A Big Decision

Are you planning to build yourself a freeze dryer? Maybe you started small with a product that seems to have great potential in the market, like freeze dried candy, or freeze dried raw pet food, or maybe even freeze dried flowers and herbs.

freeze dried raspberries techsource systems

If you are like many start up freeze drying businesses, you probably have a garage full of Harvest Right freeze dryers. A Blue Alpine Freeze Dryer is a great, low-cost, entry-level freeze dryer that makes quality product for many people. It has very limited capacity, so it is good for proof of concept and market, but not large enough for production. If this sounds familiar, now you are faced with buying a commercial size freeze dryer from an equipment maker like Parker Freeze Dry or GEA or TechSource Systems, but the price tag is too high for now. You have a couple of options, you can buy a pilot size freeze dryer, like the Big Baby Gen 2 by TechSource Systems, or you could take a crack at building your own. In that case, this article is for you!

What this article covers

This article will cover the major components you will need to build your own freeze dryer, it will also brush against some of the design requirements for proper operation and will end up touching on some of the challenges you will likely face in building your own. Will it cost you less to build your own? Probably, but not guaranteed because it depends on how many U turns you have to make along the way. U-turns can be expensive and time consuming. Remember, you can always buy, and if you finance the purchase, the cash flow from the business will more than pay for the financing. Suntorr can help you with both the machine purchase and help you find a good financing partner.

It Will NOT Be Easy

freeze drying time is money

First off, let me start by saying this project will not be as easy as you think. How do I know? I tried it and learned how many factors you have to get right, and I ended up abandoning the project at the 65% mark. I have also spoken to a number of clients who have built their own, some of whom were more successful than others. But I am not the only one who has given up on it.

Over the last 60 years, there have been many vacuum equipment makers who tried to build and sell freeze dryers under the assumption that they are simply vacuum chambers with some extra parts (I worked in the vacuum industry for many years, take my word for it, this is how they think of most vacuum processes). Most of those companies are still around, but none of them make freeze dryers anymore.

It turns out that freeze dryers are not a vacuum system with some extra parts, rather it is a phase change mass transfer system that happens to run under vacuum. In fact, the freeze drying cycle is driven by vaporization and condensation, which requires the right amount of heating and refrigeration. The physics principles that drive a freeze dryer are well known and easily calculated if you understand thermodynamics, but all too often a freeze dryer OEM does not know the thermodynamics, so they ape other freeze dryer designs and confirm by testing. It turns out that the challenge to making a good freeze dryer is in the design of the heated shelves, the refrigerated condensing plates, the sensors, the vacuum pumps, and the control system. The vacuum chamber itself is usually not a challenge to design, but it can be a challenge to find a shop to make a good one for you.

Non-Negotiable Requirements

Freeze drying is just what the name implies, it is a drying process that is done while the product is frozen. To dry anything, you need to add a precise amount of heat to the product to vaporize the water. In this case, you need to add the heat of sublimation since the product is frozen at the time, and then you need to remove the same amount of heat through the condensing plates plus a little extra for inefficiencies. This is not negotiable, it is governed by the laws of physics, so make sure you find out how to calculate the heat you need to add and remove and over what time period. If you need help with these calculations, contact me.

The Seven Critical Components in a Freeze Dryer:

  1. The vacuum chamber
  2. The heated shelves
  3. The sensors
  4. The refrigeration system
  5. The cold plates
  6. The vacuum system
  7. The control system and software

Each of these systems can be critical failure points. If you have a failure in any one of these, you will have to fix it before you freeze dry more product. Of these, the control system is the one that will be the most durable but can cause you a significant amount of anguish trying to get it programmed properly for your product.

