23.2 Hermetic, Semi-Hermetic & Centrifugal Compressors

23.2 Hermetic, Semi-Hermetic & Centrifugal Compressors

Trade D’s compressor cluster is hermetic, semi-hermetic, and centrifugal machines — including power, motor efficiency, and design pressures. Chapter 21.3 already placed centrifugals on the Class A Trade C install outline. This section is the component: how the machine is built, how it is oiled, how it unloads, why it has a crankcase heater, and which license may set it. Refrigeration & Air Conditioning Technology, 9th Edition (2021) is the technology book. F.S. 489.105 still decides who contracts the tons. FBC Mechanical 2023 Chapter 11 still wants listed vessels and relief. The Class B outline omits centrifugal compressor-system install; Class B still owes the theory for the Class A upgrade and for any question that tests the machine rather than the permit.

Quick Answer: Hermetic = welded shell, motor in the refrigerant, replace as a unit. Semi-hermetic = bolted, rebuildable, oil glass, unloaders. Centrifugal = impeller, Class A install, surge and inlet vanes. Crankcase heaters stop off-cycle refrigerant migration. Oil type follows the refrigerant. New 3-phase feeders are still an electrical contractor.

Hermetic compressors

A hermetic compressor (from “hermetically sealed”) welds the motor and the pumping mechanism inside one shell. Suction vapor usually cools the motor. There are no bolted heads to lift in the field. Residential and light-commercial scroll and small reciprocating machines are the everyday hermetics. When the motor burns, acid and carbon contaminate the entire circuit — recover, replace driers, and usually replace the compressor as a matched assembly. You do not rebuild a 3-ton scroll on the driveway.

Install points the exam actually scores:

• Level, vibration, and piping stress. A hermetic does not tolerate a discharge line used as a structural brace.
• Correct oil and charge. Most HFC/HFO comfort machines use polyolester (POE) oil, which is hygroscopic — cap the cans. Mineral oil is the old CFC/HCFC story, not a universal top-off.
• Electrical. Nameplate RLA (rated load amps), LRA (locked rotor), MCA, and MOCP. HVAC lands the existing dedicated single-phase disconnect and low-voltage controls (489.105). A burned contactor on a dedicated HVAC circuit with a breaker lock is HVAC work. A new feeder from the service equipment is not.
• Pump-down and receivers (Section 23.3) if the system is built for them; many critically charged hermetic splits are not.

Semi-hermetic compressors

A semi-hermetic (accessible hermetic) puts the motor in the refrigerant atmosphere but uses a bolted housing. Heads, valve plates, and often the motor can be serviced. Commercial reciprocating machines, some screws, and many rack compressors are semi-hermetic. Features Trade D expects you to install and recognize:

Unloading by family — do not mix the hardware:

• Recip semi-hermetic: blocked suction or hydraulic unloader heads; sometimes hot-gas bypass as a fake load.
• Scroll: tandem staging, two-step, or digital (PWM) scrolls.
• Screw: slide valve (and often a VFD).
• Centrifugal: inlet guide vanes and/or VFD — not unloader heads.

Oil, flooded starts, and crankcase heaters

Oil lubricates bearings and, on many recip machines, helps seal rings. Refrigerant moves heat. When they mix in the crankcase as a liquid, oil washes off the bearings on start — bearing wipe. That mixture happens because refrigerant migrates to the coldest part of the off-cycle system, often the compressor, and condenses there.

A crankcase heater keeps the oil warmer than the rest of the circuit during the off cycle so the crankcase is not the condenser. Typical small hermetic heaters are tens of watts; large semi-hermetics use more. The heater is generally on when the compressor is off (and often de-energized when the compressor runs, depending on the listing). After a cold night, a long power outage, or a new install sitting unpowered, energize the heater for the manufacturer’s warm-up before the first start. Florida’s Panhandle still sees 30°F nights; a warehouse in Jacksonville in January is not “too tropical for a heater.” Coastal south Florida still uses heaters because a 75°F equipment room and a 50°F evaporator can still drive migration.

Never start a compressor that slugs liquid or shows a flooded sight glass. Pump-down, heaters, accumulators (Section 23.3), and correct charge are the prevention. Jumping an oil-pressure control to “see if it will run” is how you buy a new semi-hermetic.

