6.1 Compressor Construction and Unloaders

Introduction to Positive Displacement Compressors

Industrial ammonia refrigeration systems rely primarily on positive displacement compressors to elevate the pressure of refrigerant vapor, allowing it to condense and reject heat. While rotary screw compressors are widely used for continuous, high-volume operations, reciprocating compressors remain critical for applications requiring flexibility, step-capacity control, and high-pressure differentials. A reciprocating compressor operates by drawing vapor into a cylinder, compressing it via a reciprocating piston, and discharging it through spring-loaded valves. Understanding the mechanical components, valve dynamics, and capacity control mechanisms of reciprocating compressors is essential for Certified Assistant Refrigeration Operators (CARO) to ensure safe, efficient, and reliable system operation.

Key Internal Components and Construction

The construction of industrial reciprocating compressors is robust, utilizing materials compatible with anhydrous ammonia (R-717). Because ammonia is highly corrosive to copper, bronze, and brass, all internal components are made of high-strength cast iron, carbon steel, or specialized steel alloys. Let's break down the primary construction elements:

1. Suction and Discharge Valves

The suction and discharge valves act as check valves, permitting gas flow in only one direction. They are pressure-actuated and spring-loaded, meaning they open and close based on pressure differences across the valve plate.

• Ring and Disc Valves: Large industrial reciprocating compressors use ring or disc-type valves rather than the thin reed valves found in smaller commercial units. Ring valves consist of concentric steel rings seated against a flat valve seat and held closed by small, helical springs. The flat design minimizes the clearance volume at the top of the stroke.
• Valve Seating and Lift: The springs ensure that the valve closes rapidly as the piston reaches the end of its stroke. This rapid action prevents gas from flowing back (blowback) into the suction manifold. The lift height (the distance the valve plate travels away from the seat) is carefully designed: too high of a lift leads to valve damage and noise, while too low of a lift causes excessive pressure drop and reduces compressor capacity.
• Safety Head Assembly: Unlike air compressors, industrial refrigeration compressors feature a "safety head" assembly. The discharge valve seat is held in place by a heavy safety spring rather than being bolted down. If a slug of liquid ammonia or oil enters the cylinder, the safety head lifts off its seating area, allowing the liquid to escape into the discharge manifold. This protects the cylinder head and piston from catastrophic mechanical destruction due to hydrostatic pressure.

2. Pistons and Piston Rings

The piston is the component that performs the physical work of compression. It is typically a trunk-type (automotive-style) piston made of aluminum alloy or cast iron.

• Compression Rings: Located at the top of the piston, these rings (typically two or three per piston) seal the gap between the piston and the cylinder wall. They prevent high-pressure gas from leaking down into the crankcase (known as blow-by) during the compression stroke.
• Oil Scraper Rings: Located below the compression rings, these rings scrape excess oil from the cylinder wall and return it to the crankcase, preventing the oil from entering the compression chamber and being discharged into the refrigeration system.
• Wrist Pins (Gudgeon Pins): The wrist pin connects the piston to the small end of the connecting rod. It must be highly polished steel and is pressure-lubricated to withstand the high reciprocating forces.

3. Connecting Rods and Crankshafts

The connecting rod and crankshaft assembly converts the rotational force of the drive motor into the linear reciprocating motion of the pistons.

• Connecting Rods: Forged from high-strength steel or ductile iron, the connecting rod features a split big-end bearing (crankpin bearing) lined with babbitt or aluminum inserts. The small-end (wrist pin) contains a bronze bushing. Critically, industrial connecting rods have a drilled internal oil passage running their entire length. This passage allows oil from the pressurized crankshaft to travel up the rod and lubricate the wrist pin bushing under pressure.
• Crankshafts: The crankshaft is a heavy forged-steel shaft containing crankpins (or eccentrics) for each cylinder. It is supported by main bearings (sleeve bearings or tapered roller bearings) and counterweighted to minimize vibration.
• Crankcase Shaft Seal: Where the crankshaft extends out of the compressor housing to connect to the drive coupling or belt, a mechanical shaft seal is installed. This seal prevents refrigerant and oil from leaking out, and prevents air and moisture from leaking in when the compressor operates under a vacuum. The seal consists of a stationary seal ring (usually carbon) pressed against a rotating seal ring (hardened steel), with a spring maintaining face-to-face contact. A continuous film of lubricating oil must be maintained between these faces to prevent friction, heat, and leakage.

