Dryer Life: It Has To Breathe
A review of airflow, ducting and related issues associated with safe and productive dryer performance.
Air is vital to all living things, and although a dryer isn’t alive, it too needs air in order to do its job of drying laundry in the most efficient and productive manner possible. Air is the most important aspect of the drying process. Without it, a dryer won’t function correctly. In extreme cases, a lack of air may damage the textile goods, the dryer, or your facility. Below, we take a high-level look at why air is so critical to the functioning of the dryer.
The dryer has four critical elements to the drying process, just like a washer. For a washer, the Wash Pie covers the elements of chemistry, mechanical action, time and temperature. But ultimately without water, the rest won’t work. The dryer has similar characteristics that make up the drying process. These elements form what is called the Dry Pie®.
As you see in Figure 1, the four elements of the Dry Pie® are mechanical action, time, temperature and airflow. You may ask, why is airflow the most important aspect of drying if there are four equal pieces of the Dry Pie®? Well, like water in a washer/extractor, without the airflow, your dryer won’t function. Let’s take a look at the other three elements and how they would contribute without air.
MECHANICAL ACTION
The first piece is mechanical action, or tumbling. Tumbling goods without airflow isn’t going to provide much drying action, even if there’s sufficient time and temperature applied. Taking that scenario further, if wet goods are tumbled with heat applied to them and left in the dryer for a long time, what will happen? Initially, there will be some evaporation. However, as the moisture leaves the goods, it has no place to go…remember, we’re operating without any airflow. Assuming the dryer is sealed reasonably well, the air inside the dryer basket will become fully saturated and no more evaporation will take place. This will cause an increase in temperature inside the dryer basket, and if left operating like this for too long, that heat will begin to cause problems. Most modern industrial dryers today have methods to prevent overheating, and in this situation those protective devices will most likely trigger an alarm and stop the dry cycle. If you checked inside the dryer, you would find that they have remained wet, and have close to the same amount of moisture in them as when they started. The dryer just used a fuel source (temperature), mechanical action (tumbling) and time. The problem is that there was no airflow to continually remove that evaporated moisture, allowing for continued evaporation to occur until the goods became dry. Now, a look at the opposite scenario; plenty of air and some time and mechanical action, but no heat.
AIRFLOW & DUCTING
In this situation, the goods are tumbling through a steady stream of air. Picture clothes on a line during a cool, windy day. Given enough time, even with little heat, the goods will become drier. Airflow will cause the evaporation process to begin and moisture to wick away from the goods and exhaust to atmosphere. In order to provide sufficient airflow inside a dryer, a steady stream of air must enter and exit the dryer basket. This movement of air typically is done with a motor with a wheel (or scroll) with blades, which develops a suction, pulling air from outside the dryer through the basket and blowing it out an exhaust duct. In the next section, we’ll examine how to get air into the dryer and how to move it out.
(l/r) Here are examples of gooseneck, “No Loss” and cap-style stacks that are designed to prevent rain, snow and debris from entering the dryer duct work or to prevent excessive back pressure.
For a dryer to be efficient and productive, it must be sealed relatively well. This means that air drawn into the dryer basket comes from its intended source. This is typically from one of two possible areas. The air may come from inside the plant or be drawn in through a duct from outside. This same air, now full of moisture from the drying goods, is exhausted using ductwork that ultimately exhausts outside the plant.
Since the source of air for the dryer can come from either outside or inside the plant, that source is rarely the problem with a dryer’s airflow. Usually the problem comes from not exhausting the air properly, thus causing heat buildup inside the dryer basket. Ultimately, this exhaust restriction that can lead to poor drying performance and possibly even damage to the dryer or in the worst case scenario, spark a fire.
The method of exhausting this hot, moisture-rich air is the key to efficient and productive dryers. Since this air is not being discharged into the plant, it must travel from the dryer to the outside through a closed duct. This movement of air through a duct creates pressure in the duct itself. Picture blowing through a straw with and without your finger partially over the end. The harder you try to move air through the straw with your finger partially obstructing the end, the harder it becomes. This same phenomenon is happening inside that exhaust duct that you have connected to the dryer. This restriction can lead to poor performance, low energy efficiency, and possibly even cause damage to the dryer in the form of a fire. Does this mean you have to be an engineer and design the ductwork yourself ?