POE oil after a burnout: treat acidity and replace driers. Mineral oil in a POE system (or the reverse) is a retrofit chemistry problem, not a top-off.

Centrifugal compressors — power, efficiency, design pressure, Class A

A centrifugal compressor is a dynamic machine: an impeller accelerates vapor; a diffuser (and volute) converts velocity to pressure. It is not a piston. Comfort-cooling centrifugals typically live in the over-100-ton neighborhood, which is why they sit next to the over-100-ton Class A install item. Many 40–90 ton plants are screws or tandem scrolls — do not stamp “centrifugal” on every machine over 25 tons.

Class A outline includes centrifugal compressor install. Class B omits that install bullet. Class B still needs to explain the machine, because the Class A upgrade (and a fair number of theory items) will ask. Class B may not contract a centrifugal plant or any system over 25 tons / 500,000 Btu.

Capacity control: inlet guide vanes, compressor VFD, or both. At too low a flow the impeller surges — flow breaks down, the machine hunts, and bearings pay. Stay on the manufacturer’s map. Hot-gas bypass is a last-resort fake load.

Power and motor efficiency. Input power is compressor work plus losses. Nameplate and expanded data list kW, amps, and sometimes efficiency. A more efficient motor, or a VFD that lets the impeller slow at part load, cuts kW if the condenser and evaporator actually allow the lift to fall. Hermetic centrifugals cool the motor with refrigerant; open-drive machines put a conventional motor outside with a shaft coupling — often easier to service, and motor efficiency is a NEMA-motor conversation. Either way, a several-hundred-amp 460 V feeder is electrical-contractor work under 489.105. HVAC lands flow switches, enable, purge controls, and dedicated HVAC-circuit repairs the statute actually grants.

Design pressures. The compressor housing and the chiller barrels are pressure vessels with a stamped design pressure (and often ASME stamping on large shells). Match the refrigerant:

• Low-pressure centrifugals (historically the R-11/R-123 family; many current plants use low-pressure HFOs such as R-1233zd) run the evaporator below atmospheric. Air leaks in. A purge unit exists to pull noncondensables. Ignoring a running purge is how you lose efficiency and, on older chemistry, how you vent.
• High-pressure centrifugals (R-134a / R-513A family, among others) leak out. Purge is not the same story. Relief valves still pipe outdoors per Chapter 11, not into a plenum.

Oil still lubricates gears and bearings even on machines advertised as “oil-free” at the impeller — read the listing.

Worked Class A/B power check. A 200-ton centrifugal at 0.55 kW/ton is 110 kW input (200 × 0.55) — on the order of 148 hp electrical, not a 5-ton scroll. The plant is over 25 tons and centrifugal: Class A only. A Class B qualifier who “just sets the barrel” is contracting outside 489.105. The Class B candidate still has to read 110 kW, name surge, and know that a purge on a low-pressure machine means air can leak in.

Florida scenario

A Class A shop in Orlando sets a semi-hermetic 40-ton recip on a rack and, across town, a 250-ton centrifugal. The recip gets a crankcase heater, oil-failure control left in the circuit, unloader wiring to the controller, POE oil, and a dedicated HVAC breaker lock for the listed single-circuit work the statute allows — the 460 V feeder was already there from the electrical contractor. The centrifugal gets inlet-vane and VFD control wiring, flow interlocks, a purge on a low-pressure vessel, and a design-pressure / refrigerant match on the nameplate. A Class B candidate walking the same two jobs must be able to name hermetic versus semi-hermetic versus centrifugal, explain why the heater is on overnight, and refuse the 250-ton ticket as Class B. What they must not do is jumper the oil control on the recip or treat the centrifugal as a “big hermetic scroll.”

Traps: (1) Calling a welded 3-ton scroll semi-hermetic because it has a service port. (2) Skipping the crankcase heater after a cold or unpowered sit. (3) Jumping oil-pressure safeties. (4) Treating every chiller over 25 tons as centrifugal. (5) Class B contracting a centrifugal. (6) Using evaporator 2.4 gpm/ton as if it were compressor kW.