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    "slug": "gravity-filtration-vs-vacuum-filtration-which-to-use",
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        "rendered": "Gravity Filtration vs Vacuum Filtration: Which to Use"
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        "rendered": "<p>Gravity filtration vs vacuum filtration comes down to one question: is your filtrate or your solid the product, and how fast does the cake plug? Gravity uses nothing but head pressure and a folded filter paper. Vacuum pulls the liquid through at up to -0.095 MPa. Here is where the line sits.<\/p>\n<figure>\n  <img fetchpriority=\"high\" decoding=\"async\" src=\"https:\/\/www.unionclay.com\/wp-content\/uploads\/2026\/07\/gravity-filtration-vs-vacuum-filtration-which-to-use-cover.png\" alt=\"Decision table comparing gravity filtration and vacuum filtration on driving force, cake handling, funnel size, material and vacuum rating for UnionClay ZF-10L to ZF-50L units\" width=\"1200\" height=\"675\" \/><figcaption>Gravity vs vacuum, and the four ZF unit sizes we build, on one decision table.<\/figcaption><\/figure>\n<nav class=\"toc\" aria-label=\"Table of contents\">\n<h2>On this page<\/h2>\n<ul>\n<li><a href=\"#mechanism\">What actually separates the two methods?<\/a><\/li>\n<li><a href=\"#decision-table\">The decision table: which one does your sample need?<\/a><\/li>\n<li><a href=\"#gravity-limit\">When does gravity filtration stop being viable?<\/a><\/li>\n<li><a href=\"#vacuum-spec\">What does -0.095 MPa actually buy you?<\/a><\/li>\n<li><a href=\"#material\">Stainless or ceramic: which funnel body?<\/a><\/li>\n<li><a href=\"#sizing\">How do you size a vacuum filtration unit?<\/a><\/li>\n<li><a href=\"#beyond-funnel\">What else has to be confirmed besides the funnel?<\/a><\/li>\n<li><a href=\"#faq\">Fragen und Antworten<\/a><\/li>\n<\/ul>\n<\/nav>\n<h2 id=\"mechanism\">What actually separates the two methods?<\/h2>\n<p>Both methods push liquid through a porous medium and hold the solid back. The only real difference is what supplies the driving force, and everything else follows from that.<\/p>\n<p>Gravity filtration uses the head of liquid standing in the funnel. That head is small, it shrinks as the funnel drains, and it is the same head at 200 mL as at 20 L. So gravity does not scale. A bigger funnel buys you no extra driving force, only a taller column of liquid waiting its turn.<\/p>\n<p>Vacuum filtration puts negative pressure under the filter medium. A pump evacuates the receiving flask, atmospheric pressure on top of the cake becomes the driving force, and that force stays constant from the first litre to the last. On our units the pump side is rated to pull up to -0.095 MPa, which is close to the practical ceiling for a water-ring or rotary-vane pump on an aqueous slurry.<\/p>\n<p>That is the whole physics. Most pages on this comparison stop right there and leave you to guess at the equipment. The rest of this page is the part they skip: the actual specs, and the point where one method hands off to the other.<\/p>\n<h2 id=\"decision-table\">The decision table: which one does your sample need?<\/h2>\n<p>We build the vacuum side of this comparison, so we have the parameter sheets in front of us. Below is the honest split. The gravity column describes bench glassware; the vacuum column is our ZF series, and every figure in it comes off the ZF spec sheet.