The answer is no. All manufacturers that sell industrial dryers specify the maximum back pressure (restriction in the ductwork) that their dryer can handle safely and efficiently. Once this parameter is identified, any competent mechanical firm should be able to design and calculate this back pressure, given the path your ductwork must follow to exit the plant.
Remember the example of blowing through a straw with your finger partially over the end? Now you can look at the ductwork design that your mechanical contractor has proposed and look for areas that might cause a restriction, creating added back pressure. Using the straw analogy, what happens if you bend the straw a few times? You will find that these bends create a partial restriction and increase the pressure that must be overcome to move the air efficiently out of the dryer. Ductwork acts in the exact same way. If your ductwork needs many turns and bends to make its way out of the building, there is a good chance you are going to have issues with back pressure. Does this mean you have to have a straight run out of the building? The answer is no, but to overcome this added back pressure, you must either increase the diameter of the ductwork to allow for more air flow or help the air escape by providing a secondary means to aid in pulling the air out.
The exhaust and inlet ducts connected to your dryer can cause problems since they are open to the outside elements. This means that you must prevent rain and snow from entering your dryer. To do this, many types of protection can be employed. One method is to install what’s referred to as a gooseneck on the end of the ductwork. Instead of pointing straight up at the sky, the opening points down, thus preventing the elements from entering the duct. Another method is to install a cap over the top of the duct, leaving space between the duct top and the cap.
A third method is to install a straight piece of duct, which is normally four times as long as the diameter of the duct. This is called a “No Loss” stack, and is the best course of action to prevent excessive back pressure like the first two methods might cause. The “No Loss” stack is sized approximately one-to-twoinches larger in diameter than the actual exhaust duct. Because of the four-times length, rain/water will ultimately end up on the inner wall of this stack and simply run out the bottom, instead of running down your ductwork and back into the dryer. It is the best method of preventing excessive backpressure and keeping out the elements. P
PRODUCT PARTICULARS
This article has focused generally on airflow and preventing any restrictions to the manufacturer’s specified velocity and volume, or more specifically, maximum static pressure. The last portion of this article focuses on some of the possible outcomes of a restricted exhaust duct.
As described in general terms above, restricting airflow to a dryer can lead to some big problems. The largest being the potential for fire. Remember, if the hot air cannot escape from the dryer basket, it will continue to build up heat inside the basket. Depending on the type of goods being dried, this excessive heat could cause some types of goods to combust. Shop and bar towels are highly susceptible to excess heat and have been known to cause dryer fires. Even if the dryer senses the high heat and stops the dry cycle, damage can occur. This is especially true with barrier-type garments that air has a hard time passing through in the first place. These types of garments also are typically more sensitive to high heat and can be damaged easily when temperature exceeds what’s recommended.
Another common product is microfiber. Microfiber manufacturers’ recommendations for maximum heat typically range from no heat to 150°–160° F maximum. Any exposure greater than that recommended will cause a breakdown of the microfiber’s properties (best case), causing poor performance of the product, or it may actually melt and cause damage to the product and possibly the dryer as well.
Incontinence pads are yet another barrier- type garment that has very specific maximum-heat specifications, which if exceeded can cause damage to the product and possibly the dryer as well. As you can see from these examples, knowing the heat specifications of the goods being processed, coupled with ensuring that the dryer is able to breathe correctly are critical to this vital part of the wash alley operation.
Although this article only scratches the surface on the importance of airflow and how ducting can influence that flow, we hope it provides you with guidance to help ensure that your new dryer is able to perform up to the manufactures’ specifications and protect the product you’re processing from damage. The consequences of ignoring dryer-temperature recommendations or ignoring the dryer manufacturers’ back pressure specifications could be catastrophic … leading to a fire and loss of property or life. Do your homework and that new dryer can deliver the return on investment that you were anticipating.