For Industrial Washing Systems – How Dry is Dry?

Many clients will specify that they want their washing systems to effectively wash their products, which can be a straightforward process, but they will also ask for the products to be dry on exit from the machine.

Now it may sound an odd question, but one should ask how dry is dry? Washing of products, as stated above is fairly straightforward, but not many people realize the complexities and cost implications of an air knife or drying system.

It is incorrect for any washing machine supplier to refer to their basic Air Knife section as “dryers” as the term drying implies the presence of no residual water on the washed components on exit from the system, which as most manufacturers know is extremely difficult with air knives alone.

Typically use with Box Washers or Tunnel Washers, an air knife is a blade, supplied with air via a centrifugal blower, side channel blower, or something similar that is positioned to remove excess water from product surfaces prior to exiting the machine. The blade creates a wiping effect over the product, similar to the final stage of a commercial car wash. This pushes the water to the rear or the product as it passes, and blows it back into the machine, thus clearing the product of water and minimizing water carryout from the system. Sounds easy, but there are caveats.

In order to maximize the effect of the air knife, the distance from the product should be minimized because, as with jetting, the further away from the surface the less effectiveness it is in removing moisture. This can be a particularly significant problem with items such a crates and basket where the knife is at the height of the container and away from the internal surface. Although water will be blown off the surface, it may not reach all surfaces equally

Flat surfaces are ideal for air knife effectiveness, as a standing wave can be created, and maintained over the smooth plane (see Fig 1).

FIG1

However, unless “drying” baking sheets or shallow trays, invariably most crates and plastic containers have strengthening ribs on the outer surfaces, which break up the standing wave and make the air knife less effective.

Most of the water can be removed from these areas, but due to the strengthening “ribs” complete drying is almost impossible with air knives alone. Generally, about 6-8 grams of residual water can be achieved cost effectively, depending on process speed and container design. Beyond this, heat is required to evaporate the residual liquid.

Even with flat plastic trays, another issue is that they give up heat fairly quickly, so the surfaces of the tray cool rapidly, making any heat used in the knife itself less effective (you only need to look at plastic utensils in most domestic dishwashers to see the lack of surface evaporation).

Heat can be introduced into the airflow for the knife by various means and can make a slight improvement on the dryness levels, but the energy required to do this, when offset against the improvement gain, can be costly. An air knife blower and pipework can generate up to 77-86F without adding heat due to friction and compression of the air within the system.

In order to offer complete dryness, a heat tunnel can be employed, which acts as a travelling oven, maintaining a high ambient temperature, and employing directional nozzles to assist in accelerating the evaporation of the water from the product surfaces. The trays, when subjected to this type of drying, should not exceed the supplier’s specified working temperature (generally around 140F) to prevent distortion or damage, so careful control of the system is paramount.

As with both the Air Knife and Heat Tunnel systems, the process speed and crate design will influence the dryness achieved, and a compromise may be required, due to space constraints, available budget and energy usage.

So next time you are preparing a Request for Quote, please ask yourself how dry do the containers really need to be? If you are using them immediately or for dry goods, will an air knife system be sufficient?

Unitech Washing Systems have designed, built and supplied various systems with air knives and heat tunnels successfully for many of our clients, so please do not hesitate to contact us for further information on our washing equipment.