negative pressure

A diagram would help here as this is somewhat confusing.

The pressure on the door is outside atmospheric pressure minus inside room pressure.

The force on the door would then be this difference times the area of the door. Mind your measurment units.

But if the sandtrap louver is clear, the differential pressure across the door should be zero. If you have a larged pressure drop in the duct and louver then it becomes a bit more complicated.

As above more details would help.

DEAR FRIEND,

KINDLY POST A DIAGRAM SO AS TO UNDERSTAND YOUR PROBLEM BETTER. BUT AS OF NOW PRESSURE CAN BE CALCULATED BY FORCE / UNIT AREA. HOW IS YOU SUCTION BASED ON WITH PIPE OR TUBE. PRESSURE CAN NEVER BE NEGATIVE BY THAT I MEAN TO SAY WHEN YOU CALULATE THE SIGN OF THE VALUE OF YOUR CALCULALTION WILL BE NEGATIVE OR THE FLOW OF DIRECTION WILL BE OPPOSITE.

ALSO FOR YOUR CALULATION YOU NEED FAN RPM ( REVOLUTION OER MIN), HORSE POWER IF DRIVEN WITH ENGINE, AND KW IF WITH MOTOR ( AC / DC ) CALULATION ARE DIFFERENT. BECAUSE YOU NEED TO FIRST CALULATE THE RATE AIR YOU ARE SUCKING. WE CALL IT VACCUM PRESSURE IN MARINE INDUSTRY.

HOPE I ANSWER YOUR QUESTION

SAMIP S KEWALRAMANI

+919925163385 INDIA

+6281511845495 INDONESIA/ SINGAPORE

The simplest procedure is to mount an inclined tube water manometer on the door. You can even make your own out of any transparent/translucent plastic tubing. Use coloured water in the tube, tape a mm scale along the inclined tube and read the negative pressure. Multiply the reading by sin(theta) to get the actual negative pressure in mm WG, where theta is the angle of inclination of the tube to the horizontal. http://www.engineeringtoolbox.com/u-tube-manometer-d_611.html

There is no such thing as negative pressure. There is only pressure, and less pressure. The lowest possible pressure is zero (vacuum). So in a system sucking in outside air, the highest possible differential would be 14.7 psig, which would be atmosphere on one side and total vacuum on the other...which you aren't going to get with a fan.

The actual differential pressure would depend on a variety of factors: resistance through the filter, temperature inside and outside, elevation, etc.

Negative pressure is a common term in HVAC applications. It is used in buildings to determine pressure zones and differentials compared to a baseline pressure. Very common in hospitals, labs, clean rooms, etc. where contamination is a critical metric.

Mechanical rooms will commonly experience negative pressure situations, which can be detrimental to the operation of fuel fired appliances. Refer to ASHRAE.

Duane Tilden, P.Eng

In a non-ducted configuration, the negative pressure in the room with a tightly sealed door and no cracks elsewhere in the room, will equal to the pressure drop across the louver.

First obtain the louver manufacturer's performance data sheet for the louver. Not all louvers are the same, and you will need to account for any accessories (such as BDD's, motorized dampers, birdscreens, etc) which will reduce "free area" and increase the pressure drop. Measure the height and width of the louvre which will give you the dimensional requirement on the manufacturer's chart.

Next you will need to determine the capacity of the AHU in cfm (or other equivalent volumetric rate) with the door closed. This value can either be measured or obtained from the AHU manufacturer's performance data (measuring being the most accurate).

With the measured volumetric air flow rate, determine the pressure drop of the louver from the louvers' performance curve. Based on the dimensional data of the louver (area) and the flow rate (assume the AHU flow rate equals the flow rate across the louver) the pressure drop obtained from your chart will provide you with the negative pressure at the door.

Cheers,

Duane Tilden, P.Eng

The air pressures measured include barometric pressure, static pressure, velocity pressure, total pressure and differential pressure. For evaluation of AHU performance each pressure should be measured as recommended in ASHRAE Standard 111 and analyzed together with manufactures fan curves and system effect as predicted in AMCA Standard 210.Use proper air balance calculation sheets to analyze it and record it.

Mir

I sincerely hope you have no gas or oil fired furnace / appliances in that room
with negative pressure
Negative pressure is just not good as any engineer will tell you
A positive building pressure (including the furnace room) is required

If you have gas / oil burning furnaces in that room you and the occupants are in danger
You have to have combustion air to flow through the furnace/boiler whatever

Negative pressures make the appliances back draft down the vent pipe and the
combustion gases go directly into the building and they are deadly

An outside air intake duct should be a completly sealed air tight from the air handler
to the outside wall intake and please follow all fire codes for required fire dampers
to stop outside air from feeding an internal building fire

John
Rorenet.com

After you've calculated your negative air pressure, I've discovered an inexpensive solution vs traditional make-up air systems:

http://www.cheap-makeupair.com/