6.1 Compressor Mechanics (Reciprocating, Scroll, Rotary, Screw) and Failures

6.1 Compressor Mechanics (Reciprocating, Scroll, Rotary, Screw) and Failures

The compressor is the mechanical heart of the vapor-compression refrigeration cycle. It performs two thermodynamic functions: it lowers pressure in the evaporator so refrigerant can boil at low temperatures to absorb heat, and it raises vapor pressure and temperature so heat can be rejected into an ambient medium in the condenser. Candidates preparing for the HVAC Excellence Certification must master positive displacement mechanics, compression ratio calculations, lubricant chemistries, and the diagnostic analysis of compressor failures.

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1. Positive Displacement Compressor Mechanics

All standard residential and light commercial HVAC/R compressors operate on the positive displacement principle: a fixed volume of low-pressure vapor is captured in a compression chamber, and mechanical energy physically reduces the chamber volume, forcing vapor molecules closer together and drastically increasing pressure and temperature prior to discharge.

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|                               POSITIVE DISPLACEMENT COMPRESSORS                                   |
+---------------------+---------------------+-----------------------+--------------------------------+
|    RECIPROCATING    |       SCROLL        |        ROTARY         |             SCREW              |
| - Piston & cylinder | - Involute spirals  | - Rolling piston      | - Helical meshing rotors       |
| - Crankshaft/rod    | - Orbiting motion   | - Sliding spring vane | - Male (4-lobe)/Female (6-flute)|
| - Reed valve plates | - Dual compliance   | - Ductless & mini-spl | - Slide valve (10-100% capacity)|
| - Clearance volume  | - No valves (>90% η)| - High volumetric η   | - Large commercial chillers    |
+---------------------+---------------------+-----------------------+--------------------------------+

1. Reciprocating Compressors

Reciprocating compressors utilize a rotating crankshaft driven by an electric motor to move one or more pistons linearly inside precision-honed cylinders via connecting rods and wrist pins.

• Suction Stroke: The piston moves downward from Top Dead Center (TDC) to Bottom Dead Center (BDC). The expanding cylinder volume creates a pressure drop below suction line pressure, allowing the flexible steel suction reed/flapper valve to lift open and draw in low-pressure, superheated suction vapor.
• Discharge Stroke: The piston moves upward from BDC toward TDC. The rising cylinder pressure forces the suction reed closed. When internal cylinder pressure exceeds the high-side pressure resting above the discharge valve plate, the discharge reed valve flexes open, discharging high-pressure, superheated vapor into the discharge plenum.
• Clearance Volume & Re-Expansion: In reciprocating designs, the piston cannot touch the valve plate at TDC without mechanical collision. The tiny residual space remaining between the piston crown and the valve plate is the clearance volume. High-pressure vapor trapped in this clearance space cannot be discharged; upon the downward stroke, it must first expand below suction pressure before the suction valve can open. This clearance re-expansion reduces the effective stroke volume.

Volumetric Efficiency Formula:

$\text{Volumetric Efficiency } (\eta_v) = \left( \frac{\text{Actual CFM Pumped}}{\text{Theoretical Displacement CFM}} \right) \times 100$

Compression Ratio (CR) Formula:

$\text{Compression Ratio (CR)} = \frac{\text{Absolute Discharge Pressure (psia)}}{\text{Absolute Suction Pressure (psia)}} = \frac{P_{\text{discharge (psig)}} + 14.7}{P_{\text{suction (psig)}} + 14.7}$

Worked Example: An R-410A system operates with a suction pressure of 118 psig and a discharge pressure of 385 psig. $\text{CR} = \frac{385 + 14.7}{118 + 14.7} = \frac{399.7\text{ psia}}{132.7\text{ psia}} = 3.01 : 1$

As the compression ratio rises (caused by high head pressure from dirty condenser coils or low suction pressure from restricted airflow), the clearance gas expands further down the cylinder stroke, delaying suction valve opening, drastically lowering volumetric efficiency (η_v), and causing excessive motor heat.

2. Scroll Compressors

Scroll compressors utilize two intermeshing Archimedean spiral scrolls—a stationary (fixed) scroll and an orbiting scroll driven by an eccentric motor shaft offset by 180°.

