Ruks Engineering Ltd. CT Water Systems

Introduction | Feasibility Study | Specifications | Air as Feed Gas | Drawings | Reference

Air as Feed Gas

WHY SHOULD AIR AND NOT OXYGEN BE USED AS FEED GAS TO OZONE GENERATOR IN COOLING TOWER APPLICATIONS

1. It is strongly advised to avoid using oxygen as feed gas to ozone generator system for cooling tower applications, due to following reasons;

Ozone system requires continuous production of oxygen of 90% purity, and this oxygen is produced at site. As oxygen is highly explosive, continuous production and presence of oxygen demands high level of precaution and strict protocols for operation and maintenance of the system, in the plant room. Any compromise of this may lead to potentially dangerous explosion. Adherence to such care and protocol adds substantially to capital, operation, maintenance and supervisory costs of the system. Particularly, for applications such as public buildings, where safety from explosion is very important, continuous production and presence of oxygen should be avoided. Strict statutory codes in many states stipulate oxygen systems be housed in totally separate, independent, and ventilated plant rooms, due to the threat of explosion. It is cost intensive to meet compliance to this code. We will be glad to present to you copy of this code for your evaluation.

Due to high susceptibility to explosion, insurance premiums on plants with continuous presence of oxygen may be higher.

Oxygen plants use negative pressure injection system for introduction of ozone into water stream. Venturi is used to create this negative pressure. Air under pressure (developed by air compressor) is passed through absorbers filled with molecular sieve, to produce oxygen. The pressure is then dropped down to atmospheric pressure. Movement of oxygen through the generator, and delivery pipes, to point of injection is under negative pressure. As dielectric medium have high resistance, suction force required is high. This results in high-energy consumption. For comparison, in an application with use of oxygen as feed gas, if HP of air compressor is about 10 HP, the same is about 5.5 HP if air is used as feed gas. This is a significant difference in the operating cost of the system. When factored over life of the system, the difference in operating cost is substantial.

Oxygen feed systems use Venturi injector to transfer produced ozone to water stream. This requires high pressure at inlet to Venturi, and hence needs high discharge pressure by the injection water pump, requiring higher HP for the pump. In system using air as feed gas, we may use vertical loop injection system. This requires significantly lower pump pressure, resulting in lower pump HP and lower operating cost. For comparison, in an application, if HP of side stream pump for oxygen feed system using venturi injector is about 10 HP, the same is about 3 HP for air feed system if vertical loop injection is used. This is a significant difference in the operating cost of the system. When factored over life of the system, the difference in operating cost is substantial.

Oxygen is highly corrosive. Not all of the produced oxygen is converted to ozone. For example, at ozone concentration of 6% (wt/wt in oxygen), for every pound of ozone produced, 94 Lbs of 90% pure oxygen is introduced into the condenser water system. The presence of oxygen causes rapid corrosion of copper in the condenser tubes and steel in condenser water pipes. Many installations initially designed with oxygen as feed gas have converted to air as feed gas, due to excessively high corrosion experienced. Some manufacturers of oxygen based ozone systems maintain oxygen is entirely dissolved in the water (at the point of injection of ozone into the water stream) due to the high pressure in the venturi. Only undissolved oxygen is corrosive. While this is true, it should be remembered, in a cooling tower application, water from the injection system quickly enters the tower basin, and at this point its pressure is reduced to atmospheric pressure. This pressure change releases part of dissolved oxygen back to undissolved state. These atoms of oxygen promote corrosion. Life of the HVAC system is reduced rapidly due to this. Also, pressure of water in the condenser system changes. This change in pressure releases part of tne dissolved oxygen, which promotes corrosion. This explains the reason for commencement of corrosion at points where pressure drops in the condenser water system. It is for this reason, most oxygen fed systems, use supplementary chemicals for corrosion suppression. Air fed systems do not use any type of supplementary chemicals.

Operating and maintenance cost of oxygen concentrator is added expense. This should be given consideration, as the molecular sieve of oxygen concentrator should be replaced periodically.

Benefits of using oxygen fed system for cooling tower application is almost none, while disadvantages are many.

Oxygen feed system is a good choice for high concentration applications such as municipal drinking water systems, paper and pulp bleaching plants, where ozone concentrations of 8% or higher is desired and is beneficial.