Part 2

Onion storing & drying options

Last week, we published an article on the best ways to harvest and store onions, this week we will look at a summary of the advantages and disadvantages of various drying methods. 

Drying with just outside air without heaters 
This is clear, it doesn’t work in the Netherlands' climate. With the differences in temperature between day and night, we actually suck moisture into the storage! 

Drying with limited heater capacity 
The humidity of the drying air has no effect on the development of diseases already in the onion, but it does have an effect on their spread in the store. Once you manage to keep the humidity under 65%, there isn’t enough external moisture for them and any infection is limited to the onions that are already affected. With limited heater capacity, however, the humidity only falls to 70-75%, which isn’t enough.

It also isn’t possible to achieve a high drying temperature of 30 ⁰C. A high drying temperature limits the drying time by a few days and reduces neck rot by an average of 5%. Another advantage of a high drying temperature is that infected onions rot quicker and are easier to sort out.

Alternatives: 
There are two things you can do to achieve a low humidity: you can heat the air so that it can take up more moisture and the percentage of the relative humidity falls, or you can just remove the moisture. 

Heating with a lot of gas isn’t energy efficient, and for every kilo of propane that is burnt, 1.6 kg of water is released which we then also have to remove from the store. 

This brings us to the alternative: removing the humidity from the air and attempting to keep the energy needed to do it in the store.

This method is therefore just as unsatisfactory. In order to achieve the conditions mentioned above, you can’t cut back on heater capacity and gas. 

Humidity removal with absorption dryers 
This method is often applied in market gardening. The humidity is conducted along a salt which absorbs the moisture in the air. The salt has to be regenerated so that the moisture that was trapped is removed. The method is easily applicable but can’t achieve the capacity which is needed for drying onions and so it is not a realistic option. 

Humidity removal with condensation drying 
The principle is simple: if you conduct humid air along a cold surface, the air condenses and heat is released (the condensation heat from the water). It can be compared with drinking from an ice-cold bottle that has been taken from a refrigerator on a warm summer’s day. 

In the simplest form, the refrigerator condenser block already does this -cooling is drying, and drying is cooling. However, we want to keep the product at a good temperature, so we have to warm the cooled air up again.



The principle isn’t new, the first experiments with condensation drying date back to the seventies. The technology at that time meant, however, that it wasn’t possible to reheat the cooled air without using excessive amounts of energy to make sure that the temperature of the drying air remained stable. 

It’s only in the last ten years that the technology has been picked up again, and that has resulted in systems that are able to redirect the condensation heat back to the heating blocks (which are actually the condensers of the refrigerator), keeping the air at the same temperature. This method works, but it has a number of disadvantages: 

• Lots of blocks and piping hang in the storage cell. As well as the condensers for the cooling, there are the extra blocks for heating, all with their own piping. This isn’t very good for the air flow and it also leads to a lot of ‘testing and adjusting’ to balance everything out. And for every store design, the system will be just a little bit different.

• With multiple room storages these systems require a lot of buffer capacity and complex piping with lots of valves. In practice, concessions are often made and simultaneous drying in 100% of the rooms is then impossible.

 With large scale storage, the circuit that feeds the condensers and heating is fed with water and/or glycol. Actually, you don’t want piping filled with fluids in the store as a matter of principle. When the system ages, it’s going to demand a lot of extra inspection and maintenance. 

These ‘classic’ systems can work well but they are actually based on the combination of individual pieces of fairly old technology. 

Improved condensation drying: VaccTek
From the first stores with the systems mentioned above, the method was developed further into a complete unit which combines everything in one device without external fluids: the VaccTek. You can compare it to an enormous condenser tumble dryer which we use at home. Instead of a complex process installation, it has now been reduced to a single machine. 

The challenge that had to be solved was to harmonise all the components with each other so that there was a balanced system which could do two things automatically and irrespective of the design of the storage room: 

Keeping the drying air at a required level of humidity at the press of a button while maintaining a higher temperature, independent of the conditions of the outside air. After four seasons’ experience and dozens of operational units, the method has now fully matured. 

The integration of all functions in one machine also has the advantage that it is very easy to control. After all, we’re running a store and not a factory where complex processes have to be controlled. There are now also lots of extras which make the technology more applicable:

• Suitable for every sort of box storage, both suction and blowing systems. 

• Integrated heat pump for extra drying if the condensation heat has been used up. 

• Further drying at a low temperature, so even after cooling. 

• A real ‘real-time’ metering of the amount of condensation water removed, providing an extra indication of how the drying process is going. 

The investment in these complete units is somewhat higher, but they offer 100% simultaneous drying for several rooms, better control, a heat pump and significantly lower maintenance costs over the long term. The installation and start-up is also much simpler; it is after all a machine and not a complex installation.


Placing a VaccTek in a new store. 

The method is still expensive for bulk storage. The application can be mostly found in box storage, where multiple batches of onions can be dried in one room (the drying time is almost halved). A capacity of 500 tonnes can then be used for 1000, for instance. 

Facts and fables about the condensation method: 
There is a lot of discussion about the facts and fables of condensation drying, so here are a few facts: 

Condensation drying isn’t more energy efficient than drying with heaters! 
This is true if you are drying with a high humidity, you can, of course, do that with limited heater capacity. If you want to dry your onions well, however, the humidity has to be lower than 65% and then the condensation method is by far the best method, certainly with regard to energy costs. The energy costs of drying at a relative humidity of 75% are often compared with those of condensation drying at 50-65%, but this is not a reasonable comparison, of course. If you make the comparison with drying at 30 degrees and a low relative humidity, then condensation drying is up to 70% more efficient (approx. 0.4 cents to 1.5 cents energy costs per kg). 

Condensation drying is expensive!
The initial investment is indeed higher, but that doesn’t mean that it is expensive. 

If the aim is to consistently supply a good product every year with fewer losses and lower energy consumption, then the extra investment can be justified in most cases, just in terms of cost considerations. In many countries, the method is also subject to energy-savings subsidies.

The thing that counts is the drying cost per tonne, not the extra investment per tonne. The extra investment works out at just 0.5-1 cent per kg, which is easily covered with energy savings and less failure.

However, the investment is definitely worth it, if it is measured in the confidence of being able to consistently deliver, and to deliver quality. A 100% drying guarantee offers extra commercial possibilities compared to traders or growers who do not have that option. 

Condensation drying can be produced simpler and cheaper! 
This is correct in principle. The principles applied in the VaccTek are no different basically than cool drying systems with fluid circuits. However, if it was so simple, the method would have made its breakthrough much earlier. In short, it can be simpler, but it demands a great deal of affinity and consideration from the user and it won’t work as well everywhere. Combining all the functions in one machine has proven itself to be the most effective and successful form to date. 

Condensation drying, dries too much! 
The last few years have proven that this fear is unfounded: while the neck is still open, most sorts of onion can’t be dried too much. The drying process has to be monitored, of course. As soon as the neck has closed, the humidity can be set higher, further drying can take place at a higher temperature, or the air can be cooled, so that further drying is still possible. These decisions are always for the user, regardless of the system they use. 

Condensation drying is only of interest to organic growers! 
The technique was first developed for organic farming and storage losses were dramatically reduced to levels which are usual for traditional cultivation. However, it has since been shown that condensation drying is profitable for most box storage with mechanical cooling. There is a guarantee that red onions can be dried well, for instance, which offers more opportunity to make it a permanent element of production. It guarantees drying for all crops!

For more information:
AgroVent BV
Tel. +31(0)527-636150 
Fax. +31 (0)527-636155

Publication date:



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