• Continuous Pocket Compression: The orbiting scroll does not rotate; it translates in a circular orbit. This orbital motion traps crescent-shaped vapor pockets at the outer perimeter. As the scroll orbits, these pockets continuously shrink in volume as they move inward toward the center discharge port.
• Dual Compliance Design:
  1. Radial Compliance: Centrifugal force and mechanical counterweights push the scroll flanks together radially, creating continuous gas seals along the spiral walls.
  2. Axial Compliance: Discharge gas pressure is ported into an intermediate seal cavity beneath the fixed scroll, holding the tips firmly against the base plates vertically.
• Operating Advantages:
  • No suction or discharge reed valves are required (eliminating valve flutter, breakage, and throttling losses).
  • Volumetric efficiency exceeds 90% to 95% because there is virtually zero re-expansion clearance volume.
  • Superior liquid droplet tolerance: If non-compressible liquid droplets enter the scroll, radial compliance allows the scroll flanks to separate momentarily, passing the liquid toward the center without breaking mechanical parts.

3. Rotary Compressors

Rotary compressors feature a hardened steel cylindrical roller (rolling piston) mounted on an eccentric motor shaft inside a stationary cylinder housing.

• Sliding Vane Operation: A spring-loaded sliding vane (blade) machined into the cylinder wall rides continuously against the outer surface of the rolling piston, separating the suction side from the discharge side.
• Characteristics: Low vibration, very few moving components, high volumetric efficiency at low pressure differentials. Widely utilized in ductless mini-splits, window air conditioners, and variable-refrigerant-flow (VRF) systems.

4. Screw (Helical Rotor) Compressors

Screw compressors employ two intermeshing helical rotors—a male rotor (typically 4 lobes) and a female rotor (typically 6 flutes) machined to extreme tolerances inside a stationary casing.

• Operation: Vapor enters the suction end, is trapped between the meshing helical lobes, and is compressed axially along the length of the rotors toward the discharge port.
• Capacity Modulation: Utilizes an axially sliding valve (slide valve) that bypasses a portion of the trapped vapor back to suction, providing smooth, stepless capacity modulation from 10% to 100% full load in large commercial chillers (50 to 500+ tons).

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2. Mechanical Housings and Motor Cooling

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|                                COMPRESSOR HOUSING CLASSIFICATIONS                                  |
+-----------------------------+-----------------------------+----------------------------------------+
|          HERMETIC           |        SEMI-HERMETIC        |               OPEN-DRIVE               |
| - Welded steel shell        | - Bolted cast-iron housing  | - External motor (belt or coupling)    |
| - Motor & pump on one shaft | - Fully field serviceable   | - Shaft seal isolates crankcase        |
| - Suction-gas cooled motor  | - Suction or air cooled     | - Ammonia (R-717) & transport systems  |
| - Discarded upon failure    | - Rebuildable valve plates  | - Seal leakage requires monitoring     |
+-----------------------------+-----------------------------+----------------------------------------+

• Hermetic Compressors: The electric motor and compressor mechanism are sealed inside a welded steel shell. The internal motor is cooled directly by incoming cold, low-pressure suction vapor (suction-gas cooled). If the motor burns out or mechanical failure occurs, the entire welded assembly must be replaced.
• Semi-Hermetic (Accessible) Compressors: The motor and compressor are enclosed in a bolted cast-iron body with removable cylinder heads, valve plates, bottom oil pan, and end covers. Widely used in commercial refrigeration because technicians can rebuild valves, replace pistons, and overhaul oil pumps on site.
• Open-Drive Compressors: The compressor crankshaft extends through the crankcase wall and is driven by an external power source (electric motor, diesel engine, or PTO shaft). A mechanical shaft seal lubricated with oil prevents refrigerant and pressure from escaping around the spinning shaft. Essential for anhydrous ammonia (R-717) systems, which attack the copper windings of hermetic motors.

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3. Refrigeration Lubricants and Chemical Compatibility

Refrigeration oil must lubricate moving mechanical surfaces, seal clearance spaces, cool internal components, and circulate freely through system piping without getting trapped in heat exchangers.

The Chemistry of POE Moisture Absorption

Polyolester oil is synthesized via an equilibrium reaction between an organic alcohol and a carboxylic acid: $\text{Organic Alcohol} + \text{Carboxylic Acid} \rightleftharpoons \text{Polyolester (POE Oil)} + \text{Water}$

When POE oil is exposed to ambient atmospheric moisture for as little as 10 to 15 minutes, it absorbs water molecules. In the presence of heat inside an operating system, the reaction reverses via hydrolysis, breaking the POE oil back down into alcohol and corrosive organic carboxylic acids. These acids dissolve copper tubing, attack motor winding varnish, and cause copper plating on hot steel compressor bearings. The industry standard maximum allowable moisture level in operating POE systems is 50 to 100 ppm.

