VAV system max-min air flow
VAV system max-min air flow
(OP)
A typical VAV increases air flow to Max. as the cooling load increases. As the cooling load decreases the box closes off. Then as the temperature drops below set point the reheaters on the box come on at the minimum flow rate.
I have a VAV box at a perimetre office with a hugh heat loss. At the min. air flow the supply air temp needs to be 40 deg C , this is too high. Is it ok to switch the VAV box to max. air again during heating and modulate the control valve, so basically during heating its constant volume and variable temperature????
What do you reckon..please help
I have a VAV box at a perimetre office with a hugh heat loss. At the min. air flow the supply air temp needs to be 40 deg C , this is too high. Is it ok to switch the VAV box to max. air again during heating and modulate the control valve, so basically during heating its constant volume and variable temperature????
What do you reckon..please help





RE: VAV system max-min air flow
RE: VAV system max-min air flow
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: VAV system max-min air flow
High air velocities increase latent cooling, a thing you want to avoid with heating in winter.
RE: VAV system max-min air flow
RE: VAV system max-min air flow
One question, why do you view 40C (104F) as too high? I've read guidelines that state that air over 35C (95F) will promote stratification, and it's a good number to aim for, but a higher induction diffuser and the location of your return grille can combat statification. I've seen air at temperatures of 44C (110F) and higher delivered into spaces.
RE: VAV system max-min air flow
Looks like the zone you have is a strong caididate for fixing a Parallel Fan Powered VAV box.When the VAV box turns down to its minimum position,the fan comes on as the stage 1 reheat.It draws the warm air from the ceiling plenum and pumps it back to the space.If that is insufficient,the terminal reheater comes on as stage 2 reheat.
RE: VAV system max-min air flow
Another dilemma about VAV control is that diffusers are sized for certain air throw at certain flows. Obviously typical diffusers don't change between modes, so throw characteristics tend to be compromised when the diffusers are sized for max cfm and the boxes go to min flow. This can tend to stratify warm air around the ceiling of a room. Having a heating max value resolves that...
That directly answers your question, but the other suggestions above (heater ratings, radiation heating, and windows) definitely apply.
-CB
RE: VAV system max-min air flow
http://en
which recommends Dual Maximum settings( one cooling max and one heating max) then there is a third setting for the minimum flow to comply with local codes.
There is an illustration of CB's point.
RE: VAV system max-min air flow
This leaves me with a slight control issue. I am thinking of ramping the VAV box down to the minimum setting for the first stage of heating. The second stage of heating will be to bring on the perimeter heaters- base board panel electric heaters. The glass fascade is over 20m long in an open plan office and I was thinking of controlling the little baseboard heaters off a thyristor controller (SCR) so as go ramp thier capacity up/down to match the load. On/off control would be pretty crude. Any comments? Would the manufacturers of baseboard heaters have a problem with this sort of control??
RE: VAV system max-min air flow
Why not follow the same approach, if you had electric reheat coils on the VAV boxes, just have them temper it to room temperature with an SCR discharge air controller and then the electric baseboards go "on/off" as needed?
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: VAV system max-min air flow