Evaporator (technically: the evaporator, also called the cooling coil or evaporating vessel) is the component that absorbs heat directly in a closed refrigeration cycle. It is where the refrigerant changes phase from liquid to vapour, taking heat out of the storage space and bringing the temperature down to the required range. Together with the compressor, the condenser and the expansion valve, the evaporator forms one of the four inseparable links in an industrial refrigeration system.
In cold storage design practice, the evaporator directly determines two vital indicators: how quickly temperature is pulled down to setpoint, and how evenly the cold air is distributed in the storage space. Choosing the wrong capacity or type of evaporator not only spoils goods but also overloads the compressor and shortens the life of the whole system. This article sets out the full construction, four classification methods, selection criteria and the maintenance procedure.
What is an evaporator?
An evaporator is the heat exchanger between liquid refrigerant (at low pressure and low temperature) and the environment to be cooled. Refrigerant absorbs heat from the storage space, boils and vaporises, carrying that heat out of the cold store through the circulation cycle.
Three terms are often used interchangeably in the industry and should be distinguished: “evaporator” is the correct technical name in thermodynamic terms; “cooling coil” is the common name used in cold store and freezer installation; “evaporating vessel” usually refers to the shell-and-tube type used in water chiller systems. The three terms
In a standard refrigeration system, the evaporator receives refrigerant in the liquid state after the throttle valve, With the pressure and temperature significantly reduced, the refrigerant moves through the heat exchanger tubes, absorbing heat from the air or the stored product, completely evaporating, and then being drawn back by the compressor to continue the cycle.
Telling an evaporator apart from an evaporative condenser: These are two entirely different pieces of equipment despite the similar names. The evaporative condenser is heat rejection equipment on the high-pressure side of the system, using evaporating water to cool the high-pressure compressed gas from the compressor. The evaporator sits on the low-pressure side and cools the storage space. Confusing the two on a drawing
Detailed construction of an air evaporator (industrial cooling coil)
A system evaporator A standard air evaporator in industrial cold storage today is made up of four key component groups: the heat exchange tubing, the fins, the circulation fans and the integrated defrost system.
- Heat exchange tubes: This is the core component of the evaporator, where the refrigerant flows inside to absorb heat from the environment. For small and medium systems using Freon refrigerants (R404A, R134a, R449A), the preferred material is 9.52 mm or 12.7 mm copper tube, with a thermal conductivity of up to 385 W/(m·K). Meanwhile, NH3 refrigeration systems large-capacity units usually use seamless steel tube for durability and chemical compatibility.
- Fins: These aluminium fins are 0.10 to 0.15 mm thick and increase the contact area for the evaporator. Fin pitch is calculated carefully, from 4 mm for freezer store installations up to 8 mm for chill rooms. Choosing the right fin pitch controls the rate of frost build-up and avoids icing so fast that it blocks airflow. To extend life in corrosive food environments, the aluminium fins usually carry a protective epoxy coating.
- Circulation fans: This part creates forced convection, letting the evaporator raise the heat transfer coefficient from 10 W/(m²·K) up to 60 W/(m²·K). The fan motors are specially sealed against moisture for sub-zero environments. This is the key factor in distributing cold air evenly in large-capacity cold storage.
- solutions Integrated defrost system: evaporatorTo sustain the working performance of the control cabinet and SCADA control panels to ensure the drain pan does not freeze and the condensate drains away completely.
How the evaporator works in the refrigeration cycle
Evaporator works on the principle of refrigerant phase change under low pressure. Technically, a refrigerant's boiling point is proportional to the surrounding pressure: as pressure falls, the boiling point falls with it. This is the core mechanism allowing the refrigerant to boil at deep sub-zero temperatures. In practice at -40°C when the pressure reaches around 1.43 bar (20.7 psi), this level of thermal energy is enough to hold the standard temperature for a freezer store installations at the level -25°C.
The heat exchange cycle at the evaporator runs as a closed loop through four consecutive stages:
- Intake stage: Refrigerant, in its liquid state after passing through the throttle valve, will enter the inlet. evaporator at extremely low pressure and temperature. This temperature is normally calculated to be lower than the store air temperature by 5°C to 10°C to create the necessary heat exchange differential.
- Heat absorption stage: Heat from the air inside the cold store passes through the aluminium fins, through the copper tube wall and directly into the liquid refrigerant inside. This is the stage where the evaporator performs its main cooling function for the storage space.
- Phase change stage: Once the refrigerant has absorbed enough heat, it begins to boil and change progressively to vapour along the length of the heat exchange tubing.
- Final stage: At the end of the coil, the refrigerant must reach saturated vapour or superheated vapour before being drawn back by the refrigeration compressor to continue the compression cycle.
