Laboratory Refrigerators and Freezers – Principle, Parts, Applications

Laboratory refrigerators and freezers are special cooling instruments used in research, medical and scientific laboratories. They are used for storing temperature sensitive materials. Biological samples, vaccines, chemicals and reagents are preserved in them.

They are different from normal household refrigerators. Laboratory refrigerators and freezers are made for accurate and stable cooling. They have uniform cooling, safety alarm, lock system and continuous temperature monitoring.

Laboratory refrigerators generally work at 2°C to 10°C. They are used for materials which need cool temperature but not freezing. Laboratory freezers usually work at -10°C to -25°C.

Some materials need very low temperature for storage. DNA, tissue samples and mRNA vaccines are stored in ultra-low temperature freezer. These freezers can reach up to about -86°C.

ULT freezers use special cooling system. They may have multi-compressor cascade system or Stirling engine. This helps to stop or slow down molecular activity of stored materials.

Earlier laboratory cooling was done by simple ice based storage. Later it developed into advanced cooling systems. Now the units work by thermodynamic principle and electronic control.

The history of cooling is also related with refrigerants. First generation refrigerants used ammonia and carbon dioxide. These were natural substances but had toxicity, flammability and high pressure problems.

Second generation refrigerants were CFCs. They were stable and non-toxic, but later they were found harmful for ozone layer. Then third generation HFCs were used, which protected ozone layer but had high global warming effect.

At present, fourth generation refrigerants are being used. These are more environment friendly natural refrigerants like modern safe forms of propane and ethane. Modern laboratory refrigerators and freezers also have microprocessor control, digital display, remote monitoring and automatic alarm system.

Working Principle of Laboratory Refrigerators and Freezers

Working Principle of Laboratory Refrigerators and Freezers is based on removal of heat from the inside chamber. The heat is taken from the stored materials and internal air. Then this heat is released to the outside environment.

The working is mainly done by vapour-compression cycle. In this cycle, a refrigerant moves again and again through closed system. The main parts are compressor, condenser, expansion device and evaporator.

In the first step, the compressor takes low pressure and low temperature refrigerant gas. It compresses the gas by mechanical work. Then the gas becomes high pressure and high temperature gas.

This hot refrigerant gas goes to the condenser coils. In condenser, the heat is released to the surrounding room air. The gas becomes high pressure liquid after cooling.

Then the liquid refrigerant passes through expansion device. Here the pressure suddenly decreases. Due to this expansion, the temperature of refrigerant also becomes very low.

The cold refrigerant then enters into the evaporator coils present inside the storage chamber. It absorbs heat from the chamber air and from the stored laboratory samples. During this process, refrigerant changes again into gas.

The cooled air is spread inside the cabinet by internal fans. This helps to maintain uniform temperature in all parts of the refrigerator or freezer. Then the refrigerant gas goes back to the compressor and the cycle is repeated continuously.

In Ultra-Low Temperature (ULT) freezer, very low temperature is needed. So one refrigeration cycle is not enough. These freezers generally use cascade refrigeration system.

In cascade system, two vapour-compression systems work together. The low temperature stage removes heat from the cabinet. Then this heat is transferred to the high temperature stage, which releases it to the outside environment.

Some modern ULT freezers use free-piston Stirling engine. In this system, helium gas is compressed and expanded again and again by piston and displacer. This produces cooling without using normal compressor system.

