4.2 Compressor Technologies, Compression Ratios & Refrigerant Metering Devices

Compressor Technologies, Compression Ratios & Refrigerant Metering Devices

The compressor and metering device represent the two active mechanical pressure boundaries in a vapor-compression circuit. The compressor functions as the vapor pump that raises refrigerant pressure and temperature, while the expansion device meters precise mass flow and reduces pressure to enable heat absorption. For a Kentucky HVAC contractor, understanding the internal mechanics, force balances, and operating limits of these components is vital for reliable system design and failure analysis.

---

1. Positive Displacement Compressor Technologies

All standard residential and light commercial HVAC systems utilize positive displacement compressors, which compress vapor by reducing the internal volume of the compression chamber.

+---------------------------------------------------------------------------------------------------+
|                         COMPARISON OF HVAC COMPRESSOR TECHNOLOGIES                                |
|                                                                                                   |
|   RECIPROCATING           SCROLL                  ROTARY                  SCREW (HELICAL)         |
|   - Piston & cylinder     - Fixed & orbiting      - Rolling piston /      - Intermeshing male &   |
|   - Suction/discharge       intermeshing scrolls    eccentric shaft         female helical rotors |
|     reed valves           - Continuous compression- Stationary blade      - Slide valve capacity  |
|   - Clearance volume loss - No clearance volume   - Compact, low noise      modulation (10-100%)  |
|   - Prone to slugging     - Axial/radial complianc- Mini-splits & window  - Large chillers        |
|                             tolerates light liquid  units (under 3 tons)    (50 to 1,000+ tons)   |
+---------------------------------------------------------------------------------------------------+

Detailed Mechanical Evaluation

1. Reciprocating Compressors:

   • Operation: Uses a motor-driven crankshaft, connecting rods, and pistons reciprocating inside cast-iron cylinders with suction and discharge reed valves.
   • Clearance Volume & Re-expansion: At the top of the stroke, a small space (clearance volume) remains between the piston crown and valve plate. Trapped high-pressure discharge gas must re-expand to below suction pressure before the suction reed can open, reducing volumetric efficiency at high compression ratios.
   • Vulnerabilities: Reed valves are fragile; liquid refrigerant entering the cylinder cannot compress, causing catastrophic valve fracture or connecting rod bending (liquid slugging).

2. Scroll Compressors:

   • Operation: Features two identical involute spiral scrolls—one fixed (stationary) and one orbiting driven by an eccentric crankshaft. Vapor enters outer crescent-shaped pockets that continuously shrink in volume as they move toward the central discharge port.
   • Key Advantages: Continuous compression eliminates suction/discharge valves and clearance volume, yielding nearly 100% volumetric efficiency. Scroll compressors have 60% fewer moving parts, generate minimal vibration, and feature axial and radial compliance, allowing the scrolls to separate momentarily to pass small liquid droplets without damage.

3. Rotary Compressors:

   • Operation: Utilizes a cylindrical roller rotating eccentrically inside a machined cylinder housing, sealed by a spring-loaded sliding vane.
   • Applications: High efficiency and compact footprint make rotary compressors standard in ductless mini-splits, window units, and packaged terminal air conditioners (PTACs) under 3 tons.

4. Screw Compressors (Helical Rotor):

   • Operation: Composed of two precision-machined intermeshing helical rotors (one male lobe, one female flute) inside a stationary casing. Gas is compressed axially along the flutes from suction to discharge.
   • Capacity Control: Utilizes a hydraulically actuated slide valve that bypasses uncompressed gas back to suction, providing infinitely variable capacity modulation from 10% to 100% in large commercial chillers (50 to 1,000+ tons).

---

2. Volumetric Efficiency & Compression Ratio Calculations

Volumetric Efficiency (η_v)

Volumetric Efficiency is the ratio of the actual volume of fresh suction vapor drawn into the compressor cylinder to the theoretical displacement volume of the cylinder:

Volumetric Efficiency (η_v) = (Actual Volume of Suction Vapor Pumped / Theoretical Displacement Volume) × 100%

As the compression ratio increases, volumetric efficiency drops precipitously in reciprocating compressors due to the re-expansion of clearance volume gas.

Compression Ratio (CR) Formula

Compression Ratio (CR) = Absolute Discharge Pressure (psia) / Absolute Suction Pressure (psia)

Where: Absolute Pressure (psia) = Gauge Pressure (psig) + Atmospheric Pressure (14.696 ≈ 14.7 psi)

Critical Exam Rule: NEVER compute compression ratio using gauge pressures (psig). You MUST convert both head pressure and suction pressure to absolute pressure (psia) by adding 14.7 psi.

Worked Example: Compression Ratio Calculation on an R-410A System

Field Gauge Readings:
- High-Side Discharge Pressure = 383.0 psig
- Low-Side Suction Pressure = 118.0 psig

Step 1: Convert Gauge Pressures to Absolute Pressures (psia)
- P_discharge (psia) = 383.0 psig + 14.7 psi = 397.7 psia
- P_suction (psia) = 118.0 psig + 14.7 psi = 132.7 psia

Step 2: Calculate Compression Ratio
- CR = 397.7 psia / 132.7 psia = 2.997:1 ≈ 3.00:1

Consequences of Elevated Compression Ratios (>3.5:1 in AC / >8:1 in Refrigeration)

1. Extreme Discharge Gas Temperatures: Compression generates excessive heat of compression. When internal discharge vapor exceeds 225°F (107°C), synthetic polyolester (POE) and mineral lubricating oils break down chemically (pyrolysis), producing carbon sludge, acid, and mechanical seizure.
2. Reduced Refrigerant Mass Flow & Capacity: Lower volumetric efficiency diminishes total BTUs delivered.
3. Motor Overheating: Hermetic and semi-hermetic compressor motors rely on cold suction vapor for winding cooling; low suction density causes motor thermal overloads to trip.

