Cooling towers are major items of heat-transfer equipment in the petroleum and petrochemical plants. They are designed to cool, by air, the water used to cool industrial processes. Cooling of the water by air involves evaporation of a portion
INHALATION HAZARDS IN REFINERIES 145
of the water into the air so that the remaining water is cooled by furnishing heat for this evaporation process. This cooled water is used, in turn, in heat-exchange equipment to cool other liquids and gases.
The two most common styles of cooling towers are natural draft or atmospheric type of cooling tower, and those that employ fans to move the air (known as a mechanical-draft cooling tower). Fan location is used in further classifying the tower as a forced or induced-draft cooling tower. Petroleum and petrochemical operations require large quantities of water for temperature control purposes. The water is normally circulated by pump between the heat-exchange equipment and the cooling tower. The hydrocarbon stream to be cooled can also be circulated directly through the cooling tower. Approximately 1,000 Btu is required to evaporate 1 pound of water. This is equivalent to cooling 100 pounds of water 10 O F . Thus, 1 percent of water is lost through evaporation for every 10 degrees of cooling accomplished. Additionally, a spray loss amounting to no more than 0.2 percent must be included for properly designed atmospheric or mechanical-draft towers.
Water cannot be cooled below the wet bulb temperature of the air entering the cooling tower. The performance of an individual cooling tower is governed by the ratio of weights of air to water and the time of contact between the air and water.
Commercially, the variation in the ratio of air to water is first obtained by maintaining the air velocity constant at approximately 350 fpm per square foot of active tower area and by varying the-water concentration. A secondary operation calls for varying the air velocity to meet the cooling requirements. The contact time between water and air is a function of the time required for the water to be discharged from distribution nozzles and fall through a series of gridded decks to the tower basin. Thus, the contact time is governed by the tower height. If the contact time is insufficient, the ratio of air to water cannot be increased to obtain the required cooling. A minimum cooling tower height must be maintained. Where a wide approach (difference between the cold water temperature and the wet bulb temperature of the inlet air) of 15 to 20 O F to the wet bulb temperature, and a 25 to 35 O F cooling range (difference between the temperature of the hot and cold water) are required, a relatively low cooling tower is adequate (1 5 to 20 feet). The cooling performance of a tower with a set depth of packing varies with water concentration. Maximum contact and performance have been found with a water concentration of 2 to 3 gallons of water per minute per square foot of ground area.
The problem in designing a cooling tower is one of determining the proper concentration of water to obtain desired cooling. A high cooling tower must be used if the water concentration is less than 1.6 gallons per square foot. Low towers can be employed if the water concentration exceeds 3 gallons per square foot. If the required water concentration is known, the tower area can be found by dividing the water circulation rate (gallons per minute) by the water concentration (gallons per
minute per square foot). The required tower size is dependent upon: (1) cooling range (hot water minus cold water temperature); (2) approach (cold water minus wet bulb temperature); (3) amount of liquid to be cooled; (4) wet bulb temperature;
( 5 ) air velocity through cell; and (6) tower height. Cooling towers used in conjunction with equipment processing hydrocarbons and their derivatives are potential sources of air pollution, and hence inhalation hazards, because of possible contamination of the water. The cooling water may be contaminated by leaks from the process side of heat-exchange equipment, direct and intentional contact with process streams, or improper process unit operation. As this water is passed over a cooling tower, volatile hydrocarbons and other materials accumulated in the water readily evaporate into the atmosphere. When odorous materials are contained in the water a nuisance is easily created. Inhibitors or additives used in the cooling tower to combat corrosion or algae growth should not cause any significant air pollution emissions, nor should the water-softening facilities common to many cooling towers be a problem.
The control of hydrocarbon discharges or of release of odoriferous compounds at the cooling tower is not practical. Instead, the control must be at the point where the contaminant enters the cooling water. Hence, systems of detection of contamination in water, proper maintenance, speedy repair of leakage from process equipment and piping, and good housekeeping programs in general are necessary to minimize the air pollution occurring at the cooling tower. Water that has been used in contact with process streams, as in direct-contact or barometric-type condensers, should be eliminated from the cooling tower if this air pollution source is to be completely controlled. Greater use of fin-fan coolers can also control the emissions indirectly by reducing or eliminating the volume of cooling water to be aerated in a cooling tower.
In certain refining operations, air is blown through heavier petroleum fractions for the purpose of removing moisture or agitating the product. The exhaust air is saturated with hydrocarbon vapors or aerosols, and, if discharged directly to the atmosphere, is a source of air pollution. The extent of airblowing operations and the magnitude of emissions from the equipment vary widely among refineries.
Emissions from airblowing for removal of moisture, or for agitation of products may be minimized by replacing the airblowing equipment with mechanical agitators and incinerating the exhaust vapors.
Refinery operations frequently require that a pipeline be used for more than one product. To prevent leakage and contamination of a particular product, other products connecting and product feeding lines are customarily "blinded off.
"Blinding a line" is the term commonly used for the inserting of a flat, solid plate between two flanges of a pipe connection. Blinds are normally used instead of valves to isolate pipelines because a more positive shutoff can be secured and
INHALATION HAZARDS IN REFINERIES 147
because of generally lower costs. In opening, or breaking, the flanged connection to insert the blind, spillage of product in that portion of the pipeline can occur. The magnitude of emissions to the atmosphere from this spillage is a function of the vapor pressure of the product, type of ground surface beneath the blind, distance to the nearest drain, and amount of liquid holdup in the pipeline. Emissions to the atmosphere from the changing of blinds can be minimized by pumping out the pipeline and then flushing the line with water before breaking the flange. In the case of highly volatile hydrocarbons, a slight vacuum may be maintained in the line.
Spillage resulting from blind changing can also be minimized by use of "line"
blinds in place of the common "slip" blinds. Line blinds do not require a complete break of the flange connection during the changing operation. These blinds use a gear mechanism to release the spectacle plate without actually breaking the line.
Combinations of this device in conjunction with gate valves are available to allow changing of the line blind while the line is under pressure from either direction.