6.1 Refrigeration, Chillers & HVAC Plant Equipment

Maryland Regulatory Scope: Plant Machinery Beyond Boilers

The Maryland Board of Stationary Engineers' equipment list includes chiller plants, air conditioning machinery, and refrigeration equipment. Board FAQs confirm that large refrigeration compressors — and even plants with refrigeration only — can require a licensed stationary engineer when equipment meets Title 6.5 thresholds (pressure more than 15 PSI and 30 HP or more). Refrigeration is not categorically exempt.

License grade is still based on the largest single regulated object, not the sum of every small chiller or auxiliary fan. A plant with three 25 HP reciprocating chillers and one 120 HP centrifugal machine requires a grade based on the 120 HP unit, not 195 HP aggregate.

HVACR Contractor Licensing vs Stationary Engineer Oversight

Maryland HVACR contractor licenses are a separate credential track for installation, service, and refrigerant-related work under distinct statutes and regulations. Stationary engineers should coordinate with qualified HVACR contractors for regulated refrigerant recovery, charging, and major compressor overhauls. Your stationary engineer license authorizes plant operation oversight of regulated machinery; it does not replace HVACR contractor credentials for refrigerant handling.

When Chillers Trigger Stationary Engineer Licensure

Ask three questions on every chiller plant:

1. Does any single compressor or regulated object exceed 30 HP?
2. Does regulated pressure exceed 15 PSI on that object (per Title 6.5 definitions)?
3. Does a licensed person have charge of the plant of machinery?

If yes to the statutory elements, plan for Maryland licensure even when no high-pressure steam boiler exists. Hospital campuses, university central plants, and large commercial facilities often run combined steam and chilled-water systems under one licensed engineer's oversight.

Vapor-Compression Cycle for Operators

Understanding the refrigeration cycle is essential for safe operation and for explaining abnormal alarms during exams and on shift.

1. Evaporator — low-pressure refrigerant absorbs heat from chilled water or a process fluid; chilled-water temperature rises slightly across the evaporator bundle.
2. Compressor — raises refrigerant pressure and temperature; compressor motor HP is often the nameplate figure that sets license grade when thresholds are met.
3. Condenser — rejects heat to condenser water (water-cooled) or ambient air (air-cooled); elevated condenser pressure reduces capacity and efficiency.
4. Expansion device — meters liquid refrigerant and drops pressure into the evaporator.

Centrifugal Surge and Stall

Surge is rhythmic flow reversal when discharge pressure exceeds what the centrifugal compressor can stably deliver at a given inlet condition and speed. Symptoms include vibration, loud pulsing, rising amperage swings, and possible high-discharge temperature. Operators should reduce capacity, verify condenser water flow and temperature, check for noncondensables, and avoid prolonged surge that damages impellers and bearings. Stall is a related unstable region at very low flow — treat it as an immediate capacity and conditions problem, not a nuisance alarm.

Low-Pressure Chillers and Purge Units

Older low-pressure machines (historically common with certain refrigerants) operate below atmospheric pressure on the evaporator side. Air leaks into the system raise condenser pressure and trigger purge unit activity. Excessive purge frequency signals an integrity problem requiring qualified service — not endless operator resets.

Operator Rounds, Oil, and Safety

Log on each round:

• Oil level, oil pressure, and oil temperature (where provided)
• Suction and discharge pressures and temperatures
• Motor amperage versus nameplate FLA
• Chilled-water supply/return and condenser water approach temperatures
• Purge run time on applicable machines
• Alarm history and capacity/load commands

Oil-logged machines after long shutdowns need manufacturer-approved warm-up before start to avoid liquid refrigerant in the crankcase and catastrophic bearing damage. Apply LOTO before opening pressurized vessels, removing couplings, or working on rotating assemblies.

Many refrigerants can displace oxygen and cause asphyxiation in poorly ventilated machine rooms. Know ventilation requirements, refrigerant detection (where installed), and SDS/EPA requirements. Never disable high-pressure cutouts, oil pressure trips, or motor overload protection to "keep the plant running."

Integrated Central Plants

Maryland hospitals, campuses, and commercial plants often run combined steam and chilled-water central plants. The licensed engineer oversees the regulated plant of machinery, not boilers in isolation. A night shift may involve switching chillers, adjusting cooling-tower fans, responding to steam pressure swings, and coordinating HVACR contractors for refrigerant work — all under the same safety mandate: qualified oversight of regulated equipment.

Exam Traps

• Trap: Refrigeration plants never need a stationary engineer. Correct: Board FAQ — refrigeration-only plants can require licensure when thresholds are met.
• Trap: Add all chiller HP to set grade. Correct: largest single regulated object only.
• Trap: Stationary engineers may freely recover and charge any refrigerant. Correct: HVACR credentials govern refrigerant service; engineers coordinate and operate.