solar dryer
There are several types of solar dryers, but they all generally rely on some type of solar collector to provide the heat energy that evaporates the water in the wood. Unlike solar heating for an office or home, in lumber drying it is not possible to reduce the heat requirement to the point where solar heating can be competitive. When you have a certain amount of water to remove from a certain amount of wood, you need a certain amount of total heat to do it, and that heat requirement cannot be changed. Drying times in a solar dryer are weather dependent and therefore unpredictable. In hot climates, they can degrade wood due to excessive drying. In colder climates, they are unreliable and slow. Solar dryers often use electric fans to circulate air through the wood, but the cost of running these fans is high, and because of the long drying times, then the fans must run for a long time, which makes solar drying more expensive than budgeted for. The electricity to run the fans in a solar dryer is typically more than it would take to run a dryer with a dehumidifier, since the drying time is longer.
conventional dryer
A conventional dryer uses heat that is provided by steam or hot water coils to remove water from the wood. The water extracted from the wood is converted to water vapor by evaporation and is then extracted from the dryer with the hot air. This process requires a large amount of energy and constant heating of the air, so these systems are not as energy efficient as dehumidifying dryers. To remove one liter of water from wood, a conventional dryer has to suck in about 20 cubic meters of air, heat the air, and then discard it with the evaporated water. Between heating these large amounts of air and heating the water to evaporate it, conventional dryers have a very high heat requirement. They can provide a very good quality of lumber if a good dryer control method is provided, but their energy consumption is much higher than that of a dehumidifying dryer.
vacuum dryer
The temperature at which water boils is determined by atmospheric pressure. The higher you go up a mountain, the less pressure the atmosphere exerts, so the water boils at a lower temperature than at the bottom of the mountain. Vacuum dryers take advantage of this fact to achieve drying times that are typically only a fraction of the time required by conventional or dehumidifying dryers. However, a big drawback of vacuum dryers is that the chambers are small, so the dryers cannot dry large quantities at the same time. It is necessary to provide heat to the wood continuously in a vacuum dryer. To do this, some systems use electric blankets in contact with each piece of wood, while some use heat coils or microwaves. All of these systems are extremely expensive to operate compared to conventional or dehumidifying dryers. Operating costs are typically three to four times higher than the costs of dehumidifying dryers. Initial/capital costs, as well as handling costs, are much higher due to the smaller loads. Uneven drying is also a problem with vacuum dryers. When compared on the basis of cost per thousand inches of annual production, the costs of a vacuum system are much higher than those of dehumidification. Vacuum drying can often be justified when drying thick, hardwoods.
dehumidification drying
They are energy efficient equipment and their operating cost is usually lower than other drying methods. This is true even though a dehumidifying dryer uses electrical power to drive the fans inside the chamber, as well as the blower that drives air to the dehumidifying coil and the refrigeration compressor; while a conventional system burns less expensive fuels, such as gas, diesel or biomass. But the reason a dehumidification system costs less to operate despite the fact that a conventional system burns cheaper fuel, lies in the dehumidification system's ability to conserve energy by recycling heat.
In a dehumidifying dryer, hot air (usually starting at a temperature of about 29°C) is circulated over the wood with separate circulation fans, evaporating the water contained in the wood. The hot, humid air passes through a cold cooling coil where its temperature decreases. In the cooling coil, the water evaporated into the air condenses into a liquid form and flows down the drain as a stream of cold water, rather than as a cloud of vapor carried by hot air, as in a conventional dryer.
When the air cools down in the cold coil, the system immediately uses the heat removed from the air to heat the air again. The energy efficiency of return heat is such that each time this process occurs, the air leaves the dehumidifier at an even warmer temperature than when it entered. As the temperature of the air in the dryer increases, it can reach temperatures of up to 75 - 80 °C. If the temperature is higher than desired, the operator can vent the excess heat outside. Dehumidifying dryers are very easy to operate and are very popular with beginning wood dryers. They are also popular with experienced operators who want a system that requires minimal attention for flawless drying. Dehumidification is usually the least expensive to run and install by a wide margin. Drying times with a TROCKNNER dryer are approximately the same as conventional dryers.
