How Do Ultra-Low Temperature Freezers Work?
There has been a lot of attention focused on ultra-low temperature freezing in recent months thanks to the release of COVID-19 vaccines. However, there aren’t many intuitive, detailed guides as to how ULT freezers achieve such low temperatures. Whether you’re a lab manager, technician, or just a curious reader, we aim to help you understand the tech behind these machines. Below we’ll dive into how conventional freezers work, then build off that knowledge to demonstrate how ULT freezers work. Accompanying each explanation is a diagram summarizing the process.
If you're looking to buy a ULT freezer, we have a variety of options available.
Understanding how a two-stage refrigeration system works is much easier if you first understand how a single-stage system works. The two configurations share most of the same components and refrigerants flow through them in very similar ways, but single-stage systems are easier to wrap your head around if you are a refrigeration novice.
Single-stage systems are used in freezers that only need to reach between -20C and -40C or so. The freezers in your home, restaurants, and the like use single-stage systems. Let’s dive into how they work by tracing the path that their refrigerant flows through, starting with the compressor.
Refrigerant Flow in Single-Stage Systems
The compressor, as its name implies, compresses gaseous refrigerant to drive up the pressure. It’s this pressure that keeps refrigerant flowing through the system. It’s important to note that increasing the pressure exerted on a fluid (gas or liquid) will also increase its temperature accordingly, as Amonton’s law states. Compressors are found in nearly all refrigeration systems.
After being pressurized by the compressor, the gaseous refrigerant streams into the tubes comprising the condenser. As it winds through the coils, the condenser’s fins absorb heat from the refrigerant and dissipate it. By the time the refrigerant exits the condenser, it has phased back into a liquid state.
Now on its way to cool the freezer cavity via the evaporator, the refrigerant passes through the filter drier. This component filters out contaminants like particles of metal, dirt, moisture, and even certain acids. Filtration allows the refrigerant to work more effectively when it enters the evaporator.
As it enters the evaporator, the refrigerant flows through a capillary tube. This tube is narrower than the rest of the tubing, which allows it to modulate the flow rate of the refrigerant through the evaporator.
(Here’s the exciting part, kids!) Now chilled and filtered, liquid refrigerant begins its path through the evaporator. Typically, the evaporator consists of piping crossing the height and width of the chamber’s exterior. As cold refrigerant streams through the pipes, it absorbs the heat trapped in the chamber and evaporates, thus carrying away the heat and leaving the chamber cold.
After vacating the evaporator, the refrigerant, which is now primarily gas and possibly some liquid, is collected in an accumulator. The accumulator prevents liquid refrigerant from entering the compressor. Liquids can seriously damage compressors, so the accumulator plays a vital role.
Upon exiting the accumulator, gaseous refrigerant once again enters the compressor, thus cooling it and beginning the process anew.
Two-Stage Cascade Refrigeration
Now that you understand how single-stage systems cool regular freezers, it shouldn’t be too difficult to wrap your head around two-stage cascade systems. These systems are commonly explained as two single-stage loops working in tandem, which is an apt description. Refrigerant flows through most of the same parts, and in the same order, as single-stage systems. However, there is one key component introduced in two-stage setups: the interstage heat exchanger.
Recall that in single-stage freezers, refrigerant flows from the condenser, through the capillary tube, and into the evaporator, at which point it absorbs heat from the chamber. In two-stage freezers, however, the refrigerant of the first stage serves to cool the refrigerant of the second stage. This is accomplished thanks to the interstage heat exchanger, which is a wide metal tube or box through which both first- and second-stage refrigerant flow via separate pipes.
Below is a rundown on how refrigerant flows through both stages of a two-stage cascade system.
Gaseous refrigerant is pressurized in a compressor, after which it flows into a condenser
The condenser cools the refrigerant, causing it to condense.
Now a liquid, the refrigerant is filtered by the filter drier before entering the capillary tube.
The narrow capillary tube regulates the flowrate of the refrigerant into the interstage heat exchanger.
Once in the interstage heat exchanger, the first-stage refrigerant absorbs heat from the second-stage refrigerant. The refrigerants do not mix; heat is transferred across the tubes.
The refrigerant passes through the accumulator back into the compressor, where the process begins again.
As the first-stage refrigerant follows that loop, the second-stage refrigerant follows its own loop:
- The second-stage refrigerant is pressurized by the second-stage compressor, after which it enters the oil separator.
- The oil separator removes oil from gaseous refrigerant. Oil becomes very viscous at ultralow temperatures, which may cause it to clog pipes. Additionally, removing the oil increases the volume of refrigerant flowing through the evaporator so it will absorb more heat than otherwise.
- Upon leaving the oil separator, second-stage refrigerant enters the interstage heat exchanger where it's cooled by the first-stage refrigerant flowing past it in a separate pipe.
- The second-stage refrigerant, now condensed into a liquid, is filtered by the filter drier and then flows into the evaporator.
- As it goes through the evaporator, the ultra-cold refrigerant absorbs heat from the freezer chamber and begins phasing back into a gas.
- Liquid and gas refrigerant are collected into the accumulator. As the gaseous refrigerant passes through, the liquid refrigerant is held in a reservoir, where it gradually evaporates and continues to the compressor.
An ultra-low temperature (ULT) freezer works by compressing gaseous refrigerant, cooling it to a liquid state in the condenser, and then evaporating it in the evaporator to absorb heat from the freezer chamber. This process is repeated to maintain low temperatures. In a two-stage ULT freezer, an interstage heat exchanger is used to cool the refrigerant of the second stage with the refrigerant of the first stage.
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