This chapter covers all the theoretical and practical concepts that every refrigeration journeyperson must master to succeed on the Red Seal exam. You will find the fundamental principles of thermodynamics applied to refrigeration, operating cycles, heat load calculations, and the relevant Canadian regulatory requirements.
1. Fundamental Principles of Thermodynamics
1.1 The Laws of Thermodynamics Applied to Refrigeration
Refrigeration is based on the first law of thermodynamics: energy is neither created nor destroyed; it is transformed. In a refrigeration system, thermal energy is extracted from a space to be refrigerated and rejected to a space at a higher temperature, thanks to an input of mechanical work (compressor).
The second law of thermodynamics states that heat cannot spontaneously pass from a cold body to a hot body. It is precisely to overcome this constraint that the compressor provides the work necessary to transfer heat "against the grain" of nature.
Essential Concepts:
Sensible heat: heat that causes a change in temperature without a change of state. Formula: Q = m × c × ΔT, where m is the mass (kg), c is the specific heat capacity (kJ/kg·°C), and ΔT is the temperature difference (°C).
Latent heat: heat absorbed or released during a change of state at constant temperature. For the evaporation of a refrigerant, this is referred to as the latent heat of vaporization.
Enthalpy (h): the total heat content of a substance, expressed in kJ/kg. This is the most used property in refrigeration cycle calculations.
1.2 Essential Units and Conversions
Parameter
SI Unit
Imperial Unit
Conversion Factor
Pressure
kPa
psig / psia
1 psi = 6.895 kPa
Temperature
°C
°F
°F = (°C × 9/5) + 32
Refrigerating capacity
kW
T.R. (ton of refrigeration)
1 T.R. = 3.517 kW
Mass flow rate
kg/s
lb/min
1 lb = 0.454 kg
Energy
kJ
BTU
1 BTU = 1.055 kJ
Common Trap: The ton of refrigeration (T.R.) corresponds to the amount of heat required to melt one short ton (2000 lb) of ice in 24 hours, which is 12,000 BTU/h or 3.517 kW. Do not confuse this with the metric tonne (1000 kg).
1.3 Absolute Pressure vs. Gauge Pressure
Absolute pressure (psia or kPa abs) includes atmospheric pressure (101.325 kPa at sea level). Gauge pressure (psig or kPa) is measured relative to atmospheric pressure.
Practical Application: Refrigerant property tables use absolute pressure. When you read a pressure on your manifold gauges (gauge pressure), you must add atmospheric pressure to consult the tables. At an altitude of 1500 m, atmospheric pressure is only about 84.5 kPa — a reading error can lead to a misdiagnosis.
2. The Vapor-Compression Refrigeration Cycle
2.1 The Four Main Components and Their Functions
The vapor-compression refrigeration cycle consists of four distinct stages:
25.Compression (compressor): The refrigerant in a low-pressure, low-temperature vapor state is compressed to a high pressure and high temperature. Compression is generally adiabatic (no heat exchange with the environment), which increases the enthalpy of the refrigerant.
26.Condensation (condenser): The superheated high-pressure vapor releases its heat to the cooling medium (air or water). The refrigerant changes from a vapor state to a liquid state, rejecting its latent heat of condensation. The liquid leaves the condenser slightly subcooled.
27.Expansion (expansion valve or capillary tube): The high-pressure liquid passes through an expansion orifice where its pressure drops suddenly. A portion of the liquid flashes (about 20 to 30%) and evaporates instantly, absorbing the heat necessary to lower the temperature of the mixture.
28.Evaporation (evaporator): The low-pressure liquid-vapor mixture absorbs heat from the medium to be refrigerated. The refrigerant evaporates completely, and the vapor is slightly superheated before returning to the compressor.
2.2 The Pressure-Enthalpy (P-h) Diagram
The P-h diagram is the reference tool for analyzing a refrigeration cycle. It represents pressure (vertical axis) as a function of enthalpy (horizontal axis). The characteristic curves delineate the liquid, vapor, and liquid-vapor mixture zones.
