6 replies to this topic

[mamakda]

I have normally seen that air cooled heat exchangers are normally equipped with forced draft fans while cooling towers have induced draft fans...Can anyone give me a technical reason for this ?

[Dipankarc84]

Forced draft fans in air coolers  are easier to maintain since there is easier access to the motor, fans and belts. However, induced draft air coolers are equally popular and there is no "right or wrong" answer on which one to use. Both have their share of advantages and disadvantages and in the end it all boils down to the preference of the Project Manager and the Operations/ Maintenance staff.

 

Cooling towers are relatively larger installations and forced draft fans used to exist long back I think (maybe Art Montemayer can shed some light on that?). However, the air is much more uniformly distributed in an induced draft fan and hence is preferable in Cooling towers. 

[Atttyub194]

Dear mamakda

 

Good day!

 

Please note the following

 

Induced Drat Fan

Advantages

1. Better distribution of air across the bundle.
2. Less possibility of hot effluent air recirculating into the intake. The hot air is discharged upward at approximately 2.5 times the intake velocity, or about 1,500 feet per minute.
3. Better process control and stability because the plenum covers 60% of the bundle face area, reducing the effects of sun, rain, and hall.
4. Increased capacity in the fan-off or fan failure condition, since the natural draft stack effect is much greater.

Dis Advantages

1. Possibly higher horsepower requirements if the effluent air is very hot.
2. Effluent air temperature should be limited to 220°F to prevent damage to fan blades, bearings, or other mechanical equipment in the hot air stream. When the process inlet temperature exceeds 350°F, forced draft design should be considered because high effluent air temperatures may occur during fan-off or low air flow operation.
3. Fans are less accessible for maintenance, and maintenance may have to be done in the hot air generated by natural convection.
4. Plenums must be removed to replace bundles.

Forced Draft Fan

 

Advantages

1. Possibly lower horsepower requirements if the effluent air is very hot. (Horsepower varies inversely with the absolute temperature.)
2. Better accessibility of fans and upper bearings for maintenance.
3. Better accessibility of bundles for replacement.
4. Accommodates higher process inlet temperatures

Dis Advantages

1. Less uniform distribution of air over the bundle.
2. Increased possibility of hot air recirculation, resulting from low discharge velocity from the bundles, high intake velocity to the fan ring, and no stack.
3. Low natural draft capability on fan failure.
4. Complete exposure of the finned tubes to sun, rain, and hail, which results in poor process control and stability.

Hope this will serve the purpose

 

Best regards,

 

Ahmed Attyub

[mamakda]

Thanks everyone

[Raj Mehta]

Mr. Ahmed can you please explain the following points in a bit detail, I failed to understand the logic behind those points

 

1. Induced draft : advantages 4th point

2. Induced drat : Disadvantages 1st point

3. Forced draft : advantages 1 & 4th point

4. Forced draft : disadvantages 3rd point

 

Thank you

[Dipankarc84]

Hello Raj,

 

What Mr. Ahmed tried to explain is as follows:

1. Increased capacity in the fan-off or fan failure condition, since the natural draft stack effect is much greater.

The induced draft cooler has a fan at the top of the bundle which "pulls" the air out of the cooler. As a result it has a plenum and a structure above the tube bundle which enables it to create a natural draft. Hence, even when the fans are switched off, due to the natural draft these coolers still are able to deliver some amount of cooling. This effect is much reduced in forced draft installations because the fan is placed below the bundle. The fan "pushes" the air out of the bundle. The top of the bundle is open to atmosphere and hence has a much reduced "draft"

 

     

1. Possibly higher horsepower requirements if the effluent air is very hot.

In an induced draft fan, the air sucked into the fan is hot. Remember, hotter air will have a larger volume, and a larger volume will call for a larger fan power

 

Less uniform distribution of air over the bundle

 

As I said earlier, an induced draft fan "pulls" the air out of a bundle while a forced draft fan "pushes" the air in it. The distribution of air is much more uniform when you pull it out. To understand this in a simpler way, try imagining a string lose on both ends. When you push the string from the back, it curls up randomly. But when you try pulling it from the other end, the string moves quite smoothly and linearly. The same thing happens with air.  

 

Complete exposure of the finned tubes to sun, rain, and hail, which results in poor process control and stability

 

Please read the first explanation once more. A forced draft cooler will have the tube bundle on "top" of the fan. Hence, it is exposed to atmosphere at the top. As a result, there are chances that changing weather conditions (as mentioned by Mr. Ahmed) have a direct impact on the process temperatures unlike an induced draft installation where the bundle is "covered" from the top. 

 

Hope the explanations are clear. Both types have their advantages and disadvantages. However, forced draft installations are much more common in industry, primarily because of their ease of maintenance (easier access to the fans/ motors/ gear/ belt drives etc. since they are placed below the bundle)

[breizh]

http://www.cheresour...uced-draft-fan/

 

Consider this reply to an old post !

 

Breizh