---

3. Thermostatic Expansion Valve (TXV) Operation & 3-Force Balance

A Thermostatic Expansion Valve (TXV / TEV) is a precision modulating metering device designed to regulate refrigerant flow to keep the evaporator coil fully active while maintaining a constant superheat at the coil outlet.

+---------------------------------------------------------------------------------------------------+
|                         TXV THREE-FORCE DIAPHRAGM EQUILIBRIUM                                     |
|                                                                                                   |
|                           [ SENSING BULB (Suction Line) ]                                         |
|                                          |                                                        |
|                                          | (Hydraulic / Vapor Pressure)                           |
|                                          v                                                        |
|                     ===========================================  <-- Flexible Metal Diaphragm     |
|                    |         P1: BULB OPENING FORCE            |                                  |
|                     ===========================================                                   |
|                                  ^                 ^                                              |
|                                  |                 |                                              |
|                       P2: EVAPORATOR CLOSING   P3: SPRING CLOSING                                 |
|                             FORCE                    FORCE                                        |
|                                  |                 |                                              |
|                     [ Push Pins Connected to Needle & Seat Orifice ]                              |
+---------------------------------------------------------------------------------------------------+

The Three Operating Forces

1. P1 = Sensing Bulb Pressure (Opening Force): Acts on top of the diaphragm, pushing the push pins down to open the valve orifice. As suction line temperature rises (higher superheat), bulb charge vaporizes, increasing P1.
2. P2 = Evaporator Pressure (Closing Force): Acts beneath the diaphragm, pushing upward to close the valve orifice.
3. P3 = Superheat Spring Force (Closing Force): Mechanical spring located beneath the diaphragm, providing a constant closing force established by factory calibration or an external adjustment screw.

Fundamental Force Balance Equation:
Opening Force = Closing Forces
P1 = P2 + P3

- When Superheat Increases: P1 > (P2 + P3) --> Diaphragm flexes DOWN --> Valve OPENS --> Mass flow increases.
- When Superheat Decreases: P1 < (P2 + P3) --> Diaphragm flexes UP --> Valve CLOSES --> Mass flow decreases.

---

4. Internal vs. External Equalization Rules

+---------------------------------------------------------------------------------------------------+
|                        INTERNAL VS. EXTERNAL EQUALIZER CONFIGURATION                              |
|                                                                                                   |
|   INTERNALLY EQUALIZED:                                                                           |
|   - Senses P2 directly at the valve outlet.                                                       |
|   - Valid ONLY if coil pressure drop is < 2.0 psi.                                                |
|                                                                                                   |
|   EXTERNALLY EQUALIZED:                                                                           |
|   - Senses P2 via a 1/4" line tapped into suction line downstream of coil and sensing bulb.       |
|   - MANDATORY for coils with distributors or coil pressure drops > 2.0 to 3.0 psi.                |
|   - Prevents artificial coil starvation caused by distributor nozzle pressure drop.               |
+---------------------------------------------------------------------------------------------------+

Why External Equalization is Mandatory with Multi-Circuit Distributors

Refrigerant distributors create an intentional pressure drop of 10 to 25 psi to divide liquid/flash mixture evenly across parallel coil circuits. If an internally equalized valve were used:

• The valve would sense the high pressure upstream of the distributor (P2_inlet = 135 psig) rather than the true evaporator outlet pressure (P2_outlet = 118 psig).
• This false high closing force (P2) would overpower P1, forcing the valve needle toward the closed seat, severely starving the coil, producing high superheat, and slashing cooling capacity.

---

5. TXV Sensing Bulb Placement, Insulation & Hunting

+---------------------------------------------------------------------------------------------------+
|                         PROPER SUCTION LINE BULB CLOCK ORIENTATION                                |
|                                                                                                   |
|           Lines < 7/8" OD:                        Lines 7/8" OD and Larger:                       |
|               12:00                                         12:00                                 |
|             . - ~ - .                                     . - ~ - .                               |
|         10 /         \ 2:00                           /         \                                 |
|        (  *   PIPE    *  )                        ( * 4:00   PIPE * 8:00 )                        |
|           \         /                                 \         /                                 |
|             ` - ~ - '                                     ` - ~ - '                               |
|               6:00 (NEVER AT 6:00)                          6:00 (NEVER AT 6:00)                  |
|         (Oil & liquid pool at bottom)                 (Oil traps insulate bulb)                   |
+---------------------------------------------------------------------------------------------------+

Critical Sensing Bulb Installation Rules

1. Horizontal Suction Line Position: Always mount the bulb on a horizontal run of suction line immediately adjacent to the coil outlet, upstream of the external equalizer connection and upstream of any suction line P-traps.
2. Rotational Clock Position:
   • Suction lines under 7/8" OD: Mount at 12 o'clock, 10 o'clock, or 2 o'clock.
   • Suction lines 7/8" OD and larger: Mount at 4 o'clock or 8 o'clock (45 degrees below centerline).
   • NEVER mount at 6 o'clock: Lubricating oil and condensed liquid settle at the bottom of the pipe, acting as a thermal insulator that prevents accurate gas temperature sensing.
3. Thermal Contact & Insulation: Clean the copper with emery cloth to bright metal, secure the bulb tightly using copper or stainless steel straps (never plastic zip ties), and completely insulate the bulb with waterproof closed-cell foam to isolate it from ambient air.
4. TXV Hunting: An unstable cycle where the valve alternately overfeeds (flooding) and underfeeds (starving) the coil. Caused by oversized valve porting, loose/uninsulated sensing bulb, or bulb mounted downstream of a liquid-trapping suction trap.