Reading the Diagram:
The left saturation curve represents saturated liquid (quality x = 0)
The right saturation curve represents saturated vapor (quality x = 1)
Between these two curves lies the mixture zone (0 < x < 1)
Constant temperature lines are horizontal in the mixture zone (because temperature is constant during a change of state at constant pressure)
The Four Processes on the Diagram:
1→2: Compression (vertical line upward, slightly inclined to the right — entropy increases slightly)
2→3: Condensation (horizontal line to the left through the vapor then mixture zone, down to saturated liquid)
3→4: Expansion (vertical line downward — constant enthalpy, isenthalpic process)
4→1: Evaporation (horizontal line to the right through the mixture zone, up to saturated or slightly superheated vapor)
2.3 Refrigeration Cycle Calculations
Coefficient of Performance (COP): The ratio of the refrigerating effect to the compression work.
COP = (h₁ − h₄) / (h₂ − h₁)
Where:
h₁ = enthalpy of the vapor at the compressor inlet (kJ/kg)
h₂ = enthalpy of the vapor at the compressor outlet (kJ/kg)
h₄ = enthalpy of the mixture at the evaporator inlet (kJ/kg)
Net Refrigerating Effect (NRE): The amount of heat absorbed per kilogram of refrigerant in the evaporator.
NRE = h₁ − h₄ (in kJ/kg)
Refrigerant Mass Flow Rate:
ṁ = Q_evap / NRE (in kg/s)
Where Q_evap is the required refrigerating capacity (in kW).
Compression Power:
W_comp = ṁ × (h₂ − h₁) (in kW)
Calculation Example: A system operates with the following enthalpies: h₁ = 405 kJ/kg, h₂ = 435 kJ/kg, h₄ = 250 kJ/kg. The COP is (405 − 250) / (435 − 405) = 155 / 30 = 5.17. For a refrigerating capacity of 10 kW, the mass flow rate is 10 / 155 = 0.0645 kg/s and the compression power is 0.0645 × 30 = 1.94 kW.
2.4 Superheat and Subcooling
Superheat: The difference between the actual vapor temperature and its saturation temperature at the same pressure. Measured at the evaporator outlet (or at the compressor inlet).
Useful superheat: measured in the evaporator, it ensures that the refrigerant is completely vaporized before leaving the evaporator.
Total superheat: measured at the compressor inlet, it includes the heat gain in the suction line.
Typical value: 5 to 8 °C for useful superheat in systems with thermostatic expansion valves.
Subcooling: The difference between the liquid temperature and its saturation temperature at the same pressure, measured at the condenser outlet.
Typical value: 5 to 10 °C for air-cooled condenser systems.
Insufficient subcooling indicates an undersized or dirty condenser.
Excessive subcooling may indicate an overcharge of refrigerant or an oversized condenser.
Formulas:
Superheat = Actual suction temperature − Suction saturation temperature
Subcooling = Condensation saturation temperature − Actual liquid temperature
3. Refrigerants
3.1 Classification and Nomenclature
Refrigerants are classified according to the ANSI/ASHRAE 34 standard and identified by the letter R followed by a number. The nomenclature follows specific rules:
CFCs (chlorofluorocarbons): R-11, R-12 — fully halogenated, ozone-depleting, banned in Canada since 1996.
HCFCs (hydrochlorofluorocarbons): R-22, R-123 — contain chlorine but are less damaging, being phased out gradually.
HFCs (hydrofluorocarbons): R-134a, R-404A, R-410A — chlorine-free, do not damage the ozone layer but have a high GWP.
HFOs (hydrofluoro-olefins): R-1234yf, R-1234ze — low GWP, next generation.
Global Warming Potential (GWP): A relative measure of the impact on global warming compared to CO₂ (GWP = 1). R-134a has a GWP of 1430, R-410A of 2088, R-404A of 3922.
Ozone Depletion Potential (ODP): A relative measure of ozone layer depletion compared to R-11 (ODP = 1). HFCs and HFOs have an ODP of 0.
