8.3 Air Conditioning Units, Compressors & Metering Devices

8.3 Air Conditioning Units, Compressors & Metering Devices

[!NOTE] Refrigeration Cycle Fundamentals: Mechanical vapor compression air conditioning operates on four thermodynamic stages: compression of low-pressure superheated vapor into high-pressure superheated vapor, condensation into high-pressure subcooled liquid, expansion/throttling across a metering device into a low-pressure two-phase liquid-vapor mixture, and evaporation into low-pressure superheated vapor. The compressor is the vapor pump driving mass flow, while the metering device maintains the critical pressure boundary separating the high and low sides of the system.

Air conditioning equipment selection, compressor electrical diagnostics, and expansion device calibration represent the technical core of the Alabama HACR Contractor licensing syllabus. Mastery of positive displacement compressor types, single-phase motor starting circuits (including potential relays and run capacitors), expansion valve mechanics, and modern SEER2 efficiency metrics is mandatory for both examination success and professional field engineering.

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Positive Displacement Compressor Architectures

Residential and light commercial air conditioning systems utilize positive displacement compressors, which compress refrigerant by trapping a discrete volume of vapor within an enclosed chamber and progressively reducing its physical volume.

+---------------------------------------------------------------------------------------------------+
|                         COMPARISON OF POSITIVE DISPLACEMENT COMPRESSORS                           |
+---------------------+-------------------------+-------------------------+-------------------------+
| CHARACTERISTIC      | RECIPROCATING           | SCROLL                  | ROTARY (ROLLING PISTON) |
+---------------------+-------------------------+-------------------------+-------------------------+
| Compression Action  | Piston / Connecting Rod | Orbiting Involute Spiral| Rolling Cylinder / Vane |
| Valves              | Suction & Discharge Reed| None (Port Discharge)   | Discharge Reed Only     |
| Clearance Volume    | Present (Trapped Gas)   | Zero (Continuous Sweep) | Negligible              |
| Liquid Slugging     | Extreme Susceptibility  | Superior Compliance     | Moderate Susceptibility |
| Volumetric Effic.   | Moderate (Drops at high)| Very High (95% - 100%)  | High                    |
| Primary Application | Legacy Residential/Comm | Modern Res./Light Comm. | Mini-Splits / Window AC |
+---------------------+-------------------------+-------------------------+-------------------------+

1. Reciprocating Compressors

A reciprocating compressor employs an internal crankshaft, connecting rods, and pistons sliding inside cast iron cylinders, using spring-steel suction and discharge reed valves.

• Clearance Volume & Re-Expansion: At the apex of the piston stroke (Top Dead Center, TDC), a small space remains between the piston crown and valve plate called the clearance volume. High-pressure discharge vapor trapped within this space cannot discharge; on the downward intake stroke, this trapped gas must re-expand before the cylinder pressure drops below suction pressure, delaying the opening of the suction reed valve. As compression ratio increases (such as on hot Alabama summer afternoons), volumetric efficiency declines drastically.
• Vulnerability: Liquid refrigerant or oil droplets entering the cylinder cannot be compressed. Because liquid is incompressible, liquid slugging bends or shatters reed valves, breaks connecting rods, or blows cylinder head gaskets.

2. Scroll Compressors

Scroll compressors dominate modern residential split systems and heat pumps. The assembly features two identical interleaved involute spiral scrolls:

• Orbital Mechanics: The top scroll is stationary (fixed) with a center discharge port. The lower scroll is driven by an eccentric motor shaft and orbits (it does not rotate). As it orbits, crescent-shaped vapor pockets are trapped at the outer perimeter and progressively squeezed into smaller volumes toward the center discharge port in a smooth, continuous compression cycle.
• Compliance Against Slugging (Axial & Radial Compliance): Scroll compressors feature mechanical compliance. Under high pressure or during passage of liquid refrigerant or foreign debris, the scroll plates can separate slightly along their axial plane (tips lift vertically) and radial plane (flanks pull apart horizontally). This compliance allows liquid droplets to pass through into the discharge chamber without shattering the compression elements, giving scrolls exceptional durability.
• Inverter Variable-Speed Scrolls: Modern high-efficiency systems utilize brushless DC inverter-driven scroll compressors capable of modulating capacity from 20% to 120% of nominal rating in 1-Hz increments, matching cooling capacity precisely to dynamic building load.

