The term pulse combustion originates from the intermittent (pulse) combustion of gaseous fuel in the pulse combustion engine, in contrast to continuous combustion in conventional burners.
Such periodic combustions generate intensive pressure, velocity, and to a certain extent, temperature waves propagating from the pulse combustion unit through a tailpipe (a diffuser) to the process volume of the drying chamber.
In order to excite enhanced pulsation in the drying chamber, the operating frequency of the pulse combustor engine must match the frequency of one of the natural acoustic (dynamic) modes of the process volume . When this condition is satisfied, the pulsations are in resonance and the process is resonance-driven.
Pulse Combustion Drying Technology is optimal in terms of:
The pulse combustion engine consists of an outer shell, an internal combustion chamber, a rotary air valve, a gas inlet, a gas pilot, a quench air inlet, a tailpipe and exhaust . The electric motor on top of the outer shell is used to rotate the rotary air valve as required.
For an operation cycle, air is pumped at low pressure into the outer shell and through the rotary air valve into the internal combustion chamber. At the same time, the fuel enters the combustion chamber and is ignited. A small explosion in the combustion chamber follows, creating extremely hot air (1000°C) at around 20 kPa pressure - a pulse or a sonic wave - that moves at high velocity down the tailpipe . The hot air is prevented from flowing upwards from the combustion chamber . Before it reaches the atomizing nozzle at the end of the tailpipe, the quench air is blended with the hot air to cool the air down to 650°C-750°C.
The rotary air valve closes just after ignition and as the combustion chamber empties, the rotary air valve opens again. The next charge of fuel and air enter and when they mix, the temperature of the chamber cause the next ignition. The cycle repeats at a controlled rate of around 100 times per second.
Gas-dynamic atomization takes place at the top of the drying chamber, where a low-pressure, slow-moving viscous feed (slurries and pastes) is introduced through a single pipe into the pulsating very-high-velocity gas stream. The initial droplets are quite large, as the feed introduction nozzle is a straight pipe with no restrictions. As soon as the low-energy droplets experience the high energy of the gas stream, they break up into thousands of droplets through a process of successive division. The surface area of the droplets increases dramatically and efficient evaporation of the surface moisture occurs at up to 98%. Thus, atomization and evaporation happen at exactly the same point in space and time, under extremely turbulent conditions.
In other words, within 3 to 6 milliseconds the pulsating exhaust gases with temperatures of 650°C-750°C produce ultra-fine (5-50μm) droplets and convert the surface moisture of the droplets into super-heated steam, leaving behind a very dry, fine powder with sphere shaped, smooth surfaced particles.
Although the temperature of the gas used to dry the product is very high (650°C-750°C), sufficient energy is absorbed by the evaporating water allowing very little increase in the temperature of the solid particles.