DX Coil Face Velocity? 1

[MNMaint]

I've got a dual compressor, dual circuit DX rooftop unit with equally sized compressors. Its currently a constant volume unit but I would like to convert it to a VAV unit. The manufacturer says that because the two refrigerant coils are intertwined, they each have the full face area of the two coils combined so I can only go 20% below the unit's nominal CFM. He said the face velocity across the coil is the decifding factor.

Here's my question - If only one compressor is running, why does the face velocity of the air across the coil have to be nearly the same as when both are operational? Seems to me 1/2 the load (one compressor) should be able to handle half the airflow.

Thanks!

 

[willard3]

Well, do what you want and, when the coil freezes up from low airflow, call the supplier back and apologize.

 

[MNMaint]

I was hoping to get some insight on why face velocity was an issuein a refrigernat coil to learn something. I , personally, like to know the science behind what's happening that affects equipment I'd be responsible for.

Anyone else out there have something constuctive or educational to add?

 

[MintJulep]

Well, face velocity really isn't what's important. It's a simple - if poor proxy for what is important, which is the load.

I think your manufacturer is incorrect, or there is confusion about the type of split.

http://www.trane.com/commercial/library/vol17_1/enews17_1.pdf

ASHRAE Handbook Equipment volume has all the info necessary for the interested student to work it out.

 

[msnyder95]

I agree with mintjulep. You should be able to go down much more than 20% with an intertwined coil.

 

[317069]

Why do you want to convert it to VAV unit?

 

[coolbydesign]

The face velocity of a coil is an indication of the air volume in the system.

The face velocity is a factor that determines the loading on the coil. Depending on the construction of the coil, ie rows, fin count, fin type and construction, refrigerant type, metering device, distributor style, there are maximum and minimum face velocities for the coil to perform within specification. The manufacturer should have minimum and maximum coil velocities published. The design velocity for the coil depends on application.

For a given coil,if the face velocity exceeds a threshold, (typical 500 to 600 fpm) there is the risk of water carryover off the coil. This can cause water issues in the duct or plenum.

If the face velocity drops below some threshold, (typ 225 fpm) the air through the coil may not be evenly distributed. On a water coil, this is generally not an issue. However on a DX coil (TX valve) this can cause some of the circuits to be underloaded.

As the refrigerant distributor will feed all circuits with the same mass flow of refrigerant (perfect world), If there is insufficient heat to boil all the refrigerant in one circuit it will reduce the superheat at the TX bulb. The TX valve will then underfeed the loaded portion of the coil to compensate.

It starts to get complicated after this. The underloaded circuit should have the same mass flow as the rest of the circuits, however it gets less heat. The lower heat load allows the liquid to travel further in the tube. Because the liquid has less volume than the gas, the velocity is lower. Ie there is more liquid and liquid is denser than gas.
With a lower velocity, the pressure drop in the underloaded circuit is less. (by the square of the velocity).
Now, the normally reliable distributor will feed more refrigerant to the underloaded circuit to compensate. The circuits of the coil with more heat will starve, and the underloaded circuits will overfeed.
The coil manufacturer determines this by measuring the outlet temperature of all the refrigerant circuits at the header. The velocity of the air is lowered until there is a mal-distribution of refrigerant in the circuits, as measured by differential temperature of the circuit outlets.

An excellent publication on distribution of refrigerants can be found at.

http://www.sporlanonline.com/20-10.htm

Simplified, if you reduce the air volume to a DX coil, below the manufacturers recommended threshold, bad things happen.