VAV Systems

[Pooh]

Hi All
I have had an enquiry reference VAV systems for office air conditioning, initially I thought it was a typing error and should have read VRV but apparently not. Can anybody give me an idea of the operating principles of this type of system.
I am presuming it is a mixed air system but beyond that I dont know.

Ian

[Argus]

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It can be mixed air if you need to integrate ventilation.

VAV should be VAVCT
It stands for Variable Air Volume Constant Temperature to get the full description.

It differs from a normal central-station air distribution system that uses constant air volume through fixed terminal units. In these installations, control is achieved by varying the supply air temperature. In some of these big installations, success was dependent on high-cost ductwork and good design, often using the static regain method. Sadly, skills that are in short supply nowadays.

Basically a VAV system uses purpose-designed terminal units and an adapted AHU that has a form of damper control in the inlet of the fan responding to the down-stream static pressure and volumetric flow. For this you needed a control system and a controls designer who knew what he was about.

The chiller delivers water to a coil in an AHU in the normal way. The supply air is delivered to the supply ductwork at a fixed temperature throughout the system – usually at about 12 degrees, though this can vary a little with the design.

The terminal units are able to vary the air volume according to the room temperature either through a set of controls that are system powered by the static pressure in the supply air, by pneumatic controls or by electric controls. You take your pick.
Room temperature goes up – dampers open, room temperature goes down, dampers close off.

So, in short, you achieve control by varying the air volume, not the air temperature.

It was a very big seller in the 70’s and 80’. Most large air side installations, offices etc. were VAV. This was before VRF was thought of, but a bit old-hat nowadays, unless you have a requirement for moving very large quantities of air on an intermittent basis.

The advantages are that it keeps the static pressure in the supply duct high, thereby economising on fan absorbed energy. At least, that’s the theory. This has now been largely superseded by VSD drives on the fans. Predictable load management can be transferred to the chiller.

Trox, Waterloo and others made VAV type systems, though the biggest seller was the Carrier 37 ‘Moduline’ series.


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[frank]

VAV systems also used to be accompanied by perimeter heating circuits, i.e. LPHW pipes in a trench around the perimeter of the building to offset heatloss through the external walls/windows during the heating season.

Most, if not all, VAV systems suffered from humidity control problems.

[Argus]

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Yes, true. One of the old Carrier models had a twin duct arrangement for perimeter heating. (Moduline 37 D, I think). It removed the need for all the unsightly run-round coils and kept it all in the ceiling.

But they never controlled humidity. The prevailing RH off the coil was what you got.
They could result in some drastic electrical static when used with nylon carpets and the like, if that's what you mean.



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[Pooh]

Argus
is there anybody designing and installing them now as the enquiry was from a company who needs one designing for a contract overseas I think. They rang me to pick my brains but as I have said I have not had anything to do with them.

Ian
PS It might be a nice little earner for somebody.

[Abby Normal]

Now a days, there are boxes with dampers in them that supply air to the various zone.

A zone thermostat signals more or less air to a zone to respond for more or less cooling.

Usually there is a static presure control system in place. As more and more boxes start to open up and 'demand more air', a static pressure sensor notes this and a controller will signal a VFD to ramp up the blower motor speed.

As boxes start to close off and 'demanding less air' the duct pressure would begin to build up, however the same static pressure control system would end up ramping the blower motor speed down.

Chilled water coil tires to respond by maintain a certain discharge air temperature leaving the coil.

DX cooling can be unloaded and staged to an extent as well.

[Argus]

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Ian, the world has moved on a bit since I worked on them, probably the thick end of 30 years ago.

In those days it was an early answer to curbing high fan power demands in big offices etc. nowadays the problem is more concentrated in carbon reduction - same thing, just a generation later.

So the answer is , I don't know who's putting them in now in the UK - if at all.

The big VAV contracts that used to exist are now concentrating on other technologies.

But.... I can't speak for other countries....


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[TRASH101]

Hi Ian

there are off the shelf terminal boxes available for VAV application. They just use a flow grid / temp sensor to drive a programmable damper motor. You can also get them as a dual duct i.e. heat and cool.

The central plant is pretty straight forward depending on size. You might need a bypass if the fan runs to slow (dependant on motor/ invertor selection) not really suitable for DX as you need to be able to have easy control of modulation on the cooling.

[John MacK]

A company I worked for installed them in the early '90's...

Powermaster Cooling

They might be worth contacting....

[grump]

Hi pooh,Argus has provided you with some good technical information.Here is my contribution
Watford 1976 VRV 350 terminal units, 1 no fan supplying 100% fresh air to said terminals at fixed Temperature pressure,volume,velocity,(never did get my head round the technicalities of getting air from A to B) 1 large control panel, 2TraneChillers Two gas fired boilers 2 heating pump 2 chilled water pumps 2 condensing water pumps 1 mixing valve in condensing water pipe work (150mm) 1 cooling tower. CW + HW Pipework to 350 terminals damper type control ,Frost protection etc etc
Installed approx 45 years ago ,required constant tlc from
yours truly to provide comfort conditions within the conditioned space.
Now why would any one want to go back there? I would be interested to know why your customer is so interested in antique type HVAC.
VAV works by passing the fixed temperature,pressure,volume.velocity air across a series of small orifices ,air is then pulled from the space by natural draught, heated ,cooled, mixed with main air supply the air flow is then controlled by dampers VAV which in turn control to the set point on each terminal.
All you over 50 please correct the above if you have the necessary experience as i am not at all sure that my memory can be relied on.
Grump
Forgot to mention ductwork to all 350 terminals

[ozairman]

kreuter, moduline, staefa, johnson all bring back hazy memories of my post apprenticeship days about 20 years ago and commissioning VAV boxes, retrofitting electronics to pneumatic boxes etc etc. In Australia they were all the rage and some geniuses even used to try and hook a few boxes to a DX single stage packaged air conditioner in small office buildings and try and ride the fan curves when the boxes closed down
Multi storey buildings were great doing air balances, just work out how to set the dampers on the first typical floors and bobs your fathers brother for the rest of the typical floors and more time for looking at the young office girls walking down the street.
Then after that countless service calls throughout winter to reterminate the fuses supplying the reheat banks as the electricians went marginal on the cable sizing
I still see them going into multi storey office buildings in Aus so they are not that old school.

[Abby Normal]

VAV is still fairly common on this side of the pond, its a little more advanced then moduline diffusers now a days.

Sometimes you may chose to reset the supply air temperature. Modulines and old VAV systems without VFDs wasted a lot of fan energy either bypassing a constant volume or using inlet vanes/dampers to try and choke the flow down.

Worst thing with Moduline was some hack would start cutting in take offs for regular diffusers during rennovations and kill the duct static that the whole control system ran off of.

VRV still seems to need a duct system in place for fresh ventilation air. VRV with proper ventilation seems to cost about what a chilled water system would on this side of the pond.

Never have problems freezing DX coils with them here, but then again we have some staged cooling, and there are always plenty of heat in ventilation air.

VAV could also allow some free cooling in some climates. Maybe the ventilation system used with VRV will do this also to some extent, stop rotating an ERV wheel perhaps?

Still have a bad taste from Daikin pulling the plug on the west in the years back.

Only see VRV as a retrofit where space is tight, too expensive otherwise for what you get.