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How does Ambient Temperature & Elevation affect Boiler Fan Capacity?

The amount of air in mass units (lb/h or kg/h) required to generate a given amount of steam  in a boiler is unaffected by ambient conditions or density of air as it depends only on the amount of fuel fired in Million Btu/h(HHV),excess air and fuel heating value.This can be seen from the  Million Btu method of combustion calculations .Assuming that the boiler efficiency does not change much with ambient conditions,the air mass flow and flue gas mass flow will be approximately the same at different ambient conditions. However when ambient temperature and elevation change the density of air changes,which affects the volume of air flow and the head the fan can deliver and thus it affects the fan performance.Hence it is important for boiler engineers to be aware of the density factor,while sizing fans for boilers.
The mass flow of air is given by the expression:
where W=air flow in lb/h,q=volumetric flow in acfm and r is the density of air inlb/ft3
The density r decreases with altitude as shown in the following table.

              Effect of altitude on air density
 altitude,ft density factor
  0  1
 1000  0.964
 2000  0.930
 3000  0.896
 4000  0.864
 5000  0.832
 6000  0.801
 7000  0.772
 8000  0.743

Also,the fan head H is usually expressed in in wc and the impact of  density on fan head is as follows:
H/r =constant
Hence if a fan develops 18000 acfm at 18 in wc at 1000 ft and 60 F and we would like to know what head it develops at 4000 ft.The air density at 1000 ft = 0.075x.964=0.0723 lb/ft3 and at 4000 ft,it is 0.075x0.864=0.0648 lb/ft3
where the factors 0.864 and 0.964 are from table above.Using the above equation H/r=constant,we have:
18/0.964 =H/0.864 or H=16.1 in wc,which may or may not be adequete for proper boiler operation.
The mass flow of air at 1000 ft =18000x0.0723x60=78,084 lb/h,while at 4000 ft,the fan will deliver
18000x0.0648x60=70,000 lb/h of air,which may or may not be adequete.A new curve has to be constructed for the head vs flow characteristics and then the operating point may be found.Thus at lower density the mass flow and head which a given fan can deliver are reduced.
Also,it can be shown that pressure drop in the burner is also a function of W2/r;hence when the density is lower,the burner drop increases.
Thus considering all these aspects,it is better to select the fan for the lowest density condition.Then at higher density conditions,the fan will be capable of delivering a higher mass flow and head,which can be reduced if necessary during operation by means of various methods such as dampers,vanes,fan speed adjustment etc.However not having the capacity could lead to problems if the fan has to operate in summer conditions or at low density situations.Using large margins on fans to overcome density variations is not a good idea as the turn down capability of the fan is reduced.Hence good judgement should be used while selecting fans for boilers.

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