<\/p>\n<table class=\"unionclay-spec-table\">\n<thead>\n<tr>\n<th>Decision point<\/th>\n<th>Gravity filtration<\/th>\n<th>Vacuum filtration (ZF series)<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<th>Driving force<\/th>\n<td>Liquid head only, falls as the funnel drains<\/td>\n<td>Negative pressure, up to -0.095 MPa, constant<\/td>\n<\/tr>\n<tr>\n<th>Filter support<\/th>\n<td>Folded paper in a cone<\/td>\n<td>Perforated plate, 4 mm pore diameter, under the paper or cloth<\/td>\n<\/tr>\n<tr>\n<th>Funnel body<\/th>\n<td>Glass or plastic cone<\/td>\n<td>201 \/ 304 \/ 316 stainless steel Buchner funnel, or ceramic<\/td>\n<\/tr>\n<tr>\n<th>Batch volume<\/th>\n<td>Bench scale<\/td>\n<td>10 L \/ 20 L \/ 30 L \/ 50 L funnel, up to 100 L in the range<\/td>\n<\/tr>\n<tr>\n<th>Receiver<\/th>\n<td>Open beaker or flask<\/td>\n<td>GG3.3 borosilicate glass flask, matched 10-50 L<\/td>\n<\/tr>\n<tr>\n<th>Which product survives<\/th>\n<td>Filtrate; the cake is usually discarded<\/td>\n<td>Either; the cake is recoverable and washable<\/td>\n<\/tr>\n<tr>\n<th>Pump needed<\/th>\n<td>None<\/td>\n<td>Yes, matched to funnel size and slurry<\/td>\n<\/tr>\n<tr>\n<th>Moves between rooms<\/th>\n<td>Carried by hand<\/td>\n<td>Lockable universal casters, 33-52 kg packed<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>One pattern runs down that table. Gravity wins on simplicity and loses on everything involving the solid. If you only care about the liquid coming out the bottom and the solid is a little drying agent or grit, gravity is fine and a vacuum unit wastes a pump. The moment the cake becomes your product, or starts choking the paper, the argument flips. Our <a href=\"https:\/\/www.unionclay.com\/vacuum-filtration-equipment-selection-guide\/\">vacuum filtration equipment selection guide<\/a> walks the same decision from the pump side if you are already past this point.<\/p>\n<h2 id=\"gravity-limit\">When does gravity filtration stop being viable?<\/h2>\n<p>There is no universal crystal size at which gravity dies. Anyone quoting you a number in microns is guessing, because the answer depends on your slurry, not on a table. What we can give you is the three failure modes, in the order they usually arrive.<\/p>\n<p><strong>The cake blinds.<\/strong> Fine, plate-like or gelatinous crystals build a compressible layer that seals the paper. With gravity you have a few centimetres of head fighting that layer and it loses. Flow drops toward zero and no amount of waiting fixes it. Vacuum does not fully solve blinding either, but a constant differential keeps a compressible cake moving where head cannot.<\/p>\n<p><strong>The batch outgrows the funnel.<\/strong> This is the failure most people meet first. A gravity cone at bench scale will run a beaker. It will not run 20 L in a working shift, because doubling the volume does not double the driving force. Our ZF funnels start at 10 L and step through 20 L, 30 L and 50 L because this is where gravity has already given up.<\/p>\n<p><strong>The solid is the product.<\/strong> A gravity cone gives you wet, loose solid smeared into folded paper. If you need to wash that cake, pull it dry, and scrape it out intact, you need a flat perforated plate with suction underneath. That is a Buchner geometry, and Buchner geometry without vacuum is a slow flat funnel.<\/p>\n<p>Practical rule: if two of those three apply, stop shopping for a better filter paper. If only the first applies and your volumes stay small, try a coarser medium and a re-slurry first.<\/p>\n<h2 id=\"vacuum-spec\">What does -0.095 MPa actually buy you?<\/h2>\n<p>The vacuum rating is the number on the spec sheet people misread. -0.095 MPa is gauge pressure and it is the pull the system is rated for, not a promise about your slurry. What you get across the cake depends on how much your solid resists, how well the funnel seals, and whether your liquid boils under that vacuum. Low-boiling solvents flash before you reach the rating, and no equipment spec changes that.