Exactly how does a Harvest Right Freeze Dryer work, and what's the most efficient way to freeze dry food at home? In this post, I share a review of how the Harvest Right Freeze Dryer works, breaking out the electricity use for each phase of the freeze drying cycle. Then we look at the electricity cost per load, and moisture content of different foods. I wrap up with my suggestions for optimizing load size in your Harvest Right Freeze Dryer.
Some Background on my Harvest Right Freeze Dryer and Home Freeze Drying Setup
I purchased my Harvest Right Freeze Dryer in November, 2016. It is the middle sized unit as that was the only model offered at the time. I started with a JB Industries Eliminator Vacuum Pump, and have just switched to the new Harvest Right Pump. As of this writing, I have processed 101 loads of food through the freeze dryer with the JB pump.
The features of my setup include:
- It is located in our laundry room because of the noise. My wife would prefer in the garage, but I do woodworking there and it would get covered in sawdust.
- There is a “non-GFCI” dedicated 20 amp electrical outlet. Based on the data I've collected, I wouldn't recommend a standard household 15 amp circuit. My dryer pulls over 16 amps at certain times during operation.
- To figure out electricity costs for freeze drying a load of food, I hooked up a power meter. The meter allows me to monitor voltage, AC current draw, power (watts) and electrical energy used (kW-hrs).
- I built a small wooden stand for the vacuum pump to sit on. This makes it easy to get to the pump drain valve, so I never have to move the pump. It also reduces stress on the vacuum hose from the dryer to the pump.
- I place a JB Industries oil collection tank (Model DV-T1) under the pump stand. When a load finishes, I drain the oil from the vacuum pump immediately. I remove the water from the oil and filter the oil after each batch. I have done this from the very first load and Harvest Right now recommends doing this.
- The dryer and the vacuum pump sit on a 25” x 37” plastic utility cart. The cart makes it easy to move around, if needed, plus it provides storage.
Review of Basic Harvest Right Freeze Dryer Operation Phases
The Harvest Right Freeze Dryer uses three different processing phases: freezing, drying and final drying.
During the freeze cycle, the freeze dryer refrigeration cycle cools the vacuum chamber and the food to -40°F (also -40°C). (One day I plan to put a thermocouple in the dryer and check it, just for curiosity.)
The factory default for this phase is 9 hours. Harvest Right says you can reduce this to 7 hours, if the food is already frozen when placed in the dryer. I have never changed this process time in any of the loads of food that I have run.
When the freeze cycle is completed, the dryer turns on the vacuum pump. The front panel displays the pressure in the vacuum chamber in units of mtorr. (For reference purposes 1 psi is equal to 51715 mtorr.) The dryer does not display the actual vacuum level until the dryer pressure has dropped below 2500 mtorr (0.05 psia). It takes my dryer 4 minutes and 25 seconds to drop below 2500 mtorr. This time has been very consistent, within a few seconds difference for each load. The dryer continues reducing the chamber pressure until it reaches 500 mtorr (0.01 psia).
When the freeze dryer reaches 500 mtorr, it turns on heaters under the food trays. Ice at these pressures does not melt, but goes directly to vapor. This is called sublimation. The heaters cause the food to give off more vapor, which collects on the chamber walls as ice. The freeze dryer removes moisture from the food by turning it into a vapor. It then traps that vapor on the chamber walls as ice.
When the heaters are turned on at 500 mtorr, the vacuum pressure will continue to go down for awhile. Then it begins to rise, as the vapor produced by the heating of the food overcomes the ability of the vacuum pump to maintain vacuum level. When the chamber pressure reaches 600 mtorr, the heaters are turned off. However, the food is still warm and continues to give off more vapor. This causes pressure to continue to rise.
Depending on the type and amount of food in the freeze dryer, the pressure will increase to 750 – 800 mtorr. The food cools off (because the heaters are off) and the vacuum pump starts reducing chamber pressure again. When the chamber pressure reaches 500 mtorr, the heaters come back on and the cycle repeats itself. With a large load of food, my dryer initially has the heaters on for about 4 and a half minutes and the heaters off for about 20 minutes. This means that one cycle takes about 25 minutes.
3. Final Drying
After hours of repeating this heater on/heater off drying process, the food will have so little moisture left that the vacuum pump is able to keep the vacuum chamber below 600 mtorr even with the heaters on.
When the Harvest Right freeze dryer “brain” sees that the dryer pressure remains below 600 mtorr with the heater on, it switches to the Final Dry phase. This is another fixed time phase. The factory default is 7 hours. You can change this time using the touch screen, but I typically use the factor default.
During this final dry cycle, the refrigeration system continues to maintain a low temperature. The heaters are cycled on and off to maintain the temperature of the heated trays, rather than the vacuum chamber pressure as in the Drying Phase.
Harvest Right Freeze Dryer Electrical Usage
When I had my house built, put in a 20 amp “non-GFCI” outlet to run a freezer. I didn’t want the freezer getting shut off because I put too much load on a standard 15 amp household circuit and tripped the breaker. We never bought the freezer, so my Harvest Right freeze dryer is now connected to that outlet.
I use an electrical power meter to measure voltage, current draw, power (watts), and energy used (kw-hours). Power use for the freeze dryer is listed below.
During the freezing cycle, my dryer starts out pulling 8.3 amps. It slowly reduces this to about 6.4 amps at the completion of the cycle. The dryer is using an average of 330 watts of power during freezing, and it takes approximately 3 kW-hrs of energy for the factory default 9 hour freezing cycle.
