3 replies to this topic

[slwy]

Hi I have a problem understanding the pressure drop across fans.

 

I have a furnace using balanced draft and a preheater (which is a shell and tube heat exchanger).

 

A shell and tube heat exchanger has pressure drops of about 20-30kPa.

Is it right that the pressure drop across the FD fan is the sum of pressure drops across the preheater, burner, ducting etc..

If that is so, the pressure drop across the FD fan will be more than 20kPa.

 

However, I read that typical pressure drops across FD fans are about 0.03bar (3kPa) range.

So is it wrong that my fan is having such high pressure drops..

 

 

Another question is that in using a balanced draft, does that mean that the pressure in the combustion chamber must be slightly negative and the pressure in the stack must be slightly positive compared to atmospheric pressure? (since fluid flows from high pressure to low pressure)

 

I am really confused. Please help.

[thorium90]

So where did you read that FD fans can only have 3kPa of pressure drop? 20kPa seems fine. There is no shell and tube heat exchanger inside a furnace. Its just the tubes exposed to the hot flue gas.

 

In balanced draft, there are FD and ID fans. FD fans blow air in and ID fans suck air out. The balance between the two creates whatever pressure is inside.

[Art Montemayor]

My responses are in RED.

 

I have a furnace using balanced draft and a preheater (which is a shell and tube heat exchanger).

If your furnace requires a balanced draft, then it must be employing a forced draft (FD) air blower and an induced draft (ID) flue gas blower.  You must maintain a constant draft (flow of gases) in the furnace in order to establish flame and furnace stability.  You achieve this by “balancing” the gas flow, using throttling of the fan flows or speed changes – since both fans are normally centrifugal fans.

 

A shell and tube heat exchanger has pressure drops of about 20-30 kPa.

This is not necessarily true and seems too low.  Where did you obtain this information?

 

Is it right that the pressure drop across the FD fan is the sum of pressure drops across the preheater, burner, ducting etc..

It depends where you locate the FD fan.  You don’t furnish a flow diagram or P&ID, so we can’t tell.

 

However, I read that typical pressure drops across FD fans are about 0.03bar (3kPa) range.

This is not necessarily true and seems too low.  Where did you read this?

 

Another question is that in using a balanced draft, does that mean that the pressure in the combustion chamber must be slightly negative and the pressure in the stack must be slightly positive compared to atmospheric pressure?  (since fluid flows from high pressure to low pressure)

That depends on how your system is designed for balanced flow to take place.  It also depends on where you are measuring the pressure.  You must be specific and the best way is to furnish an accurate flow diagram or P&ID.

 

The bottom line in any combustion process taking place in a furnace is that you must maintain a constant feed of oxidant (oxygen) to the burners while you also must allow for the produced combustion gases to exit the furnace at the same rate that they are being produced.  This is nothing complex or nuclear, just simply common sense for the process to work continuously and safely.  To mechanically carry this out, centrifugal fans are used at the inlet to the furnace and another at the outlet (FD & ID fans).

 

 

[ankur2061]

slwy,

 

I would recommend that you have a look at the following link for sizing calculations (motor rating) for a FD fan for fired heaters:

 

http://www.cheresour...eaters-boilers/

 

Regards,

Ankur.