Positive pressurisation

Contamination may arise from use of solvents within the laboratory or from an adjacent laboratory with a shared ventilation system and can lead to airborne contamination of sample vials and other equipment. Particular care should be taken in the area where samples are handled and transferred and during the preparation of concentrated standard solutions. Many laboratories, particularly where large volumes of solvents are regularly used, find it necessary to have special room, often positively pressurised, for the preparation and analysis of samples for VOCs. Recently decorated rooms can also be a source of VOCs from surface coatings. Contamination can often be intermittent with the wind direction being the controlling factor. 

Installation of a radon sump system equipped with a fan is suggested as the most effective and best choice for high levels of radon existance [29]. For this, a sump which is a small empty space about the volume of a bucket is dug imder the solid floor and a pipe is routed from it to the outside air. The sump and the fan connected at the exit of the pipe to suck air, both help to alter the air pressure below the floor and to pass and to release it harmlessly into the atmosphere. There are also applications where the fan is replaced by a blowing system to facilitate the removal of the remaining of radon in the soil. It is also possible to increase the circulation of air beneath the floor (improved ventilation under suspended timber floors with or without a fan via an air-brick) or at loft level or even by using positive house ventilation as a whole ( positive pressurisation is most effective if house is very air-tight). 

Positive pressurisation below floor slabs (a particular form of active venting)... 

Positive pressurisation is a particular form of active system that works by pumping air into the void or void former below a floor slab to create a zone of... 

Underfloor natural (passive) venting wherever possible (or car park). Where passive ventilation is not feasible or cost effective use active venting or positive pressurisation... 

Subfloor ventilation with active abstraction/ pressurisation (venting layer can be a clear void or formed using gravel, geocomposites, polystyrene void formers etc.) 2.5 There have to be robust management systems in place to ensure the continued maintenance of any ventilation system Active ventilation can always be designed to meet good performance Mechanically assisted systems come in two main forms extraction and positive pressurisation... 

No single method can provide two types of protection are xmtrue. For example a positive pressurisation system does not provide two levels of protection (a barrier and a ventilation system). If it stops working a second level of protection, such as a membrane, is required to protect a building... 

Detailed analysis of venting and positive pressurisation systems... 

Assume positive pressurisation is required to resist an in groxmd gas pressure of 10 Pa with a factor of safety of 2. So design gas pressure = 20 Pa. Say pressure at diffuser is 350 kPa, so pressure drop over 10 m to edge of building is 330 kPa. 

All types of floor slab as above. Proprietary gas resistant membrane and passively ventilated or positively pressurised underfloor sub-space. 

Reinforced concrete cast in s/tu floor slab (suspended, non suspended or raft). Proprietary gas resistant membrane and ventilated or positively pressurised underfloor sub-space, oversite capping and in ground venting layer and in ground venting wells or barriers,... 

Either the escape stairway/s and corridors shouid be protected by a positive pressurisation system conforming to BS 5588-4 for smoke controi or... 

Packs filled at high altitudes then transported to lower altitudes (positive pressure). Can apply to both non-pressurised and pressurised aircraft. 

This test can only be applied to flexible packs, as the pack is put under positive pressure by some form of weight/force. A full pack is employed and loss detected by actual leakage. Also, a filled container can be stored on its side for 24 h at 38° C (after filling at a lower controlled temperature) (see CAN/CSA—276. 1-M 90 (Canada) using a three-quarters full container). A positive air pressure test (using a full pack in a reinforced chamber capable of being pressurised) can also be employed. 

In actual practice a pack may suffer from positive and negative pressures due to changes in environmental air pressures. This may occur due to either transportation via air in pressurised and unpressurised aircraft or changes in air pressure by ground height. 

Placing packs under a positive pressure external to the pack can provide a useful indication of possible ingress. The positive pressure atmosphere may use air, water under pressure, or a concentration of any gas (or liquid) which can be pressurised (see below for further details). For tests involving combined pressure and vacuum, comments apply as for egress. 

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