The Vacuum Chamber

freeze drying components diagram labeled back

The vacuum chamber has two main issues, leak tightness and footprint. It can be surprising how much a vacuum leak can inhibit the proper operation of your process cycle. Your freeze dryer can tolerate small leaks, but they are cumulative and once the total leak rate exceeds the pumping speed of your vacuum pump at operating target pressure, you will no longer be able to achieve the results that you want in your freeze dryer. Finding out THAT you have a leak is easy. Simply pump down the chamber, isolate the vacuum pump, and plot pressure over time. This method will also tell you what the total leak rate is, however, it will not show you WHERE the leak is, which is the most critical information. If you do not have a vacuum leak detector, you must guess where the leak is. Always start at the most likely places for leaks, where the chamber is breached by doors, ports, and vacuum feed throughs. Sometimes you can detect a leak at these locations by dribbling acetone across the suspected locations and observing whether or not you have pressure spikes in your chamber. Naturally you want to have your chamber drawn to as deep a vacuum as you can if you’re doing this method otherwise you won’t see any pressure spikes. The best alternative, particularly if you’re doing commercial freeze drying is to buy a helium leak detector. Helium leak detectors will enable you to find and fix the leak very quickly, which can be worth a great deal of money. Most helium leak detectors cost over $20K when new, but they can be rented, or you can hire a service tech to bring one to your facility and find the leaks for you.

The Heated Shelves

Problems with heated shelves revolves around the heating method and the type of sensors used to measure shelf temperature. There are two primary choices for heating shelves. One is electrically heated shelves. The other is fluid heated shelves. Each of these methods has a distinct advantage over the other in that fluid heated shelves are very unlikely to overheat the product but if you spring a leak, pressurized silicone oil or antifreeze will spray all over.

freeze drying heated shelf sensors

Electrical shelves can overheat if the sensor or relay fails, but they provide quick control, allow for better zonal temperature control, and never douse the product with heating/cooling fluid. Some electrically heated shelves can even be quickly removed for easy cleaning. Every freeze dryer brand has their own way of heating the product. An important thing to know is that most of the heat transfer into the product happens by radiative heat rather than by conduction. In vacuum, there is no convective heat transfer. All of the cooling of the product happens by evaporative cooling, and the temperature that cooling takes place at is driven by the pressure in the chamber and happens at the vapor point of the water in the product.

The Sensors

freeze dryer thermocouple sensor

Failure of temperature sensors in a freeze dryer is one of the more frequent causes of lost product and machine down time. Some sensors are better than others, but durability is most often driven by how the sensors are used in the machine.

Sensor type: There are primarily three types of temperature sensors used in freeze dryers. Resistance Temperature Detector (RTD), Thermocouple, and Micro Electro Mechanical System (MEMS). The RTD changes resistance as temperature changes. A steady voltage is supplied to the RTD which changes with the resistance, thereby providing a voltage signal that will be converted to a digital signal by the display or PLC, a thermocouple generates a particular voltage for a particular temperature which becomes a direct signal that will be converted to a digital signal by the display or PLC, and the MEMS sensor generates digital gray code that interfaces directly with a digital display or PLC.

Sensors on long flexible wires to insert into your product are usually the first to fail. Measuring temperature this way will give you variable results depending on where the probe end up resting. If inserted into the product, it will show temperatures below freezing until the water is sublimated out, but if that is used to control heater temperature, it can lead to burned product because there is no direct feedback to the system regarding how hot the heaters are. Sensors that are embedded into the shelves provide much more consistent readings and can be used to directly limit the temperature of the shelf to prevent burning. Depending on how the sensor is embedded, the expansion and contraction of the shelf can wear through the sensor wire causing failure.

The Refrigeration System

freeze dryer rack techsource systems

The refrigeration system can either be durable or it can be a huge pain. It depends on the quality of the refrigeration system. There are dozens of refrigeration system suppliers from Asia and frankly, they fail often and if they fail, parts are almost impossible to come by. They also don’t come with very good documentation to let you know how to operate them properly or to maintain them properly. There’s also some debate whether fluid chillers or direct freon or is a better method of cooling your ice bank condensers. Direct freon is a little bit simpler and therefore somewhat less expensive. However, chilled fluid has some distinct performance advantages over direct freon cooled condensers. See here for more details about this comparison.