3.2 Canadian Regulatory Requirements
The Ozone-Depleting Substances and Halocarbons Regulations (ODSHR) — enacted under the Canadian Environmental Protection Act (1999) — governs the management of refrigerants in Canada.
Key Requirements:
Any technician handling refrigerants must hold a qualification certificate issued by an accredited training organization.
Refrigerant leaks must be repaired within prescribed timeframes depending on the leak rate:
Systems over 50 kg: repair within 30 days if the leak exceeds 10% per year
Systems under 50 kg: repair within 30 days if the leak exceeds 25% per year
Maintenance records must be kept and maintained up to date.
Refrigerant recovery is mandatory before any opening of the circuit.
3.3 Refrigerant Blends and Temperature Glide
Zeotropic blends (R-404A, R-410A, R-407C) exhibit temperature glide: their saturation temperature varies at constant pressure. This phenomenon is due to the difference in volatility of the components.
Practical Consequences:
The saturation pressure no longer corresponds to a single temperature but to a range of temperatures.
The dew point temperature (saturated vapor) differs from the bubble point temperature (saturated liquid).
Charging must be done in the liquid phase to avoid charging a mixture whose composition has changed.
Thermostatic expansion valves must be selected for zeotropic blends.
Azeotropic blends (R-500, R-502): behave like a pure substance — no glide, constant saturation temperature at a given pressure.
4. System Components
4.1 Compressors
Open compressors: The motor is external; the shaft passes through the crankcase via a shaft seal. Used for large capacities (ammonia).
Semi-hermetic compressors: The motor and compressor are in the same housing, which is accessible (bolted). Used for medium capacities.
Hermetic compressors: The motor and compressor are sealed within a welded shell. Used for small capacities (refrigerators, freezers).
Types of Compressors:
Reciprocating (piston): the most common, good efficiency, sensitive to liquid slugging.
Rotary vane: compact, used in residential applications.
Screw: large capacities, continuous operation, used in industrial refrigeration.
Centrifugal: very large capacities, used in comfort air conditioning and industrial refrigeration.
Compression Ratio: The ratio of absolute discharge pressure to absolute suction pressure. A ratio that is too high (> 10:1) leads to excessive discharge temperatures and a drop in volumetric efficiency.
4.2 Condensers
Type
Principle
Advantages
Disadvantages
Air (natural convection)
Air circulates naturally
Simple, economical
Low efficiency
Air (forced draft)
Fan forces air
Efficient, compact
Noise, filter maintenance
Water (shell and tube)
Water circulates through tubes
Very efficient
Water consumption, treatment
Evaporative
Water sprayed over the condenser
Very efficient in dry climates
Maintenance, legionella risk
Condenser Temperature Difference (TD): The difference between the condensing temperature and the entering air temperature. Typical value: 10 to 15 °C for air-cooled condensers.
4.3 Evaporators
Classification by Refrigerant Feed:
Direct expansion: the refrigerant expands directly into the evaporator.
Flooded: the evaporator contains a liquid reservoir; the refrigerant is fed continuously.
Dry expansion: the refrigerant is completely vaporized before the outlet.
Classification by Air Circulation:
Natural convection: cold plates, static coolers.
Forced convection: fans, air coolers.
Evaporator Temperature Difference (TD): The difference between the entering air temperature and the evaporation temperature. Typical values: 8 to 12 °C for cold rooms, 5 to 8 °C for refrigerated display cases.
4.4 Expansion Devices
Capillary tube: A tube of small diameter (0.5 to 2 mm) and predetermined length. Provides a fixed resistance to flow. Used in small systems (domestic refrigerators, freezers). Does not adapt to load variations.
Thermostatic expansion valve (TXV/EXV): Maintains a constant superheat at the evaporator outlet. Composed of a sensing bulb, a diaphragm, and an orifice. The bulb is attached to the suction line at the evaporator outlet.
Thermostatic Expansion Valve Adjustment:
The bulb pressure (P_bulb) opposes the evaporation pressure (P_evap) and the spring pressure (P_spring).
