Burnham Series 8H/8HE Boiler Manual De Propietario

7)   If a tapered or graduated manifold vent is desired, 

such as in Figure 1-6 the horizontal and vertical 

portion of the vent serving five modules is also 

complete with 6) above.  However, to size the 

manifold vent at an intermediate position such as 

CV3 in Figure 2-4, use Figure 2-13 for the MBH 

Input of the modules served by that position of the 

manifold vent.  The Input MBH of each module 

can be determined from Figure 2-3.  In this case 

CV3 serves two modules having an input of 

920 MBH.  Enter figure 2-13 at an input of 920 

MBH.  Move right to H = 10 Ft. and find CV3 

= 14” diameter.  Mark this vent diameter size on 

the drawing of the boiler room.  Thus a graduated 

manifold vent design is complete.  It is possible 

with this procedure to reduce the manifold vent 

size after each module.  However, from a practical 

standpoint, the cost of fittings may offset the lower 

cost of smaller vent pipe.

2.10    All of the above procedure is based on data found 

in the ASHRAE Guide, 1975 Equipment Volume, 

Chapter 26.  The basic chimney equation is expressed 

as follows:

where:  I = Operating heat input, BTUH
 

    di = Inside diameter of the common vent or    

         manifold vent

 

M =   Mass flow input ratio, lb. of products per            

1000 BTU of fuel burned.  A value of 1.60 was  

   used based on 5.3% CO

2

 after dilution.  An    

   additional 15% dilution was added for each    

   unfired module.

 

∆P =   Pressure difference or loss in the system              

acting to cause flow, inches of water.  Use  

        

     0.537 inches water per 100 Ft. of pipe.

B  =    Sea level barometer used —29.92” Hg

K   =   Resistance loss coefficients, dimensionless.

Tm =  Temperature in vent system at average  

 

     conditions, °Fabs.

  The serious Engineer should become familiar with the 

above basic equation and the range of the variables that 

may be encountered.  The tables in Figures 2-12 thru 

2-14 should not be extrapolated.  If system conditions 

do not fall within the limit of the tables, vent sizes must 

be calculated using the chimney equation above as 

described in the ASHRAE Guide.

               (di)² 

(∆PB) 

0.5

I = 4.13 x 10

5  

x   M       x    (KTm)