4.2 Major System Components

Major System Components

The vapour compression refrigeration cycle relies on four major components working in harmony: the compressor, condenser, expansion device, and evaporator. A profound understanding of these components, their various types, and their specific operational characteristics is fundamental for any Category I technician diagnosing, servicing, or installing RACHP equipment.

Compressors

The compressor is the heart of the system. Its primary function is to draw in low-pressure, low-temperature superheated refrigerant vapour from the evaporator and compress it into a high-pressure, high-temperature vapour. This compression provides the necessary pressure differential to drive the refrigerant through the circuit and elevates the temperature of the vapour above the ambient cooling medium, allowing heat rejection in the condenser.

There are three primary types of compressors encountered in stationary commercial and industrial equipment:

Reciprocating Compressors

Reciprocating compressors use a piston and cylinder mechanism, much like a car engine. As the piston moves down (suction stroke), the suction valve opens, drawing in low-pressure vapour. As the piston moves up (compression stroke), the suction valve closes, and the discharge valve opens when the cylinder pressure exceeds the discharge line pressure.

• Clearance Volume: A critical concept in reciprocating compressors is the clearance volume—the small space remaining at the top of the cylinder when the piston reaches top dead center (TDC). High-pressure vapour trapped here must expand as the piston drops before new suction vapour can enter, which reduces volumetric efficiency at high compression ratios.
• Applications: Widely used in commercial refrigeration, cold rooms, and smaller chillers. They can be hermetic (welded steel casing), semi-hermetic (bolted cast iron casing allowing internal repairs), or open-type (driven by an external motor via belts or a coupling).

Scroll Compressors

Scroll compressors consist of two spiral-shaped scrolls: one fixed and one orbiting. The orbiting motion creates crescent-shaped pockets of vapour that are progressively squeezed toward the center discharge port.

• Characteristics: Scroll compressors have fewer moving parts, no suction or discharge valves, and operate continuously rather than in pulses. This results in smoother, quieter operation and higher efficiency compared to reciprocating types. They also exhibit "radial compliance," meaning the scrolls can separate slightly to pass small amounts of liquid refrigerant or debris without catastrophic damage.
• Applications: Dominant in air conditioning, heat pumps, and medium-temperature commercial refrigeration.

Screw Compressors

Screw compressors use two meshing helical rotors (male and female) housed in a closely fitting casing. As the rotors turn, vapour is drawn into the spaces between the lobes and compressed axially as the volume decreases toward the discharge end.

• Characteristics: These provide large swept volumes and continuous, smooth compression. They rely heavily on oil injection for sealing the clearances between the rotors, cooling the compression process, and lubrication.
• Applications: Used in large industrial refrigeration and high-capacity water chillers.

Condensers

The condenser's role is to reject the heat absorbed in the evaporator and the heat of compression to a secondary cooling medium (air or water). Inside the condenser, the high-pressure, high-temperature vapour is de-superheated, condensed into a liquid, and finally subcooled. Subcooling (lowering the liquid temperature below its saturation point) is vital as it prevents flash gas in the liquid line and increases the system's cooling capacity.

Air-Cooled Condensers

These are the most common type, consisting of copper or aluminum tubes surrounded by aluminum fins to increase the heat transfer surface area. Fans force ambient air across the coil. Their efficiency is heavily dependent on the ambient air temperature; higher ambient temperatures lead to higher condensing pressures and reduced system capacity.

Water-Cooled Condensers

Water-cooled condensers are typically shell-and-tube or brazed plate heat exchangers. Water flows through one circuit while refrigerant condenses in the other.

• Characteristics: Because water has a higher specific heat capacity and is generally cooler than summer ambient air, water-cooled systems operate at lower condensing pressures, yielding higher efficiency. However, they require cooling towers or dry coolers, water pumps, and rigorous water treatment regimes to prevent scaling, corrosion, and biological growth (e.g., Legionella).

Evaporators

The evaporator absorbs heat from the conditioned space or fluid. Low-pressure liquid refrigerant enters the evaporator and boils into a vapour as it absorbs heat.

Direct Expansion (DX) Evaporators

In a DX evaporator, refrigerant flows through tubes and progressively boils off. By the time the refrigerant reaches the exit, it must be completely vaporized and slightly heated above its saturation temperature (superheated). Superheat guarantees that only 100% vapour enters the suction line, protecting the compressor from liquid slugging. DX evaporators are ubiquitous in air conditioning and commercial refrigeration.

Flooded Evaporators

Flooded evaporators are fundamentally different. The tubes are submerged in a pool of liquid refrigerant. As heat is transferred, the liquid boils, and the resulting vapour rises to the top of the shell and is drawn off by the compressor. A float valve maintains a constant liquid level. These are highly efficient because the entire heat transfer surface is wetted, but they require a significantly larger refrigerant charge. They are typically used in large industrial chillers.

Expansion Devices

The expansion device creates the pressure drop in the system, regulating the flow of high-pressure liquid from the liquid line into the low-pressure evaporator.

Capillary Tube

A simple length of copper tubing with a very small internal diameter. The pressure drop is determined by the tube's length and bore.

• Characteristics: It has no moving parts and cannot adjust to varying load conditions. It equalizes system pressures during the off-cycle, allowing the use of low-starting-torque compressors. Systems with capillary tubes are "critically charged"—the exact mass of refrigerant is vital for correct operation.

Thermostatic Expansion Valve (TEV)

A TEV modulates the flow of liquid refrigerant to maintain a constant superheat at the evaporator outlet.

• Mechanism: A sensing bulb strapped to the suction line contains a thermostatic charge. As the suction line temperature rises (load increases), the pressure in the bulb increases, pushing down on a diaphragm to open the valve wider, admitting more refrigerant. Conversely, if superheat falls, the valve throttles back.
• External Equalization: For large evaporators with significant pressure drops, an external equalizer line connects the underside of the diaphragm to the evaporator outlet, ensuring accurate superheat control regardless of the pressure loss across the coil.

Electronic Expansion Valve (EEV)

EEVs use a precision stepper motor to drive a needle valve, controlled by a microprocessor. The controller calculates superheat using a pressure transducer and a thermistor located at the evaporator outlet. EEVs offer unparalleled accuracy, can operate over a massive range of capacities, and allow the system to function at much lower condensing pressures during cold weather, saving significant energy.