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Towers
Sizing of plume abatement coils
Plume abatement coils are used to reduce / prevent visible plumes of moisture exiting a wet cooling tower fan stack . Typically , this is done by mixing in dry , heated ambient air with the moist air stream exiting the drift eliminators . The cold ambient air passes over the “ airside ” of the coil and is heated by transferring heat from a hot fluid source flowing on the “ tube side ” of the coil . The article will present the methodology and equations required to budget size the overall plume abatement coil surface area required .
About the author
Bob Giammaruti is CEO of SHECO Industries and has over 30 years experience in the management , sale , design , and manufacturing of heat exchanger equipment . Bob holds a Masters and Bachelors of Mechanical Engineering from Drexel University and is the author / co-author of multiple technical papers and US Patents .
By Bob Giammaruti , SHECO Industries
Plume abatement coils can be used in both counter flow or cross flow cooling towers as shown in Figures 1 and 2 . The plume abatement coils sit above the water distribution system and drift eliminators on the perimeter / sides of the tower . Dry air from ambient is drawn through the plume abatement coils and is mixed with the moist air exiting the drift eliminators . The resultant air mixture is then expelled from the cooling tower via the fan and fan stack . The following is a basic method to size plume abatement coils used in wet cooling towers . It should be noted that the following method is accurate to only +/ - 10 % and should only be used for scoping and / or budget calculations .
Basic coil design and assumptions A typical plume abatement coil is shown in Figures 3 and 4 . As shown in Figures 1 or 2 , ambient air enters the plume abatement coil and is heated via a hot fluid to a predetermined temperature needed to mix with the moist air and abate the plume . For this sizing calculation , the following assumptions have been made :
• The heat exchanger is a one-pass cross flow with both fluids unmixed .
• The process fluid is water from the hot water basin .
• The tube side fouling factor ( f ) is 0.00026 sqm K / W ( 0.0015 sqft hr F / Btu )
• The air to be heated is ambient air .
• Tube side pressure drop is between 0 and 240 N / sqm ( 0 and 5 lb / sqin )
• Process tube material is carbon steel with extended wrap on aluminum fins .
• Process tubes are 25.4 mm ( 1.0 inch ) in diameter and between 0.89 mm ( 0.035 inch ) and 2.77 mm ( 0.109 inch ) thick .
• Extended fins are 15.88 mm ( 0.625 inch ) high , 0.41 mm ( 0.016 inch ) thick and have a spacing of 394 fins / m ( 10.0 fins / inch ).
• Tubes are in a staggered pitch pattern with the transverse pitch ( Pt ) being 63.5 mm ( 2.50 inch ) and the longitudinal pitch ( Pl ) being 55.0 mm ( 2.165 inch ) as shown in Figure 5 .
Coil thermal sizing The method employed here to thermally size the coil will be the NTU Method as described in Compact Heat Exchangers ( Reference 1 ) as well as in the Fundamentals of Heat and Mass Transfer ( Reference 2 ) as applied to plume abatement coils . The following are the equations to determine the total surface area required to thermally size the coil . The required duty of the coil ( s ) can be determined as follows :
Q a
= M a cp a
( T ai
– T ao ) ( Eq . 1 )
Next , a heat balance is performed where Q a = Q h
:
Q a
= Q h = Q = M h cp h
( T hi – T ho
) ( Eq . 2 )
From here , we solve for M h assuming a temperature difference between T hi and T ho
:
M h
= Q / ( cp h ( T hi
– T ho )) ( Eq . 3 )
Figure 1 . Counter flow cooling tower with plume abatement Figure 2 . Cross flow cooling tower with plume abatement www . heat-exchanger-world . com Heat Exchanger World October 2022
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