fan start damper position 2

[weaver101]

When supply and return fans are starting, recircule, fresh air and exhaust dampers are open or closed, which is right?

 

[imok2]

A Typical Mixed Air Control


The Control system energizes when the fans are turned on

The MA temperature is maintained by modulating mixing dampers.

OA and EA dampers close and RA damper opens when fan is off, and modulate for MA temperature Setpoint control when fan is on.

There is a damper control and setpoint for minimum ventilation damper position when MA temperature is satisfied

NOTE: This is not 22% OA or OA damper open 22%, it is the control program output value necessary to position the OA damper such that the design OA cfm is maintained to provide a healthy CO2 level in the building


Features

1. The proper proportions of OA and RA are admitted to prevent the MA temperature from dropping below the desired MA temperature.
2. A minimum quantity of OA, determined by the setting of the minimum position switch is assured at all times.

 

[weaver101]

Thanks for quick responce.

But, I mean only starting position, not normal operation.
When supply and return fans begin to start, high current occurs. to prevent the fan motors, some consultants advice closed damper starting, others advice open damper starting. Please, declare your opinion.

Thanks.

 

[macmet]

I have occasionally closed dampers on start-up when the fan is sized at a high operating temperature and on start-up the temperature is lower.

I believe this is because of the more dense cold air causing motors to trip on the higher load.

 

[mauricestoker]

Or it might have been tripping out on freezstat so the coils don't bust.

 

[imok2]

Weaver101 I thaught that's what I said that the OA and EA dampers are closed at start up, However they will ramp up to maintain MA temp which should be about 60*F. The timing on ramp up can/should able to be controlled.

 

[mauricestoker]

IMOK,

Sounds like exactly what you said, but stated a different way. OA damper won't open up after freezstat until after warm-up. Just a lot of variations by AHU and application. If a relative negative environment, you reverse the proof of flow (dP and CT) from supply side to exhaust side. If the heating or cooling coil is included, you modulate the valve to meet MA discharge. If you have separate minimum OA and economizer, well, ad nauseum.

First step I usually look at is after control circuit is energized, motors are locked out until smoke control dampers meet end switch. I like having proof of flow as well as end switch, but different designers do different things.

No one-size fits all answer, but I've never encountered change in air density being a factor on start up draw.

 

[imok2]

mauricestoker
You are quite right that's why I said " A Typical Mixed Air Control" because there are many varations so I didn't want to get too technical but tried to focus on the mixing dampers. All we can do is try as best we can to help :>)

 

[MintJulep]

Fan power is proportional to the mass of the airflow cubed.

Closed dampers = less airflow = lower motor current at start up.

For across-the-line starts at any rate. Less important if the fan is driven via VFD and has soft-start.

 

[weaver101]

Thanks you all...