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Multilayer Insulation

CryoWest News
July 31, 2019

Multilayer Insulation

This article, written by Dr. John Weisend, was originally published in the Summer 2010 issue of Cold Facts as part of his series, Defining Cryogenics.

Multilayer insulation (also referred to as superinsulation) is a key component in the reduction of heat leak to cryogenic systems due to thermal radiation.

MLI consists of a series of uncooled reflective surfaces placed in the vacuum space between two surfaces, one warmer than the other. Generally speaking, for ideal systems the reduction in thermal radiation heat leak scales as 1/(N+1) where N is the number of layers in the MLI. However, most systems are far from ideal and great attention to detail is required to properly use MLI.

Most MLI systems consist of thin reflective layers (such as aluminized Mylar) separated from each other by thermally insulating layers. The insulating layers may be made of such things as silk or nylon netting. Other MLI designs omit the separate insulating layers, aluminize only one side of the Mylar and crinkle the Mylar so that only point contacts exist between the layers.

Since the goal here is to reduce the heat leak to the cold surface, the MLI must be designed to reduce the thermal radiation heat leak without providing a thermal short that would increase the heat leak caused by thermal conduction. MLI systems are frequently made into blankets of MLI for easier installation.

The requirement that the MLI not add to the conduction heat leak in the cryogenic system has a significant impact in its use. Great care must be taken not to thermally short the cold and warm layers of the MLI together; say by simplemindedly wrapping a MLI blanket back on itself. Attempting to put too many layers in a given space can result in tightly compacted MLI and higher conduction heat leaks.

Penetrations through the MLI for supports, pipes or instrumentation lines must be carefully handled to avoid both clear lines of sight through the MLI and the thermal shorting of the MLI layers. Since the MLI will thermally contract as it cools, seams and joints need to be designed so that gaps in the MLI don’t open up upon cool down.

MLI is used in a vacuum space, and thought should be given to the impact of the MLI on the pumping  down of the vacuum. In many cases, the layers in the MLI are perforated to increase pumping speeds. As long as the perforations in the layers aren’t lined up with each other they won’t greatly impact the performance of the MLI.

A very good explanation of MLI is given in “Multilayer Insulation Systems” by T. Nast found in the Handbook of Cryogenic Engineering (1998). Recent research on MLI includes: “Robust Multilayer Insulation for Cryogenic Systems”, J. Fesmire et al., “Synthesis of the Multilayer Cryogenic Insulation Modeling and Measurements”, J. Polinski et al. both published in Advances in Cryogenic Engineering Vol. 53 (2008) and “Venting & High Vacuum Performance of Low Density Multilayer Insulation”, M. Riesco et al. in Advances in Cryogenic Engineering Vol. 55 (2010).

July 31, 2019
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dosing
July 18, 2019

Liquid Nitrogen Dosing

Liquid nitrogen dosing systems are commonly found in bottling facilities where non-carbonated beverages are made (water, juices, teas, etc.). It is becoming ever more important to use dosing systems to pressurize bottles; giving them strength for stacking.

Over the past 20 years, beverage makers have reduced the weight of bottles to reduce costs and environmental impact. The most common material used for beverage bottles is Polyethylene terephthalate; commonly called PET. The rising cost of polymers and the impact that plastic bottles have on the environment has led most beverage producers to “Light-weight” bottles; reducing the weight of PET polymer used to produce bottles. This results in thinner, weaker bottle walls.

It would be impractical to simply fill the bottles and allow them to be handled without some means of strengthening them. In order to handle bottles after filling, they must be stacked so they can be transported from the bottling facility to customers. Weak bottles at the bottom of a pallet will buckle under the pressure from the bottles above; resulting in a dangerous situation and high losses.

The solution is to pressurize the bottles. Nitrogen is the perfect solution, because it is inert and available in the liquid phase. When liquid nitrogen vaporizes to gas, it expands to 682 times the liquid volume. This property is exploited with dosing systems which add a droplet of liquid nitrogen and trap it in the bottle by capping the bottle immediately.

That droplet, trapped in the bottle expands as it vaporizes and increases the pressure of the bottle. The increase in pressure has the effect of strengthening the bottle; a process called “rigidification”.

An additional benefit that liquid nitrogen dosing offers is some protection from oxygen, which can lead to spoilage of many non-carbonated drinks.

 

Source: https://www.lindeus.com/en/processes/controlled_and_modified_atmospheres/liquid_nitrogen_dosing/index.html

July 18, 2019
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frozen foods
July 18, 2019

The Benefits of Nitrogen for Food Processing

Chemistry is an essential component for the food processing industry, which looks to improve product quality, reduce costs, and increase output. One such option is the use of nitrogen. As Chris Johnson, Food Industry Specialist for Air Products, discussed in a seminar in October 2009, nitrogen is proving a beneficial ingredient for the prepared foods sector, with a range of creative applications, including freezing, packaging, mixing, coating, and grinding. Nitrogen can be used to process a wide variety of foods, including meat, poultry, seafood, fruits, vegetables, pasta, dairy products, baked goods, and prepared meals. 

