This chapter covers all the knowledge required for the Red Seal exam concerning compressors — the heart of any refrigeration system — as well as their prime movers (electric motors, couplings, transmissions). You will find operating principles, installation and maintenance procedures, efficiency and power calculations, as well as applicable regulatory requirements in Canada. Each section is structured to correspond directly to the objectives of the national training plan (NFPA) for the trade.
Compressor Classification
Open, Semi-Hermetic, and Hermetic Compressors
Type
Motor
Shaft Seal
Typical Applications
Advantages
Disadvantages
**Open**
External, coupled
Present (packing or mechanical seal)
Industrial systems, ammonia, large air conditioners
Motor replaceable without opening the circuit, easy to maintain
Risk of leakage at the seal, bulky
**Semi-Hermetic**
Integrated into the crankcase, accessible
None (no shaft seal)
Commercial, medium industrial refrigeration
No shaft leakage, repairable on site
Motor cooled by refrigerant, risk of overheating
**Hermetic**
Integrated, welded
None
Residential, light commercial
No leakage, compact, economical
Not repairable, complete replacement required
Exam Tip: The shaft seal of an open compressor must be replaced if a refrigerant leak is detected at that location. The procedure requires recovering the refrigerant, removing the coupling, then replacing the seal while respecting the torque specified by the manufacturer.
Reciprocating Compressors (Piston)
The reciprocating compressor operates on the positive displacement principle. The piston moves in the cylinder in a back-and-forth motion, creating a volume variation that draws in, compresses, and discharges the refrigerant.
Theoretical Four-Stroke Cycle:
14.Intake — the piston descends, the suction valve opens, refrigerant enters the cylinder.
15.Compression — the piston rises, both valves are closed, pressure increases.
16.Discharge — the discharge valve opens, compressed gas leaves the cylinder.
17.Expansion — residual gas in the clearance volume expands before the next cycle.
Swept Volume vs. Displaced Volume: The swept volume (Vₛ) is the theoretical volume displaced by the piston over its full stroke. Volumetric efficiency (ηᵥ) is the ratio between the volume of gas actually drawn in and the swept volume. It is always less than 1 due to clearance volume, valve pressure drops, and gas heating.
ηᵢ = indicated efficiency (typically 0.80 to 0.90)
Rotary Compressors
Rotary compressors use rotational motion rather than reciprocating motion. The main types are:
Vane (Rotary Vane): an eccentric rotor turns inside a cylindrical stator. Vanes slide in slots and divide the space into chambers of variable volume. Used in small residential systems.
Scroll: two interleaved spirals, one fixed, one orbiting, create pockets of gas that move toward the center while compressing. Widely used in air conditioning and heat pumps.
Screw: two helical rotors (male and female) mesh and compress the gas. Used in medium and large industrial systems.
Advantages of screw compressors: vibration-free operation, high flow rate, superior volumetric efficiency, reduced maintenance (no valves).
Centrifugal Compressors
The centrifugal compressor is a dynamic compressor (non-positive displacement). The gas is accelerated by the rotation of an impeller, then slowed in a diffuser where kinetic energy is converted into pressure.
Characteristics:
Very high flow rates (from 500 TR and up)
Moderate discharge pressure
Performance curve with risk of surge at low flow
Used in large central air conditioning systems (water chillers)
Exam Tip: Surge occurs when the flow drops below the minimum point on the performance curve. The compressor then alternates between discharge and suction, causing severe vibrations and potential damage. Modern systems are equipped with anti-surge controllers that open a hot gas bypass valve to maintain minimum flow.
Prime Movers and Transmissions
Electric Motors
The motors used in refrigeration systems are primarily three-phase induction motors (industrial and commercial) or single-phase (residential and light commercial).
Nameplate Ratings:
Voltage: 208 V, 230 V, 460 V, 575 V (Canada)
Frequency: 60 Hz
Service factor (SF): 1.15 for most refrigeration motors
Insulation class: F (155 °C) or H (180 °C)
IP rating: IP54 or IP55 for humid environments
Full-Load Current Calculation:
I = (P × 1000) ÷ (√3 × U × cos φ × η)
Where:
I = current (A)
P = power (kW)
U = line-to-line voltage (V)
cos φ = power factor (typically 0.80 to 0.90)
η = motor efficiency (typically 0.85 to 0.95)
Example: Three-phase motor of 15 kW, 460 V, cos φ = 0.85, η = 0.90.
