Airflow Resistance

From the energy and noise point of view, care must be taken to prevent undue airflow resistance. This is achieved by not normally having more than five tube rows. 

For small-scale laboratory work, the exhaust surface is often made as a separate section added to the side of a table or put into a large hole in a table. These tables usually have a sheet metal surface that is resistant to the chemicals used and is easily cleaned. Many circular holes are cut into the metal surface to allow for airflow. This perforation makes the pressure difference over the table quite high and at the same time gives an even distribution of the airflow over the entire surface. These types of exhaust surfaces could be formed to suit different working conditions, e.g., the surface could be made to fit into a sink or to be placed below and around a balance. Using side walls that are not too high, on three or four sides, transforms the table to a partial enclosure, which increases... 

A maximum resistance, when clean, of 25 mm WG when operated at rated airflow capacity... 

For HEPA filters, the major design paranieter is the system airflow, which can be used to establish the size and number of HEPA filters required. The penetration, resistance, and airflow are determined by the manufacturer before the filter is shipped from the factory." Table 13.7 gives the standard sizes for HEPA filters. 

Design airflow capacity at clean filter resistance ... 

Rotary viscous A continuously rotating oil-coated roll of material. The filter resistance is kept low by a motor drive that ensures a clean surface is always presented to the airflow. 

Air filter resistance, final The maximum pressure drop allowed across a filter to ensure that the design airflow rate is achieved. 

Airflow rate controller Any mechanical, pneumatic, or electrical device that controls the air flow rare into Or out of a space by increasing or decreasing the flow resistance. See Dampers and Valve. 

Fan energy The energy required by a fan in order for it to provide a given airflow rate against a set resistance. 

Rapping A method used on bag or other filters to dislodge the dust cake from the fabric when the airflow resistance exceeds a certain value. 

Very small particles can be collected but large ones (above 5 pm) can cause arcing across the narrow gap between collector plates. A coarse filter should first remove these large particles. Precipitators are not normally suitable for wet environments since excessive moisture can cause electrical tracking and failure. Resistance to airflow is negligible. 

The resistance to airflow which causes static pressure drop. It is similar to electrical resistance and voltage drop... 

Lowe, H. J. and Christie, D. G. Inst. Mech. Eng. Symposium on Heat Transfer (1962). Paper 113, 933. Heat transfer and pressure drop data on cooling tower packings, and model studies of the resistance of natural-draught towers to airflow. 

Figure 17.3 Airway obstruction, (a) Normal, patent airway with radial traction offered to it by surrounding airways. Resistance in this airway is low and air flows through it freely, (b) The airway is obstructed by the presence of excess mucus and airway resistance is increased. Airflow is reduced, (c) Thickening of the airway wall due to inflammation or edema narrows the lumen of the airway. The decrease in airway radius increases airway resistance and decreases airflow, (d) Destruction of surrounding airways results in the loss of interdependence, or radial traction. Without the structural support offered by surrounding airways, the central airway collapses and airflow through it is reduced.

Airflow through the airways is proportional to the gradient between atmospheric pressure and alveolar pressure (AP) and inversely proportional to the airway resistance (R). 

Factors determining resistance to airflow are also analogous to those determining the resistance to blood flow and include viscosity, length of the airway, and airway radius. Under normal conditions, the viscosity of the air is fairly constant and the length of the airway is fixed. Therefore, airway radius is the critically important physiological factor determining airway resistance ... 

Asthma is characterized by variable symptoms such as wheeze, shortness of breath and coughing and is usually associated with airway inflammation, with variably reduced spirometric indices [4, 5], with increased non-specific airway responsiveness (AR) to spasmogens [6, 7] and increased levels of semm immunoglobulin E (IgE) and eosinophils [8-10]. The symptoms of asthma are primarily due to excessive airway narrowing, which leads to an increased resistance to airflow, especially during forced expiration, and produces characteristic spirometric findings. A cardinal feature of asthma is that airway narrowing is reversible either spontaneously or as the result of therapy. 

The underpressure created in the respiratory tract is the driving force for the airflow through an inhalation device. The attainable underpressure and the rate of the airflow both depend on the total resistance in the airways and inhaler. The pressure drop achieved during inhalation is furthermore a function of the anatomy of the lungs, the effort made by the patient, pathological factors and the presence of exacerbations (e.g. in case of asthma). 

A large proportion of the airflow resistance in the airways (internal resistance is of-... 

For nebulizers and MDIs, the external resistance (Re) is quite low. Different approaches have been made to describe the external airflow resistance of DPIs. Olsson and Asking [99] derived an empirical relationship between flow rate () and pressure drop (AR), AR = for a number of inhalers (such as Rotahaler , Spinhaler and Turbuhaler ) in which they define the proportionality coefficient (C) as the airflow resistance. This relationship differs only slightly from the general (theoretical) equation for orifice types of flow constrictions ... 

Table 3.7. Airflow resistances of some marketed dry powder inhalers in kPa° min 1...

Several authors measured attainable pressure drops as a function of the external resistance for different groups of volunteers. Healthy male subjects (during maximal inspiration) are able to create a pressure drop (on average) of 6.7 kPa through an airflow resistance of 0.038 kPa° min 1, (which is in the range of that of marketed DPIs, see Table 3.7), whereas females are able to create a pressure drop of 3.8 kPa under the same resistance [22]. Differences between different groups of patients depend on the degree to which pulmonary func-... 

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