 

Freezing 

Cryogenic (i.e., very low-temperature) freezing results in smaller weight losses from dehydration than traditional mechanical freezing. Frozen food processors can thus preserve the quality of their products with a freezing system that uses environmentally friendly liquid nitrogen. 

 

Due to its own extremely cold temperature, liquid nitrogen can enable a cryogenic system to freeze food within minutes, instead of the hours traditionally required with other systems. The faster freezing causes the formation of small ice crystals, which then help ensure product moisture and quality are maintained longer. 

 

Further, liquid nitrogen can be used to remove heat from other processes to help prevent microbial growth, reduce cycle times, and/or increase production throughput. 

 

For example, Excellent Foods in Burbank, Wash., used a liquid nitrogen freezer for its Apple Fries products. These peeled, cut, coated, deep-fried, and finished apple wedges needed moisture locked in before going to market. With an immersion tunnel system, they took only three to four minutes from peel to freeze. 

 

Custom Pack in Biloxi, Miss., switched over the years from canning shrimp to a variety of freezing techniques, including the use of a cryogen straight tunnel freezer for a time. Eventually, the company settled on cryogenic technology with a nitrogen-based immersion freezer to process individually quick-frozen (IQF) shrimp. This proved particularly well-suited for the wet and fleshy seafood. As a result, production capacity increased from 5,000 to 8,000 pounds per hour. 

 

Holten Meat in Suaget, Ill., began using liquid nitrogen freezing tunnels in 1974. The technology allows the company to ship its frozen Thick N’ Juicy hamburger patties and maintain their freshness 90 days from code date, providing a much longer shelf life than fresh patties. 

 

Packaging 

Gaseous nitrogen can be used in a wide range of packaging processes, including Modified Atmosphere Packaging (MAP), to help preserve the quality of food. 

 

MAP is a preservation technique for minimally processed foods, including fish, meat, fruits, and vegetables. The specific gas or mix of gases in the package depends on that package’s materials, the product, and the temperature at which it will be stored, but in all cases the purpose of the technique is to change the composition of the air around the food. 

 

Food products spoil in different ways, whether through microbial growth, discoloration, oxidation, or moisture loss. By replacing the breathable atmosphere in a package with a pure gas or a gas mixture, including nitrogen, the shelf life of products can be significantly extended, helping them look, smell, and taste good far longer than they would otherwise.

 

Mixing, coating, and grinding 

In mixing applications, liquid nitrogen is used to chill sauces and gravies to stop the cooking process while reducing cool cycle times. 

 

In coating applications, the low temperature of liquid nitrogen allows it to enrobe individually quick-frozen (IQF) products in sauces while they freeze, helping produce an even coating. 

 

During grinding, meanwhile, liquid nitrogen can be used to eliminate frictional heat to help improve the throughput of mills, while also preventing the loss of flavor and aroma components in food additives, ingredients, and functional foods. 

 

Specialized equipment can be coupled with liquid nitrogen to help optimize IQF and non-IQF food freezing, crust freezing, chilling, coating, mixing, and forming. Regardless of size, both the equipment and the nitrogen can be configured in a mode that matches the scale of the food processing operation.

 

Source: http://www.airproducts.com/~/media/downloads/article/b/en-benefits-of-nitrogen-american-chemistry-reprint-332-10-040.pdf

July 18, 2019
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ice cream
July 18, 2019

Liquid Nitrogen Ice Cream

It’s been described as the five minute way to make amazing ice cream. No cranking… no ice and rock salt mixtures… and you can forget about those electric ice cream makers. All you need is a little liquid nitrogen (-320°F), some creamy ingredients, and an appetite for exploring the science of food. Keep reading to learn the secrets for finding and safely using liquid nitrogen to make one of the most trendy desserts on the cooking show scene.

Experiment

Using liquid nitrogen to make ice cream is nothing new. Science teachers have been making “instant ice cream” much to the delight of their students for years (because they love the sounds of ooohs & ahhhs that naturally follow this amazing feat!). But making liquid nitrogen ice cream (or Nice Cream as we call it) has become a craze that’s sweeping the trendy cooking shows and the avant-garde chefs who practice molecular gastronomy. In pop culture, molecular gastronomy is the merging of scientific tools and techniques with culinary arts – using tools such as centrifuges, liquid nitrogen, microscopes, or lasers in the preparation of food items. So, it only makes sense that making ice cream amidst clouds of smoke and a big container of liquid nitrogen is the “in” thing to do among ice cream aficionados.

Liquid Nitrogen Basics

The actual “how to’s” of making liquid nitrogen ice cream are quite simple, and you’ll find the complete guide, including tips and tricks, later in the article. However, let’s start with the most asked questions: What is liquid nitrogen and where do you get it? Back to the chemistry lab we go… You might remember from science class that most of the air we breathe is nitrogen (about 78%) with 20% being oxygen and the remaining 2% a potpourri of gases. Liquid nitrogen is -320°F or -196°C (in other words, it’s cold). While liquid nitrogen is used for many temperature-related applications, it’s particularly useful for freezing foods because nitrogen is odorless, colorless, and tasteless.