I = (15 × 1000) ÷ (1.732 × 460 × 0.85 × 0.90) = 15,000 ÷ 609.5 = 24.6 A
Motor Protection
Overload protection is mandatory according to the Canadian Electrical Code, Part I (C22.1). Relevant rules include:
Rule 28-302: motor overload protection — the device must be set at 125% of the full-load current for motors with a service factor of 1.15 or greater, and at 115% for others.
Rule 28-308: short-circuit and ground-fault protection — the fuse or circuit breaker must be sized according to Table 29 of the Code.
Rule 28-310: disconnection must be visible and accessible.
Exam Tip: For a hermetic compressor motor, the current used for protection sizing is the equipment nameplate current rating, not the motor current alone. Rule 28-312 specifies the adjustments for hermetic compressors.
Couplings and Transmissions
Transmission Type
Application
Advantages
Disadvantages
**Direct coupling**
Open compressors, large systems
No transmission losses, precise alignment required
Angular misalignment: ≤ 0.05 mm per 100 mm of diameter
Belt Tension: The deflection at the center of the span should be approximately 1/64 of an inch per inch of span length (approximately 1.5 mm per 100 mm). A belt that is too tight damages bearings; too loose, it slips and overheats.
Performance Calculations and Efficiencies
Volumetric Efficiency
The overall volumetric efficiency (ηᵥ) of a reciprocating compressor is calculated:
ηᵥ = 1 − C × [(P₂/P₁)^(1/n) − 1]
Where:
C = clearance volume ratio (clearance volume / swept volume, typically 0.02 to 0.05)
P₁ = absolute suction pressure (kPa)
P₂ = absolute discharge pressure (kPa)
n = polytropic exponent (typically 1.1 to 1.3 for refrigerants)
Example: Compressor with C = 0.04, P₁ = 300 kPa, P₂ = 1200 kPa, n = 1.2.
The compression ratio is the ratio between the absolute discharge pressure and the absolute suction pressure.
R = P₂ ÷ P₁
Typical Values:
Air conditioning: 2.5 to 3.5
Commercial refrigeration: 4 to 8
Freezing: 8 to 12
Above 12: two-stage compression required
Exam Tip: An excessively high compression ratio causes excessive discharge temperature, oil degradation, and a drop in volumetric efficiency. The solution is two-stage compression with intercooling (liquid injection or heat exchanger).
Power and Efficiency
Indicated Power (Pi): power developed inside the cylinder.
Pi = (Pm × Vs × N) ÷ 60
Where:
Pm = indicated mean effective pressure (kPa)
Vs = swept volume (m³)
N = rotational speed (rpm)
Shaft Power (Pa): power supplied to the compressor by the motor.
Pa = Pi ÷ ηmechanical (ηmechanical ≈ 0.85 to 0.95)
Coefficient of Performance (COP):
COP = Refrigeration capacity (kW) ÷ Power input (kW)
Energy Efficiency Ratio (EER):
EER = Refrigeration capacity (Btu/h) ÷ Power input (W)
Electrical installations for refrigeration systems must comply with the following rules:
Rule 26-256: wiring of refrigeration equipment — conductors must be sized at 125% of the equipment nameplate current rating.
Rule 28-602: control of compressor motors — the starter must be sized according to the equipment nameplate current rating.
Rule 28-604: overload protection for hermetic compressors — maximum setting of 140% of the equipment nameplate current rating.
Rule 26-258: equipment must be connected using compliant cable or conduit.
CSA B52 — Mechanical Refrigeration Code
This Canadian standard governs the design, installation, and maintenance of refrigeration systems. Key points:
System classification according to refrigerant (groups A1, A2, A3, B1, B2, B3) and charge (low, medium, high).
Sections 6 and 7: requirements for machinery rooms and technical spaces.
Section 9: piping and connections — copper tubing must comply with the applicable standard and be supported at regular intervals.
Section 11: pressure testing — systems must be subjected to a pressure test of 1.1 times the maximum working pressure (MWP) for both high and low pressure sides.
CSA B149.1 — Natural Gas and Propane Installation Code
Applicable when refrigeration systems use gas burners (absorption) or when gas appliances are installed in the same room. Relevant rules include:
Rule 5.8: ventilation of rooms containing gas appliances.
Rule 6.3: venting of combustion products — venting systems must be sized according to Table 6.3.
Ozone-Depleting Substances Regulations (ODSR)
Although this is a federal regulation (Environment and Climate Change Canada) and not a code, it is essential for the exam:
Prohibition of intentional release of refrigerants (CFCs, HCFCs, HFCs).
Obligation to recover before any intervention on the circuit.
Record keeping for quantities of refrigerant purchased and recovered.
Mandatory technician certification (categories I to IV depending on charge and system type).
