What is “static pressure”?

We were running the BOHS module course P601 “Commissioning and Thorough Examination and Testing of Local Exhaust Ventilation Systems” last week. One of the concepts delegates often find difficult to get to grips with is “static pressure”, which is one of the main engineering measurements carried out during the testing of a local exhaust ventilation (LEV) system.

In essence, the static pressure in a point in the ventilation system is the atmospheric pressure inside the duct. As a fan increases the pressure inside the duct on the exhaust side, and as air moves from high to low pressure, air is expelled from the duct. This results in a reduction of pressure in the duct on the other side of the fan so that it is lower than atmospheric pressure, causing air to flow into the system (and, providing the hood is well designed, drawing the contaminants in with it).

Although it would be possible to measure the “atmospheric” pressure inside the duct, which would be the “true”, or absolute, static pressure, we don’t do that.  Typically the static pressures inside the system are only slightly lower than atmospheric pressure – in many cases the the difference between the inside and outside will be less than 1 KPa than an atmospheric pressure which at standard conditions is 101 kPa. Instead, we measure the difference between the absolute pressure at the point under consideration and the ambient atmospheric pressure outside.  This is relatively easy to achieve using a manometer either in conjunction with a pitot tube or by holding a tube at right angles to a hole in the duct, with the other end of the tube connected to a suitable manometer.

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Measuring static pressure in a duct using a pitot tube

This is really the differential static pressure and although we usually refer to the measurement as the “static pressure”, using this term is not strictly correct. However, this has become commonplace in ventilation testing. Use of the differential rather than absolute static pressure  has its advantages. The absolute pressure varies depending on ambient conditions, which change from day to day. Also the change in absolute pressure along a system is relatively small compared to atmospheric pressure and so can be difficult to quantify in practice (it is not easy to measure a small change and the change in the absolute pressure compared to atmospheric pressure is typically less than 1%).

The absolute static pressure before the fan (the suction side) is lower than atmospheric pressure, so the differential static pressure is negative. On the exhaust side of the fan, the absolute pressure is higher than atmospheric pressure, so the differential static pressure is positive.

Static pressure measurements are easy to make and can allow judgements to be made about the performance of the system and help to diagnose and  locate problems. We’ll come back to this in a future post.

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