Furnace forced draft fan low suction flow protection 2

[JeffBabs]

Dear colleagues, we will be replacing two very old FDFs of one of our furnaces. These old FDFs are have discharge damper controls and are now undersized due to several debottlenecking and duty increase of the furnace. The new FDFs to be installed will have a capacity of 1600 acmm each at 511 mmAq. They will have the following features:

-individual venturi flow meters
-separate air suction towers and discharging at common discharge ducting
-suction damper control will be used (in contrast to the old discharge damper control)
-will be normally operating in parallel
-they will have no online vibration monitoring and protection system(only daily routine check by operations and weekly PM by maintenance)

The blowers were already manufactured and the capacity and performance tests are acceptable as per requirement and the expected operating points are far from the surge region. Our systems engineering, however, are not sure whether to install an individual blower low-low flow protection. As a mechanical engineer, I am accustomed to compressors having surge protection system and low-low flow trips. As for these centrifugal fans, I was suggesting to no longer have a protection system for individual blowers (i.e. protecting them from low flow) since they are less sensitive to these conditions (compared to compressors). Moreover, if there will be a low flow protection, I am not sure what will be the trip and alarm set-point. (take note that furnace protection from lack of combustion air is separate and is already in place). Does anyone have a similar experience with regards to this flow protection concern and what was the approach you used?

 

[willard3]

You must explain your acronyms. What is an FDF, what are the units of acmm and etc.

 

[JeffBabs]

willard3,
FDF - forced draft fan as mentioned in title
acmm - actual cubic meter per min
mm Aq - is mm of water

 

[SAK9]

Are you using inlet vane control for volume control?With inlet vane control you have a new fan curve for each position with curve shifting down as damper closes.With outlet damper control,fan curve stays the same and it is the system curve that moves up/down the fan curve.Variable speed control will create new curve for each fan speed!So surge point may differ depending on the mode of volume control.What will be the level of accuracy of venturi flow meters under low flow conditions?Rather than introducing low flow protection,it will be more sensible to limit damper closure to say 30% or whatever is an allowable minimum based on actual field tests.This will prevent unplanned shutdowns.

 

[JeffBabs]

SAK9,
We will be using inlet vane control. You suggestion of clamping the damper opening at a minimum percentage is noted and also reminds me of what some of our air compressors have. Thanks for the suggestion, this helps as lot.