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Cooling Towers
N z
Number of Rows ( N ) 0 to 0.4 4 0.4 to 0.5 5 0.5 to 0.7 6
Table 2 – Number of required tube rows
And , the number rows can be assumed to be per Table 2 . The number of tubes is calculated as follows :
Figure 5 . Plume abatement coil tube lay out
able U values is approximately between 2,248,500 and 2,453,000 J / hr sqm C ( 110 to 120 Btu / hr sqft F ) depending on the tube wall thickness . For the tube OD , tube pitch , fin density , fin material and hot fluid assumed above , the overall heat transfer coefficient for this set of calculations can be found in Table 1 as a function of tube wall thickness . Now , we can determine the approximate overall bare tube surface area required as follows :
A = NTU C min / U ( Eq . 10 )
Next , we need to determine the number of tube rows in the coil . This can be determined by the following equation ( Reference 3 ):
N z
= [( T hi
-T ho
)/( T hi
-T ci
)]( U c
/ U ) ( Eq . 11 )
Where :
U c
= 2,044,061 J / hr sqm C ( 100 Btu / hr sqft F )
N t
= A / A t
= A / [( pi )( OD / w )( L )] ( Eq . 12 )
Where : w = 1,000 mm ( 12 in )
Based on the number of tube rows determined by equation 11 and Table 2 , the number of tubes per row can be calculated as follows :
N r
= N t / N ( Eq . 13 )
From here , the overall width of the heat exchanger surface can be determine as follows :
W = N r
( Pt / w ) ( Eq . 14 )
Now , the value given in equation 14 is for the overall width of the exchanger . Given this is for a plume abatement application , the overall heat transfer surface is typically broken down into smaller sections for easier manufacturing , transport and installation . The approximate width of an individual bundle can be determined as follows :
W b
= ( W / N s ) + G ( Eq . 15 )
Figure 6 . Effectiveness ( E ) versus NTU for CR range of 0.1 to 1.0
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