser before the compressor starts to prevent freezing and high head pressure.

What does a steam trap do in a steam distribution system?

Answer: Discharges condensate and non-condensable gases while holding back live steam

Steam traps remove condensate and air from steam lines without letting steam escape.

A hydraulic elevator car slowly sinks below the landing while parked. Which component is the most likely culprit?

Answer: Internal valve or cylinder packing leakage

Hydraulic cars drift when fluid bypasses through leaking control valves or past cylinder seals.

When an electric fire pump is tested weekly under NFPA 25, how long must it run under no-flow (churn) conditions?

Answer: 10 minutes

Current NFPA 25 requires electric fire pumps to run at least 10 minutes during churn tests, while diesel pumps run 30 minutes.