There are two distinct means to “dry” or remove moisture from a surface or object: evaporation and mechanical stripping.
Evaporation is an action causing a change in the state or phase of the liquid to a gas or vapor.
The change in phase of liquid water to water vapor requires either considerable energy and/or time to transpire. Under normal conditions, only the top layer (boundary layer) of water molecules evaporates to become water vapor. The application of heat hastens this phenomenon by exciting the water molecules, resulting in greater molecular movement and speeding the phase change process, but only within the dimension of the outer single molecule boundary layer of the liquid. The process of engaging the whole mass of the liquid in a phase change to gas or vapor state is boiling — at temperatures that are obviously outside of the realm of drying cars. Common applications of evaporative drying are clothes dryers, dishwashers, restroom hand dryers, and hairdryers. The point is that all of these evaporative approaches to drying are time consuming or require less than reasonable energy inputs.
This is the stripping of the moisture by means of a stream of air or other contact.
Stripping offers the potential to quickly and efficiently separate the liquid from the surface by mechanical action. This can include contact with the moist surface with towels, chamois, or other absorbent materials to wick away the moisture. The focus of this discussion is the use of air streams colliding with the water (impingement) that is attached to a vehicle’s surface to cause the separation and removal of the water.
Mechanical stripping of moisture is ideally suited to the requirements of car washing. This is the basis of operation for self-service, in-bay dryers. It is economical, simple in practice, and can function very effectively. Ideally, the water is stripped or sliced from the surface of the vehicle and converted to airborne droplets, mist and/or water vapor, and blown away. There are a few variables that affect the performance of mechanical “air” dryers — surface condition, profile and orientation (water runs downhill, right?), temperature, and the force and direction (angle of impingement) of the air stream at impact with the liquid to be removed.
The surface condition of the vehicle dictates the power of attachment forces between the water and the surface. The condition of this water can range from beaded (loosely bonded to the surface) to flat (more powerfully bonded to the surface). Itis well known that dirty, oxidized paint and un-waxed vehicles don’t dry well compared with clean, well-sealed finishes. Lower temperatures also change the surface tension of water, reduce “beading,” and require more stripping energy for drying. “Beaded water” rests on the vehicle in the shape of droplets forming “sides or walls” that catch the force of the air. See illustration above.
On the other hand, “flat water” resists drying in two ways — higher attachment to the surface and an obviously poor impact profile. However, if the force of the air stream is sufficient, small “waves” will be created in the water surface changing the profile enough to create impact points and allow stripping to occur. See illustrations on page 20.
Obviously, under these less than ideal conditions, the drying (stripping) process takes longer (multiple actions have to occur), and a great deal more force is required at the point of impact (impingement).
This brings the discussion to performance, which typically begs the question of power, specifically horsepower, and the amount required for best results. The snappy answer is: just enough to dry the car; anything more is a waste of energy. The serious discussion of the matter involves how the input power can be most effectively focused on the point of impingement. Here’s a great example of force concentration: a 100-lb. lady in stiletto high heels, walking with a normal gait, can create an impact force on flooring more than 30 times greater than a 250-lb. man walking across the same floor in typical street shoes — and do it in style, of course. The difference is the focus or concentration of the lady’s weight — gravitational (normal) force in an area that is 50 times smaller or more “focused” than the big heel of the man’s shoe. The same principles can be applied to drying to achieve optimum results efficiently. Here’s how:
First, let’s begin with a layman’s explanation of the manner that fluids act and react to external forces. We’re all familiar with the action of water as it exits a nozzle under pressure. Eventually, it falls to the ground after covering a considerable distance. During this brief “flight,” it initially exhibited substantial force (impact pressure) that was much more noticeable close to the nozzle and was rapidly reduced by the resistance of air and action of gravity over ti
Schlieren photograph of air stream exiting a round orifice.
Now visualize this water stream exiting a pressure gun or hose into a large body of water — beneath the surface of a lake or ocean, for example. Because the viscosity and specific gravity of the two fluids are matched the process of entropy occurs more quickly and differently. The water-into-water jet would travel a much shorter distance — and a great deal of commotion or turbulence would be “visible.” Gravity would no longer cause the water to drop to the bottom of the large body of water but instead the molecules would eventually just seem to come to a rest and swim about with the rest of their cohorts.
This is how air behaves in our “above-water” world. The process of loss of power (entropy) in air streams occurs as an inverse square relationship. Simply, it occurs much more quickly at first, more slowly over time. For each unit of distance traveled in “free air” the loss in velocity and force is one square less. This faster loss of velocity and force is caused by turbulence — vortices and eddies that quickly form when jets of fluids (including gases like air) are injected into masses of similar fluids. A picture of an air stream exiting a nozzle using a special photographic process is shown above.