In operation, the degree of superheat is regarded as the most important technical parameter of the evaporator. Superheat is the difference between the actual vapour temperature measured at the coil outlet and the theoretical boiling temperature at the corresponding pressure. On projects Tan Long has delivered, the standard superheat value is usually set at 5°C to 8°C. Controlling this figure closely ensures the refrigerant has fully evaporated before entering the compressor unit, completely preventing liquid slugging, which can seriously damage the valve and piston system.
Evaporator types in industrial refrigeration systems
Refrigeration engineers classify evaporators by four criteria: refrigerant feed principle, cooling medium, air convection method, and structural form.
Classification by refrigerant feed principle
This is the most important classification, directly affecting performance and system design.
| Criteria | Dry expansion evaporator (DX) | Flooded evaporator |
|---|---|---|
| Principle | A thermostatic or electronic expansion valve (TXV/EEV) holds the liquid level at 75–80% of tube length | A float valve holds the liquid flooded at 100% of the tube surface throughout |
| Heat transfer coefficient | 3,000 to 5,000 W/(m²·K) | 5,000 to 8,000 W/(m²·K), 20 to 30% higher |
| Control | Easy to operate, regulated by TXV or EEV | Needs a liquid separator and float valve, more complex |
| Suitable applications | Small and medium cold stores (under 500 m²), Freon systems | Seafood processing plants, large freezer stores (over 1,000 m²), NH₃ systems |
| Suitable refrigerants | R404A, R449A, R134a, R22 | NH₃ (R717), CO₂ (R744), large-scale Freon |
| Investment costs | 15 to 25% lower | Higher, but saves electricity over the long term |
From Tan Long's real installation experience, cold storage for preserving seafood of more than 1,000 metric tons usually uses flooded coils with NH₃ refrigerant, saving 15 to 20% of electricity compared with a DX system of the same capacity. Chill rooms of 200 to 500 m² for vegetables and fruit suit DX coils using R449A, which are simple to operate and maintain.
Classification by cooling medium
By cooling medium, evaporators fall into three main groups: air cooling evaporators, liquid cooling evaporators (also called chiller evaporators, for water or glycol solution), and direct contact evaporators, used in certain special freezing processes.
Condenser Air evaporators are the most common type in cold stores, freezer stores and air blast freezer, and industrial freezing cabinets. The chiller evaporator is used in glycol water chilling systems serving food processing plants, breweries and industrial processes needing precise water temperature control.
Classification by air convection method
Natural convection coils have no fans; the heavier cold air falls and the lighter warm air rises. This type has a low heat transfer coefficient and is used mainly in dry goods rooms sensitive to humidity swings, such as wine cellars or specialist pharmaceutical stores. Forced convection coils have one to four fans and are the
Classification by structural form
Fishbone Evaporator Herringbone evaporators have the heat exchange tubes staggered in a herringbone pattern, increasing the contact time between air and the cold surface, which suits seafood storage rooms needing high humidity. Bare tube evaporators are used in large-capacity NH₃ freezer stores, being easy to clean and corrosion-resistant. Plate evaporators contact freezer, in direct contact with the product for rapid freezing.
The role of the evaporator in an industrial refrigeration system
In a thermodynamic cycle, the evaporator is the single most important point of heat absorption. It can be said that every other component in the system operates in order to optimise the heat exchange that takes place here. Specifically, the refrigeration compressor maintains the low pressure so the refrigerant boils at the required temperature, the expansion valve drops the refrigerant pressure before it enters the coil, and the condenser rejects the absorbed heat to the outside environment.
The heat exchange efficiency of the evaporator has a direct and proportional effect on the business's economics. When the evaporator runs at high efficiency, the refrigerant's boiling temperature stays close to the air temperature inside the cold storage. This reduces the pressure difference the compressor has to handle, saving 3% to 5% of electricity for every extra 1°C of boiling temperature. Conversely, if the evaporator runs inefficiently because of thick ice or fouling, the compressor is forced to work at a lower suction pressure, wasting electricity and failing to hold the storage temperature in freezer store installations.
Calculating and selecting the wrong evaporator capacity leads to serious technical consequences:
- If the capacity is too small: if the coil cannot handle the heat load, the refrigerant will not evaporate fully. The excess liquid drawn back to the compressor causes liquid slugging, which can destroy the compression valves and pistons within a few hours of operation.
- If the capacity is too large: Installation evaporator an oversized unit not only raises the initial investment pointlessly but also drives up the superheat of the vapour returning to the compressor. This raises the end-of-compression temperature, wasting electricity and considerably shortening the system's life.
To keep the system running sustainably, coordinating evaporator and integrated refrigeration system design and installation is the prerequisite that Tan Long's engineering team focuses on in every project.
