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newSchoolWorks.com › VoTechWorks › Building & Construction › HVAC/R › Commercial Refrigeration › Lesson 4
BLD-HVAC07-07 · Lesson 4 · 16 clock hours · demo

Commercial Compressors: Types, Operation, and Service

Objective. Students will identify reciprocating, scroll, and semi-hermetic compressors used in commercial refrigeration, explain the function of each internal component, and perform an oil-level and crankcase-heater check.

Commercial Compressors: Types, Operation, and Service

The compressor is the heart of any refrigeration system. It raises refrigerant vapor from low suction pressure to high discharge pressure, driving the refrigeration cycle forward. In commercial refrigeration you will encounter three compressor types regularly: reciprocating, scroll, and semi-hermetic reciprocating. Knowing how each works—and how to check its condition—is a core service skill.

Reciprocating Compressors

A reciprocating compressor uses pistons moving back and forth inside cylinders, much like a car engine. On the downstroke, low-pressure refrigerant vapor is drawn in through the suction valve. On the upstroke, the vapor is compressed and pushed out through the discharge valve. Multiple cylinders increase capacity. Key internal parts include:

Scroll Compressors

A scroll compressor uses two spiral-shaped scrolls. One scroll is fixed; the other orbits eccentrically. The orbiting motion traps pockets of vapor between the scrolls and continuously moves them toward the center, where pressure rises and compressed vapor exits through the discharge port. There are no suction or discharge reed valves. Advantages include smoother operation, fewer moving parts, quieter sound, and good efficiency at part load. Scroll compressors are direction-sensitive: running one backward (reversed motor leads) produces no useful compression and can damage the unit.

Semi-Hermetic Reciprocating Compressors

A semi-hermetic (also called serviceable hermetic) compressor encloses the motor and compressor in a bolted, gasketed housing that can be opened for field service—unlike a welded hermetic compressor, which must be replaced when it fails internally. Semi-hermetics are common in medium and large commercial refrigeration systems (walk-in coolers, display case racks). The motor windings are cooled by suction refrigerant vapor, so proper suction superheat is critical to motor longevity. Internal components—pistons, valves, crankshaft, oil pump—are essentially the same as an open reciprocating compressor.

Crankcase Heater Check

Before starting any compressor that has been off for more than a few hours:

1. Confirm the crankcase heater has been energized for at least the time specified in the service manual (often several hours for larger compressors).

2. Feel the crankcase with the back of your hand—it should be noticeably warm to the touch.

3. Use a clamp ammeter on the heater wire to verify current draw matches the rated wattage.

4. If the heater is cold or drawing no current, tag out the compressor and replace the heater before starting.

Starting a compressor with cold, refrigerant-saturated oil can bend connecting rods, crack pistons, or strip valve seats.

Oil-Level Check

1. With the system running at stable conditions, look through the oil sight glass on the crankcase.

2. Oil should appear at the level specified by the manufacturer—commonly between one-quarter and three-quarters of the sight glass.

3. Foaming oil indicates refrigerant flashing out of the oil; investigate for refrigerant migration or low superheat.

4. An oil level that is too low suggests oil loss (oil trapping in the system or a leak) and requires investigation before adding oil.

5. Use only the refrigerant-compatible oil type listed on the compressor nameplate or in the service manual.

Reading Operating Pressures and Amperage

Connect a manifold gauge set at the suction service valve and discharge service valve. Record suction pressure, discharge pressure, and compare to the expected values for the refrigerant and the system's design operating conditions. Then clamp an ammeter around one of the compressor power leads and record current draw. Compare to the nameplate RLA (rated load amps) and LRA (locked rotor amps). High amperage with low discharge pressure often points to a valve failure or worn pistons. High amperage with high discharge pressure may indicate a refrigerant overcharge or condenser problem.

Safety

Key terms8

Semi-hermetic compressorA compressor whose motor and compression assembly are enclosed in a bolted, gasketed housing that can be opened for field service, unlike a welded hermetic unit.
Reed valveA thin, flexible metal flap in a reciprocating compressor that acts as a one-way check valve, opening on pressure difference to allow refrigerant to enter or leave the cylinder.
Oil sight glassA transparent window fitted to the crankcase of a reciprocating or semi-hermetic compressor that allows a technician to observe the oil level while the compressor is running.
Crankcase heaterAn electric resistance heater mounted on the compressor crankcase that keeps the oil warm during off cycles, preventing refrigerant from migrating into and diluting the oil.
Flooded startA condition in which liquid refrigerant mixed with crankcase oil is drawn into the compressor cylinders at startup, risking serious mechanical damage.
RLA (Rated Load Amps)The full-load current draw stamped on the compressor nameplate, used as a baseline when evaluating measured amperage during operation.
Scroll compressorA compressor that uses one fixed and one orbiting spiral scroll to continuously compress refrigerant vapor from the outer edge toward a central discharge port, with no reed valves.
Suction superheatThe number of degrees by which the suction vapor temperature exceeds the saturation temperature at suction pressure; critical for protecting the compressor motor windings in semi-hermetic designs.

