Air supply unit

Air supply units designed for the introduction of air with a high impulse, e.g., an anemostat, are nor suitable for air shower systems. Such units mat cause a vertical up flow under them and thereby an effect opposite the intention of the air shower. 

Manufacturers of thermal anemometers provide small rigs for their calibration. They typically consist of a nozzle, an air supply unit, and a regulating valve. The probe is placed into the nozzle jet. The reference velocity is calculated from the nozzle upstream pressure and nozzle characteristics. Due to its small size, this type of rig can be used only for hot-wire or other thermal anemometers. ... 

Clean air is introduced into the FLEC with (PTFE) tubing from an air supply unit as described by, e.g., Wolkoff et al. (1995). The air flow into the cell is controlled by needle valves. The outlet is a union cross with an overflow tube to avoid false air intake during... 

Supplied air escape units — Supplied air units, such as emergency squads might use, are expensive and require a significant level of training to be able to put them on quickly and use them properly. However, small air-supplied units are available at very reasonable prices which only need to be pulled over one s head and activated to provide 5 minutes of air. This is usually sufficient time in which to escape the immediate area of an accident. 

Dry compressed air is used for smaller units like those used for swimming pools. Air supplied units will produce a lower ozone concentration. 

If oxygen content is below 19.5%, entry is prohibited except for essential emergency repairs using an air supply unit entry is prohibited if oxygen content exceeds 23.5%. 

Air Supply. Oxygen in excess of stoichiometric requirements for complete combustion is needed because incineration processes are not 100% efficient and excess air is needed to absorb a portion of the combustion heat to control the operating temperature. In general, units that have higher degrees of turbulence such as Hquid injection incinerators require less excess air (20 to 60%) while units with less mixing such as hearth incinerators require... 

Batch Process. In the batch process (Fig. 5), the feedstock is preheated in a tube furnace or heater placed between the feedstock storage and the blowing vessel. The air supply is provided by a variety of blowers or compressors and a vertical-tower vessel is preferable for air-blowing. Knockout dmms, water scmbbers, incinerators, furnaces, and catalytic burning units have been used for fume disposal (32). Steam is used for safety and to ensure positive fume flow to the incinerator. 

Blowing hot air A typical arrangement is shown in Figure 31.9. The bus duct enclosure is provided with inlet and outlet valves at suitable locations. Hot air is supplied through the inlet valve until the interior of the enclosure is completely dry. The blowing equipment comprises an axial flow fan. a heater unit, an inlet air filter unit, pressure... 

The noise from the fan is propagated in the duct to the openings inwards or outwards in the premises. The ventilation noise often propagates into the surrounding area from the supply air and exhaust air terminal units in the rooms. The ducts in themselves may also be important sources of noise, particularly if they are poorly insulated or otherwise designed in such a way that noise generation may occur. 

Local ventilation in industry usually differs from the description above in that it is connected to a local exhaust hood (Chapter 10), which has a capture efficiency less than 100%. The capture efficiency is defined as the amount of contaminants captured by the exhaust hood per time divided by the amount of contaminants generated per each time (see Section 10.5). Figure 8.3 outlines a model for a recirculation system with a specific exhaust hood. Here, the whole system could be situated inside the workroom as one unit or made up of separate units connected with tubes, with some parts outside the workroom. For the calculation model it makes no difference as long as the exhaust hood and the return air supply are inside the room. 

In work areas, the noise is usually generated by different sources, such as air-handling units, refrigerating plants, etc. (especially extract and supply air fans). 

To determine if the local supply air reaches the breathing zone, a tracer gas is used. The tracer concentrations at various heights below the supply unit are recorded as a percentage of the supply duct tracer concentration. This provides information on the degree of mixing taking place between the and ambient air. 

Blow-through unit An air-handling unit downstream of the supply fan. 

Although these units find initial application in areas of limited water, they have not been limited to this situation. In many instances they are more economical than cooling tower systems and have been successfully applied in combination with cooling towers (see Figure 10-184). Economic comparisons should include such items as tower costs, basin, make-up facilities, water treatment, pumps for circulation, power supply, blow down, piping, etc. For small installations of air-cooled units, they should be compared... 

