Temperature Controlled Systems: Refrigerators and Freezers
Have you ever completed the temperature mapping / qualification testing for a walk-in refrigerator or freezer and felt good about the data? That is until someone asked you to relate what the numbers mean in relation to the system and what was happening inside the unit. You knew the results met the acceptance criteria, but maybe weren’t so sure how they related to system performance.
Cooling and Heating…How Does It Work?
All Temperature Controlled Units (TCUs) have five main components:
- Microprocessor
- TCU Engine
- Compressor
- Condenser
- Evaporator
An area containing all of the main components listed above with the exception of the evaporator is located outside the controlled environment space (e.g. outside the cold room).
The compressor pumps refrigerant through the temperature-controlled system. It works with the engine to increase pressure triggering the refrigerant to change phase from a gas to a liquid. Due to the phase change the refrigerant will heat up during compression. The liquid refrigerant is then pumped to the condenser. The condenser removes the heat absorbed by the refrigerant from the compressor and temperature-controlled area. As the heat is being absorbed the refrigerant approaches thermal equilibration with the ambient air outside the controlled environment space. Once the refrigerant has been cooled to a liquid it flows through an expansion valve into the evaporator. The evaporator then transfers heat out of or into the controlled environment space to control the area temperature. The refrigerant changes back to a gas as it absorbs heat. The heated refrigerant then flows back through the compressor and the cycle starts over. As you might have guessed, the heating process of a refrigeration unit is usually the opposite of the cooling process. The evaporator becomes the condenser and vice versa.
What about the defrost cycle you ask?
The defrost cycle is used to prevent the system from forming ice. When and how often the defrost cycle is used is based on measurements of the coils temperature at given time intervals. A timer closes the circuit which then allows air to flow to a temperature sensor. If the temperature falls within the temperature range that would allow the formation of ice, the unit will start a defrost cycle. Typically, an electric heater is used to defrost the evaporator. Obstruction of airflow due to evaporator icing may also trigger the defrost cycle to begin.
Parting Thoughts
An actively temperature-controlled system is designed to maintain the air temperature within a defined space, thereby insulating the stored goods from the effects of outside conditions. To achieve this, the air inside the system must be able to flow throughout the area being controlled. If your data is meeting acceptance criteria then great, everything is probably fine. But if it’s not then your issue probably lies with a critical design parameter such as: sufficient capacity for heat exchange, proper airflow, thermal integrity, and air velocity. All of these are as crucial to a temperature control system, including monitors and alarms, as they are to the refrigeration process.
Written By: Nathan Roberge, Consultant III
Reference:
- Active Temperature-Controlled Systems: PDA Technical Report No. 64; 2013
