Heat Sources
WOOD-WASTE FIRED STEAM BOILER
Steam heating is good for large drying plants with more than 10 units of 75 m3 and are spread over 100 m length. Steam can flow over long distances without using a pump. For hot water and thermal oil heating, circulation pumps are required, and flow distribution must be well planned. Steam heating is also necessary if the timber needs to be steamed for colour change, if the timber has water based chemical impregnation, if timber has resin and needs steam to soften the resin, or if the timber needs high temperature drying.
It is important to note that running a steam boiler, the water supply must be adequate as water loss due to flash steam and steam spray will be quite high. If the condensate from the dryers is not returned to the boiler, the losses will be 100% of the rated steam production per hour. Together with the water loss, softening and treatment chemicals will also be lost. The quality of the water is most important. If the water is very hard and contains many dissolved solids, it must be treated properly by a professional water treatment company. Water samples must be sent to the laboratory for analysis. Hard water with dissolved solids can ruin the boiler tubes very quickly.
Tritherm does not manufacture steam boilers, but we do work with different manufacturers in various countries.
We are happy to provide a guideline for sizing the boiler capacity.
- For drying tropical hardwood – use 4 kg of steam per m3 of timber, if you are in the tropics.
- For drying timber with water based chemical impregnation – use 5 kg of steam per m3 of timber.
- For countries with mild winter, add 1 more kg of steam per m3 of timber to the calculation in Item #1 and #2.
- For countries with severe winter, add 2 more kg of steam per m3 of timber to the calculation in Item #1 and #2.
(TO CALCULATE THE HEAT CAPACITY FOR THERMAL OIL, HOT WATER HEATER, HOT AIR GENERATOR, OR HEAT PUMP IN KCAL/HR, JUST MULTIPLY THE STEAM CAPACITY IN KGS BY 580 KCAL/KG)
WOOD-WASTE FIRED THERMAL OIL HEATER
Thermal oil heating is good for high temperature drying and for ThermaWood heat treatment process requiring temperatures over 200°C. Thermal oil has a low heat transfer coefficient, half that of water. The oil flow needs to be fast for effective heating. For a 1,000,000 KCal/hr thermal oil heater for example, a pump capacity of 100 m3 per hour with 30 Kw motor power is required. Whereas, for a hot water heater, only a 15 Kw circulation pump is required.
If 1 dryer out of 8 dryers is running, the circulation pump will still have to run at full capacity. A wood fired thermal oil heater requires an emergency circulation pump (diesel engine driven) in case of power outage. An emergency pump with at least 25% of the rated flow rate is required. During a power outage, the oil circulation stops. The remaining wood waste in the combustion chamber will continue to burn or amber. The oil film temperature will likely exceed the oil breakdown temperature resulting in severe oxidation and the cracking of the oil. This will result in outgassing and hardening of the oil which can result in a fire and an explosion. It is important to send oil samples to the laboratory to check for oil degradation.
There should be at least a by-pass valve that can be opened , either manually or automatically to allow the oil from the elevated expansion tank to flow back to the ground level receiver tank through the heater. This will cool down the furnace piping slowly, avoiding overheating. It is important to engage a competent supplier who understands thermal oil heating safety and can train the operators professionally.
Tritherm has designed and installed several wood-fired thermal oil heaters with automatic feeding. Thermal oil piping has to be welded by skilled welders as leakage after filling with oil will be troublesome. After compelting the pipe installation, compressed air pressure test should be carried out.
WOOD-WASTE FIRED HOT WATER HEATER
Hot water heating for timber drying is safe and economical. It is safe because it is an open system like a water kettle. It is economical because there is no water lost as the water is recirculated in the system and water treatment is done only once a year. The water can be heated to 95°C which is sufficient to dry timber up to a temperature of 75°C. For hard and medium hardwood that has not been treated with water based chemicals and has no wood resin, a hot water system is very much suitable.
Tritherm has built many wood-waste fired hot water heating systems. Automatic Feeding of Sawdust with clean combustion is available.
WOOD-WASTE FIRED HOT AIR GENERATOR
For single dryer use, a hot air generator is the most economical heat source available. Our hot air generator has a 3- pass design and uses high temperature SS310 heat exchanger tubes that can resist high temperature creep.
Wood off-cuts can be manually fed into the furnace. The combustion flue gas will transfer the heat to the drying air indirectly through the stainless heat exchanger tubes. The generated hot air enters the dryer at the top of the drying chamber at about 105°C. The air returns to the hot air generator from the bottom of the chamber for reheating.
Hot Air Generator can be equipped with an optional automatic sawdust feeder complete with mini sawdust storage silo. A pellet burner can also be attached to the furnace if wood pellets are available.
HEAT PUMP
A Heat Pump is an electrically operated device that moves heat from one place to another using a refrigerant as the carrier. The principle is much like the air conditioner or the refrigerator. While a refrigerator moves heat from inside the fridge to the outside, a Heat Pump moves heat outside air (ambient) to inside the dryer room.
When a refrigerant is compressed from a low-pressure vapour to a high-pressure liquid, lots of heat is generated. Using the new generation of scroll compressor with R134a refrigerant, 1 kW of electrical power to the compressor can provide heating power of 3 Kw. This energy ratio is called Coefficient of Performance (COP). The old generation of dehumidifiers, that used the old rotary or piston compressor, have a COP of about 2.0.
During expansion in the Evaporator, heat is absorbed by the refrigerant from the surrounding as it expands. The air passing through the evaporator will be cooled and can be directed to cool an office nearby. The heat absorbed will then be lost again during compression when the vapor changes to liquid phase at high temperature and pressure. The refrigerant will then release the heat through a Heat Exchanger placed inside the dryer.
During expansion in the Evaporator, heat is absorbed by the refrigerant from the surrounding as it expands. The air passing through the evaporator will be cooled and can be directed to cool an office nearby. The heat absorbed will then be lost again during compression when the vapor changes to liquid phase at high temperature and pressure. The refrigerant will then release the heat through a Heat Exchanger placed inside the dryer.
Since the drying heat is generated electrically, it will be wasteful to vent the humid and hot air without recovering the heat. For this purpose, Tritherm has designed a Heat Recovery Ventilator (HRV) to preheat the incoming fresh air by the outgoing hot air.
The below illustration gives you an idea how the dryer is heated by heat pump and how the Heat Recovery Heat Exchanger is placed.