<\/p>\n<p>What the rating does tell you is the ceiling on the differential you can apply, and that it stays flat across the run. That is why the ZF units hold their throughput from 10 L to 50 L while a gravity cone does not.<\/p>\n<p>Underneath the paper sits a perforated plate with 4 mm pores. That plate is structural, not the filter: it holds the medium against the differential so the paper does not tear into the flask when the cake loads up. The 4 mm figure is constant from ZF-10L through ZF-50L, so the support does not change as you scale. Only the funnel diameter does.<\/p>\n<figure>\n  <img decoding=\"async\" src=\"https:\/\/www.unionclay.com\/wp-content\/uploads\/2026\/07\/gravity-filtration-vs-vacuum-filtration-which-to-use-fig1.png\" alt=\"Bar chart comparing UnionClay ZF vacuum filtration funnel diameters: ZF-10L at 300 mm, ZF-20L at 350 mm, ZF-30L at 400 mm and ZF-50L at 500 mm\" width=\"1200\" height=\"675\" \/><figcaption>Funnel diameter across the ZF range. Filter area scales with the square of this figure, which is why throughput climbs faster than capacity.<\/figcaption><\/figure>\n<p>That chart carries the point of this whole page. Going from ZF-10L to ZF-50L multiplies the funnel capacity by five, but the funnel diameter only goes from &#966;300 mm to &#966;500 mm. Filter area follows the square of the diameter, so the bigger unit is not simply five times the smaller one. If your bottleneck is cake area rather than tank volume, size on the diameter column, not the litre column.<\/p>\n<h2 id=\"material\">Stainless or ceramic: which funnel body?<\/h2>\n<p>Once you have decided on vacuum, the funnel body is the next real fork, and it is a chemistry question, not a budget question.<\/p>\n<p>Stainless is the default. Our <a href=\"https:\/\/www.unionclay.com\/product\/stainless-steel-vacuum-filtration-unit-10l-100l\/\">stainless steel vacuum filtration unit in the 10L-100L range<\/a> is offered in 201, 304 and 316, and that choice matters more than the capacity you pick. 201 is the economy grade and belongs on neutral aqueous work. 304 is the general lab default. 316 adds molybdenum and is what you specify when chlorides are in the picture \u2014 brine, hydrochloric traces, seawater-derived feed. Putting 201 in front of a chloride stream will pit it, and no vacuum rating saves you from that.<\/p>\n<p>Ceramic is the answer when even 316 is not enough. The <a href=\"https:\/\/www.unionclay.com\/product\/ceramic-vacuum-filtration-unit-10l-100l\/\">ceramic vacuum filtration unit covering the same 10L-100L band<\/a> exists for feeds that attack steel outright or for work where any metal pickup contaminates the product. Strong acids, and anything where trace iron ruins your analysis, push you here. Ceramic gives up the toughness of steel in exchange, so it is a deliberate trade, not an upgrade.<\/p>\n<p>Both bodies drain into a GG3.3 borosilicate glass flask, so you can watch the filtrate and the receiver is not the weak link in the chemical chain. There is no point specifying a 316 funnel and then draining it into a flask that cannot take the same feed. The full <a href=\"https:\/\/www.unionclay.com\/product-category\/vacuum-filtration\/\">vacuum filtration category<\/a> lists both bodies with their current configurations.<\/p>\n<h2 id=\"sizing\">How do you size a vacuum filtration unit?<\/h2>\n<p>Here is the ZF parameter sheet as we publish it. Size against this, not against a nominal batch number.