X hits on this document

PDF document

State of California Department of Pesticide Regulation - page 68 / 104

211 views

0 shares

0 downloads

0 comments

68 / 104

60

minimal amount of chemical treatment. An example of this type of water cooling system is the radiator used on water-cooled automobile engines.

Once-Through Systems. Once-through systems require an abundant source of cool water, such as from a well, river, stream, or sometimes the ocean. The cool water removes heat and is then disposed of, usually back into the water source. Chemical treatment of the water for control of microorganisms is very costly since the water is not reused and chemicals are lost in the water discharge. Discharging water treatment chemicals in this way becomes a source of environmental pollution.

Open Recirculating Systems. Most systems commonly in use on large installations are the open recirculating type. These systems are more economical to install than closed circulating systems and waste much less water and treatment chemicals than once-through systems. However, because the water in these systems is open to the air, this water becomes vulnerable to microorganism contamination. In addition,tomake upwaterloss from evaporation,leaks,andblowdown,thesesystems require periodic addition of water. Blowdown is a process where operators drain off some of the water in the system and add fresh water to reduce the concentration of salts, minerals, and other dissolved materials that build up due to evaporation.

Pumps move warm water from heat exchangers, located in the heat-generating area, to the cooling tower. In the tower, the water flows over and through a suitable tower fill consisting of wooden slats or molded honeycombed sections. This cooling tower fill provides a large surface area where the cascading water contacts counter- flowing air, providing maximum evaporation and subsequent cooling. As the cooled water reaches the tower basin, pumps recirculate it through the heat exchangers, completing the cycle. Typically, the water cooling tower has a large fan to move air across the cascading water in the tower fill. In some styles, a natural draft of air, rather than a fan, cools the water.

PROBLEMS ASSOCIATED WITH WATER COOLING TOWERS

In addition to harboring disease organisms, cooling systems are subject to several problems caused by water quality or microbial growths. These include:

  • scale deposits

  • corrosion

  • fouling

  • wood deterioration

Such conditions reduce the efficiency of heat transfer, damage components made of wood or steel, and restrict the flow of water through the system. Proper chemical treatment, coupled with periodic blowdown to replace some of the water, reduces these problems.

Scale Deposits. Scale deposits are the result of the precipitation of concentrated minerals in the water. These minerals adhere to surfaces of the water cooling tower and inner surfaces of associated plumbing. Higher water temperature increases scale problems because minerals precipitate more quickly. Scale deposits interfere with proper heat transfer and may prevent adequate water circulation. Scale deposits also provide places that harbor bacteria and other microorganisms.

Corrosion. Corrosion of metal components occurs more readily in open recirculating systems because of bacterial action and contact of the water with air. At the water/air interface, water picks up oxygen, corrosive gases, and other airborne

MICROORGANISM CONTROL IN WATER COOLING SYSTEMS

Document info
Document views211
Page views211
Page last viewedFri Dec 02 18:32:06 UTC 2016
Pages104
Paragraphs2536
Words34926

Comments