This chapter covers two families of essential equipment in refrigeration and air conditioning systems: heat exchangers (evaporators, condensers, intercoolers) and flow control devices (thermostatic expansion valves, capillary tubes, electronic valves). For the Red Seal exam, you must not only understand the thermodynamic principles but also know how to select, install, diagnose, and maintain these components in accordance with Canadian standards. Mastery of heat load calculations, energy balances, and safety criteria is directly assessed.
Heat Exchangers: Fundamental Principles
Heat Transfer Mechanisms
Three modes of transfer govern the operation of all heat exchangers:
Conduction: transfer through a solid (tube wall, fin). Governed by Fourier's law: Q = k × A × ΔT / L, where k is thermal conductivity (W/m·°C), A is surface area, ΔT is temperature difference, L is thickness.
Convection: transfer between a surface and a moving fluid. The convection coefficient h (W/m²·°C) depends on velocity, viscosity, and roughness.
Radiation: generally negligible in refrigeration heat exchangers, except in evaporative condensers exposed to sunlight.
The total thermal resistance R_total = 1/(h_i × A_i) + L/(k × A_m) + 1/(h_o × A_o) + fouling factors. The overall coefficient U (W/m²·°C) is the inverse of R_total. The fundamental design equation is:
Q = U × A × ΔT_m
where ΔT_m is the logarithmic mean temperature difference (LMTD) for a counter-flow or parallel-flow heat exchanger:
ΔT_m = (ΔT_1 − ΔT_2) / ln(ΔT_1 / ΔT_2)
Calculation example: A water-cooled condenser receives refrigerant at 40 °C and condenses it at 35 °C. Water enters at 25 °C and leaves at 30 °C. In counter-flow: ΔT_1 = 40 − 30 = 10 °C, ΔT_2 = 35 − 25 = 10 °C. ΔT_m = 10 °C (special case where ΔT_1 = ΔT_2). If water entered at 20 °C and left at 30 °C: ΔT_1 = 10 °C, ΔT_2 = 15 °C, ΔT_m = (15 − 10) / ln(15/10) = 5 / 0.405 = 12.3 °C.
Heat Exchanger Classification
Type
Configuration
Typical Application
Advantages
Disadvantages
Shell and tube
Bundle of tubes inside a shell
Water-cooled condensers, liquid coolers
Robust, easy to clean
Bulky, expensive
Plate
Stacked corrugated plates
Brazed plate heat exchangers (BPHE)
Compact, high efficiency, low refrigerant charge
Sensitive to fouling, difficult to repair
Finned tube
Tubes with aluminum or copper fins
Air-cooled evaporators, air-cooled condensers
Large surface area, lightweight
Difficult to clean fins
Double tube
Two concentric tubes
Small capacities, liquid-to-liquid
Simple, easy maintenance
Limited surface area
For the exam, remember that brazed plate heat exchangers are increasingly common in systems from 2 to 100 tons, as they offer an excellent U coefficient (up to 6000 W/m²·°C) and reduce refrigerant charge, which meets the requirements of CSA B52 (Mechanical Refrigeration Code) regarding charge limits.
Evaporators
Role and Classification
The evaporator absorbs heat from the medium being cooled. Distinguish between:
Direct expansion (DX) evaporators: refrigerant vaporizes inside the tubes; air or water circulates outside.
Flooded evaporators: liquid refrigerant completely covers the tubes; requires a liquid separator and level control.
Recirculation evaporators: liquid is pumped in excess (recirculation rate of 3 to 5 times); the separator returns unvaporized liquid.
Air-Cooled Evaporators
Air-cooled evaporators (unit coolers) are classified by service temperature:
High temperature (above 0 °C): fruit storage, air conditioning.
Medium temperature (0 °C to −10 °C): cold rooms for dairy products.
Low temperature (−10 °C to −40 °C): freezing, blast freezing.
Defrost is critical. Four methods:
32.Air defrost (off-cycle): the fan continues to circulate ambient air. Simple, but slow and ineffective below 0 °C.
33.Electric defrost: heating elements in the drain pan and on the fins. Controlled by timer or differential pressure.
34.Water defrost: water spray over the fins. Effective, but requires drainage and water treatment.
35.Hot gas defrost: cycle reversal or injection of hot gas into the evaporator. Fastest, but complex.
Exam rule: hot gas defrost requires a liquid receiver (accumulator) sized to contain all the refrigerant during the defrost cycle, in accordance with CSA B52.