Parts/Components of Laboratory Refrigerators and Freezers

The main parts of laboratory refrigerators and freezers are as follows-

A. Core Refrigeration System Components

1. Compressor
   Compressor is used to pump the refrigerant through the system. It starts the cooling process. In Ultra-Low Temperature (ULT) freezers, two compressors may be used together in cascade system.
2. Condenser coils
   Condenser coils are present for removing heat from the refrigerant. Here the hot vapour refrigerant releases heat to outside air. Then it changes into liquid form.
3. Evaporator coils
   Evaporator coils are present inside the cooling chamber. It absorbs heat from the inside part of refrigerator or freezer. This makes the chamber cold.
4. Expansion device
   It may be a valve or capillary tube. It controls the flow of refrigerant. It also decreases the pressure of refrigerant and helps in cooling.
5. Heat exchangers
   Heat exchangers are used for transfer of heat. In cascade freezers, plate heat exchangers are used between high stage and low stage system.
6. Refrigerants
   Refrigerants are cooling fluids used in the system. They circulate through compressor, condenser, expansion device and evaporator. Hydrocarbons, propane and ethane may be used as environment friendly refrigerants.

B. Alternative Cooling Components

1. Free-piston Stirling engine
   It is used as an alternative cooling system in some freezers. It has moving piston and displacer. It can produce very low temperature without normal compressor.
2. Thermosiphon
   Thermosiphon is a sealed copper tube. It is connected with Stirling engine and passes around the cabinet interior. It absorbs heat from chamber and carries it away.

C. Cabinet and Structural Components

1. Outer cabinet
   Outer cabinet is the strong outer covering of the refrigerator or freezer. It is usually made up of heavy gauge steel. It protects the inner parts.
2. Inner chamber
   Inner chamber is the storage space where samples are kept. It is made up of stainless steel or seamless polypropylene. It is easy to clean.
3. Insulation layer
   Insulation layer is present between outer cabinet and inner chamber. It may be made up of polyurethane foam or vacuum insulation panels. It prevents entry of outside heat and keeps cold inside.
4. Door gaskets and seals
   Door gaskets are rubber strips present around the door. They make airtight sealing. It prevents the escape of cold air from inside.
5. Fans
   Fans are used for air movement. Condenser fan cools the compressor and condenser area. Evaporator or circulation fan spreads cold air uniformly inside the chamber.
6. Shelves, drawers and baskets
   These are present inside the chamber. They are used for keeping samples, reagents and materials in arranged way. They may be adjustable.
7. Casters or levelers
   Casters are wheels present at the bottom. Levelers are adjustable legs. They help in movement and proper balancing of the unit.

D. Control, Monitoring and Security Components

1. Thermostat or microprocessor controller
   It controls the temperature inside the unit. It starts or stops the cooling system according to the set temperature. It is the main control part.
2. Digital display panel
   It is present outside the unit. It shows the temperature and other settings. It may be LCD or touchscreen type.
3. Temperature sensors
   Temperature sensors measure the inside temperature. Thermistors or buffered probes are used. These may be kept in glycol or glass beads to show sample like temperature.
4. Alarm system
   Alarm system gives warning when temperature goes out of range. It also works during power failure or if door remains open. It gives sound and light signal.
5. Digital data logger or temperature recorder
   It records the temperature continuously. It is used for quality control and record keeping. It also helps during audit and compliance work.
6. Door locks
   Door locks are used for safety of stored materials. It may be key lock or keyless lock. It prevents entry of unauthorized person.
7. Sensor ports
   Sensor ports are small holes in the unit. They are used for passing external monitoring cables or instruments into the chamber.
8. Internal electrical outlets
   Some refrigerators have electrical outlets inside the chamber. These are used for giving power to instruments kept inside. It is common in chromatography refrigerators.

E. Emergency Backup Systems

1. CO₂ or LN₂ backup system
   This system is used when mechanical cooling fails. Liquid carbon dioxide (CO₂) or liquid nitrogen (LN₂) is released into the freezer through solenoid valve and nozzle. It helps to maintain freezing temperature for some time.
2. Rechargeable battery backup
   Battery backup gives temporary power during power failure. It keeps alarm and temperature monitoring system working. This helps to protect the stored samples.

