Paper Presented at the 6th International Heat Pipe Symposium – 2000, Chiang Mai, Thailand
November 5-9, 2000

Dehumidifier Heat Pipes for Rice Drying and Storage
By Khanh Dinh, President


Heat pipes have been used in various applications from cooling electronics, energy recovery to enhancing dehumidification in air conditioning; most of these applications have been in the High Tech domains. Today I would like to present to you an exceptional application: heat pipes for rice drying, to help the Farmers of the World.

Why Rice? Of the various types of crops, rice is one of the most important. Although one of the youngest species of grains cultivated by human, rice feeds billions of people all over the world, and its consumption is ever increasing. Rice combines so many virtues that it can be considered as the ultimate gift to mankind.

Why Rice Drying? The need comes from rice’s popularity: the demand for rice is so paramount that farmers around the world, especially those in Asia, grow two or even three crops per year. Traditional sun drying is no longer appropriate because it is likely that at least one or two harvests will occur during the rainy season. Sun drying, although the most traditional and natural way of drying rice paddy, in itself does not yield the best or the highest production since it could not be effectively controlled. Rain damage can take a heavy toll on the farmers who solely rely on the sun for drying.


The advantage of mechanical dryers has been established for many years. Just about all the wheat and corn produced in America is mechanically dried. However, drying under a tropical climate is much more difficult.

Most rice dryers rely on heating air, then blowing the air through wet rice paddy to evaporate water from the rice. This process requires large amount of energy to supply the heat for water evaporation as well as to heat the air, which is used as the carrier. The performance of traditional dryers depends heavily on ambient conditions, especially ambient relative humidity. On a sunny day, a batch of rice can be dried in six to ten hours. On a rainy day, it could well take three times as long and three times as much the energy. The irony is when you do have the need for drying, the dryers function efficiently at their worst.

The drying system presented today is a complete departure from the traditional way to build a dryer. This system uses two technologies to achieve an almost impossible task: make dry air out of wet air, and make heat out of humidity. Dehumidifier heat pipes are used to produce dry air, and a latent heat pump is used to produce low cost heat energy, from the very ambient air.


The concept of heat pipes for dehumidification is derived from a NASA spin-off technology. In NASA’s space program, the shuttle had to be dry upon its landing in Florida, where the weather is quite humid. Our company has developed, almost 15 years ago, a special air-conditioning/dryer machine to perform such a task. That machine was the first machine to use a Dehumidifier Heat Pipe. The concept is better explained in the following illustrations:

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With this concept in mind, the engineers at Heat Pipe Technology of Gainesville, Florida developed a special dehumidification/air conditioning line of products which used heat pipes to increase the dehumidification efficiency of vapor compression in air conditioning. Such products are now widely available on the market to control humidity at low energy cost.

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Custom Wrap-Around Heat Pipes

U-Shaped Heat Pipes

Heat Pipe Technology has since become well known in the field of heating, ventilating and air conditioning (HVAC) and dehumidifier heat pipes are used all over the world to provide dry cool air at great energy savings. Several heat pipes are installed in Thailand including one at an electronic manufacturing facility here in Chiang Mai, and we have many other systems installed in Bangkok.

Some years ago I had the opportunity to present this application of heat pipes in dehumidification at the Heat Pipe Symposium in Melbourne, Australia. Today I have the opportunity and great pleasure to introduce to you a very "down to earth" application, which can help many farmers in Asia to preserve and conserve the grains of their hard labor, the DaikaTM Heat Pipe Rice Dryer.


DAIKATM, which mean "great goodness," is the trade name of this new rice dryer. This new invention is created with the full faith of benefiting humankind, by facilitating the post harvest processing and the storage of rice. This new invention promises to save energy, and to perform the almost impossible: it works better when the air is most humid and it works best when it is raining!

Let us now talk about rice drying. As harvested, rice with its husk (or paddy) from the field contains about 25% to 30% moisture. To conserve paddy, one has to bring the moisture content down to 13-15%. Huge amounts of water must be evaporated from the paddy. In the process of drying rice, we need heat. But too much heat damages the quality of paddy: the grain becomes brittle and breaks at the milling stage, and the polished rice could be yellowed. Ideally rice paddy should be dried below 120F (49C) to prevent the loss of quality and breakage at millings. Rice seeds have to be dried at even lower temperature of around 100F (38C) to preserve the germs. At such low temperatures if the ambient air is wet, drying times will lengthen. For example, when it rains, it can take a whole day to dry at 120F and even longer at 100F. This results in high energy consumption and low production.

If we can create dry air, we can achieve faster drying rates at lower temperatures. This is exactly what the DaikaTM Dryer does. It creates dry air out of wet air and uses the heat of condensation of the water vapors as the source of heat.