3. Rotary Compressors

A rotary compressor features a cylindrical roller (rolling piston) driven by an eccentric shaft inside an outer stationary cylinder. A spring-loaded sliding vane separates the suction chamber from the discharge chamber. Rotary compressors have minimal clearance volume and operate with low noise and vibration, making them the standard choice for ductless mini-split and package terminal air conditioning (PTAC) systems.

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Single-Phase Motor Starting Components & Circuits

Residential single-phase alternating current (240 VAC, 60 Hz) creates a pulsating magnetic field rather than a rotating magnetic field. To create the rotational phase displacement necessary to start a single-phase compressor motor, manufacturers utilize split-phase capacitor circuits.

                               POTENTIAL STARTING RELAY CIRCUIT

            [Line 1] ────────────────────────────────────────┬───────── (Run Winding - R)
                                                             │
                                                     [Run Capacitor]
                                                             │
            [Line 2] ───────┬────────────────────────────────┼───────── (Common - C)
                            │                                │
                            │                  ┌─────────────┴────────┐
                            │                  │   [Start Capacitor]  │
                            │                  └─────────────┬────────┘
                            │                                │
                            │        [Terminal 1] ───/ ─── [Terminal 2] (Normally Closed Contacts)
                            │                                │
                            │                                ├───────── (Start Winding - S)
                            │                                │
                            └─────── [Terminal 5] ───────────┘
                                   (Relay Coil across 2 & 5)

Capacitors: Start vs. Run

1. Run Capacitors: Heavy-duty, oil-filled metallized polypropylene capacitors in round or oval aluminum cans rated for continuous duty (370V or 440V AC). Typical capacitance ranges from 30 to 80 microfarads ($\mu\text{F}$). Wired permanently between the Run (R) and Start (S) windings, the run capacitor shifts the phase angle of the start winding current by approximately 90 electrical degrees, creating continuous rotating magnetic flux that optimizes running torque, lowers amperage draw, and corrects motor power factor.
2. Start Capacitors: Dry electrolytic capacitors housed in black plastic/Bakelite shells rated for intermittent duty (250V to 330V AC). Typical capacitance is very high—100 to 300 microfarads ($\mu\text{F}$). A start capacitor is wired in series with the start winding only during initial rotor acceleration (1 to 3 seconds), providing the massive starting torque required to break away high-friction compressor loads.

• Bleed Resistor Requirement: A start capacitor must always feature a 15,000 to 20,000 ohm, 2-watt bleed resistor soldered across its terminals. The resistor bleeds off trapped DC residual voltage following start-up, preventing contact arcing and extending capacitor life.

Potential Starting Relays (PSR)

A Potential Starting Relay is the precision electro-mechanical switch used in Capacitor-Start Capacitor-Run (CSCR) circuits and professional "hard start kits" to drop out the start capacitor.

• Operating Principle (Back-EMF): The relay coil is constructed of thousands of turns of hair-thin copper wire, giving it exceptionally high electrical resistance (typically 3,000 to 10,000 ohms). The coil is connected across Terminal 2 (Start Winding) and Terminal 5 (Common). The relay contacts between Terminal 1 and Terminal 2 are NORMALLY CLOSED and wired in series with the start capacitor.
• Back-Electromotive Force Generation: As the compressor motor accelerates from a standstill toward its synchronous speed (3,600 RPM on 60 Hz 2-pole motors), the spinning magnetic fields of the rotor induce a counter-voltage inside the start winding known as Back-Electromotive Force (Back-EMF). Back-EMF acts like an electrical generator; at 75% to 80% speed, the voltage across Start and Common exceeds line voltage, climbing to 300 to 450 VAC on a standard 240V system!
• Pick-Up and Drop-Out Voltages:
  • Pick-Up Voltage: The precise back-EMF voltage required to energize the relay coil and magnetically pull contacts 1 and 2 OPEN, taking the start capacitor out of the circuit. This occurs when the rotor achieves 75% to 80% of rated speed.
  • Drop-Out Voltage: The voltage threshold below which the coil magnetic field collapses, allowing contacts 1 and 2 to spring-reset closed. When the compressor shuts down, back-EMF drops to zero, dropping out the relay coil and resetting contacts 1 and 2 closed ready for the next restart cycle.