The Cold Plates

The cold surface in your freeze dryer, whether provided by plates or other geometric shapes, are what do the majority of the pumping of water vapor. Consider that vacuum is created by removal of gases (which are nothing more than substances above their boiling points) from a sealed volume like your vacuum chamber, for example. Removal of gases can be done by mechanically pumping them out using a compressor (your vacuum pump) or by changing them into another phase, ie. liquid or solid. Your cold surface creates vacuum by converting water vapor to ice, effectively removing the gaseous form of water vapor from your system. Put succinctly, your cold plates are vacuum pumps ---for water vapor.

How effective are the cold plates vs. the mechanical vacuum pump. Depending on the size of your vacuum pumps, the cold plates can pump water vapor more than 1000 times faster than your mechanical pumps. If you want to know more about this, I did these calculations to back it up. The cost of pumping water vapor this way is mostly in refrigeration because temperatures of -40C or lower are required for good operation. Furthermore, the location, orientation, and surface area of the cold surfaces also play critical roles in your overall success. The surface area and temperature enable your machine to maintain the vacuum you need, the orientation and location of the cold plates determine how easy or annoying the regeneration step is (melting the ice off the cold plates).

freeze drying open showing cold plates

The Vacuum System

If the cold plates pump so much faster than the vacuum pump, then why do you even need a vacuum pump? To pump the non-condensable gases. Where do these non-condensable gases come from? Two places, the chamber is initially filled with air and must be pumped down at the start of your cycle. You must establish vacuum, then the cold plates will maintain vacuum as you generate water vapor throughout the cycle. After initial pump down, the non-condensable gases come from the product in forms of dissolved gases like CO2, O2, N2, H2, etc. The majority of non-condensable gases comes from vacuum leaks. This means that sizing of vacuum pumps for a freeze dryer is more about your normal leak rate than it is about the size of your freeze dryer. A leak tight chamber allows a smaller vacuum pump, a lower operating pressure, or both.

The vacuum system is also a critical failure point, particularly when the vacuum pump is an oil-filled vacuum pump, which means that the compression mechanism is bathed in oil both to lubricate and to seal the vacuum compression cycle. Oil sealed vacuum pumps are prone to problems with applications where a significant amount of moisture is being pumped, which is exactly the case in freeze drying where you have almost 100% saturated moisture atmosphere. In these cases, the oil sealed vacuum pump will condense water inside of the pump as it pumps up to atmospheric pressure. That condensed water will then both hurt the performance of the pump and the ultimate pressure (vacuum) and it will make it prone to premature failure due to compromising the quality of the oil in the pump and by rusting the pump mechanism prematurely. If you have already been freeze drying, you are probably very familiar with this problem.

Dry vacuum pumps, which means pumps that operate without oil in the compression chamber of the pump, are an excellent way to avoid the pitfalls of the rotary vane pump, or the piston pump. There are two types of dry vacuum pump widely used in freeze drying, the dry screw and the dry scroll type vacuum pump. Both of these pump types have a higher capital cost than the equivalent sized oil sealed types, but if chosen correctly, have a much lower cost of ownership. The caution here is that you must choose carefully, when it comes to dry vacuum pumps, some brands perform flawlessly while others can fail after a short period of time. Needless to say, a more expensive pump that fails shortly is NOT a frugal choice. If you want input from somewhere other than the salesmen of the various brands, ask engineers@suntorr.com. We are independent from any vacuum equipment brand, but have extensive experience with the major brands.

Control System and Software

The last main component in your freeze dryer is the control system with software. Any commercial freeze dry is going to need to be controlled by a PLC or equivalent electronic control system. The choice of PLC in the controller is largely just driven by the OEM’s preference, although availability of spares or repair parts could be a concern. The big issue with the PLC controllers is the ability to program them, reprogram them, or troubleshoot them. If you’re buying a commercial freeze dryer and that restore OEM will not let you get into the control system and troubleshoot and or modify the control system that should raise a red flag. Most importantly, ensure that whatever freeze dryer control system you use, ensure that it is intuitive, easy to use, easy to create custom recipes for every variation of product that you might have, and that you can monitor it both locally and remotely.

freeze dryer dry vacuum pump by Leybold used in Techsource systems

Summary

Are you still going to try to build your own freeze dryer? You certainly can, but it's always better to leverage decades of experience and knowledge. Just give us a shout and we'll be glad to talk to you about your needs!