Types of Laboratory Refrigerators

Types of Refrigerator

The types of laboratory refrigerators are as follows-

1. Household or domestic refrigerators
   Household refrigerators are sometimes used in laboratory. They are used only for storing non-flammable aqueous solutions. They are not much recommended for laboratory use because inside electrical parts may ignite flammable vapours.
2. Flammable material storage refrigerators
   These refrigerators are used for storing flammable or explosive materials. They have spark-free inner chamber. Internal switch, light or thermostat are not present inside, because these may produce spark.
3. Explosion-proof refrigerators
   Explosion-proof refrigerators are used in hazardous location laboratories. These are used where the surrounding air may contain explosive vapour. They have spark-free inside and outside parts. Motors are enclosed and the unit is directly hard-wired to power source.
4. Standard laboratory refrigerators
   Standard laboratory refrigerators are used for routine laboratory storage. Reagents, cell culture media, proteins and non-critical biomedical samples are stored in it. It is used for daily laboratory work.
5. Vaccine and pharmacy refrigerators
   These refrigerators are made for storing vaccines and pharmacy materials. They maintain accurate temperature as required by CDC and WHO standards. They may have glass doors and adjustable basket drawers.
6. Blood bank refrigerators
   Blood bank refrigerators are used for storing blood and plasma. They have fast temperature recovery. They are made for heavy liquid load like blood bags. They also have strict temperature recording system.
7. Chromatography refrigerators
   Chromatography refrigerators are used for keeping chromatography instruments inside the cold chamber. They have internal electrical outlets and access ports. This helps to operate the instrument inside the refrigerator.
8. Refrigerator and freezer combinations
   These units have both refrigerator and freezer compartments. They are used when cooling and freezing storage both are needed in one laboratory-grade unit.
9. Compact refrigerators
   Compact refrigerators are small size refrigerators. They are used where space is less. They may be built-in, countertop or under-counter type and are useful in small laboratories.
10. Manual defrost refrigerators
    Manual defrost refrigerators need manual removal of ice. Ice forms on the coils and user has to remove it. It needs more maintenance.
11. Auto defrost refrigerators
    Auto defrost refrigerators remove ice automatically. They use heating element for melting ice from coils. It is easier to maintain than manual defrost type.

Types of Freezer

The types of freezer are as follows-

1. Ultra-Low Temperature (ULT) freezer
   ULT freezer is a high performance freezer. It maintains very low temperature between -40°C to -86°C. It uses multi-compressor cascade system or Stirling engine. It is used for long term storage of important biological samples.
2. Upright freezer
   Upright freezer is vertical type freezer. It has front opening door. It has shelves inside for keeping samples in arranged way. Samples can be taken out easily.
3. Chest freezer
   Chest freezer is horizontal type freezer. It opens from the top. It keeps cold air inside for longer time. It is used for bulk deep-freeze storage.
4. Undercounter and countertop freezer
   These are compact freezers. They are used where space is less. Undercounter freezer is kept below laboratory bench and countertop freezer is kept on the counter.
5. Flammable material storage freezer
   This freezer is used for storing volatile liquids and flammable materials. It has spark-free and flame-resistant inner chamber. It prevents internal ignition.
6. Explosion-proof freezer
   Explosion-proof freezer is used in hazardous location. It has spark-free inside and outside parts. It is used where outside atmosphere may contain explosive vapour. It is hardwired with power source.
7. Blood bank and plasma freezer
   This freezer is used for storage of blood and blood plasma. It maintains required freezing temperature. It keeps blood products safe for use.
8. Pharmacy and vaccine freezer
   This freezer is used for storing vaccines and pharmaceutical products. It maintains temperature between -50°C to -15°C. It helps to protect the activity of vaccines and medicines.
9. Liquid nitrogen or cryogenic freezer
   Cryogenic freezer uses liquid or vapour phase nitrogen. It can reach very low temperature up to -196°C. It stops cellular degradation and is used for long term specimen storage.
10. Manual defrost freezer
    Manual defrost freezer needs removal of frost by user. Ice collects inside and it has to be removed manually. It needs more maintenance.
11. Auto defrost freezer
    Auto defrost freezer removes frost automatically. It uses heating cycle for melting ice. It is easier to maintain than manual defrost freezer.
12. Combination refrigerator/freezer
    This unit has both refrigerator and freezer compartments. It is used when cooling and freezing storage both are needed in one laboratory device.

