Forced draft vs Induced draft

[mazamm]

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 ?

 

[pennpiper]

This part of your question is not accurate. It is rare but some Cooling Towers are designed with forced draft fans near grade in the sidewall of the structure. I tried to find a photo or drawing for you but was not successful.





prognosis: Lead or Lag

 

[mazamm]

Wy this is incorrect. I have observed it in my refinery. My supervisor told me to search for the reason. Is this something to deal with motor power?

 

[DRWeig]

There is no technical reason to my knowledge. Either way makes air got through the exchanger.

You can buy either type. HVAC air handling units can be draw-thru or blow-thru relative to the heat exchangers. Cooling towers can be draw-thru, blow-thru, and cross-flow.

I suppose in some circumstances you might want to consider whether you want the motor waste heat in the air entering the exchanger or added to the air downstream. The temperature levels the motor is exposed to may play a role -- do you want your motor in the warmer air or the cooler air?

Best to you,

Goober Dave

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[macmet]

I know that some (all?) air cooled condensers have fans that can function as FD or ID fans. I believe the fans normally draw air from below and "push" it out the top, but the rotation is reversed at times so you are pulling warm air over the fans to keep them from freezing. I only have second hand knowledge of ACCs though, so I'm sure someone can explain that better.

 

[rakuday]

One reason for using induced draft fans in cooling towers are, water will be falling down from the fill and will be collected in the sump at the bottom. Fans cannot be fixed in the bottom of the fill, as it increases the complexity of construction.

Whereas for heat exchangers, the fluid being cooled will usually flow thru tubes, hence the mode (forced / induced) can be chosen depending upon other criteria. For additional information regarding the forced & induced draft coolers refer below link:

http://www.hudsonproducts.com/products/finfan/tech.html

Hope this helps

 

[mazamm]

Thanks rakuday..i was also thinking the same thing which u told about water coolers.
This cleared my confusion

 

[lappygamer]

Fans are volumetric machines. The amount of power required to move a gas is a function of the volumetric flow rate of the gas. In order to reduce the size of the motor required for the fan I would guess that the fans are placed where gas temperatures and consequently volumetric flow rate are the lowest.

 

[mazamm]

lappygamer

Volume flowrate(Q): Q= Velocity * Area
For a given fan, the velocity is same. Since both hot and gases will occupy same area So how can you say that lower temperatures yield low volumetric flow rates and consequently low Horsepower??

(though it is 100% true that large volumetric flow rates will require large motor motor)

 

[lappygamer]

mazamm,

The velocity and the volumetric flow rate are dependent on the density of the gas.

Let's compare two cases. In both cases, the mass flow of gas through the system is 100,000 lb/hr.

Case 1: At the inlet to an air pre-heater the gas temperature is 60F. The density of the gas is 0.076 lb/ft3. This would be a volumetric flow rate of 21,930 ACFM.

Case 2: At the outlet of an air pre-heater the gas temperature is 500F. The density of the gas is 0.041 lb/ft3. This would be a volumetric flow rate of 40,650 ACFM.

The mass of air through the system is identical but the volumetric flow rate is dependent on the temperature of the gas at the point you are evaluating. Putting the fan where the temperature is 500F instead of 60F will require 85% motor power.

 

[lappygamer]

That last part should read "85% more motor power."

 

[mazamm]

cheers mate

 

[mazamm]

I was not keeping the mass-flow constant