The principle of operation relies on two technologies. First is a heat pipe enhanced dehumidifier to extract moisture from the air. Second is a high efficiency latent heat pump. The dehumidifier produces cool dry air, which is then heated up by the heat pump. In this process the heat source comes from the heat of condensation of the water vapor in the wet air. Therefore, the dryer is self-regulating, and the more humid the air, the better the dryer works!


Until today just about all dryers work on the principle as illustratred below.

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Drying in Humid Climate

In this system, the drying air supplied to the grains depends on the ambient air. The supply air is wet when the ambient air is wet. The drying psychrometric is as shown here.

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Traditional  Process


The concept of dehumidifier drying is not new. Numerous studies have shown its advantages, especially for low temperature drying.

A dehumidifier as illustrated provides dry air and performs high quality drying. But such system, as normally used in an air conditioner, consumes large amount of electricity to cool down the air and to heat it back up, resulting in high energy bills.

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Dehumidifier Drying


In this application, the presence of the heat pipe drastically reduces the need for the cooling coil capacity while the moisture removed is still the same as the Dehumidifier Drying as illustrated previously. The psychrometric process is illustrated below:

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Dehumidifier Heat Pipe Drying Process

In this process, air going into the precooling section of the heat pipe gets free cooling (from point 1 to 2) The cooling coil now can cool the passing air down with much less energy. (from point 2 to 3) The cold air coming out of the cooling coil is then reheated freely (from point 3 to 4) before going into the condenser to be heated to appropriate drying temperature (from point 4 to 5). Air going in the rice bin gains large amounts of moisture from point 5 to 6.

Since the heat source heat for the condenser is provided by the compressor which operates with a COP of 3 or better, large amount of energy is saved.

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Electric Heat Pump with Dehumidifier Heat Pipes

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IC Engine Powered Heat Pump with Dehumidifier Heat Pipes

This design presents an even more energy efficient system. Where electricity is not available, an engine can be used to power the compressor and blower. The waste heat from the engine can be captured to provide higher drying capacity.

Compared to a dryer using direct burning of the same amount of fuel, this system typically provides three times the drying capacity when the waste heat from the exhaust is used, and twice the drying capacity when only the heat from the cooling system is used (to avoid fumes from the exhaust of the engine).


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Capacity 8 tons of paddy/day
Power Used 288 kWh
Cost of Drying USD23 (1,000 baht)
Ambient Air 30C, 70% RH
Drying Air 45C, 18% RH
Paddy In 28% moisture content
Paddy Out 14% moisture content
Typical Head Yield 10% above sun dried
Estimated Whole Grain 200-300 Kg/day

Example of a Return of Investment for a DAIKA 8 Based on a 100-Day per Year Operation :

DAIKA 8 Drying Compared to Natural Drying

Days of Operation (per year) 100 days
Total Rice Paddy Dried 800 tons
Extra Whole Grain Produced 25 tons
Extra Value Produced $5,000
Less Cost of Energy $2,300
Benefit $2,700/year
Return of Investment (for $8,000 capital) 3-year payback time


Even though the drying system presented here uses a static bin, the DaikaTM Dryer can also be connected to a grain recirculation system for continuous drying. The dryer can be used in a closed loop fashion, especially with fluidized systems where much energy is still available in the air leaving the fluidized bed.

The DaikaTM Dryer can also be connected to a storage bin or warehouse for long tem storage of grains, bran, seeds with a minimum amount of energy, compared to other refrigerated storage systems because the DaikaTM system does not cool the air as much.


In reality, the DaikaTM Dryer can be used to dry many other agricultural products such as soybean, coffee, pepper, corn, wheat, etc. Even Longan has been successfully dried with the DaikaTM Dryer. One highly beneficial application lies in seed drying since seeds need to be dried at lower temperature than food products. Low temperature is necessary to preserve life of the seeds. At our facility in Florida, we are experimenting with peanut seeds, soybean seeds, and grass seeds with excellent results. In most seed drying applications, we see the possibility of reducing drying time in half and cutting the energy cost to one-third.

In rice drying, we typically obtain a 50% saving in drying cost compared to other method of drying. Most importantly, we routinely experience a 10% increase in whole grain yield. This means that if this technology is applied worldwide, the production of whole grain rice will increase by 10% with less land to be cultivated, and less fertilizer and insecticides to be used.

In a world where cultivated land is lost to erosion, pollution and urbanization, we hope that this invention will come to help the people that our high tech societies often forget--the people who feed us and our children, the Farmers of the World.

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Last modified: February 27, 2004 14:24 -0500