Squeeze or Spin — Tunnel Washer Extractor Options
We size up the pros and cons of batch press and centrifugal extractors
No matter what tunnel washer you’re using, the downstream equipment is equally important to overall system performance. This article presents a brief, high-level overview of extraction methods. In general, there are two types of extraction equipment commonly used in batch tunnel systems. The first is a batch press extractor; the second is a centrifugal extractor.
Batch Press Extractors
Press extractors are really quite simple machines as far as the function they perform. They accept a load from the tunnel into a round basket and then squeeze water out of the goods by means of a hydraulic ram with a rubber membrane attached. However, the actual process is a bit more complex. Removing as much water as possible within the parameter of the programmed tunnel cycle time is a critical aspect of the overall system performance. The reason is that the less moisture you remove via the press means longer drying time and/or ironer slowdowns that reduce efficiency.
The press cycle as noted above is governed by the overall cycle time of the tunnel. The press cycle must be programmed to be slightly shorter than the tunnel cycle so it is always ready when the tunnel cycle ends. Some manufacturers’ presses link the controls together between press and tunnel, thus allowing the press to stay in sync with the tunnel cycle. This may mean short-cycling the press to ensure that it’s ready for the incoming load. Unfortunately, many of the time functions of the press are fixed, such as lowering/raising the basket, ram up/down movement, and cake-ejection time. This typically leaves only the time under high pressure variable that can be adjusted, so shorter tunnel cycles inevitability mean less time under high pressure and thus more moisture left in the goods.
There are two ways that manufacturers try to achieve this goal of reduced cycle times without sacrificing moisture extraction: larger baskets and/or higher maximum pressure. Each method has its pros and cons. First let’s take a look at enlarging the basket diameter to reduce the height of the cake. This allows water to more easily escape during the high pressure cycle of the press. However, as the diameter becomes larger and the cake height continues to decline, this results in uneven distribution of goods as they flow into the basket, which can become a problem. Therefore most manufacturers have settled into basket diameters for presses with rated-load capacities of 150- 250 lbs. (68-113 kg) with press basket/ membrane diameters of between 39-52 inches (99-132 cm). However, as discussed above, a larger diameter isn’t the complete answer. A balance of the right basket diameter (cake height) coupled with proper formula-development creates the optimal relationship between pressure, time, material type and moisture extraction.
Increasing the maximum pressure definitely has advantages, up to a point. That point is reached when the maximum pressure at the membrane reaches between 40-45 bar. Above this pressure point, there is very little improvement in moisture removal, especially in a smaller basket diameter. Pressing laundry when confined in a basket creates an environment similar to a filter press. Water becomes trapped between the fibers and no amount of pressure within reasonable rates in a given time will increase the removal rate. Only time can allow this increase in pressure to work. The other disadvantage to running at higher maximum pressures is the increased wear and tear on the equipment and the linen.
Higher pressures also wreak havoc on different goods types, ranging from setting creases to linen damage. Using pressures exceeding 45 bar will set creases in fabrics, including terry products. Unless rinse is kept at ambient (room temperature), these creases may set permanently. The downside to holding rinse temperatures at room temperature is that using higher temperatures in the final rinse allows the fibers in the product to open up allowing for increased moisture removal. Holding rinse temperature at ambient levels isn’t the only downside of using higher pressure presses.
As textiles continue to evolve, many new blends of polyester and cotton products are entering the market. This trend is especially prevalent in higher-end hospitality products like sheets and pillow cases. When these blends and higher thread-count products are subjected to pressures greater than 25-30 bar, these products can be permanently damaged with thread breaks called “blowouts.” Since water is incompressible, the high pressure from the extraction press causes the water to push through the tiny gaps between the threads, resulting in small holes in the product. There are a number of ways to combat this phenomenon, and they all involve using less pressure or ramping up to a lower pressure setpoint very slowly. Another method is the use of tamp cycles, but this shortens the overall press time under high pressure, resulting in less moisture removal and a possible slowdown at the ironers.
As you can see from the discussion above, higher pressure isn’t always the answer. Operators need to factor in basket diameter, the types of goods you’re processing and formula capabilities before you make a decision solely on high pressure. Remember, high pressure isn’t necessarily the best solution!
The other type of extractor is the centrifugal extractor. This type of extractor uses centrifugal force identical to a washer to remove moisture from the goods. The tunnel transfers goods into the extractor and it begins to spin, gradually ramping up to a high g-force extraction process. Much more time is needed to remove an equivalent amount of water as the press extractor, so tunnel cycles are typically programmed for longer periods or multiple centrifugal extractors are required to match the shorter tunnel cycles. The extra extractor(s), conveyance devices and additional controls all factor in to additional capital costs for this type of system. However, some types of goods cannot be extracted with the press extractor and require the use of centrifugal extractors. An example of this is the processing of walk-off mats in a tunnel in an industrial laundry. A press extractor would damage the mats, thus necessitating that the operator complete the extraction in a centrifugal extractor. Another example from the industrial laundry side could be uniforms. A press extractor could damage buttons on shirts and zippers on pants, although some can undergo processing in a press extractor at low-pressure settings. Even if a portion of your goods mix involves these types of items, there may be another solution that would allow for faster cycle times in your tunnel and the use of a press extractor.
Washer/Extractor Assist option
That option could be to use washer/extractors for the goods that can’t undergo processing in a press extractor, while running the rest of your mix in a tunnel with a press extractor. This option depends on the percentage of these types of goods in the makeup of the overall mix. It also assumes that you can process a percentage either by using existing washer/ extractors or by making a relatively small investment in new machinery. Taking these goods out of the tunnel and lowering cycle times could provide enough additional productivity to justify the added investment in washer/extractors, if the laundry doesn’t already have them in place.
There is much more science behind the press and centrifugal extractors, and we encourage readers to look more deeply into the technical details of this topic.
DIEDRICH WILLERS is product engineer-tunnels and presses for G.A. Braun Inc., Syracuse, NY. Contact him at 800.432.7286 or firstname.lastname@example.org.