Switching to a vacuum steam heater can improve product quality and avoid equipment wear-and-tear.
All concentrated milk products are typically preheated as much as practical just before they are finely dispersed into the hot air stream within a spray dryer. Atomization is most commonly achieved with a high pressure spray nozzle, but can also be done using a rotary atomizing wheel. In either case, powder is produced when the residual moisture is evaporated from the preheated concentrate within the spray dryer.
In order to maximize the performance of the spray dryer, the concentrated milk should be preheated to a practical level, so that evaporation can take place quickly following atomization. This preheating step elevates the concentrate temperature to near the atmospheric boiling point, and is best accomplished in an indirect shell-and-tube steam heater with a small approach temperature. This achieves gentle heating, and no heat damage to the solids.
Unfortunately, many older spray dryers utilized steam preheaters that were designed to operate at or above atmospheric pressure (>14.7 psia, and >212 ºF), and consequently have a large approach temperature. If, for example, the target concentrate temperature into the spray dryer is 150 ºF, and 212 ºF steam is used in the heater shell, there will be a large (62 ºF) approach temperature. This will heat damage the solids and the heater tubes will foul rapidly, resulting in a reduced spray dryer capacity after only several hours of operation. To make matters worse, these heaters experience considerable thermal stresses, so they frequently develop tube-side and shell-side cracks.
The Vacuum Steam Heater to the Rescue
The solution is to replace this type of heater with a vacuum steam heater, which is typically installed with current design spray dryers. When properly designed, such a heater will operate at a much lower (15 ºF) approach temperature, with a shell temperature of ~165 ºF. The shell of the heater would be operating under vacuum in this case and there would be no appreciable heat damage to the solids at these conditions. The thermal performance of such a heater will be consistent and allow the spray dryer to also receive a consistently preheated feed.
Do these conditions exist in your facility and is it time to improve system performance? The solution is straight-forward, and a simple process assessment will confirm that this upgrade will be beneficial.
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