Maintenance and Troubleshooting Procedures
Compressor Commissioning
137.Preliminary checks: alignment, belt tension, rotation direction (for three-phase), oil level, crankcase pressure.
138.Crankcase preheating: the crankcase heater must be energized at least 12 hours before start-up to prevent liquid slugging (refrigerant migration into the oil).
139.Start-up: check starting current, oil pressure (differential of 100 to 300 kPa depending on manufacturer), discharge temperatures.
140.Adjustments: high and low pressure switches, oil flow controller, oil differential pressure switch.
Common Fault Diagnosis
Symptom
Probable Cause
Check
Compressor does not start
HP pressure switch open, overload tripped, defective contactor
Test continuity, check pressures, measure voltage
Short cycling (frequent starts)
LP pressure switch misadjusted, TEV too small, insufficient charge
Check suction pressure, subcooling
Abnormal noise
Liquid slugging, worn bearings, broken valves
Listen with stethoscope, check crankcase temperature
146.Preparation of the new compressor: check the nameplate, install the crankcase heater, purge the holding charge of nitrogen.
147.Installation: braze with a nitrogen flow (5 to 10 L/min) to prevent internal oxidation.
148.Leak test: pressurize with nitrogen to the MWP, check with an electronic leak detector or soap solution.
149.Evacuation: down to 500 microns (0.5 Torr) or less, hold for 30 minutes.
150.Charging: introduce refrigerant in liquid phase on the liquid side, top up in vapor phase on the suction side.
151.Commissioning: follow the start-up procedure described above.
Pitfalls to Avoid
154.Confusing absolute pressure and gauge pressure in compression ratio calculations. Always use absolute pressures (gauge + 101.3 kPa).
155.Forgetting the service factor (SF) when sizing protection. A motor with an SF of 1.15 can operate at 115% of its rated power, but the protection must be set accordingly.
156.Neglecting the 125% rule for conductor sizing. The equipment nameplate current rating must be multiplied by 1.25 to determine the minimum conductor ampacity.
157.Using temperature instead of pressure to check a system charge. The charge is verified by subcooling (condenser) and superheat (evaporator), not by pressure alone.
158.Ignoring the crankcase heater during extended shutdown. Without preheating, refrigerant migrates into the oil and causes liquid slugging at start-up.
159.Confusing compressor types in questions about applications. Centrifugal compressors are not used for small systems; hermetic compressors are not repairable on site.
160.Forgetting unit conversions: 1 TR = 3.517 kW = 12,000 Btu/h. Exam questions often mix units.
161.Not knowing the compression ratio limits: above 12, two-stage compression is mandatory to avoid excessive discharge temperatures.
162.Neglecting ventilation requirements for machinery rooms according to CSA B52. Ventilation must be calculated based on the refrigerant charge and safety group.
163.Confusing the Canadian Electrical Code rules: Rule 28-604 (hermetic compressor protection) differs from Rule 28-302 (standard motor protection). The setting percentages are not the same.
Summary
Compressors are classified into three main categories by construction (open, semi-hermetic, hermetic) and four types by operating principle (reciprocating, rotary, screw, centrifugal).
Volumetric efficiency depends on clearance volume, compression ratio, and polytropic exponent. It is always less than 100%.
Compression ratio is calculated using absolute pressures. Values above 12 require two-stage compression.
Electric motors must be protected in accordance with the Canadian Electrical Code, Part I: conductors at 125% of the nameplate current, overload protection per Rules 28-302 and 28-604.
CSA B52 governs mechanical installation: system classification, pressure testing at 1.1 × MWP, machinery room ventilation.
ODSR mandates refrigerant recovery and technician certification.
Maintenance procedures include checking alignment, belt tension, crankcase preheating, and evacuation down to 500 microns.
Power calculations use refrigeration cycle enthalpies and efficiencies (indicated, mechanical, volumetric).
Unit conversions are essential: 1 TR = 3.517 kW = 12,000 Btu/h.
Know the typical values: volumetric efficiency (85–95%), compression ratio (2.5–12), service factor (1.15), microns for vacuum (500).
Review the Code rules cited in this chapter — questions often focus on exact percentages (125%, 115%, 140%).
Practice converting units quickly: kPa ↔ psi (1 psi = 6.895 kPa), kW ↔ TR, °C ↔ °F.
For troubleshooting questions, reason in terms of possible causes: electrical (supply, protection), mechanical (valves, bearings), thermodynamic (charge, expansion valve, condenser).
This chapter covers all the essential concepts to pass the "Compressors and Prime Movers" section of the Red Seal exam. Review each section, redo the example calculations, and consult the cited standards to delve deeper into regulatory details.