Water-Cooled Evaporators (Liquid Coolers)
Liquid coolers (chillers) use either shell and tube (water side in tubes) or brazed plates. The fouling factor is crucial: it represents the additional thermal resistance due to deposits. Typical values are:
Fluid
Fouling Factor (m²·°C/W)
Clean city water
0.00009
Treated river water
0.00018
Seawater
0.00026
Cooling tower water
0.00035
Excessive fouling reduces heat transfer, increases discharge pressure (condenser) or suction pressure (evaporator), and can cause compressor damage.
Condensers
Types and Selection
Type
Cooling Fluid
Typical Capacity
Considerations
Air-cooled
Ambient air
1 to 500 tons
Fans, noise, high ambient temperature
Water-cooled (shell and tube)
City or tower water
10 to 5000 tons
Water treatment, water cost
Evaporative
Air + water
10 to 2000 tons
Water consumption, basin maintenance
Hybrid (air-cooled + desuperheater)
Air + water
5 to 200 tons
Heat recovery
The evaporative condenser combines water evaporation and air cooling. The latent heat of vaporization of water (approximately 2257 kJ/kg) allows a large amount of heat to be rejected with reduced air flow. The condensing pressure is typically 5 to 10 °C above the ambient wet-bulb temperature, compared to 10 to 15 °C above the dry-bulb temperature for an air-cooled condenser.
Air-Cooled Condensers: Surface Area Calculation
The required heat exchange surface area is calculated by:
A = Q_cond / (U × ΔT_m)
where Q_cond = Q_evap + W_comp (total heat rejected). For a 10-ton system (35.2 kW) with a COP of 3.0: Q_evap = 35.2 kW, W_comp = 35.2 / 3.0 = 11.7 kW, Q_cond = 46.9 kW. With U = 25 W/m²·°C and ΔT_m = 12 °C, A = 46,900 / (25 × 12) = 156 m².
Common trap: forgetting to add compressor power to the condensing load. The heat rejected at the condenser is always greater than the refrigeration capacity.
Subcooling and Desuperheating
The condenser has three zones:
52.Desuperheating: hot gas cools from discharge temperature to saturation temperature.
53.Condensation: phase change at constant temperature (rejects approximately 80% of total heat).
54.Subcooling: liquid cools below saturation temperature (rejects 5 to 10%).
Subcooling is essential to prevent flash gas formation in the liquid line. It is measured at the condenser outlet: subcooling = saturation temperature − liquid temperature. A typical value is 5 to 8 °C. Insufficient subcooling indicates a low refrigerant charge or a fouled condenser.
Flow Control Devices
Role and Functions
The flow control device (expansion valve) must:
Reduce refrigerant pressure from condensing pressure to evaporating pressure.
Control refrigerant flow so the evaporator is properly fed.
Maintain adequate superheat at the evaporator outlet to protect the compressor.
Superheat is the difference between the actual gas temperature at the evaporator outlet and the saturation temperature corresponding to the suction pressure. Typical superheat is 5 to 10 °C for a thermostatic expansion valve.
Thermostatic Expansion Valve (TXV)
The TXV is the most common type. It consists of:
Sensing bulb: attached to the evaporator outlet, contains a fluid (charge) that exerts pressure on the diaphragm.
Diaphragm: separates the high-pressure side from the low-pressure side.
Adjustment spring: sets the static superheat.
Seat and needle: modulate the flow.
The opening force (bulb pressure P_b) must overcome the closing force (evaporating pressure P_evap + spring pressure P_spring):
P_b = P_evap + P_spring
Static superheat is the minimum superheat required to begin opening (typically 3 to 5 °C). Total superheat is the superheat at the operating point (5 to 10 °C). Operating superheat = total superheat − static superheat.
TXV adjustment: turning the adjustment screw clockwise increases spring pressure, thus increasing superheat (less flow). Counterclockwise decreases superheat (more flow). One full turn changes superheat by approximately 2 to 3 °C depending on the model.
Bulb charge types:
Charge Type
Behavior
Application
Liquid (MOP)
Maximum operating pressure limited
Compressor protection at startup
Gas
Pressure follows bulb temperature
Wide temperature range
Adsorption
Fast response, resists liquid slugging
Variable load systems
Cross-charged
Limits maximum pressure
Low temperature applications
Electronic Expansion Valve (EEV)
The electronic expansion valve uses a stepper motor or pulse-width modulated valve. It is controlled by an electronic regulator that measures superheat via two sensors (pressure and temperature) or via gas temperature and evaporating pressure. Advantages:
Superheat accuracy of ±0.5 °C (versus ±2 °C for a TXV).
Fast response to load variations.