During drying, the freezer keeps the refrigeration system running, plus it turns on the vacuum pump, plus it intermittently operates resistive heater under the food tray shelves. My electrical meter is not a peaking meter, so I don’t get the instantaneous inrush current when the vacuum pump comes on. All I measure is the steady state value.
In any case, with the pump running and the heaters are off the dryer is drawing almost 13 amps. When the heaters are turned on the current draw goes up to just slightly over 16.1 amps. When the heaters are off the power used is about 625 watts. With the heaters on the power jumps up to about 1350 watts due to the resistive characteristic of the load. The heaters are on approximately 20 percent of the time and the average power consumed is about 725 watts. If the drying cycle runs for 10 hours the dryer would use about 7.25 kW-hrs during this cycle.
Final Drying Cycle
The Final Drying cycle for the Harvest Right freeze dryer is very similar in electrical use to the Drying Cycle. The refrigeration unit and vacuum pump run continuously, and heaters are cycled on and off to maintain food drying temperatures. The average power consumption is still around 725 Watts. The 7 hour factory Final Drying Cycle uses approximately 5 kW-hours of electrical energy.
The following table summarizes the electrical current, power and cost of operation for each dryer phase:
Harvest Right Freeze Dryer Electrity Cost per Load
My cost to run a small load of food requiring 10 hours of drying time would be $1.57. I tend to run larger loads of food, as I will discuss in a bit, which typically take 18 hours to dry. My average electrical usage is 21 kW-hours/load and I pay about $1.90 per load, except in the summer time when my electrical rates go through the roof.
Moisture Content of Foods for Freeze Drying
I accurately weigh the food that I put into the dryer both before and after freeze drying to determine the moisture content of the foods I dry.
The following table gives the values for moisture content of the foods that I have measured. Fruits and vegetables are all dried raw (except as noted) and meats are all dried cooked. Cooking meats removes a significant amount of water. The moisture content in the table is the moisture content of the meats after cooking.
Using a Harvest Right Freeze Dryer Efficiently
My goal is to operate my dryer in such a way that I dry the most amount of food for the least amount of money and the least amount of operating time on the dryer. Just like your car, the more you operate your freeze dryer, the sooner it needs parts and maintenance.
There are many factors that probably affect dryer efficiency but I look at it this way:
- The Freeze and Final Dry cycles are fixed times and are not dependent on the amount of food put into the dryer. These two cycles combined take 16 hours of dryer operation for every load, big or small.
- If you double the amount of water that the dryer has to remove, it takes more than twice as long to remove that extra water. In the range of 40 to 75 ounces of water this effect is very small. (See Figure 1 for this data.) There is a lot of scatter in the data because I am drying all kinds of different foods, which have different textures. I'm sure that some foods dry easier than others.
- Below about 75 ounces of removed water, the drying time is pretty linear. Above this, the drying time seems to increase significantly. This may be because the ice formed in the dryer is coming into contact with the heating trays and is getting reprocessed. The Harvest Right manual warns about this. I have not run many loads above 80 ounces, but I am going to try to run some really large loads and see how long it takes.
Comparing Dry Times and Load Sizes
Using the data shown on Figure 1, we can compare the time it takes to dry 35 and 70 ounces of water. The drying times, on average, are 7 hours and 15.5 hours respectively. However when we combine the drying times with the fixed Freezing and Final Drying times things change significantly. The total time for all three cycles to remove 35 ounces becomes 23 hours and the total time for removing 70 ounces is 31.5 hours.
Running two 35 ounce loads would take 46 hours of total dryer operation or 14.5 hours more than running one larger load. To see this effect graphically I calculated the amount of water removed per hour of dryer operation and plotted this data which appears as Figure 2. More water removed per hour of operation is what I/we are looking for. Remember, that this is for total hours of operation factoring in the Freezing and Final Drying times as well as the Drying Cycle.
Optimizing Load Size
For my dryer, small loads of food result in water extraction rates of about 1.5 ounces per hour of operation. At 80 ounces of water removed the water removal rate has increased to 2.4 ounces per hour. However, the data shows that this effect is leveling off and probably doesn’t get higher than this. I expect that as I increase moisture content significant above 80 or 90 ounces of water the amount of water removed will start to decrease.
Looking at the shape of the curve in Figure 2, my dryer appears to be approaching maximum dryer moisture removal rate of 2.40 ounces/hour around 80 ounces of removed water.
Using the moisture content of food that I have measured in the past, I can calculate how much weight of any of those foods I need to put into my Harvest Right Freeze Dryer to have it extract 80 ounces of water. For example, poached chicken breasts have a moisture content of 70%. I can then load 80/.7 = 114 ounces of chicken breasts into the dryer and, on average, have the dryer extract 80 ounces of water. Doing this keeps me operating the dryer near the point of best efficiency. It also allows me to estimate with pretty good accuracy just how long the dryer will take to complete the total drying process.
More Information on Home Freeze Drying
There are several more posts about home freeze drying on the site, or you can go directly to the Harvest Right website for current pricing and other specification.
If you choose to purchase a Harvest Right freeze dryer through my site, I receive an affiliate payment at no extra cost to you. (Thank you.)
This is a guest post by one of our Common Sense Home readers, Dennis Alexander. Dennis shared some detailed comments on the original Harvest Right freeze dryer review, so I asked him to share a little more about his experience with home freeze drying with the Harvest Right Freeze Dryer. – Laurie Neverman, editor