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RECOMMENDATIONS FOR
PROPER FILTER PRESS OPERATION

Proctor Maple Research Center
(Condensed Version)

INTRODUCTION

Consumers of pure maple syrup demand a high quality product. Like most food items, appearance can affect the value of maple syrup and the customer’s perception of quality. Proper filtration is essential to remove sugar sand (niter) in order to ensure a quality finished product. The two main categories of filtration are gravity filters and pressure filters (filter presses). Both methods can produce excellent results; the major difference is the rate of processing. Filter presses are designed to filter large volumes of syrup in a relatively short amount of time.

The plate filter press works by pumping unfiltered syrup through a series of filters coated with filter aid under pressure. The result is an attractive “polished” product. This value-added product generally commands a higher price in the marketplace.

FILTER PRESS STACK AND FILTER PLATES:

The filter press frame acts as a support for the filter plates. The filter press is made up of a “stack” of waffle and cake plates. Waffle plates act to distribute the syrup evenly while providing a firm backing for the DE cake and filter paper. Cake plates have a cavity that allows the cake of DE to form.

Filtering capacity increases with the size and number of plates used. The method for tightening the plates is usually a pair of threaded bars with heavy-duty wing-nuts. A pan or container is needed to catch the syrup that weeps from the press. Many sugar-makers add a set of stainless steel quick couplers to the inlet and outlet of the press.

DIATOMACEOUS EARTH (FILTER AID):

Diatomaceous earth works in a filter press by forming a complex matrix that traps suspended solids while allowing maple syrup to pass through. DE is essential to the operation of a filter press. DE, the skeletal remains of single-celled sea creatures called Diatoms accumulate on the ocean floor over millions of years forming thick layers. These layers are mined, sifted, and sorted by particle size. DE is also used in the beer and wine industry, it is the gritty substance in toothpaste and DE is used in pool filters. Not all DE is food grade and not all grit sizes will work for filtering maple syrup. Make sure that DE is purchased from a maple equipment distributor.

CAUTION: DE is a finely ground, abrasive material and should always be handled with care. Do not inhale the dust and avoid eye and skin contact.

 

Table 1-1: General recommendations for Diatomaceous Earth needed for various filtering batch sizes in 7” or 10” plate filter presses (More DE may be needed for dark or late season syrup)
Gallons 7” Plate Press 10” Plate Press
40 5 cups 8 cups
45 6 cups 9 cups
50 7 cups 10 cups
55 8 cups 11 cups
 

FILTER PAPER:

The purpose of filter paper is to catch the DE suspended in syrup and provide a surface for the DE cake to form. During filter press set-up filter paper is placed between each plate. Filter papers have two holes that match the inlet and outlet hole in the plates. It is important that the correct size papers are used and that the alignment of the holes matches that of the plates. Lightly moistening the papers will help keep them in place during setup. Although filter paper has two distinct sides, or textures, there is no detectable difference in filtering performance.

PRESSURE GAUGE AND BYPASS VALVE:

The pressure gauge is used to monitor back-pressure in the filter press which can lead to filter paper failure. The gauge is plumbed into the pipes that carry the syrup to the press and should be able to read a range of at least 0-50 Pounds Per square Inch (PSI).

A simple stainless steel ball valve can be used as a bypass valve. The valve is placed before the stack. A food grade hose or stainless steel pipe long enough to reach your syrup reservoir should be connected to your bypass valve.

SETUP OF A PLATE FILTER PRESS:

Make sure all parts of the press are clean and dry, and that there are no obstructions in any pipes or plates that carry syrup. This is especially important for new filter presses. Take extra time to make sure no debris or oils from the manufacture of the press remain.

Without the proper plate orientation, the press will not function correctly. None of the plates should appear misaligned or sticking out of the stack. The plates should always alternate between hollow and waffle plates. The first and last plates in a press are waffle plates.

  1. Begin with the first plate in the stack attached to the frame; place one sheet of filter paper against the waffle then slide on a cake place, sandwiching the paper between the two plates. Some filter press frames do not have a set of guides to keep the filter papers in alignment. If this is the case, use a pair of appropriately sized dowels (1/2 inch for most presses) in the inlet and outlet holes of each plate to keep the papers at the right height during set up.
  2. Place a filter paper on the other side of the cake plate and cover it with another waffle plate.
  3. Repeat this process until all plates are used. Plates must alternate. At no time should two cake plates or two waffle plates be used next to each other.
  4. The end plate is a waffle plate made from thicker metal then the rest of the stack. Attach this plate and secure using the manufacturer’s hardware. Tighten down the stack. Remember, a filter press is under pressure; if the stack is loose during filtering, syrup may squirt out between the plates.
  5. Make sure all the quick connects are closed; this includes the tubing from the syrup reservoir to the filter press and out of the filter press to the drum.

CHARGING THE PRESS:

The filter press works by completely coating the filter papers with a cake of DE. This process is called charging the press. Without completely coated papers, unfiltered syrup can mix with filtered syrup.

