SELECTING THE DRYER FOR YOUR APPLICATION
The selection process for the best dryer for your application requires a close examination of your operation and what improvements would reduce the cost of production. With the rising costs of resin and energy, the proper selection of a dryer could pay for itself in less than one year. Also, remember that you may not use the same materials in the future as currently and should purchase for the long run.
The first question is whether central or beside-the press drying fits your process the best. You can also consider a combination where the drying is done near the press using a central material system to load the dryers. Central drying and conveying systems are ideal where the runs are longer, fewer materials, and no regrind is returned to the dryer.
Beside –the-press dryers are ideal for short runs, reuse of regrind, and flexibility. These dryers are normally supplied with a closed-loop conveying package to move the dried material to the press. This eliminates the contamination of the dried material with wet ambient air.
For a group of small molding machines, a hopper bank may be ideal. A typical hopper bank includes hoppers sized for a standard run with the ability of pre-drying material for the next job, reducing the idle time for the press. A properly designed system allows the control of the drying temperature for each hopper and valves to shut the hopper off for cleaning.
Almost every dryer manufacturer claims their dryer is the most energy efficient. While this cannot be true, an in house trial is suggested where several dryers are concerned. It is generally accepted that compressed air dryers are less energy efficient due to the cost of compressed air and the potential of having to add air compressors for several dryers. These dryers are ideal for occasional use or where few molding machines are used.
The costs of producing rejects is always more expensive than the cost of the dryer. This is especially true with the more costly resins that tend to be harder to dry. The replacement of inefficient dryers could greatly reduce operating costs due to reject levels and energy consumption.
Sizing the dryer system is critical. The four criteria for proper drying are air flow, temperature, residence time, and dew point level. Air flow and temperature are related parameters. The only way to heat the pellets is the transfer of the BTU’s from the air flow to the pellets. The lower the air flow, the longer it takes to heat the pellets to the drying temperature resulting in longer drying times. Higher drying temperatures also require higher air flows. We recommend 1 cfm per pound of usage to handle all resins.
The drying hopper size determines the residence time of the material which has to be long enough to dry the resin. To size the hopper, multiply the hourly usage by the drying time and select the closest hopper size. For example, a usage of 50 pounds per hour of resin that takes 3 hours to dry would require a 150 pound capacity hopper.
Often overlooked is the dew point level the dryer achieves. The migration of the moisture out of the pellet is the difference in vapor pressure inside the pellet as compared to the surrounding air. The lower the dew point, the greater the difference and the faster and more complete the drying of the pellet. For several years, minus 40 degrees has been the industry standard for acceptable dew point levels. Our dryers provide much lower levels independently recorded at minus 80For below resulting in much faster drying. Unfortunately, the meters that read this low are relatively expensive and are not usually found in most molding shops.
Downtime of the molding machine should be of major concern to today’s molders. Because of the shorter runs we see today, the downtime waiting for new dried material can be a significant factor in profitability. Our dual hopper series addresses this situation by having the next material drying in the second hopper while processing using the other hopper. We have had customers increase their machine time from 60% to 95% using this equipment. In some cases, it has resulted in eliminating the purchase of additional molding machines while increasing output.
The introduction of regrind at the press continues to be important as resin prices increase. By using a press side dryer, the regrind can be loaded from the grinder into the dryer reducing material and handling costs. We have introduced models that volumetrically add the regrind much more accurately than a proportional loader resulting in a higher, more accurate regrind percentage. This percentage can be adjusted while running with results seen in as little as one shot.
There are a few different designs of dryers on the market. The oldest and most successful is the dual bed dryer that uses a valve to shift the air stream from one bed to the other for continuous operation. A second style rotates the beds over a manifold to shift the air from process to regeneration. Both models use discreet molecular sieve and a traditional regeneration cycle. Depending upon the specific design, low dew points can be achieved. Service is relatively easy with desiccant changes in minutes and affordable.
The wheel design uses a combination of desiccant and silica gel powders fused onto a carrier that rotates over a manifold for process and regeneration cycles. The wheel typically rotates every 5 minutes because of the very small amount of desiccant material to absorb the moisture.
There is also a vacuum dryer that rotates the material in canisters over a manifold to heat, draw a vacuum, and for processing. The drying of the pellets is accomplished by heating the material and then pulling a vacuum to assist in removing the moisture from the pellets. This style appears to be limited in its application as it will not dry all materials and will not process regrind.
One way to evaluate a dryer is to look at how many moving parts there are and what the associated wear on seals, etc there will be. Our philosophy is to minimize the moving parts and make servicing easy. Floor space may also be a consideration especially for the larger sizes. Look carefully at the actual size of the dryer size in question as many advertisements are misleading on this and efficiency of operation.
The hopper design is often overlooked when selecting a drying system. A quick look at how the air is introduced into the hopper will indicate how uniformly the pellets are heated and whether the material flow will be ideal. The air spreader cone should be perforated and large enough to uniformly introduce the air to the pellets and be low enough in the hopper to heart material down to the bottom of the hopper. Small cones may not spread the air properly and cause rat holing of the material.
Cleaning of the hopper will be important if you do several material changes per day. Look for a hopper design with large access doors and easily removable spreader cones. Our hopper design does not incorporate any gaskets around the door to eliminate contamination from damaged seals. Hoppers requiring tools to access the inside may not be the best for JIT processing.
If you are planning to buy several dryers, we suggest narrowing your selection down to 2-3 suppliers and asking for a trial machine for your operators to evaluate. Remember, they are the ones using the equipment. If possible, go to the factory to see how the dryers are made. The attention to detail will be very apparent. If this is not possible, ask for references in your area and see if you can visit their facility.
Price is always an issue when buying any equipment. Make sure the quotes you receive are for comparable equipment. Check to make sure the air flow, hopper size and any options are clearly spelled out in the quote. In some cases, options from one company may be included as standard on another design.
Pick a dryer company that has a good service record and has the parts you may need in the future. Is their website helpful? Can you access the manuals and electrical schematics on their website for use at night or on the weekends?
Selecting the best dryer can make a significant effect on your costs and quality. The time spent on selection will pay itself off within a year.
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