Sizing a QF® Duct System

Nordfab offers assistance to those who have never designed a ducting system. We can assist you in determining the correct pipe size and configuration that will supply you with the correct flow. Please contact Nordfab if you need assistance.

Below is an example of system sizing (intended as a guideline only). Dust characteristics, machinery constraints, and other system requirements can affect performance and must be considered.

Skip example and go to  Air Volume Chart

Example for sizing QF duct

EXAMPLE

Always work from your machines back toward the filter. Suppose that you have a 5" drop that rises and runs back to join with a 6" drop as sketched above. What size branch will you need?

The 5" pipe carries 615 CFM at 4500 FPM, (See chart below). The 6" pipe will need 885 CFM at the same velocity (See chart). Added together you have a total of (615 + 885) 1500 CFM coming together. 

Looking again at the chart under 4500 FPM, you find that 1500 CFM is not listed, but falls very close to the 1570 CFM listed for an 8" pipe. This indicates that the 5" joined to the 6" will require an 8” pipe to carry all of the material at the right velocity. The branch, therefore, will be 8" on the downstream end reducing down to a 5" with a 6" branching off of it. That is listed as an 8-6-5 branch.

Example for sizing QF duct

Air Volume Chart

USING THE CFM/FPM CHART

This chart allows you to choose the correct pipe size for the CFM that is required.

Different materials need to be conveyed at different velocities to prevent the material from falling out of the air stream. See reference chart

For example, wood chips and saw dust flow well at 4500 feet per minute. Referring to the chart, a 4" pipe will convey 395 CFM at 4500 FPM. This indicates that a 4" pick-up on a machine will use 395 CFM from the filtering system. 

Or working in reverse, if you know that a machine will require approximately 400 CFM to remove the waste, then you should design a 4" pipe for the application.

AIR VOLUME IN DUCTS IN CUBIC FEET PER MINUTE (CFM) 
  Velocity in Feet Per Minute (FPM)    
Duct Ø  2000 2500  3000  3500  4000  4500  5000  5500  6000  6500 7000 
3 98 123 147 172 196 221 245 270 295 319 344
4 175 218 262 305 349 393 436 480 524 567 611
5 273 341 409 477 545 614 682 750 818 886 954
6 393 491 589 687 785 884 982 1080 1178 1276 1374
7 535 668 802 935 1069 1203 1336 1470 1604 1737 1871
8 698 873 1047 1222 1396 1571 1745 1920 2094 2269 2443
9 884 1104 1325 1546 1767 1988 2209 2430 2651 2872 3092
10 1091 1364 1636 1909 2182 2454 2727 3000 3272 3545 3818
11 1320 1650 1980 2310 2640 2970 3300 3630 3960 4290 4620
12 1571 1963 2356 2749 3142 3534 3927 4320 4712 5105 5498
13 1843 2304 2765 3226 3687 4148 4609 5070 5530 5991 6452
14 2138 2673 3207 3742 4276 4811 5345 5880 6414 6949 7483
15 2454 3068 3682 4295 4909 5522 6136 6749 7363 7977 8590
16 2792 3491 4189 4887 5585 6283 6981 7679 8377 9076 9774
17 3152 3941 4729 5517 6305 7093 7881 8669 9457 10246 11034
18 3534 4418 5301 6185 7069 7952 8836 9719 10603 11486 12370
19 3938 4922 5907 6891 7876 8860 9845 10829 11814 12798 13782
20 4363 5454 6545 7636 8727 9817 10908 11999 13090 14181 15271
21 4811 6013 7216 8418 9621 10824 12026 13229 14432 15634 16837
22 5280 6599 7919 9239 10559 11879 13199 14519 15839 17159 18478
23 5770 7213 8656 10098 11541 12983 14426 15869 17311 18754 20197
24 6283 7854 9425 10995 12566 14137 15708 17279 18849 20420 21991
26 7374 9217 11061 12904 14748 16591 18435 20278 22122 23965 25809
28 8552 10690 12828 14966 17104 19242 21380 23518 25656 27794 29932
30 9817 12272 14726 17180 19635 22089 24543 26998 29452 31906 34361
32 11170 13962 16755 19547 22340 25132 27925 30717 33510 36302 39095
34 12610 15762 18915 22067 25220 28372 31525 34677 37830 40982 44135
36 14137 17671 21206 24740 28274 31808 35343 38877 42411 45945 49480
38 15751 19689 23627 27565 31503 35441 39379 43316 47254 51192 55130
40 17453 21816 26180 30543 34906 39269 43633 47996 52359 56723 61086
42 19242 24053 28863 33674 38484 43295 48105 52916 57726 62537 67347
44 21118 26398 31677 36957 42237 47516 52796 58075 63355 68634 73914
46 23082 28852 34623 40393 46163 51934 57704 63475 69245 75016 80786
48 25132 31416 37699 43982 50265 56548 62831 69114 75397 81681 87964
50 27270 34088 40906 47723 54541 61359 68176 74994 81811 88629 95447
52 29496 36870 44244 51618 58991 66365 73739 81113 88487 95861 103235
54 31808 39760 47712 55664 63617 71569 79521 87473 95425 103377 111329
56 34208 42760 51312 59864 68416 76968 85520 94072 102624 111176 119728
58 36695 45869 55043 64216 73390 82564 91738 100912 110085 119259 128433
60 39269 49087 58904 68722 78539 88356 98174 107991 117808 127626 137443
62 41931 52414 62897 73379 83862 94345 104828 115310 125793 136276 146759
64 44680 55850 67020 78190 89360 100530 111700 122870 134040 145210 156380
66 47516 59395 71274 83153 95032 106911 118790 130669 142548 154427 166306
68 50439 63049 75659 88269 100879 113489 126099 138709 151318 163928 176538
70 53450 66813 80175 93538 106900 120263 133625 146988 160350 173713 187075
72 56548 70685 84822 98959 113096 127233 141370 155507 169644 183781 197918
74 59733 74667 89600 104533 119466 134400 149333 164266 179200 194133 209066
76 63006 78757 94509 110260 126011 141763 157514 173266 189017 204768 220520
78 66365 82957 99548 116139 132731 149322 165914 182505 199096 215688 232279
80 69812 87265 104719 122172 139625 157078 174531 191984 209437 226890 244343

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