Laboratory Refrigerators and Freezers Operating procedures

1. Food and drinking materials should not be stored in laboratory refrigerators and freezers. These units are used only for laboratory materials. It should not be used for human food storage.
2. Before storing chemicals, compatibility should be checked. Different chemicals should not be kept together if they can react with each other. This prevents leakage, reaction and hazard.
3. All containers are closed tightly before keeping inside. Loose covering like aluminium foil, cork and ungreased glass stoppers should not be used. Screw cap bottles are used and the cap is tightened properly.
4. The screw cap may be wrapped with parafilm. The container can also be kept inside plastic bag. This helps to prevent leakage and contamination.
5. Volatile chemicals should be allowed to come at room temperature before sealing. If sealed when cold, pressure may build up inside the container. This may cause leakage or breakage.
6. Secondary containment should be used for liquid chemicals. Plastic trays or catch pans are kept on the shelves. If leakage occurs, the liquid remains inside the tray.
7. An updated inventory should be kept outside the refrigerator door. It shows what materials are stored inside. This helps in easy checking and safe handling.
8. Only required amount of chemicals should be stored. Unnecessary excess chemicals should not be kept for long time. Some stored compounds may degrade with time.
9. Biological materials like vaccines should be kept in original packing. They should be protected from light. Older stock or nearest expiry stock is kept in front for first use.
10. The outside of the unit should be labelled clearly. Labels like No Food or Drink, Chemical Storage Only or Not For Flammable Storage should be written as required.
11. Every stored container should be labelled. The label should contain name of material, owner name or initials, date of preparation or receiving and hazard warning.
12. Water resistant label and ink should be used. Normal ink may be removed by moisture or condensation. Then information may be lost.
13. Flammable liquids should not be stored in ordinary refrigerator. Liquids having flash point below 100°F or 37.8°C are kept only in UL-approved flammable storage or explosion-proof refrigerator.
14. Standard domestic refrigerator should not be used for flammable chemicals. Internal spark may cause fire or explosion.
15. The refrigerator or freezer should be connected to grounded outlet. Extension cord should not be used. Direct and safe power connection is needed.
16. The unit should not be placed near laboratory exits. Exit path should remain open. This is needed during emergency.
17. Minimum and maximum temperature should be recorded at the start of each working day. This shows whether the unit is working in proper temperature range or not.
18. The unit should be cleaned and defrosted regularly. Manual defrosting is done at least once in a year or when excess ice is formed.
19. Frost-free refrigerators should be avoided for chemical storage. Drain tube and heating parts may create spark hazard.
20. Door seals and gaskets should be checked regularly. If seal is damaged, cold air may leak out. Secondary containment trays should also be inspected.
21. During power failure, extra care should be taken. Temperature inside may rise quickly. Chemicals may decompose and toxic or flammable vapours may form.
22. After power failure, the door should be opened carefully. Any unusual smell should be noticed. This may indicate chemical leakage or vapour formation.
23. If spill occurs inside the unit, all materials are removed first. The unit is fully defrosted before cleaning. Interior is cleaned with suitable solvent or 10% bleach solution for biological spill.