Stable operation at low load (modulation 10 to 100%).
Possibility of liquid level control in the evaporator.
Disadvantages: higher cost, requires electrical supply and controller, risk of liquid slugging if the temperature sensor is poorly positioned.
Capillary Tube
The capillary tube is a copper tube with an internal diameter of 0.5 to 2.5 mm and a length of 1.5 to 6 m. It acts as a constant friction orifice. Characteristics:
Fixed flow rate: does not adapt to load variations.
Pressure equalization: after shutdown, pressures equalize in 2 to 5 minutes, which reduces compressor starting torque.
Critical refrigerant charge: the system must be charged precisely (often ±5%).
Liquid slugging protection: a suction accumulator is often required.
The capillary tube is used in small systems (< 5 tons): refrigerators, freezers, dehumidifiers, small air conditioners.
Float Expansion Valve
Two types:
High-pressure float (liquid side): maintains a constant level in the liquid receiver, feeds the evaporator by gravity. Used in large ammonia systems.
Low-pressure float (evaporator side): maintains a constant level in the flooded evaporator. The float is in a separate chamber connected to the evaporator.
Comparison of Flow Control Devices
Criterion
TXV
EEV
Capillary Tube
Float
Cost
Medium
High
Very low
High
Superheat accuracy
±2 °C
±0.5 °C
None (fixed flow)
N/A (level)
Load adaptation
Good
Excellent
None
Good (level)
Pressure equalization
No (except special model)
No
Yes
No
Maintenance
Low
Medium (electronic)
None
Medium
Typical application
1 to 100 tons
1 to 500 tons
< 5 tons
> 100 tons (NH₃)
Sizing and Selection
Evaporator Selection Criteria
98.Total heat load (W or tons): includes gains through walls, infiltration, products, fans, lighting, people.
99.Air or fluid temperature: determines the evaporating temperature (typically 8 to 12 °C below air temperature for an air-cooled evaporator).
100.Relative humidity: for storage applications, a lower ΔT (6 to 8 °C) maintains higher humidity.
101.Defrost: method chosen based on temperature and humidity.
102.Available space: dimensions, weight, access for maintenance.
105.Design ambient temperature: for Canada, design values vary by region (e.g., 35 °C in Toronto, 30 °C in Vancouver, 32 °C in Montreal). Consult ASHRAE climate data.
106.Temperature difference (TD): air-cooled condenser: 10 to 15 °C; water-cooled condenser: 5 to 8 °C; evaporative condenser: 5 to 7 °C above wet-bulb.
107.Altitude: above 600 m, air density decreases, which reduces air-cooled condenser capacity by approximately 2% per 300 m.
Quick Sizing Rule
For an air-cooled condenser, rated capacity is given for a TD of 15 °C. If the actual TD differs, capacity varies approximately according to:
Example: A condenser rated at 50 kW at TD 15 °C operates at TD 12 °C. Actual capacity = 50 × (12/15)^0.8 = 50 × 0.84 = 42 kW. Therefore, a larger condenser is needed, or a higher condensing pressure must be accepted.
Canadian Standards and Codes
CSA B52 — Mechanical Refrigeration Code
CSA B52 is the reference standard in Canada for the design, installation, and maintenance of refrigeration systems. Key points for heat exchangers and expansion valves:
Article 4.2: Heat exchangers must be designed for the maximum working pressure on the refrigerant side. The design pressure must be at least equal to the refrigerant saturation pressure at 54 °C (130 °F) for direct expansion systems.
Article 5.1: Evaporators and condensers must be equipped with overpressure protection devices (relief valves) if the internal volume exceeds 0.03 m³ and the refrigerant is flammable or toxic.
Article 6.3: Brazed plate heat exchangers must be certified by an accredited organization (CSA, UL) and bear a nameplate indicating maximum pressure, maximum temperature, and refrigerant.
Article 7.2: Expansion valves must be sized for the maximum system flow at minimum condensing pressure and maximum evaporating pressure.
Canadian Electrical Code (CE Code), Chapter V
The Canadian Electrical Code, Chapter V (CSA C22.1 standard) applies to electrical installations of refrigeration systems. Relevant rules:
Rule 8-200: Calculation of electrical demand for compressor and fan motors. The load must be calculated at 125% of the full-load current of the largest motor, plus 100% of all other motors.
Rule 26-250: Motor overload protection. Protection devices must comply with manufacturer specifications.
Rule 28-602: Disconnection and protection of control circuits. Control transformers must be protected against overcurrent.