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4. Compressor Failure Modes and Systematic Diagnostics

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|                                 COMPRESSOR FAILURE MECHANISMS                                      |
+---------------------+---------------------+-----------------------+--------------------------------+
|    FLOODED STARTS   |   LIQUID SLUGGING   |  LOSS OF LUBRICATION  |     THERMAL OVERHEATING        |
| - Off-cycle migrate | - Solid liquid slug | - Low vapor velocity  | - Discharge temp > 225°F       |
| - Oil foaming       | - Broken reeds/rods | - Trapped in coils    | - Oil carbonization & sludge   |
| - Bearing wipeout   | - Hydraulic shock   | - Short-cycling wear  | - Acid formation (HF/organic)  |
+---------------------+---------------------+-----------------------+--------------------------------+

1. Off-Cycle Migration and Flooded Starts

• Physics of Migration: During the system OFF-cycle, refrigerant vapor naturally migrates to the coldest location in the system. Because refrigeration oil has a powerful chemical affinity for refrigerant vapor, vapor condenses into liquid inside the compressor crankcase, settling as a dense liquid layer beneath the oil.
• The Flooded Start Event: When the compressor starts, crankcase pressure drops instantaneously. The liquid refrigerant boils explosively, creating severe oil foaming. The oil pump picks up this refrigerant foam instead of liquid oil, starving crankshaft journals and causing catastrophic bearing seizure within seconds.
• Prevention: Install a crankcase heater (CCH) that maintains oil temperature 20°F to 30°F higher than the coldest system component, or implement an automatic pump-down cycle.

2. Liquid Slugging

• Mechanism: The entry of solid slugs of non-compressible liquid refrigerant or liquid oil directly into the compression chamber while running.
• Consequences: Because liquids cannot be compressed, cylinder pressure spikes to thousands of PSI within a fraction of a stroke. This hydraulic shock instantly fractures suction and discharge reed valves, bends or snaps connecting rods, breaks wrist pins, and fractures scroll tip seals.

3. Loss of Lubrication & Oil Return Velocity

• Refrigerant vapor carries oil droplets through the entire circuit. To ensure oil returns up vertical suction risers, vapor velocity must be maintained at 1,000 to 1,500 FPM in horizontal lines and 1,500 to 2,500 FPM in vertical risers.
• Low load, low refrigerant charge, oversized piping, or short-cycling traps oil in the evaporator, starving the compressor crankcase.

4. Overheating and Oil Carbonization

• Discharge Line Temperature Rule: The temperature of the discharge line, measured 6 inches from the compressor service valve, must never exceed 225°F (107°C).
• Internal discharge valve port temperatures run 50°F to 75°F hotter than the external line. At internal temperatures exceeding 275°F to 300°F, mineral and POE oils thermally decompose (carbonize), forming abrasive carbon sludge and corrosive hydrofluoric (HF) acid that destroy valve seats and bearings.

5. Motor Burnout Remediation (Cleanup Procedure)

An electrical motor burnout occurs when winding insulation degrades, generating an intense electric arc that decomposes refrigerant and oil into toxic, highly acidic sludge.

1. Diagnostic Verification: Perform an oil acid test using a chemical test vial. Classify burnout as mild (discoloration/slight odor) or severe (black sludge/strong acrid odor).
2. Recovery: Recover contaminated refrigerant using dedicated recovery equipment with inline filter-driers.
3. Component Replacement: Replace the failed compressor, contactor, and liquid line filter-drier.
4. Burnout Filter-Drier Installation: Install an oversized liquid line filter-drier with high-acid activated alumina core and a temporary suction line burnout filter-drier equipped with dual gauge ports.
5. Evacuation & Operation: Evacuate system to < 500 microns. Recharge with virgin refrigerant. Run system for 4 hours and measure pressure drop across the suction drier (replace if ΔP > 3 psi). Re-test oil with an acid test kit after 24 to 48 hours; once acid-free, recover refrigerant, remove the suction burnout drier, and install a standard permanent line section.