31.2 Repairing Hermetic, Semi-Hermetic, Centrifugal Compressors & Blowers

31.2 Repairing Hermetic, Semi-Hermetic, Centrifugal Compressors & Blowers

Trade F’s next cluster is repairing the machines Chapter 23.2 installed: hermetic, semi-hermetic, and centrifugal compressors, plus blowers. The install chapter taught welded vs bolted vs impeller. This section is the service decision: replace the hermetic, rebuild the semi-hermetic, Class A work on the centrifugal, and repair the blower without turning a forward-curved wheel into a grenade. Refrigeration & Air Conditioning Technology, 9th Edition (2021) is the book. F.S. 489.105 still fences 25 tons / 500,000 Btu for Class B. The Class B outline omits centrifugal compressor-system install; Class B still owes repair theory for the upgrade exam and for any question that tests the machine rather than the permit. EPA 608 still sits under every opened refrigerant circuit (Section 31.1).

Quick Answer: Hermetic = welded, replace as a unit after 608 recovery. Semi-hermetic = bolted, rebuild valve plates, gaskets, oil, sometimes pistons — still recover first. Centrifugal = impeller, bearings, seals, purge; Class A plant. Blowers: bearings, wheel-to-inlet gap, rotation arrow, belt alignment — dirt is unbalance, not extra cfm.

Why “repair” is not “jumper the overload”

A compressor that is out on internal overload or a blower that is noisy is a diagnosis (Trade E) plus a repair method (Trade F). Jumping a safety, adding oil through a suction port “until the knock stops,” or beating a wheel until it spins is not a repair. LOTO the disconnect, recover if the refrigerant circuit will be opened, then use the method that matches the housing. New three-phase building feeders remain electrical-contractor work; HVAC lands dedicated HVAC-circuit repairs with a breaker lock, existing single-phase disconnect reconnects, and low-voltage controls.

Hermetic compressors — replace, do not field-rebuild

A hermetic compressor welds the motor and pump inside one shell. Suction vapor cools the motor. There are no bolted heads. Field “rebuild” means cut it out and put a new one in. Typical replacement sequence:

1. LOTO. Verify zero energy.
2. Recover to the 608 level in Section 31.1. Do not cut a charged 410A process tube.
3. Cut or unbraze the process tubes. Nitrogen purge while heating so you do not send scale into the new machine. Plug the old compressor — residual oil is still a hazardous waste story, not a drain-on-the-roof story.
4. Ohm the new compressor: single-phase C–R–S (R–S highest, C–R run/lowest). Any pin to shell is grounded — reject it. Three-phase windings should be equal.
5. Match oil type to the refrigerant (POE with 410A/HFC; mineral with many remaining HCFC machines). Do not pour mineral into a POE 410A replacement.
6. Replace the liquid-line filter-drier on every compressor change. If the old machine burned (acid, carbon, burnt-varnish smell): acid-test the oil, install a suction-line filter-drier, consider a burnout drier, and do not skip the deep vacuum.
7. Nitrogen test, 500 micron vacuum, weigh in the charge (or the nameplate amount after a burnout when recovered refrigerant is contaminated — do not put acid oil back).
8. Energize the crankcase heater before the first start if the machine sat cold (Chapter 23.2). Check amp draw vs RLA, superheat, and subcooling.

A wound-on-stator hermetic that is open or grounded is a replace. You cannot pull the stator on the roof. Scroll hermetics are still hermetics: the shell is welded even when the machine is large.

Semi-hermetic compressors — bolted, rebuildable

A semi-hermetic has a bolted housing, typically an oil sight glass, an oil pump, and often unloaders. After recovery (or isolation with front-seated service valves plus recovery of the crankcase), you may:

• Replace valve plates, reed valves, and head gaskets.
• Inspect pistons, rings, cylinders, rods. Score and aluminum in the oil is a deeper rebuild or a replacement.
• Change oil, acid-test, replace the oil filter/screen if the machine has one.
• Service the oil pump and confirm net oil pressure (Section 31.4) after start.
• Megger motor windings (do not megger through solid-state boards).
• Torque head bolts in sequence dry or lubed per the compressor OEM — made-up torque is how you warp a head and add a leak.