Exam trap: The CE Code requires that electric defrost heaters be on a separate circuit with overcurrent protection calculated at 125% of rated current, and that the defrost termination thermostat be wired in series with the contactor.
CSA B149.1 — Natural Gas and Propane Installation Code
This standard applies to gas-fired absorption refrigeration systems and gas water heaters used for regeneration. Key points:
Article 5.4: Gas appliances must be installed with a minimum clearance of 150 mm from combustible surfaces.
Article 6.2: Ventilation of rooms containing gas appliances must comply with combustion air flow requirements.
Other Relevant Standards
CSA C22.2 No. 120: Safety requirements for commercial refrigerators and freezers.
ASHRAE 15: Safety standard for refrigeration systems (adopted by reference in CSA B52).
CAN/CSA C273: Energy efficiency of liquid chillers.
Installation and Commissioning Procedures
Installing a Thermostatic Expansion Valve
135.Bulb position: attach the bulb to a horizontal line at the evaporator outlet, at the 3 o'clock or 9 o'clock position (never at the bottom, to avoid oil pockets). The bulb must be in direct contact with the tube, thermally insulated from the ambient.
136.External equalization: for evaporators with a distributor or with pressure drop > 0.3 bar, use an externally equalized TXV. The equalization line must be connected at the evaporator outlet, after the bulb.
137.Installation direction: the expansion valve must be installed with flow in the direction indicated by the arrow. The liquid line must be free of sharp bends that could create vapor pockets.
138.Filter drier: install a filter drier upstream of the expansion valve to protect the seat and needle from particles and moisture.
Commissioning a System with a TXV
140.Charge verification: measure subcooling (5 to 8 °C) and superheat (5 to 10 °C).
141.Superheat adjustment: if superheat is too high (> 10 °C), turn the screw counterclockwise (decrease superheat). If too low (< 3 °C), turn clockwise.
142.Operation verification: observe the cycle. The compressor must not receive liquid (liquid slugging). Suction pressure must be stable.
143.Bulb charge test: warm the bulb with your hand — suction pressure should increase. Cool the bulb with ice — pressure should decrease.
Troubleshooting Common Faults
Symptom
Probable Cause
Verification
High superheat, low suction pressure
Insufficient refrigerant charge
Measure subcooling
High superheat, normal suction pressure
TXV undersized or obstructed
Check filter, needle
Low superheat, high suction pressure
TXV oversized or bulb improperly attached
Check bulb contact
Fluctuating suction pressure
Hunting
Readjust superheat, check equalization
Frost on liquid line
Freezing expansion valve (moisture)
Replace filter drier
Heat Balance Calculations
Evaporator Balance
The refrigeration capacity of an evaporator is calculated by:
Q_evap = ṁ_r × (h_outlet − h_inlet)
where ṁ_r is the refrigerant mass flow rate (kg/s), h_outlet is the gas enthalpy at the outlet (kJ/kg), h_inlet is the liquid enthalpy at the inlet (kJ/kg).
Example: R-134a, flow rate of 0.05 kg/s, enthalpy at inlet (liquid at 35 °C) = 249 kJ/kg, enthalpy at outlet (superheated gas at 10 °C) = 410 kJ/kg. Q_evap = 0.05 × (410 − 249) = 8.05 kW.
Condenser Balance
Q_cond = ṁ_r × (h_gas_inlet − h_liquid_outlet)
With the same flow rate: h_gas_inlet (at 70 °C) = 430 kJ/kg, h_liquid_outlet (at 35 °C) = 249 kJ/kg. Q_cond = 0.05 × (430 − 249) = 9.05 kW. The difference (9.05 − 8.05 = 1.0 kW) represents the compressor work.
Heat Exchanger Effectiveness
Effectiveness ε = Q_actual / Q_max, where Q_max is the maximum possible transfer (limited by the fluid with the smallest heat capacity). For an air-cooled evaporator:
Example: air entering at 20 °C, leaving at 12 °C, evaporating temperature 5 °C. ε = (20 − 12) / (20 − 5) = 8 / 15 = 0.53 (53%). An effectiveness above 60% generally indicates an oversized heat exchanger or insufficient air flow.
Preventive Maintenance
Heat Exchangers
Cleaning air-cooled condensers: brush the fins, blow with compressed air (from inside to outside). Frequency: quarterly in urban environments, monthly in industrial environments.
Cleaning water-cooled condensers: chemical descaling (5-10% phosphoric acid) or mechanical cleaning (rotary brush). Check the fouling factor.
Inspecting evaporators: check frost accumulation, fin condition, condensate drain pan drainage.
