Room Pressure Control

[SAK9]

Guys,
I have been told by a controls engineer that active pressure control is not the right solution for small rooms.according to his experience,small rooms experience large swings in pressure when active pressure control is used.More stable pressure control can be achieved for small rooms by using volumetric offset,according to him.Could any one please confirm if this is correct and if yes why?

 

[wilg]

Volumetric offset maintains a constant difference
(offset) between supply and exhaust airflow rates.
This constant volumetric difference will result
in positive or negative pressures in spaces in
spite of changing duct pressures.

Volumetric offset is sometimes combined with pressure
override which prevents the terminal devices from
going out of control during long periods of room
imbalance (keeping doors open).

Pressure offset is used to maintain space pressures
relative to a corridor (or reference space) with
changing duct pressures.

With Volumetric offset the volume of individual
terminal supply units can be summed and flow rates
can be adjusted to maintain desired constant
space velocities.

The controller accepts a series of input analog signals
and performs the required calculations and outputs a
series of analog signals to maintain environment control
of the local space. The calculations are prepared using a
proprietary logic based program with appropriate
PID control loops. This results in a stable and rapid
control signal to all terminal devices.

Volumetric offset usually has field adjustable
set points and the number of inputs and outputs
are selectable to the application.

 

[ChasBean1]

Facts nicely presented by Wilg. Personal preference? I side with your controls guy. Cfm offset is better. It will always maintain the right flow direction; accept the fact that magnitude will vary when people enter/exit the room. Do you really want to maintain a room 0.05" (as an example) when someone opens the door? You'd need about 12,000 cfm differential to maintain that pressure across an open door. The pressure magnitude can be established as desired via sealing the room (door sweeps, gaskets) AFTER establishing flow offset.

More importantly, establishing pressure control could kill needed air exchange rates and could vary the required cfm over the life of the space (e.g., by someone banging up the door or by a gasket peeling off). If pressure control were a building-wide practice, how do you size your AHUs/exhaust fans?

One vote here for the control guy. Good luck, CB

 

[lilliput1]

I have 32 years experience in Engr consulting firm & we have always used CFM differential instead of direct pressure control for reasons as stated by Wilg & ChasBean1. In addition note that direct pressure control require a common pressure reference but in the real world there will be no ideal common reference that has a fixed pressurization value. It changes & there would be pressure drop associated with the pneumatic tubing piped to the reference resulting in response lag, hysterisis & inaccuracy. Use CFM differential controls with adequate control range between operating differential & alarm setpoints. Provide access door to allow cleaning of sensors. Avoid use of grid type straighteners if possible because they are dirt traps. If they can't be avoided, use filter at return/ehaust grilles. Use DDC controls & have the BAS printout room no., actual SA CFM, actual return CFM, actual Exhaust CFM, setpoint CFM differential, actual CFM differential. Reguire the BAS to flag an alarm if actual differential is below or above the desired operating range. This should be printed out on a format that could be used for daily or weekly check of the system.

 

[Accystan]

You might want to check out the products produced by a company called TSI Inc. at

http://www.tsi.com/hvac/lab/products/room/sureflow.htm

Their room pressure control product basically utilizes the offset principle but the offset is monitored/controlled by a “through the wall device”. The room to be controlled (negatively or positively) is connected by a small tube to a reference space and, most importantly, provides closed loop control. I believe there are other manufacturers with products based on the same principle but can’t recall their names.

 

[lilliput1]

You can use the TSI device as an alarm. However when we used it for hospital isolation room, we had to add a key switch to disable it (avoiding nuisance alarm)when the room is being cleaned with doors open & room is vacant.

 

[wilg]

lilliput1
Could that have been set up with a disreard
protocol at the workstation ?

 

[lilliput1]

Local is better in our case because the head nurse in the nurse's station at each ward can take care of it.

 

[wilg]

lilliput1
I suppose disregard protocols at workststations
can be a bad idea, I was field checking a
roof that had noisy dust collectors on it
and accidently ended up being trapped in
a steel shed because the door was slammed shut
by a wind gust which broke the door latch.
I pulled a disconnect on a large fan which sent an
alarm signal to a central control station
and a mechanic showed up 5 minutes later and it
took 2 men 15 minutes to get me out of there.
If they had that fan on disregard at the time,
I would not be writting this, no one knew
where I was at the time and it would have
taken maybe 10 or 20 days before anyone
would have needed to go into that shed.