VAV AC system

[Lc_shi]

Hi
VAV AC system is introducing into our market. I know little about it. Who can offer some background knowledge related?

rgds
LC

[botrous]

Hi Luchian . . .
VAV (Variable Air Volume ) systems are automated dumpers that controls the flow of the air to the load (space to cool) the thermostat of the space commands the VAV , as soon as the tempreture rises , the VAV opens gradually so that the room tempreture become equal to the desired tempreture , it consists of a cylinder or box with a disc in it and a motor , the motor take the signal from the thermostat and acts according to it .
When the VAV closes , that means less load ==> the pressure of the air in the ducts will rise and the thermostat of the return air will send a signal that the air tempreture is in the good range, that will be compensated by a VSD (and a dumper )that controls the fan speed at the AHU , so as many as VAVs closes the fan speed of the AHU will react to it to equilbre the air pressure in the system , that of course results in less load to the coil of the AHU and the cycle continues till it arrives to the compressor that can be controlled by a VSD and all the systems acts in harmony for more confort and more effiency.

That's a breif

Here are some links :
http://www.cse-distributors.co.uk/ab...an_control.htm
http://www.trane.com/Commercial/Equi...ine.aspx?pl=37

Hope this helps ....
I will see if i have any other information and will send it to you.

Regards

[botrous]

Some additional links
http://www.thermalproductscorp.com/A...FlowSystem.pdf
http://www.thermalproductscorp.com/A...ion_PItoSP.pdf

[Greg W]

VAVs can have water or electrical reheat coil/elements
to add heat to conditioned zones. VAVs without reheats rely on hot deck to add heat, Ive worked on lots of vav systems and Dont think much of them, Generaly they are over controled and time consumming. The best system is cold deck with zone reheats. Add a VSD if you want to get perticular.

[US Iceman]

Hi LC,

Another item to consider for VAV systems is not to use direct expansion (DX) coils for the cooling requirements.

I have seen a lot of these with compressor problems due to the frequent system changes that can occur during operation. TXV's do not respond well to frequent, or large changes in capacity.

A VAV system does work well on chilled water systems, but the chiller should be capable of low load operation.

In simple terms, a VAV system is similar to an automobile heating and cooling system. You can vary the fan speed to increase the heating or cooling effect, or, you can change the amount of heating or cooling (the temperature of the temperature).

Most VAV systems respond to changes in room temperature by varying the volume of air provided to the space.

Regards,
US Iceman

[Greg W]

I hear you , Chilled water is the only way with VAV.
hey ICE many jobs for A/C dudes over they

[US Iceman]

Hi Greg,

On this side of the pond, it is the same as everywhere else: it is hard to find a good man (or women too).

A good tech or engineer is always in demand.

Another fringe benefit is that the US women all love the Aussie accent.

US Iceman

[frank]

Another point to note is that the perimeter heating circuit installed to add the additional heat can lead to really low RH% when the VAV valves start to shut, due to the release of residual heat.
Most occupants complain of dry eyes or noses.

[botrous]

Another point to note is that the perimeter heating circuit installed to add the additional heat can lead to really low RH% when the VAV valves start to shut, due to the release of residual heat.
Most occupants complain of dry eyes or noses.

Any system has it's own probems

[Lc_shi]

Thanks for all your input. I got much from these.
dry eyes or noses? is there any humidifier with VAV ?

rgds
LC

[techguy]

Hi Ic shi

Most Vav systems I've worked on have Large Ahu's with VCD's on the fan inlets or Vsd's on the fan motors which operate to maintain constant static pressure in the supply ductwork.Which varies as the dampers in the vav units modulate depending on the load demand in the space As Frank pointed out Humidification is a major problem and large humidifiers have to be installed Cooling and heating is supplied by Chilled water and LPHW from boilers. The controls are always over engineered and never maintained due to the cost of this ,the humidifiers need constant maintenance because they are constantly operating The operating and maintenance costs of these systems are huge, any facilities manager I have ever worked with that has a VAV site complained constantly about the overheads. From a service engineers point of view though you'll never be out of work with VAV systems.Give me a VRF system any day.

Regards
T

[wambat]

Ic-Shi for a detailed read on VAV Systems go here
http://www.energy.ca.gov/reports/200...3-082_A-11.PDF

[frank]

Thanks for all your input. I got much from these.
dry eyes or noses? is there any humidifier with VAV ?

rgds
LC

Yes there is but these are usually down at the central plant so individual rooms do not gain