The syrup to be filtered must be heated to 200° Fahrenheit. Draw off three to five gallons of hot syrup into a stainless steel bucket or clean food grade pail used ONLY in maple production. This will be your precharge syrup. Begin by stirring half the DE into the pail of hot syrup (see Table 1-1). For example, a 45 gallon batch of syrup would require 4-1/2 cups, of DE to charge a 10 inch press. Combine the DE and hot syrup until completely mixed. Add the remaining 4-1/2 cups of DE to the batch of syrup waiting to be filtered. Pump the bucket with syrup/DE mixture through the press. Catch the syrup exiting the press in another clean bucket at the outlet of the press. Pump the entire 3-5 gallons of syrup through the press before checking the clarity of the syrup leaving the press. The syrup should appear polished and free of any suspended solids. If any cloudiness is present; first shut off the pump, then return the cloudy syrup to the first pail and repeat filtering until syrup comes out clear. Very small air bubbles created by the pump can make the syrup look cloudy. When the syrup is clear, add it back into the batch of syrup waiting to be filtered. This will prevent low temperature syrup from entering a storage container.

Connect the press to the unfiltered syrup (finishing pan). Due to loss of heat while filtering syrup; heat your syrup to 200° F to maintain the minimum temperature of 180° needed to sterilize syrup storage containers.

FILTER PRESS OPERATION:

The press is now ready to operate. Connect the press to the unfiltered syrup (finishing pan). Make sure the drum or container to be filled is clean, dry and ready for use.

Make sure the bypass valve is closed and the return line to the unfiltered syrup (finishing pan) is in place. Turn on the pump.
As it runs, look over the entire press and make sure everything is operating smoothly. Monitor the pressure gauge for increases in pressure. If the pressure rises dramatically upon start-up then the press may have been set up incorrectly. Normal operating pressure for a 10-inch plate press with seven cake plates is between 0-20 PSI. This range is meant as a guideline. Presses may occasionally run into problems at lower pressures especially if papers or plates are not seated correctly.

Once back-pressure is observed, the plates have begun to clog. This results in a slower rate of filtering and the imminent possibility of a ruptured paper. The bypass-valve can be used to relieve some of the back-pressure, but it will decrease the filtering rate. When the valve is opened, some of the syrup entering the stack is diverted back to the reservoir; thus reducing the load on the filter press. The valve can help finish filtering a batch of syrup, but will not allow filtering to continue indefinitely. A general rule of thumb is to clean your press if the bypass valve is used.

FILTERING:

Sugarmakers who collect syrup into a finishing pan or some other holding container before filtering are using the “batch filtering” method.

When charging the press during batch filtering, half the total DE used to filter the batch of syrup is added to the charging bucket of syrup. The remaining DE is mixed with the remaining syrup to be filtered. This insures that DE will be constantly added to the existing cake in the filter press. By adding DE continually to the filter press, more surface area is created during filtering. The impurities (niter) being filtered are spread out and filtering capacity is increased.

If more then one batch of syrup is sent through the same set of cakes and papers, care must be taken to prevent excessive cooling of the press.

Sugarmakers who will only produce a gallon of syrup an hour may encounter problems with clogging or a noticeable drop in temperature within the press. A press that has cooled can, in turn, cool filtered syrup to below 180° F. Gravity will naturally pull down ward on the filter cakes over time. If the press is left idle for too long, gaps may be created in the DE cake allowing unfiltered syrup to pass through.

CLEANING THE PRESS:

When filtering is complete, a small amount of syrup will remain in the press. This syrup can be recovered by pumping a few gallons of hot water through the press. Catch the discharge in one of the buckets used for charging the press. The mix can be held in a bucket until the next boil, added to the evaporator syrup pan or returned to the clean finishing pan to be incorporated with the next batch. If you add this concentrate to your evaporator from the filter press, be careful not to add to much at one time and scorch your pan.

Only clean water should be used to clean the filter press. Water should be run through the pump and the plates should be thoroughly scrubbed and then rinsed with hot water. It is important to be sure that all surfaces are clean and all of the small holes in the filter plates are clear of obstructions.

TIPS AND TROUBLE SHOOTING:

  • Watch for the appearance of back-pressure; this indicates clogging in the press. Sudden spikes of pressure indicate imminent paper failure.
  • Occasionally niter is too small to filter. If this is the case, let the barrel stand for a month and then carefully pump out syrup without distributing niter that settled out. Filter the syrup a second time. This usually occurs at the beginning of the season.
  • Look for seeps and weeps of syrup beyond normal. Check pressure, check stack tightening hardware.
  • Do not run pump without fluid. This causes excessive wear and tear.
  • Make sure papers are aligned correctly.
  • Check for proper plate alignment.
  • Use DE; add more for darker or late season syrup.
  • Avoid wearing loose fitting clothes around pulleys.
  • Operate the press hot.
  • Keep syrup temperature leaving the press above 180° F.
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