Capacity Control and Unloader Systems

Refrigeration loads fluctuate constantly. Running a compressor at full capacity during low-load conditions causes short-cycling, high energy consumption, and unstable system pressures. Reciprocating compressors manage this using unloader systems that disable specific cylinders.

Suction Valve De-pressor Mechanism

The most common capacity control method in reciprocating compressors is holding the suction valve open.

• Depressor Pins and Finger Lifters: Inside the suction port of each unloadable cylinder, a set of depressor pins (or a finger lifter) is positioned under the suction valve ring. When the cylinder is unloaded, these pins are forced upward, lifting the suction valve plate off its seat and holding it open.
• Unloaded Operation: As the piston moves upward on the compression stroke, the gas in the cylinder is not compressed. Instead, it is pushed back through the open suction valve into the suction manifold. Because no compression takes place, the cylinder does no work, reducing the compressor's cooling capacity and significantly lowering the electrical current draw on the motor.

Actuation and Control Mechanisms

Unloader mechanisms are actuated either hydraulically or electromagnetically:

1. Hydraulic Unloader Control: This system utilizes the compressor's net oil pressure. When the compressor is off, oil pressure is 0 psig. Internal springs force the unloader piston to push the depressor pins upward, holding the suction valves open. This ensures an unloaded start, which reduces the startup torque and current draw on the motor by up to 70%. Once the compressor starts and the oil pump builds net oil pressure, oil is directed into the unloader cylinder. This pressure overcomes the spring force, retracting the depressor pins and allowing the suction valve to seat, thereby loading the cylinder.
2. Electric/Solenoid Control: Solenoid valves are installed in the unloader control lines. An external controller (such as a microprocessor or pressure switch) monitors suction pressure. When the pressure drops below a setpoint, the controller activates the solenoid valve. The solenoid then routes pressurized oil (or discharge gas) to the unloader piston to load or unload the cylinder as needed.

Worked Example: Net Oil Pressure Calculation

Net oil pressure is the actual pressure delivering oil to the bearings and unloader system. It is defined by the following equation:

$P_{\text{net}} = P_{\text{pump discharge}} - P_{\text{crankcase}}$

If the oil pump discharge gauge reads 85.0 psig and the compressor suction pressure (which represents crankcase pressure) reads 20.0 psig, the net oil pressure is calculated as:

$P_{\text{net}} = 85.0 \text{ psig} - 20.0 \text{ psig} = 65.0 \text{ psi}$

This compressor has sufficient oil pressure, as standard industrial systems require a minimum net oil pressure of 30.0 psi to actuate unloaders and protect bearings from wear.

Maintenance and Safety Precautions

• Valve Inspection: Compressor valves must be inspected periodically for wear, carbon deposits (pitting), and hair-line cracks. Leaking valves severely reduce compressor efficiency and increase operating temperatures.
• Crankshaft End-Play: Technicians must measure crankshaft end-play using a dial indicator to ensure main thrust bearings are within manufacturer specifications.
• Shaft Seal Care: A shaft seal that has been dry for a long period will fail immediately upon startup. If a compressor has been idle, the shaft should be rotated manually by hand to coat the seal faces with oil.
• LOTO Requirements: Prior to opening any compressor housing for valve or bearing service, lock-out/tag-out (LOTO) protocols must be enforced on the motor disconnect, and the refrigerant must be fully evacuated.