<\/p>\n<table class=\"unionclay-spec-table\">\n<tbody>\n<tr>\n<th>Model<\/th>\n<td>ZF-10L<\/td>\n<td>ZF-20L<\/td>\n<td>ZF-30L<\/td>\n<td>ZF-50L<\/td>\n<\/tr>\n<tr>\n<th>Funnel type<\/th>\n<td>Stainless steel Buchner funnel<\/td>\n<td>Stainless steel Buchner funnel<\/td>\n<td>Stainless steel Buchner funnel<\/td>\n<td>Stainless steel Buchner funnel<\/td>\n<\/tr>\n<tr>\n<th>Funnel capacity<\/th>\n<td>10L<\/td>\n<td>20L<\/td>\n<td>30L<\/td>\n<td>50L<\/td>\n<\/tr>\n<tr>\n<th>Funnel size (mm)<\/th>\n<td>&#966;300*H200<\/td>\n<td>&#966;350*H220<\/td>\n<td>&#966;400*H240<\/td>\n<td>&#966;500*H280<\/td>\n<\/tr>\n<tr>\n<th>Werkstoff<\/th>\n<td>201\/304\/316 stainless steel<\/td>\n<td>201\/304\/316 stainless steel<\/td>\n<td>201\/304\/316 stainless steel<\/td>\n<td>201\/304\/316 stainless steel<\/td>\n<\/tr>\n<tr>\n<th>Pore diameter<\/th>\n<td>4mm<\/td>\n<td>4mm<\/td>\n<td>4mm<\/td>\n<td>4mm<\/td>\n<\/tr>\n<tr>\n<th>Flask capacity<\/th>\n<td>10L<\/td>\n<td>20L<\/td>\n<td>30L<\/td>\n<td>50L<\/td>\n<\/tr>\n<tr>\n<th>Glass material<\/th>\n<td>GG3.3 borosilicate glass<\/td>\n<td>GG3.3 borosilicate glass<\/td>\n<td>GG3.3 borosilicate glass<\/td>\n<td>GG3.3 borosilicate glass<\/td>\n<\/tr>\n<tr>\n<th>Vacuum<\/th>\n<td>up to -0.095Mpa<\/td>\n<td>up to -0.095Mpa<\/td>\n<td>up to -0.095Mpa<\/td>\n<td>up to -0.095Mpa<\/td>\n<\/tr>\n<tr>\n<th>Mobility<\/th>\n<td>Lockable universal casters<\/td>\n<td>Lockable universal casters<\/td>\n<td>Lockable universal casters<\/td>\n<td>Lockable universal casters<\/td>\n<\/tr>\n<tr>\n<th>Packing weight (kg)<\/th>\n<td>33<\/td>\n<td>39.5<\/td>\n<td>48<\/td>\n<td>52<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Three things in that sheet decide the model for you.<\/p>\n<p>The funnel and flask capacities are matched one to one at every step: a 10 L funnel drains into a 10 L flask, a 50 L funnel into a 50 L flask. That means the unit is sized for a single batch to land in the receiver without a mid-run stop. If your batch is 30 L, the ZF-30L is the model that runs it in one pass. Skip the ZF-50L if your batch never exceeds 20 L \u2014 you gain nothing but a taller funnel to reach into and a heavier frame to push.<\/p>\n<p>The funnel height climbs with the diameter, H200 to H280. That is your slurry depth headroom. A thick, high-solids slurry needs the height even when the litre figure says you could go smaller.<\/p>\n<p>The packed weight runs 33 kg at ZF-10L to 52 kg at ZF-50L. Every model rides on lockable universal casters, so this is a rolling floor unit, not a bench instrument. Check your door widths before the unit ships \u2014 a 52 kg frame does not go up onto a bench.<\/p>\n<p>Beyond 50 L the range continues to 100 L. At that end sizing stops being a table lookup and turns into a pump and cycle-time discussion, which is a conversation to have with us rather than a column to read. Our <a href=\"https:\/\/www.unionclay.com\/company-catalog\/\">product catalog<\/a> carries the full range with the current configuration set.<\/p>\n<h2 id=\"beyond-funnel\">What else has to be confirmed besides the funnel?<\/h2>\n<p>A vacuum filtration unit is a system, and the funnel is only the part with the model number on it. Our source sheet lists six items to confirm before a configuration is final.<\/p>\n<p><strong>Funnel capacity.<\/strong> Covered above: match it to a single batch.<\/p>\n<p><strong>Receiver volume.<\/strong> Nominally matched, but confirm it if you are washing the cake. Wash liquid lands in the same flask as the mother liquor, and a receiver sized for the batch alone will fill.<\/p>\n<p><strong>Material grade.<\/strong> 201, 304, 316 or ceramic, decided by your chemistry.<\/p>\n<p><strong>Vacuum connection.<\/strong> The port and hose have to match what your pump presents. This is the detail that most often delays commissioning, because the unit and the pump are frequently bought separately.<\/p>\n<p><strong>Discharge method.<\/strong> How the cake comes out. People forget this one entirely, then discover it on the first run while scraping a 50 L funnel by hand.<\/p>\n<p><strong>Pump requirement.