An example would be where the consultant had already invited competitive quotations for supplying air-handling units so that he could design a particular make into the system and ensure that his client received the best value for money. The bidder would include in his bid, either separately shown or in the body of his figures, a sum to cover profit and on-cost in handling the equipment. The contractor appointed would be responsible for organizing delivery and would give a warranty on the equipment, in accordance with the contract. This aspect is amplified in Section 8.20. 

The second method makes use of the pressure energy of the primary (fresh) air supply to induce room (secondary) air circulation. This air, at a pressure of 150-500 Pa, is released through nozzles within the coil assembly, and the resulting outlet velocity of 16-30 m/s entrains or induces room air to give a total circulation four or five times as much as the primary supply. This extra air passes over the chilled water coil. Most induction units are wall mounted for perimeter cooling, but they have been adapted for ceiling mounting. 

Such units may use chilled water or direct-expansion refrigerant, and will have the air filter at the top. It may not be possible to introduce outside air through it, so the room will have a pressurized fresh air supply, which will be filtered to remove fine dusts which may affect the computers. Computer room units work with a very high sensible heat ratio of 0.95 or more, so they have large coils to keep the ADP up near the dew point of the room air. Most will have an inbuilt steam humidifier to replace any moisture which is removed on the coil. 

Considering the countercurrent flow of water and air in a tower of height z (Figure 13.15), the mass rate of flow of air per unit cross-section G is constant throughout the whole height of the tower and, because only a small proportion of the total supply of water is normally evaporated (1 -5 per cent), the liquid rate per unit area ll can be taken as constant. The temperature, enthalpy, and humidity will be denoted by the symbols 6, H, and Tf respectively, suffixes G, L, 1, 2, and / being used to denote conditions in the gas and liquid, at the bottom and top of the column, and of the air in contact with the water. 

Filtered air may be used to purge a complete room, or it m be confined to a specific area and incorporate the principle of laminar flow, which permits operations to be carried out in a gentle current of sterile air. The direction of the airflow may be horizontal or vertical, depending upon the type of equipment being nsed, the type of operation and the material being handled. It is important that there is no obstruction between the air supply and the exposed product, since this may resnlt in the deflection of microorganisms or particulate matter fiom a non-sterile surface and canse contamination. Airflow gauges are essential to monitor that the correct flow rate is obtained in laminar flow units and in complete suites to ensure that a positive pressure fiom clean to less clean areas is always maintained. 

Humidification can be done at the system level by having humidifier section at the air-handling unit supplying fresh air. The humidifier can be electric or gas type. Another method is to use evaporative pads such as GLASdek from Munters. This eliminates the use of gas or electricity for humidification. 

Outdoor air is treated and supplied by a central air-handling unit. The required amount of air is delivered into the return air plenum of the fan coil units. The air-handling unit is provided with a hot water coil and a chilled water coil. Both the coils are served by central reversing heat pump. A total enthalpy wheel can reduce the energy consumed for treating outdoor air. 

The exhaust air of the building is collected in three exhaust air handling units. Indirect evaporative coolers exploit the remaining cooling capacity of the exhaust air and cool the supply air in the dehumidifier via a water loop. This cold recovery makes the system more efficient. Depending on ambient... 

Start-up of the exhaust ventilation system and critical equipment must be sequenced to prevent a hazardous condition. In addition, the starting of the supply air handling unit and the exhaust fan services each room shall initiate simultaneously to avoid placing the room under positive pressure. Automatic transfer switching will be used. 

Each hood intended for toxic work must have a face velocity of 100 linear feet per minute. When many hoods are employed, the volume of tempered air that must be supplied (summer and winter) is quite large. The required airhandling equipment is so massive that minor misadjustments may make it difficult to get out of a room because of air pressure on a door. One way to deal with this is to vent the doors and keep the hallways at a slightly higher pressure than the labs. When an airhandling (supply) unit falls short, the hallway provides needed makeup air. Computers can operate air handling systems more precisely than can traditional systems and an alarm system that pinpoints defective elements for early repair can help avoid gross imbalances. 

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