Criteria for choosing the right evaporator for a cold store
Refrigeration engineers base evaporator selection on four groups of parameters: cold store heat load, operating temperature, refrigerant type, and the storage environment conditions.
- Calculating the cooling load is the first step. Cold store heat load comprises four components: heat transmitted through the envelope (walls, ceiling, insulated panel floor), heat brought in by incoming product, heat generated by fan motors and lighting, and heat infiltrating through the door during loading and unloading. Total heat load is expressed in kW (or BTU/h, with 1 kW = 3,412
- Design Evaporating Temperature (evaporating temperature, Te) is usually set 8°C to 12°C below the cold store air temperature, depending on the application. For example, a freezer store held at -18°C typically uses a Te of -28°C to -30°C; a chill room at 2°C to 4°C uses a Te of -8°C to -10°C. This temperature difference directly affects the heat exchange surface area required and the system's energy efficiency.
- The type of product stored determines the fin pitch and the fin surface coating. Fresh frozen seafood needs a wide fin pitch (6 to 8 mm) because of its high moisture content and rapid icing. Pharmaceuticals and vaccines need coils with an antibacterial epoxy coating and tight humidity control. Vegetables and fruit need gentle convection (air speed below 2 m/s) so the product does not wilt.
Common industrial evaporator brands in Vietnam
The industrial cooling coil market in Vietnam currently has four main brand groups: European (Güntner of Germany, Alfa Laval of Sweden), Taiwanese (Kewely), Chinese (Zhongli, Wuxi) and domestically manufactured units.
Kewely evaporators (Taiwan)
Kewely is widely used in chill room and freezer store projects in Vietnam thanks to its flexible operating range from -35°C to 10°C and compatibility with a range of refrigerants (R22, R404A, R449A, R134a).
The two models Tan Long deploys most often on medium and large cold storage projects are the DL 21.0/105 and the DD 14.9/80. The DD 14.9/80 has a cooling capacity of 14.9 kW (50,867 BTU/h), runs on 380V/3-phase/50 Hz, has a 6 mm fin pitch and suits freezer stores from -18°C to -22°C with volumes of 150 to 300 m³. The DL 21.0/105 has a capacity of 21.0 kW
Zhongli evaporators (China)
The Zhongli range is 20 to 30% cheaper than Kewely, suiting small and medium cold storage projects or where investment budget is the priority. Two popular models are the DD-11.2/60 (11.2 kW, i.e. 38,222 BTU/h, dimensions 1,780 x 540 x 740 mm, 220V/single phase) and the DJ-1.2/8 (1.2 kW, i.e. 4,094 BTU/h, area
Real-world experience from Tan Long's projects shows Zhongli coils run reliably for the first 3 to 5 years with maintenance on schedule, suiting cold stores for agricultural produce and processed food in the 0°C to -18°C range. For projects requiring more than 10 years of life or in highly corrosive environments,
Evaporator maintenance and defrost: the correct technical procedure
Why cooling coils ice up, and the consequences of not defrosting in time
Ice forming on the fin surface is an inevitable part of cold store operation. When humid air inside the store contacts the coil surface at below 0°C, water vapour in the air condenses and freezes into a frost layer on the aluminium fins.
A 5 mm frost layer has a thermal conductivity of roughly 0.5 to 2.3 W/(m·K), far below aluminium (237 W/(m·K)), forming an unwanted insulating layer. When the ice exceeds 10 mm, airflow through the coil drops by 30 to 50%, heat exchange efficiency falls sharply and the store fails to reach temperature. Left without defrosting for a long period,
The defrost cycle is set at 2 to 4 times a day depending on humidity and the product stored. Fresh seafood stores with high humidity need defrosting 4 times a day; dry food stores or chill rooms for vegetables need 2 times a day.
The standard three-step defrost and evaporator cleaning procedure
Sustaining the performance of the evaporator is vital to optimising cold store operating cost. Below is the in-depth maintenance procedure Tan Long's team applies on real projects.