PTC Thermistors (Solid-State Start Assist)

A Positive Temperature Coefficient (PTC) thermistor is a solid-state ceramic disc wired in parallel with the run capacitor. When cold, its resistance is low (20 to 50 ohms), allowing starting current to bypass into the start winding. Within 0.2 to 0.5 seconds, passing current heats the ceramic disc, causing its internal resistance to spike into tens of thousands of ohms, choking off starting current.

• Limitation: A PTC thermistor must cool down completely (3 to 5 minutes off-cycle) to return to low resistance. If a compressor short-cycles, the hot PTC thermistor cannot assist, causing the compressor to lock on its internal thermal overload.

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Refrigerant Expansion & Metering Devices

The metering device regulates the mass flow rate of liquid refrigerant entering the evaporator coil and creates the pressure drop that allows liquid to flash into a low-temperature boiling mixture.

+---------------------------------------------------------------------------------------------------+
|                         EXPANSION & METERING DEVICES COMPARED                                     |
+---------------------+-------------------------+-------------------------+-------------------------+
| CHARACTERISTIC      | FIXED ORIFICE / PISTON  | THERMOSTATIC EXP. (TXV) | ELECTRONIC EXP. (EEV)   |
+---------------------+-------------------------+-------------------------+-------------------------+
| Operation           | Fixed Restriction Hole  | Modulating Mechanical   | Microprocessor Stepper  |
| Superheat Control   | Fluctuates with Load    | Maintains Constant SH   | Ultra-Precise Dynamic SH|
| Charge Sensitivity  | Extremely Critical      | Moderate (Tolerant)     | High System Efficiency  |
| Seasonal Efficiency | Baseline (13-14 SEER)   | High (15-18 SEER)       | Maximum (20+ SEER2)     |
| Equalizer Needed    | No                      | External on High Drop   | Electronic Transducer   |
+---------------------+-------------------------+-------------------------+-------------------------+

1. Fixed Orifice (Piston / Capillary Tube)

A fixed restriction featuring a calibrated brass orifice piston or narrow-bore copper capillary tube.

• Operating Physics: Refrigerant flow rate is strictly governed by the physical diameter of the orifice hole and the pressure differential between the high-pressure liquid line and low-pressure evaporator.
• Limitations: Non-modulating. As outdoor ambient temperature rises or indoor heat load fluctuates, a fixed orifice cannot adjust flow. Systems with fixed metering are highly sensitive to refrigerant charge; charging must be verified strictly using the Superheat Method with manufacturer charging charts.

2. Thermostatic Expansion Valve (TXV)

A TXV modulates the mass flow of refrigerant entering the evaporator in exact proportion to thermal load, maintaining a constant superheat at the evaporator outlet to protect the compressor from liquid flooding while maximizing coil surface utilization.

                         THERMOSTATIC EXPANSION VALVE FORCES

                              [Sensing Bulb (P1: Opening)]
                                           │
                                           ▼
                             ┌───────────────────────────┐
                             │      Flexible Diaphragm   │
                             └─────────────┬─────────────┘
                                           │
                       ▲                                   ▲
                       │                                   │
               [Evaporator Inlet                   [Adjustable Spring
               Pressure (P2: Closing)]             Pressure (P3: Closing)]

• Three Opposing Mechanical Forces:
  1. $P_1$ (Bulb Pressure): Downward opening force generated by the thermal expansion of volatile liquid inside the sensing bulb clamped to the suction line. As suction line temperature rises (superheat increases), bulb pressure increases, forcing the valve open.
  2. $P_2$ (Evaporator Pressure): Upward closing force acting against the underside of the diaphragm, exerted by refrigerant evaporating inside the coil.
  3. $P_3$ (Spring Pressure): Upward closing force exerted by an internal spring (often factory set or field adjustable). The spring value dictates the superheat setpoint: $P_1 = P_2 + P_3 \quad \Longleftrightarrow \quad \text{Equilibrium Condition}$
• External Equalizer Tube:
  • When an evaporator coil exhibits a high internal pressure drop (greater than 2 to 3 PSI across distributor tubes, feeder circuits, and headers), internal valve pressure ($P_2$) at the valve outlet is significantly higher than the pressure at the coil outlet.
  • An external equalizer tube routes suction pressure from the outlet of the evaporator coil to the underside of the diaphragm. This cancels out coil pressure drop, preventing the TXV from starving the coil and generating artificially elevated superheat.
• Sensing Bulb Mounting Rules:
  • Bulb must be clamped firmly to a clean, bare horizontal copper suction line immediately downstream of the evaporator outlet and wrapped tightly with waterproof insulation.
  • On suction lines smaller than 7/8" OD, mount the bulb on top of the line.
  • On suction lines 7/8" OD or larger, mount the bulb at the 10 o'clock or 2 o'clock position (45° above center). NEVER mount the bulb at the bottom (6 o'clock position), where compressor oil puddling acts as an insulator and causes sluggish, erratic flooding.