Balance: P_bulb = P_evap + P_spring
Turning the adjustment screw clockwise increases the spring tension, which increases the superheat.
Opening superheat: the minimum superheat at which the expansion valve begins to open. Typical value: 3 to 5 °C.
Electronic expansion valve (EEV): Controlled by an electronic controller that measures temperature and pressure at the evaporator outlet. Offers precise superheat control and adapts to load variations.
Float expansion valve: Used in flooded systems. The float maintains a constant liquid level in the evaporator.
5. Heat Load
5.1 Components of the Heat Load
The heat load of a cold room is calculated by adding up all sources of heat:
137.Transmission through walls: Q = U × A × ΔT, where U is the heat transfer coefficient (W/m²·K), A is the surface area (m²), and ΔT is the temperature difference between the outside and inside (°C).
138.Air infiltration: due to door openings. Depends on the number of openings, the temperature difference, and the relative humidity.
139.Stored products: sensible heat (cooling of products) + latent heat (freezing) + respiration heat (for fruits and vegetables).
140.Internal equipment: fans, lighting, motors, electric defrost.
141.Occupants: heat given off by people (approximately 250 W per person in moderate activity).
142.Miscellaneous: forklifts, packaging, etc.
5.2 Transmission Load Calculation
Formula: Q = U × A × ΔT
The U factor is the inverse of the total thermal resistance: U = 1 / R_total, where R_total = R_internal + R_wall + R_insulation + R_external.
Example: A cold room measuring 10 m × 8 m × 4 m (height) is maintained at −20 °C. The exterior walls (total surface area of 144 m²) have a U factor of 0.25 W/m²·K. The outside temperature is 30 °C.
Safety margin: 10 to 15% added to the total to account for unforeseen circumstances.
Diversity factor: not all equipment operates at full load at the same time.
Compressor running time: the compressor does not run continuously. A system is sized to operate approximately 16 to 18 hours per day, leaving a margin for defrost and maintenance.
The Canadian Electrical Code, Part I (CSA C22.1) applies to all electrical installations in Canada. The relevant rules for refrigeration systems include:
Rule 8-200: Demand calculation — motor loads must be calculated according to Tables 44 and 45.
Rule 26-250: Protection of motor circuits — each motor must be protected against overcurrent.
Rule 26-252: Motor overload protection.
Rule 28-600: Disconnection and protection of refrigeration and air conditioning equipment.
Key Requirement: All refrigeration equipment must have an accessible disconnecting means that cuts off all ungrounded conductors.
6.2 CSA B52 Code
The CSA B52 — Mechanical Refrigeration Code establishes safety requirements for the design, installation, and operation of refrigeration systems. Key points:
Classification of systems according to refrigerant type and location (categories A1, A2, A3, B1, B2, B3 based on toxicity and flammability).
Refrigerant charge limits according to occupancy classification.
The CSA B149.1 — Natural Gas and Propane Installation Code applies to systems using flammable refrigerants such as propane (R-290) or isobutane (R-600a). Requirements:
Charge limits for occupied spaces.
Required ventilation.
Electrical equipment certified for flammable atmospheres (Class I, Division 2).
6.4 ASHRAE 15 Standard
The ANSI/ASHRAE 15 — Safety Standard for Refrigeration Systems is adopted by reference in several Canadian codes. It establishes:
Maximum allowable refrigerant concentrations in the event of a leak.
Mechanical ventilation requirements for machinery rooms.
High superheat + low suction pressure: indicates a lack of refrigerant OR an underfeeding expansion valve (obstruction, discharged bulb, undersized orifice).
Low superheat + low suction pressure: indicates an expansion valve open too wide OR a flooded evaporator (insufficient superheat).
High superheat + normal suction pressure: indicates a correct refrigerant charge but an excessive heat load (open doors, warm products, defective fans).
Low subcooling + high condensing pressure: indicates a dirty condenser or a defective fan.
High subcooling + high condensing pressure: indicates an excessive refrigerant charge.
8. Pitfalls to Avoid
196.Confusing gauge pressure and absolute pressure: always convert before using refrigerant property tables.