- Step 1: Pump down: Close the liquid feed valve at the coil inlet. Let the compressor keep running to draw all the remaining liquid refrigerant in the coil back to the receiver until suction pressure falls to around 0.5 bar (7.3 psi), then stop the compressor. This step prevents pressure
- Step 2: Defrost: Activate the defrost heater (or the hot gas valve). Defrost time is 20 to 45 minutes depending on ice thickness and heater capacity. During defrost, the store thermometer will rise temporarily by 2°C to 5°C, which is normal. The defrost termination thermostat is usually set to end the defrost when the coil temperature
- Step 3: Dry the coil and drain the oil: After the ice has melted, let the fans run for 5 to 10 minutes to dry the fin surfaces before restoring the liquid feed. Use a soft nylon brush to sweep dust off the fins; never use hard objects or metal wire, which bend the aluminium fins and permanently reduce the heat exchange area. If the dust is thick, use a light-pressure water jet (below 5 bar, i.e. 7
Oil draining is a quarterly maintenance task. Compressor lubricating oil partly dissolves in the refrigerant and accumulates gradually in the coil, occupying space inside the heat exchange tubes. Oil settling at 1% of the coil tube volume reduces heat transfer efficiency by roughly 2 to 3%. Drain the oil from the drain valve at the base of the liquid separator into the oil recovery vessel
Evaporator applications by industry in Vietnam
The choice evaporator must be based on the characteristics of each product type and the operating environment. Below is a summary of the typical technical standards applied on Tan Long's projects:
| Industry | Storage temperature | Recommended coil type | Refrigerant | Specific notes |
| Seafood freezer store | -18°C to -25°C | Flooded, 6 to 8 mm fin pitch | NH₃ or R449A | Epoxy coating against sea salt corrosion |
| Chill room for vegetables and fruit | 2°C to 8°C | DX, gentle convection, air speed < 2 m/s | R134a or R449A | Humidity controlled at 85% to 95% RH |
| Pharmaceutical store (GDP) | 2°C to 8°C | DX, fully redundant (100% standby) | R134a | Continuous temperature logging |
| Food processing | -5°C to -15°C | Flooded, or DX + EEV | NH₃ or R404A | Designed to support daily CIP cleaning |
| Air Blast Tunnel Freezers | -35°C to -40°C | Large-capacity DX, high-speed fans | R404A or R507A | Defrost 6 to 8 times a day |
| Port cold storage | -18°C to -23°C | Large-scale flooded NH₃ | NH₃ (R717) | NH₃ leak detection system mandatory |
Evaporator is the component that directly determines cooling effectiveness, product storage quality and total operating cost. Choosing the right type, calculating the capacity accurately and keeping to a strict maintenance cycle are the three key factors for a freezer store installations to run sustainably.
At Tan Long we not only supply evaporator to international standards but also integrate smart control technology through PLC and SCADA systems for real-time temperature monitoring. If your business needs in-depth advice on selecting equipment matched to your actual heat load, our engineering team is ready to analyse it and propose the best solution.
Frequently asked questions about evaporators
Why is the cooling coil running but the store not reaching temperature?
The three most common causes are: a thick layer of ice on the fins that has not been defrosted, an expansion valve that is stuck or incorrectly set (insufficient liquid feed), and a refrigerant leak reducing suction pressure. Check the suction pressure on the compressor suction gauge: if it is abnormally low against design, the cause is usually low refrigerant charge or a faulty expansion valve.
Should I choose a dry (DX) or flooded cooling coil?
Cold stores under 500 m² and Freon systems: choose DX, which is simpler and lower in investment cost. Large freezer stores above 1,000 metric tons, industrial-scale seafood processing plants, or NH₃ systems: choose flooded coils to optimise energy efficiency and reduce long-term electricity costs.
How long before an evaporator needs replacing?
The average life of a stainless steel or copper coil in a food cold store environment is 10 to 15 years with maintenance on schedule. Signs that early replacement is needed include: refrigerant leaks at coil tube welds that cannot be repaired, aluminium fins corroded across more than 30% of the area, and a marked drop in cooling capacity despite full cleaning and defrosting.
How does an evaporator differ from an evaporative condenser?
The evaporator sits on the low-pressure side, cooling the storage space by evaporating refrigerant to absorb heat. The evaporative condenser sits on the high-pressure side outside the building, using evaporating water to reject heat from the high-pressure compressed gas and turn it into high-pressure liquid. The two have entirely opposite positions, functions and principles within the refrigeration cycle.
Related pages
- What is a cassette evaporator? Construction, principle and how to choose
- Cold store condensers: construction, types and how to choose
- Horizontal Shell and Tube Condensers: Classification and Advantages and Disadvantages
- What is a condenser? Structure and operating principle
- evaporative condenser
Is your business in need of a survey, upgrade, or new design for your industrial refrigeration system?
With over 18 years of practical experience leading more than 29 major national projects, Tan Long's team of HVAC engineers is committed to providing accurate thermal load calculations, optimizing the COP efficiency coefficient, switching between natural refrigerants NH3/CO2, and saving 20–30% in operating electricity costs.
Engineer Nguyen Toan Thinh (Technical Director)
With over 18 years of practical experience in directing, consulting, designing, and constructing heavy industrial refrigeration systems, ASRS automated warehouses, and IQF freezing systems across more than 29 major national projects.