Safety First

Okay, there is one tiny little negative to using liquid nitrogen to make ice cream… it’s dangerous if you’re not careful. If you touch the stuff, you can easily freeze tissue and cause severe damage. That’s why it’s not readily available to the public, and you’ll need to get some basic training on how to handle liquid nitrogen before you get started. As Mr. Wizard warns, “Never let liquid nitrogen touch your skin, and be sure to wear your safety gloves and glasses, kids.” You probably won’t see your favorite celebrity chefs wearing safety glasses or gloves, but it’s a must for kitchen chemists like you and me. Contact with liquid nitrogen can cause injury … it can be dangerous if not used properly.

Here’s the best advice… Go to your local high school or university and pay a visit to the science department. Ask around to find out who does the science demos with liquid nitrogen each year for the students and ask if you can come in as a spectator to watch. You’ll learn a lot from watching someone who has experience handling the liquid nitrogen (and you’re likely to learn a few cool tricks at the same time).

Where to Buy Liquid Nitrogen

It’s not as easy as running to the local grocery store to buy milk. Liquid nitrogen must be stored and transported in a special container called a Dewar, otherwise it will evaporate very quickly. Your best bet is to search your local Yellow Pages for a supplier of liquid nitrogen. You can also call a pediatrician to find out where the doctor purchases liquid nitrogen to burn off warts (now that’s appealing when you’re talking about ice cream). Welders use liquid nitrogen as well as some farmers (something to do with artificial insemination). Try contacting the chemistry or physics department of your local college or university to find out where they purchase it. Because of liability issues, you probably will not find too many people who are likely to just give you a container of LN2 (short for liquid nitrogen), and you’ll need to buy it from a company who distributes it. Many times these companies will let you borrow or rent one of the 5-liter Dewars to transport your secret ice cream making ingredient. You can budget about $1.50 to $2.00 a liter and you’ll need about 5 liters to make up a couple quarts of ice cream.

The Recipe

There are hundreds and hundreds of recipes for ice cream, so you’ll just need to find one that you like. But one of the benefits of making ice cream using liquid nitrogen is that you can test the batch in just a few minutes. Here’s a basic recipe to which you can add anything to make your signature Nice Cream.

Let’s start by making a small batch of ice cream until you master the mixing technique. The general rule is to use twice as much cream as milk and sweeten to taste with vanilla and sugar.

  1. In the large steel bowl, mix one quart of Half and Half with one pint of milk.
  2. Add the vanilla and sugar. Stir until the sugar dissolves.
  3. At this point, you can add anything you like – get creative. It’s easy to replace the vanilla with any flavor you wish. Add chocolate chips, crunch up a candy bar, toss in frozen berries, nuts, marshmallows, cookie dough… whatever sounds good to you!
  4. It’s time for the liquid nitrogen. Put on your safety glasses and gloves… do it! Your assistant chefs should also be wearing their safety gear as well. You might want to put the bowl in your sink just in case it overflows (which it probably will) or spread out some plastic on the table. It’s impossible to measure how much liquid nitrogen to add, so just pour about a quart (that’s equal to about 1 liter) of liquid nitrogen into the bowl. Expect lots of cool fog and the sounds of oohs and ahhs from the audience.
  5. DO NOT STIR THE MIXTURE… just yet. Let the liquid nitrogen do its job of partially freezing the mixture. If you stir too soon, the concoction will bubble over and you’ll have a mess (and you’ll waste some of your ice cream).
  6. Stir the mixture with the wooden spoon. Some of the cream will be completely frozen while other parts will still be liquid. Keep stirring until you have a consistent mixture, and then add more liquid nitrogen.
  7. Let it freeze for a few seconds before stirring.
  8. Stir, stir, and stir some more until you have a consistent mixture. 
  9. Keep adding small amounts of liquid nitrogen until you have the perfect creamy consistency.

How Does It Work?

The Science Behind the Freeze

The secret to the creamy ice cream is all in the rapid freezing of the mixture. The liquid nitrogen causes the fat and the water particles to stay very small, giving the ice cream its creamy consistency. The goal is to avoid ice crystals – similar to what you get when you make ice milk. According to food science experts, rapid freezing preserves the nutrients in food (a quick-frozen vegetable that’s three months old will likely have a nutrient composition closer to harvest levels than a five day old fresh vegetable. The faster you freeze, the less you destroy tissue structures that could lead to a more rapid loss).

Additional Info

Brain Freeze

One final thought… It’s easy to get distracted by the plumes of smoke and the excitement of making instant ice cream, and you forget about common sense and safety. In the teaching profession, we call it a “brain freeze.” Keep your head above the clouds, take all of the safety precautions, and you’ll create an unforgettable memory… and some awesome ice cream at the same time.

Source: https://www.stevespanglerscience.com/lab/experiments/liquid-nitrogen-ice-cream/

July 18, 2019
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