Types of Air Curtains and Their Application
According to the application and aerodynamic pattern, the following types of air curtains have been designed:
• Air curtains with heated indoor air
• Air curtains with unheated indoor air
• Combined air curtains with indoor air
• Air curtains with unheated outdoor air
• Air curtains for cooled rooms
• Air curtains for gates with long passages
• Air curtains for process equipment
The first four air curtain types are installed in outer apertures of gates in heated rooms and also in unheated rooms where a standard temperature should be maintained in the working zone. They are designed to prevent the ingress of outdoor air during the cold season of the year. Air curtains for cooled rooms are designed to prevent the ingress of warm outdoor air. If the heating program for the room is to be replaced by the cooling program during the year, it is advisable to equip the gate with both an air curtain that prevents ingress of cold and one that prevents ingress of heat.
Air curtains for process equipment are designed to prevent toxic constituents from entering the room through apertures of the equipment.
The traditional design pattern (to be described in Section 7.7.5) is recommended for air curtains with heated indoor air. The air curtain may be doublesided with horizontal supply (Fig. 7.86a), or it may be single-sided (Fig. 7.86b) with horizontal or vertical supply. In all cases, the air curtain is a flat jet discharged at an angle toward the pressure side of the opening.
Air curtains with heated indoor air are used for relatively small gates (up to 3.6 x 3.6) in genial climate and in buildings without skylights, and also in case constructions prevent the installation of combined air curtains (e. g., not enough space at the gate). Air curtains with heated indoor air guarantee the necessary temperature of the air mixture entering through the gate, but they also consume a relatively large amount of thermal energy.
Air curtains with unheated indoor air have a similar air supply arrangement. Only the design of the air intake duct might differ; it may be extended to take in warm air, for example, from the upper overheated zone of the room. Air curtains with unheated indoor air are recommended in case the standard temperature of the air coming in through the aperture of the open gate can be maintained without heating the air in the curtain, as in the following cases:
• Rooms with an overheated upper zone of over 2 xC (if it is possible to use the air from the upper overheated zone of the room)
• Rooms with excess heat (if it is technically possible and reasonable to utilize it)
• Rooms with a low standard air temperature in the working zone near (he gates (8 °C and below)
Air curtains with unheated indoor air are more restricted in application than air curtains with heated indoor air, but they do not require the installation of air heaters in the curtain devices.
Air curtains with unheated outdoor air are recommended with a onesided lateral supply of outdoor air in the form of a flat jet at an angle to the plane surface of the gate aperture toward the outdoor air (Fig. 7.86b). In
This case the outer surface of the enclosure should not have any obstructions on the opposite side that might prevent the jet from flowing along the surface of the gate. This free length should be no less than the approximate width of the gate.
Air curtains with unheated outdoor air find application in unheated rooms and also in case there are no strict hygiene requirements for the microclimate in the gate zone (no working places near to the gate, e. g.). Air curtains with unheated outdoor air should not be used in double-wing gates, in humid rooms, and in cases of transport with an open driver’s cab through the gate.
Air curtains with unheated outdoor air do not provide for the necessary microclimate in the immediate vicinity of the gate, but since no thermal energy is used, they reduce heat losses from the room.
Combined air curtains with indoor air are recommended with a doublesided supply of heated and unheated air fed to the room in the form of plane jets at an angle of 15° to one another (Fig. 7.86c).
Delivery ducts for unheated (outdoor) air are located in the room right against the gate. Delivery ducts for air heated in the air curtain device (indoor) are installed inside the room behind the outdoor delivery ducts, with a clearance for exhaust air between the outdoor and indoor delivery ducts of the air curtain.
Recommended applications of combined air curtains with indoor air include
• Severe climate areas
• Locations with several gates (three or more)
Combined air curtains with indoor air cut down expenses due to rational utilization of energy by the heated jets. Thermal energy savings are 25-60% depending on the dimensions of the gate and climatic region, and the reduction in expenditures is 30-70%.
Air curtains for cooled rooms are recommended with a one-sided supply of outdoor air from above in the form of a flat jet at an angle to the plane surface of the gate toward the cold indoor air (Fig. 7.87).
Air curtains for cooled rooms are used in all types of rooms with artificial cooling of air: vegetable stores, cold rooms, freezers, air-conditioned plants and storehouses, etc. Installation of air curtains for cooled rooms considerably reduces cold losses through the open gate and also reduces undesirable variations in temperature in the gate zone inside and outside the cooled room.
Air curtains for gates with long passages use a pattern of air supply “curtains in a channel” (Fig. 7.88). The operating principle is based on the complete conversion of the jet impulse to counterpressure that prevents outdoor air from bursting into the room.
The air is fed against the bursting airflow or at a minor angle. The jet of the curtain developing in the channel is dampened, and from the end of the channel it turns to the opposite direction. This creates a closed circulation proof against outside effects.
The length of the passage should be chosen so that it prevents the air from being released outside. To reduce the length of the passage, the air is normally supplied in the form of incomplete spray jets with nozzles of special design. Another alternative is to feed outdoor air into the passage (Fig. 7.89).
Air curtains for process equipment are designed to prevent the ingress of toxic constituents (gases, aerosols, heat flows) into the room through open apertures of the process equipment. They also support necessary parameters of technological processes in the plant. Processes running in the technological equipment are classified as isothermal (e. g., spray-painting chambers) and nonisothermal (e. g., heat dryers). In isothermal processes one uses damper-type air curtains for process equipment in combination with an exhaust system (Fig. 7.90). In nonisothermal processes one uses a circulation system based on the “curtains in a channel” operating principle (Fig. 7.91).
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