In the market for a used ULT freezer? Here are 6 things to consider before you buy.
While the vast majority of ultra-low temperature (ULT) freezers use dual-stage cascade refrigeration systems, freezers driven by Stirling engines have been capturing more of the ultra-cold storage market. Stirling freezers are notably more efficient, compact, and simpler than traditional cascade freezers. They're not without drawbacks, however — for one thing, finding used Stirling freezers can be a challenge. You might have to drop the cash on a new unit.
If you're deciding between a cascade compressor freezer and a Stirling freezer, then you may want to consider the information below.
How do dual-stage cascade compressor ULT freezers work?
Dual-stage cascade ULT freezers contain two refrigeration loops, or "stages". One loop cools the other, which in turn allows the second loop to absorb enough heat from the chamber that it can reach ultra-low temperatures. The first, or "high", stage absorbs heat from the second, or "low", stage via a heat exchanger. After being cooled in the heat exchanger, the low-stage refrigerant then flows through the evaporator to absorb heat from the chamber. Here's a more thorough explanation of how these freezers work.
While this technology is very effective, it still has room for improvement. As you'll read below, Stirling engine freezers address many of the problems plaguing dual-stage cascade freezers. Before covering that, though, let's take a look at the pros and cons of cascade compressor ULT freezers
Benefits of cascade compressor ULT freezers
Nearly all of the benefits of these freezers can be derived from one simple fact:
Dual-stage cascade ULT freezers are much more common than Stirling engine freezers.
This competition between freezer manufacturers helps drive down prices, gives space for third-party resources, and allows for a wide variety of models and configurations.
Since cascade freezers are far more commonplace than Stirling freezers, it's easier to find used models available for less than new units. If you're trying to stretch your budget, you'll have a easier time doing so with a cascade freezer.
There are far more avenues to find product literature and support for these systems than Stirling engine freezers because they're so common. If you want to save money, going with third-party options for support can be considerably cheaper than having the manufacturer send a service technician out to your facility.
Upright, chest, undercounter, dual-door...there is a lot of variance among the hundreds of models of cascade ULT freezers. Stirling freezers, on the other hand, are only sold by one or two companies with narrower product offerings.
Having been in use for over 30 years, cascade freezers have been proven time and time again. You know what to expect with these systems.
Drawbacks of dual-stage cascade ULT freezers
Dual-stage cascade freezers aren't great for the environment, and their power requirements aren't great for your budget.
Cascade freezers have a lot of parts, which means that they also have a lot of potential failure points. This makes troubleshooting very difficult and time-consuming. Potentially, you may also have to spend more money replacing parts over the years than you typically would with a Stirling engine freezer.
Because they have two refrigeration loops, cascade freezers take up a lot of space. If your lab is especially tight on free space, squeezing in a cascade freezer could be problematic for you.
How do Stirling engine ULT freezers work?
Stirling engines are closed-cycle engines that use heat differential to move pistons, which in turn crank the flywheel and drive the engine. Here is a very detailed explanation of how they work.
In ULT freezers, however, these engines work a bit differently than a standard Stirling. We recommend reading Stirling Ultracold's explanation of their engine design to learn more about how Stirling ULT freezers work.
These engines are suited particularly well for ULT freezing because they work best in settings that require a mostly consistent throughput. They're not built for "stop and go", varying performance like automobile engines are. Thus, keeping a chamber at a set temperature is an excellent usage of Stirling technology.
Benefits of Stirling engine ULT freezers
According to some studies, Stirling engine freezers "promise to reduce energy consumption of ULT freezers by 50% or more." Not only will this make your lab more environmentally-friendly, it will also save you quite a bit of money in the long run.
With far fewer parts than cascade compressor units, a Stirling engine ULT freezer is less prone to failure and, if it does, will be easier for you or a service technician to troubleshoot.
Because they have so few parts, Stirling engine systems can be condensed into very compact shells and save you valuable lab space.
Drawbacks of Stirling engine ULT freezers
Given that there are only a handful of Stirling freezer manufacturers, most notably Stirling Ultracold, it follows that there is less variation in the models available for purchase. If you require a particular capacity that is not offered by any current Stirling freezers, you may have to go with a cascade compressor system.
It's no secret that buying pre-owned equipment can be considerably more cost-effective than buying new equipment. Unfortunately, used Stirling freezers aren't very common on the used market. If you're looking to save as much money as possible upfront, then purchasing a used Thermo Revco or VWR cascade freezer is likely your best route.
While all signs point to Stirling engines being a major step forward in ultracold storage tech, they're still quite new and relatively unproven. If you want to err on the side of caution, then picking up a cascade freezer from a reputable brand like Thermo Revco could be the best route to go down.
Which is right for you?
While you may pay more for a Stirling freezer upfront, it won't cost as much to run as a cascade freezer of equal size would. Additionally, if you're trying to be mindful of your lab's environmental impact, a Stirling freezer would help lighten your carbon footprint.
If, however, you have a very limited budget or simply can't find a Stirling freezer that fits your desired specifications, then purchasing a used cascade compressor is your best bet.
Have questions about your ULT freezer? Contact us at firstname.lastname@example.org or (567)-221-0615 for assistance.