airflow

[bangoman]

hie all.Still on commissioning .Im doing airflows for a new install heat pumps ducted systems.Im using a 4 " vane anemometer to get the velocities that correspond to the airflow needed at the supply registers.My formular is as below ,
airflow =velocity * duct surface area
velocity=airflow/duct surface area

I will then adjust my blower speed to give me the velocity that corresponds to my desired airflow .

Am I on the right track?

[Gary]

The higher the airflow, the higher the coil temperature, the higher the efficiency.

On the other hand, the coil temperature must be low enough to achieve the desired humidity.

Generally speaking, the coil leaving air temperature must be 20F/11K (or more) below the temperature at the thermostat.

This can be demonstrated on a psychrometric chart:

Find the desired room temperature along the bottom. draw a vertical line to where it intersects with the desired humidity. From there, draw a horizontal line to where it intersects with the 100% humidity line, then down vertically to find the needed leaving air/coil temperature.

[Brian_UK]

airflow =velocity * duct surface area
velocity=airflow/duct surface area

Airflow = velocity * duct free area

If you are taking readings at the supply terminals be aware of the jetting effect caused by the terminal vanes or slots.

Taking readings at the extract or air inlet terminals can sometimes be more accurate.

[frank]

Airflow = Volume (m3/s, cuft/min etc)

Volume = Velocity x Area

Velocity = Volume/Area

Area = Volume/Velocity

Know any 2 you can find the 3rd.

[bangoman]

Brian ! by duct free area what do u mean ? wher do I find a duct free area that I can measure velocity

[TRASH101]

Brian ! by duct free area what do u mean ? wher do I find a duct free area that I can measure velocity

He means the cross sectional area of the duct.

How accurate do you want your results because measuring the velocity at the grill/louver with just an anemometer is a bit "rough". You might try making a hood for the grill/louver but you should find a correction factor for that sort of thing.

If you want to have confidence in your results I suggest you hire or buy the tools for the job (i.e. a balometer)

[Brian_UK]

Brian ! by duct free area what do u mean ? wher do I find a duct free area that I can measure velocity
Say you have a (metric) duct sized at 300 x 200mm with an air terminal at the end with 5 x 5mm bars across it.

Duct area = 300 x 200 = 0.06 sq.m

Five 5mm x 300 bars = 0.0075 sq.m

Gross area = 0.06 sq.m
Free area = 0.06 - 0.0075 = 0.0525sq.m

If you measured a face velocity at the grille/terminal of 2m/s and used the duct gross area then t you would say that you had an air volume of 2 x 0.06 = 0.12 cu.m/s.

In fact you should use the free area so the calculation goes 2m/s x 0.0525sq.m = 0.105cu.m/s and shortfall of 13%.

[bangoman]

Brian and All thank u so much for the posts .Im becoming a better engineer evrytime .Thanxs again

[bangoman]

Hi all.We have a capture hood at work though its for bigger registers,is it ok if i fabricate an extension and use it on smaller registers .

[Brian_UK]

Yes, but be careful that you do not over-throttle the system.

Too much resistance of the hood will give you false readings.

If it is for only the odd job then you can make them from cardboard quite easily.

[bangoman]

brian ,these are side wall registers nt ceiling ones .does it really matter.

[Brian_UK]

Yes, it matters. Airflow is airflow, it doesn't matter where it's coming from ;)

[bangoman]

thanxs brian just that my boss a work told me that the hood was only designed for the ceiling registers bt ddnt understand wat he meant by that

[Brian_UK]

Probably meant that you could reach the ceiling with the hood without needing ladders ;)

The proper way to make use of a hood is to compare the anemometer measured airflow rate with an accurate duct traverse measured airflow rate.

This will give you a correction factor for the anemometer/hood and provide greater accuracy.