Applications of Laboratory Refrigerators and Freezers

• Laboratory refrigerators and freezers are used for storing biological samples. DNA, RNA, cell lines, stem cells, embryos and tissues are kept in them.
• These are used in research, diagnosis and biobanking. The samples are kept for long time. It prevents damage and degradation of samples.
• They are used for storage of blood and plasma. Whole blood, platelets and plasma are kept in hospitals and blood centres. It helps to maintain the quality of blood materials.
• They are used for storing vaccines and medicines. Insulin, chemotherapeutic drugs, live-virus vaccines and mRNA vaccines are kept at required temperature.
• Proper cold storage helps to keep the activity of vaccines and drugs. The storage is done according to CDC and WHO guidelines.
• In biopharmaceutical manufacturing, they are used for cold storage. Biologics, protein drugs and large molecule therapies are kept during production.
• They are used for storage of laboratory chemicals and reagents. Enzymes, cell culture media, electrophoresis buffers and staining solutions are kept in them.
• Flammable and hazardous chemicals are stored in special refrigerators. Spark-free or explosion-proof refrigerators are used for volatile and explosive chemicals.
• In chromatography work, special refrigerators are used. The chromatography instrument can be kept inside the cold chamber. It helps in temperature sensitive testing.
• They are used for clinical and medical supplies. Testing kits, breast milk and other delicate medical materials are stored in cold condition.
• In forensic laboratory, they are used to store biological evidence. The evidence is kept safely before testing. It prevents degradation of forensic samples.

Advantages of Laboratory Refrigerators and Freezers

• Laboratory refrigerators and freezers maintain accurate temperature. The temperature remains stable inside the chamber. This is needed for vaccines, DNA, reagents and other sensitive materials.
• They have microprocessor and digital thermostat. These parts control the cooling properly. So sudden temperature change is reduced.
• They recover temperature quickly after door opening. Forced air circulation helps to bring back the set temperature. This protects the samples from heat effect.
• They have alarm system and temperature recorders. Alarm gives warning during power failure, open door and out of range temperature. This helps to save valuable samples.
• Digital data logger records the temperature continuously. It helps to keep proper record. It is useful for quality control and laboratory audit.
• They have door locks for safety. Only allowed persons can open the unit. This protects costly samples and restricted medical materials.
• Some units have key operated temperature setting. It prevents unwanted change in set temperature. So the stored materials remain safe.
• Special laboratory refrigerators are safer than domestic refrigerators. Flammable storage refrigerators have spark-free inside parts. Explosion-proof refrigerators have spark-free inside and outside parts.
• They are useful for storing volatile and explosive chemicals. These units reduce chance of ignition and fire hazard.
• Many units have battery backup. During power failure, alarm and monitoring system can still work. Some freezers also use LN₂ or CO₂ backup for maintaining low temperature.
• Modern units save more energy. They may use variable speed compressor, Stirling engine and natural refrigerants like propane or ethane. This reduces electricity use and environmental damage.
• They are strong and durable. Heavy gauge steel, thick insulation and magnetic door gasket are used. These prevent cold air from escaping.
• The inner chamber is easy to clean. Seamless inner surface helps in proper cleaning. This is useful for laboratory hygiene.
• They are useful for long term sample preservation. Low and stable temperature slows down cellular degradation and bacterial growth. This gives reliable samples for diagnosis, clinical trial and research work.