Open-drive machines (external motor, shaft seal) are a third family. A leaking shaft seal is a seal job after recovery or pump-down, not a “semi-hermetic head gasket.” Do not call a scroll hermetic a semi-hermetic because it is large.

Worked rebuild vs replace. A 15-ton semi-hermetic on a legal Class B rack has broken discharge reeds (high suction, low head, amps down). Recover, pull the head, replace the valve plate and gaskets, new drier, nitrogen, vacuum, charge, verify net oil pressure and superheat. The same symptom on a welded 5-ton hermetic is a compressor changeout, not a valve-plate kit. Sight-glass oil at one-half to three-quarters while running is the usual teaching picture; a full glass after a long off-cycle is not proof the oil pump is healthy.

Centrifugal compressors — impeller machines, Class A plant

A centrifugal is a dynamic compressor: impeller, diffuser, inlet guide vanes and/or a VFD, not reciprocating heads. Low-pressure machines (R-123 and similar) run below atmosphere on the suction side and use a purge to pull air and moisture out. Surge is the part-load failure mode: flow breaks down, the machine hunts, howls, and heats. Surge is not fixed by jumpering a high-pressure cutout, and it is not a recip unloader click.

Repair work the outline cares about:

• Bearings and shaft clearance.
• Impeller damage (erosion, foreign object).
• Shaft seals on open-drive centrifugals.
• Oil sump, oil pump, oil cooler, oil heater — centrifugal oil is specific; do not pour POE 410A oil into an R-123 sump.
• Purge unit repair (compressor, condenser, drift). A purge that runs continuously is air in, not “normal.”
• Inlet-vane linkage: vanes that do not actually stroke will surge or limit capacity.

Class A contracts the centrifugal plant and any system over 25 tons / 500,000 Btu. Class B still has to explain impeller, purge, surge, and oil — the Class B outline dropped install of centrifugal compressor systems, not the physics. A 40-ton centrifugal in a Miami hospital is Class A work even if the repair is “only a purge compressor.”

Repairing blowers

A blower (fan) is not a compressor. Trade F still scores repairing blowers because a dirty or misaligned wheel looks like a “low-capacity refrigerant” call.

Repair sequence:

1. LOTO. Block the wheel — residual spin stores energy.
2. Inspect the inlet cone / vortex gap. A wheel sucked into the housing or sitting too far back leaks around the inlet and loses cfm.
3. Set screws on the shaft flat; a second screw often locks the first. A wheel that walks is a bearing and housing destroyer.
4. Bearings: pillow-block, cartridge, or sleeve. Lubricate only bearings that have fittings, with the grease the OEM named. Do not pack an over-greased bearing “until it weeps” as a monthly habit.
5. Clean the wheel. Dirt is unbalance. Do not bend blades back to “true” them.
6. Rotation must match the arrow on the housing. Three-phase: swap two leads to reverse (inside the HVAC electrical fence on a dedicated circuit).
7. Belts: deflection commonly taught as about 1/64 inch per inch of span (use the drive listing). A straightedge across both sheaves is alignment. Missing belts on a multi-belt drive overload the remaining belts.
8. After repair, measure amps vs FLA and external static. A forward-curved wheel overspeeded to “make cfm” after a dirty coil is how you yield the wheel.

Florida HVAC scenario

A Class B qualifier in Orange County has a 4-ton packaged unit with a welded hermetic that is grounded pin-to-shell and a forward-curved indoor blower that howls. Correct path: recover 410A into a yellow-top cylinder, replace the hermetic (new POE machine, new drier, nitrogen, vacuum, weigh-in), replace blower bearings, reset the wheel to the inlet gap, lock set screws, confirm rotation. Wrong path: try to unbolt a welded shell, pour oil into the suction, and speed the blower with a smaller motor sheave until amps exceed FLA. Class B is legal on 4 tons. An 80-ton centrifugal next door with a purge that never stops is Class A, plus a leak-and-air-in diagnosis — not a Class B weekend rebuild.

Traps: (1) Rebuilding a hermetic. (2) Replacing a semi-hermetic because you did not know valve plates exist. (3) Class B bidding a centrifugal over 25 tons. (4) Treating surge as a high-pressure cutout problem. (5) Overspeeding a forward-curved wheel. (6) Wrong oil after a compressor change. (7) Skipping recovery because “it is only a head gasket.”