Level 1, Unit 6, Section 2 – Types of Compressors

Unit 6 — Refrigeration System Components

Section 2 — Types of Compressors

Section 2 Overview

Not all compressors work the same way. Section 2 develops the knowledge of compressor types required of Level 1 Refrigeration and Air Conditioning Mechanics (313A/313D): how each design achieves compression, what makes it suited to its application, how its capacity is controlled, and what service considerations are unique to each type.

2.0.1 — General Learning Outcomes

Upon successful completion of this section, the apprentice will be able to:

Distinguish positive displacement compressors from dynamic (centrifugal) compressors; explain how each category achieves pressure increase; and identify the common components shared across most compressor types.
Describe the operating principle of reciprocating compressors — piston and cylinder action, suction and compression strokes, valve plate function, and clearance volume — and compare hermetic, semi-hermetic, and open-drive configurations by accessibility, application, and service implications.
Explain the operating principles of scroll and rotary compressors; describe the design features that make each type quiet and vibration-free; and identify the capacity ranges and typical applications where each is preferred.
Describe how twin-screw and single-screw compressors compress refrigerant using helical rotors, and explain the slide-valve capacity control method used in screw compressors; identify the capacity ranges and HVAC/R applications where screw compressors are used.
Explain how centrifugal compressors use impeller velocity and diffuser pressure recovery to compress refrigerant; describe surge and how it is avoided; and identify the large-capacity applications where centrifugal compressors are the preferred choice.

2.0.2 — Section 2 — Lessons at a Glance

Section 2 begins with the components and classification principles common to all compressor types, then examines each of the five HVAC/R compressor types in its own lesson.

2.1

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Common Compressor Components

Positive displacement vs. dynamic classification; the components shared across most compressor types — housings, bearings, motor windings, suction and discharge connections — and how each supports compression.

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2.2

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Reciprocating Compressors

Piston-and-cylinder compression, suction and compression strokes, valve plate operation, volumetric efficiency, and clearance volume. Hermetic, semi-hermetic, and open-drive configurations: features, applications, and service access.

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2.3

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Scroll Compressors

Fixed and orbiting scroll members; progressive pocket compression; continuous, pulse-free refrigerant flow. Advantages over reciprocating designs, sensitivity to liquid refrigerant and reverse rotation, and residential and light-commercial applications.

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2.4

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Rotary Compressors

Rolling piston and spring-loaded vane operation; simultaneous suction and compression in a single cylinder revolution. Compact hermetic design, low noise, and use in small residential and window AC equipment.

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2.5

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Screw Compressors

Twin-screw and single-screw designs; helical rotor compression; slide-valve capacity control from 10–100%. Oil flooding, oil separation, and the four oil cooling methods. Industrial and large commercial HVAC/R applications.

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2.6

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Centrifugal Compressors

Impeller velocity and diffuser pressure recovery; single- and multi-stage designs; inlet guide vane and variable speed capacity control; surge and surge avoidance. Large-tonnage water chillers and industrial refrigeration.

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2.0.3 — Choosing the Right Compressor Type

No single compressor type is best for every application. Each design involves trade-offs between capacity range, efficiency at full and part load, refrigerant compatibility, cost, noise, serviceability, and tolerance for operating upsets. Understanding these trade-offs is what allows a technician to make sense of manufacturer documentation, recognize when a compressor is being operated outside its design envelope, and recommend an appropriate replacement.

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How Section 2 Connects to the Rest of Unit 6

Section 1 established what the compressor does and how to keep it running. Section 2 extends that foundation by explaining how different compressor designs accomplish the same fundamental task — and why design differences matter for installation, service, and troubleshooting.

The capacity control methods introduced here — cylinder unloading, slide valves, inlet guide vanes, and variable speed drives — reappear throughout Unit 6 as system control strategies, and connect directly to the electrical fundamentals covered in Unit 4.