Limitations of Laboratory Refrigerators and Freezers

• Laboratory refrigerators and freezers use more electricity. Ultra-Low Temperature (ULT) freezers consume very high power. So running cost becomes high.
• These units release heat into the room. For this, good electrical supply and HVAC system is needed. If room cooling is poor, the surrounding area may become hot.
• Mechanical parts may wear out with time. Cascade refrigeration system has many moving parts. Oil used in the system may move into tubes and cause oil clogging.
• Stirling engine system also has moving parts. Springs and other parts work continuously. So wear and tear can occur slowly.
• Power failure is a major problem. If electricity stops or cooling system fails, the inside temperature starts increasing. Sensitive and costly samples may get damaged.
• Backup system is needed for safe storage. LN₂ or CO₂ backup system may be used. But these systems are costly and need maintenance.
• Older freezers may use HFC refrigerants like R-404A or R-508B. These have high global warming potential. So they are harmful for environment.
• Standard household refrigerators are not safe for chemical storage. They have light switch, defrost heater and thermostat inside. These may produce spark and ignite flammable vapours.
• Natural refrigerants also have some problems. Ammonia is toxic and corrosive to copper. Hydrocarbons like propane are flammable. Carbon dioxide works at very high pressure.
• Temperature may not remain fully constant in some freezers. Compressor starts and stops again and again. This creates small up and down change in temperature.
• These units need regular monitoring and calibration. For medical and laboratory standards, proper temperature record is needed. IQ, OQ and PQ validation may also be required.
• Manual defrost models need frequent cleaning. Ice buildup should be removed. If ice remains, cooling efficiency becomes low.
• Initial cost is high. Medical-grade refrigerators, freezers and cloud monitoring systems are costly. So small laboratories may face difficulty.
• Skilled technician is needed for installation and repair. If technician is not available, maintenance becomes difficult. Calibration and repair may also take more time.

Precautions

• Food and drinks should not be kept in laboratory refrigerators and freezers. These units are only for laboratory materials. Label should be written outside like No Food or Drink or Chemical Storage Only.
• Flammable chemicals should not be kept in ordinary refrigerator. Domestic refrigerators have thermostat, light switch and other spark producing parts. These may ignite vapours and cause explosion.
• Chemicals having flash point below 100°F or 37.8°C should be stored in proper unit. UL-approved flammable material storage refrigerator or explosion-proof refrigerator is used for them.
• All containers should be sealed properly before storage. The cap should be tight and the container should be kept in stable position. Aluminium foil, cork and ungreased glass stopper should not be used as cap.
• Screw top containers should be closed properly. They may be wrapped with parafilm or kept inside plastic bag. This helps to prevent leakage.
• Liquid chemicals are kept in secondary containment. Plastic trays or secondary containers are placed on shelves. If leakage occurs, the liquid remains inside the tray.
• All stored materials should be labelled and dated. The label should contain name of material and other needed information. Updated inventory is kept outside the door.
• Only compatible chemicals are kept together. Chemicals which can react with each other should not be stored side by side. Only required amount should be stored.
• Chemicals should not be stored for very long time without checking. Some compounds may degrade slowly. They may also form hazardous decomposition products.
• Frost-free refrigerators should be avoided for chemical storage. Their defrost heater may make spark. Drain tube may carry spilled chemical near compressor area.
• The unit should be grounded properly. Extension cord should not be used. It should be connected with safe and permanent power source.
• The refrigerator or freezer is kept in well ventilated place. It should not be placed near laboratory exit. Exit path should remain free for emergency.
• Warning label should be placed near plug and circuit breaker. Do Not Unplug or Do Not Stop Power to Circuit Breaker should be written. This prevents accidental power loss.
• Temperature should be monitored continuously. Digital data logger (DDL) with buffered probe is used. Probe may be kept in glycol, so it shows temperature like stored materials.
• Air thermometer alone should not be depended always. Air temperature changes quickly when door is opened. Buffered probe gives more proper reading of stored materials.
• The unit should be cleaned and defrosted regularly. Manual defrost is done at least once in a year or when ice is more. Ice buildup reduces cooling efficiency.
• Door seals and magnetic gaskets should be checked. If seal is loose or broken, cold air escapes. Grounding connection should also be checked.
• In units having gas backup system, hoses and connections should be inspected. It is checked about every 30 days. Leakage or loose connection should not present.
• After power failure, the unit should be opened carefully. Inside temperature may increase and vapours may form. Unusual smell or vapour should be noticed.
• If spill occurs inside the unit, all materials are removed first. The unit is defrosted before cleaning. Cleaning method is selected according to chemical, biological or radiological spill.
• For biological spill, 10% bleach solution may be used. For chemical spill, suitable compatible cleaning solvent is used. Safety precautions should be followed during cleaning.

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