197.Neglecting the temperature glide of zeotropic blends: the saturation temperature is not unique — use the dew point temperature for superheat and the bubble point temperature for subcooling.
198.Charging a zeotropic blend in the vapor phase: this changes the composition of the refrigerant in the system. Always charge in the liquid phase.
199.Forgetting the safety margin in heat load calculations: an undersized system will never be able to reach the required temperature.
200.Ignoring the ODSHR requirements: refrigerant recovery is mandatory before any opening of the circuit, regardless of the type of refrigerant.
201.Confusing typical superheat and subcooling values: superheat is measured at the evaporator (5 to 8 °C), subcooling at the condenser (5 to 10 °C).
202.Not checking the compression ratio: a ratio that is too high can damage the compressor — check the manufacturer's limits.
203.Forgetting the altitude factor: lower atmospheric pressures at altitude change gauge pressure readings and system performance.
204.Using the wrong conversion table: 1 T.R. = 3.517 kW = 12,000 BTU/h — do not confuse with electrical power units.
205.Neglecting the CE Code requirements for motor circuits: Rules 26-250 and 28-600 impose specific protections — know them by heart.
Summary
Refrigeration is based on the first two laws of thermodynamics: transferring heat from cold to hot requires an input of work (compressor).
The vapor-compression cycle consists of four stages: compression, condensation, expansion, evaporation. Each stage is visualized on the P-h diagram.
The COP measures the efficiency of the cycle: COP = (h₁ − h₄) / (h₂ − h₁). A high COP means better energy efficiency.
Superheat (5 to 8 °C) ensures that the refrigerant is completely vaporized; subcooling (5 to 10 °C) ensures that liquid reaches the expansion valve without flash gas.
Refrigerants are classified according to their ODP and GWP. Canada regulates their use through the ODSHR and requires qualification certificates.
Zeotropic blends exhibit temperature glide — charging must be done in the liquid phase.
The heat load includes six components: transmission, infiltration, products, equipment, occupants, miscellaneous. A margin of 10 to 15% is added.
Applicable Canadian standards: CE Code (CSA C22.1), CSA B52, CSA B149.1, ASHRAE 15.
Diagnostics rely on the systematic comparison of measured parameters (pressures, temperatures, superheat, subcooling) with design values.
Review Questions
219.A refrigeration system has a COP of 4.5 and a refrigerating capacity of 15 kW. What is the electrical power absorbed by the compressor?
Answer: W = Q / COP = 15 / 4.5 = 3.33 kW
221.A thermostatic expansion valve is set for 6 °C of superheat. The suction pressure corresponds to a saturation temperature of −15 °C. What temperature should the thermometer indicate on the suction line at the evaporator outlet?
Answer: T = −15 + 6 = −9 °C
223.An air-cooled condenser has a condensing temperature of 40 °C and an entering air temperature of 25 °C. What is the condenser TD?
Answer: TD = 40 − 25 = 15 °C
225.An R-134a system has a discharge pressure of 1200 kPa (gauge). What is the approximate condensing temperature? (Use the tables: at 1200 kPa abs = 1301 kPa, T_sat ≈ 50 °C)
226.A cold room has a total wall surface area of 200 m² (U = 0.3 W/m²·K) and is maintained at 2 °C. The outside temperature is 28 °C. What is the transmission load?
CSA C22.1 — Canadian Electrical Code, Part I (Rules 8-200, 26-250, 28-600)
CSA B52 — Mechanical Refrigeration Code
CSA B149.1 — Natural Gas and Propane Installation Code
ANSI/ASHRAE 15 — Safety Standard for Refrigeration Systems
ANSI/ASHRAE 34 — Designation and Safety Classification of Refrigerants
Ozone-Depleting Substances and Halocarbons Regulations (ODSHR) — Canadian Environmental Protection Act (1999)
This chapter constitutes the essential theoretical foundation for the Red Seal exam. Make sure you master cycle calculations, typical operating values, and regulatory requirements before moving on to the following chapters on specific systems and installation techniques.
