24 HONEYWELL 3
measurement where the laser is pointed. Honeywell’s Optical Caliper design has several advantages. Given that Honeywell’s QCS has always scanned fully off sheet, it uses the Coanda slots to capture the sheet as the measurement comes on sheet. There is a second feature, a shaped vacuum nozzle. The Coanda slots are about two centimeters longer than the start of the nozzle. Purpose of the Coanda slots is to capture a large-scale area early as the sensor approaches the specimen, while the measurement is coming on sheet (Figure 4). Then the local area where the measurement is to take place is stabilised by one cleverly shaped vacuum nozzle, with one leading edge shaped like a wing to provide MD and CD smoothing of the sheet. The purpose is to present the paper to the laser, held over a wear resistant zirconia button in a perfectly-presented fashion. While these features of the
sheet stabilisation work together, they are not dependent on each other for optimisation. The Coanda slots can be set to capture the sheet, paying attention only to edge capture, regardless of flutter and curls. Depending on the amount of flutter calming required, the nozzle to vacuum can be set as light as possible for smooth readings. Since the settings of these two devices are independent, commissionig is effectively set once and forget, for the sheet and grade range of the paper machine.
Results After months on control, dry end operators of a recycled newsprint machine in Sweden indicated
September 2012
that the “sensor works well” and “the profile on screen is same as on paper” when they feel the hardness by hand. Based on their reel building experiences and comparing with past experiences, they feel that, “it’s as good as you get for newsprint”. Winder operators at the same mill experienced no problems running the PM reels. Using this combination of laser triangulation and sheet stabilization, Honeywell achieved good caliper agreement with the mill lab, and with contacting caliper on sheets with more than 30 per cent ash, with no clogging and no sheet damage.
Who needs this? This new measurement joins the Honeywell family of sensors that includes a variety of contacting caliper solutions. The papermakers with the greatest need for the Optical Caliper measurement are the high tonnage producers of lightweight sheets. Typical grades that suit this description are publication and advertising, newsprint and magazine papers. This type of paper is very competitive, easily damaged, and requires high production efficiency. Maximizing yield is critical to survival. For so many grades, CD Caliper and good Reel Building are at or near the top of the list for control requirements, and a reliable caliper measurement is the key. Common problems with contacting caliper measurements on these lightweight sheets include:
• Pick damage: Fillers and recycled furnish are common in these grades to achieve economic targets and, to some degree, print quality targets. Those fillers, and even the fibres themselves,
can contain debris which may cause picks on pinching type of traditional caliper measurement. Holes and occasional paper breaks are inevitable. An Optical Caliper measurement reduces or eliminates the chance of defect generation. Tonnage yield is higher with less winder losses, and quality of the paper more meaningful to high speed presses. • Density: At high speeds especially, sheet flutter challenges the ability of the contacting style measurement to reliably pinch and hug the sheet. There can be many reasons, but one example of a process issue is higher loss of fillers at the wet-end, usually on the edges. This causes varying density which is not easily controlled on a CD basis. A lower density sheet can be more flutter-prone, thus creating a different dynamic environment for contacting caliper sensors between the edges and middle of the sheet. Having one puck pinching pressure setting in multiple dynamic situations is a challenge, where non-pinching design for sheet stabilisation would not have this concern. • Tolerance to ash – target shift: With less concern for pick damage, and higher tolerance for variable density, the papermaker can be free to add fillers or improve the paper’s retention in place of more expensive fibres. Significant economics can be achieved for every percentage point of ash increased to displace fibre.
Summary The Optical Caliper completes the family of caliper measurements for the overall papermaking community. While the issues presented here are focused on a
Pulp Paper & Logistics
Using this combination of laser triangulation and sheet stabilization, Honeywell achieved good caliper agreement with the mill lab, and with contacting caliper on sheets with more than 30 per cent ash, with no clogging and no sheet damage
specific group of producers, the other groups of users may find the traditional contact caliper measurement cost effective, and reliable. Honeywell’s designs for
contacting caliper, while not discussed here, are among the best performing, and lightest touch in the industry. Both options are available. Optical Caliper measurement from Honeywell provides an additional solution to help papermakers achieve higher operational efficiency when presented with challenging process problems. It has been field- proven in demanding situations with excellent correlation to both laboratory and previous contacting caliper measurements. Its long-term stability has given confidence to the customers to use it for closed loop caliper control. More information from Pak
Hui, Product Manager, and Tom Steele, Senior Marketing Manager, at Honeywell Process Solutions. www.
honeywellprocess.com
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