Heat, Odor, and Moisture Removal
Removal of concentrated heat, odor, or objectionable vapors by ventilation is best
carried out by locating the exhaust outlets as close as possible to the heat source.
When concentrated sources of heat are present, canopy hoods will remove the heat more efficiently.
Figure 13.1 shows a canopy-hood installation over a kitchen range. Grease filters
reduce the frequency of required cleaning. When no grease is vaporized, they may be eliminated.
Greasy ducts are serious fire hazards and should be cleaned periodically. There are on the market a number of automatic fire-control systems for greasy ducts.
These systems usually consist of fusible-link fire dampers and a means of flame smothering CO2, steam, foam, etc.
Figure 13.2 shows a double hood.
This type collects heat more efficiently;
i.e., less exhaust air is required to collect a given amount of heat. The crack area is arranged to yield a velocity of about 1000 ft /min.
A curtain of high-velocity air around the periphery of the hood catches the hot air issuing from the range or heat source. Canopy hoods are designed to handle about 50 to 125 ft3 /min of exhaust air per square foot of hood. The total amount of ventilation air should not yield more than 60 changes per hour in the space.
Where hoods are not practical to install and heat will be discharged into the room, the amount of ventilation air may be determined by the following method:
Determine the total amount of sensible heat generated in the premises lights, people, electrical equipment, etc. This heat will cause a temperature rise and an increase in heat loss through walls, windows, etc. To maintain desired temperature conditions, ventilation air will have to be used to remove heat not lost by transmission through enclosures.
With Eq. (13.24), we can calculate the amount of ventilation air required by assuming a difference between room and outdoor temperatures or we can calculate this temperature gradient for a given amount of ventilation air.
The same method may be used to calculate the air quantity required to remove any objectionable chemical generated. For example, assume that after study of a process we determine that a chemical will be evolved in vapor or gas form at the rate of X lb /min. If Y is the allowable concentration in pounds per cubic foot, then Q = X/Y, where Q is the ventilation air needed in cubic feet per minute.
Where moisture is the objectionable vapor, the same equation holds, but with X as the pounds per minute of moisture vaporized, Y the allowable concentration of moisture in pounds per cubic foot above outdoor moisture concentration.
Once, the amount of ventilation air is determined, a duct system may be designed to handle it, if necessary.
Ventilation air may be provided by installing either an exhaust system, a supply system, or both.
In occupied areas where no unusual amounts of heat or odors are generated, such as offices and shipping rooms, a supply-air system may be provided, with grilles or ceiling outlets located for good distribution. When the building is tight, a relief system of grilles or ducts to the outside should be provided. But when the relief system is too extensive, an exhaust fan should be installed for a combination supply and exhaust system.
All air exhausted from a space must be replaced by outside air either by infiltration through doors and windows or by a fresh-air makeup system. Makeup air systems that have to operate during the winter season are often equipped with heating coils to temper the cold outside air.
Natural Ventilation
Natural ventilation in buildings is accomplished by use of windows, louvers, skylights, roof ventilators, roof monitors, jalousies, intake hoods, etc. They should be located to admit fresh air only and not near sources of smoke, dust, odors, or polluted air from adjacent sources. Discharge vents should also be provided to eliminate vitiated air from the building. The outlet locations must not discharge toward other fresh-air intakes of the building or its neighbors. In multifloor buildings, vertical vent shafts, or risers, are used to supply ventilation air throughout the building.
Mechanical Ventilation
Mechanical ventilation is almost always preferred over natural ventilation because of reliability and the ability to maintain specific design requirements, such as air changes per hour and face velocities for exhaust hoods. Natural ventilation permits wide variations in ventilation-air quantities and uncertain durations of ventilation.
(In critical areas, such as in carcinogenic research laboratories, natural ventilation is never relied upon.) For this reason, mechanical ventilation systems are almost always used where ventilation requirements are critical and must be highly reliable.
Mechanical ventilation is often required by various building codes for various applications as follows:
1. Control of contaminants in the work area for health protection and compliance with OSHA standards for achieving the legal limits set on employee exposure to specific toxic and hazardous substances
2. Fire and explosion prevention for flammable vapors
3. Environmental protection
4. Reuse of valuable industrial materials
5. Human comfort removal of heat, odors, and tobacco smoke
6. Humidity control
7. Corrosive fumes and noxious gases
Mechanical ventilation may be a single system without heating, cooling, filtration, humidification, dehumidification, etc., or it may include various combinations of these functions. In other words, the systems can be heating-ventilating units or heatingventilatingair-conditioning (HVAC) units.
In many complex and specialized buildings, certain functional areas will be required to have various degrees of positive pressurization, negative pressurization, or balanced atmospheric conditions. The ventilation air as a part of the system supply air is used to provide the positive and balanced pressures. An exhaust system is utilized to maintain the negative-pressure areas. In many designs, air lost by pressurization is exfiltrated from the system and does not become part of the returnair stream.
Recirculation of ventilation air is prohibited from certain areas, such as toilets, bathrooms, biology labs, chemistry labs, hospital operating rooms, mortuary rooms, isolation rooms, and rooms with flammable vapors, odors, dust, and noxious gases.
In all ventilation systems, a quantity of air equal to the ventilation air should leave the building. If this is not accomplished, then the building will become pressurized, and the ventilation air will exfiltrate through available doors, windows, cracks, crevices, relief vents, etc. Since in many cases this is undesirable and unreliable, exhaust systems are usually employed. The exhaust, in many cases, may be part of a complete HVAC system.
