Inlet dampers are preferred over discharge damper. Airflow is more uniform at the inlet. Discharge damper would involve system loss depending on the length of the staright duct to the damper. Less straight duct length, more loss. However fire codes may require smoke dampers at the supply fan ischarge and at the return fan inlet for fans over 15,000 CFM.
I Think there are two issues while comparing inlet and outlet dampers.
The most important point is that an inlet damper changes the fan curve. Closing inlet damper brings the fan head-flow curve down, with the shut off head more or less unchanged. Along with this shift, the surge line also gets shifted towards the y-axis, which would mean that the fan can be safely operated with inlet damper at very low flow conditions without causing surging.
In the case of discharge damper, the fan curve remains the same, but the system curve changes. There is no change in surge line, hence risky to operate at very low flows.
The second advantage of an inlet damper control is the lower energy consumption compared to discharge damper.
Another important consideration relates to the ability of the system components to withstand +ve & -ve pressures. I have seen an air handler collapse because an inlet damper was shut during start up. The casing was tested to withstand in excess of the fan +ve discharge pressure only. If the damper had been placed on the discharge side of the fan the unit would have been OK.
To prevent damaging ductwork,Provide an end switch at each damper and wire to the fan starter circuit so if any damper is closed, the end switch is open and the fan is not allowed to start. Another way would be to provide pressure switches to shut down fan if discharge or intake pressure exceed set limits.
Outlet damper would increase static pressure at discharge, as you have pointed out. The system curve would move towards y-axis when you close the discharge damper.
Are we comparing inlet vanes to a discharge damper for fan volume control? Or are we discussing isolation dampers on the inlet and outlet of an airhandling unit?
Regardless, I'd recommend a VFD over inlet or discharge dampers for volume control.
I was curious about volume control. We have systems with fans reaching 400hp and prefer to use dampers than vfd's on these units. On smaller fans we use vfds.
When you use either suction or discharge dampers, the fan static head can be matched with the system resistance. The extra head is absorbed at the damper. However, when you use variable speed devices, your fan static head reduces and this will be a problem if your system resistance is predominantly static in nature (for ex. aeration, boiler draught fans etc).
We use these vfd's on boilers. Why are boiler draft fans static? I would expect that the resistance would be proportional to air flow so that if the fan is turned down the resistance drops as well.
I always thought that when vfd's were used the system curve moved toward the y-axis along the same static pressure curve. If this is the case then we should have no issues with turn down - which we do not seem to have.
You wouldn't happen to have any literature on vfd's for fans similar to the link for dampers you provided earlier?
Well I have been doing some reading...and since speed is related to frequency I believe the pressure curve will move down as the freq. is lowered. So, if this is correct then my earlier understanding was mistaken.
I would still be interested in any links/reports/books about vfd's though.
The performance curve of a fan changes with both inlet dampers and VFDs. You can get better advantage with VFDs if the resistance is purely (or significantly) dynamic.
However, high stack applications require minimum static head for effective purging of the combustion system. If your flow reduction incidentally matches with the reduction in static head then you will not have any problems. Otherwise, you may have to run your fan at higher flowrates (than required) to match system resistance.
This Fans link gives you some idea about fan operation with various controls.