Critical Importance of Nozzle to Surface Distance
Notice that the air stream becomes visually turbulent within the distance of one or two diameters of the nozzle and that this process of decay continues even more rapidly at the right hand side of the photograph (above). This photo shows how critical nozzle to surface distance is to the performance of drying with air. More distance to vehicle means more input (horsepower) power – and lots of it. In a wash tunnel, typical rack-mounted dryers require “brute force” levels of input power to produce the desired level of drying due to the larger distances required to provide adequate vehicle clearance.
Spread the Effort for Maximum Effect
Now that we’re all committed to moving our hypothetical, handheld nozzle as close as possible to the vehicle surface, the thought occurs that we may end up “sweeping the floor with a toothbrush.” The answer is to design a nozzle with a slot and not a round orifice, i.e., trade the little brush for a nice push broom. The slot shaped nozzle allows the user to keep the nozzle close to the vehicle surface and still dry large areas with power and speed. The increased power required to maintain a given level of flow only increases in proportion to the area of the opening. A 10 inch by 1/8 inch slot roughly comprises a 1-1/4 square inch (actual 1.22 sq. in.) opening as does a 1-1/4 inch diameter round orifice. A correctly designed slot-shaped tool can be worked within a fraction of an inch from the vehicle surface and dry a 10-inch wide “swath.” The round orifice must be held a distance of 12 inches or more from the surface to achieve this parameter — and at an enormous loss of force at the point of impingement. If the round nozzle is moved closer to achieve equal stripping power, the drying “swath” becomes tediously small.
Remember the “wave” illustration presented earlier? The shape of the “waves” mirrors the shape of the nozzle. Round-shaped nozzles create round-shaped waves — much like the ring waves created when a stone is tossed into a still pond. In actual drying use with round nozzles, this is seen as “blowing a hole in the water.”
The adjacent, unaffected water is “free” to flow back into the areas that were just dried. This is exacerbated by the turbulence of the airflow and a phenomenon known as the “Coanda Effect,” which is explained below. When the air stream from a round nozzle is aimed at the surface some of the air moves along the surface in the intended direction but the remaining airflow streams backward along the surface in a totally unintended direction — causing “re-wet.”
The slot-shaped nozzle uniquely creates a laminar (sheet) flow of air. In this case, the streamlines of flow become and maintain a more orderly straight-line travel across the surface directing the stripping action with precisi
This laminar flow developed from slot-shaped orifices is the basis for industrial air-knife technology — a technique employed for a wide variety of drying and stripping applications over decades. Bottling plants, painting and coating facilities, papermaking and printing plants, machining centers, and agricultural processors — to name a few — widely use air knives to rapidly and efficiently remove moisture and excess liquids from processes and products with absolute control.
There is another phenomenon of fluids that is useful in tool design for improving dryer stripping performance. In the earlier explanation of stripping: air molecules colliding with the surface water, it may have seemed as though these air molecules were acting like sub-microscopic baseballs or tennis balls rebounding against the masses of water and causing the release of the moisture from the surface. The
The Coanda Effect.
impact part is valid but fluids (liquids and gases) exhibit another unique aspect of flow and can be utilized for our benefit. When a fluid flows over or against a solid surface, there is an affinity to attach and flow along that surface; this is known as the Coanda Effect.
This is easily demonstrated by placing the bottom of a spoon adjacent to the flow from a sink faucet. Instead of bouncing away from the spoon’s curved surface, the faucet’s flow attaches to and is redirected along its curve in a curious and unexpected manner as shown in the photo below. Dryer nozzle shapes can be tweaked to take advantage of this effect to create a venturi and thus provide even more impact velocity and force.
The final answer to the “power” question: make and offer something that is small enough to fit in your customer’s hands, yet more than strong enough to thoroughly dry their cars or bikes — to the last drop, if desired. Your distributor or supplier would be happy to work out a customized set-up to boost your business with this game-changing service.
Mike Doyle is a 30-year veteran of the car wash industry having started with Doyle Vacuum Cleaner Company in the 1970s. He developed the Power Dry product while at Doyle in the early 1980s, which was the industry’s first handheld dryer for both vacuum islands and car wash bays. He founded Industrial Vacuum Systems with Joe Doyle in 1991. Mike and Joe Doyle presently own Car-Dry, manufacturers of the Blasto-Dry handheld dryer.