PIPE DUCT SELECTION

TABLES TO AID IN PIPE SELECTION

DISCLAIMER: This information is displayed for general approximation only.  It does not constitute a recommendation for a given application.  Local codes and equipment manufacture's instruction override this "public domain" information

 

CORRECTED PRESSURE DROP (FRICTION) PER 100 FT. DUCT LENGTH

ADJUSTMENT OF DUCT DESIGN PRESSURE FOR EQUIVALENT LENGTH (NORMAL RESIDENTIAL DUCT CALCULATION SETTING - .10) PER 100 FT
(1200 CFM MAX) .08 RETURN

PRESSURE DROP (IN. OF WATER COLUMN)

TOTAL EFF                                                                                                                                         LENGTH                                                                                                                                            
DUCT & FITTING                                                                                                                                    
IN FEET            .05    .06    .08    .10    .125   .14    .16    .18    .20    .25    .30    .34    .375    .40     .50    .625     .75    1.00
35-44              .13    .15    .20    .25    .32    .35    .40    .45    .50    .63    .75    .85    .95    1.00    1.25    1.50    1.90    2.54
45-54              .10    .12    .16    .20    .25    .28    .32    .36    .40    .50    .60    .68    .76     .80    1.00    1.26    1.52    2.02
57-64              .08    .10    .13    .17    .21    .23    .27    .30    .33    .42    .50    .57    .63     .67     .83    1.05    1.26    1.68
65-74              .07    .09    .11    .14    .18    .20    .23    .26    .29    .36    .43    .49    .54     .57     .72     .90    1.08    1.44
75-84              .06    .08    .10    .13    .16    .18    .20    .23    .25    .31    .38    .43    .47     .50     .63     .79     .94    1.26
85-94              .06    .07    .09    .11    .14    .16    .18    .20    .22    .28    .33    .38    .42     .45     .56     .70     .84    1.12
95-104             .05    .06    .08    .10    .13    .14    .16    .18    .20    .25    .30    .33    .38     .40     .50     .63     .75    1.01
105-114            .05    .05    .07    .09    .11    .13    .15    .16    .18    .23    .28    .31    .34     .37     .47     .57     .69     .91
115-129            .04    .05    .07    .08    .10    .12    .13    .15    .17    .21    .25    .28    .31     .33     .46     .51     .62     .82
130-149            .04    .04    .06    .07    .09    .10    .11    .13    .14    .18    .21    .24    .27     .29     .36     .45     .54     .72
150-169            .03    .04    .05    .06    .08    .09    .10    .11    .13    .16    .19    .21    .24     .25     .31     .39     .47     .63
170-189            .03    .03    .04    .06    .07    .08    .09    .10    .11    .14    .17    .19    .21     .22     .28     .35     .42     .56
190-214            .03    .03    .04    .05    .06    .07    .08    .09    .10    .13    .15    .17    .19     .20     .25     .31     .37     .50
215-239            .02    .03    .04    .05    .05    .06    .07    .08    .09    .11    .13    .15    .17     .18     .22     .28     .33     .44
240-264            .02    .02    .03    .04    .05    .06    .06    .07    .08    .10    .12    .14    .15     .16     .20     .25     .30     .40
265-289            .02    .02    .03    .04    .04    .05    .06    .07    .07    .09    .11    .12    .14     .15     .18     .23     .27     .36
290-324            .02    .02    .03    .03    .04    .05    .05    .06    .07    .08    .10    .11    .12     .13     .17     .20     .24     .33
325-374            .02    .02    .02    .03    .04    .04    .05    .05    .06    .07    .09    .10    .11     .11     .14     .18     .21     .29
375-424            .01    .02    .02    .03    .03    .04    .04    .05    .05    .06    .08    .09    .09     .10     .13     .16     .19     .25
425-474            .01    .01    .02    .03    .03    .03    .04    .04    .05    .06    .07    .08    .08     .09     .11     .14     .17     .22
475-524            .01    .01    .02    .02    .03    .03    .03    .04    .04    .05    .06    .07    .08     .08     .10     .13     .15     .20
525-574            .01    .01    .02    .02    .02    .03    .03    .03    .04    .05    .06    .07    .07     .07     .09     .11     .14     .18
575-625            .01    .01    .01    .02    .02    .02    .03    .03    .03    .04    .05    .06    .06     .07     .08     .10     .13     .17

 

MAXIMUM DUCT VELOCITY (FEET PER MINUITE)


      MAIN DUCT*           BRANCH                                      
SUPPLY    RETURN      SUPPLY     RETURN         APPLICATION             
1000........800........500........500.......APARTMENTS, RESIDENTS       
1300.......1100.......1000........800.......AUDITORIUMS, THEATERS       
1500.......1300.......1200.......1000.......HOSPITAL ROOMS, HOTEL ROOMS 
2000.......1500.......1600.......1200.......BANKS, LIBRARIES, OFFICES,  
                                            MEETING ROOMS, RESTAURANTS, 
                                            RETAIL STORES               
3000.......1000.......2200.......1500.......INDUSTRIAL                   

*Use branch duct velocities where sound control is critical.