Worked examples3

1. A semi-hermetic compressor has a nameplate RLA of 18 A. Your clamp ammeter reads 21.6 A at normal operating conditions. Calculate the percentage above RLA and state what this indicates.
  1. Subtract RLA from measured current: 21.6 A − 18 A = 3.6 A above RLA.
  2. Divide the excess by RLA and multiply by 100 to get percentage: (3.6 ÷ 18) × 100 = 20%.
  3. Compare to acceptable limits. A reading 20% above RLA is significantly elevated and warrants investigation (overcharge, high condensing pressure, or an electrical issue).
Answer: The compressor is drawing 20% above its RLA. This is outside normal tolerance and the technician should check discharge pressure, condenser condition, and supply voltage before continued operation.
2. You are checking a crankcase heater rated at 120 V, 40 W. What current should your clamp ammeter read if the heater is operating correctly?
  1. Use the power formula: I = P ÷ V.
  2. Substitute values: I = 40 W ÷ 120 V = 0.333 A.
  3. Round appropriately: expect approximately 0.33 A on the clamp ammeter.
Answer: Approximately 0.33 A. Any reading significantly lower or equal to zero indicates the heater element has failed and must be replaced before starting the compressor.
3. A reciprocating compressor runs on a medium-temperature rack. Suction pressure is 40 psig and discharge pressure is 180 psig. The technician clamps the ammeter and reads only 8 A against an RLA of 22 A. What does this pattern suggest?
  1. Note that current is far below RLA — the compressor is doing very little work.
  2. Note that suction pressure is not especially low and discharge pressure is not especially high, suggesting the refrigerant circuit is present but compression is inefficient.
  3. Low amperage with relatively normal pressures that are not separated by the expected ratio points to internal valve failure: worn or broken reed valves allow high-pressure gas to leak back into the suction side, reducing compression work and current draw.
  4. Document the measured values and flag the compressor for valve plate inspection after recovery and lockout/tagout.
Answer: The data pattern is consistent with failed or leaking suction or discharge reed valves. The compressor is not building adequate compression ratio, so it draws far less current than normal. The compressor should be taken offline, refrigerant recovered, and valves inspected.

The math in this lesson2

Learners compute percent above RLA for a 21.6 A reading on an 18 A compressor.
7.RP.3A-CED.4
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor teardown demonstration of a semi-hermetic reciprocating compressor cutaway: students sketch and label pistons, valves, crankshaft, and oil pump in their notebooks. · Guided inspection activity: in groups of three, students visually inspect operating scroll and reciprocating compressors on training rigs, record suction and discharge pressures, amperage draw, and oil sight-glass level on a service record form. · Fault-finding drill: instructor introduces one simulated fault per group (flooded start, tight suction service valve, overheating) and students diagnose using pressure gauges and clamp ammeter, then document diagnosis and corrective action.
Materials: Semi-hermetic compressor cutaway, training rigs with scroll and reciprocating compressors, manifold gauge sets, clamp ammeters, service record forms, NCCER HVAC Trainee Guide Level 3, compressor service manuals
Check: Students submit completed service record form for their compressor inspection; instructor checks that all data fields are accurately recorded and fault diagnosis is correctly supported by measured values. Pass requires all data fields complete and correct diagnosis.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

HVAC Excellence / NATE Core – Compressor Operation
1. What is the PRIMARY purpose of the crankcase heater on a commercial refrigeration compressor?
NATE – Refrigeration Systems / NCCER HVAC Level 3 Compressors
2. During a semi-hermetic compressor teardown demonstration, you notice the discharge reed valves are cracked. Which operating symptom would MOST likely be observed at the manifold gauges?
NATE – Commercial Refrigeration / EPA 608 Core
3. A scroll compressor is installed and starts, but the system shows little pressure difference between suction and discharge. The most likely cause is:
NCCER HVAC – Safety / EPA 608 Core Safety
4. Before connecting manifold gauges to an operating compressor, which personal protective equipment (PPE) is MOST important to protect against high-pressure refrigerant exposure?
NATE – Commercial Refrigeration / HVAC Excellence
5. You observe the oil sight glass on a running reciprocating compressor and notice the oil is foaming. What does this MOST likely indicate?
NCCER HVAC Level 3 / HVAC Excellence – Compressor Types
6. Which compressor type uses an orbiting spiral element and has NO suction or discharge reed valves?
NATE – Electrical Fundamentals / NCCER HVAC
7. A technician's clamp ammeter reads 28 A on a compressor with a nameplate RLA of 20 A. What is the percentage above RLA?
EPA 608 Universal – Core Regulations
8. EPA Section 608 of the Clean Air Act requires technicians to do which of the following BEFORE opening a refrigerant circuit for compressor service?
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