3. Electronic Expansion Valves (EEV)

EEVs replace mechanical diaphragms and bulbs with direct digital control. A bipolar stepper motor moves a precision needle pin in increments of 480 to 2,000 discrete steps.

• Microprocessor Control: An integrated controller reads suction pressure from a solid-state pressure transducer and temperature from an electronic thermistor multiple times per second, calculating real-time superheat using embedded psychrometric refrigerant algorithms. The PID control loop drives the stepper motor to maintain a precise superheat setpoint (typically 8°F to 12°F) across a 10% to 100% capacity range.

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Energy Efficiency Standards: SEER2 & EER2

Effective January 1, 2023, the United States Department of Energy (DOE) enacted updated testing procedures codified under Appendix M1 to Subpart B of 10 CFR Part 430, replacing legacy SEER, EER, and HSPF metrics with SEER2, EER2, and HSPF2.

+---------------------------------------------------------------------------------------------------+
|                         LEGACY (M) VS. MODERN (M1) EFFICIENCY STANDARDS                           |
+---------------------+-----------------------------------+-----------------------------------------+
| METRIC              | LEGACY TEST PROCEDURE (M)         | NEW DOE M1 PROCEDURE (M1)               |
+---------------------+-----------------------------------+-----------------------------------------+
| External Static P   | 0.10 to 0.20 in. w.g. (Unreal)    | 0.50 in. w.g. (Realistic Ductwork)      |
| Blower Power Impact | Minimal blower motor wattage draw | Significantly higher blower power draw  |
| Numerical Rating    | Legacy SEER / EER                 | SEER2 / EER2 (approx. 4.5% - 5% lower)  |
| SEER to SEER2 Equiv | 14.0 SEER Legacy                  | Equal to approx. 13.4 SEER2 Modern      |
| Minimum SEER2 (AL)  | 14.0 SEER (Pre-2023 South Zone)   | 14.3 SEER2 (or 15.2 SEER2 < 45k BTU split)|
+---------------------+-----------------------------------+-----------------------------------------+

The Engineering Significance of 0.50 in. w.g. Static Pressure

Under legacy SEER testing, outdoor condenser and indoor air handler packages were tested against an external static pressure (ESP) of only 0.10 to 0.20 inches water column (in. w.g.). Real-world ductwork systems in Alabama homes—equipped with return air filters, evaporators, transitions, and diffusers—typically operate at total external static pressures of 0.50 to 0.80 in. w.g.

• By elevating the mandatory testing static pressure to 0.50 in. w.g., the DOE M1 standard forces indoor blower motors to work against realistic resistance, increasing fan electrical wattage and dropping the calculated efficiency ratio.
• Consequently, a system rated at 14.0 SEER under the old test produces a rating of approximately 13.4 SEER2 under M1 testing.

SEER2 vs. EER2 Definitions

1. Seasonal Energy Efficiency Ratio 2 (SEER2): Measures the total cooling output in BTUs delivered across a standardized seasonal cooling profile divided by the total electrical energy consumed in Watt-hours (Wh): $\text{SEER2} = \frac{\text{Total Seasonal Cooling (BTU)}}{\text{Total Seasonal Electrical Energy (Wh)}}$
2. Energy Efficiency Ratio 2 (EER2): Measures instantaneous steady-state cooling efficiency at peak design conditions: 95°F outdoor dry-bulb, and indoor conditions of 80°F dry-bulb / 67°F wet-bulb: $\text{EER2} = \frac{\text{Net Cooling Capacity (BTU/hr)}}{\text{Total Electrical Power Input (Watts)}}$

• Contractor Relevance: In Alabama's hot climate, EER2 is a critical engineering specification. While SEER2 reflects weighted seasonal performance during moderate spring and autumn days, EER2 reflects how efficiently equipment operates during the 95°F+ peak afternoon heat waves that push electric utility distribution grids to peak capacity.