CFM=  BTUH                                       
    1.08xT.D.       (TEMPERATURE DIFFERENCE - DRY BULB)

 

 
DUCT SIZE/CAPACITY (100 EQUIVALENT FEET)
SUPPY OR RETURN SIZES


 CFM     ROUND      RECTANGULAR                                       
  50.......5".........8 X 6
  75.......5".........8 X 6
 100.......5".........8 X 6........3 1/4 X 10......................(110)
 125.......6".........8 X 6........3 1/4 X 12......................(140)
 150.......7".........8 X 6........3 1/4 X 14.........(160) (STUD SPACE)
 175.......7".........8 X  6
 200.......7".........8 X 6
 250.......8".........6 X 10.........8 X 8......................6 X 12
 300.......9".........6 X 10.........8 X 8......................6 X 12
 350.......9".........6 X 10.........8 X 8......................6 X 12
 400.......9".........6 X 10.........8 X 8......................6 X 12
 500......10"........10 X 10........12 X 8......................8 X 12
 600......12"........10 X 10........12 X 8......................8 X 12
 700......12"........12 X 10........14 X 8.....................10 X 12
 800......12"........12 X 10........16 X 8.....................10 X 12
 900......14"........14 X 10........18 X 8.....................12 X 12
1000......14"........14 X 10........18 X 8.....................12 X 12
1200......14"........16 X 10........22 X 8.....................14 X 12
1400......16"........18 X 10........24 X 8.....................16 X 12
1600......16"........20 X 10........28 X 8.....................18 X 12
1800......16"........24 X 10........30 X 8.....................20 X 12
2000......18"........26 X 10........32 X 8.....................20 X 12
2500......18"........30 X 10........20 X 14....................24 X 12
3000......20"........36 X 10........24 X 14....................28 X 12
3500......22"........40 X 10........28 X 14....................32 X 12
4000......24"........26 X 16........32 X 14....................38 X 12        

                      8" JOIST - 375 CFM; 10" JOIST - 525 CFM (RETURN)

 
KEY FACTORS IN DUCT DESIGN LAYOUT

1.  Aspect Ratio of Duct
     a.  Relationship of the long side to the short side.
     b.  Keep the ratio as low as possible, round is best - less friction -
         loss for a given cross sectional area.
     c.  Stack of oval is not the best.
     d.  Space limitation will limit the size of round pipe.
     e.  8" sided duct is the modern trade-off from an equipment and clearance stand point.


2.  Duct Heat Gain or Loss
     a.  Ducts with large aspect ratio's have more heat gain or loss.
     b.  Ducts carrying small air quantities at low velocity have the greatest heat gain or loss.
     c.  The addition of insulation to a duct can significantly affect heat gain or loss.
     d.  Ducts that run through unconditioned space should be insulated.

3.  Duct Friction And Turbulence
     a.  Static pressure constantly decreases as air flow through a duct system.
     b.  This decrease in pressure is partly the result of friction (air rubbing against duct walls).
     c.  Static pressure losses may also be caused by turbulence within the duct, especially near
         fitting that change
         the direction or reduce the flow of the air (air-chums).
 
EFFECTIVE LENGTHS OF COMMON FITTINGS


 1.  Duct Fittings
     500 - Offset starting collar..............10'
     501 - Straight starting collar............35'
     502 - Round starting collar...............35'
 
2.  Round Branch Fittings (Takeoff)
     502 - Round starting collar...............35'
     502 - Round starting collar w/elbow...35'-50'
     575 - Square to round straight............15'
     572 - Square to round elbow...............40'
     575 - Square to round elbow...........40'-45'
     576 - Round to round..................20'-25'
     577 - Round to round..................20'-25'
     651 - Round to round..................20'-25'
     (576,577,651 Takeoff are adjustable and have a
      wider mouth than body.)

3.  Round Elbows and Angles
     200-203 Elbows.........................5'-10'  
     207-210 Angles........................10'-20'

4.  Register Boots
      608 - Universal, straight..................5'
            610 - 90 degree, elbow....................30'
      612 - End.................................50'


 

  

  Duct Elbows - Radius Throat
      Up to 11".....10'
      12" - 21".....15'
      22" - 27".....20'
      29" - 33".....25'
 
   Square Throat 
      Up to 11".....40'
      12" - 15".....57'
      16" - 21".....75'
      22" - 27"....100'
      28" - 33"....125'
 
     
The above charts when used in conjunction with a "duct calculator" can be a big aid to designing proper air distribution systems.  All capacities are based on total effective feet of 100 (duct and all fittings in the run).  The common, residential design perimeter for friction loss is .10 drop per 100 feet on the supply side and .08 on the return side.  Each fitting is assigned a drop in pressure value due to friction by means of its relationship to straight pipe (effective feet). 

Each of the above drawings lists a numerical value which shows the effective feet of this fitting (equivalent length of straight duct due to friction imposed on air flowing through this fitting.)  Page 32 continues with more fittings diagrams and a list of the effective length of common Champion fittings.