<\/strong> The pump has to hold the rated vacuum against the leak rate of the assembly at your funnel size. An undersized pump turns a -0.095 MPa unit into a slow one, and the equipment gets the blame.<\/p>\n<p>Send us those six answers and the model selects itself. Send only a litre figure and we will ask for the other five anyway.<\/p>\n<h2 id=\"faq\">Fragen und Antworten<\/h2>\n<h3>Is vacuum filtration always faster than gravity filtration?<\/h3>\n<p>Not always, but it is the safe assumption. Vacuum applies a constant differential up to -0.095 MPa where gravity applies a falling head, so vacuum normally wins by a wide margin. The exception is a low-boiling solvent that flashes under vacuum, where the pull is capped by the liquid rather than by the pump, and a coarse, free-draining slurry can run acceptably fast under gravity alone.<\/p>\n<h3>Can I use a Buchner funnel without a vacuum pump?<\/h3>\n<p>Mechanically yes, usefully no. A Buchner funnel is a flat perforated plate \u2014 ours are 4 mm pore diameter \u2014 and a flat plate has no cone to concentrate the head. Without suction underneath it filters more slowly than an ordinary gravity cone. The flat geometry only pays off when there is a differential across it.<\/p>\n<h3>What vacuum level do I need for filtration?<\/h3>\n<p>Our ZF units are rated to pull up to -0.095 MPa, and that is the ceiling, not a target you must hit. What you actually need depends on how much your cake resists and how volatile your filtrate is. Specify the rating so you have headroom, then let the slurry decide where it settles.<\/p>\n<h3>Does a bigger funnel mean proportionally faster filtration?<\/h3>\n<p>No, and this catches people out. Going from ZF-10L to ZF-50L raises capacity fivefold but the funnel diameter only goes from &#966;300 mm to &#966;500 mm. Filter area scales with the square of diameter, so area and volume do not climb at the same rate. If throughput rather than batch volume is your constraint, size on diameter.<\/p>\n<h3>Should I choose stainless steel or ceramic?<\/h3>\n<p>Stainless unless your chemistry rules it out. 304 handles general lab work and 316 handles chlorides. Move to ceramic when the feed attacks steel outright or when any metal pickup would contaminate your product. Both bodies are available across the same 10L-100L band, so the decision costs you nothing in capacity. Our <a href=\"https:\/\/www.unionclay.com\/faq\/\">equipment FAQ<\/a> covers the related configuration questions.<\/p>\n<h2 id=\"verdict\">The short version<\/h2>\n<p>Run gravity if the filtrate is your product, the solid is incidental, and the batch fits in bench glassware. The moment two of those three stop being true, you are buying a vacuum unit and the only remaining questions are size and material.<\/p>\n<p>For size, match the funnel to one batch: ZF-10L, ZF-20L, ZF-30L or ZF-50L, and up to 100 L beyond that. Do not buy a stage up as insurance \u2014 the frame gets heavier and the funnel gets deeper for no gain you will use. For material, let the chemistry pick: 201 for neutral aqueous, 304 as the lab default, 316 where chlorides are present, ceramic where steel is not an option at all.<\/p>\n<p>Then confirm the other five items \u2014 receiver volume, vacuum connection, discharge method, pump requirement, and grade \u2014 before anything is final. If you want the reasoning laid out across our whole equipment line rather than filtration alone, our <a href=\"https:\/\/www.unionclay.com\/company-catalog\/\">equipment selection guide<\/a> is the place to start.<\/p>",
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        "rendered": "<p>Gravity filtration vs vacuum filtration: gravity gives falling head, vacuum pulls to -0.095 MPa. See our decision table on real 10L-50L ZF unit specs.<\/p>",
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