Ventilation of Kitchens
In estimating the requirements for the ventilation of kitchens, the following two methods should be considered:
1. It is customary to allow a complete change of air every 2 minutes.
2. In many cases it is desirable to have all the extracted air leave via hoods or canopies located over ranges, steam tables, urns, dishwashers, and so on.
The first method applies only to average conditions, and modification from this average should be made depending on the kitchen size and the heat- and vapor-producing equipment.
In the second method, the air volume should be calculated from the hood entrance velocity rather than the air-change method.
Light cooking requires an entrance velocity of only 50 feet per minute, while severe conditions may run to 150 feet per minute or higher.
The size of a hood will depend on its dimensions. For example, a hood 3 feet by 8 feet would have an area of 24 square feet using an average velocity of 100 feet per minute.
Where quiet operation is essential, the blower should be selected on the basis of a low outlet velocity. This will also result in lower operating costs.
If space is limited and noise is not a factor, smaller units with higher outlet velocities may be necessary. This may result in a lower initial cost.
General Ventilation Rules
The American Society of Heating and Ventilating Engineers offers the following recommendations for designing and installing an adequate natural ventilation system:
1. Inlet openings in the building should be well distributed and should be located on the windward side near the bottom, while outlet openings are located on the leeward side near the top. Outside air will then be supplied to the zone to be ventilated.
2. Inlet openings should not be obstructed by buildings, trees, signboards, and so on, outside or by partitions inside.
3. Greatest flow per square foot of total openings is obtained by using inlet and outlet openings of nearly equal areas.
4. In the design of window-ventilated buildings, where the direction of the wind is constant and dependable, the orientation of the building, together with amount and grouping of ventilation openings, can be readily arranged to take full advantage of the force of the wind. Where the wind’s direction is variable, the openings should be arranged in sidewalls and monitors so that, as far as possible, there will be approximately equal areas on all sides. Thus, no matter what the wind’s direction, there will always be some openings directly exposed to the pressure force and others to a suction force, and effective movement through the building will be ensured.
5. Direct shortcuts between openings on two sides at a high level may clear the air at that level without producing any appreciable ventilation at the level of occupancy.
6. In order for temperature difference to produce a motive force, there must be vertical distance between openings. That is, if there are a number of openings available in a building, but all are at the same level, there will be no motive head produced by temperature difference, no matter how great that difference might be.
7. In order for the forces of temperature difference to operate to maximum advantage, the vertical distance between inlet and outlet openings should be as great as possible.
Openings in the vicinity of the neutral zone are less effective for ventilation.
8. In the use of monitors, windows on the windward side should usually be kept closed, because if they are open, the inflow tendency of the wind counteracts the outflow tendency of temperature difference. Openings on the leeward side of the monitor result in cooperation of wind and temperature difference.
9. In an industrial building where furnaces that give off heat and fumes are to be installed, it is better to locate them in the end of the building exposed to the prevailing wind. The strong suction effect of the wind at the roof near the windward end will then cooperate with temperature difference to provide for the most active and satisfactory removal of the heat and gas- laden air.
10. In case it is impossible to locate furnaces in the windward end, that part of the building in which they are to be located should be built higher than the rest so that the wind, in splashing, will create a suction. The additional height also increases the effect of temperature difference to cooperate with the wind.
11. The intensity of suction or the vacuum produced by the jump of the wind is greatest just behind the building face. The area of suction does not vary with the wind velocity, but the flow due to suction is directly proportional to wind velocity.
12. Openings much larger than the calculated areas are some- times desirable, especially when changes in occupancy are possible or to provide for extremely hot days. In the former case, free openings should be located at the level of occupancy for psychological reasons.
In single-story industrial buildings, particularly those covering large areas, natural ventilation must be accomplished by taking air in and out of the roof openings. Openings in the pressure zones can be used for inflow, and openings in the suction zone, or openings in zones of less pressure, can be used for outflow. The ventilation is accomplished by the manipulation of openings to get airflow through the zones to be ventilated.