Oil analysis: take a compressor oil sample to detect acidity (acid number < 0.05 mg KOH/g) and moisture.
Flow Control Devices
TXV: check diaphragm tightness, needle condition, bulb attachment. Replace the filter drier if the pressure differential exceeds 0.7 bar.
EEV: check electrical connections, temperature sensor signal (Pt1000 resistance or NTC thermistor), controller programming.
Capillary tube: check that it is not pinched or obstructed. A partially obstructed capillary tube causes high superheat and low suction pressure.
Traps to Avoid
170.Confusing superheat and subcooling: superheat is measured at the evaporator outlet (low-pressure side), subcooling at the condenser outlet (high-pressure side). Never reverse the two in a diagnosis.
171.Forgetting compressor power in the condenser balance: Q_cond = Q_evap + W_comp. A candidate who calculates Q_cond = Q_evap loses points.
172.Using LMTD instead of simple temperature difference: for an air-cooled evaporator, LMTD does not apply in the same way. Use the TD method (air temperature − evaporating temperature) for air-cooled evaporators.
173.Neglecting external equalization: an internally equalized TXV on an evaporator with a distributor or large pressure drop causes excessive superheat. The exam often tests this distinction.
174.Ignoring CSA B52 requirements for relief valves: any heat exchanger over 0.03 m³ containing a Group A2L, B2, or B3 refrigerant must have a relief valve sized according to Annex A of the standard.
175.Calculating refrigerant charge without considering heat exchanger volume: total charge includes liquid in the condenser, receiver, liquid line, and evaporator. An incorrect calculation leads to abnormal subcooling.
176.Confusing TXV bulb charge types: MOP (Maximum Operating Pressure) charge limits evaporating pressure at startup, but it also reduces valve capacity at high pressure. Do not use it for variable load systems.
177.Forgetting the fouling factor in sizing: a heat exchanger sized without a fouling factor will be undersized in actual service. The fouling factor must be added to the thermal resistance.
178.Not checking material compatibility: copper and aluminum must not be in direct contact in an ammonia (NH₃) system. Use separators or compatible materials.
179.Neglecting air purging of water-cooled condensers: accumulated air reduces heat transfer surface and increases condensing pressure. Install an automatic purge at the highest point.
Summary
Heat exchangers transfer heat by conduction, convection, and radiation, governed by the equation Q = U × A × ΔT_m.
Evaporators are classified by service temperature and feed method (direct expansion, flooded, recirculation). Defrost is essential below 0 °C.
Condensers reject total heat (evaporation + compressor work). Subcooling (5 to 8 °C) protects the expansion valve against flash gas.
Flow control devices include the TXV (most common), EEV (precise), capillary tube (fixed flow), and float (large systems).
Superheat (5 to 10 °C) is the key diagnostic parameter. TXV adjustment is done via the spring screw.
Applicable Canadian standards are CSA B52 (refrigeration mechanics), CE Code Chapter V (electrical), and CSA B149.1 (gas).
Heat balance calculations use refrigerant enthalpies. The fouling factor must be included in sizing.
Preventive maintenance includes cleaning heat exchangers, checking expansion valves, and oil analysis.
Review Questions (Red Seal Exam Style)
190.An air-cooled condenser has a rated capacity of 40 kW at TD 15 °C. What is its capacity at TD 10 °C?
196.An internally equalized TXV is used on an evaporator with a distributor. What problem results?
a) Excessive superheat at the outlet
b) Insufficient subcooling
c) High condensing pressure
d) Liquid slugging at the compressor
(Answer: a — the pressure drop in the distributor is not compensated)
202.According to CSA B52, what is the minimum design pressure for an R-410A evaporator?
a) The saturation pressure at 54 °C
b) The saturation pressure at 40 °C
c) 1.5 times the working pressure
d) The maximum compressor discharge pressure
(Answer: a — the saturation pressure at 54 °C is the standard reference)
208.An R-134a system has a flow rate of 0.08 kg/s. The enthalpy at the evaporator inlet is 250 kJ/kg and at the outlet is 405 kJ/kg. What is the refrigeration capacity?
a) 12.4 kW
b) 10.8 kW
c) 14.2 kW
d) 9.6 kW
(Answer: a — 0.08 × (405 − 250) = 12.4 kW)
214.What is the effect of a clogged filter drier upstream of a TXV?
a) High superheat and low suction pressure
b) Low superheat and high suction pressure
c) High subcooling and low condensing pressure
d) No effect on operation
(Answer: a — the restriction reduces liquid flow to the evaporator)
