Technical Details Regarding Pulse Drying
On this page we explain a few subjects related to pulse combustion spray drying.
The PCS Pulse Dryer is more efficient than a conventional spray dryer for four main reasons, as follows:
- Higher Solids. On most feedstocks, the PCS Dryer can handle a higher solids loading and viscosity than a conventional spray dryer. This has a dramatic effect on the cost per dry pound of powder.
- Higher Heat Transfer Rate. In a PCS Dryer, the wet feed experiences a turbulent, pulsating gas stream having a contact temperature up to 1500 F greater than a conventional spray dryer (See table). Heat transfer is very high, and drying is complete within a few seconds (See chart). On some materials, thermal efficiency is as high as 1400 Btu per pound of water removed.
- Lower Dryer Exit Temperatures. The high rate of heat transfer and the very rapid drying allow pulse dryers to operate at lower dryer outlet temperatures, which in turn reduce product degradation and increase thermal efficiency.
- Lower Maintenance. The PCS Dryer uses no high-pressure pumps, and no high-RPM equipment. Unlike conventional dryers, there are no wear parts in the atomization process.
PCS is an exceptionally good choice when handling an abrasive or corrosive feed since we use no high-pressure pump, no nozzles, and no rotary disk. Atomization is accomplished in the exhaust jet of the pulse combustion burner, and the liquid is pumped to our atomizer at low pressure, using a simple, low-maintenance pump.
When the nozzles or rotary disks of a conventional spray dryer begin to wear, the dynamics of atomization change and the dryer begins to produce a different particle. In a pulse dryer, there are no atomizer parts to wear out; every droplet sees the same atomization energy and the same differential temperature. Thus, particle size distributions tend to be tighter than those obtained from a conventional spray dryer. On a related note, the mean particle size tends to be somewhat smaller than that of a conventional spray dryer.
The PCS atomizer has no restrictions in the feed stream, and is completely modular and variable. The operator can easily make major changes to the atomization process to optimize the powder. We have been able, for instance, to move average particle size from 10 microns to 100 and back to 10 with process changes only, although not all products are as controllable. Changing the atomizer on our dryer takes five minutes and only a few simple tools.
High Inlet Temperatures
High inlet temperatures can be used without scorching the feed material because the extreme turbulence in the atomizing zone of the dryer causes a high rate of heat transfer, evaporating the water quickly. This rapid evaporation cools the process, protecting the solid particles during their short residence time in the drying zone. Egg whites can be pulse-dried at an inlet temperature of 1000F without any damage to this delicate protein.
The pumping pressure in the PCS Dryer’s feed system is usually less than 1 psi, and is never more than the pressure required to move the feed through an open pipe with no strictures. The highest air and fuel pressure delivered to the PCS Dryer is 6 psi. The highest rotational speed is found in our off-the-shelf fans and blowers.
Special noise abatement systems are not required for a PCS Dryer. Ambient noise is less than 85dB around our pulse combustion dryer when in operation. Previous generation pulse dryers were unacceptably noisy, with typical noise levels around 130dB. This was because the pulse combustors were fabricated with lightweight metal (from their aircraft heritage), and because the inlet end of the combustors were open to the atmosphere. PCS’s engineers completely changed the design of the combustor, replacing the lightweight metal with thick-walled Inconel, and then encased the entire combustor in an outer shell of quarter-inch stainless steel. These two major upgrades reduced operating noise levels to less than 85 dB.