The phrase pairing the two words freeze and dry is one of the more straightforward and clear descriptions of a process in industry. To freeze dry a blueberry, an ice cream sandwich, the latest-greatest-pharmaceutical or a formaldehyde-infused complex carbon chain; one must first freeze the liquid water (or other liquid compound) into solid ice that is integrally bound within the food or other moist substance.
Next, the material must be dried. That is, the constituent part of the thing must have all the moisture removed from it, just as if blow drying a freshly showered mane. The tricky part is, of course, that the moisture is now embedded within the substrate as a frozen ice brick, and it must remain coldly solid even while it is being dried away from the remaining core stuff in order to preserve the molecular pith of the desired final product.
It turns out that freeze drying anything at all, despite the straightforward word pairing, is pretty darn difficult to pull off.
We view the molecular compound which combines two parts hydrogen to one part oxygen (H2O) as either solid or liquid since those are the two states in which our eyeballs allow us to see ice or water, respectively. Though it is, rarely do we consider transparent and ethereal steam, the vapor phase of H2O, as being equally prevalent in our lives.
The key to freeze drying is sublimation (sub-lah-MAY-shun). And the key to sublimation, that is the phase change of a solid directly into a gas vapor without going through the liquid phase, is pressure; or more aptly, the lack of pressure. In the case of H2O, it takes a low pressure, or at least what we humans here on Earth think of as low pressure, somewhere on the order of a one-thousandth of an atmosphere (~1 mbar) or lower, to sublimate ice directly into steam.
Vacuum Chamber to Pump: Decreasing Pressure with Undulating Temperature
Let’s put some frozen strawberries on a tray and place them in a vacuum chamber. Once the embedded ice starts to sublimate when we bring down the chamber pressure to 1 mbar; the solid water will begin to get even colder than frozen. This is because, just like a pot of water boiling on the stovetop, it takes sustained heat energy to drive the sublimating phase change. So, our freeze-drying vacuum chamber also needs some heaters to sustain the temperatures in order to maintain sublimation.
Water expands by a factor of about 1600 times when it changes phases from a liquid or solid into a gas at standard atmospheric pressure. But we are not at atmospheric pressure inside this chamber. Instead, recall that we’re at just 1/1000th of that. Therefore, the strawberry’s embedded ice is expanding by a factor of at least 1.6 million, or even by a greater factor if the chamber is operating at a lower pressure such as one-half or one-tenth of a mbar, as many do.
This leads us to two realities. One, we must carefully control the rate of heating so that we don’t sublimate too hard and explode the strawberry into bits. We are going for a gentle frozen simmer, as opposed to a rolling boil. Two, we need to re-deposit the sublimated ice back out of the gas stream prior to the inlet of the vacuum pumping system if we want its relatively limited volumetric pumping capacity to have a chance in keeping up with the onslaught of the expanded cold steam.
For this second reason, we must install a cold trap in our freeze dryer. Commonly supplied in pairs to allow for longer batch cycles, these ammonia-based refrigeration evaporators alternate back and forth, similar to the operation of a dual-tower desiccant compressed air dryer. While one cold trap is on-line depositing ice crystals out of the vapor stream, the other is offline and warmed at atmospheric pressure, ice melting to water to be drained away from the system.
In the context of the industrial vacuum system in its entirety, the cold trap can itself be considered a vacuum source, just one that uses thermodynamics in lieu of mechanical pumping. In fact, the collapsing vapor as frost deposits against the evaporator tubes is the single most important aspect regarding performance of the vacuum system at large. It also has huge ramifications on the maintenance requirements of the mechanical systems yet to come downstream. Not only does the cold trap reduce the volume of gas that the mechanical vacuum system must deal with by the same ~1.6MM:1 ratio, but it also eliminates a huge majority of what would become liquid water either inside or in the exhaust system of the vacuum pump.
The relative high pressure of <1 mbar in the cold trap’s -100℉ pushes us along on our continuing journey to the lowest pressure we’ll see in our vacuum system, that at the inlet throat of the mechanical vacuum pump. Or, in the case of a production operation, the first in, potentially, a series of rotary lobe vacuum boosters. Moving away from the cold trap, the temperature begins to naturally rise again just by the fact that the system piping is passing through a, relatively speaking, scorching +68℉ room temperature environment.
Vacuum Systems and Condensable Vapor Load: Temperature Control
Our vacuum pumping system must now do the heavy lifting of compressing the sum total of the non-condensable nitrogen and oxygen which may have leaked in from the world, the helium or argon which may have been purposefully added for product quality and control, plus any of the condensable water vapor which flew by the cold trap (nothing is 100% efficient), to back up to just slightly above local atmospheric pressure.
Mechanical vacuum boosters and pumps are happiest compressing vapor only, as opposed to liquids and especially (duh) solids. Once we compress our resulting gas mixture to around 1,013+ mbar, any of the condensable vapors will want to do just that—condense—unless we keep them nice and hot. At a pressure of around 0.5 mbar at the vacuum inlet throat, the required compression ratio across the vacuum pumping system will be around 2000:1. If just 3-tenths of a mbar less at 0.2 mbar, then the compression ratio more than doubles to 5000:1. Thankfully, if there’s one byproduct that these magnitudes of compression ratios translate to, it is heat energy.
In the case of typical operating pressures and cold trap deposition efficiencies combined with a fully dry-running vacuum system, there should be ample heat-of-compression energy available to temper the vacuum pump cooling such that the targeted exhaust temperatures remain high enough to maintain the vapor phase throughout the pump, into the exhaust system and down and away from the vacuum pump’s discharge port.
Lubricant or water-sealed mechanical backing pump technologies may also be used, though each presents unique application challenges, since its operating temperatures must be maintained significantly lower than its dry counterparts. The addition of inert sweep gas or ambient ballast air may be necessary to clear the pump internals and exhaust system of condensing liquids. In any case, to maximize pump life and minimize maintenance costs it is likely desirable to add such gas packages for proper start-up, control and/or shutdown procedures.
Freeze dry systems are some of the more thermodynamically active vacuum applications around, with operating pressure requirements firmly categorized in the medium vacuum realm. The thermodynamic lessons learned from the principles and subsystems in play can and should be applied broadly to your own industrial vacuum system whether it’s freeze-drying or some other process.
About the Author
Bryan Jensen is a mechanical and aeronautical engineer with over 20 years of application experience in a wide variety of industrial manufacturing processes focusing on the compressed air and gas industry. He grew up in Alaska, trained in the Midwest and started his career as a NASA materials research engineer in the Deep South before returning to the Pacific Northwest. He currently heads the Engineered System Solutions team for Rogers Machinery Company, based in Portland, Oregon.
For more information or any questions, call Rogers Machinery Company at (503)-639-0808 or visit the Engineered System Solutions page at www.rogers-machinery.com.
Originally published in the May 2024 issue of Blower & Vacuum Best Practices Magazine.
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